DATA SET DOCUMENTATION
1. Title
1.1 Field Sunphotometer Data
1.4 March 19, 1993
2. Investigator
2.1 Principal Investigator: Michael Spanner, NASA Ames Research Center;
Co-Investigators: Robert Wrigley, Rudolf Pueschel, Philip Russell and
John
Livingston, NASA Ames Research Center
2.2 Measurement of aerosol optical properties and atmospheric correction
of remotely sensed data acquired during Hapex-II/Sahel.
2.3 Contacts
2.3.1 Michael Spanner
Robert Wrigley
2.3.2 NASA Ames Research
Center, Mail Stop 242-4
Moffett Field, CA 94035, USA
NASA Ames Research
Center, Mail Stop 242-4
Moffett Field, CA 94035 USA
2.3.3 415-604-3620
415-604-6060
2.3.4 Spanner@gaia.arc.nasa.gov
Wrigley@gaia.arc.nasa.gov
2.3.5 415-604-4680
415-604-4680
2.4 Please acknowledge the NASA Ames Research Center investigation,
Michael Spanner Principal Investigator if these data are used or
referenced.
3. Introduction
3.1 The goal of this overall investigation was to measure aerosol optical
properties from both ground and aircraft based Sunphotometers during
the Hapex-II/Sahel experiment. These measurements are to be used to:
1) measure the magnitude and variability of the aerosol optical depth in
both time and space; 2) Determine the optical properties of the Sahelian
aerosols; and, 3) atmospherically correct some remotely sensed data
acquired during the Hapex-II/Sahel experiment. The data described in
this documentation are the field Sunphotometer data.
3.2 The phenomenon being measured is the atmospheric aerosol optical
depth. The parameters include total optical depth, aerosol optical depth,
airmass, atmospheric pressure and temperature.
3.3 We made measurements of atmospheric aerosol optical depths from
the field Sunphotometer during 7 days of the Hapex-II/Sahel experiment.
These days were September 1, 3, 6, 8, 10, 13 and 17. Measurements were
made throughout the day during periods when cloudiness was at a
minimum. We acquired data approximately every five minutes. These
data were collected to coincide as well as possible with aircraft and
satellite remote sensing measurements, in particular the NS001 TMS,
ASAS and SPOT. The overall quality of the data appear to be excellent.
The instrument was well calibrated both before and after the experiment
at Mauna Loa Observatory, Hawaii.
The field Sunphotometer data will be used in conjunction with the
Airborne Tracking Sunphotometer to determine the magnitude and
variability of the aerosol optical depth in both time and space. From
the
aerosol optical depth, we will invert the data using an algorithm
developed by King et al., 1978, to derive the size distribution of the
Sahelian aerosols. We will then use Mie theory to calculate the aerosol
phase function and single scattering albedo. Finally, we will use the
atmospheric correction algorithm of Wrigley et al., 1992 to
atmospherically
correct selected NS001 TMS, ASAS and SPOT data collected during the
Hapex-II/Sahel experiment.
Atmospheric correction of SPOT data will utilize the aerosol properties
derived from surface optical depth measurements. Atmospheric correction
of NS001 and ASAS data will utilize aerosol properties derived from the
airborne optical depth measurements as well as those from the surface
measurements.
4. Theory of Measurements: The instrument measures direct beam solar
radiation in 10 channels in the visible and near infrared wavelengths.
The solar radiation data are collected in the form of voltages. The
instrument was calibrated both before and after the experiment at the
Mauna Loa Observatory, Hawaii using the Langley plot technique. For
calibration, data are collected at a number of solar angles from low
solar
elevation (air mass=5) to high solar elevation angle (airmass = 1.8). A
regression is developed between log voltage and airmass. This regression
equation is then extrapolated to an airmass of 0. This value at zero is
called the zero airmass intercept voltage, and is the value that is used
for
calibration of the instrument.
The voltages measured by the instrument during Hapex-II/Sahel were
converted to total optical depth using the zero airmass intercept
voltages
calculated during the calibrations.
The Rayleigh optical depth is
subtracted from the total optical depth. Rayleigh optical depth is
calculated using pressure and temperature measured at the field site.
Finally NO2 and Ozone optical depth are subracted from the total minus
Rayleigh optical depth. NO2 abundance was obtained from climate tables
based on Noxon, 1979, and convolved with absorption coefficients at the
field Sunphotometer wavelengths. Ozone optical depth was calculated
using ozone abundances from the TOMS satellite instrument convolved
with absorption coefficients at field Sunphotometer wavelengths. The
result of this processing is the aerosol optical depth measured in 9
channels (not including the 940 nm water vapor channel) at
approximately 5 minute intervals.
5. Equipment
5.1 The instrument consists of a 10 channel filter wheel Sunphotometer
with a heater, an amplifier to measure voltages, a 12 volt power supply
and a tripod.
5.1.1 The instrument is mounted on a tripod on the ground.
5.1.2 The mission was undertaken to measure the aerosol optical depths
at the Hapex-II/Sahel field sites for the overall purpose of atmospheric
correction of remotely sensed data.
5.1.3 The primary quantity being measured is aerosol optical depth.
5.1.4 The instrument measures energy in the direct beam of the Sun.
From the calibrations developed before and after the experiment, these
voltages are converted to aerosol optical depth, which is a measure of
the
extinction of the direct Solar beam by aerosols and particulates in the
atmosphere.
5.1.5 The Field Sunphotometer has a 2.0 degree field of view and is
heated to 40 degrees C to maintain temperature stability. It has a
filter
wheel design with 10 filters. The nominal wavelengths and the full
width half maximum (FWHM) for the instrument are presented in the
following table.
Wavelength
nm
FWHM
nm
371.5
400
440
520
610
670
780
867
938
1029
10
10
10
12
11
10
10
10
11
10
The data are recorded manually from a LCD display on the external
instrument amplifier.
5.1.6 Manufactured by Dr. John Reagan, Department of Electrical and
Computer Engineering, University of Arizona, Tucson, Arizona, 602-6216203.
5.2 Calibration
5.2.1 Factors which may affect calibration are instrument variations that
may occur between calibrations. We have not noticed any significant
drifts in calibration during the time period of the experiment.
5.2.1.1 We believe that the aerosol optical depths are accurate to a
value
of .01 in aerosol optical depth.
5.2.2 The instrument was calibrated at Mauna Loa Observatory, Hawaii
on July 10-15 and October 27 to November 5, 1992.
5.2.3 The following table shows the calibration coefficients, corrected
for
Earth-Sun distance, calculated during calibration at Mauna Loa
Observatory, Hawaii, on July 10-15, October 27 to November 5, and the
mean values used for the Hapex-II/Sahel experiment in August and
September, 1992.
Wavelength
nm
July 10-15
October 27November 5
Mean for
Experiment
371.5
400
440
520
610
670
780
867
1029
49.622
280.957
711.623
1506.656
1691.857
1708.381
1778.811
1476.614
962.177
48.588
289.219
706.755
1508.178
1742.386
1762.495
1836.128
1519.870
980.237
49.105
285.088
709.188
1507.417
1717.122
1735.438
1807.470
1498.242
971.207
6. Procedure
6.1 Data are collected manually with the field Sunphotometer. The
instrument is pointed at the Sun using the Solar image in the cross
hairs.
As the filter wheel is turned through each of the ten channels, the
beginning time, instrument temperature, voltages and gain, and finally
the
ending time are recorded. This sequence takes approximately one minute.
This process is repeated approximately every 5 minutes for the course of
the data collection period, typically from 2-6 hours.
6.2 Spatial Characteristics
6.2.1 The field of view of the instrument is 2 degrees. The
Sunphotometer measures the direct beam of solar radiation.
6.2.2 See #6.2.1 above
6.3 Temporal Characteristics
6.3.1 Data were acquired during 7 days of the Intensive Observation
Period. The days, times and locations were:
Date
September
September
September
September
September
September
September
September
1, 1992
3, 1992
6, 1992
8, 1992
10, 1992
13, 1992
17, 1992
17, 1992
Time (GMT)
Site
06:53-09:41
13:17-17:04
10:38-14:42
13:14-16:06
08:30-10:30
10:14-15:46
08:10-10:31
12:54-15:31
WCS-fal
WCS-fal
WCS-fal
WCS-fal
WCS-fal
WCS-fal
SS-fal
SS-tig.
6.3.2 Data were acquired approximately every 5 minutes during the data
acquisition periods.
7. Observations
8. Data Description
8.2 Type of data
8.2.1
8.2.2
8.2.3
8.2.4
Variable
Source
Description
Range
Units
Date
Day/Month/
Year
September 1-17,
1992
N/A
N/A
Time
Hours Minutes.
Seconds
0-2400
GMT
Watch
Site
Super Site,
WCS-fal,
SS-fal, SS-corn
N/A
H2SIS
Instrument
Reagan1,
Reagan2
N/A
N/A
N/A
Solar
zenith
algorithm
N/A
7-78 degrees
Degrees
Solar
Airmass
N/A
1-4.5
Dimension
less
Solar
Rayleigh
N/A
Reagan
Tau (371.5 nm)
algorithm
.4836-4863
Dimension
Rayleigh
Reagan
.3495-.3514
algorithm
N/A
less
Dimension
8.2.5
Tau (400 nm)
algorithm
less
Rayleigh
Reagan
Tau (440 nm)
algorithm
N/A
Rayleigh
Reagan
Tau (520 nm)
algorithm
N/A.
Rayleigh
Reagan
Tau (610 nm)
algorithm
N/A
Rayleigh
Reagan
Tau (670 nm)
algorithm
N/A
Rayleigh
Reagan
Tau (780 nm)
algorithm
N/A
Rayleigh
Reagan
Tau (867 nm)
algorithm
N/A
Rayleigh
Reagan
Tau (938 nm)
algorithm
N/A
.2356-.2369
Dimension
less
1187-.1193
Dimension
less
.0619-.0622
Dimension
less
.0423-.0425
Dimension
less
.0229-.0230
Dimension
less
.0147-.0148
Dimension
less
.0108
Dimension
less
Rayleigh
N/A
Reagan
Tau (1029 nm)
algorithm
.0075
Aerosol
N/A
Reagan
Tau (371.5 nm)
algorithm
.2249-.7654
Aerosol
Reagan
Tau (400 nm)
algorithm
.2172-.7852
N/A
Dimension
less
Dimension
less
Dimension
less
Aerosol
Reagan
Tau (440 nm)
algorithm
N/A
Aerosol
Reagan
Tau (520 nm)
algorithm
N/A
Aerosol
Reagan
Tau (610 nm)
algorithm
N/A
Aerosol
Reagan
Tau (670 nm)
algorithm
N/A
Aerosol
Reagan
Tau (780 nm)
algorithm
N/A
Aerosol
Reagan
Tau (867 nm)
algorithm
N/A
Aerosol
Tau (938 nm)
N/A
.2323-.7993
Dimension
less
.2259-.7924
Dimension
less
.2137-7883
Dimension
less
.2096-.7957
Dimension
less
.2036-.8146
Dimension
less
.1855-.8018
Dimension
less
Aerosol
N/A
Reagan
Tau (1029 nm)
algorithm
Not Calculated
N/A
.1607-.7656
Dimension
N/A
less
8.3 Example of data entry: There are 2 records of header information
followed by 1 blank record in the data file provided to Hapex-II/Sahel
Information System.
01/09/92 653.00 WCS-fal Reagan1 72.8521 3.3453 0.4855 0.3508 0.2365
0.1191 0.0622 0.0230 0.0148 0.0108 0.0075 0.3829 0.3580 0.3509 0.3338
0.3145
0.3072 0.2970 0.2780 1.0057 0.2418
9. Data Manipulations
9.1 Formulas for processing data can be found in Spanner et al., 1990.
9.1.1 Description of algorithms can be found in 9.1.
9.2 Data Processing Sequence
9.2.1 The steps for processing are as follows: 1) Input data into
computer,
2) Run program developed by John Reagan's lab called Pdata7 to format
data, 3) Run program Atten7 to calculate all the variables including
solar
zenith angle, airmass, Rayleigh Optical depth and Instantaneous optical
depth (Total optical depth minus Rayleigh optical depth), 4) calculate
NO2 and ozone optical depths from Noxon et al., 1979 and TOMS data,
respectively, 5) subtact NO2 and ozone to derive aerosol optical depth.
The ozone abundance used to calculate ozone optical depth was 286
Dobson units, as determined from the TOMS satellite instrument. The
following table shows the values calculated for NO2 and ozone optical
depth which were subtracted from the Instantaneous optical depth to
derive the aerosol optical depth.
Wavelength
NO2 Tau
Ozone Tau
371.5
400
440
520
610
670
780
867
938
1029
0.0025
0.0028
0.0025
0.008
0.0001
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0009
0.0129
0.0343
0.0114
0.0003
0.0
0.0
0.0
9.2.2 The processing sequence has not changed over time
9.2.3 No special corrections or adjustments have been made
10.
Errors
10.1 Calibration errors would be the main source of error in the
derivation of aerosol optical depth.
10.2 Quality Assessment
10.2.1 Data were compared with Airborne Tracking Sunphotometer and
with data collected by R. Halthore of the GSFC ASAS Investigation (C.
Walthall, P.I.).
10.2.2 Overall, we feel that these data are of high quality. Two good
calibrations of the instrument were performed before and after the HapexII/Sahel field collection effort. The aerosol optical depths are
accurate to
a value of approximately 0.01.
10.2.3 Quantitative error estimates were not performed
11. Notes
11.1 The aerosol optical depth at 938 nm was not calculated because this
channel primarily measures absorption due to water vapor. We intend
to calibrate the 938 channel in the future, but at this time, the channel
is
not calibrated and therefore, the water vapor overburdens were not
calculated for the 938 channel. The values listed in the 938 channel
should be ignored.
11.2 The values of aerosol optical depth are accurate instantaneous
values
of aerosol optical depth. These data were taken every 5 minutes,
therefore, under conditions of rapid variability in cloudiness or haze,
the
data may not be internally consistent or appropriate. It is useful to
calculate averages of aerosol optical depth over periods of time, for
example one-half hour, to get a more accurate measure of the average
conditions at a site.
12. References
12.1 Portable Radiometer Data Reduction Manual for use with PDATA7
and ATTEN7, Ian C. Scott-Fleming, Department of Electrical and
Computer Engineering, University of Arizona, Remote Atmospheric
Sensing Lab, Tucson, Arizona, 85721, Contact: Dr. John Reagan 602-6216203.
12.2 Wrigley, R.C., M.A. Spanner, R.E. Slye, R.F. Pueschel and H.R.
Aggarwal, 1992, Atmospheric Correction of Remotely Sensed Image Data
by a Simplified Model, Journal of Geophysical Research, 97(D17):1879718814.
Bruegge, C.J., R.N. Halthore, B. Markham, M. Spanner and R. Wrigley,
1992, Aerosol Optical Depth Retrievals Over the Konza Prairie, Journal of
Geophysical Research, 97(D17):18743-18758.
Spanner, M., R. Wrigley, R. Pueschel, J. Livingston and D. Colburn, 1990,
Determination of atmospheric optical properties for the First ISLSCP
Field
Experiment (FIFE), Journal of Spacecraft and Rockets, 27:373-379.
Noxon, J., 1979, Stratospheric NO2, 2, Global behavior, J. Geophys. Res.,
84:5067-5076.
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.