BOREAS RSS-11 Ground Network of Sunphotometer Measurements Summary The BOREAS RSS-11 team operated a network of five automated (Cimel) and two hand-held (Miami) solar radiometers from 1994 to 1996 during the BOREAS field campaigns. The data provide aerosol optical depth measurements, size distribution, phase function, and column water vapor amounts over points in northern Saskatchewan and Manitoba, Canada. The data are useful for the correction of remotely sensed aircraft and satellite images. The data are provided in tabular ASCII files. Table of Contents 1. Data Set Overview 2. Investigator(s) 3. Theory of Measurements 4. Equipment 5. Data Acquisition Methods 6. Observations 7. Data Description 8. Data Organization 9. Data Manipulations 10. Errors 11. Notes 12. Application of the Data Set 13. Future Modifications and Plans 14. Software 15. Data Access 16. Output Products and Availability 17. References 18. Glossary of Terms 19. List of Acronyms 20. Document Information 1. Data Set Overview 1.1 Data Set Identification BOREAS RSS-11 Ground Network of Sunphotometer Measurements 1.2 Data Set Introduction A ground-based sunphotometer network was installed within the BOReal Ecosystem- Atmosphere Study (BOREAS) study areas to characterize the size and distribution of atmospheric aerosols and column water vapor amount and to aid in the correction of aircraft and satellite imagery. Automated sunphotometer data are available fairly continuously from early 1994 through 1996 for two sites in the Northern Study Area (NSA) and two in the Southern Study Area (SSA). Data from a fifth automated instrument located at the Flin Flon airport at the Manitoba/Saskatchewan border; this instrument was moved in 1996 to Paddockwood, SK, in the SSA. Measurements made by two additional hand-held instruments, operated during the 1994-96 time period at each of the study areas, are also a part of this data set. A local observer made the hand-held measurements near noon when the Sun was not obscured by any clouds. 1.3 Objective/Purpose The purpose was to establish a data set of aerosol, water vapor, and ozone atmospheric properties in the BOREAS region for a 3-year period for use by BOREAS in atmospheric correction of remotely sensed data. Another goal was to establish an aerosol climatology of the region by specifying, for example, the annual cycle of aerosol loading across the region. 1.4 Summary of Parameters Parameters: Direct solar irradiance (340, 380, 440, 500, 670, 870, 1020nm) Sky radiance (440, 670, 870, 1020nm) Aerosol optical thickness Column water vapor amount 1.5 Discussion A network of five automated (Cimel) and two hand held (Miami) solar radiometers were operated during the 1994-1996 time period for BOREAS to provide aerosol optical depth measurements, size distribution, phase function, and column water vapor amounts. The purpose was to establish a data set of aerosol, water vapor and ozone atmospheric properties in the BOREAS region for a three year period for use by BOREAS in atmospheric correction of remotely sensed data. 1.6 Related Data Sets BOREAS RSS-03 Reflectance Measured from a Helicopter-Mounted Barnes MMR BOREAS RSS-12 Airborne Tracking Sunphotometer Measurements BOREAS RSS-12 Automated Ground Sun Photometer Measurements in the SSA BOREAS RSS-18 Ground Sunphotometer Measurements in the SSA An additional data set collected and used in comparison was sunphotometer measurements made by Norman O’Neill (RSS-19) at the Canada Centre for Remote Sensing (CCRS). These data were not submitted to the BOREAS Information System (BORIS). 2. Investigators 2.1 Investigators Name and Title Brian L. Markham Physical Scientist 2.2 Title of Investigation Characterization of Atmospheric Optical Properties for BOREAS 2.3 Contact Information Contact 1 --------- Brian L. Markham NASA/GSFC Greenbelt, MD (301) 286-5240 (301) 286-0239/1757 (fax) Brian.L.Markham.1@gsfc.nasa.gov Contact 2 --------- Joel Schafer NASA/GSFC Greenbelt, MD (301) 286-4547 (301) 286-0239 (fax) Joel.S.Schafer.1@gsfc.nasa.gov Contact 3 --------- Jaime Nickeson BORIS RSS Team Representative NASA/GSFC Greenbelt, MD (301) 286-3373 (301) 286-0239 (fax) Jaime.Nickeson@gsfc.nasa.gov 3. Theory of Measurements Sunphotometers measure direct surface irradiance. Because direct solar irradiance depends on the atmospheric transmission, the components affecting this transmission such as aerosol optical thickness (AOT), water vapor column abundance, ozone absorption, and molecular scattering can either be estimated or bemeasured. Optical thickness is calculated from spectral extinction of direct beam radiation at each wavelength based on Beer's Law: where V? = digital voltage measured at wavelength ? Vo = extraterrestrial voltage m = optical airmass ? = total optical thickness R = relative Earth-Sun distance Attenuation due to Rayleigh scatter, and absorption by ozone and gaseous pollutants is estimated and removed to isolate the AOT (?). The dependence of transmission, Tw, on water column abundance can be written as the following [Halthore et al., 1992, Bruegge et al, 1992]: where W is the vertical column abundance (cm), and the constants a and b depend on the wavelength position, width and shape of the filter function, and atmospheric conditions. Sky radiance measurements are inverted with radiative transfer equations [Nakajima et al., 1983] to provide aerosol properties of size distribution and phase function over the particle size range of 0.1 to 5 µm. 4. Equipment 4.1 Sensor/Instrument Description 4.1.1 Collection Environment The automated sunphotometers operated during non-winter months according to their diurnal schedule (section 4.1) when there was no precipitation and the atmosphere is clear enough to allow the instrument to focus on the Sun with a four-quadrant detector (AOT roughly <6.0). Precipitation activates an externally mounted wet sensor that causes the instrument to remain in the parked (nadir) position. 4.1.2 Source/Platform All instruments were mounted on the roof of a building or hut, with a clear view of the sky above 5 degrees elevation angle. The handheld instruments were held by the human observer. 4.1.3 Source/Platform Mission Objectives The purpose of the building roof was to cover the building and provided a convenient place to mount the sunphotometers. The objectives of the human observer during the measurement times was to obtain good measurements and varied at others. 4.1.4 Key Variables Direct solar irradiance Sky radiance 4.1.5 Principles of Operation Eight interference filters are located in a filter wheel that is rotated by a direct drive stepping motor. A thermistor measures the temperature of the detector, allowing compensation for any temperature dependence in the silicon detector. 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 and directs the sensor head to within approximately 1 degree of the Sun. Then, a four-quadrant detector tracks the Sun precisely prior to a programmed measurement sequence. 4.1.6 Sensor/Instrument Measurement Geometry The Cimel automatic sunphotometers take direct solar irradiance and sky radiance measurements with a 1.2-degree FOV. 4.1.7 Manufacturer of Sensor/Instrument The automatic sunphotometers were made by: Cimel Electronique 5 Cite de Phalsbourg 75011 Paris, France The hand-held sunphotometers were made by: University of Miami Dept. of Meteorology Coral Gables, FL 33124 4.2 Calibration A number of automatic sunphotometers from the Goddard Space Flight Center (GSFC) were regularly taken to the Mauna Loa Observatory in Hawaii, where the Langley method of calibration [Shaw, 1983] was used for absolute calibration. Instruments used for BOREAS were calibrated by intercomparison with reference instruments using measurements conducted on the top of a building at GSFC on clear days with low aerosol loading. On occasion, in-field Langley calibrations were performed. For the 940-nm channel that includes water absorption, calibration was performed using the following procedure. At 940 nm, the measured digital voltage is: where t is the extinction due to Rayleigh and aerosol scatter and water vapor absorption, and Tw, again, is water vapor transmission. Combining this relation with that of Section 3 describing yields: Plotting the left-hand side of the above equation against mb gives a straight line with the desired y-intercept ln(Vo) and slope Wb [Halthore et al 1997]. This modified Langley method was used only for calibrating the water vapor band. Hand-held instruments are intercalibrated at the beginning and end of the season with collocated automatic instruments on clear days of low aerosol. 4.2.1 Specifications The automatic instruments used were Cimel CE-318s which, during 1994, had 10-nm bandpass filters in the visible and near-infrared with center wavelengths at 340, 380, 440, 670, 870, 940, and 1020 nm, with an additional 50-nm bandpass filter centered at 940 nm. The two 940-nm channels were to be used for column water vapor abundance determination. The wider bandpass filter proved unnecessary for determinating water vapor column abundance, so it was replaced with a 10-nm bandpass filter centered at 500 nm in spring of 1995. In addition to measuring solar irradiance with a field of view (FOV) of 1.2 degrees, these instruments measure the sky radiance in four spectral bands (440, 670, 870, and 1020 nm) along the solar principal plane (i.e., at constant azimuth angle, with varied solar zenith angles) up to nine times a day and along the solar almucantar (i.e., at constant solar zenith angle, with varied azimuth angles) up to six times a day. The two hand-held sunphotometers have four channels (500, 670, 870, and 940 nm). They are capable of only the direct solar measurements and require manual data entry. They have a peak hold feature that allows them to record the highest voltage response when pointed in the general direction of the Sun. 4.2.1.1 Tolerance If the aerosol conditions were considered to be constant during the Langley procedures at Mauna Loa, then deviation of measurements from the linear regression line gives an indication of the sunphotometer precision. Triplet variability from three reference instruments deployed at Mauna Loa was calculated based on 168, 264, and 288 observations, respectively. For all wavelengths, the variability of a triplet was always less than 1%, and generally about 0.3%, suggesting that uncertainty due to instrument precision is minimal [Holben et al., 1996]. 4.2.2 Frequency of Calibration The instruments were intercompared at GSFC on clear days with reference Cimels calibrated by Langley or modified Langley techniques at Mauna Loa. This process was done before and after each field campaign, and the reference instrument (#13) was taken to Mauna Loa every 6 months. Occasionally, filter changes during deployment or short-term contamination by spider webs in the collimator warranted the use of in-field Langley calibrations. 4.2.3 Other Calibration Information The instrument referred to below as #13 is the reference instrument used for BOREAS. 1994 Cimel Sun data are calibrated as follows: The ratios of the post season to preseason calibrations follow for each instrument in the order. The bracketed numbers are for instrument #31 and #32 1020 870 670 440 340[380] 380[340] #11 NSA-YJP -- interpolated between the May cross comparison with #13 and the November cross comparison with #13 (with the exception of the June 26 through July 4 -- a spider web incident that was calibrated with onsite Langley techniques for 26-Jun-1994). 0.99233306 1.00224738 0.99804205 1.0118189 0.97430213 1.01370528 #12 Flin Flon-- interpolated between the May cross comparison with #13 and the November cross comparison with #13. 0.96776048 0.97619891 0.98253527 0.95644371 0.89236603 0.90532343 #31 YJP-SSA -- interpolated between the May cross comparison with #13 and November cross comparison with #13. 0.99090514 0.99927597 0.98348781 0.99160097 0.99534901 0.94812591 #32 Waskesiu -- used May cross comparison with #13 for full BOREAS season (with exception of 21-25 July-1994 spider web incident calibrated with onsite Langley techniques) -- the November cross comparison with #13 was not valid for BOREAS because of an apparent change in the instrument during its return. #6 Thompson Zoo/Prince Albert -- used May cross comparison with #13 until 09- Jun-1994; used cross comparison with #13 in July until 11-Jul-1994; interpolated between 11-July-1994 cross comparison with #13 and 12-Oct-1994 cross comparison with #13. 1.00103256 0.98735627 0.96687931 0.74577641 1.02234866 mid season 1.01655301 1.016745 0.99002756 0.74729656 1.05768801 post season 1995 Cimel sunphotometer data are calibrated as follows: The time and date are given for each zero airmass voltage (Vo) and for each wavelength and the seasonal change in filter response is given as well. #6 Flin Flon Wavelength 1020 870 670 500 DATE: 08-May-95 12517.458 12897.622 13899.506 13655.055 TIME: 22:32:37 DATE: 05-Nov-95 12582.104 12927.327 13833.724 13723.94 TIME: 15:58:58 Post/preseason 1.005 1.002 0.995 1.005 #11 Waskesiu Wavelength 1020 870 670 500 DATE: 07-May-95 12885.336 9326.284 14941.283 18483.47 TIME: 14:41:54 DATE: 29-Dec-95 13404.445 9648.732 15809.717 18588.241 TIME: 17:31:53 Post/preseason 1.04 1.035 1.058 1.006 #12 Thompson Wavelength 1020 870 670 500 DATE: 08-May-95 13327 13223 12849 17887 TIME: 15:49:24 DATE: 07-Dec-95 13291 13418 13149 17854 TIME: 16:37:45 Post/preseason 0.997 1.015 1.023 0.998 #31 YJP-SSA Wavelength 1020 870 670 500 DATE: 07-May-95 13329.603 13645.573 14900.62 18606.043 TIME: 15:49:23 DATE: 29-Dec-95 13620.063 14025.419 15261.078 18132.432 TIME: 17:31:58 Post/preseason 1.022 1.028 1.024 0.975 #35 YJP-NSA Wavelength 1020 870 670 500 DATE: 08-May-95 19516.054 14028.61 22462.976 9513.898 TIME: 22:21:34 DATE: 07-Jun-95 19819.766 14270.977 22896.422 9808.112 TIME: 11:12:32 DATE: 02-Dec-95 19953.95 14316.853 22858.584 9665.12 TIME: 16:34:38 Post/preseason 1.022 1.021 1.018 1.016 #6 Flin Flon Wavelength 440 380 340 940 DATE: 08-May-95 18147.869 19241.212 19757.94 22399.36 TIME: 22:32:37 DATE: 05-Nov-95 18344.145 20505.336 18284.42 20831.22 TIME: 15:58:58 Post/preseason 1.108 1.066 0.925 0.93 #11 Waskesiu Wavelength 440 380 340 940 DATE: 07-May-95 18296.37 21366.285 33970.048 21292.029 TIME: 14:41:54 DATE: 29-Dec-95 19135.821 22185.209 30718.195 21590.383 TIME: 17:31:53 Post/preseason 1.046 1.038 0.904 1.014 #12 Thompson Wavelength 440 380 340 940 DATE: 08-May-95 16964 24463 32590 29583.1 TIME: 15:49:24 DATE: 07-Dec-95 17465 25282 31927 28451.3 TIME: 16:37:45 Post/preseason 1.03 1.033 0.98 0.962 #31 YJP-SSA Wavelength 440 380 340 940 DATE: 07-May-95 16585.035 17619.795 32936.416 23163.098 TIME: 15:49:23 DATE: 29-Dec-95 15980.247 18374.366 28336.05 22616.378 TIME: 17:31:58 Post/preseason 0.964 1.043 0.86 0.976 #35 YJP-NSA Wavelength 440 380 340 940 DATE: 08-May-95 18035.776 6789.077 33650.941 32735.451 TIME: 22:21:34 DATE: 07-Jun-95 17950.115 7319.054 34008.235 32404.751 TIME: 11:12:32 DATE: 02-Dec-95 18170.429 7149.50 32357.469 31599.466 TIME: 16:33:38 Post/preseason 1.007 1.053 0.962 0.965 1996: Cimel sunphotometer data are calibrated as follows: The time and date are given for each zero airmass voltage (Vo) and for each wavelength, and the seasonal change in filter response is given as well, where appropriate. #12 Thompson Wavelength 1020 870 670 500 DATE: 27-Apr-96 13420.234 13491.316 13087.821 18244.657 TIME: 20:05:38 DATE: 04-Nov-96 13557.683 13635.545 12996.507 18142.348 TIME: 17:32:28 Post/preseason 1.01 1.011 0.993 0.994 #12 Thompson Wavelength 440 380 340 940 DATE: 27-Apr-96 17474.941 25292.378 30613.31 29172.311 TIME: 20:05:38 DATE: 04-Nov-96 17271.154 24877.615 30012.10 28021.078 TIME: 17:32:28 Post/preseason 0.988 0.984 0.98 0.961 #10 Paddockwood Wavelength 1020 870 670 500 DATE: 31-Mar-96 12449.513 8361.421 11602.164 15602.52 TIME: 15:59:22 #10 Paddockwood Wavelength 440 380 340 940 DATE: 31-Mar-96 15192.358 20776.75 26675.68 20253.18 TIME: 15:59:22 #31 YJP SOUTH Wavelength 1020 870 670 500 DATE: 29-Dec-95 13620.063 14025.419 15261.078 18132.432 TIME: 17:31:58 #31 YJP SOUTH Wavelength 440 380 340 940 DATE: 29-Dec-95 15980.247 18374.366 28336.050 22616.378 TIME: 17:31:58 #35 YJP NORTH Wavelength 1020 870 670 500 DATE: 24-Apr-96 19600.860 14273.589 22715.018 9523.237 TIME: 15:20:45 DATE: 20-Dec-96 19295.777 13997.458 22520.035 9453.443 TIME: 18:32:59 Post/preseason 0.984 0.981 0.991 0.993 #35 YJP NORTH Wavelength 440 380 340 940 DATE: 24-Apr-96 18016.307 6759.045 30243.207 31001.937 TIME: 15:20:45 DATE: 20-Dec-96 18160.719 6866.885 29525.022 29548.983 TIME: 18:32:59 Post/preseason 1.008 0.976 1.016 0.953 #11 Waskesiu Wavelength 1020 870 670 500 DATE: 24-Apr-96 13622.427 9649.122 15575.813 18969.570 TIME: 16:13:30 DATE: 02-Dec-96 13602.240 9565.654 15565.203 18980.755 TIME: 19:10:59 Post/preseason 0.999 0.991 0.999 1.001 #11 Waskesiu Wavelength 440 380 340 940 DATE: 24-Apr-96 19495.370 29884.542 22098.695 21705.495 TIME: 16:13:30 DATE: 02-Dec-96 19322.585 28941.093 21926.102 21078.678 TIME: 19:10:59 Post/preseason 0.991 0.992 0.968 0.971 5. Data Acquisition Methods A preprogrammed sequence of measurements is taken by these instruments starting at an airmass of 7 in the morning and ending at an airmass of 7 in the evening. During the large airmass periods, direct Sun measurements are made at 0.25 airmass intervals; at smaller airmasses, the interval between measurements is typically 15 minutes. The almucantar measurements are taken at 0.5-degree intervals near the Sun (within 6 degrees) and increase from 2 to 10 degree intervals away from the solar position. The data are collected and transmitted via the Geostationary Environmental Satellite (GOES) at 1-hour intervals to a computer at Wallops Island Flight Facility [Holben et al., 1996]. The hourly transmitted 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 Vitel transmitter adds a time stamp at the time of transmission, as does the Wallops receiving station. The transmitter also checks for parity errors and signal strength of the transmission. After data are downloaded from the central receiving station, a status report and a troubleshooting report are automatically generated and e- mailed to appropriate system and instrument managers. The status report provides a comprehensive assessment of the operation of the radiometer and the Vitel transmitter for the data transmitted with the current download. Network managers then have sufficient information to assess the operation of individual stations. Within Demonstrat, a package of user-friendly UNIX software, the raw data (voltages) are converted to AOT or precipitable water using the relevant instrument-specific calibration coefficients. 6. Observations 6.1 Data Notes The data sets are generally complete from May to October or September with some exceptions noted below. 1994: #11 YJP-NSA 26-Jun - 04-Jul collimator obstruction #32 Waskesiu, SK 21-Jul - 25-Jul collimator obstruction 1995: #11 Waskesiu, SK 01-Aug - 22-Aug collimator obstruction #31 YJP-SSA 04-Sep - 07 Sep #31 YJP-SSA 12-Sep - 22-Sep #31 YJP-SSA 01-Jun - 01-Jul removed because of local forest fire 1996: #10 Paddockwood 18-Apr - 27-Apr * Installed in February * 02-Sep - 14-Sep #11 Waskesiu, SK 01-May - 12-May 04-Sep - 30-Sep #31 YJP-SSA 23-Sep - 02-Oct 15-May - 29-May 04-Sep - 08-Sep (440-nm unusable) 12-Sep - 23-Sep (440-nm unusable) #35 YJP-NSA 24-Sep - 03-Oct The hand-held records are generally complete for the whole year depending on availability of the observer. 6.2 Field Notes None given. 7. Data Description 7.1 Spatial Characteristics 7.1.1 Spatial Coverage Deployment Sites Latitude Longitude UTM Easting UTM Northing 1994: Flin Flon, MB 54.67777N 101.67843W 327315.2 6062229 NSA-YJP 55.89575N 98.28706W 544583.9 6194706.9 NSA-Thompson Airport 55.78344N 97.83359W 573151.1 6182593.1 SSA-YJP 53.87581N 104.64529W 523320.2 5969762.5 Prince Albert Airport 53.20004N 105.68383W 454320.6 5894742.2 SSA-Lake Waskesiu 53.91672N 106.06717W 429909.5 5974783.3 1995: SSA-YJP 53.87581N 104.64529W 523320.2 5969762.5 NSA-YJP 55.89575N 98.28706W 544583.9 6194706.9 SSA-Lake Waskesiu 53.91672N 106.06717W 429909.5 5974783.3 Flin Flon, MB 54.67777N 101.67843W 327315.2 6062229 NSA-Thompson Zoo 55.75N 97.8867W 571137 6178837 1996: SSA-YJP 53.87581N 104.64529W 523320.2 5969762.5 NSA-YJP 55.89575N 98.28706W 544583.9 6194706.9 SSA-Lake Waskesiu 53.91672N 106.06717W 429909.5 5974783.3 SSA-Paddockwood School 53.50951N 105.56697W 462400 5929100 NSA-Thompson Zoo 55.75N 97.8867W 571137 6178837 In addition, a hand-held sunphotometer was operated at Lake Waskesiu and Thompson during each year. 7.1.2 Spatial Coverage Map Not available. 7.1.3 Spatial Resolution Each sunphotometer takes measurements that are generally representative of the local area under quiescent conditions. During forest fire episodes, however, the spatial resolution depends on the nature of smoke dispersion. 7.1.4 Projection Not Applicable 7.1.5 Grid Description Not Applicable 7.2 Temporal Characteristics 7.2.1 Temporal Coverage Automatic sunphotometer data are acquired at 0.25 or 0.5 airmass intervals from an airmass of 7 up to 2. Air mass is approximately equal to 1/cos(zenith angle). Then, measurements are made every 15 minutes until an afternoon airmass of 2, where time intervals are again dictated by airmass fractions. 7.2.2 Temporal Coverage Map The following are the operation periods for the Cimel automatic sunphotometers during the 1994, 1995, and 1996 BOREAS campaigns: 1994 OPERATION PERIODS Flin Flon, MB 19-May - 13-Sep NSA-YJP 18-May - 26-Jun, 4-Jul - 01-Nov SSA-YJP 23-May - 13-Oct Waskesiu, SK 25-May - 21-Jul, 25-Jul - 13-Oct Thompson, MB 08-Jun - 13-Jun, 27-Jul - 10-Sep Prince Albert, SK 17-May - 06-Jun, 20-Jul - 26-Jul, 12-Sep -18-Sep 1995 OPERATION PERIODS Flin Flon, MB 18-May - 07-Sep NSA-YJP 16-May - 03-Nov SSA-YJP 25-May - 01-Jun, 01-Jul - 12-Sep, 22-Sep - 21-Nov Waskesiu, SK 24-May - 01-Aug, 22-Aug - 04-Nov Thompson, MB 15-May - 29-Oct 1996 OPERATION PERIODS Paddockwood, SK 27-Feb - 07-Nov NSA-YJP 14-May - 24-Sep, 30-Sep - 09-Oct SSA-YJP 10-May - 14-May, 28-May - 20-Sep, 01-Oct - 23-Oct Waskesiu, SK 08-May - 14-May, 23-May - 04-Sep, 29-Sep - 27-Oct Thompson, MB 14-May - 09-Sep The following are the peration periods for the hand-held sunphotometers during 1994, 1995, and 1996 BOREAS campaigns. Gaps may indicate extended cloudy conditions, instrument problems, or operator unavailability. 1994 OPERATION PERIODS Waskesiu, SK 04-Jan - 03-Mar, 22-Mar - 26-Sep, 10-Oct - 22-Dec Thompson, MB 01-Jan - 29-Jun, 01-Aug - 21-Oct 1995 OPERATION PERIODS Waskesiu, SK 01-Jan - 11-Apr, 04-May - 02-Sep, 21-Sep - 08-Oct Thompson, MB 04-Feb - 20-Dec 1996 OPERATION PERIODS Waskesiu, SK 03-Jan - 31-Dec Thompson, MB 03-Jan - 31-Dec 7.2.3 Temporal Resolution Cimels operated approximately every 15 minutes from morning to evening. Hand- held sunphotometers were operated near noon only. 7.3 Data Characteristics Data characteristics are defined in the companion data definition file (r11sunpd.def). 7.4 Sample Data Record Sample data format shown in the companion data definition file (r11sunpd.def). 8. Data Organization 8.1 Data Granularity All of the RSS-11 Ground Network of Sunphotometer Measurements are contained in one dataset. 8.2 Data Format(s) The data files contain numerical and character fields of varying length separated by commas. The character fields are enclosed with a single apostrophe marks. There are no spaces between the fields. Sample data records are shown in the companion data definition files (r11sunpd.def). 9. Data Manipulations 9.1 Formulae Equation Used for Calculation of optical depth Calculation of precipitable water Calibration of water vapor channel Cloud-screening procedure 9.1.1 Derivation Techniques and Algorithms Optical thickness is calculated from spectral extinction of direct beam radiation at each wavelength based on Beer's Law: where V???= digital voltage measured at wavelength ? m = optical airmass ? = total optical thickness R = relative Earth-Sun distance Attenuation due to Rayleigh scatter and absorption by ozone and gaseous pollutants is estimated and removed to isolate the AOT (?). The dependence of transmission, Tw, on water column abundance can be written as the following [Halthore et al., 1992, Bruegge et al., 1992]: where W is the vertical column abundance (cm), and the constants a and b depend on the wavelength position, width and shape of the filter function, and atmospheric conditions. Sky radiance measurements are inverted with radiative transfer equations [Nakajima et al., 1983] to provide aerosol properties of size distribution and phase function over the particle size range of 0.1 to 5 µm. 9.2 Data Processing Sequence 9.2.1 Processing Steps The automatic radiometers acquire data regardless of sky conditions, except for rain, and thus require cloud-screening procedures. Three quality control schemes are considered to reject data obtained under marginal conditions. The Cimels perform three scan sequences spaced 30 seconds apart, and thus acquire three AOT measurements at each wavelength. If any of these triplets exhibit a coefficient of variation of more than 12%, the data derived from all channels are rejected. Secondly, data exhibiting an increasing or nearly flat AOT with wavelength between 440 nm and 870 nm are considered cloud-contaminated. Therefore, data with an Angstrom coefficient, a, of less than or equal to zero are removed. The Angstrom coefficient is calculated by: for ? at 440 nm and 870 nm, where ? is wavelength. Finally, the remaining data are plotted along with the direct Sun observations acquired during almucantar measurements exhibiting high azimuthal symmetry about the solar plane, which are expected to represent cloudless conditions. A regression line through these almucantar observations is plotted as well. The abscissa shows the AOT at 440 nm, while the ordinate depicts the ratio of this AOT to the Angstrom coefficient described above. The core of the remaining data set is found to follow the trend of the almucantar regression, and outliers are removed by subjectively drawn polygons. Hand-held data are cloud-screened in a similar, but less rigorous, manner because the manual instruments do not take triplet measurements. The same plotting technique is used with axes of x: [500nm AOT] and y: [500nm / a (500/870)], although no almucantar measurements are available to plot simultaneously. 9.2.2 Processing Changes None. 9.3 Calculations None. 9.3.1 Special Corrections/Adjustments None. 9.3.2 Calculated Variables Water vapor, AOT. 9.4 Graphs and Plots None. 10. Errors 10.1 Sources of Error Error Result ----------------------- ---------------------------------------------------- Tracking misalignment Overestimation of AOT and/or reduced reproducibility Filter degradation Generally underestimation of AOT Detector response Over- or underestimation of AOT Temperature stability Over- or underestimation of AOT at 1020 nm Collimator obstruction Overestimation of AOT Failure to find peak Overestimation of AOT response (hand-held) 10.2 Quality Assessment All data have been cloud-screened and checked for almucantar symmetry. 10.2.1 Data Validation by Source Pre/postdeployment intercomparison was made with reference instruments, pre/postdeployment filter calculations were made and agreement of proximal instruments was verified 10.2.2 Confidence Level/Accuracy Judgement Data are generally good, with an absolute AOT accuracy of typically ±0.02 for the automatic sunphotometers and ±0.04 for the hand-held sunphotometers. Precipitable water measurements have agreed within 10% of radiosonde measurements [Halthore et al., 1997]. 10.2.3 Measurement Error for Parameters and Variables The Cimels make three direct Sun measurements at each wavelength in a 30-second scan sequence. This group of three measurements is referred to as a triplet. The coefficient of variability of the triplets for three reference instruments deployed at Mauna Loa was calculated based on 168, 264, and 288 observations, respectively. For all wavelengths, the variability of a triplet was always less than 1%, and generally about 0.3%, giving an estimate of the instrument's reproducibility [Holben et al., 1996]. The calibration coefficients, Vo are usually determined by averaging the y-intercepts from three to seven Langley plots at Mauna Loa Observatory. The averaged Vo values from all calibration sessions at Mauna Loa have a coefficient of variability of ~0.25% to 0.5% for the visible and near-infrared wavelengths, ~0.5% to 2% for the ultraviolet, and ~1.0% to 3.0% for the water vapor channel [Holben et al., 1996]. The overall accuracy of AOT measurements is expected to be in the range of 0.02 at an airmass of 1.0. The hand-held sunphotometers were calibrated by intercomparison with colocated Cimels. The estimated level of uncertainty for these instruments is greater (0.04) because of less frequent recalibration. For the sky radiance measurements, calibration was performed at the NASA GSFC Calibration Facility using a calibrated integrating sphere to an accuracy of +/- 5%. For the 940-nm channel that includes water absorption, calibration was performed using a variant of the modified Langley method as described in Halthore et al. [1997]. The method used is similar to that described elsewhere; for instance, Bruegge et al., 1992b, and Halthore et al., 1992b. Column amounts of precipitable water derived from sunphotometer measurements at BOREAS have compared favorably with radiosonde observations [Halthore et al., 1997] to within +/- 10%. 10.2.4 Additional Quality Assessments None given 10.2.5 Data Verification by Data Center BORIS personnel have reviewed portions of the actual parameter values and generated plots for use in visually spotting any anomalous values. 11. Notes 11.1 Limitations of the Data None given. 11.2 Known Problems with the Data Most data are good, though occasionally the AOT at 1020 nm will exceed that at 870 nm, or the AOT at 500 nm will exceed that at 440 nm for very low aerosol conditions (AOT 500 < 0.04), when sensitivity to aerosol is minimal and calibration errors are greatly accentuated. Most of the apparent errors are on the order of 0.01 in AOT, so it is typically a minor concern. On most instruments, the 340-and 380-nm channels are less reliable than the longer wavelengths, likely because of the nature of the filter design, and at times, they yield AOT values that are lower for 340 nm than 380 nm. Triplet variation is larger for instrument #31, which is currently suspected to result from a 4-quadrant detector that is misaligned or otherwise functioning inadequately. Similar problems in other instruments at GSFC, not involved with BOREAS are being investigated. 11.3 Usage Guidance None given. 11.4 Other Relevant Information None given. 12.0 Application of the Data Set The AOT data can be used as input for standard radiative transfer programs such as LOW-resolution Radiative Transfer Code (LOWTRAN-7), MODerate Resolution Radiative Transfer Code (MODTRAN), or the Second Simulation of the Satellite Signal in the Solar Spectrum (6S) to perform atmospheric correction of remotely sensed data. 13. Future Modifications and Plans Possible fully objective cloud-screening routine. 14. Software 14.1 Software Description "Demonstrat" is a package of user-friendly UNIX software developed at GSFC used for data analysis and can be found at spamer.gsfc.nasa.gov. Within Demonstrat, the raw data (voltages) are converted to AOT or precipitable water using the relevant instrument-specific calibration coefficients. 14.2 Software Access Guest accounts are available and accessible via X-term window. Contact persons: Brent Holben (Brent.N.Holben.1@gsfc.nasa.gov) or Ilya Slutsker (Ilya.Slutsker.1@gsfc.nasa.gov). 15. Data Access 15.1 Contact Information Ms. Beth Nelson BOREAS Data Manager NASA GSFC Greenbelt, MD (301) 286-4005 (301) 286-0239 (fax) beth@ltpmail.gsfc.nasa.gov 15.2 Data Center Identification See Section 15.1. 15.3 Procedures for Obtaining Data Users may place requests by telephone, electronic mail, or fax. 15.4 Data Center Status/Plans The RSS-11 sunphotometer data are available from the Earth Observing System Data and Information System (EOSDIS) Oak Ridge National Laboratory (ORNL) Distributed Active Archive Center (DAAC). The BOREAS contact at ORNL is: ORNL DAAC User Services Oak Ridge National Laboratory Oak Ridge, TN (423) 241-3952 ornldaac@ornl.gov ornl@eos.nasa.gov 16. Output Products and Availability 16.1 Tape Products None. 16.2 Film Products None. 16.3 Other Products The data are available in tabular ASCII files. 17. References 17.1 Platform/Sensor/Instrument/Data Processing Documentation None. 17.2 Journal Articles and Study Reports Ahern, F.J., R.P. Gauthier, P.M. Teillet, J. Sirois, G. Fedosejevs, and D. Lorente, 1991, Investigation of continental aerosols with high-spectral- resolution solar-extinction measurements, Appl. Optics, 30, 5276-5287. Bruegge, C.T., J.E. Conel, R.O. Green, J.S. Margolis, R.G. Holm, and G. Toon, 1992, Water vapor column abundance retrievals during FIFE, JGR, 97(D19), 18759- 18768. Bruegge, C.T., R.N. Halthore, B. L. Markham, M. Spanner, and R. Wrigley, 1992, Aerosol optical depth retrievals over the Konza Prairie, JGR, 97(d19), 18743- 18758. d'Almeida, G.A., and P. Koepke, 1998, An approach to a global optical aerosol climatology, in Aerosols and Climate, edited by P.V. Hobbs and M.P. McCormick, Deepak Publishing, Hampton, VA. Halthore, R.N., B.L. Markham, R.A. Ferrare, and T.O. Aro, 1992, Aerosol optical properties over the midcontinental United States, JGR, 97(D17), 18769-18778. Halthore, R.N., T.F. Eck, B.N. Holben and Brian Markham, 1997, Sunphotometric measurements of atmospheric water vapor column abundance in the 940-nm band, JGR. Hansen, J.E. and A.A. Lacis, 1990, Sun and dust versus greenhouse gases: An assessment of their relative roles in global climate change, Nature, 345, 713- 719. Hegg, D.A., P.V. Hobbs, R.J. Ferek, and A.P. Waggoner, 1995, Measurements of some aerosol properties relevant to radiative forcing on the east coast of the United States, J. Appl. Meteor., 34, 2306-2315. Holben, B.N., T.F. Eck and R.S. Fraser, 1991b, Temporal and spatial variability of aerosol optical depth in the Sahel region in relation to vegetation remote sensing, Int. J. Remote Sens., 12, 1147-1163. Holben, B.N., T.F. Eck, I. Slutsker, D. Tanré, J.P. Buis, A. Setzer, E. Vermote, J.A. Reagan, Y.J. Kaufman, T. Nakajima, F. Lavenu and I. Jankowiak, Automatic sun and sky scanning radiometer system for network aerosol monitoring, Rem. Sens. Environ. (in press) Holben, B.N., Y.J. Kaufman, A. Setzer, D. Tanré, and D.E. Ward, 1991a, Optical properties of aerosol from biomass burning in the tropics, BASE-A, in Global Biomass Burning, the MIT Press, Cambridge, MA, 403-411. Kaufman, Y.J., A. Setzer, D. Ward, D. Tanré, B.N. Holben, P. Menzel, M.C. Pereira, and R. Rasmussen, 1992, Biomass burning airborne and spaceborne experiment in the Amazonas (BASE-A), J. Geophys. Res., 97(D13), 14581-14599. Kaufman, Y.J., B.N. Holben, D. Tanré, and D.E. Ward, 1994, Remote sensing of biomass burning in the Amazon, Remote Sensing Reviews, 10, 51-90. Lenoble, J., 1993, Atmospheric Radiative Transfer, Deepak Publishing: Hampton, VA, pp 532. Markham, B.L., J.S. Schafer, B.N. Holben, and R.N.Halthore, 1997, "Atmospheric aerosol and water vapor characteristics over north central Canada during BOREAS", JGR (in press) Nakajima, Y., M. Tanaka and T. Yamauchi, 1983, Retrieval of the optical properties of aerosols from aureole and extinction data, Appl. Opt., 22, 2951- 2959. Reagan J.A., K.J. Thome, and B.M. Herman, 1992, A simple instrument and technique for measuring columnar water vapor via Near-IR differential solar transmission measurements, IEEE Trans. Geosci. Remote Sensing, 30, 825-831. Sellers, P.and F. Hall. 1994. Boreal Ecosystem-Atmosphere Study: Experiment Plan. Version 1994-3.0, NASA BOREAS Report (EXPLAN 94). Sellers, P.and F. Hall. 1996. Boreal Ecosystem-Atmosphere Study: Experiment Plan. Version 1996-2.0, NASA BOREAS Report (EXPLAN 96). Sellers, P., F. Hall and K.F. Huemmrich. 1996. Boreal Ecosystem-Atmosphere Study: 1994 Operations. NASA BOREAS Report (OPS DOC 94). Sellers, P., F. Hall and K.F. Huemmrich. 1997. Boreal Ecosystem-Atmosphere Study: 1996 Operations. NASA BOREAS Report (OPS DOC 96). Sellers, P., F. Hall, H. Margolis, B. Kelly, D. Baldocchi, G. den Hartog, J. Cihlar, M.G. Ryan, B. Goodison, P. Crill, K.J. Ranson, D. Lettenmaier, and D.E. Wickland. 1995. The boreal ecosystem-atmosphere study (BOREAS): an overview and early results from the 1994 field year. Bulletin of the American Meteorological Society. 76(9):1549-1577. Sellers, P.and F. Hall. 1997. BOREAS Overview Paper. JGR Special Issue (in press). Shaw, G.E., 1983, Sun photometry, Bull. Am. Meteorol. Soc., 64, 4-10. Thome, K.J., B.M. Herman and J.A. Reagan, 1992, Determination of precipitable water from solar transmission, J. Appl. Met., 31, 157-165. Twomey, S., 1991, Aerosols, clouds and radiation, Atmos. Environ., 25(A), 2435- 2442. Ward, D.E., R.A. Susott, J.B. Kauffman, R.E. Babbit, D.L. Cummings, B.Dias, B.N. Holben,Y.J. Kaufman, R.A. Rasmussen and A.W. Setzer, 1992, Smoke and fire characteristics for cerrado and deforestation burns in Brazil: BASE-B experiment, J. Geophys. Res., 97(D13), 14601-14619. 18. Glossary of Terms Aerosol optical depth - a dimensionless measure of the extinction of the direct solar beam due to scattering and absorption by atmospheric particulates. Air mass - approximately equal to 1/cos(solar zenith angle) Almucantar - sunphotometer procedure that measures the sky radiance in four spectral bands (440, 670, 870, and 1020 nm) at constant solar zenith angle, with varied azimuth angles ranging from 0.25-degree intervals near the solar azimuth position to 10-degree intervals far from the solar azimuth position. Asymmetry parameter - average cosine of the scattering angle weighted by the phase function. Phase function - a dimensionless measure of the relative scattering intensity of a particle as a function of angle relative to the original propagation direction. Rayleigh scatter - scatter of solar energy by gaseous molecules that is highly predictable for a given atmospheric pressure. 19. List of Acronyms 6S - Second Simulation of the Satellite Signal in the Solar Spectrum AOT - Aerosol Optical Thickness ASCII - American Standard Code for Information Interchange BOREAS - BOReal Ecosystem-Atmosphere Study BORIS - BOReas Information System CCRS - Canada Centre for Remote Sensing DAAC - Distributed Active Archive Center EOS - Earth Observing Satellite EOSDIS - EOS Data and Information System FOV - Field of View GOES - Geostationary Operational Environmental Satellite GSFC - Goddard Space Flight Center LOWTRAN - LOW resolution radiative TRANsfer code MODTRAN - MODerate resolution radiative TRANsfer code NASA - National Aeronautics and Space Administration NSA - Northern Study Area ORNL - Oak Ridge National Laboratory PANP - Prince Albert National Park SSA - Southern Study Area URL - Uniform Resource Locator YJP - Young Jack Pine 20. Document Information 20.1 Document Revision Date Created: 20-Jun-1996 Revised: 19-Mar-1998 20.2 Document Review Dates BORIS Review: 19-Sep-1997 Science Review: 16-Oct-1997 20.3 Document ID 20.4 Citation If this data set is referenced by another investigator, please acknowledge the paper by Markham, et al (in press), listed in Section 17. 20.5 Document Curator 20.6 Document URL Keywords: ---------------- Aerosol Optical Thickness Water Vapor Volume Distribution Phase Function RSS11_Sun_Photo.doc 04/17/98