measurement of aerosol optical thickness using a narrow
TRANSCRIPT
Measurement of Aerosol Optical Thickness
Using a Narrow Band Sun Photometer
Prepared for ATMS 748
Atmospheric Measurements
University of Nevada
Reno
David DuBois
September 8, 1998
1
1. INTRODUCTION
This section describes the historical background of measuring the aerosol content of theatmosphere through the use of the concept of turbidity. The section concludes with abrief theoretical basis for this project.
1.1 Historical Introduction
The application of photometry to estimate atmospheric extinction has its origin in 1725with Pierre Bouguer (Middleton 1961). Bouguer used the moon as the source ofradiation to measure the transmissivity of the atmosphere. He used the formula
eTIE cos/"�
where T is the transmissivity, E is the optical index of the material in question, O� is thethickness of the atmosphere, and I is the angle of the beam of primary illuminationmeasured from the solar zenith. The transmissivity of the atmosphere has a valuebetween 0 and 1, where 0 corresponds to a perfectly opaque atmosphere and 1corresponds to a perfectly transparent atmosphere. This expression is now namedBouguer's Law. Bouguer compared the transmission of light through the atmosphere tothat of light passing through colored glass. To get the transmissivity, he then solved forthe quantity EO through the use of two different measurements of I. Angstrom (1929,1961, 1964, 1970) researched the problem and refined a mathematical framework tocharacterize the atmospheric optical properties. The product EO is called the "turbidity"and is a dimensionless and positive number. This quantity is also known as the opticaldepth or optical thickness in the literature. The AOT has been, and continues to be, animportant measurement and modeling parameter in the air pollution, atmosphericradiation and climatology fields. "Aerosol optical depth is the principle variabledescribing aerosol effects in classical radiative transfer calculations," (WMO 1991). Amore rigorous definition of turbidity used widely today and bringing out more of thephysics involved (Liou 1980) is presented in the next section.
1.2 Theoretical Basis
Incident solar radiation at the top-of-the-atmosphere, FO(f), is attenuated by variousmolecules and atmospheric aerosols as it propagates down to the earth's surface. Thisattenuated flux reaching the ground is denoted as FO(0). The ratio of the incident surfaceflux over the incident top-of-the-atmosphere flux defines the total atmospherictransmissivity or T. The transmissivity can be approximated using Bouguer's Law as:
mMR
eF
FT )]()([
)()0( OWOW
O
O �� f
Where WR(O) is the Rayleigh optical depth, WM(O) is the aerosol optical depth, m is the airmass or secI (1/cosI) with I being the solar zenith angle. This equation is equivalent tothe equation first stated by Bouguer except that the EO is expanded out to better
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understand the components of the total extinction that was measured by the sunphotometer. Furthermore, the aerosol optical depth or AOT can be expressed in terms ofthe extinction coefficient
³f
0
),()( dzzeM OEOW
where Ee(O,z) is the vertical profile of the aerosol extinction coefficient as a function ofwavelength. The integral in the AOT equation above integrates from the ground, z = 0,to the top of the atmosphere at z = f. The variable FO(0), at a wavelength of 530 nm, wasthe measured parameter in this experiment. This expression is the same as the EO in thefirst equation above. Since the definition of extinction is the sum of scattering andabsorption, the expression for Ee(O,z) can be expanded to
),(),(),( zzz ase OEOEOE �
where Es(O,z) is the aerosol scattering coefficient and Ea(O,z) is the aerosol absorptioncoefficient. Considering the wavelength of the measurements and the general aerosolcontent, the major contributor to the extinction is from aerosol scattering during themeasurement period. Particles with sizes that are on the order of the wavelength ofvisible light contribute the most to light scattering. The parameter E e(O,z) is a complexfunction of particle composition, size, index of refraction and shape. Efficient lightscatterers are typically composed of ammonium nitrate (NH4NO3), ammonium sulfate((NH4)2SO4) and ammonium bisulfate (NH4HSO4). Efficient light absorbing particles arecomposed of elemental carbon (black carbon) and organic carbon. Typically, the carbonparticles are located near sources of combustion, either classified as anthropogenic orbiogenic. Exceptions to this are long-range transport of forest fire effluent or burningfuel smoke. Mineral particles such as fugitive dust can contribute to both solar scatteringand absorption depending on the size of the particles and their composition. If their sizesare on the order of the wavelength of light they will contribute to scattering. Someminerals, such as hematite, have relatively high indices of refraction relating to theirability to absorb radiation. The effects of dust on climate have become importantrecently as uncertainties in the net aerosol radiative forcing in global climate models(GCMs) have attracted attention (Andreae 1996). The particle extinction is alsogenerally dependent on the type of airmass. A moist airmass in the troposphere will tendto have more light scattering aerosols than one that is dry. The magnitude of the turbiditycan become very large for optically thick atmospheres. Examples of optically thickconditions are commonly found in stratus and cumulus clouds, smoke plumes and heavyfogs. The exact vertical profile of Ee(O,z) is in general a complex and dynamic parameterthat varies from place to place and in time. In-situ measurements of aerosol extinction inarid environments point toward a decrease in aerosol concentration as a function ofheight (Pinnick et al. 1993). Lidar measurements in desert environments also show asharp decrease in aerosol concentration as a function of height in the lower troposphere(Spinhire et al. 1980). The conclusion drawn from these measurements is that most ofthe light extinction can be attributed to aerosols in the lowest portion of the troposphere.
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The exception to this is when volcanic ash and gases are thrust into the upper troposphereand lower stratosphere. During this measurement, no volcanic activity was present. Theturbidity measures the column integrated attenuation due to extinction from aerosols andgaseous species. The term turbidity and aerosol optical thickness (AOT) are synonymouswith both being logarithmic indices of atmospheric optical attenuation in a column. Thisis true only for wavelengths outside of the strong gaseous absorption regions in theinfrared.
1.3 Measurement Technology
Turbidity or AOT estimates have evolved from the crude eye measurements to thethermopile and semiconductor detectors used in instruments today. Thermopile detectorsare used today in some instruments because of their electrical stability. Modern planarsilicon junction photodiode semiconductor sensors have proven reliable as substitutes forthe older technology. These sensors have high efficiencies and spectral sensitivities inthe wavelength range of 300 to 1100 nm. Filter technology has advanced allowingnarrow bandpass values as low as a few tens of nanometers. These narrow bandwidthfilters are usually constructed using interference or thin film filter technology. Moreoften than not, these filters incur a substantial cost to the system and may degrade in timedue to the environmental exposure.
An economical alternative to the filter based photometer systems is the use of asemiconductor diode or photodiode sensitive to visible radiation. The use of aphotodiode as the detection mechanism in a sun photometer has been proposed bynumerous investigators. Photodiode photometers have been used to measure cloudoptical depth (Raschke and Cox 1983). The use of LED in the light sensing configurationhas been successfully demonstrated (Mims 1992; Carlson 1997). Instead of operating aLED in the normal way by applying a voltage across the diode to emit light, the lightsensing configuration requires the measurement of the current across the diode whensunlight is applied. The primary advantages of using light emitting diode (LED) sensorsin photometers are the compact size, inexpensiveness, availablility, reliability and theinherent narrow spectral bandpass.
An important application of aerosol optical thickness is to measure the climatalogicalaerosol distribution spatially and temporally. A recent measurement study of the ARMCART sites over a 250 x 350 km2 area showed that the annual variations of median AOT,at 500 nm, were found to be 0.30 over the five sites (Nash and Cheng, 1998). This is animportant finding since these spatial variations are smaller than the grid spacing of globalclimate models (GCM). The measurement of AOT during volcanic eruptions has been avaluable tool in mapping the effects of ash plumes globally (Mendonca et al. 1978;Spinhirne and King 1985; Dutton and DeLuisi 1983; Ryznar and Baker 1983; Michalskyand Stokes 1983). These particles may be injected into the stratosphere, giving them longlifetimes and affecting the global balance of radiation. One example is the eruption ofMt. Pinatubo during 1991. During the eruption nearly 30 million tons of aerosols werereleased into the stratosphere. These aerosols were detected by not only satelliteobservation platforms but also ground based photometers and spectrometers. Based onsatellite observations, three years after the eruption, nearly all of the Mt. Pinatubo
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aerosols were gone. In the study of global change and its application to global warming,the temperatures of the Earth or any other planet depends mainly on the amount ofsunlight received, the amount of sunlight reflected into space and the extent to which theatmosphere retains heat.
The purpose of this project was to construct an inexpensive sun photometer using off-the-shelf electronic components to measure atmospheric aerosol optical thickness. Thisproject provides an opportunity to apply knowledge in electronics, electro-optical devicesand atmospheric physics to solve a long-standing problem in the measurement of aerosoloptical thickness. An effort was made to quantify errors, limitations and accuracy of theinstrument using an appropriate statistical and theoretical basis.
1. INSTRUMENT DESCRIPTION
The measurement of the AOT was accomplished by first measuring the sun's irradianceas attenuated by the atmosphere over a period of several hours. This was accomplishedby using an Eppley solar tracker to point the photometer and pyrheliometer at the sun.
Of critical importance in designing a measurement system is a basic understanding of thecharacteristics of the quantities to be measured and a grasp of the equipment's technicalrequirements to perform the measurement. An instrument meeting the goals for thisproject was a simple radiometer with a narrow spectral bandpass and narrow field ofview. The radiometer needed to be sensitive to radiation in the visible spectrum (400 to700 nm) and have a narrow spectral width <50 nm. The signal to noise ratio designneeded to be high in the visible in order to insure reliable data. The instrument's physicalrequirements were only that it had to be mounted on the Eppley sun tracker and enclosedto protect the electronics from the heat of the sun and moisture.
The initial sun photometer instrument was based on the TERC (TERC 1998) VHS-1.This instrument was primarily provided as a project for educational purposes andhobbyists interested in the measurement of atmospheric aerosol optical properties. TheTERC VHS-1 photometer was built according to plans provided on the internet. Thedesign was very simple but limited. Its primary limitation was the enclosure's durability.This instrument was not intended to be operated outdoors continually and in directsunlight. Another instrument was designed that was durable and thermally stableallowing it to be mounted on the Eppley sun tracker.
The three basic blocks for the instrument used in this project are:x Detector (LED in reverse operation)x Current to voltage converter/amplifier (operational amplifier)x Voltage measurement
2.1 LED Detector
The detector was a commonly used LED (in reverse mode) available at retail RadioShack stores (catalog number 276-022A). The LED had a green appearance indicating anemission of green light. This diode was encapsulated in a 5 mm round type (T-1 3/4)
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epoxy package with a polished rounded emitting end. The emitting end of the diode wassanded off to make a flat surface therefore eliminating the rounded lens-like appearance.The manufacturer designed the polished rounded end to maximize the light output in alldirections. Leaving the rounded end would degrade the measurements by reflecting aportion of the incident solar radiation away from the diode. The sun photometer detectorwas designed to view only a small portion of the sky. The field of view or half angle (T½)was defined as 3º 4' 55.851" or 3.082181º. The half angle T½ is defined in the followingfigure.
T1/2 Outputaperture
LED
This half angle corresponds to a solid angle : = 9.089x10-3 sr. To put this intoperspective, the sun subtends a solid angle of about : = 7.0x10-5 sr. The sunphotometer's field of view is approximately 11 times that of the sun's angular size. Thissolid angle design insured that the entire LED was illuminated with about a 1 mm of spillover around the sensor's edge. The figure below shows the LED's position.
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2.1 Theory of LED Detection
By joining two types of semiconductors through a p-n junction, the LED emits light whena current is passed through them. These devices are capable of converting a flow ofelectrons into light and vice versa. The photovoltaic effect requires a potential barrierwith an electric field in order to operate (Dereniak and Crowe 1984). An incident photonof energy greater than or equal to the energy gap Eg can create a hole-electron pair. Theelectric field of the junction will not allow the hole-electron pair to recombine. Thephotoelectrons are therefore available to produce a current through an external circuit.
Eg
N Region
P Regionphotoelectron
hole
electron
Ec
Ev
(�ILHOG
Io
In this figure Io is the potential barrier energy, Ec and Ev are the conduction and valenceband energies defining the band gap energy Eg in electron volts (eV). The basicrequirement for photovoltaic operation is that the incident photons have enough energy toexcite photoelectrons across the energy gap. It is also necessary to have the detectortemperature low enough so that the electrons are not thermally excited across the gap.This energy requirement can also be expressed as
gEhc
h t O
Q
where h is Planck's constant, c is the speed of light, O is the incident radiation wavelengthand Q is the incident radiation's frequency. For a wavelength of 525 nm, thecorresponding junction energy gap is 2.36 eV. This wavelength defines the long-wavelength cut-off for the material. For wavelengths shorter than O, the incidentradiation is absorbed by the semiconductor and hole-electron pairs are generated (Sze1981). The thermal constraint is such that the thermal energy is less than the junctionenergy or
gEq
kT��
where q is the charge of a single electron in Coulombs (1.6x10-19 C) and T is the absolutetemperature in Kelvins. When this relation is valid, the material remains an insulator.The relatively narrow spectral bandwidth is the result of the allowed transitions betweenthe valence and conduction bands in the material.
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This figure shows the allowed transitions between the conduction and valence bands inGaAs1-xPx where the x=0.4 refers to red, x=0.65 is orange, x=0.85 is yellow and x=1.0 isgreen light (Sze, 1981). A similar diagram for the semiconductor GaP can be shown.
Green LEDs are manufactured from gallium phosphide (GaP) using either vapor phase orliquid phase epitaxy. Most green LEDs currently manufactured have dominant emissionwavelengths between 565 to 570 nm. To the eye, this spectral region appears green toyellowish-green. Special LEDs can be constructed using a nitrogen free process thatemits at a dominant wavelength of 555 nm. In a green emitting GaP LED the mainemission peak broadens on the long wavelength side with increasing nitrogenconcentration without a corresponding shift in the peak wavelength. For the LED used inthis project, the peak emission wavelength is at 555 nm with a spectral line half width of30 nm. Its detection wavelength is 525 nm with the same spectral line half width. LEDoperating lifetimes are long, typically on the order of 105 hours. The lifetime is definedas the time elapsed to reach 50% decrease of the external efficiency. LED detection canbe operated in either photovoltaic or photoconductive modes. The photovoltaic modeallows the highest signal to noise ratio since it eliminates the noise from the load resistor.On the other hand, a photoconductive design gives greater sensitivity than an LED in aphotovoltaic mode. In this project the LED was operated in the photovoltaic mode.
2.2 Sun Photometer Amplification
The current to voltage conversion and amplification was accomplished using a singleoperational amplifier (op-amp). The particular op-amp was the LM741, an 8 pin dip
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package. The operational amplfier and LED were connected using an experimenter's"breadboard" which allowed flexibility in the installation. The supply voltage on the 741op-amp required r18 volts which was provided by two 9 volt batteries.
2.3 Eppley Pyrheliometer
The Eppley normal incidence pyrheliometer (NIP) is an instrument for measuring theintensity of direct solar radiation at normal incidence (Eppley 1998). The NIPincorporates a wire-wound thermopile at the base of a tube, the aperture of which bears aratio to its length of 1 to 10. This ratio corresponds to an angle of 5q 43' 30". The insideof this brass tube is blackened and suitably diaphramed. The tube is filled with dry air atatmospheric pressure and sealed at the viewing end by an insert carrying a 1 mm thickInfrasil II window. Two flanges, one at each end of the tube are provided with a sightingarrangement for aiming the pyrheliometer directly at the sun. Sensitivity of the NIP isapproximately 8PV/(W m-2) and was provided by the manufacturer. The calibration, lastcompleted in 1974, was given as 6.72x10-6 volts/(W m-2). I believe that the calibrationmay have changed from 1974 to 1998 resulting in some unknown calibration value. Thishowever does not affect the results of the study since the sun photometer was the primarysubject.
2.4 Data Acquisition
Three sources of data acquisition were used in this project: 1) manual pen and paper, 2)strip chart recorder and 3) LabVIEW¥ and A/D electronic interface to computer. Withthe exception of the LabVIEW¥ system, all other activites took place on the roof of theDesert Research Institute SAGE building, approximately 30 feet above ground level.
The manual method involved pointing the photometer at the sun, reading a voltage off ofthe photometer and noting the time of the reading. This method did not allow the hightemporal resolution measurements of AOT over short periods of time. Measurementerrors are introduced easily in this method if the voltage varies while the reading takesplace.
A standard chart recorder was used operating with paper rates 5 to 10 cm per hour. Thechart recorder was the primary data collection method in saving the Eppley NIP output.The chart recorder allowed both an easy visual check of the alignment of the instrumentsand to see how the radiation was changing with respect to earlier times in the day.
LabVIEW¥ is a graphical programming development environment based on the Gprogramming language for data acquisition and control, data analysis, and datapresentation (National Instruments 1998). In LabVIEW¥, virtual instruments, or Vis,replace the writing programs to gather data from an A/D card in the computer.LabVIEW¥ front pane user interfaces were created, giving interactive control of thesoftware system. To specify the functionality, block diagrams were assembled to createthe VI. A 50 ohm cable of approximately 100 feet was tethered from the photometer tothe computer located on the second floor of the SAGE building.
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3. SOURCES OF NOISE
In the analysis of the output from the photometer during operation, it was apparent thatthere were fluctuations in the voltage. These fluctuations originated from severalelectrical sources. It was assumed that the solar output collected on the ground during aclear day would not vary appreciably (less than one percent) over several seconds.
Sources of electronic noise are 1) capacitive coupling of the wires within the sunphotometer, 2) inductive coupling that depends on and is proportional to current, 3)grounding problems, 4) temperature dependence on LED performance and 5) thermalnoises from resistor (shot noise, contact noise, popcorn noise). Shaw (1983) notesimportantly that solid state optical detectors are temperature sensitive. These solid statedevices, such as LEDs, are known to have different wavelength responsivities at varioustemperatures. A remedy for this problem would be to either measure the temperature ofthe detector or construct a type of climate control within the instrument. The temperaturefluctuations of the instrument were kept to a minimum by enclosing it in a white plasticbox. The first construction of this instrument included the enclosure of the electronicsinside a light gray metal box, but the temperature of the box's surface became noticeablyhotter when compared to the white plastic. The fact of the temperature variation of theelectronics complicates the discussion of quantitative errors in the measurement, sincetemperature was not measured inside the instrument enclosure. Uncorrelated noisesources are added together, on a power basis, and is the sum of the squares of each noisesource voltage.
223
22
21
2ntotal vvvvv ���� �
As an indication of the noise level while recording solar radiation during the afternoon,30 seconds of data was collected at a rate of approximately 20 Hz. The plot of the data isshown below.
April 18, 1998 (980428_4.txt) all 30 seconds of data from Sun Photometer II
4.80
4.81
4.82
4.83
4.84
4.85
4.86
4.87
4.88
4.89
4.90
1 28 55 82 109
136
163
190
217
244
271
298
325
352
379
406
433
460
487
514
data point
ampl
ifier
out
put v
olta
ge
10
Over this 30 seconds collection of data, the mean voltage was 4.849 while the standarddeviation was 0.008. One of the ways to visualize the noise level is to calculate thefluctuations about the mean as in the expression:
'vvv �
This analysis will illustrate the distribution of errors about the mean, shedding some lighton the statistics of the noise. A histogram of the fluctuations over the 30 second intervalis shown below.
0
20
40
60
80
100
120
-0.05 -0.04 -0.03 -0.02 -0.01 0.00 0.01 0.02 0.03 0.04
fluctuation voltage about mean
freq
uenc
y
This plot clearly shows a distribution centered around the 0.0 fluctuation or meanvoltage. The shape of the distribution is also close to that of a Gaussian. The affects ofaveraging the signal over longer periods of time would smooth the high frequencyfluctuations. High time resolution measurements of the AOT would not give any newinformation about the atmospheric aerosols since the high frequency variations are due toelectronic instrumentation noise sources.
4. CALIBRATION
One of the largest sources of systematic error is from the calibration known as theLangley method (Shaw 1983). The sun photometer in this report was not calibratedagainst an absolute calibration source such as a laboratory blackbody. Instead, thephotometer was calibrated by the use of a Langley plot in the measurement of the direct,clear sky sun. A day in which there were no clouds or visible haze was chosen as acalibration day. Voltages were collected throughout the day in order to ascertain severalvalues of the air mass. The voltage data from the Langley plot was used to find theextraterrestrial constant, the signal the sun photometer would give at the top of theatmosphere. The Langley plot was constructed by plotting the air mass as a function ofthe log of the LED signal. The LED signal was corrected for the dark voltage when theLED was covered. A linear regression of the data in this graph reveals the ET orextraterrestial constant for the instrument. For the TERC VHS-1 instrument, the ET was1.8474 while for the Sun Photometer II it was 1.9803. The correlation coefficients, r2, for
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both values of ET were more than 0.999 in both cases. Correlation coefficients r2 > 0.999are required to insure that the signal is primarily from molecular scattering rather thanclouds or large amounts of aerosols. Performing Langley plots for the other days wherethere were clouds and haze showed a nonlinear plot of air mass verses signal.
5. MEASUREMENTS
The TERC VHS-1 and sun photometer II were tested in the field during several weeks inApril 1998. Early TERC VHS-1 measurements were done manually by hand-holding thesensor and aligning it to the sun. Output from the photometer was measured using adigital voltmeter and manually written on paper. Later measurements utilized an Eppleysun tracker and a digital data acquisition system. An Eppley precision normal incidencepyrheliometer (NIP) was operated along with the narrow band sun photometer. TheEppley NIP has a narrow field of view (5 degrees) and measures the direct solarirradiance. The voltages from the Eppley pyrheliometer were recorded on a strip chartrecorder. Both sun photometer and pyrheliometer were pointed at the sun with theEppley sun tracker.
Measurements started on March 18, 1998 with the TERC VHS-1 sun photometer. Datawas collected with this instrument until April 25, 1998. After noticing the limitations ofthis instrument, another instrument was created that eliminated most of the pitfalls of theTERC VHS-1 instrument. This instrument was named Sun Photometer II and containedthe same electronics and optics as the TERC VHS-1 and was attached to the sun trackerby the 20th of April, 1998. The first day of operation of the Sun Photometer II was onApril 20 and was successful.
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Overall, solar irradiance data was taken over 13 days and with three instruments. Thetable below shows the days that AOT was monitored and by which instrument.
INSTRUMENT
day TERC VHS-1 Sun Photometer II EppleyNIPMarch 18, 1998 xApril 7, 1998 xApril 15, 1998 xApril 16, 1998 xApril 17, 1998 xApril 18, 1998 x xApril 20, 1998 x x xApril 21, 1998 x x xApril 23, 1998 x x xApril 25, 1998 x x xApril 26, 1998 x xApril 27, 1998 x xApril 28, 1998 x x
These days represent a window of clear sky over the Reno basin. Many days prior to themeasurements were clouded-over due to the effects of El Nino. AOT collected by theSun Photometer II over the period April 17 through April 28 is shown below.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
4/17/98 0:00 4/19/98 0:00 4/21/98 0:00 4/23/98 0:00 4/25/98 0:00 4/27/98 0:00 4/29/98 0:00
date & time (PDT)
AO
T (
525
nm)
The AOT measured during the time period before April 25 shows considerably lessaerosol extinction than the remaining days. This time period was unique and showed aninfluence by agricultural burns in eastern Idaho and portions of Oregon and Washington(Reno Gazette-Journal, 1998). Compared to the previous week, the AOT was as low as0.02 on the 21st of April. An exception was on April 18th where the AOT was high notdue to agricultural burning haze but from high clouds over the area. It was alsonoteworthy that confirmed reports of large amounts of Asian dust were observed to be
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affecting the Pacific Northwest during this period. It was hypothesized that the affect ofthe agricultural burning had as much of an impact on the AOT as the dust transportedover the Pacific Ocean. The AOT during April 25 to April 28 is shown below.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
4/25/98 12:00 4/26/98 0:00 4/26/98 12:00 4/27/98 0:00 4/27/98 12:00 4/28/98 0:00 4/28/98 12:00 4/29/98 0:00
date
AO
T (
52
5 n
m)
The plot above shows a decrease in AOT during mid-day and an increase during theafternoon. The diurnal changes varied as much as a factor of two from these three daysof measurements. One possible explanation is due to the actual aerosol diurnal variation.This may be explained from the basis of the diurnal characteristics of mixing in the loweratmosphere and the build up of aerosols when the mixing layer is lower during themorning and evening hours. During the day the surface heats up, increasing the depth ofthe mixed layer, while the effects of convection and turbulence mixes the aerosol layer.
6. DISCUSSION
It is a known fact that the behavior of aerosol scattering is dependent on the wavelengthof the radiation, particle size distribution, particle concentrations, particle morphologyand the refractive index. So far, a few researchers have modeled all of these componentsfor atmospheric particles with simple geometry. Publication of experimentalinvestigations using monochromatic radiation are also few in number but are steadilyincreasing due to the interest in LIDAR system performance and analysis. Analyticmodels for atmospheric particle optics and evolution have only emerged recently sincethe use of fast inexpensive computers. The main complication is that there is no closedform for the analytic solution of multiple scattering and numerical techniques are the onlyway to solve them. Even if the equations were likely to be solved, an added complicationin the inversion calculations is that it involves calculation along a path with unknownconcentrations and size distributions. Taking this into consideration, the analysis of thisdata set used a simple turbidity expression to estimate the AOT.
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Solar radiation, having wavelengths in the visible, is attenuated primarily by aerosols andclouds. Very little molecular absorption takes place in the spectrum 400 to 700 nm.Radiation is scattered away from the sun to observer's line of sight through Rayleighscattering by molecules and this effect decreases as the wavelength increases. Rayleighscattering plays an important role in the extinction of light at the wavelength of the sunphotometer and cannot be ignored. The figure below shows the affect aerosols andmolecules have on the transmitted solar radiation reaching the ground during typicalconditions in the midlatitudes.
0.0E+00
5.0E-05
1.0E-04
1.5E-04
2.0E-04
2.5E-04
400 600 800 1000 1200 1400 1600 1800 2000
wavelength (nm)
Sur
face
Sol
ar Ir
radi
ance
(W
cm-2
nm
-1) attenuated solar
incident solar
The lighter colored curve corresponds to the solar radiation incident at the top of theatmosphere. This plot was created from MODTRAN2 (Berk et al. 1989; Anderson et al.1993) and is based on the 1976 U.S. Standard Atmosphere, normal aerosol loading andno clouds. The calculation used a rural aerosol with a surface visibility of 23 km and aspectral resolution of 2 cm-1. In this case the sun illuminated the earth at a 50 degreesolar zenith angle which corresponds to the conditions during the late morning (10:00 amPDT) and afternoon (4:00 pm PDT). This example was to illustrate the fact that afraction of the solar radiation reaching the earth is attenuated by the combination ofmolecular and aerosol extinction. The wavelength of 530 nm appears on the black curveat the peak emission. It is also informative to note that the radiation reaching the earth'ssurface at that wavelength is attenuated by the atmosphere almost entirely by atmosphericaerosols. Molecular absorption effects, such as from water vapor, start to be significantat wavelengths greater than 700 nm.
The AOT value in this study was calculated based on the simple approximation formonochromatic attenuation of light though a linear medium. Beer's or Bouguer Law wasapplied to the wavelength of 530 nm. This wavelength was the peak detection
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wavelength sensitivity of the LED used in this project. The AOT was calculated basedon the application of Bouguer's Law
mMR
eF
F )]()([
)(
)0( OWOW
O
O ��
f
where we can replace FO(0) with "signal" and FO(f) with the extraterrestrial constant, ET.Recall that FO(f) is the solar flux that would be observed at the top of the atmosphere.Then this equation becomes
mMR
eET
signal )]()([ OWOW ��
Now taking the natural logarithm of this equation gives:
mET
signal MR )]()([)ln( OWOW ��
Rearranging the terms gives:
AOTmmETsignal R �� � )()ln()ln( OW
Now solving for AOT gives the following expression:
m
msignalETAOT
R )()ln()ln( OW��
Since the measured signal is the sum of the dark voltage and the actual signal from thesun, we can replace the "signal" with "sun voltage - dark voltage" in this equation. Alsothe Rayleigh optical thickness, WR(O) is evaluated to be 0.10599 (sea level) at awavelength of 530 nm. To adjust this value for any pressure, p in millibars, theconversion factor (p/po) was applied to the Rayleigh optical thickness. The value for po
was taken as 1013.25 mb which is the standard atmosphere sea level pressure. The basicequation for the AOT was then:
m
mppvoltagedarkvoltagesunETAOT
)/(10599.0)ln()ln( 0���
where ET was the extraterrestrial constant for the sun photometer and m was the air mass.This was the practical equation that was used in the calculation of AOT in the Excelspreadsheet formula. The air mass calculation was defined as
Isin
1 m
16
where the angle I was the solar elevation angle during the measurement. The value of mvaries from a maximum of 1 during solar noon to zero at sunset and sunrise. The AOT asa normalized quantity with respect to air mass was used to compare with othermeasurements. The equation for AOT only applies to a wavelength of 530 nm since itincludes the value of Rayleigh scattering explicitly. The air mass calculation was fromthe publication by J.K. Page (1986). Values based on these equations were compared tothe values that can be extracted from the U.S. Naval Observatory's web page athttp://aa.usno.navy.mil/AA/data/docs/AltAz.html. The calculations did show somedifferences between the two calculations. It is an unresolved matter as to which set ofcalculations can be trusted.
7. CONCLUSION
It has been shown that a simple instrument can measure aerosol optical thickness. It wasfortuitous to have several documented sources of aerosols to attenuate the solar radiationduring this experiment. The agricultural burning lasted long enough to observe the affectover the week of April 25 through April 28. The AOT varied between 0.14 to 0.50during the burning episode. Normal AOT for this area varied from around 0.10 to 0.20based on the few days of data taken before the external aerosol sources were present.More data is necessary to characterize the total aerosol AOT climatology for this area.The sun photometer built for this project proved sensitive enough to resolve extremelythin to sub-visual clouds of dust and aerosols. During the measurements, a dust devil thatlofted dust up to a hundred meters was detected by the sun photometer while the plumedrifted by.
The high AOT was expected to be the sum of the Asian dust and the transport ofagricultural wood burning from the Pacific Northwest. It is not known how much effectthe Asian dust had on the attenuation of sunlight over the measurement area. Satellitemeasurements confirmed the presence of dust that was transported from China. Severalair quality groups and satellite researchers have documented the presence of dusttransported through the Pacific Northwest, West Coast and points inland. The highaerosol amounts appeared to be only affecting the air aloft over the Reno area since thesurface air quality measurements did not rise appreciably over the period (Jennison1998). This is probably due to the fact that the suspended combustion particles are smalland on the order of the wavelength of light. This was confirmed with the observation ofthe sun aureole during the period of highest noticeable haze. The sun appeared to bewhite for most of the day. The sunsets during this time did not show a red glow butrather a yellow to orange hue. A photo is included in this report to show the affect of thehaze on the setting sun. The trajectory model HYSPLIT_4 (Draxler and Hess 1998)provided particle back-trajectories starting at 00z, April 25 and going back 96 hours. TheHYSPLIT_4 trajectory was provided by the NOAA Air Resources Laboratory in SilverSpring, Maryland. HYSPLIT_4 is a single particle Lagrangian integrated trajectorymodel that uses gridded meteorological data to calculate the positions of a pollutant intime. The model calculations were based on NCEP model output called the "FNLdatabase"(Stunder 1997). The FNL database was used because of its coverage of the
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northern hemisphere and was readily available from NOAA. The figure below showsone such model calculation at a height of 5500 meters.
This trajectory calculation shows that a particle had traveled from the Aleutian Islands 96hours previous to April 25, 1998. Other investigators have performed trajectorycalculations farther than 96 hours and show it originated in the Peoples Republic ofChina. Another trajectory calculation at 1.5 km for the same conditions showed quitedifferent results. The 1.5 km level corresponds roughly to the 700 mb pressure level andwould be more useful in identifying a source of pollution to the eastern Sierra. The 1.5km trajectory back to 96 hours brings a particle from the northern California coast. Thisfigure shows the 1.5 km back-trajectory. The results from these trajectory calculationsshow that sources impacting the measurement site, at the surface, was probably from thewest coast or Pacific Northwest. The aloft trajectory calculations, at 5500m, do showthat it was possible to be impacted from Asian dust farther in time than 96 hours. Areport from EarthAlert (EarthAlert 1998) indicated that on "April 21, 1998 -- A howling
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dust storm whipped up by gale-force winds lashed China's northwestern region ofXinjiang over the weekend, killing at least 12 people. The storm knocked out power andwater supplies as it swept through 10 cities and districts. In Tacheng city, flying rocksshattered windows as the high winds downed trees. The China Daily reported that acoating of yellow dust from nearby deserts was left in most areas in the wake of thestorm."
Husar (1998) has investigated the origin of the Asian dust from AVHRR satellitemeasurements and has tracked the dust plume from China to the U.S. The figure belowshows the results of his work.
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An extension of this work would include the capability to monitor several wavelengthsthrough the addition of several more LEDs to the photometer. Five other LEDwavelengths ranging from the blue (430 nm), yellow (585 nm), orange (620 nm), red(660 nm) and the near infrared (1100 nm) were investigated. Although simple toconstruct, the sun photometers using these additional wavelengths were not built. TheseLEDs are very inexpensive, costing no more than $2.00 and available at Radio Shackstores. Other LEDs can be used that have narrower spectral bandwidths and in otherspectral regions such as the near infrared (900 to 1200 nm) and into the blue visible (400to 430 nm). Detectors are not limited to the LED type and can include avalanchphotodiodes (APDs), Silicon photodiodes, InSb, PbS, ge, CdSe, and CdS. Many of thesedevices may require modulation and cooling to 77K in order to achieve sensitivity todetect solar radiation. The cost varies in these types of sensor materials depending on thesensitivity, size and electronic packaging.
To further improve the data quality, a calibration with an absolute radiometric standardwould be ideal. However, a calibration with a much simpler source of photons could bearranged by the use of a standard high output incandescent light bulb. The light bulbwould have to be at least 100 W in order to achieve enough photon flux for the LED.
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