5.4. ushuaia, argentina (6/8/05 – 6/10/08)uv.biospherical.com/report_0506/chap5_4.pdf · and...

NSF UV SPECTRORADIOMETER NETWORK 2005-2006 OPERATIONS REPORT

PAGE 5-38 BIOSPHERICAL INSTRUMENTS INC.

5.4. Ushuaia, Argentina (6/8/05 – 6/10/08)

This sections describes quality control of solar data recorded between 6/8/05 and 6/10/08. There were no site visits in 2006 and 2007. Solar data recorded between 6/8/05 and 6/21/06 were assigned to Volume 15, data covering the period 6/22/06 – 6/5/07 are part of Volume 16, and the period 6/6/07 – 6/10/08 was assigned to Volume 17. Opening and closing calibrations of the 3-year period were performed on 6/7/05 and 6/11/08, respectively. A total of 53840 scans are part of the Ushuaia datasets of Volumes 15-17. Only 2.5% of all possible scans were lost due to technical problems, mostly due to extended power outages. 5.4.1. Irradiance Calibration

The on-site irradiance standards used during the reporting period were the lamps M-698, M-766, 200W008, and 200W026. The four lamps have the following history: lamps 200W008 and 200W026 were calibrated by Optronic Laboratories (OL) on 11/19/96 and 3/28/01, respectively. Lamp 200W026 was used less than 30 times (total burn time of less than 20 hours) since its calibration in 2001. The lamp can be considered a long-term standard based on its sparing use. The calibration of lamp M-698 was established in 1999 by comparing several absolute scans from lamps M-698 and M-874. (The latter lamp was the traveling standard at that time.) Lamp M-766 has an OL calibration from October 1992. It was recalibrated in 1999 like M-698. The traveling standard used during the site visit in June 2005 was lamp M-764. It was calibrated by OL on 3/28/2001. The traveling standards used during the site visit in June 2008 were the lamps 200W017 and 200W038. Their calibrations were established in June 2006 and April 2008, respectively, using a set of four 1000-W FEL lamps with serial numbers H-011, H-013, H-023, and H035. These FEL lamps have been calibrated by NOAA's Central UV Calibration Facility (CUCF) at Boulder, Colo, and their irradiance scale refers to the detector-based NIST scale from 2000 (NIST2000) (Yoon et al., 2002). Since all NSF network data refer to the source based NIST scale from 1990 (NIST1990) (Walker et al., 1987), the irradiance values of the four FEL lamps were first converted to the NIST1990 scale before their calibration was applied to the two traveling standards. See Chapter 5.5 of the 2007-2008 Operations Report for more details on the transfer. The site standards M-698, M-766, and 200W008 were recalibrated on 7/16/08 against the traveling standards 200W017 and 200W038 using scans from the June 2008 site visit. Figure 5.4.1 shows a comparison of site standards M-698, M-766, 200W008 (with their new calibration scales applied), and the traveling standards 200W017 and 200W038 with lamp 200W026. Calibrations of all lamps agree to within ±0.7%. We note that this is a remarkable result considering that the irradiance scales of all lamps but 200W026 are traceable to the CUCF irradiance scale, whereas lamp 200W026 is traceable to the OL irradiance scale from March 2001. This results confirms the consistency of the scales provided by CUCF and OL and also underlines our ability to preserve these scales over a long time. Figure 5.4.2 shows a comparison of traveling standard M-764 and Ushuaia lamps M-698, M-766, and 200W008 with standard 200W026 at the beginning of the season. The following calibration data was applied: M-764: OL 3/28/2001; M-698: BSI 6/17/08; M-766: BSI 5/25/00; 200W008: BSI 11/19/96; 200W026: OL 3/28/01. Results presented in Figure 5.4.2 supports the following conclusions: (1) There is a difference between the calibrations of traveling standard M-764 and 200W026 of approximately 2% in the UV and 1% in the visible. As both lamps were calibrated by OL at the same time and both lamps were used only infrequently since their calibration, a difference of 2% in the UV is comparatively large. However, this difference is still within the combined calibration uncertainty of the two lamps. A part of the difference may also be caused by the fact that lamp M-764 has been frequently shipped to network sites in support of site visits. (2) Lamps M-698 and 200W026 agree to within ±0.7%. This indicates that the calibration of M-698 established on 6/17/08 did not change significantly between June 2005 and June 2008. (3) Calibrations of lamps M-766 and 200W008 agree to within ±2.5% with the calibration of lamp 200W026.

CHAPTER 5: QUALITY CONTROL AND CALIBRATION STANDARDS

BIOSPHERICAL INSTRUMENTS INC. PAGE 5-39

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Figure 5.4.1. Comparison of traveling standards 200W017, 200W038 and Ushuaia lamps M-698, M-766, and 200W008 with standard 200W026 at the end of the season (6/18/08). The irradiance scales of all lamps but 200W026 are traceable to the CUCF irradiance scale, adjusted for the difference between the NIST2000 and NIST1990 scales. The irradiance scale of lamp 200W026 is traceable to the OL irradiance from March 2001, which at this time was also traceable to the NIST1990 irradiance scale.

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Figure 5.4.2. Comparison of traveling standard M-764 and Ushuaia lamps M-698, M-766, and 200W008 with standard 200W026 at the beginning of the season (6/7/05).



The on-site standards M-698, M-766, and 200W008 were compared five times with the (long-term) standard 200W026 during the reporting period. Results indicate that calibrations of lamps M-698 and 200W026 were consistent to within ±0.5% on all five occasions. Variation for the pair M-766 and 200W026 were in the order of ±1.5%, and scans of 200W008 and 200W026 were only consistent at the ±3% level, suggesting that lamp 200W008 is unreliable. Because of the instabilities of lamps M-766 and 200W008, calibrations of solar data of the period 6/8/05 – 6/10/08 were only based on lamps M-698 (BSI calibration from 7/16/08) and 200W026 (OL calibration from 3/28/01). 5.4.2. Instrument Stability

The stability of the spectroradiometer over time was primarily monitored with bi-weekly calibrations utilizing the on-site standards M-698 and 200W026, and daily response scans of the internal irradiance reference. The stability of the internal lamp is monitored with the TSI sensor, which is independent of possible monochromator and PMT drifts. By logging the PMT currents at several wavelengths during response scans, drifts in monochromator throughput and PMT sensitivity can be detected. Figure 5.4.3 shows the changes in TSI readings and PMT currents at 300 and 400 nm, derived from response scans performed between 6/8/05 and 6/10/08. The system was extraordinarily stable over this 3-year period: TSI measurements increased only by about 2%, indicating that the internal lamp became only slightly brighter. PMT currents varied to within ±2%. Measurements of the internal lamp cannot detect changes in the throughput of the instrument’s fore optics. Analysis of absolute scans indicated somewhat larger changes in responsivity than results from the response scans. To correct for these changes, the instrument’s calibration was broken into four periods, which are summarized in Table 5.4.1. Figure 5.4.4 presents ratios of irradiance spectra applied to the internal lamp during periods P2 – P4, referenced to the spectrum for Period P1. There were step-changes in the order of 2-4% between Periods P1 – P3. System responsivity in the UV did virtually not change between September 2005 and June 2008. Figure 5.4.5 presents ratios of standard deviation to average calculated from the individual absolute scans in the four periods. These “relative standard deviation spectra” are useful for estimating the variability of calibrations within a given period. The variability is typically less than 1.0% for wavelengths above 320 nm, indicating good consistency of individual absolute scans. Data for Periods P1 and P2 are noisier than for Period P3 and P4 because of the lesser number of absolute scans.

Table 5.4.1 Calibration periods for Ushuaia Volumes 15-17.

Period name

Period range Volume Number of Absolute scans

Remarks

P1 06/06/05 - 08/08/05 15 4 P2 08/09/05 - 09/15/05 15 4 P3 09/16/05 - 06/05/07 15 and 16 22 P4 06/06/07 - 06/12/08 17 21



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Figure 5.4.3. Time-series of TSI signal and PMT currents at 300 and 400 nm during measurements of the internal reference lamp performed at Ushuaia between 6/8/05 and 6/10/08.

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Figure 5.4.4 Ratios of irradiance assigned to the internal reference lamp in Periods P2 - P4, referenced to the irradiance of Period P1.



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Figure 5.4.5. Ratio of standard deviation and average calculated from the absolute calibration scans measured at Ushuaia between 6/8/05 and 6/10/08.

5.4.3. Wavelength Calibration

Wavelength stability of the system was monitored with the internal mercury lamp. Figure 5.4.6 shows the differences in the wavelength offset of the 296.73 nm mercury line between pairs of consecutive wavelength scans for the period 6/8/05 and 6/10/08. In total, 1089 scans were evaluated. For 96.7% (94.3%) of the scans is the difference in the wavelength offset to neighboring scans less than ±0.055 nm (±0.025 nm). Changes larger than ±0.1 nm (2.8% or 31 scan-pairs) were mostly caused by power outages and subsequent operator intervention. Affected data were adjusted accordingly. After data were corrected for day-to-day wavelength fluctuations, the wavelength-dependent bias between this homogenized data set and the correct wavelength scale was determined with the Version 2 Fraunhofer-line correlation method (Bernhard et al., 2004). The resulting monochromator non-linearity correction function is shown in Figure 5.4.7. After data had been wavelength corrected using the shift-function described above, the wavelength accuracy was tested again with the Version 2 Fraunhofer-line correlation method. Results for noontime scans are shown in Figure 5.4.8 for four wavelengths in the UV and one in the visible. Wavelength shifts are typically smaller than ±0.05 nm; the standard deviation of the residual shifts at 320 nm is 0.036 nm. There is some temporal variation of the wavelength-shift pattern, which will be reduced during Version 2 data processing. Few outliers occur when spectra are distorted due to changing cloud cover. The wavelength stability is not worse during cloudy conditions but the validation is subject to larger uncertainties.



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Figure 5.4.6. Differences in the measured position of the 296.73 nm mercury line between consecutive wavelength scans for the period 6/8/05 and 6/10/08. The labels of the horizontal axis give the center wavelength shift for each column. The 0-nm histogram column covers the range from -0.005 to +0.005 nm. “Less” means shifts smaller than -0.105 nm; “more” means shifts larger than 0.105 nm.

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Figure 5.4.7. Monochromator non-linearity correction function for the Volumes 15-17 period at Ushuaia. The error bars indicate the 1σ standard deviation of the wavelength shift.



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Figure 5.4.8. Wavelength accuracy check of final data at four wavelengths in the UV and one in the visible by means of Fraunhofer-line correlation. The noontime measurement has been evaluated for each day of the Volumes 15-17 period.

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Figure 5.4.9. The 296.73 mercury line as registered by the PMT from external and internal sources. The wavelength scale is the same as applied for solar measurements, i.e., it is based on a combination of internal scans and the Fraunhofer-correlation method. It is assumed that the wavelength registration of the monochromator did not shift between internal and external scans, which were close in time.



Although data from the external mercury scans do not have a direct influence on data products, they are an important part of instrument characterization. Figure 5.4.9 illustrates the difference between internal and external mercury scans collected during the site visits in 2005 and 2008. The wavelength scale of the figure is the same as applied during solar measurements. The peak of the external scans agrees approximately with the nominal wavelength of 296.73 nm, whereas the peak of the internal scans is shifted by about 0.08 nm to shorter wavelengths. External scans have a bandwidth of about 1.04 nm FWHM; the bandwidth of the internal scan is 0.75 nm. External scans have the same light path as solar measurements and represent the monochromator’s bandpass at 296 nm relevant to solar scans. 5.4.4. Missing Data

Volume 15 (6/9/05 – 6/21/06): A total of 18278 solar scans are part of the Ushuaia Volume 15 dataset. These are 96% of all possible solar scans. About 3% of all scans were missed due to technical problems, mostly power outages lasting several hours. (Shorter power outages are bridged by the system’s UPS). A listing of missing data is provided in Table 5.4.2. Inconsistency between GUV and SUV indicate in the table could be caused by snow accumulation on one of the instruments. Volume 16 (6/22/06 – 6/5/07): A total of 17653 solar scans are part of the Ushuaia Volume 16 dataset. These are 97% of all possible solar scans. About 1.5% of all scans were missed due to technical problems. More details are provided in Table 5.4.2. Volume 17 (6/6/07 – 6/10/08): A total of 17909 solar scans are part of the Ushuaia Volume 17 dataset. These are 95% of all possible solar scans. About 1.5% of all scans were missed due to technical problems and 1% due to roof repairs; see Table 5.4.2 for details. Table 5.4.2 Missing scans of Ushuaia Volumes 15–17.

Time Period Scans missing

Reason

Volume 15 Throughout period 181 Calibration absolute, wavelength and response scans 06/14/05 7 Change storage medium 06/25/05 30 Power outage 06/27/05 32 SUV and GUV not consistent 07/08/05 17 SUV and GUV not consistent 07/18/05-07/19/05 49 SUV and GUV not consistent 07/28/05 6 Software error 08/25/05 20 Power outage 09/04/05 11 Power outage 09/17/05 14 Power outage 11/23/05 30 SUV and GUV not consistent 12/10/05 50 Power outage 12/23/05-12/25/05 164 Power outage / Wavelength setting beyond correctable range 12/31/05 6 Unknown 01/11/06 15 Power outage 01/14/06-01/15/06 47 Power outage 03/08/06 15 Power outage 04/02/06 35 Power outage 04/08/06 31 Power outage 04/21/06 12 Power outage



Volume 16

Throughout period 162 Calibration absolute, wavelength and response scans 06/30/06 32 SUV and GUV not consistent; no PSP signal 07/10/06 3 Power outage 07/21/06-07/22/06 46 Full storage medium 08/15/06-08/16/06 120 Incorrect GPS time; scans overwritten 10/09/06 4 Troubleshooting network connection 11/12/06 28 Power outage 04/05/07-04/06/07 38 Power outage 05/26/07 33 SUV and GUV not consistent 05/27/07 23 Incorrect GPS time; scans overwritten

Volume 17 Throughout period 200 Calibration absolute, wavelength and response scans 06/15/07 9 Unknown 06/29/07-06/30/07 57 Software error 07/26/07 4 Binary data files corrupted 07/29/07-08/25/07 10 Roof repair 08/27/07 4 Change of storage medium 08/27/07-08/31/07 58 Roof repair 09/01/07 12 Power outage 09/01/07-09/05/07 59 Roof repair 09/06/07-09/22/07 22 Roof repair 09/21/07 4 Binary data files corrupted 11/22/07 12 Power outage 12/01/07-12/02/07 77 Power outage 12/16/07 46 Power outage 01/30/08-01/31/08 22 Power outage 02/03/08-02/04/08 47 Power outage 03/06/08-03/07/08 20 Full storage medium; change of medium 04/07/08-04/08/08 91 Incorrect GPS time; scans overwritten 04/20/08 6 Power outage 05/02/08 9 Power outage 05/09/08-05/11/08 75 Incorrect GPS time; scans overwritten 05/23/08 20 Unknown

5.4.5. GUV Data

The GUV-511 radiometer, which is installed next to the SUV-100, was calibrated against final SUV-100 measurements following the procedure outlined in Section 4.3.1. Separate calibration factors were established for data of every volume. Factors of all channels but the 320-nm channel were consistent to within ±1%. The 320-nm channel drifted by 3% over the 3-year period. Drifts in published GUV data are smaller than 1%. Data products were calculated from the calibrated measurements (Section 4.3.2). Figure 5.3.10. shows a comparison of GUV-511 and SUV-100 erythemal irradiance. For solar zenith angles smaller than 80°, measurements of the two instruments agree to within ±5.3% (±1σ). The GUV tends to underestimate erythemal irradiance at large solar zenith angles. We advise data users to use SUV-100 rather than GUV-511 data whenever possible, in particular for low-Sun conditions.



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Figure 5.4.10. Comparison of erythemal irradiance measured by the SUV-100 spectroradiometer and the GUV-541 radiometer. SUV-100 measurements are based on “Version 0” (cosine-error uncorrected) data. Figure 5.4.11 shows a comparison of total ozone measured by the GUV-511 radiometer, the SUV-100 (Version 2 data set; see www.biospherical.com/NSF/Version2), and the Ozone Monitoring Instrument (OMI) installed on NASA’s AURA satellite (data version 8.5, Collection 3). GUV-511 ozone values were calculated as described in Section 4.3.3. There is generally good agreement between the three data sets. SUV-100 measurements are on average 2.8% larger than OMI observations and GUV data exceed OMI measurements by 1.8% on average. The reason of these systematic differences is partly caused by the way the atmospheric ozone and temperature profiles are used by the three inversion methods. We also note that OMI version 8.5 ozone columns are slightly lower than the original OMI data release.



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Figure 5.4.11. Comparison of total column ozone measurements from GUV-511, SUV-100 (Version 2 data), and OMI. GUV-511 and SUV-100 measurements are plotted in 15 minute intervals. For calculating ratios of data sets, only GUV-511 and SUV-100 measurements concurrent with OMI overpass data were evaluated.

5.4. ushuaia, argentina (6/8/05 – 6/10/08)uv.biospherical.com/report_0506/chap5_4.pdf · and...

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