introduction instrumentation d.s. bigelow, j.r. slusser, j...
TRANSCRIPT
Interim Report 1996: USDA Ultraviolet Radiation Monitoring Program
D.S. Bigelow, J.R. Slusser, J.H. GibsonNatural Resource Ecology Laboratory
Colorado State UniversityFort Collins CO 80521
January 1997
USDA Agreement # 94-34263-0687
Executive Summary
The discovery of the Antarctic ozone hole in 1985, accompanied by large increases in surface ultraviolet-B(UVB) radiation, has raised serious questions about the continued protection of the earth's living systemsfrom the harmful effects of UVB radiation. To assess the potential for damage that increased UVBradiation might have on agricultural crops and forests, the USDA established a UVB monitoring networkand began measuring UV radiation with broadband UVB-1 pyranometers in 1994. Because of the limitedinformation available from broadband instruments a new multi-spectral instrument has been developed toprovide spectral UVB data at network sites in support of biological and atmospheric science research. This7 wavelength UV Multi-filter Rotating Shadowband Radiometer (UV-MFRSR), manufactured by YankeeEnvironmental Systems, has now been installed at 10 of the network's 20 sites. Plans call for an expansionof the network to at least 26 sites, each equipped with a UV-MFRSR, UVB-1, and VIS-MFRSR.
Twelve of the UVB broadband instruments were recalibrated at least once since 1994 allowing comparisonof their spectral responses over time. The comparisons indicate a non-linear drift in the spectral responseof the detectors. This suggests possible non-reproducibility in the laboratory characterizations andunderscores the need to have calibrations carried out at the newly established NOAA interagency supportedcalibration facility. The stability of the multi-spectral instrument's filters’ transmission and centerwavelength is a factor critical to obtaining accurate multi-spectral data. In a test of the stability of themulti-spectral instrument's filters, Langley plots for VIS-MFRSRs were obtained for clear days. It wasdetermined that many units exhibited a decline in the Langley intercept which is indicative of either changesin filter spectral response or degradation of the diffuser. Work in progress will permit routine adjustmentsfor common wavelength mis-registration and drift. The application of cosine corrections and laboratorycalibrations to the raw VIS-MFRSR and UV-MFRSR data has been implemented in a prototype stage andis slated for general use in early 1997. Finally, the success rate of querying sites by phone has beenincreased by developing new software.
Because the current laboratory calibrations of the UV-MFRSR are in some doubt an alternative means ofchecking calibration is being investigated. The method requires knowing the extraterrestrial irradiance,currently measured to within 5% agreement by different satellites, the relative response of the detector, andthe Langley voltage intercept. Initial comparisons at Table Mountain CO of UV-MFRSR irradiancescalibrated using this method agree to an average of 7% with a radiative transfer model and to within an
IntroductionNetwork Expansion
InstrumentationCalibration and QAData Management
ResearchMeetings
Future WorkReferences
Appendix AAppendix BAppendix CAppendix DAppendix E
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average of 14% of a Brewer spectroradiometer. It is recommended that further investigation of the Langleymethod of calibration be moved to a research site at Mauna Loa Observatory Hawaii.
The network continues to intercompare different instruments at its Central Plains Experimental Range COsite. Two new UV-MFRSRs and a Smithsonian 18 channel SR-18 will be added to the site in 1997. DuringJune 1996 the network co-sponsored a third Annual Spectroradiometer Intercomparison at Table MountainCO. The intercomparison brings together instrumentation from various federal programs in an attempt toidentify bias in calibrations, angular response and spectral response of the various instruments.
An increase in the number of data requests has necessitated the streamlining of codes and the acquisition ofnew, more efficient database management software. The transfer of network data to interested clients hasbeen facilitated by the establishment of an ftp directory which is updated every two weeks. Daily plots ofmulti-spectral UV-MFRSR and broadband UVB-1 data are also routinely made available on the World-Wide-Web (http://uvb.nrel.colostate.edu/UVB ) the day after the data is measured. To further increase theefficiency of data dissemination and storage, the network has acquired a CD ROM writer.
Ozone has been successfully retrieved using data from the newly deployed UV-MFRSR under clear skies,and work is underway to retrieve ozone columns during cloudy weather. We are also collaborating withNASA Goddard to provide ground truth from our site at Table Mountain CO for their TOMS/ADEOSsatellite data. This collaboration uses a model to predict surface UV irradiances for all of the US and mayresult in effectively extending the range of our network beyond 26 sites.
Conferences attended by the network personnel included the NASA/TOMS Intercomparison in GreenbeltMD, the International Radiation Symposium in Fairbanks AK and the Synthetic Spectra Workshop, FortCollins CO. Presentations were made by network staff at all these conferences.
The network recommends expanding to at least 26 sites to provide the minimum grid-based networkoriginally proposed. It is recommended that wherever possible the network continue to collocate withestablished UV instrumentation to allow comparisons of measurements. The NOAA Calibration Facility inBoulder CO should continue to be given full support by the network as it will eventually perform all of thenetwork's UV calibrations including cosine corrections and recharacterizations after field use. Monitoringfilter stability through continued analysis of Langley intercepts and frequent recalibration should also bemaintained.
Given the network's recent advances in UV instrumentation and the successful demonstration of the use of models to generate synthetic spectra we believe it is timely to convene a group of biologists and botanistsstudying the effects of UV radiation on biological systems for the purpose of reaching a consensus onexactly what sort of data products will be needed from the network to further their research.
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Interim Report: USDA Ultraviolet Radiation Monitoring Program, 1996
D.S. Bigelow, J.R. Slusser, J.H. Gibson
Introduction
The USDA Ultraviolet Radiation Monitoring Program was established in 1993 to provide the USDepartment of Agriculture with the information necessary to determine if changing levels of ultravioletlight have an effect on food and fibre production in the United States. Prior to the establishment of theprogram only limited information was available to make such an assessment and the geographicdistribution and quality of this information was insufficient to meet the requirements of the agency (Gibson,1991; UVB Monitoring Workshop, 1992). Two different but complimentary actions were taken by theagency to begin to obtain the information necessary to make its assessment. The first solicited proposals forthe development of an improved spectroradiometer (see Appendix A) and the second action which isreported in this document, established an ultraviolet radiation monitoring program.
The primary objective of the USDA Ultraviolet Radiation Monitoring Program is to provide information tothe agricultural community about the geographic and temporal climatology of UVB irradiance. Its data isintended to assist scientists in relating changes in stratospheric ozone to changes in ultraviolet light and toimprove the understanding of the factors which control ultraviolet light. Both are critical in assessing theimpacts of changing UV light on agricultural systems. Since the establishment of the network the data hasfound use with model developers, human health effects researchers, ecosystem scientists and those seekinga ground truth measurement for satellite systems.
The initial network of twelve stations was established with broadband meters and ancillary measurementsof temperature, humidity and seven wavelengths of visible light produced by the Multi-Filter RotatingShadowband Radiometer (MFRSR) (Harrison et al., 1994), the latter instrument serving as a intelligentdata logging and communications system for all of the measurements taken at a single site. Since thatbeginning the network has expanded to 20 stations and the broadband meters have been supplemented witha seven wavelength ultraviolet rotating shadowband radiometer referred to in this document as a UV-MFRSR.
Accomplishments in 1996
Climate Network Expansion
It has been estimated that 20-40 monitoring locations are necessary to adequately define a useful nationalUV climatology (Gibson, 1992). During 1996 the network expanded from 12 to 20 sites which included theestablishment of a research site near Boulder, Colorado at Table Mountain, the site of the annual NorthAmerican Spectroradiometer Intercomparisons. Locating at the Intercomparison site provides the networkwith continuous access to a spectroradiometer which can be used to validate proposed routine products ofthe USDA monitoring program such as synthetic spectra and optical depth retrievals (see Research Sectionbelow).
1 The naming of products in this report does not constitute an endorsement by the USDA. Products are identified solelyfor the convenience of the reader.
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Figure 1 Locations of Sites in the USDA UVB MonitoringProgram in 1996
Other sites established in 1996 include GrandCanyon, Arizona; Baton Rouge, Louisiana;Wye, Maryland; Grand Rapids, Minnesota;Mead, Nebraska; Big Bend, Texas; Underhill,Vermont and Lake Dubay, Wisconsin. Current sites in the network are shown inFigure 1.
Additional sites chosen but not yet installedinclude Poker Flat, Alaska; Mauna Loa,Hawaii; Fort Peck, Montana and EvergladesNational Park in Florida. It is anticipated thatthe network will be complete and fullyinstrumented by the end of 1997.
Instrumentation (Hardware)
Visible Multi-Filter Rotating Shadowband Radiometers (VIS-MFRSR) - The VIS-MFRSR is a 7 channel,10 nm full-width, half-maximum (FWHM) passbanded shadowbanded radiometer that measures visiblelight at 415, 500, 610, 665, 860 and 960 nominal wavelengths. Yankee Environmental Systems (YES) ofTurners Falls, Massachusetts became the sole vendor of the VIS-MFRSR instrument during 1993 throughan exclusive licensing agreement with the instrument's developers at the State University of New York atAlbany (SUNY) and the Pacific Northwest Laboratory (PNL). This change in vendor resulted in a numberof improvements to the shadowband instruments which were not entirely compatible with the network'sexisting polling activities and the instruments originally purchased from PNL. The acquisition anddeployment of the new YES instruments beginning in 1996 required that many of these differences beaddressed.
The YES shadowbands support higher modem speeds which lead to reduced phone bills and faster datatransfers, thereby reducing the risk of polling failures due to line interruptions. Coincidently the networkwas required to move away from the Hayes 2400 baud Pocket Modems1 originally supplied as standardequipment with the PNL shadowbands. Not only were they no longer manufactured but new FederalCommunications Commission guidelines specified that modems were to no longer be powered by telephoneline power as is the case with the pocket modem. Finding a replacement modem however, proved difficultbecause of the wide variety of phone line quality throughout the network and the quirkiness of theshadowband’s communication system. Four brands of modems were evaluated in various combinations ofhost and remote machine configurations. A US Robotics External 28,800 Sportster1 model proved to bethe most robust for the network's applications and is now being used throughout the network. New versions of firmware controlling the operation of the shadowband also appeared with the YESshadowbands. In addition to fixing a few obscure operational bugs, the new firmware gave the instrumentsmore functionality (band angle control, expanded memory management, automatic head and boardidentification coding) and a new access password. Unfortunately, the new firmware was not backwardly
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compatible with older PNL equipment and required that clock crystal speeds of the older PNL units beboosted from 5Mhz to 12Mhz. The network is in the process of making this upgrade but has beenhampered by new problems identified in the upgraded equipment. These include a band wobble, solved witha firmware modification, and a time keeping problem requiring frequent clock adjustments (twice per week)to the upgraded units. Modifications are significant enough that the network must maintain atroubleshooting inventory of both PNL and YES shadowband parts. PNL and YES shadowband hardwarecomponents are not considered to be interchangeable.
Ultraviolet Multi-Filter Rotating Shadowband Radiometers (UV-MFRSR) - In addition to the new locations, the network began expanding the instrumentation at each site to include a new seven channelultraviolet version of the visible Multi-Filter Rotating Shadowband Radiometer. This new shadowbandinstrument manufactured by YES contains 2 nm FWHM filters at 300, 305, 317, 325, 332 and 368 nmnominal wavelengths. Ten sites were equipped with the UV-MFRSR in 1996. Prototype instruments werefirst made available to the network by YES during the 1995 North American SpectroradiometerIntercomparison and then throughout the following year. The first production instrument was deployed tothe Davis, California site in August of 1996.
With the addition of a second shadowband at each polling location (the UV-MFRSR) the network faced theprospect of adding a second phone line to each site. However, additional phones lines are not alwaysavailable in remote locations. Phone switches were investigated for these locations and found to besuccessful in most circumstances but it was discovered that the line voltage requirements of phone switchesvary widely. Numerous switches were tested before an acceptable one was found to work in most of thenetworks locations. It is anticipated that the network will be able to meet its polling requirements with acombination of phone switches and a minimal number of extra phone lines.
Broadband UVB Pyranometers - Original network monitoring locations were equipped with only a VIS-MFRSR and a broadband UVB pyranometer. The pyranometers, YES model UVB-1 measuringbroadband UV between 280 and 320 nm, were only envisioned as a temporary UVB monitoring instrumentuntil a higher quality instrument became available. Because of this they were only deployed at the network'soriginal 12 sites. In the spring of 1996 the network decided to retain the broadband pyranometers at itsoriginal sites and add the broadband to its standard array of instrumentation. This decision was based uponthe observation that broadband meters continue to be used by many researchers including those involved inhuman health effects of ultraviolet light. The continuation of broadband meters in the network also helps tomaintain an historical reference to past studies which have used the meters.
Ancillary Instruments - Each site continues to be equipped with a temperature/ humidity sensor and adownward looking LI-COR1 photometer for indicating the presence or absence of snow cover. A barometerhas additionally been installed at the Table Mountain research site to allow pressure to be factored into theozone retrieval and synthetic spectra models being evaluated at this site.
Smithsonian SR-18 UVB Radiometer - The network took delivery of its first SR-18 in late May of 1996and added it to the instrumentation at its Colorado site at the Central Plains Experimental Range in June.This instrument is a 2 nm FWHM, multi-filter radiometer with 18 channels that span the range of 290-324nm. The instrument has run continuously at the site with only minor interruptions due to a failed powersupply. Five additional units are to be delivered in 1997. Work with this instrument has been curtailedbecause of its inability to communicate effectively with the network's host computer. Currently an operatormust be sent to the site to copy data onto floppy disks. These disks are then carried or mailed to the
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network for processing. Additional software also needs to be developed to handle undocumentedinconsistencies with returned fixed length records before any of the data can be routinely evaluated by thenetwork.
High Resolution Spectroradiometers - The Competitive Research Grants Office of USDA-CSREES andthis project have supported the development of a high resolution spectroradiometer to measure UVBradiation. This development is under the direction of Dr. Lee Harrison at SUNY. Current plans call forthe establishment of approximately six "research" or reference sites using these high resolution scanninginstruments. Installation and operation of these instruments including site support and data acquisition willbe performed by Harrison. These instruments are designed to have a wavelength range of 280-400nanometers and a band pass of 0.1 nanometers (Gibson 1991). The development of the instrument hasbeen delayed by problems of acquiring a commercial monochromator meeting desired specifications. Sucha monochromator is now available and will be available for testing at the 1997 North AmericanSpectroradiometer Intercomparison. Information on the progress on the development of this instrument isdetailed in Appendix A.
Table 1. Instrumentation Available at Each USDA UVB Monitoring Location: End of 1996.
State/Site Name VIS-MFRSR UV-MFRSR Broadband
Arizona, Grand Canyon pnl X
California, Davis pnl X X
Colorado, Central. Plains Exp. Range X X X
Colorado, Table Mountain X
Georgia, Griffin pnl X
Illinois, Bondville pnl X X
Louisiana, Baton Rouge pnl
Maine, Presque Isle X X X
Maryland, Wye pnl
Michigan, Douglas Lake X X
Minnesota, Grand Rapids X X X
Nebraska, Mead pnl
New Mexico, Jornada pnl X
New York, Geneva pnl X
Ohio, Oxford pnl X
Texas, Big Bend pnl X X
Vermont, Underhill X X X
Utah, Logan pnl X X
Washington, Pullman pnl X X
Wisconsin, Lake Dubay X
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Table 1 above lists the instrumentation installed at each network site at the end of 1996. Unless otherwisenoted equipment has been manufactured by Yankee Environmental Systems (YES). The networkanticipates that a complete set of multi-filter, broadband and ancillary instruments will be available at eachsite by the end of 1997.
Calibration and Quality Assurance
Visible Multi-Filter Rotating Shadowband Radiometers - The most significant achievement with VIS-MFRSR calibration and quality assurance during 1996 was the locating of the original board calibrationsfor the 20 units purchased from PNL in 1993. Unfortunately, corresponding head calibrations are stillmissing. The board calibrations however, permit an approximate calibration of the first two years ofnetwork data by matching recent YES head calibrations with the older board calibrations. Softwaredeveloped by the instrument design group at SUNY which applied calibration to the VIS-MFRSR datastreams was useless prior to this time.
Seven of the 20 original PNL heads were re-calibrated by YES in 1996 permitting calibration to be appliedto 7 of the 12 sites that were operating prior to 1996. However, because the network rotates VIS-MFRSRheads throughout the network, it will not be able to provide calibrated data from any of its sites thatcontinue to use PNL heads until the remaining 13 heads cycle through the YES calibration facility. This isexpected to be completed in early 1997.
The lack of available laboratory calibration cited above prompted the network to explore the use of analternative method for calibrating its VIS-MFRSR data. In the absence of laboratory-provided calibrationdata, VIS-MFRSR calibration may be derived through a Langley analysis of each instruments specificchannels (see the Research Section below for a more thorough discussion of the Langley technique). However, this method depends upon the stability of the passbands of the VIS-MFRSR over time.
Using Langley analysis software supplied by the SUNY-Albany VIS-MFRSR group (Harrison andMichalsky, 1994), the network examined all of its data for suitable Langley events and arranged thereturned intercepts by head for further analysis. In addition to the review of filter stability the Langleycalculations provided a quality check of the integrity of the application software which matches cosinecharacterization results with individual data records. Mismatched record keeping or improper softwarelogic prevents the execution of the Langley analysis program.
Figure 2 illustrates the results of this stability check for head # 8725. This head was first located at thenetwork's Colorado site and then at the network's Utah site hence the break in the data record. Thedownward drift of Vo at each wavelength suggests that the filters are exhibiting some instability, especiallyearly in their life. Though most heads do not as yet have as extensive an operating history as the onedisplayed, most appear to exhibit similar behavior. In some cases, where all filters appear to drift together,the downward drift may be due to the deterioration or soiling of the diffuser. In those cases where theredoes not appear to such uniformity in the decline, the 610 and 665 nm wavelengths typically exhibit thestrongest downward trends. Preliminary indications suggest that a more extensive Langley history willneed to be developed before it is clear how to compensate for the apparent drifts in filter response. Furtheranalysis of filter stability is planned for 1997.
Early in 1996, the instrument development group at SUNY reported that some units were displaying atime-keeping problem when they were collecting data at a high rate and were being polled frequently.
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94.0 94.5 95.0 95.5 96.0 96.5 97.0
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Vo for Unit 8725 through December 1997
USDA UVB Monitoring Network: January 1997
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Figure 2 Time Series of Langley Regression Intercepts for USDA Head # 8725
When it appears, the problem shifts the time of a 3 minute average forward or backward by one or morerecords. Days then appear to have more or less 3-minute records than would be expected based upon theminutes of daylight computed for each day at each site. This introduces an error in all downstream dataprocessing that relates the angular corrections of data to the time of day at individual sites.
A computer program was developed by the group to check for this error. When the program was applied tothe historical USDA data files there appeared to be time discrepancies in most of the USDA raw data files.Through some extensive detective work it was found that the time problem was actually caused by aparticular sequence of network polling commands that instrument firmware was supposed to prevent. Thecommand sequence has since been amended and updated firmware is now available which performs asdesigned. After a preliminary evaluation of the affected data the network concluded that the errorintroduced is not serious enough to warrant a correction to the data at this time.
During the above upgrading of firmware in PNL manufactured VIS-MFRSRs the network discovered thatthe upgrade caused an intermittent band motion problem. The problem was further identified in all newlymanufactured YES shadowbands using this new "K" series prom. As the band returns to it index (home)position it intermittently shakes, sometimes causing the instrument to lose track of its rotation index. Thisresults in an instrument shutdown. The problem was resolved with the release of a second prom ('M')
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which all but two network sites have received. Since the problem was discovered early the affect on thenetwork has been minimal.
Ultraviolet Multi-Filter Rotating Shadowband Radiometers - Calibration and quality assurance of the UV-MFRSR follows the procedures outlined for the VIS-MFRSR instruments. Because of its later deploymentin the network it has not suffered the problems noted in the VIS-MFRSR discussions above. Laboratorycalibration has been available for the instruments and there do not appear to be any time-keeping errors inthe data files. Filter stability, though having even less Langley history than the VIS-MFRSR appears to becomparable to that noted in the VIS-MFRSR data records. Finally band motion problems were resolvedwith the installation of the series "M" prom.
Broadband UVB Pyranometers - Extensive reviews of the broadband data were conducted in 1996 inpreparation for the release of the data to the network's ftp archive. One such review was an examination ofthe nighttime values or dark counts returned by the instrument when the signal should be zero. It wasdiscovered that data from four of the nine initial sites and all of the sites the network shared with theQuantitative Links monitoring program (Maine, Illinois and Ohio), had non-zero dark counts as a result ofa ground loop in the initial wiring scheme used at these sites.
Table 2 lists the brief periods when dark count corrections were deemed necessary and presents the mediannighttime dark count value for each time period listed. Multiple time periods at the Maine and Washingtonsites result from attempts to remediate the problem which only achieved partial solution.
Table 2. Periods of Dark Counts Corrections at USDA UVB Monitoring Sites
Sites Approximate Dates Median
dark counts
Griffin, GA August 1994 - September 1994 81
Bondville, IL August 1993 - November 1994 26
Howland, ME October 1993 - July 1995 27
July 1995 - period of record 16
Presque Isle, ME October 1995 - November 1995 90
Geneva, NY August 1994 - September 1994 83
Oxford, OH August 1994 - December 1994 24
Pullman, WA July 1994 -August 1994 62
August 1994 - September 1994 69
September 1994 - October 1994 54
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1015
2025
3035
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9401 9402 9403 9404 9405 9406 9407 9408 9409 9410 9411 9412
coun
ts
Year and Month
Figure 3 Monthly Distribution of Dark Counts Resulting from a Ground Loop atthe Howland, Maine Station
Sc( CN
P50) [( TT
P50) � slope of the regression of T
P50dailyversus N
P50daily ] 1
Figure 3 illustrates the problem at the Howland, Maine Quantitative Links site using monthly distributionsof daily nighttime data. As shown, dark counts display a strong seasonal (actually temperature)dependency. Though similar at each site, regression analysis suggested that a separate regression be usedto correct data at individual sites. Separate linear regressions were developed of the form
where the corrected counts (Sc) are equal to the count of a 3-minute (or 1-minute) record (C) minus themedian dark count of the time interval being corrected (NP50), less a temperature dependency correction. Thetemperature dependency correction is computed as the product of the temperature of the 3-minute (or1-minute) record (T) minus the median nighttime temperature of the time period (TP50) times the slope of aregression of the daily nighttime median temperatures versus the daily median dark counts. A second review, conducted in preparation of placing broadband data into the ftp archive, involvedidentifying the explicit calibration lineage of the broadband meters. This was deemed necessary todocument how the network calibrated, collected and processed its broadband data. This resulted in thecreation of the broadband measurement document given in Appendix B and in the determination that thecalibration constant used by the network to convert raw signal voltages to calibrated units of watts/meter2
was in error. Prior to the review (April 1996) the network had been dividing instead of multiplying its rawcounts by a predetermined conversion factor. This resulted in a necessary correction of 19682 to all
9
previously released data. All clients were notified of the mistake and necessary correction, and the problemand solution were also posted to the network's World-Wide-Web site.
A final aspect of the review was a comparison of the spectral responses of those broadband meters that hadbeen recharacterized since their original deployment to sites. By 1996 twelve of the broadband meters hadbeen recalibrated at least once and four had been through the calibration facility 4 times. Although onlypreliminary conclusions can be drawn from such a small data set, it would appear that characterizationsperformed in 1994 and 1995 are likely biased towards the shorter wavelengths. In all of the twelve pairedrecharacterizations the second spectral response displayed a marked shift towards the shorter wavelengths.This shift appears to reverse however in those meters that had cycled through the facility two more times. Most likely, these changing responses represent imprecision in the recharacterization process rather thanactual changes in the meters themselves. There does not appear to be any other explanation which wouldsupport non-linear trends in these meters. The spectral responses of individual meters will continue to betracked until it can be determined what, if any, corrections may be necessary to fully qualify the network'sbroadband data.
Ancillary Instrumentation - Temperature and humidity measurements continue to lack calibration. A handheld, portable temperature /humidity probe was acquired as a field reference instrument during 1996 andfield technicians began using it to adjust local instruments. However, there has been no attempt to maintaina traceable calibration of this instrument. This will be accomplished at some future date.
Calibration constants printed on the calibration certificates of the downward looking LI-COR photometerswere averaged to create a standardized unit conversion constant which is now being applied to all LI-CORraw voltages in the network. Because the use of this instrumentation is only to determine the presence andabsence of snow the network has concluded that any further calibration of this instrumentation inunwarranted.
Smithsonian SR-18 - Nothing has been done to establish calibration or quality assurance procedures for theSR-18. The development of calibration and quality assurance procedures for this instrument is one ofmany tasks facing the network in 1997.
Intercomparisons - The network's Central Plains Experimental Range site (Colorado) continues to be thenetwork's primary site for prototyping, testing and characterizing network instrument configurations. Inaddition the site serves as the primary location for quality assurance studies of network precision and bias.During 1996 there were 6 broadband meters, two new seven channel UV-MFRSRs, one VIS-MFRSR andone new Smithsonian SR-18 Radiometer located at the site. An additional filter instrument, a GUV-511,manufactured by Biospherical Instruments1 is scheduled to be installed in early 1997. Duplicated andcollocated instrumentation data produced in 1996 will be used in 1997 to establish network precision andbias estimates.
Calibration Facility - A critical issue in the establishment of the monitoring program, and one that hasplagued previous efforts to monitor UVB radiation, is an understanding of the spectral characteristics of theinstruments employed using a National Institute of Standards and Technology (NIST) traceablecalibration. The Radiometric Physics Division of NIST produced the initial characterizations of thenetwork's broadband meters and Harrison's group at SUNY produced the characterizations of the initialPNL manufactured VIS-MFRSRs. However, it became apparent very early in the life of the network thatthese facilities did not have the capacity to handle the volume of instrumentation that would be deployed in
10
the USDA network. As a result all new and previous instrumentation is now routinely characterized andcalibrated by YES. YES calibration facilities however are also limited and it is anticipated that they toowill not be able to meet the needs of the network over the long-term.
The USDA is not alone in its desire to establish a more permanent, NIST traceable calibration facility.Because of this need and the availability of expertise in the radiation measurement programs conducted bythe NOAA-ARL labs in Boulder, CO, NOAA was selected to construct a calibration facility under thedirection of John DeLuisi and Patrick Disterhoft. This calibration facility is supported by the USDA(subcontract to NOAA), USEPA, NOAA and NSF. To assure the highest possible quality, NIST isproviding radiometric standards, technical guidance and oversight.
As a part of this facility NOAA has established a field site on Table Mountain north of Boulder, CO wherevarious instruments can be compared against the solar irradiance. This site also serves as a permanentlocation for instrument intercomparisons including the Annual North American InteragencySpectroradiometer Intercomparisons. In June of 1996 the network helped co-sponsor a third annual NorthAmerican Spectroradiometer Intercomparison. Results of the intercomparison will be reported at a laterdate. Results of the 1st North American Spectroradiometer Intercomparison (1994) will be published in latespring of this year by NIST (Thompson, et al. 1997). Intercomparison data sets are available through theNIST ftp server (lasulite.nist.gov). An additional field site is planned at a high altitude site west of Boulder,CO operated by the University of Colorado’s Institute for Arctic and Alpine Research INSTAAR. Thiswill provide clearer sky conditions for instrument evaluation and Langley calibrations.
Presently the calibration facility is modifying a McPherson double monochromator to perform responsivityscans in the UV region which will determine the wavelength response characteristics of broadband andnarrowband filter instruments. For the YES UVB-1, four orders of magnitude of signal range have beenobtained with a 0.8 nm bandpass. This will permit the determination of the top of the long wavelengthplateau. The determination of filter functions for the VIS-MFRSR is still limited. Using the samebandpass and acquiring the signal straight from the preamplifier, only two orders of magnitude arecurrently available. When an improved amplifier is installed, another 1.5 to 2 orders of signal rangeshould be possible. Since calibration and cosine measurement facilities have been completed, fullcharacterization (filter functions, cosine response, and calibration) of the broadband and VIS-MFRSRsshould be available in late April of 1997. With the acquisition of a new high resolution, high throughputdouble monochromator in early 1997, work to develop the capability to measure the filter functions of theUV-MFRSR will begin. The UV instruments with 2 nm passband interference filters will require themonochromator to operate at a slit width of less than the 0.8 nm available with the McPherson. Ideally theslit width of the monochromator should be one fortieth that of the interference filter passband or in the caseof the UV-MFRSRs, one fortieth of 2 nm. When the incorporation of the new monochromator is complete,full characterization of the UV instruments can begin.
11
Data Management
Over the past year data management and programming tasks have been organized into four majorcategories. These include software support for polling individual sites, the application of calibrationinformation to network data, improved accessibility to network data, and data archiving. A fifth category,that of systems administration of the project’s more than a dozen mixed platform computers, printers andtape systems, continues to occupy two-thirds of a person's time. This is largely due to the project’s locationoff-campus which necessitates the self-maintenance of both the computers on the local network and the off-campus network itself.
Support for Polling Sites - Data management tasks in support of site polling activities are given the highestpriority in the network. Failures in polling or corruption of data files either during the transfer ofinformation or within the shadowband’s data logging system immediately trigger an investigation andremedial action involving two or three of the project’s staff. If you do not first collect the data, there willbe nothing to show for the time spent measuring the solar radiation.
The updating of data tables that hold and track each instrument’s location and configuration was completedin time for the deployment of the network’s first UV-MFRSRs. Software previously prototyped in the Perlscripting language was re-written into C and the entire polling process was made more efficient, faster andautomated thereby taking full advantage of the network's data base management system (Ingres1). Now assoon as data is entered in the system by the field technicians it immediately becomes available to the entiredata system. This has the further benefit of providing the information necessary to keep the Web pagescurrent each day.
During the transition from Hayes modems to US Robotics modems and while gaining experience with thenew modems the network experienced brief periods of less reliable polling than was previously enjoyed. Tocompensate the network implemented a dual modem/machine, smart polling system that tracks the resultsof the polling through each modem and attempts to re-poll sites when failures have occurred. This strategyrestored the reliability of network polling and it has now been implemented as a permanent feature of thenetwork’s polling strategy. The success rates of retrieving VIS-MFRSR data from each of the network'ssites is given in Appendix C.
Application of Calibration Information to Data Records - Cosine characterizations performed first by PNLand SUNY and later by YES were moved into a data base management system during 1996 therebyallowing the network to automate the process of selecting and applying the most recent (or appropriate)cosine correction constants to any selected USDA network data. The automation of this selection processenabled the network to include cosine corrected data in its World-Wide Web data presentation and tosupport cosine correction software previously developed for the Unix, IBM and MacIntosh computingplatforms, all of which are in use by our data users. More efficient C programs replaced prototype Perllanguage applications during the streamlining of the cosine correction data processing applications.
A similar processing strategy has been used to automate the application of calibration data to both the VIS-MFRSR and UV-MFRSR data streams and it is anticipated that this process will move from the prototypeto production stage in early 1997. Presently all available calibration data for both VIS-MFRSR and UV-MFRSR instrumentation have been moved into the data system and prototype programs (in Perl) are beingtested which apply calibration to selectable network data sets.
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Improved Accessibility to Network Data - The primary means of distributing network data continues to bevia the World-Wide-Web site maintained by the network (http://uvb.nrel.colostate.edu/UVB/). The numberof information inquiries via other means (E-mail, telephone, letter, etc) however, also continues to grow(see Appendix D).
Information "clients" have increasingly been interested in the network's historical data, both broadband andVIS-MFRSR, but the historical data was only available via a formal data request to the network. Toremedy this the network created a permanent ftp archive for broadband data which is updated twice permonth. An easy to use link from the Web page to the ftp directory gives data users access to all of thenetwork’s broadband data. Users may also access the data directly through standard ftp connections.
The network also expanded its daily display of VIS-MFRSR and broadband data during 1996 to give thescientific community the ability to plot, on demand, all of the network’s data by the day, week, or month.Ancillary data (temperature, humidity and surface reflectance) along with the new UV-MFRSR data arealso now available alongside the shadowband data and all shadowband plots now routinely display cosinecorrected data.
Documentation of the broadband measurement and calibration procedures have been added to the Web siteas well as a listing of all project personnel. It is anticipated that further documentation of VIS-MFRSR andUV-MFRSR measurement and calibration procedures will be added to the site in 1997.
With increased usage of the Web site by a wider variety of clients (see Appendix D) execution speed andefficiency have become more important to the network. To meet this challenge the network has replacedmuch of its Perl computer code with faster, more efficient, C-language code and has modularized much ofits computer code to eliminate redundancies and cross program dependencies. The network has also beguna transition from an Ingres to an Oracle1 data base management system (DBMS). After carefulconsideration of the network's ongoing and future needs, it was concluded that the Ingres product was notkeeping pace with advances in software inter-connectivity. The network appeared to be increasinglycreating work-arounds for data transfer among software packages and graphical user interfaces. Whilethere will be a learning period to become familiar with the new DBMS, the network expects to experiencelong-term gains in data programming and processing efficiency.
Data Archiving - With more than 12,000 raw data files and more than 200 calibration characterizationfiles now collected, it has become important for the network to effect a long term strategy for archiving itsdata. Primary raw data files are now used only infrequently as the network utilizes more highly processedmonthly data sets for its primary data files. Appendix E contains a portion of the network's overall datasystem design indicating how both raw and processed data are arranged in the network's computer system.The network has acquired a CD ROM writer and will begin moving data files currently contained in theraw/ data directories to this media. Other materials to be moved to this archive include polling log files(archive/ directory) and original scanned images from each of the network's sites. As the data basemanagement system becomes the source of primary network data, monthly data files now used as theprimary source of network data will also be moved to CD ROM archives.
13
I8 ' Io,8exp(&j J8,i mi) 2
ln I8 ' ln Io ,8 & mJ 3
ln V8 ' ln Vo ,8 & mJ 4
I8 'V8m I8F8d8
Vo,8m F8d85
Research
The Langley Method of Calibration - It has been assumed that in the absence of explicit calibrationmeasurements or as a complementary check of lamp calibrations performed by YES and soon to beconducted by a newly established NOAA Calibration Facility, a Langley calibration technique may be usedas an alternate method of calibration. This has not however been well tested in the UV. The attenuation ofthe direct radiation as it passes through the earth’s atmosphere is described by the Beer-Lambert Law(Craig, 1965).
where I8 is the direct irradiance at the ground, Io,8 is the extraterrestrial irradiance, J is the optical depth forthe ith absorber and m is the slant path (or airmass) through the atmosphere. The equation assumesunchanging atmospheric optical conditions throughout the day and uniform horizontal mixing of theabsorbers, conditions typically met only at clean, high elevation sites. Taking the natural log of both sides
and, if instead of irradiance we measure uncalibrated voltages, the equation is
where V8 is the measured voltage of a particular channel and Vo, 8 is the extrapolated voltage intercept atzero airmass.
A plot of the natural log of voltage due to the direct beam at one filter wavelength versus the optical pathlength or airmass (typically the secant of the solar zenith angle) results in a straight line whose slope is theoptical depth of the atmosphere at that wavelength and whose intercept at zero airmass is the voltage thedetector would register if it were pointed towards the sun at the top of the atmosphere.
To transform this measured voltage at the ground (V8) to a calibrated irradiance we must know theextraterrestrial solar irradiance Io, 8 and the filter function or relative response of the filter/photodetectorcombination, F8 (Tug and Baumann, 1994). Recent measurements of Io, 8 reviewed by Cebula et al., (1996)reveal differences of approximately 4% in the region between 300 and 350 nm. Compared with othererrors of the Langley method of calibration, these differences in the measured extraterrestrial flux aresmall. The irradiance at the ground then is:
14
Figure 1 Langley plot for the 317.7 channel October 10, 1996 at Table Mountain, CO. The rangeof the airmasses is 2 to 6. The slope is the atmospheric optical depth, J, at this wavelength.
Below in Figure 4 is a plot of ln voltage versus airmass for October 10, 1996 at Table Mountain for the317.7 channel. Langley plots were analyzed using the Harrison and Michalsky (1994) algorithm on cleardays at the Central Plains Experimental Station, CO (July and August 1996) and at Table Mountain, CO(September and October 1996).
Table 3 gives the average Langley voltage intercept for the 6 channels used along with the standarddeviation and percent standard deviation of the 16 Langley events. No Langley plots are available for the300 nm channel because of insufficient signal to noise.
Table 3. Average of Sixteen Langley Voltage Intercepts July through October, 1996
Filter 305.4 311.5 317.7 325.5 332.5 367.8
Vo 11727 4316 6480 2230 2073 1024
Std. Dev 841 320 500 179 214 84
% S.D. 7% 7% 8% 8% 10% 8%
Using these voltage intercepts, the SUSIM extraterrestrial solar flux (VanHoosier et al., 1988) and themeasured filter functions in equation 5, calibration factors were developed. These factors were then used todetermine global irradiances for noon values measured by the UV-MFRSR at Table Mountain, CO. Tocheck the validity of these irradiances, the results were compared to both a model (Stamnes et al, 1988) runfor the appropriate conditions (solar zenith angle [SZA]), albedo, aerosols, elevation) and the irradiances
15
J ' Jray % Jozo % Jaer 6
measured by a collocated Brewer spectrophotometer (Bais et al., 1996). The comparison is summarized inTable 4. The Brewer only measures out to 365 nm so no comparison is made at 367.8 nm.
Table 4. Comparison of UV-MFRSR Using Langley Calibration with Model and BrewerSpectrophotometer October 10, 1996 Table Mountain, CO SZA=47.8E
Filter CW (nm)UVRSR Model Brewer
(Watts/m2/nm)
305.4 0.041 0.044 0.037
311.5 0.143 0.149 0.119
317.7 0.239 0.244 0.197
325.5 0.335 0.336 0.307
332.5 0.398 0.443 0.364
367.8 0.556 0.606 ---
The irradiances of the UV-MFRSR using the Langley calibration factors and the radiative transfer modelagree to within 11% at all wavelengths and with an average difference of 7.7%. Shaw (1982) demonstratedthat at the very clean high altitude Mauna Loa Observatory extrapolations of Vo, 8 were constant to within astandard deviation of 1.2 % at 380 nm and 0.4% at 415 nm. This is due to extremely stable opticalconditions of the observatory. Extrapolations to zero airmass in the ultraviolet below 380 nm are expectedto be less constant for two reasons: changes during a day in the ozone column amount (which absorbsstrongly below about 330 nm) will alter the optical depth, and more scattered light around the sun’s disk atshorter wavelengths (McKenzie and Johnston, 1995).
Whether the technique will produce Langley derived irradiances as good as Shaw (1982) will only beknown with additional work. These results however, do suggest that the application of the Langleytechnique to wavelengths in the UV is promising. Further work should also address the reasons whyirradiances measured by the Brewer are consistently 8 to 17% lower than the UV-MFRSR or the model. There may be a bias in the Brewer's irradiance calibration.
Retrieving Ozone from the UV-MFRSRs - Measurement of ozone is critical in constructing syntheticspectra from the 7 filter measurements of the UV-MFRSR as well as to quantify the relative contribution ofozone, clouds and aerosols to UVB attenuation. We have investigated a method for extracting ozonecolumn amounts for clear skies from the 311 nm and 317 nm channels of the UV-MFRSR using theLangley method described above. Presently it is only valid during clear days when the ozone and aerosolsare constant temporally and spatially.
The UV-MFRSR returns the direct normal irradiance, I, at 7 ultraviolet wavelengths. A plot of ln(V)versus airmass yields a straight line (Langley plot) with a slope of the optical depth, J, which is comprisedof
16
J1 ' Jray1 %Jozo1 %Jaer1 7
& (J2 ' Jray2 % Jozo2 % Jaer1) 8
J1 &J2 ' (Jray1 & Jray2) &' (Jozo1 & Jozo2) 9
" ' m"(8) S(8)F(8) d8
mS(8)F(8) d810
Jozo ' P"ozo 11
P '((J1&J2)&(Jray1&Jray2))
"ozo1& "ozo2
12
where Jray is the Rayleigh or scattering optical depth, Jozo is the ozone optical depth, and Jaer is the aerosoloptical depth.
Two channels with center wavelengths close together (311 and 317 nm) were chosen and total opticaldepths, J1 and J2, at these wavelengths were computed using the Langley method outlined above. Theaerosol optical depth for both channels were assumed to be equal and the Rayleigh optical depth computedas per methods outlined by Stephens (1993) allowing a solution for (Jozo1 - Jozo2).
Next, the effective ozone cross section, "& was computed, weighted by the extraterrestrial solar flux, S(8),and the filter function of the photometer, F(8).
The ozone optical depth, Jozo, is the product of the ozone column, i, and the effective ozone cross section"&.
Thus combining equations 9, 10, and 11 the ozone column, i, can be determined.
17
Using this technique ozone column abundances were determined from direct normal solar irradiancesmeasured by the UV-MFRSR located at the Central Plains Experimental Range, CO on the mornings ofJuly 11, 22, and 23. These values are compared with those measured at 10:00 a.m on the same dates bythe NOAA Dobson spectrophotometer located at Boulder, CO about 100 km SW of the site (Table 5). Theagreement is to within an average of 7% suggesting that ozone retrievals are possible from the UV-MFRSRchannels and therefore the network has promise for using synthetic spectra as a means of establishingstandard wavelength reporting of its multi-filter measurements.
Table 5. Comparison of Column Ozone Derived from the UVRSR and a Dobson
Date UVRSR (DU) Dobson (DU)
July 11 310 283
July 22 303 283
July 23 299 283
NASA /USDA-UVB Cooperation in Model Validation - NASA has expressed a desire to utilize the USDAUV-MFRSR measurements as a set of ground-based measurements upon which they can evaluate theirmodel-based calculations of UVB in their TOMS satellite program. As an early indication as to how thecomparisons might be made we made a number of tests using the Stamnes (1988) model in parallel withthe NASA model as a general means of isolating the causes of differences in predicted irradiances. Theradiative transfer models were run with identical input parameters (scattering optical depth, albedo andSZA) with the goal of obtaining irradiances that agree to within 1% for various input conditions.Preliminary results using data from the Table Mountain, CO site suggest that TOMS overflights may beused as a daily reference to USDA instrument stability. Once perfected the NASA satellite measurementsand algorithm will extend the range of coverage of UV measurements and predictions to cover the entirecontinental United States.
Conferences/ Workshops/Meetings
Project staff participated in a number of conferences, workshops and symposia in 1996. Participation inthese events as well as more informal meetings with colleagues and collaborators provides the network withpeer review and discussion of its methodology and results, and ensures that network products are perceivedas high quality and useful to the community whose needs it is striving to fulfill.
Central Plains Experimental Range Third Annual Symposium; Colorado State University, Fort Collins,CO. January 11, 1996. This annual symposium brings together all of the scientists who conduct researchat the Central Plains Experimental Range (CPER) in eastern Colorado. The CPER is the primary site usedby the UVB Monitoring Network to test new and additional instrumentation and instrument configurations. The exchange of information between project staff and others working on the CPER provides the networkwith an invaluable source of site characteristics as well as furnishes the network with a wealth of new ideasfor the use and presentation of its data.
18
1996 North American Spectral Radiometer Intercomparison; Boulder, CO. June 18 - 25, 1996. Thisannual intercomparison is sponsored in part by the network and serves to establish the comparability andbias of network data to those projects using alternate types of instrumentation. In addition, theintercomparison provides the supporting documentation necessary to determine the causes of the differencesbetween the various instrumentation that participates the intercomparison. Two of the network's new UV-MFRSRs were included in the intercomparison to determine their comparability to spectroradiometers.
Current Problems in Atmospheric Radiation: International Radiation Symposium (IRS) 1996; Universityof Alaska, Fairbanks, AK. August 19-24, 1996. The IRS Symposium is a once every four year,international conference of radiation scientists. Dr. James Slusser, Dr. James Gibson and David Bigelowpresented a poster at the conference titled USDA UVB Monitoring Program, which introduced the radiationcommunity to the monitoring program and presented some examples of what might be accomplishedthrough the use of the data. An abstract of the presentation will be published along with others included in Session 8 titled"UV Radiation and Modeling".
WWW Access to CA-OpenIngres Databases. CAWorld; New Orleans, LA. August 25-29, 1996. Thisworkshop was developed and conducted by Bill Davis and another Colorado Sate University staff memberto acquaint data base management practitioners with methods and Ingres tools available for use withCommon Gateway Interface (CGI) scripts. These scripts are used to create on-the-fly HyperText MarkUpLanguage (HTML) forms and documents. The presentation was cited in the trade journal DBMSMagazine.
Measuring Ultraviolet Light With the Shadowband Radiometer: the Effects of Ozone, Clouds andAerosols; Colorado State University, Natural Resource Ecology Laboratory, Fort Collins, CO.September 27, 1996. This seminar given by Dr. James Slusser, introduced researchers at Colorado StateUniversity and our department to some of the work being done within the USDA Ultraviolet RadiationMonitoring Program.
TOMS/ADEOS UV Data Product Validation Product Workshop; NASA/Goddard Space Flight Center,Greenbelt, MD. October 8-9, 1996. NASA invited scientists with a history of involvement with ground-based UV instruments to this workshop to discuss how ground-based instruments might be used to validateUV estimates derived from the US/Japanese TOMS/ADEOS satellite program. The workshop served tointroduce the UV scientists to the satellite program and introduce NASA scientists to the nuances ofground-based UV measurement, calibration and data interpretation. Dr. James Gibson and Dave Bigelowmade presentations to the attendees outlining the status and availability of network data.
UVB Monitoring Program Project Retreat; Estes Park, CO. October 15-17, 1996. With the recentincrease of project staff and the imminent expansion of the network, both in the number of sites andamount of instrumentation at each site, it was concluded that a short retreat would be beneficial inreestablishing program priorities and bringing all of the project staff to a common level of understanding ofthe projects scope, goals and vision.
Synthetic Spectra Workshop; Colorado State University, Fort Collins, CO. December 5-6, 1996.Scientists from NCAR, NOAA, NASA, Canada's AES, and SUNY Albany met in Ft. Collins to discuss theways to construct the entire solar spectrum reaching the earth's surface from a set of 7 UV-MFRSR filtermeasurements. Two major approaches were presented.
1. Derive ozone from the ratio of two channels, e.g. 311 and 368 nm (similarly described above in theResearch Section). Derive effective cloud optical depths by taking the ratio of the observed totheoretical clear-sky irradiances at 368 nm. Then iterate cloud thickness at 368 in the model untilthe observed ratio is achieved. Retrieved ozone and cloud amounts can then be used as input to radiative transfer models to generate synthetic spectrum. This approach is described by Dahlback(1996) and similar to Stamnes et al. (1991).
2. Using all 7 channels, measure the global transmission, which for each channel is the ratio of themeasured global voltage to the Langley zero airmass intercept. Then, using a newly establishedmodel by Min and Harrison (private communication) construct synthetic spectra attenuating theextraterrestrial solar flux by multiplying it by a function using 6 wavelength dependent parameters. The filter functions are multiplied by the first guess synthetic spectrum to generate 7 modeltransmissions which are compared with the measured transmission. The 6 parameters are adjusteduntil the differences between the measurements and the model are at minimum.
Initial results presented using both techniques are very promising. Starting with filter "measurements"constructed from a synthetic spectrum, however, the second algorithm returned a virtually identicalsynthetic spectrum. The algorithm is fast and reliable, and can be run "as needed" meaning there is no needto store the synthetic spectra.
A critical use for the algorithm is to transform measured filter irradiances to common wavelength values. Currently each channel registers irradiances measured at slightly different center wavelengths, makingcomparisons of different data sets impossible. By using this algorithm and standard set of filter functions,all measured irradiances may be transformed to a standard set of wavelengths.
Recommendations and Future Work
Much of the effort of building a UV monitoring program over the past years has centered aroundfamiliarizing project personnel with the operation of VIS-MFRSRs and identifying promising locations fornetwork instrumentation. Along the way a number of "improvements" have been introduced into basic VIS-MFRSR technology and the network has by and large been requested to absorb the cost of implementingthese changes into the routine procedures of network operations. While these improvements are appreciatedand encouraged they have highlighted the differences between technology proven in the visible spectra andnew challenges of working in the UV.
! The network must continue to investigate those procedures that have been developed for thevisible spectra to ensure UV results reported by the network are valid.
As outlined in the preceding pages a number of outstanding issues have been and will need to be brought toresolution to achieve this.
! The network needs to continue to expand into at least the 26 site, grid-based, network that wasoriginally proposed if it hopes to enjoy the spatial coverage it set out to define.
Some locations that offer special opportunities such as the Table Mountain and Central PlainsExperimental Range in Colorado and the Mauna Loa, Hawaii sites need to be enhanced to provide thenetwork with specific research and quality assurance information.
! The establishment of a research/network site at Mauna Loa Observatory appears to be the only
way the network will be able to credibly establish that Langley methods can be applied to the UVand to demonstrate that synthetic spectra will indeed be reflective of actual measurements.
We recommend setting up 2 UV-MFRSR instruments atop the 11,300 foot elevation Mauna LoaObservatory. One will be the normal network shadowband instrument. The second instrument will bemounted on a sun-tracker with a collimating tube which limits the field of view to about 2.0E (the sunsubtends 0.5E). This tracker follows the sun's motion through the sky with great precision. Setting 2 UVinstruments atop the Mauna Loa Observatory will allow study of the UV photometer performance underexceptionally clean and aerosol free skies (Shaw, 1979) enabling the network to: 1) reduce standarddeviation in Langley voltage intercepts which will improve Langley calibrations and allow comparisonswith laboratory results; 2) compare results with tracker and shadowbands including testing of the cosineresponse of the shadowband; 3) compare calibrations derived from Langley methods with the New Zealandspectrometer (Bodhaine et al., 1996); 4) study aerosol optical depth in ultraviolet
! Standardization and equity of peer calibration laboratories need to be initiated and supported toestablish network accuracy and to ensure equity through time of the UV measurements made by thenetwork.
Laboratory capacity for instrument calibration and characterization is minimal and the network has alreadybeen forced to use facilities at SUNY-Albany, Yankee Environmental Systems and soon the NOAACalibration Facility in Boulder, CO to complete its quality assurance initiatives. Both calibrationtechniques and facilities need to be evaluated and standardized if the network is to continue to provide aquality product to its scientific customers. Continued support for the NOAA Calibration facility is criticalto these needs.
! The tracking of filter stability needs to continue in order to establish a firm basis for implementingroutine algorithms for correcting and normalizing instrument response.
! Work in progress with ozone retrievals and synthetic spectra generation needs to continue so thatthe network can establish a routine procedure for achieving commonality in the wavelengths itreports for its filter instruments.
To bring these procedures to fruition the network needs to advance its work with ozone retrievals to includecloudy skies and expand its capability of producing synthetic spectra. It is more typical for there to be clouds than clear sky so ozone retrieval under cloudy conditions must beconsidered. To a first approximation clouds are spectrally neutral. Using a discrete ordinate radiativetransfer code, Stamnes et al (1991) demonstrated that ozone column could be effectively retrieved from theratio of measured spectral (0.5 nm resolution) global irradiances at 340 nm and 305 nm. We are confidentthat a method of retrieving ozone columns based on the ratio of measured filter global irradiances at 368nm and 311 nm can be achieved and tested by comparing ozone columns derived from the UV-MFRSR atTable Mountain with those measured by the Dobson at Boulder 10 miles to the south.
! Collocation with both peer and identical instrumentation needs to continue so that networkprecision, bias and its comparability to complementary research programs is established.
The installation of two GUV-511 Biospherical Instruments filter instruments at the Central PlainsExperimental Range, CO, will provide the network with an ongoing independent check of UV-MFRSRcalibrations and help establish a direct relationship between the second most used filter instrumentcurrently used in the world. There are over 30 of these instruments in operation throughout North and
South America as well as Europe and Antarctica. Unlike the UV-MFRSR, the GUV-511 has noshadowband, so it measures only the total horizontal irradiance. Daily comparisons of both instruments’305 nm channels and the UV-MFRSR 368 nm to the GUV-511 380 nm channels are a high priority.
The Smithsonian SR-18 instrument has been running smoothly at the CPER since July 1996. Howeverthere have been data transfer problems which prevent polling the instrument and downloading the data. These need to be resolved but since most likely there will be no more than 6 of these instruments inoperation, comparison of the SR-18 with the UV-MFRSR must be considered a lower priority thancomparison with the GUV-511. We will continue to collect data from the SR-18 in anticipation of havingthe resources to make a detailed analysis of this important instrument.
! We recommend that a workshop for biological scientists who use UV data be conducted in 1997.
We recommend that scientists investigating the effects of UV radiation on plants and animals as well ashumans and other living organisms, be invited to discuss their current research and discuss what form ofdata from the USDA UVB Monitoring Network might be most useful to them. This could for exampleinvolve writing algorithms to transform the 7-channel, 3-minute averages into time and wavelength energyintegrals and weighted biological doses. Alternately there could be the need for daily maxima or othermeasures that can be extracted from the data stream with suitable algorithms. The UV effects scientificcommunity is still fairly new in addressing problems of calibration, experimental design, and comparison ofresults. It is expected that the workshop will further all of these important issues.
22
References
Bais, A.F., C.S. Zefros, and C.T. McElroy, Solar UV measurements with the double- and single-monochromator Brewer Ozone Spectrophotometers, Geophys. Res. Lett. 23, 833-836, 1996.
Bodhaine, B.A., R.L. McKenzie, P.V. Johnston, D.J. Hoffmann, E.G. Dutton, R.C. Schnell, J.E. Barbes,S.C. Ryan and M. Kotkamp, New ultraviolet spectroradiometer measurements at Mauna Loa Observatory,Geophys. Res. Lett., 23, 2171-2142, 1996.
Cebula, C.B., G.O. Thuiller, M.E. VanHoosier, E. Hilsenrath, M. Herse, G.E.. Brueckner, and P.C.Simon, Observations of the solar irradiance in the 200-350 nm interval during the ATLAS-1 mission: Acomparison among three sets of measurements - SSBUV, SOLSPEC, and SUSIM, Geophys. Res. Lett.,23, 2289-2292, 1996.
Craig, R. A., The Upper Atmosphere: Meteorology and Physics, 509 pp., Academic Press, New York,1965.
Dahlback, A., Measurements of biologically effective UV doses, total ozone abundances, and cloud effectswith multichannel, moderate bandwidth filter instruments, Appl. Opt., 35, 6814-6520, 1996.
Gibson, J.H. (ed), Criteria for Status-and-Trends Monitoring of Ultraviolet (UV) Radiation:Recommendations of the UV-B Monitoring Workshop (March 1992), National Atmospheric DepositionProgram, Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO, 1992.
Gibson, J.H. (ed), Justification and Criteria for the Monitoring of Ultraviolet (UV) Radiation: Report ofUV-B Measurements Workshop (April 1991). National Atmospheric Deposition Program, NaturalResource Ecology Laboratory, Colorado State University, Fort Collins, CO, 1991.
Harrison, L., J. Michalsky and J. Berndt, Automated Multi-Filter rotating shadow-band radiometer: aninstrument for optical depth and radiation measurements. Appl. Opt. 33, 5118-5125. 1994.
Harrison, L., J. Michalsky, Objective algorithms for the retrieval of optical depths from ground-basedmeasurements. Appl. Opt. 33, 5126-5132, 1994.
McKenzie, R.L. and P.V. Johnston, Comment on “Problems of UV-B radiation measurements in biologicalresearch. Critical remarks on current techniques and suggestions for improvements” by H. Tüg andM.E.M. Baumann, Geophys. Res. Lett., 23, 1157-1158.
Shaw, G.E., Aerosols at Mauna Loa: Optical Properties, J. Atm., Sci. 36, 862-869, 1979.
Shaw, G. E., Solar spectral irradiance and atmospheric transmission at Mauna Loa Observatory, Appl.Opt. 21, 2007-2011, 1982.
Stamnes, K., C. Tsay, W. Wiscombe and K. Jayaweera, Numerically stable algorithm for discrete-ordinate-method radiative transfer in multiple scattering and emitting layered media. Appl. Opt., 27, 2502-2509, 1988.
23
Stamnes, K, J. Slusser and M. Bowen, Derivation of total ozone abundance and cloud effects from spectralirradiance measurements, Appl. Opt., 30, 4418-4426, 1991.
Thompson, A. , T. Early, J. DeLuisi, P. Disterhoft, D. Wardle, J. Kerr, J. Rives, Y. Sun, T. Lucas, T.Mestechkina and P. Neale, 1994 North American Interagency Intercomparison of Ultraviolet MonitoringSpectroradiometers. J. Res. NIST. (To be published May/June 1997).
Tug, H. and M. Baumann, Problems of UV-B radiation measurements in biological research. Criticalremarks on current techniques and suggestions for improvements” Geophys. Res. Lett., 21, 689-692, 1994.
UV-B Monitoring Workshop: A Review of the Science and Status of Measuring and MonitoringPrograms, Science and Policy Associates, Inc., Washington, DC, March 10-12, 1992.
VanHoosier, M. E., et al., Absolute solar spectral irradiance 120 nm - 410 nm (results from the SolarUltraviolet Spectral Irradiance Monitor - SUSIM - Experiment onboard Spacelab 2), Astrophys. Lett..Commun., 27, 163-167, 1988.
24
Figure 2 Raw Cd & Hg line spectrum used for testing.
Appendix A. Testing of an Instrument's SA U-1000 Double Monochromator
Testing of an Instruments SA U-1000 Double Monochromator
Lee Harrison & Jerry Berndt, ASRC
The US Military Academy, West Point NY, owns an early Instruments-SA U-1000 double monochromator. Capt.Augustus Fountain at West Point has been very gracious in letting us impose on him to do the tests reported here. Thisinstrument is equipped with 1,800 g/mm gratings, and a cosecant drive (so that it's natural mechanical scale iswavenumber rather than wavelength). Thus it is not an exact duplicate of the model variant we would use for UVspectroradiometry (that would use 3,600 g/mm gratings and a sine-drive), but we were unable to locate an instrumentwith the sine-drive that we could gain access to, and so elected to test the West Point instrument as certainly beingmore instructive than testing none at all.
Previously in our laboratory here we had developed a variant of the electronic drive-control system used for the RSIMonochromator-based field instrument that is more flexible in the range of stepping motor systems it can handle. (Theone we fabricated for the RSI-Instrument was optimized [and limited] to that specific application.) We tested this drivesystem against a 1 meter Acton monochromator we have at ASRC that is a fairly good surrogate of the ISA in so faras drive electronics a-re concerned. This drive system also included the signal acquisition and digitization from asilicon photodiode detector; we wished to provide our own drive system to guarantee that motion/sampling sequenceswere completely controlled and understood, and substitute our own detector to avoid both potential interface difficultiesand any chance of destruction of our host's PMT detector. A Si photodiode would not be the detector of choice foratmospheric UV spectroradiometry, but is ideal for laboratory use when testing against bright line sources, as itpossesses linearity over wide dynamic range, effectively instantaneous recovery from high signal levels, and can't bedamaged by inadvertent large exposures.For the testing the instrument was illuminated with a low-pressure Cd & Hg discharge lamp to provide a spectrum with
accurately known line positions. The raw line spectrum as observed by the U-1000 is shown in figure 1. In this figurewavenumber is decreasing to the right; the line features used for subsequent wavenumber/wavelength testing are listedin table 1 that follows. This spectrum shown does not include several features that were discarded because they weremultiple lines too close to resolve. All of the wavelengths cited in the table are taken from standard tables for the linesmeasured in air at I bar and 15' C, and the wavenumbers are then directly computed.
1 Note that these are wavenumbers in air computed from the referenced wavelength. This is appropriate forthe analysis done here, as the monochromator is air-filled, and the fundamental physics of the grating diffractionare controlled by the wavelength at the grating surface. This is in contrast to the common (but not uniformlyapplied) convention that optical properties expressed in wavenumber are for the reciprocal lengths in vacuum.
25
Because these lines have been so commonly uses as secondary wavelength standards we are confident that the citedvalues are accurate to one least significant digit shown in the wavelength column.
Table 1: Cd & Hg emission lines used for our tests
Wavenumber1 Wavelength, nm Stepcount 27696.918 361.051 152622 27678.826 361.287 153536
27396.133 365.015 167683 24712.348 404.656 301827 22944.463 435.835 390194 21375.926 467.816 468585 20833.680 479.992 495715 19662.512 508.582 554268 18312.537 546.074 621772 17332.254 576.959 670789 15531.641 643.847 760830
The stepcount in table 1 is arbitrary, but was established by the number of steps required to move from a position closeto the short-wavelength limit of the instrument. The entrance and exit slits were adjusted to 100 µm, and for simplicitythe two intermediate slits between the stages were left wide open. The 100 µm slits yielded optical slitwidths rangingfrom approximately 0.027 nm for the shortest wavelengths, to 0.057 for the 643 nm line. These narrow opticalslitwidths make accurate line centroid retrieval a relatively easy task given reasonable signal-to-noise.
Figure 2: 61 repeated scans of the 439.8 nm emission line1-9 in black, 10-20 in red, 21-30 in green, 31-43 in blue, 44-61 in purple.The optical slitwidth is =0.030 nm FWFM
2Our drive operated the instrument's SLO-SYN M061FDO8 stepping motor with a conventional L/4R driveat the manufacturer's rated current of 3.8 ampere. At this current the motor was only mildly warm to touch, whileeither stepping or stationary. ISA's spectra-drive system uses much higher currents, and the motor becomes too hotto grip. We are uncomfortable with the thermal stress on this motor for a field instrument, and would considermodifications if the ISA instrument is selected. Our conservative power to the motor coupled with a wish tooperate at speed where we were sure steps would not be lost produced a stepping rate where a full scan and retracetook approximately 80 minutes.
26
Figure 4 Detail of figure 2. Note the recurring feature at 301,870 steps.
We checked the primary focus adjustment, and were pleased to see that it had remained so accurately focused duringtwo years without use that no adjustment on our part improved the slit-flinction. Visual inspection suggested minorslit-misalignment, that we did not try to remove.
For the tests the entire spectrum range shown in table 1 was scanned in the order shown, with a 1000 step domainapproximately centered on each peak being digitized and stored with a sample every 2 steps. Following themeasurement of the longest wavelength peak the drive would retrace to a position 1000 steps below the first storedposition, and repeat the cycle. All stepping was done with a period of 3 msec between steps. This is much slower thanthe instrument can be operated2 but was chosen to ensure no loss of motion. The repeated 61 scans of the fourth of theemission lines at 435.8 nm is shown in figure 2.
Examination of figure 2 makes it immediately apparent that the wavelength reproducibility clusters, with sequencesof scans grouping together. The first 10 scans are most erratic, and have been discarded for further analysis on thepresumption that an instrument that has remained idle for so long should be permitted a warm up. (These 10 discardedscans represent slightly more than 12 hours of continuous running.)
Within the best clusters reproducibility was excellent (as good as 0.001 nm 1 sigma), but then there would be a "jump"to a new domain. Obviously on seeing a behavior like this the immediate hypothesis that comes to mind is thatsomehow we are "losing" steps ... either due to failure of the driving/counting system to accumulate them properly, orthat for whatever reason the motor could not actually make them (driven too fast, etc.).
3 It also demands a choice about the domain of the integration, that obviously cannot be 0 to - in practice.T'his interacts with the choice of baseline removal.
27
CS x xdx
S x dx=
∞
∞
∫∫
( )
( )
0
0
Figure 5 Linear Center-fit technique - 439.8 line
We present figure 3 to demonstrate using the evidence within the data that neither of the above simple explanationsmake sense: the signal resolution and noise floor are so good that you can see small repeating "glitches" observableon the shoulders of the peaks where the derivative of the signal is large. Note that these features stay in exactly thesame place on all scans relative to step count, while the peaks wander back and forth. They can't be due to noise.There are several possible mechanical explanations; we think the most probable to be very small local machiningimperfections of the cosecant bar. Alternative explanations include local cyclic error in the mechanical reduction fromthe motor to the leadscrew, or a local binding spot or "drunken" turn on the leadscrew. For any of these causes the dataremain a- compelling demonstration that the stepping count and motion input to the drive mechanism were accurate.
If our assignment of the physical cause to imperfections of the cosecant bar is correct then the fidelity of thedrive/leadscrew mechanism itself is excellent. The wander in the wavelength reproducibility would then be due toangular shifts either in the connection of the cosecant bar to the grating shaft, or to grating shifts relative to the gratingshaft. (Note that with respect to the latter we were operating the instrument with the intermediate slits wide open, sothat a grating could make a small shift without affecting the throughput.)
All following analyses are based on the retrieval of the effective center of the line shapes present in the data. Thereare a variety of common algorithms to do so, and the optimal use and true accuracy of such methods remain vigorouslyargued issues among optical metrologists. Given the high signal-to-noise in these signals, narrow optical slitwidth,relatively flat baselines, and the absence of overlapping peaks the accurate recovery of the effective line centers is mucheasier for these data than it can be in the general case. All data presented here were analyzed by two methods, andresults compared as a test of potential method-dependent error.
The first method is commonly known as the "centroid method" or "method of moments." It consists of computing
where S(x) is the measured signal at position x. This method is simple to code and fast, and makes no assumptionsabout the peak shape or symmetry thereoe However it does maximally weight the impact,of the wings of the peak, andhence also is maximally sensitive to baseline removal (if done), or baseline magnitude3
4 The conversion from step-scale to wavelength is discussed subsequently.
28
The second method is the double linear-fitting technique shown in figure 4: the regions from 20% to 80% of themaximum amplitude to either side of the peak were individually fitted with a least-squares line, and then position ofthe intercept of the two lines was taken as the center fit. This algorithm works well when (as is the case here) the peaksare isolated, signal-to-noise is high, and the instrument slit-function is a reasonable approximation to the idealsymmetric triangle.
For both methods linear baseline removal was done on the data before the centroid or center-fit was computed. Theresults were then compared, and all peaks of all scans except three peaks in scan 7, had centroids and center-fits thatagreed4 to 0.002 nm or better. Two of the three discrepant peaks showed anomalous noise in the tails that wouldpreferentially bias the centroid algorithm. The third had a asymmetric peak that would cause the accuracy of eithermethod to be in doubt. The question of why scan 7 was peculiarly affected remains moot, but we think that major line-power transients in the building at its time are the likely explanation. In any event this scan is one of the first ten thatwe will not include in the linearity and reproducibility analysis.
Figure 5: Temperature and centroid ªª stepcount vs. scan number
Figure 5 show the difference of the centroid positions for peaks four through eleven (404 through 643 nm) for eachscan from scan 3 (arbitrarily chosen) and the temperature of the instrument casting. During the three days of testingthe instrument temperature ranged from 20.5 to 21.7 'C with a time-series that shows only a weak diurnal component.The data in figure 5 make it apparent how anomalous the first 10 scans are relative to all the rest. Neglecting the firstten an obvious anticorrelation of temperature vs. centroid is apparent for all the wavelengths. Using multivariateregression it explains = 40% of the variance in the 404 nm position, and less than 25% of the variance at 643 nm.
The same data are replotted in figure 6 as a classic scattergram of A stepcount vs. temperature. The time-line trajectorythrough these is also shown for the sixth peak. Again this plot emphasizes how anomalous the first 10 scans arerelative to the others.
29
Figure 6: Centroid ªª stepcount vs. temperature
We then analyzed the centroid data as follows:
1 . A linear fit of centroid stepcount vs. the wavenumber (ftom table 1) was performed to obtain thecoefficients m, b for
centroid_stepcount = m * wavenumber + b
this establishes the wavenumber calibration scale for the instrument. Note that wavenumber is theindependent variable in this least-squares problem because the variance to be minimized is that ofthe centroid step-count from the fitted line.
2. Given m, b then the following can be trivially computed:
M wavenumber = (centroid_stepcount -b)/m - wavenumber;
M wavelength = 10,000,000 /((centroid_stepcount -b)/m) - wavelength;
The M quantities are the deviations from the linear fit in question. This process was done twice, once allowing eachscan to have an individual linear fit, and then using a single linear fit from scans 10 - 60 for all the data. The resultsof the deviations with respect to wavelength are shown in figures 7 and 8 respectively.
30
Figure 7: Deviation from linearity of individually fitted scans.
Figure 8: Deviation from linearity given a single fit for all the scans.
31
Figures 7 and 8 contain the basic information about the wavelength reproducibility and linearity of the instrument.T'he deviations from linearity are systematic and reproducible; they are less than 0.01 nm for all observed lines shorterthan that at 643 nm, and 0.02 nm in that case. The cosecant mechanism has a control slope that must go to infinityas wavenumber goes to zero, and so such a mechanism will be expected to get in trouble at longer wavelengths (thisis one of several reasons why a sine mechanism is preferred).
The total wavelength-assignment error budget of the instrument can be seen in figure 8. Your eye notes the extremalrange of the variances in this figure; were one-standard-deviation error bars plotted rather than all the points then thedeviations around the means would appear half as large. Again, momentarily neglecting the 643 nm line, the totalerror budget is made up of roughly equal contributions from the deviations from linearity and reproducibility. Thisworst-case error budget is less than 0.01 nm, except for the linearity deviation of the 643 nm line.
Readers should note that this apparent error budget could be reduced by applying temperature dependent correctionsfor the deviations, that as noted above can explain somewhat less than half the variance in the reproducibility over therange of temperatures tested. Further, we can attempt to remove the nonlinearity residuals; a fourth-order fit isshown in figure 8. An optimist applying both of these can reduce apparent standard deviations of the residuals below0.005 nm at all wavelengths, with only rare outliers exceeding 0.009 nm.
While I find this tempting, I think it is a bit too optimistic for my taste given only the data presented. In the caseof the temperature corrections one would like to see a somewhat larger temperature range, and series that betterdecouple the temperature from other possible time-dependent effects. In the case of the linearity one would wish tosee a greater density of line features in the spectrum, so that one could be sure that the fit was truly removing a functionthat was a fairly large scale feature of the system, rather than simply aliasing a fit through errors that might either belocally random or periodic with high spatial frequencies.
Figure 9: Optical Slit-Widths vs. WavelengthThe bold magenta line is the theoretical FWHM in wavelength for 100 µm slits, on a double additiveI m monochromator with both gratings turning "tip toward output"
Figure 9 shows the apparent FWHM optical slit-widths retrieved from the data by the double linear-fit algorithm. Atthe shorter wavelengths the agreement with the theoretical prediction is excellent. The discrepancy at the 435.8 nmline is expected because this emission line is an extremely broad one (listed in the line catalogs as "nebulous"). Thelarge discrepancy at 546 nm is due to the fact that this line saturated the A/D converter, so that the apparent line shape
32
is trapezoidal and the amplitude at which to determine the FWRM is underestimated. (Under these conditions thedouble linear-fit algorithm can still retrieve the line center with some loss of accuracy.) However we don't understandthe discrepancies at 576 and 643 nm. Both are narrow emission features; the 643 nm Cadmium line is one of thenarrowest discharge emissions known, and was the standard for interferometry before the advent of lasers.
We emphasize that these optical slit-widths were extremely reproducible. Figure 10 shows the fractional variation ofslit-width determined for each emission line, vs. the scan sequence. (These are color-coded as black, brown, red ....blue, violet, and then black with dots, and brown with dots, for the emission lines in short to long wavelength order.)The slitwidths for all of the lines except the two longest appear to be stable to ±2%. The time variation of the stepcountposition for the 435 nm line is superimposed; there are no significant correlations with it.
Figure 10: Variation of Optical Slit-Widths vs. Scan
An alternative explanation for the variation in the apparent line widths is to assume that the slitwidths were perfectlystable, and that the noise seen in figure 10 is a conservative test of the reproducibility of the linear-fit retrievals, sincethe width is computed by differencing two fits for each emission line. Given this assumption, then the uncertainty ofthe individual line center retrievals used for figures 7 and 8 would be approximately 21/2x0.02x0.03 nm (at I sigma)= 0.00085 nm. Thus the reproducibility of the line centers is an instrument effect rather that an artifact of the retrieval,but this basic premise is clear simply by inspection of figure 2. Probably there is some variation in slitwidth, but it isless than 2% for all wavelengths except the longest (where the motion of the cosecant bar is stressed, and greaterirregularity is expected and seen here), and below our ability to resolve. In any event these data suggest that variationin the slit position is not a likely explanation of the clustering line positions.
The first-order theory of monochromator aberrations is wavelength independent, and hence would not explain theapparent broadening of the optical slit-widths at longer wavelengths. This remains a minor mystery to be sorted out,but we don't preclude analytical error on our part.
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Appendix B. Quality Assurance Procedures for BroadBand Measurements (Excerpt from the network'sWorld-Wide-Web Application Pages: QA: Characterizations of USDA Broadband UV-B Pyranometers)
Quality Assurance
The USDA UV-B monitoring program recognizes the importance of instrument stability in establishing along-term climatological record. The network documents the stability of its broadband instruments throughannual calibrations and annual recharacterizations of each instrument's spectral response. Initial instrumentcharacterizations were first established by submitting each of the network's initial broadband meters to theNational Institute of Standards and Technology (NIST) for an evaluation of their cosine and spectral response.Plots of the NIST characterizations are available at the end of this document.
The cosine response of a broadband meter is a measure of the departure of the angular response of theinstrument from that of Lambert's cosine law. This law states that the response of an ideal detector to constantand uniform light should decrease in proportion to the cosine of the angle of incidence of the light. In practicethe response is a function of the design geometry and manufacturing of the meter. It is anticipated that oncedetermined, the cosine response characteristic of an individual instrument will not change unless the instrumentbecomes damaged. In field applications the stability of cosine response, once characterized, should only bedependent upon maintaining it in a level plane. In the USDA monitoring program this is checked and adjustedannually. The spectral response of a broadband meter is a measure of the instrument's response to light at specificwavelengths - typically generated with a high resolution scanning monochromater and xenon arc source. Twoimportant quality attributes of the instrument can be determined and tracked through this characterization; theinstrument's central wavelength stability and stability of its characteristic shape. Because of the importanceof documenting uniformity in these parameters over the life of the USDA's monitoring program, spectralcharacterizations are remeasured approximately annually when each instrument is returned to the manufacturerfor recalibration. Calibration of the USDA broadband meters follows the theory of Grainger et al., 1993. That is, a calibrationconstant for a selected broadband meter was derived from a regression of the integrated spectral response ofa spectroradiometer against the signal produced by the selected meter. This meter serves as the primaryreference for the network. Results of the regression yielded a relationship of 1.968 ± 0.11 (Watts/meter2)/Volt.The relationship is referenced to the 296 nm monochromatic radiation peak of the broadband's spectralresponse function and integrates energy over the range of 280 - 320nm. It should be noted that referencing toother peak wavelengths and ranges will result in a different relationship (constant). Annual calibrations followASTM E_824 methodology ending with the adjustment of each test meter's signal to the signal of a referenceinstrument maintained according to ASTM method E_816 by the calibration facility. The merits of thisapproach are discussed by DeLuisi et al. (1992). Presently, the calibration facility is located at YankeeEnvironmental Systems, Inc. in Turners Falls, MA.
34
Broadband Cosine Response
Figure 1 illustrates a typical cosine response of a Yankee Environmental Systems UVB-1 Pyranometer usedin the USDA UV-B monitoring program. The black continuous line of the plot represents an ideal cosineresponse. The colored line is the actual cosine response function of meter serial number 930203 which wasdetermined on the date listed in the legend in the center of the plot.
Figure 1: Typical Cosine Response of the YES UVB-1 Broadband Pyranometer
It should be noted that the network does not apply cosine correction to its broadband measurements. Graingeret al. (1993) have noted that the R-B type meters are "nearly azimuthally independent" citing standard errorsof relative cosine responses of less than 0.1% for solar zeniths angles < 65 degrees and less than 2.7%everywhere else.
Broadband Spectral Response
Figure 2 illustrates three successive spectral response characterizations of a single Yankee EnvironmentalSystems UVB-1 Pyranometer. A legend in the upper right corner of the plot indicates the serial number of themeter being displayed, its characterization date and the agency that performed the characterization. The blackline of the plot represents the original spectral characterization of the instrument by NIST. Differences in thedynamic range of subsequent overlays reflect differences in laboratory capabilities. Most characterizationfacilities are not able to achieve the 10-5 measurement thresholds that have been achieved at NIST. This plot also illustrates the differences in spectral response that will be noted when differing materials are usedfor the meter's transparent dome. The red line (the plus symboled plot and highest line in the shorter
35
wavelengths) represents a dome constructed with UV-grade fused silica whereas the blue line (triangle symboland line with the shortest spectra), one with Schott 280 glass. Initial characterizations of the USDA meters at NIST (black line and open circle symbol) indicate that themeters were constructed with Schott 280 glass. It should be noted that all instruments currently used by thenetwork have UV-grade fused silica domes.
Figure 2: Sample Spectral Response of a YES UVB-1 Broadband Pyranometer
NIST characterization of the initial broadband instruments deployed by the USDA network indicated that 8of 13 instruments exhibited peak spectral responses at 298 nm while 3 instruments had a peak response at 296nm. Two instruments peaked at 300 nm.
Calibration of Broadband Meters
The USDA monitoring program applies the 1.968 (Watts per m2)/Volt calibration constant uniformly to allits raw voltages to convert them to units of energy. It has been suggested that calibration accuracy may beimproved through the use of zenith specific correction factors (Yankee Environmental Systems, 1995). Thefollowing table presents these corrections for a Yankee broadband meter calibrated using a referencedintegrated wavelength range of 280-320nm (1.968 (W/m2)/V)) with a peak spectral response at 296nm. Itshould be noted that referencing to other peak wavelengths and ranges will result in different correction factors.
36
Table 1. Correction Ratios For Various Solar Zeniths Angles
Solar Zenith Angles Ratio
21.8 0.969
25.0 0.992
30.0 1.019
35.0 1.047
40.0 1.070
45.0 1.100
50.0 1.138
55.0 1.188
60.0 1.267
65.0 1.351
70.0 1.453
[From Yankee Environmental Systems, Inc]
Because this correction is based on clear sky measurements and specific assumptions about the relationshipof direct to diffuse radiation (Green et al., 1980) the USDA has decided not to apply zenith specificcorrections to its broadband measurements. At this time, the network recommends that individual data usersdetermine the validity of the above assumptions in the context of their use of the data and then if appropriateand necessary, apply the specific correction.
NIST Characterizations of USDA UVB Broadband Pyranometers
Ten of the Yankee UVB-1 Broadband Pyranometers that are deployed in the USDA UV-B Monitoring Programwere characterized by the National Institute of Standards and Technology (NIST) for their spectral response,cosine response and linearity prior to their field installation. Since the original characterizations, the instrumentshave been reevaluated by Yankee Environmental Systems to determine their stability. Documentation of thisstability (or instability) can be demonstrated by viewing the preceeding plots.
Figures 3 and 4 display the original cosine and spectral responses of the networks original broadband meters.Variability in the plots reflects the between instrument variance of the measurements.
37
Figure 3: NIST Cosine Characterization of the USDA's YES UVB-1 Broadband Pyranometer
Figure 4: NIST Spectral Characterization of the USDA's YES UVB-1 Broadband Pyranometer
38
References
American Society for Testing Materials, 1994. ASTM Method E_824. Standard Test Method for Transfer ofCalibration From Reference to Field Radiometers. West Conshohocken, PA 19428. American Society for Testing Materials, 1995. ASTM Method E_816 Standard Test Method for Calibrationof Pyrheliometers by Comparison to Reference Pyrheliometers. West Conshohocken, PA 19428.
DeLuisi, J., Wendell, J., Kreiner, F. 1992. An Examination of the Spectral Response Characteristics of SevenRobertson-Berger Meters After Long-Term Field Use. Photochemistry and Photobiology 56(1) pp115-122.
Grainger, R.G., R.E. Basher, R.L. McKenzie 1993. UV-B Robertson-Berger meter characterization and fieldcalibration. Applied Optics, 32(3) pp343-349.
Green, A.E.S., Cross, K.R., Smith, L.A. 1980. Improved Analytic Characterization of Ultraviolet Sky.Photochemistry and Photobiology 31, pp59-65. Yankee Environmental Systems, Inc. 1995. UVB-1 Ultraviolet Pyranometer Installation and User Guide,Turners Falls, MA 01376
39
Appendix C. Completeness of USDA Ultraviolet Radiation Monitoring Network Data Records by Site asa Percent of the Days of Network Operation
The network collected 6589 data files from its site polling activities in 1996. Corrupted files (unusable)remained approximately the same as in previous years - 1.4% (91 files). Corrupted files however, don't alwaysresult in missing data due to the redundancies built into the polling routines.
The annual percentages of captured data given in the Table below are based upon the number of hours ofoperation where gaps in the data did not exceed 60 minutes. New sites are those starting up for the first timein 1996. New sites coming on-line in 1996 had completeness percentages ranging from 90% (Maryland) to100% (Texas and Minnesota).
Site/ Year 1993 1994 1995 1996
California 95.1 98.8 99.6
Colorado 92.6 98.7 97.2
Georgia 98.2 96.5 98.1
Illinois 82.0 88.6 95.2 97.4
Maine - Howland (closed) 66.8 85.4 83.3 79.8
Maine - Presque Isle 95.9 94.3
Michigan 97.6 89.9 95.5
New Mexico 94.4 98.8 97.6
New York 96.6 97.5 99.0
Ohio 95.1 95.4 99.2 99.1
Utah 100 98.6
Washington 91.3 99.5 97.0
Annual Site Median 82.0 94.8 98.1 97.5
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Appendix D.. Usage of the USDA World Wide Web Site and Other Data Requests Through 1996
Usage of the USDA Ultraviolet Radiation Monitoring Program's data continues to grow in spite of the fact that the program has never registered itsWeb Server with internet indexing services or advertised its presence except at a very limited number of scientific meetings. Since its beginning in1994 the program's Web site has been visited by more than 223 companies, 171 internet service providers (local network providers who give accessto individuals), 181 institutions of higher education, 19 non-profit organizations and agencies from more than 12 states and 52 countries. Both ftpand http access is increasing (see below) indicating that many users are beginning to obtain data from the ftp directory without substantial guidancefrom project staff.
Data and information requests via Email has also increased substantially over the past year from none to approximately 30 in 1996. Nearly half ofthese requests are from K-12 students seeking general information about UV light and Ozone. No doubt most students have visited the Web site andare seeking additional information they perceive as not being available through standard reference venues. Requests include everything from data tohelp with classroom projects. Scientists, graduate students comprise most of these rest of the requesters. Typically they are seeking summarized orderived information from the program such as cumulative doses, daily maximums, etc. Many of these "products" are planned for the future but notyet available through the project.
Internet Access SummaryThe Internet Access Summary listed below is excerpted from the USDA's Ultraviolet Radiation Monitoring Program's World Wide Web Server. Because it is excerpted hotlinks (listed in green) are not available with this document. Additional information cited in the hotlinks is availablethrough a password protected Web application. Please contact the network for additional details for accessing this information.
The tables summarize the accesses made to http and ftp internet servers for the USDA Ultraviolet Radiation Monitoring Program. The tables list thenumber of hourly accesses per yearly quarter. A single hourly access is defined as one user accessing one or more project files during an hour (i.e.12:00 - 12:59). The ftp table additionally lists beneath the number of accesses the number of megabytes of data transmitted. Subsequent tables listthe data transmitted in kilobytes.For information on who has these web pages listed as hotlink on their own web pages, see Internet Referral Statistics.
41
HTTP Time Series Analysis of All Domains
UVB 1\95 2\95 3\95 4\95 1\96 2\96 3\96 4\96
US Domains 23 73 234 280 438 438 475 654
Intl Domains 10 59 52 110 146 160 257
Unresolved IPs 9 10 78 122 181 213 307 312
Totals 32 93 371 454 729 797 942 1223
FTP Time Series of All DomainsUVB 3\95 4\95 1\96 2\96 3\96 4\96
US Domains13627
32665
13622
17328
24550
509167
Intl Domains18< 1
5< 1
3710
6< 1
152
4< 1
Unresolved IPs281
262
422
12925
174
Totals15427
35966
19934
22130
38977
530171
42
Appendix E:. Data System Design of the USDA Ultraviolet Radiation Monitoring ProgramTo obtain access to the USDA - UVB file system, users may use ftp to uvb.nrel.colostate.edu logging on as anonymous. This will
put the user in the /pub directory. The user can then move into the /UVB directory which is the equivalent of the information/ directory listed below. The anonymous ftp UVB/data/ subdirectory is a pointer to the USDA - UVB public data directory. This directory contains the quality documenteddata of the USDA UVB project which is currently available.
Overall System Design
The overall structure of the USDA UVB data system is hierarchical. It consists of a top level Unix directory named 'urn' with five primarysubdirectories (Figure 1). Two other directories, named information/ and public/ are designed for external users to find their way aroundinformation located in the other directories. The directory data/ is the primary repository of USDA UVB data. The directories bin/, source/ anddocuments/ are designed as the primary repositories of executables and basic information for the USDA UVB project. The dbms/ directory containsall information required to recreate and maintain the data base management system used by the project for meta-data. This includes calibration data. Unix file system level links make the appropriate portions of these the directories available to external data users through the information/ directory.
*** Design Features *** All programs and documentation are maintained in a common place to minimize the use of depreciated code anddocuments. The goal is to be able to effect a change in a single location that will update all data/document users.
Data Custody
Data Custody of Signals Captured by the VIS-MFRSR and UV-MFRSR at USDA UVB Monitoring Locations
Devices plugged into the VIS-MFRSRs and UV-MFRSRs follow one of four data custody chains during the transfer of their signals to the data basemanagement system. Two of the chains, the VIS-MFRSR and UV-MFRSR chains, are nearly identical but are discussed separately to facilitatediscussion. For the purposes of this discussion "channels" will refer to the output of the data acquisition portion of the instrument as returned by theSUNY/ASRC (State University of New York/Atmospheric Sciences Research Center in Albany) developed programs "xtty" and "tu".
43
Figure 1. Overall structure of the USDA UVB Data System
urn/ | | bin/ ----- source/ ---------dbms/ ---------------------------- documents/ ----------------------- data/ information/ - public/ | | | | | |--- poll.pl |---- ingres_report.writer/ |----- system_diagram |---- archive/ | ----www |--- xtty |---- ingres_systems/ | |---- raw/ | ---- ftp |--- rsrsplit |---- table_structures/ |----- admin/ |---- mtm/ |--- interpc |----- catl/ |---- characterizations/ |--- interp |----- manl/ |---- completeness/ |--- tu |----- procedures/ |--- callang |--- jd/ph |--- Splus |--- tar_archives
All of the data from the USDA UVB monitoring program begins as packed 13-bit signed quantities stored on board the instrument itself. Thestructure of this packed data structure is to some extent dependent upon the configuration of the instrument. That is, the actual data structure isencoded within each returned data stream according to a strict set of rules. Unpacking algorithms must use these rules to determine how theinformation was stored on board the system. Table 1 explains the rules and presents the generic packing structure.
*** Design Features *** On-board data acquisition is very compact and makes data files "self-documenting". All of the capabilities of theinstrument can be utilized (and reconfigured) without fear of losing the documentation of the record structure.
44
Table 1. VIS-MFRSR Type-II Header Structure (From Jim Schlemmer http://grunt.asrc.albany.edu/~rsr/mfrsr.shtml)
Byte Number Description Remarks
0 Firmware Revision Number Rev. 13 is the only revision that is currently recognized as having a type-II file structure.
1 - 2 Unit ID The unit identification number
3 - 4 Head ID The head identification number. Normally, this is set by a unique identifying chip in the head. There isa command to set it manually, however.
5 - 6 Longitude Values are 16 bit fraction of a circle. For example, a hex value of 34ED is 13549 decimal. A 16 bitvalue can range from 0-65535 so we calculate the fraction of a circle as 13549/65536. We take thisquotient and multiply by 360 to yield 72.43 degrees. All longitude values are west of Greenwich and alllatitude values in the Northern Hemisphere are between 0 and 90, those in the Southern Hemisphereare between 90 and 180 (90=equator, 180=south pole, so 135 is 45 degrees south).
7 - 8 Latitude
9 Flags Byte These 8 bits control shadowband operation thus: bit 7 : Immediate output mode bit 6 : Low-power mode bit 5 : Enable Shadowband operation bit 4 : Not Used/Reserved bit 3 : Create "dummy" data bit 2 : Enable system voltage watchdog bit 1 : Output averaged observations bit 0 : System is in halted state
10 - 11 Sampling Rate 16-bit unsigned integer representing sampling rate in seconds.
12 - 13 Averaging Interval 16-bit unsigned integer representing the averaging (integration) rate in seconds.
14 - 15 System Clock: Date 16-bit unsigned integer representing the system date in days. This value is the number of days sinceJan 1, 1900 (that date being day 1). Range is 0 to 65536 but values of less than about 30000 are suspectsince no shadowbands where around back then.
16 - 18 System Clock: Seconds 24-bit unsigned integer representing the system time in seconds. This value is the number of secondsinto the current day, GMT. Range is 0 to 86400.
45
19 Number of Diodes Indicates the number of active VIS-MFRSR filter diodes. Note that this field is only meaningful if theEnable Shadowband bit is set in the FLAGS register. (This used to be controlled by the high-orderbyte of the photocal field in type-I headers.)
20 - 23 Day-Time Channels These 32 bits are toggled to turn sampling on/off for the instrument channels that run only during thedaytime. The 13-bit channels associated with these bits have a range of -4096to +4096
24 - 27 All-The-Time Channels These 32 bits are toggled to turn sampling on/off for the instrument channels that run all the time. The13-bit channels associated with these bits have a range of -4096 to +4096.
28 - 30 Counters These 24 bits are toggled to turn sampling on/off for the instrument channels that are attached todigital sensors. The 13-bit channels associated with these bits have a range of -4096 to +4096.
31 - 33 Data Size The number of bytes in the remainder of the file that are significant. Due to the fixed, 128-byte packetsize of the xmodem protocol used by the RSR when transferring data, it is usually necessary for theRSR to pad the last packet (which will usually be shorter than 128 bytes) with zeros. The data size tellsunpacking programs when to stop. Note: If this number is 0, it implies that the data file isnon-authentic. That is, a convention is followed in which any program that creates RSR data files,should clear this byte as an indication that it is not strictly pure.
34 Errors The 8 bits of this byte represent error conditions on the VIS-MFRSR
Each instrument is polled nightly using a Unix based scheduler that calls a SUNY/ASRC polling program named "xtty". This program consultssite-instrument specific configuration parameters stored in the monitoring network's data base via a wrapper perl script called "poll.pl" and copiesthe packed data currently stored on the instrument to the /data/raw/ directory on the programs server. Error codes generated by the "xtty" programare logged in the /data/archive/ directory and pre-selected message categories are Emailed to a technician responsible for QC followup.
*** Design Features *** Using a wrapper allows the primary packing/unpacking software to be maintained by the SUNY/ASRC group withoutserious competition. A goal of the network is to maintain a common unpacking code so that various computingplatform-specific software tools (Bandaid-for the Mac; DOSBand/WinBand for DOS machines; tu/callang for Unixmachines) can all share commonly formatted data files. Since the instrument storage is only queried and not dumped(cleared) we can poll data at a rate that is twice as frequent as is necessary thereby giving us the redundancy necessary toensure no loss of data due to communication failures. Daily polling allows instrument malfunctions to be caught andcorrected on a daily basis thus minimizing downtime due to instrument failure or corruption.
46
Packed raw data files are routinely processed with a filtering program named "rsrsplit" that sorts and merges packed data records obtained fromsequential polling routines into unique data sets void of all duplication. The splitter preserves the unique data record documentation within eachpolled data set by merging only sequential data that share the exact same structure and header information (ie same longitude, latitude, head andboard identification, sampling frequency, etc). These files, known as mtm files, then serve as the primary data files for subsequent data processing. Raw files obtained via the nightly polling are archived.
The USDA UVB monitoring program uses the SUNY/ASRC derived unpacking program "tu" as its primary unpacking tool. A C-program wrappernamed "interpc" calls the "tu" program and re-arranges its output to a standardized parameter-ordered output stream. This ordering is necessary asthe unpacker (tu) is unaware of what instruments are associated with what channels or if an auxiliary instrument has been associated with more thanone channel during an instruments tenure at a monitoring location. The wrapper (interpc), via the network's data base, keeps track of whatinstrument signal is associated with what auxiliary channel. Table 2 summarizes the processing of each VIS-MFRSR signal brought into the datasystem and identifies which data custody chain that piece of data follows.
*** Design Features *** Use of the wrapper ensures that the basic SUNY/ASRC unpacker code is simple and universal to all classes ofshadowbands. The wrapper also provides infinite flexibility in customizing output streams to meet project needs.
47
Table 2. Signal Processing Through the VIS-MFRSR
Sensor Parameter Device Channel Output to logger Transforms in "tu"
EngineeringTransforms ("interp")
Clock date GMT [1]Time since midnight
1 JAN 1900
none YYMMDD (date)
Clock time GMT [2] none none
Clock time [3] decimal hour (time)
VIS-MFRSR unused
VIS-MFRSR total horizontal 0-2.000 volts [4]0-4096 mv
none none
VIS-MFRSR total-horizontal 0-2.000 volts [5] none none
VIS-MFRSR total-horizontal 0-2.000 volts [6]Average of two side
measurements
subtracted from
nadir measurement
and added back to
the blocked
measurement
Average of
zenith-corrected
differences
returning 0-4096
mv
none none
VIS-MFRSR total-horizontal 0-2.000 volts [7] none none
VIS-MFRSR total horizontal 0-2.000 volts [8] none none
VIS-MFRSR total-horizontal 0-2.000 volts [9] none none
VIS-MFRSR total-horizontal 0-2.000 volts [10] none none
VIS-MFRSR diffuse-horizontal 0-2.000 volts [11] none none
VIS-MFRSR diffuse-horizontal 0-2.000 volts [12] none none
VIS-MFRSR diffuse-horizontal 0-2.000 volts [13] none none
VIS-MFRSR diffuse-horizontal 0-2.000 volts [14] none none
VIS-MFRSR diffuse-horizontal 0-2.000 volts [15] none none
VIS-MFRSR diffuse-horizontal 0-2.000 volts [16] none none
VIS-MFRSR diffuse-horizontal 0-2.000 volts [17] none none
48
VIS-MFRSR direct-normal 0-2.000 volts [18] none none
VIS-MFRSR direct-normal 0-2.000 volts [19] none none
VIS-MFRSR direct-normal 0-2.000 volts [20] none none
VIS-MFRSR direct-normal 0-2.000 volts [21] none none
VIS-MFRSR direct-normal 0-2.000 volts [22] none none
VIS-MFRSR direct-normal 0-2.000 volts [23] none none
VIS-MFRSR direct-normal 0-2.000 volts [24] none none
QLINKS (precipitation) [25]
Vaisala humidity 0-1.000 volts [26] none [a] (%RH)
Vaisala temperature 1000 - 1154 mv [27] none [b] (deg C)
QLINKS PIR case temperature [28]
QLINKS PIR dome temperature [29] [c] (w/m2)
QLINKS/PAW PIR/Epply pyranometer at Pawnee [30]
LI-COR 210SZ 0-100 mv [31] 0-1000 mv none none (deg C)
QLINKS [32]
VIS-MFRSR Reference Voltage [33] [d] (volts)
VIS-MFRSR Head temperature [34] [e] (deg C)
Yankee dome temperature ±2.500 volts [35] 0 - 2500 mv none [f] (deg C)
Yankee UVB-1 signal ±2.500 volts [36] 0 - 2500 mv none [g] (w/m2)
49
Engineering Transforms for the VIS-MFRSR and other Ancillary Measuring Devices
NOTE: [value] means the number returned by the VIS-MFRSR through the ASRC program `convert'. [a] Relative Humidity = [value]/10
[b] USDA and QLINKS sites use different circuit boards to process the Vaisala temperature signal. - For USDA temperature = ([value] - 2893) * 0.090) - For QLINKS temperature = (0.05280 * [value] - 0.4263)
[c] This channel is not used by USDA sites. QLINKS sites use this channel for an Epply pyrgeometer (PIR; precision infrared radiometer). The Pawnee uses this channel for an Epply model 4 8-48 pyranometer (Black and White; combined direct and diffuse radiation). - For Pawnee; watts/meter squared = [value]/10.884 - For QLINKS; watts/meter squared = factory_calibration_constant * [value] + board_offset + 5.6724e-08 * (273.1 + [case temperature])^4 where delta =[case_value[28]/thermister_value[33]] * 1000) and case temperature = -57.63238 + (292.12769 * delta) - (769.21277 * delta^2) + (1489.01428 * delta^3) - (1502.90637 * delta^4) + (648.54034 * delta^5) [d] Volts = [value]/1000
[e] Degrees C = -48.63338 + (477.12256 * delta) - (1838.90784 * delta^2) + (4631.11377 * delta^3) - (5864.18701 * delta^4) + (2952.99707 * delta^5); where delta = [value]/5000
[f] UVB sensor temperature = ([value] * 0.0469) + 6.6 [g] [value] * 0.001968
50
VIS-MFRSR Chain:
The first 24 channels of the instrumentation follow the VIS-MFRSR data custody chain. The first three channels provide a time stamp that includesboth date and time while the 4th through 24th channels provide outputs of solar irradiance at specific wavelengths. Channel [1] returns a date, whilechannel [3] returns a time. The second channel [2] provides a decimal rendition of the time in hours. Time is kept internally as UniversalCoordinated Time (UTC) and the internal clock is reset each time an authorized polling authority queries the VIS-MFRSR's data acquisitionfacilities. Unauthorized authorities only query the data acquisition system and do not reset any parameters. Errand clocks are simply reset andtracked by authorized authorities and returned times are not corrected for the offset. Offsets are however, returned as a diagnostic after each poll.
The shadowband instrument itself utilizes channels [4] through [24] of the instrument's data storage capacity. Channels [4] through [17] receive 0 to2.000 volts output from the amplifiers in the VIS-MFRSR sensor head. These voltages are passed through precision resistors to effectwavelength-specific gains which range from 1.43 to 8.03. Resistors "tune" the signal to output ranges of 0-4096 millivolts. The system is designedto effect an A/D conversion of 1.0 millivolts to one count. Significant digits in all subsequent "in-board" calculations are preserved by multiplyingthe counts by a factor of 4 and then performing integer computations. All final 12-bit variables are divided by 4 prior to its storage in theVIS-MFRSR's data acquisition chip.
AUXiliary Chain:
The METSUITE includes channels [25] through [31]. The suite is so named because it is configured for the convenience of connecting typicalmeteorological instrumentation. In the USDA network, channels [25] and [29-30] are not used. However, these channels are "reserved" forprecipitation measuring devices ([25]) and for pyrgeometer (PIR) outputs. This reservation is based upon a desire to keep network channel outputscomparable with a sister radiation network run by the Department of Energy under its ARM program (Quantitative Links).
Channel [26] and [27] contain humidity and temperature measurements made with a Vaisala model HMP 35A probe. Counts from theVIS-MFRSR's data system are converted to % RH by dividing the signal by 1000. Temperature measurements in degrees C are obtained by conditioning the output signal of the device through a linearity correction loop and based upon the manufacturers stated specification of 100 ohmssignal at 0 degrees C.
Channel [31] presents the signal in watts/meter squared from a downward looking LI-COR 210SZ radiation sensor. The sensor is intended only as acrude indication of the gross changes in cover and hence albedo at a monitoring location (eg presence or absence of snowfall).
51
Use of the Data Base Management System
Figure 2. Functional Description of Table Structures in the USDA UVB Data Base Management System `*' signifies tables currently in use PRIMARY DATA TABLES L |-------------------| o | locations_info .|*-- Site identification and classification VIEWS: PSEUDO TABLES CREATED By o T |-------------------| information A PERMANENT Data Base Definition k a | people_info |*-- Name/Addresses/Mailing Labels -----------| |---------------| u b |-------------------| |----->| site_contacts | -- site operators p l | requestlg_info | -- tracking of data and documentation | |---------------| e |-------------------| requests | | |
| | |---------------| -- current|-------------------| | | instrumentation
E | mfrsr_tracking |*-- polling configurations of VIS-MFRSR's....| |---------------| and polling q T |-------------------| |----->| site_status | parameters u r | uvrsr_tracking...|*-- polling configurations of UV-MFRSR's | |---------------| i a |-------------------| | | | p c | yankee_tracking |*-- tracking of Yankee UVB-1 deployment |----->|---------------| -- Equipment,moves m k |-------------------| | | site_history | chronology at e i |vaisala_tracking...|*-- tracking of Vaisala deployment | |---------------| sites n n |-------------------| | | | t g | licor_tracking |*-- tracking of LI-COR deployment ...........| |---------------|
|-------------------| | || | |---------------||-------------------|
C | broadbands_cosine |*-- cosine characterization data for broadbands a |-------------------| l T |broadbands_spectral|*-- spectral response of the broadbands i a |-------------------| b b | mfrsr_cosine |*-- cosine correction data for the VIS-MFRSR's r l |-------------------| a e | uvrsr_cosine |*-- cosine correction data for the UV-MFRSR's t s |-------------------| i | board_calibrations|*-- gains and offsets of both VIS-MFRSR and UV-MFRSR boards o |-------------------| n | head_calibrations |*-- gains, offsets and center wavelength characterizations
|-------------------| of both VIS-MFRSRs and UV-MFRSRs | | |-------------------|
| ? | -- Primary data from the shadowband |-------------------| instruments | ? | -- Primary broadband data
|-------------------| | ? | -- Primary ancillary on-site measurement data ----->|---------------| |-------------------| | clear_days | | ? | -- Primary data from the scanning instrument |---------------|
|-------------------|
52
References
Harrison, L., J. Michalsky and J. Berndt. 1994. The Automated Multi-Filter Rotating Shadowband Radiometer: An Instrument for Optical Depthand Radiation Measurements, Applied Optics, 33, 5118-5125.
Harrison, L. 1991 Rotating Shadowband Radiometer Instruction Manual Atmospheric Sciences Research Center, State University of NewYork-Albany, Albany, NY 12205, May 1991, Software Versions 8.0e