Optical Thickness Cross-Calib. (FIFE)
(Optical Thickness Calibration).
The Optical Thickness Calibration Data Set contains Rayleigh, aerosol, ozone, and total optical thickness data obtained from three different groups. These groups measured atmospheric transmission, in particular, the aerosol optical thickness, using at least three different instruments (e.g., a solar transmissometer SXM-2, a Reagan sunphotometer, and an airborne tracking sunphotometer) and three different analysis procedures.
During the 1989 field campaign for FIFE there were three independent groups measuring atmospheric transmission, in particular, the aerosol optical thickness, using at least three different instruments and three different analysis procedures. The objective of this study was to collocate the instruments in space and time so that a comparison could be made of the aerosol optical thickness derived by the three groups.
Rayleigh, aerosol, ozone, and total optical thickness.
These data enable the intercalibration of the sunphotometers used in 1989 and therefore, the direct comparison of the data collected by these instruments. Measurements were taken with three instruments (Solar transmissometer SXM-2, Reagan sunphotometer, and Airborne Tracking Sunphotometer) positioned side by side on August 4, 1989. Only the SMX-2 and Reagan sunphotometer were used for measurements on August 6, 1989. Optical thickness was measured with each of the instruments.
Bruegge et al. (1992a) reported on the comparison study involving data acquired in the early afternoon of August 4, 1989 using the Reagan, SXM-2, and Airborne Tracking Sunphotometers (ATSP) at Manhattan Airport. When the data at 440 nm were compared for a 1-hour period, the aerosol optical depth was found to drop by about 0.01. All instruments followed the same temporal profile during periods of simultaneous measurements. Generally, the data were in agreement to within 0.005.
Dr. Carol J. Bruegge
Jet Propulsion Laboratory
Dr. Rangasayi N. Halthore
NASA Goddard Space Flight Center/Hughes STX Corp.
Mr. Brian L. Markham
NASA Goddard Space Flight Center
Dr. Michael A. Spanner
NASA Ames Research Center
Aerosol Optical Thickness Calibration.
Dr. R.N. Halthore
Dr. C.J. Bruegge
Jet Propulsion Lab.
Dr. M.A. Spanner
NASA/Ames Res. Ctr
Moffett Field CA
The Optical Thickness Calibration data were obtained by C.J. Bruegge, R.N. Halthore, B.L. Markham, and M.A. Spanner.
Sunphotometers or solar transmissometers have long been used to measure atmospheric transmission. The basic idea is that since for time scales in order of a day, solar radiance I_o outside the atmosphere is a constant in narrow bands (approx. 10 nm) in the visible and near-infrared wavelengths, changes in radiance I measured at the ground by the sunphotometer are the result of changes in atmospheric radiance, and more importantly, transmission. Thus, applying Bouger's law of attenuation,where:
Note: water vapor is not included in this analysis, which refers to channels that avoid water bands. If the sunphotometer responds with a voltage V for an incident radiance I as in I = KV, where K is a constant, thus implying linearity in the instrument response, equation (1) becomes, after some manipulation,
For a measurement of voltage V at a known airmass of m (= sec(theta), approximately where theta is the solar zenith angle), all the terms on the right-hand side of equation (2) are known, and therefore, aerosol optical thickness can be estimated. Note that V_o is the calibration coefficient to be discussed in the Calibration Section, and Rayleigh and ozone optical thickness are calculated thus:
with p as the measured surface pressure and P_o is the sea level pressure of 1.013 bars. The wavelength of observation lambda is in micrometers or microns. Ozone optical thickness is estimated by multiplying the ozone column abundance in Dobson units (DU or matm-cm) obtained from climatological charts (i.e., the standard relation between ozone abundance, latitude and time of year) by the absorption coefficient tabulated below:
________________________________ Wavelength Ozone Absorption (nm) Coefficient in [matm^-1][cm^-1] ________________________________ 441 3.36E-6 522 4.8E-5 557 9.73E-5 613 1.19E-5 671 4.55E-5 781 4.61E-6 872 6.17E-7 1030 0.0For a description of the theory involved in water column abundance measurements in channels that include the 940 nm water vapor band, the reader should consult Bruegge et al. (1992b).
SXM-2 and Reagan sunphotometers were located on the ground, while the ATSP was in its usual location atop the C-130 aircraft parked nearby.
The objective was to compare the measured aerosol or total optical thickness as measured by different groups.
Aerosol optical thickness, pressure, total optical thickness, and optical thickness due to ozone absorption.
Sunphotometers used in this study typically employ a silicon photodiode detector, which may be temperature controlled. A filter wheel having 6 - 10 bands throughout the visible and near-IR at preselected wavelengths that usually avoid water band absorption. Sometimes bands are specifically chosen to include water absorption to monitor water column abundance.
Sunphotometers are designed to measure solar radiance at the ground surface, in a narrow viewing angle as determined by a baffled long tube. The narrow viewing angle, typically 1-2 degrees field of view ensures full coverage of the solar disk, while eliminating much of the sky radiance.
See the documents listed in the Sensor/Instrument Description Section.
Equation (2) in the Theory of Measurements Section can be written as the following:
In the above equation, the terms on the left-hand side can be plotted against m_aer, the aerosol airmass. If the aerosol optical thickness does not change during portions of the day when the airmass is changing the most (sunrise and sunset), such a plot would yield a straight line whose y-intercept would be a function of the required calibration coefficient: ln(V_o/R^2), and whose slope is -tau_aer.
This is known as the Langley plot method of calibration. The requirement that tau_aer not change during measurements is most easily met at mountain sites, which are above the boundary layer and in the morning when convective motions have had not enough time set in. For a further discussion, see Halthore and Markham (1992).
In 1987 and 1989, all of the sunphotometers were calibrated using the Langley plot method (or a minor variant on the theme). The present effort, however, was conducted to perform another method of calibration-the intercomparison method. In this method, if there are two instruments (e.g., 1 and 2) of identical bands, situated side by side and measuring the aerosol optical thickness at the same time, the calibration coefficient of one of the instruments (e.g., 2) can be computed from equation (2) as:
Where the subscripts _1 and _2 refer to the instruments 1 and 2 respectively. Instrument 1 is the reference instrument in this case. If the bands do not overlap, appropriate extrapolation/interpolation in wavelength for the optical thickness must be done and with the help of equation (2), V_o is determined.
The advantage of having three instruments is obvious. Aerosol optical thickness derived from three independent calibrations can be compared to have the following outcomes:
Further intercomparisons with other instruments better calibrated than the three used here must be done to resolve the differences.
In the data reported here, it can be seen that the aerosol optical thickness derived from the three instruments agreed to within the uncertainties of any one instrument.
Not available at this revision.
Not available at this revision.
The Langley plot method of calibration was performed at mountain sites before and after the 1989 IFC. Calibration check using the intercomparison method was done once during the 1989 IFC.
Optical thickness data were collected simultaneously by the SXM-2, Reagan, and ATSP sunphotometers at the same site and times during the 1989 IFC. Surface pressure was measured with a high-accuracy barometer made by Atmospheric Instrumentation Research, Inc.
The FIFE study area, with areal extent of 15 km by 15 km, is located south of the Tuttle Reservoir and Kansas River, and about 10 km from Manhattan, Kansas, USA. The northwest corner of the area has UTM coordinates of 4,334,000 Northing and 705,000 Easting in UTM Zone 14.
All the data were acquired at a single location in Manhattan airport near the tarmac where the C130 aircraft was parked.
The sitegrid is XMH2-SPS with coordinates as follows:
This is point data. However, the data represent the optical thickness for distances of about 0.5 km from the instrument collecting the data.
Data were collected on 2 days during the IFC. Measurements were made by all three investigators on August 4, 1989, but only two of the investigators collected data on August 6, 1989. Below are the dates and investigators for the measurements:
Investigators Date ------------- -------- Halthore, Bruegge, Spanner AUG-04-89 Halthore, Bruegge AUG-06-89
The spectra were taken by Spanner every minute on August 4, by Bruegge about every minute on the August 4 and 6 August, and by Halthore every minute on the August 4 , and three measurements on the August 6. There are 10 spectra on August 4 between 1843 and 1852 GMT which coincide in time for all three instruments.
The SQL definition for this table is found in the OT_CALIB.TDF file located on the CD-ROM Volume 1.
Parameter/Variable Description Range Units Source
SITEGRID_ID This is a FIS grid location code. Site grid codes (SSEE-III) give the south (SS) and the east (EE) cell number in a 100 x 100 array of 200 m square cells. The last 3 characters (III) are an instrument
STATION_ID The station ID designating the location of the observations.
OBS_DATE The date of the observations, in the format (DD-MMM-YY).
OBS_TIME The time that the observation was [GMT] taken, in GMT. The format is HHMM.
INSTR_ID The code name for the instrument used to make the observations.
SURFACE_PRESS The surface pressure at the time [millibars] of the observation.
SURFACE_TEMP The surface temperature during [degrees the observation. Celsius]
SOLAR_ZEN_ANG The Solar Zenith Angle for this [degrees] observation.
WAVLEN The wavelength at which the [nm] observation was made.
OZONE_OPTCL_THICK The Ozone Optical Thickness of the atmosphere due to Ozone molecules in the air.
RAYLEIGH_OPTCL_THICK The Rayleigh Optical Thickness, caused by molecular scattering.
AEROSOL_OPTCL_THICK The Aerosol Optical Thickness, of colloidal particles suspended in the atmospheric column.
TOTAL_OPTCL_THICK The Total Optical Thickness, on a vertical path from surface to space.
MIE_CONSTANT_1 MIE_1 and MIE_2 are used to compute AEROSOL_OPTCL_THICK at arbitrary wavelengths.
MIE_CONSTANT_2 MIE_1 and MIE_2 are used to compute AEROSOL_OPTICAL_THICK at arbitrary wavelengths.
ABUNDANCE_OF_OZONE The column abundance of ozone, at standard temperature and pressure.
WEATHER A comment on the weather conditions at the time of the observation.
FIFE_DATA_CRTFCN_CODE The FIFE Certification Code for * the data, in the following format: CPI (Certified by PI), CPI-??? (CPI - questionable data).
LAST_REVISION_DATE data, in the format (DD-MMM-YY).
The primary certification codes are: EXM Example or Test data (not for release). PRE Preliminary (unchecked, use at your own risk). CPI Checked by Principal Investigator (reviewed for quality). CGR Checked by a group and reconciled (data comparisons and cross-checks).
The certification code modifiers are: PRE-NFP Preliminary - Not for publication, at the request of investigator. CPI-MRG PAMS data that are "merged" from two separate receiving stations to eliminate transmission errors. CPI-??? Investigator thinks data item may be questionable.
SITEGRID_ID STATION_ID OBS_DATE OBS_TIME INSTR_ID SURFACE_PRESS ----------- ---------- ---------- -------- -------- ------------- XMH2-SPH 417 04-AUG-89 1841 SMX-2 963.60 XMH2-SPH 417 04-AUG-89 1841 SMX-2 963.60 XMH2-SPH 417 04-AUG-89 1841 SMX-2 963.60 XMH2-SPH 417 04-AUG-89 1841 SMX-2 963.60 SURFACE_TEMP SOLAR_ZEN_ANG WAVLEN OZONE_OPTCL_THICK RAYLEIGH_OPTCL_THICK ------------ ------------- ------- ----------------- -------------------- 440.0 552.0 557.0 612.0 AEROSOL_OPTCL_THICK TOTAL_OPTCL_THICK MIE_CONSTANT_1 MIE_CONSTANT_2 ------------------- ----------------- -------------- -------------- .1580 .1340 .1170 .1020 ABUNDANCE_OF_OZONE WEATHER FIFE_DATA_CRTFCN_CODE ------------------ ------------------------ --------------------- 300.00 CPI 300.00 CPI 300.00 CPI 300.00 CPI LAST_REVISION_DATE ------------------ 14-JUL-93 14-JUL-93 14-JUL-93 14-JUL-93
This is point data acquired at a single location at the Manhattan airport. Data were collected on 2 days during the IFC. The data represent the optical thickness for distances of about 0.5 km from the instrument collecting the data.
A general description of data granularity as it applies to the IMS appears in the EOSDIS Glossary.
The CD-ROM file format consists of numerical and character fields of varying length separated by commas. The character fields are enclosed with a single apostrophe. There are no spaces between the fields. Each file begins with five header records. Header records contain the following information: Record 1 Name of this file, its table name, number of records in this file, path and name of the document that describes the data in this file, and name of principal investigator for these data. Record 2 Path and filename of the previous data set, and path and filename of the next data set. (Path and filenames for files that contain another set of data taken at the same site on the same day.) Record 3 Path and filename of the previous site, and path and filename of the next site. (Path and filenames for files of the same data set taken on the same day for the previous and next sites (sequentially numbered by SITEGRID_ID)). Record 4 Path and filename of the previous date, and path and filename of the next date. (Path and filenames for files of the same data set taken at the same site for the previous and next date.) Record 5 Column names for the data within the file, delimited by commas. Record 6 Data records begin.
Each field represents one of the attributes listed in the chart in the Data Characteristics Section and described in detail in the TDF file. These fields are in the same order as in the chart.
The voltage response of the instrument V to a radiance L at the ground can be written as a function of the exoatmospheric solar radiance response V_o through the Bouger's Law:where:
If changes in tauare negligible during an air mass change, such as during mornings at a mountain site, a plot of ln V versus m gives as the y-intercept the required exoatmospheric radiance response ln V_o. The latter is the well-known Langley plot method of calibration.
The aerosol optical thickness derived from a voltage measurement using the above equation is compared for each instrument. Of the three instruments, if data from two agreed and the other did not, the latter could be calibrated using:
where all the quantities on the right-hand side of the equation are known and tau is derived from the other instruments. This is the intercomparison method. If none of the instruments agreed, then, aerosol optical thickness derived from the most reliable instrument/calibration is used to calibrate the other two. In the data set presented here, the derived aerosol optical thickness values all agreed to within the uncertainties of the measurements (see the Confidence Level/Accuracy Judgment Section).
Morning data runs were used to produce Langley plots.
Errors can arise during measurement or calibration. During calibration, a requirement of the Langley plot method is that the atmospheric optical thickness remains constant during the period of maximum airmass change. At mountain sites, this requirement is usually met on clear days, but not always. The effect of varying atmospheric conditions is reflected in the quality of the Langley plots (see Halthore and Markham, 1992). The calibration coefficient, which is the y-intercept in Langley plots, can be obtained to a consistency better than 1% and we take this as the uncertainty in our measurements. A 1% uncertainty in V_o translates to an uncertainty in the aerosol optical thickness of 0.01 at airmass of 1. The equation is:
Uncertainties in the Rayleigh optical thickness and ozone optical thickness are negligible for the conditions encountered in FIFE.
For sunphotometers that do not employ constant temperature detectors, a major source of variation and hence uncertainty arises due to inadequate temperature compensation for the response. Furthermore, for sunphotometer that do not employ auto-tracking or peak hold features, another major source of uncertainty arises due to imperfect pointing. For measurements reported here, these are not expected to contribute to absolute errors in the measurements of aerosol optical thickness.
Not available at this revision.
All the data submitted to FIFE have an uncertainty of 1% in the calibration which translates to an absolute uncertainty in aerosol optical thickness of (0.01) per airmass.
FIS staff applied a general Quality Assessment (QA) procedure to these data to identify inconsistencies and problems for potential users. As a general procedure, the FIS QA consisted of examining the maximum, minimum, average, and standard deviation for each numerical field in the data table. An attempt was made to find an explanation for unexpected high or low values, values outside of the normal physical range for a variable, or standard deviations that appeared inconsistent with the mean. In some cases, histograms were examined to determine whether outliers were consistent with the shape of the data distribution.
The discrepancies, which were identified, are reported as problems in the Known Problems with the Data Section.
The data verification performed by the ORNL DAAC deals with the quality of the data format, media, and readability. The ORNL DAAC does not make an assessment of the quality of the data itself except during the course of performing other QA procedures as described below.
The FIFE data were transferred to the ORNL DAAC via CD-ROM. These CD-ROMs are distributed by the ORNL DAAC unmodified as a set or in individual volumes, as requested. In addition, the DAAC has incorporated each of the 98 FIFE tabular datasets from the CD-ROMs into its online data holdings. Incorporation of these data involved the following steps:
Each distinct type of data (i.e. "data set" on the CD-ROM), is accompanied by a documentation file (i.e., .doc file) and a data format/structure definition file (i.e., .tdf file). The data format files on the CD-ROM are Oracle SQL commands (e.g., "create table") that can be used to set up a relational database table structure. This file provides column/variable names, character/numeric type, length, and format, and labels/comments. These SQL commands were converted to SAS code and were used to create SAS data sets and subsequently to input data files directly from the CD-ROM into a SAS dataset. During this process, file names and directory paths were captured and metadata was extracted to the extent possible electronically. No files were found to be corrupted or unreadable during the conversion process.
Additional Quality Assurance procedures were performed as follows:
As errors are discovered in the online tabular data by investigators, users, or DAAC staff, corrections are made in cooperation with the principal investigators. These corrections are then distributed to users. CD-ROM data are corrected when re-mastering occurs for replenishment of CD-ROM stock.
Parameter Sitegrid Stn Date Instrmnt Description of problem --------- -------- --- -------- -------- --------------------------- SURFACE_PRESS XMH2-SPH 417 4-AUG-89 SMX-2 272 values have 0.96 SURFACE_TEMP XMH2-SPB 416 4-AUG-89 REAGAN Only 2 values, 40 & 28 SOLAR_ZEN_ANG C-130 Only records for C-130 OZONE_OPTCL_THICK XMH2-SPB 416 6-AUG-89 REAGAN 6 values greater than 0.041 AEROSOL_OPTCL_THICK XMH2-SPB 416 4-AUG-89 REAGAN 4 values greater than 0.55 6-AUG-89 REAGAN 6 values greater than 0.597 TOTAL_OPTCL_THICK 4-AUG-89 C-130 274 values greater than 1.4
The purpose of the data in this data set is to give an idea of the discrepancies between the values of different instruments when they are collocated in space and time. This hopefully will assist an investigator in assessing the quality of the measurements during the rest of the IFCs.
The Optical Thickness Calibration Data Set can be used to study the discrepancies between the aerosol optical thickness measurement values of different instruments when the instruments are collocated in space and time.
The FIFE field campaigns were held in 1987 and 1989 and there are no plans for new data collection. Field work continues near the FIFE site at the Long-Term Ecological Research (LTER) Network Konza research site (i.e., LTER continues to monitor the site). The FIFE investigators are continuing to analyze and model the data from the field campaigns to produce new data products.
Software to access the data set is available on the all volumes of the FIFE CD-ROM set. For a detailed description of the available software see the Software Description Document.
ORNL DAAC User Services
Oak Ridge National Laboratory
Telephone: (865) 241-3952
FAX: (865) 574-4665
ORNL Distributed Active Archive Center
Oak Ridge National Laboratory
Telephone: (865) 241-3952
FAX: (865) 574-4665
Users may place requests by telephone, electronic mail, or FAX. Data is also available via the World Wide Web at http://daac.ornl.gov.
FIFE data are available from the ORNL DAAC. Please contact the ORNL DAAC User Services Office for the most current information about these data.
Optical Thickness Calibration data are available on FIFE CD-ROM Volume 1. The CD-ROM filename is as follows:
Note: capital letters indicate fixed values that appear on the CD-ROM exactly as shown here, lowercase indicates characters (values) that change for each path and file.
The format used for the filenames is: ydddMULT.sfx, where y is the last digit of the year (e.g., 7 = 1987, and 9 = 1989), and ddd is the day of the year (e.g., 061 = sixty-first day in the year). The filename extension (.sfx), identifies the data set content for the file (see the Data Characteristics Section) and is equal to .OT for this data set.
Bruegge, C.J., R.N. Halthore, B.L. Markham, M. Spanner and R. Wrigley. 1992a. Aerosol optical depth retrievals over the Konza Prairie. J. Geophys. Res., 97:18743-18758.
Bruegge, C.J., J.E. Conel, R.O. Green, J.S. Margolis, G. Toon and R.G. Holm. 1992b. Water-vapor column abundance retrievals. J. Geophys. Res., 97:18759-18768.
Fraser, R.S., R.A. Ferrare, Y.J. Kaufman, S. Mattoo. 1989. Algorithm for atmospheric corrections of aircraft and satellite imagery. J. Int. Remote Sens., 13:541-557.
Halthore, R.N., and B.L. Markham. 1992. Overview of Atmospheric Correction and radiometric calibration efforts during FIFE. J. Geophys. Res., 97:18731-18742.
Halthore, R.N., B.L. Markham, R. Ferrare and O. Aro. 1992. Aerosol optical properties over the mid-continental United States. J. Geophys. Res., 97:18769-18778.
King, M.D., and D.M. Bryne. 1976. A method for inferring total ozone content from spectral variation of total optical depth obtained with a solar radiometer. J. Atmos. Sci., 33:2242-2251.
King, M., D. Bryne, B. Herman, and J. Reagan. 1978. Aerosol size distributions obtained by inversion of spectral optical depth measurements. J. Atmos. Sci., 35:2153-2167.
Shaw, G.E., J.A. Reagan, and B.M. Herman. 1973. Investigations of atmospheric extinction using direct solar radiation measurements made with a multiple wavelength radiometer. J. Appl. Meteorol., 12:374-380.
Contact the EOS Distributed Active Archive Center (DAAC) at Oak Ridge National Laboratory (ORNL), Oak Ridge, Tennessee (see the Data Center Identification Section). Documentation about using the archive and/or online access to the data at the ORNL DAAC is not available at this revision.
A general glossary for the DAAC is located at Glossary.
A general list of acronyms for the DAAC is available at Acronyms.
May 10, 1994 (citation revised on October 14, 2002).
Warning: This document has not been checked for technical or editorial accuracy by the FIFE Information Scientist. There may be inconsistencies with other documents, technical or editorial errors that were inadvertently introduced when the document was compiled or references to preliminary data that were not included on the final CD-ROM.
Previous versions of this document have been reviewed by the Principal Investigator, the person who transmitted the data to FIS, a FIS staff member, or a FIFE scientist generally familiar with the data.
April 17, 1996.
Bruegge, C. J., R. N. Halthore, B. L. Markham, and M. A. Spanner. 1994. Optical Thickness Cross-Calib[ration]. (FIFE). Data set. Available on-line [http://www.daac.ornl.gov] from Oak Ridge National Laboratory Distributed Active Archive Center, Oak Ridge, Tennessee, U.S.A. doi:10.3334/ORNLDAAC/60. Also published in D. E. Strebel, D. R. Landis, K. F. Huemmrich, and B. W. Meeson (eds.), Collected Data of the First ISLSCP Field Experiment, Vol. 1: Surface Observations and Non-Image Data Sets. CD-ROM. National Aeronautics and Space Administration, Goddard Space Flight Center, Greenbelt, Maryland, U.S.A. (available from http://www.daac.ornl.gov).