******************************************************************************** * * * Readme file for the AERONET Data Sets * * * ******************************************************************************** B.N. Holben, Code 923 T.F. Eck, Code 923 Y.J. Kaufman, Code 913 I. Slutsker, Code 923 A. Smirnov, Code 923 E. Vermote, Code 923 NASA/GSF, Greenbelt, MD 20771 D. Tanre I. Jankowiak Lab d'Optique Atmospherique, U.S.T. de Lille, 59655-Villeneuve d'Ascq, France J.P. Buis CIMEL Electronique, 5 cite de Phalsbourg, Paris, France A. Setzer Instituto de Pesquisas Espaciais, Sao Jose dos Campos, SP, Brazil J.A. REAGAN University of Arizona, Tucson, AZ 85721 T. NAKAJIMA University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153, Japan F. LAVENU Laboratoire D'Ecologie, Ecole Normale Superieure, Paris, France Revision Date: August 1998 TABLE OF CONTENTS 1.0 Introduction to AERONET Data Set 1.1 Background of the AERONET Project 2.0 Automatic Sun and Sky Scanning Spectral Radiometer 2.1 General Description of Instrumentation 2.2 Measurement Concept for AERONET Data Set 2.3 Instrument Precision 2.4 Instrument Calibration 2.5 Data Accuracy 2.6 Data Transmission 2.7 Processing System 2.8 Instrument and Network Status 2.9 Data Processing 3.0 References 1.0 Introduction to AERONET Data Set AERONET (AErosol RObotic NETwork) is an optical ground based aerosol monitoring network and data archive supported by NASA's Earth Observing System and expanded by federation with many non-NASA institutions. The network hardware consists of identical automatic sun-sky scanning spectral radiometers owned by national agencies and universities. Data from this collaboration provides globally distributed near real time observations of aerosol spectral optical depths, aerosol size distributions, and precipitable water in diverse aerosol regimes. The data undergo preliminary processing (real time data), reprocessing (final calibration ~6 mo. after data collection), quality assurance, archiving and distribution from NASA's Goddard Space Flight Center master archive and several identical data bases maintained globally. The data provide algorithm validation of satellite aerosol retrievals and as well as characterization of aerosol properties that are unavailable from satellite sensors. The concept and description of a remote sensing aerosol monitoring network initiated by NASA, developed to support NASA, CNES and NASDA's earth satellite systems under the name AERONET and expanded by national and international collaboration is described. Recent development of weather resistant automatic sun and sky scanning spectral radiometers enable frequent measurements of atmospheric aerosol optical properties and precipitable water at remote sites. Transmission of automatic measurements via the geostationary satellites GOES and METEOSATS' Data Collection Systems allows reception and processing in near real-time from approximately 75% of the earth's surface and with the expected addition of GMS, the coverage will increase to 90% in 1998. NASA developed a UNIX based near real-time processing, display, and analysis system providing internet access to the emerging global data base. Information on the system is available on the project homepage at http://spamer.gsfc.nasa.gov. The philosophy of an open access data base, centralized processing and a user friendly graphical interface has contributed to the growth of international cooperation for ground-based aerosol monitoring and imposes a standardization for these measurements. The system's automatic data acquisition, transmission, and processing facilitates aerosol characterization on local, regional, and global scales with applications to transport and radiation budget studies, radiative transfer modeling, and validation of satellite aerosol retrievals. Accurate knowledge of the spatial and temporal extent of aerosol concentrations and properties has been a limitation for assessing their influence on satellite remotely sensed data (Holben et al., 1992) and climate forcing (Hansen and Lacis, 1990). With the exception of the AVHRR weekly ocean aerosol retrieval product (Rao, et al., 1989), the voluminous 20 year record of satellite data has produced only regional snapshots of aerosol loading and none have yielded a data base of the optical properties of those aerosols which are fundamental to our understanding of their influence on climate change. With the advent of the EOS era of laboratory quality orbiting spectral radiometers, new algorithms for global scale aerosol retrievals and their application for correction of remotely sensed data will be implemented (Kaufman and Tanré, 1996). However the prospect of fully understanding aerosols influence on climate forcing is small without validation and augmentation by ancillary ground-based observations as can be provided by radiometers historically known as sun photometers. Following is a description of a new sun-sky scanning radiometer system that standardizes ground-based aerosol measurements and processing, can provide much of the ground-based validation data required for future remote sensing programs and may provide basic information necessary for improved assessment of aerosols impact on climate forcing. 1.1 Background on the AERONET Project The technology of ground-based atmospheric aerosol measurements using sun photometry has changed substantially since Volz (1959) introduced the first handheld analog instrument almost four decades ago. Modern digital units of laboratory quality and field hardiness can collect data more accurately and quickly and are often interfaced with onboard processing (Schmid et al., 1997 , Ehsani, 1998, Forgan, 1994, and Morys et al., 1998). The method used remains the same, that is a filtered detector measures the spectral extinction of direct beam radiation according to the Beer-Lambert-Bouguer law: V[lambda] = Vo[lambda] d2exp-([tau][lambda]m)*ty (1) where: V=Digital voltage Vo=extratrestrial voltage m=optical airmass [tau]=total optical depth [lambda]=wavelength d=ratio of the average to the actual earth-sun distance ty=transmission of absorbing gases The digital voltage (V) measured at wavelength ([lambda]) is a function of the extraterrestrial voltage (Vo) as modified by the relative earth sun distance (d) and the exponent of the total spectral optical depth ([tau][lambda]) and optical airmass (m). The total spectral optical depth is the sum of the Rayleigh and aerosol optical depth after correction for gaseous absorption. The multi-filter rotating shadowband radiometer (MFRSR) employs a different strategy. It measures spectral total and diffuse radiation to obtain the direct component from which aerosol optical thickness is computed using the Beer-Lambert-Bouguer law. The instrument nominally measures at one minute intervals and has been shown to be reliable over long periods of time. The measurements are networked to a common server by a modem interface and the data processed by a common analysis system (Harrison et al., 1994). It is widely used in the United States principally for the DOE ARM sites. As the number of measurements from the MFRSR network increases, the impact of aerosol loading on the radiation balance should be more clearly understood especially when taken in concert with other ground, airborne and satellite measurements. Sky scanning spectral radiometers, that is radiometers that measure the spectral sky radiance at known angular distances from the sun, have expanded the aerosol knowledge base most importantly through inversion of the sky radiances to derive aerosol microphysical properties such as size distribution and optical properties such as phase function (Nakajima et al. (1983, 1996), Tanré et al. (1988), Shiobara et al. (1991), and Kaufman et al., (1994)). This technique requires precise aureole measurements near the solar disk and good straylight rejection. Historically these systems are rather cumbersome, not weather hardy and expensive. The CIMEL and PREDE (French and Japanese manufacturers respectively) sun and sky scanning spectral radiometers overcome most such limitations, and provides retrievals from direct sun measurements of aerosol and water vapor abundance in addition to aerosol properties from inversion of spectral sky radiances. Since the measurements are directional and represent conditions of the total column atmosphere, there are direct applications to satellite and airborne observations as well as atmospheric processes. As has been demonstrated by the shadowband network and satellite remote sensing in general, prompt delivery of the data for analysis is fundamental for obtaining a comprehensive, continuous data base and allows assessment of the collecting instruments health and calibration. To achieve this goal, minimize costs and expand the coverage globally, we use the simple and inexpensive Data Collection System (DCS) operating on the geosynchronous GOES, METEOSAT and GMS satellites providing nearly global coverage in near real-time at very little expense (NOAA/NESDIS, 1990). Finally there are the very contentious issues of processing the data archive. Although the Beer-Lambert-Bouguer law is very straightforward, its implementation has as many variations as there are investigators who use it. The central problem being agreement on the accuracy which the aerosol optical thickness is derived. The uncertainties in computation of the airmass (m), the calculations for the Rayleigh and ozone optical depths ([tau]r, [tau]o) and water vapor expressed as total column abundance or precipitable water (Pw) as well as strategies for calibration of the instruments and monitoring the long term change in calibration all combine to preclude any globally accepted processing scheme. Perhaps even more debatable are the aerosol properties derived from inversions of the sky radiances with the radiation transfer equation. Our solutions make the raw data and calibration data available to the user and provide a basic processing package (of published, widely accepted algorithms) with sufficient friendliness and flexibility that all data may be accessed globally through common forms of electronic communication on the internet. Following is the Aerosol Robotic Network (AERONET) version of a ground-based aerosol monitoring system that offers a standardization for a ground-based regional to global scale aerosol monitoring and characterization network. We have assembled a reliable system and offer it as a point of focus for further development of each component. As an example of the system's performance under a variety of conditions, we present data collected in the Brazilian Amazon during the dry season and Mauna Loa, Hawaii. Owing to the fundamental importance of these and similar data for basic aerosol research, aerosol forcing research and validation of retrievals from space based platforms, we are emphasizing this system for a regional to global scale network of these observations. Our philosophy is for an open, honor system whereby all contributed data may be accessed by anyone but publication of results requires permission of the contributing investigators. We have designed and implemented a system that promotes these goals. 2.0 Automatic Sun and Sky Scanning Spectral Radiometer Most if not all sun photometer networks have had limited success when people are required to make routine observations. Therefore an automatic instrument is a fundamental component for routine network observations. The measurement protocol must be reasonably robust such that unwanted data may be successfully screened from useful data, data quality and instrument functionality may be evaluated and the instrument should be self-calibrating or at the least collects data for its calibration. Following is our assessment of the CIMEL CE-318 instrument that meets these criteria of a field hardy, transmitting, sun and sky scanning spectral radiometer which is used in the AERONET program. 2.1 General Description of Instrumentation The CIMEL Electronique 318A spectral radiometer manufactured in Paris, France is a solar powered weather hardy robotically pointed sun and sky spectral radiometer. This instrument has approximately a 1.2 degree full angle field of view and two detectors for measurement of direct sun, aureole and sky radiance. The 33 cm collimators were designed for 10-5 straylight rejection for measurements of the aureole 3 degrees from the sun. The robot mounted sensor head is parked pointed nadir when idle to prevent contamination of the optical windows from rain and foreign particles. The sun/aureole collimator is protected by a quartz window allowing observation with a UV enhanced silicon detector with sufficient signal-to-noise for spectral observations between 300 and 1020 nm. The sky collimator has the same field of view but an order of magnitude larger aperture-lens system allows better dynamic range for the sky radiances. The components of the sensor head are sealed from moisture and desiccated to prevent damage to the electrical components and interference filters. Eight ion assisted deposition interference filters are located in a filter wheel which is rotated by a direct drive stepping motor. A thermister measures the temperature of the detector allowing compensation for any temperature dependence in the silicon detector. A polarization model of the CE-318 is also used in AERONET. This version executes the same measurement protocol as the standard model but takes additional polarized solar principal plane sky radiance measurements hourly at 870 nm (Table 1 and 2). The sensor head is pointed by stepping azimuth and zenith motors with a precision of 0.05 degrees. A microprocessor computes the position of the sun based on time, latitude and longitude which directs the sensor head to within approximately one degree of the sun, after which a four quadrant detector tracks the sun precisely prior to a programmed measurement sequence. After the routine measurement is completed the instrument returns to the "park" position awaiting the next measurement sequence. A "wet sensor" exposed to precipitation will cancel any measurement sequence in progress. Data are downloaded under program control to a Data Collection Platform (DCP) typically used in the geostationary satellite telemetry system, (see Section 2.6 Data Transmission). 2.2 Measurement Concept for AERONET Data Since the instrument was first available in 1992, the measurements protocols have evolved to a point in which we feel maximum information content is achieved within the constraints of the hardware and software available for the network system and the goals of the aerosol climatology data base. The radiometer makes only two basic measurements, either direct sun or sky, both within several programmed sequences. The direct sun measurements are made in eight spectral bands (anywhere between 340 and 1020 nm; 440, 670, 870, 940 and 1020 nm are standard) requiring approximately 10 seconds. A sequence of three such measurements are taken 30 seconds apart creating a triplet observation per wavelength. Triplet observations are made during morning and afternoon Langley calibration sequences and at standard 15-minute intervals in between (Table 1). The time variation of clouds are typically greater than that of aerosols causing an observable variation in the triplets that can be used to screen clouds in many cases. Additionally the 15-minute interval allows a longer temporal frequency check for cloud contamination. Table 1. Measurement sequences of the CIMEL Sun/Sky scanning spectral radiometer. -------------------------------------------------------------------------------- |Spectral Range| Target| No. Obs. | Obs. |Application | nm | | | Interval | -------------------------------------------------------------------------------- BASIC | 340 to 1020 | Sun | 1 each | ~ 8 sec. for .8 | AOT, Pw, DIRECT | | | [lambda] | [lambda] | [alpha] SUN | | | | | -------------------------------------------------------------------------------- Triplet | 340 to 1020 | Sun | Three | 3 @ 30 sec.apart, |AOT,Pw,[alpha] Obs. | | | direct sun | 1 min. total | & | | | | |cl[omicron]ud | | | | | screening -------------------------------------------------------------------------------- Standard | 340 to 1020 | Sun | Variable: | Ea. 15 min. m=2AM | AOT, Pw, Measure- | | | depends on | to m=2PM | [alpha] ment Seq. | | | day length | | -------------------------------------------------------------------------------- Langley | 340 to 1020 | Sun | 16 AM & PM |m=7-5,incr. of .5m |Langley, Cal., | | | between m 7|m=5-2,incr. of .25m| AOT, Pw, | | | & 2 | | [alpha] -------------------------------------------------------------------------------- BASIC SKY | 440 to 1020 | Sky | 1 each | none | Sky Radiance | | | [lambda] | | -------------------------------------------------------------------------------- Langley | 440 to 1020 | Sky | 16 between | m=7-5, .5 |Stability of Sky | | | m 7 & 2 | m=5-2, .25 |Langley Plot -------------------------------------------------------------------------------- Almucantar| 440 to 1020 | Sky |72 (Table 2)| >8/day:m=4,3,2,1.7|Size Dist. and | | | | hrly. 9AM to 3PM |P([theta]), | | | | |AOT, [alpha] -------------------------------------------------------------------------------- Polariza- | 870 | Sky |42 (Table 2)| hourly m=3AM to |Size Dist. and tion | | | | m=3PM |P([theta]) -------------------------------------------------------------------------------- Principal | 440 to 1020 | Sky |42 (Table 2)| hourly m=3AM to |Size Dist. and Plane | | | | m=3PM |P([theta]), | | | | |AOT, [alpha] -------------------------------------------------------------------------------- Sky measurements are performed at 440, 670, 870 and 1020 nm (Table 1). A single spectral measurement sequence (Langley sky) is made immediately after the Langley airmass direct sun measurement, 20 degrees from the sun. This is used to assess the stability of the Langley plot analysis according to O'Neill et al., 1984. Two basic sky observation sequences are made, the "almucantar" and "principal plane". The philosophy is to acquire aureole and sky radiances observations through a large range of scattering angles from the sun through a constant aerosol profile to retrieve size distribution, phase function and aerosol optical thickness (AOT). An almucantar is a series of measurements taken at the elevation angle of the sun for specified azimuth angles relative to the position of the sun. The range of scattering angles decrease as the solar zenith angle decreases thus almucantar sequences made at an optical airmass of 2 or more achieve scattering angles of 120 degrees or larger. Scattering angles of 120 degrees are typical of many sunsynchronous viewing satellites thus a measure of the satellite path radiance is approximated from the ground station. During an almucantar measurement, observations from a single channel are made in a sweep at a constant elevation angle across the solar disk and continues through 360 degrees of azimuth in about 40 seconds (Table 2). This is repeated for each channel to complete an almucantar sequence. More than four almucantar sequences are made daily at an optical airmass of 4, 3, 2, and 1.7 both morning and afternoon and, an almucantar is made hourly between 9 AM and 3 PM local solar time for the standard instrument and skipping only the noon almucantar for the polarization instrument. A direct sun observation is made during each spectral almucantar sequence. Table 2. Almucantar and Principal Plane sequences for the standard and polarization instruments. -------------------------------------------------------------------------------- | Sun | Sky (°) -------------------------------------------------------------------------------- ALMUCANTAR | 0° |6.0,5.0,4.5,4.0,3.5,3.0,2.5,2.0,-2.0,-2.5,-3.0,-3.5,-4.0, Azimuth angle | |-4.5,-5.0,-6.0,-8.0,-10.0,-12.0,-14.0,-16.0,-18.0,-20.0, relative to sun | |-25.0,-30.0,-35.0,-40.0,-45.0,-50.0,-60.0,-70.0,-80.0, | |-90.0,-100.0,-110.0,-120.0,-130.0,-140.0,-160.0,-180.0 | |Duplicate above sequence for a complete counter clockwise | |rotation to -6 -------------------------------------------------------------------------------- PRINCIPAL PLANE:| 0° |-6.0,-5.0,-4.5,-4.0,-3.5,-3.0,-2.5,-2.0,2.0,2.5,3.0,3.5, Standard | |4.0,4.5,5.0,6.0,8.0,10.0,12.0,14.0,16.0,18.0,20.0,25.0, Scattering Angle| |30.0,35.0,40.0,45.0,50.0,60.0,70.0,80.0,90.0,100.0,110.0, from sun | |120.0,130.0,140.0 (negative is | | below the sun) | | -------------------------------------------------------------------------------- PRINCIPAL PLANE:| - |-85.0,-80.0,-75,-70,-65.0,-60.0,-55.0,-50.0,-45.0,-40.0, Polarization | |-35.0,-30.0,-25.0,-20.0,-15.0,-10.0,-5.0,5.0,10.0,15.0, Scattering Angle| |20.0,25.0,30.0,35.0,40.0,45.0,50.0,55.0,60.0,65.0,70.0, from sun | |75.0,80.0,85.0 (negative is in | | the anti solar | | direction) | | -------------------------------------------------------------------------------- The standard principle plane sequence measures in much the same manner as the almucantar but in the principal plane of the sun where all angular distances from the sun are scattering angles regardless of solar zenith angle. This measurement sequence begins with a sun observation, moves 6 degrees below the solar disk then sweeps through the sun taking about 30 seconds for each of the four spectral bands.(Table 2). Principal plane observations are made hourly when the optical airmass is less than 2 to minimize the variations in radiance due to the change in optical airmass. Polarization measurements of the sky at 870 nm are an option with this instrument. The sequence is made in the principal plane at 5 degree increments between zenith angles of -85 and +85 degrees. The configuration of the filter wheel requires that a near-IR polarization sheet is attached to the filter wheel. Three spectrally matched 870 nm filters are positioned in the filter wheel exactly 120 degrees apart. Each angular observation is a measurement of the three polarization filter positions. An observation takes approximately 5 seconds and the entire sequence about 3 minutes. This sequence occurs immediately after the standard principle plane measurement sequence. 2.3 Instrument Precision We define the precision of the instrument as its ability to accurately reproduce results from multiple measurements under constant conditions using standardized techniques. Three methods will be used to assess the radiometric precision: (1) The variability of the digital numbers (DN) from the spectral response acquired from the two meter diameter integrating sphere at Goddard Space Flight Center which is used to determine the gain and offset calibrations of the sky radiance channels, (2) examination of dark current values taken during each sky radiance measurement and (3) the triplet variability of the DN's taken from Mauna Loa Observatory Langley observations was used to evaluate the sun channels. All instruments are routinely calibrated with Goddard's two meter integrating sphere at least twice per year and the reference instruments approximately monthly. Each calibration session consists of three sequential measurements at four lamp levels (radiance levels). The sphere's precision is not well known however the absolute accuracy is ~5. % or less (Walker et al., 1991). Assuming the sphere has perfect precision we may use these data to estimate the precision of the sky channels. The percent deviation from the mean of each sequence was averaged from all the sequences since 1993 for each of the three reference instruments. In all but 1 case, the variability was much less than 1% of the mean value (Table 3A). Given these results, some of the variability in Table 3A could be attributed to the uncertainty in the precision of the integrating sphere and the potential for variability in the data collection procedure. Over 3000 dark current values were examined for each instrument and the average values computed by wavelength for the sun and both sky (aureole- 2 to 6 degrees = sky1 and dark sky 6 to 180 degrees=sky2) observations. The dark current values for the sun observations averaged less than 1 count compared to typical measurement values of 2000 to 15000 counts depending on wavelength, optical depth and airmass (Table 3B), thus for typical conditions the dark current is insignificant. The sky observations have a higher dark current value ranging from 2 to 14 counts with standard deviations of approximately the same magnitude. Typically this is about 1% of the signal and is subtracted prior radiance computation. Table 3. The DNs were used to compute (A) the % variation from the mean for the sky channels, (B) the mean Dark current values for all measurement conditions and (C) the % variation of the mean triplet values during selected Mauna Loa Langley Plots for three field and reference instruments. ------------------------------------------------------------------------------- (A) |Inst.|Inst.|Inst.| |Mean%| | | | | | | | | | Integrating|#2 |#13 |#32 | |Var. | | | | | | | | | | Sphere |Mean%| |Mean%| |Var. | | | | | | | | | | |Var. | | | | | | | | | | | | | | ------------------------------------------------------------------------------- [lambda] |1.02 | .87 |.67 |.44| n |1.02|.87|.67|.44|n|1.02|.87|.67|.44| n (µm) | | | | | | | | | | | | | | | ------------------------------------------------------------------------------- 12 Lamps | - | - | .1 |.3 | 3 | - | - |.1 |.1 |9| - | - |.5 |.3 |4,8 ------------------------------------------------------------------------------- 6 Lamps | 2.7 | .8 | .7 |.4 | 7 | - |.5 |.1 |.1 |9| .1 |.1 |.1 |.2 | 8 ------------------------------------------------------------------------------- 2 Lamps | .2 | .3 | .2 |.3 | 8 | .1 |.1 |.1 |.2 |9| .3 |.2 |.2 |.3 | 8 ------------------------------------------------------------------------------- 1 Lamp | .1 | .1 | .1 |.4 | 7 | .1 |.1 |.1 |.4 |9| .1 |.1 |.1 |.4 | 8 ------------------------------------------------------------------------------- ----------------------------------------------------------------------- (B) |Inst.|Inst.|Inst.| |Mean| | | | | | | Dark |#2 |#13 |#32 | | DN | | | | | | | Current |Mean | |Mean | | | | | | | | | | DN | | DN | | | | | | | | | ----------------------------------------------------------------------- |sun |sky1 |sky2 | n |sun |sky1|sky2| n |sun|sky1 |sky2| n ----------------------------------------------------------------------- 1020 nm |1.17 |11.98|7.16 |3201|1.29|6.01|4.00|3889|.43|14.04|8.00|2703 ----------------------------------------------------------------------- 940 nm |.64 | - | - |3201|.22 | - | - |3889|.05| - | - |2703 ----------------------------------------------------------------------- 870 nm |.73 |8.07 |4.36 |3201|.59 |3.62|2.87|3889|.21|9.17 |6.17|2703 ----------------------------------------------------------------------- 670 nm |.56 |4.52 |2.02 |3201|.15 |1.93|1.14|3889|.11|6.40 |4.15|2703 ----------------------------------------------------------------------- 440 nm |.60 |4.94 |2.10 |3201|.15 |2.02|1.16|3889|.10|5.57 |3.31|2703 ----------------------------------------------------------------------- 380 nm |.56 | - | - |3201|.01 | - | - |3889|.06| - | - |2703 ----------------------------------------------------------------------- 340 nm |.77 | - | - |3201|.23 | - | - |3889|.05| - | - |2703 ----------------------------------------------------------------------- Sky1=small aperature collimator for measurements from 2 to 6 degrees from sun Sky2=large aperature collimator for measurements from 6 to 180 degrees from sun ----------------------------------------------- ----------------------------------------------- (C) |Inst. |Inst.|Inst. | | | Mauna Loa| #2 | #13 | #32 | | | Langley | | | | | | Plots | | | | | | ----------------------------------------------- Sun |Mean | |Mean | |Mean | |Var. %| n |Var. %| n |Var. %| n ----------------------------------------------- 1020 nm | .2 | 288 | .3 | 168 | .1 | 264 ----------------------------------------------- 940 nm | .2 | 288 | .3 | 168 | .2 | 264 ----------------------------------------------- 870 nm | .3 | 288 | .4 | 168 | .2 | 264 ----------------------------------------------- 670 nm | .3 | 288 | .3 | 168 | .2 | 264 ----------------------------------------------- 440 nm | .3 | 288 | .3 | 168 | .2 | 264 ----------------------------------------------- 380 nm | .7 | 288 | .5 | 168 | .6 | 264 ----------------------------------------------- 340 nm | .9 | 288 | .7 | 168 | 1.0 | 264 ----------------------------------------------- Langley plots from NOAA's Mauna Loa Observatory have been made to determine the spectral extraterrestrial voltage (Vo[lambda]) for these instruments since 1993. The observatory's high altitude and isolation from most local and regional sources of aerosols provides a very stable aerosol and irradiance regime in the mornings (Shaw, 1983). The Langley Plot is a log of the DN taken during these times plotted against the optical airmass between a range of 5 and 2. The intercept is the calibration coefficient and the slope the optical thickness. If the aerosol loading is constant, these points plot as a straight line. The deviation of these points from the linear regression line is a measure of the precision of the instrument although it does include atmospheric variation which we are assuming is negligible at Mauna Loa during the selected Langleys. Table 3C shows the average variability of a triplet is less than 1% and is most typically 0.3% for all three instruments. This is in agreement with the precision estimated from the integrating sphere analysis. 2.4 Instrument Calibration Calibration refers to the determination of the calibration coefficients needed to convert the instrument output (DN) to a desired output, in this case aerosol optical thickness (AOT) and radiance (W/m2/sr/µm). The calibration accuracy is the level of accuracy with which a desired output is achieved using defined comparison procedures. Calibration is frequently traced back to the variability with which the calibration coefficients are determined to achieve that unit output. Thus instrument calibration is a combination of the instrument precision, the calibration procedure and the algorithms used. In this section, we will discuss the variability of the calibration coefficients we determine for the sky channels from the two meter integrating sphere, the spectral Vo from the Mauna Loa Langleys and the change in the calibration coefficients as a function of time. We will also discuss the intercomparison procedure for transferring the Vo calibration coefficients from a reference instrument to a field instrument and the computation of the resultant variability. The sphere calibration procedure given in the previous section allows us to compute a gain and offset for each sky wavelength. The mean dark current DN is typically between 0 and 14 counts (the median DN is zero to one for the sun channels) (Table 3b) which is subtracted from the DN thus giving an offset of zero. The Instrument DNs are plotted against the exitant radiance from the integrating sphere and a gain is computed from the linear regression fit through the origin. The mean gain is computed from three regression gains made for each session. The accuracy of the sphere is reported as ±5% (Walker et al., 1991) thus the calibration coefficient accuracy can be no better than 5% plus the variability of the three regressions (precision) or conservatively ±~ 5.5%. (Unpublished studies of the two meter integrating sphere in 1997 indicate the absolute accuracy is between 1 and 3% depending on wavelength.) The Vo calibration coefficients are typically computed from an average of five or more Langley plots obtained at the Mauna Loa Observatory. The variability of the retrieved mean Vo as measured by the coefficient of variation (CV, standard deviation/mean) indicates the combined uncertainty of the atmosphere, instrument and the repeatability of the calibration procedure. The averaged Mauna Loa Langleys Vo obtained during all calibration sessions have a CV of ~0.25 to 0.50% for the visible and near-IR wavelengths, ~0.5 to 2% for the UV to ~1 to 3% for the water vapor channel (Table 4 and continuing observations). The Mauna Loa (MLO) calibration is conducted with two simultaneously operating reference instruments. Comparisons are made between ratios of raw spectral voltages as a check for instrument repeatability. A diurnal variation of less than 1% of the ratioed voltages is considered acceptable. Approximately monthly the MLO master instruments are swapped with two reference instruments located at GSFC. The GSFC reference instruments are used for intercomparison with field instruments. Monitoring voltage ratios is continued for all master instruments and field instruments during the calibration procedure. Table 4. The mean CV in percent by wavelength (nm) of the Mauna Loa derived Langley Vo for all of the wavelengths used in the reference CIMEL sun photometers. ---------------------------------------------------------- Inst.No. | 1020 | 940 | 870 | 670 | 500 | 440 | 380 | 340 ---------------------------------------------------------- | CV % | CV %| CV %| CV %| CV %| CV %| CV %| CV % ---------------------------------------------------------- 2 | .19 |2.39 | .14 | .18 | .22 | .22 | .33 |2.10 ---------------------------------------------------------- 13 | .27 | .89 | .29 | .44 | .90 | .40 | .77 | .63 ---------------------------------------------------------- 32 | .26 |3.19 | .19 | .24 | .23 | .29 |1.10 | .48 ---------------------------------------------------------- 37 | .29 |2.23 | .21 | .32 | .28 | .28 | .32 | .43 ---------------------------------------------------------- 101 | .26 | .70 | .40 | .23 | .10 | .22 | .32 | .37 With respect to the long term stability of the calibration coefficients, the optical interference filters are the limiting factors. Periodic sphere gains and mountain top Langley calibration coefficients have been determined since 1993. The results are typical for interference filters. On average, there has been a decrease from 1 to 5% per year and, after 2 years, there has been a rapid decay in some filters (Table 5). However, starting in 1997 we installed ion assisted deposition (IAD) iinterference filters in all instruments with the expectation of improved filter stability with time, which in fact is noted in Table 5 for instrument #11. Since the % decrease in the time dependent calibration coefficients is usually greater than the uncertainty of a semiannual Vo determination we use a linear interpolation of the Vo between calibration dates. This requires that the instrument calibration coefficient be followed closely. Thus, until more information is available, we calibrate instruments on a 6 month rotation and change filters after two years of field use. Therefore the percentage changes which occur between Vo calibrations are actually a factor of 2 to 3 smaller than shown in Table 5 since these values are on a % change per year. Most instruments cannot be calibrated at Mauna Loa and a well calibrated integrating sphere with sufficient radiometric output is not common, therefore most instruments are calibrated at Goddard Space Flight Center with the two meter integrating sphere and intercomparisons against the Goddard reference instrument with a Mauna Loa derived Vo. Intercomparisons are made by solving Eq. 1 for the field instrument Vo based on the reference instrument [tau]a during simultaneous observations (time difference of less than 5 seconds), under clear stable atmospheric conditions ([tau]a440 less than 0.15). The CV of the Vo computed from these comparisons is typically larger than the reference instrument uncertainty. The total error is the uncertainty attributed to the field instrument calibration coefficient due to transfer of calibration from the reference instrument plus the error from the reference instrument defined from the Mauna Loa calibration. As with the reference instruments, calibration coefficients are then linearly interpolated between the calibration tie points unless independent information suggests a different method as in the case of a change in filters at which time new calibration comparisons must be made. The spectral voltage ratios of the field instrument are compared to the reference instruments during several days. Variations throughout a large range of optical airmass (typically 1.5 - 6) of less than ±1% are considered acceptable. Table 5. The decay rate of zero airmass voltages, Vo, (%/yr) is shown for filters less than two years old for each reference instrument. ----------------------------------------------------------------------- | 1020 | 940 | 870 | 670 | 500 | 440 | 380 | 340 ----------------------------------------------------------------------- #2 | -2 | -1 | 2 | 2 | 3 | -4 | 11 | 3 6-10/95 | | | | | | | | ---------------------------------------------------------------------- #13 | 5 | -31 | 2 | 0 | ND | 2 | 23 | 11 6-9/94 | | | | | | | | ---------------------------------------------------------------------- #13 | 10 | 5 | 10 | 11 | ND | 15 | 20 | 15 9/94-6/95| | | | | | | | ---------------------------------------------------------------------- #32 | 4 | 6 | 7 | 2 | 2 | 4 | 26 | 5 6-10/95 | | | | | | | | ---------------------------------------------------------------------- #11 | -4 | 8 | -1 | 0 | 0 | 0 | -3 | -2 6/97-1/98| | | | | | | | ---------------------------------------------------------------------- Measurements of the spectral temperature sensitivity of the instrument in a temperature controlled chamber showed agreement with the manufacturers published temperature sensitivity of the detectors. To date, only the 1020 nm channels showed significant temperature variation (0.25%/oC ±0.05%/oC ) warranting a correction to a reference temperature in the processing. However, for polarization instruments, measurements indicate that the plastic polarizing filter introduces a temperature sensitivity of ~0.20%/oC to the polarized 870 nm radiance measurements. 2.5 Data Accuracy Various instrumental, calibrational, atmospheric and methodological factors that influence the precision and accuracy of optical depth determination have been pointed out clearly in a series of publications (Shaw, 1976; Reagan et al., 1987 and Russel et al., 1993) and attempts to account for or minimize these are described in previous sections. Instrument uncertainty due to electro-optical precision is for all practical purposes insignificant (Table 3) for a properly operating instrument. The variability of the atmosphere is characterized by the variability of the triplet optical thicknesses which may at times be cloud contaminated. This uncertainty is computed, can be used as a screening tool, and may be retrieved from the AERONET data base. Additionally the uncertainty due to calibration is trackedwith all time dependent data and may also be retrieved from the data base. Typically the total uncertainty in AOT from a newly calibrated field instrument under cloud free conditions is <±0.01 for [lambda] >440 nm and <±0.02 for shorter wavelengths. Uncertainty in the water vapor retrieval is limited by larger uncertainty in the Vo for the 940 nm channel and by the uncertainty of the radiosonde intercomparisons, typically less than 12%. The uncertainty of the sky radiance data is more difficult to ascertain since these only constitute single observations and no absolute self-calibration procedure is implemented between the sphere calibrations. Based on the sphere calibration, the uncertainty in the sky radiance at the time of calibration is assumed <±5% for all four channels at the time of calibration. Scattering aerosol optical depth is directly related to the aureole brightness and thus the accuracy is a function of the sky calibration. We feel that for low optical depth monitoring the sky brightness may retrieve scattering optical depths with less absolute error than traditional extinction approaches (Table 6) assuming perfect straylight rejection and a uniformly distributed aerosol in the aureole. Development of an in situ sky calibration procedure is under evaluation, (Nakajima et al., 1996). Table 6. The absolute value (and % error) of the extinction optical depth and scattering optical depth at airmass of 2 clearly illustrate the possible advantages of using the scattering optical depth for low optical depth ranges. ---------------------------------------------------------------- Calibration error | 0% | 1% | 5% ---------------------------------------------------------------- [tau] sct | 0.059 (0%) | 0.058 (1%) | 0.056 (5%) ---------------------------------------------------------------- [tau] ext | 0.059 (0%) | 0.054 (8.5%) | 0.033 (44.1%) ---------------------------------------------------------------- 2.6 Data Transmission Data are transmitted from the memory of the sun photometer via the Data Collection Systems (DCS) to either of geosynchronus satellites GOES-E, GOES-W, or METEOSAT (GMS is anticipated in 1998) and then retransmitted to the appropriate ground receiving station. The data can be retrieved for processing either by modem or internet linkage resulting in near real-time acquisition from almost any site on the globe excluding poleward of 80° latitude. The DCS is a governmental system operated for the purpose of transmitting low volume environmental data from remote sites for various institutions and government agencies. Each station on the GOES and METEOSAT networks has been assigned a user ID and transmission time window passing up to 30 kbytes per day in 24 and 48 individual transmissions at hourly and half-hourly intervals respectively. During each transmission, a packet of data and status information are time stamped by the radiometer, the transmitter and the central receiving station (Wallops Island, VA, USA for GOES; Darmstadt, Germany for METEOSAT; and Tokyo, Japan for GMS). Typically the data are maintained in the receiving station computers for 3 to 5 days before they are overwritten. The data are retrieved daily from the central receiving station which we term near real-time. 2.7 Processing System A fundamental component of the AERONET system is a package of user friendly UNIX software that provides near real-time information on the status and calibration of the instruments, provides data processing with referenced and generally accepted processing algorithms, provides an orderly archive of the data and provides convenient electronic access for all users to the raw and processed data base. We shall discuss these aspects of the current operational state of the software and future enhancements. 2.8 Instrument and Network Status The radiometer data stream includes date, time, temperature, battery voltage, wet sensor status and time of transmission as well as several levels of identification numbers. The DCP adds a time stamp at the time of transmission as does the DCS receiving station plus checks for parity errors and signal strength of the transmission. After data are downloaded from the central receiving station, a status report and a trouble shooting report are automatically generated and e-mailed to appropriate system and instrument managers and an internet homepage provides these information to the entire community. The status report provides a comprehensive assessment of the operation of the radiometer and DCP for the data transmitted with the current download. Network managers then have sufficient information to assess the operation of individual stations. To more quickly identify trouble spots, a troubleshooting report is generated that lists by instrument only information that fails to meet normal operating thresholds i.e. low battery voltage, transmission time error, missed transmission etc. This approach can identify remote station problems quickly often leading to same day resolution. Documentation of the status report is available under the AERONET home page http://spamer.gsfc.nasa.gov. 2.9 Data Processing There is lack of agreement on corrections, calibration procedures, data analysis procedures etc. often caused by divergent error tolerances or specific requirements of various investigators. We have implemented a series of processing algorithms on a UNIX server that have been published in the open literature and/or are generally accepted by the scientific community (Table 7). These algorithms impose a processing standardization on all of the data taken in the network facilitating comparison of spatial and temporal data between instruments. The archival system allows the user community to access either the raw or processed data via internet for examination, analysis and/or reprocessing as needed. The archival browse algorithms are collectively known under the program name "demonstrat" which graphically provides access to all aspects of the data base and through the AERONET homepage (http:/spamer.gsfc.nasa.gov). The program operates on a workstation called "spamer.gsfc.nasa.gov". The algorithms within "demonstrat" comprise three principal categories; time dependent retrievals such as AOT and Pw, calibration assessment, and sky radiance retrievals. There are a growing number of sub processing algorithms within each of these. As importantly, `demonstrat' allows all data to be retrieved through "FTP" and e-mail access for personal computer analysis and/or reprocessing as the user requires. As new and improved approaches and models are accepted within the community the processing may be applied uniformly to the network wide data base. Additionally access to the data base through demonstrat provides an opportunity for testing new algorithms and models for an increasingly diverse set of measurements for a variety of locations and conditions. The following figures were obtained directly from the `demonstrat' output to illustrate the access to the data base. -------------------------------------------------------------------------------- Table 7. The algorithms, inputs, corrections and models used in computing the aerosol optical thickness, Pw, spectral irradiance, and sky radiance inversions are referenced. -------------------------------------------------------------------------------- Variable, algorithm Comments References or correction -------------------------------------------------------------------------------- BASIC COMPUTATIONS ------------------ Penndorf, 1957 Rayleigh Optical Depth, Edlen, 1966 [tau]r refractive index of Burcholtz, 1995 air depolarization factor Input elevation in m Young, 1980 Solar Zenith Angle, [theta]o Michalsky, 1988 Earth sun distance, d Iqbal, 1983 Ozone amount, O3 Table lookup by 5° lat. long. London et al., 1976 Aerosol optical air mass, ma Kasten and Young, 1989 Rayleigh optical air mass, mr Kasten and Young, 1989 O3optical air mass, mo Komhyr et al., 1989 CORRECTIONS ----------- Temperature, T ~0.25%/°C for 1020 nm Hamamatsu Inc. and Lab specific for each inst. measurements Water Vapor for 1020 AOT from Pw retrieval, Lowtran Kneizys et al, 1988 Rayleigh, all wavelengths from elevation O3 abs. coef. [lambda] > 350 nm Vigroux, 1953 O3 abs. coef. [lambda] < 350 nm Bass and Paur, 1984 Time, t Cimel, UTC, DAPS time stamps, *Refer to Homepage ±1 second RETRIEVALS ---------- Spectral direct Sun AOT, Beer's Law Shaw, 1983 Langley Plots Pw: (a, k, Vo) Modified Langley Bruegge et al., 1992; Reagan et al., 1992 Scattering AOT From spectral sky radiance Nakajima et al., 1983 Size Dist., Phase function From spectral sky radiance Nakajima et al., 1983 Size Dist. From spectral direct sun AOT Twitty, 1975; Halthore and Fraser, 1987, King 1978 MODELS ------ Spectral2 (irradiance) parameterized spectral RT Bird and Riordan, 1986 6-S (Linkage) analytical, RT Vermote et al., 1995 PROCEDURES ---------- Cloud Screening Thresholds, [lambda] AOT & t *Refer to Homepage Climatology, Direct Sun AOT, Pw, Wavelength Exp. *Refer to Homepage Climatology, Sky Size Dist., Phase function, *Refer to Homepage g -------------------------------------------------------------------------------- *Homepage reference: http://spamer.gsfc.nasa.gov -------------------------------------------------------------------------------- 3.0 References Bass, A.M. and R.J. Paur, 1984, The ultraviolet cross-section of ozone:1. The measurements, in Atmospheric Ozone, edited by C.S. Zerefos & A. Ghazi, pp. 606-610, Reidel, Dordrecht. Bird, R.E. and C. Riordan, 1986. 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