This investigation is designed to demonstrate the feasibility of determining the aerosol content in the atmosphere from contrast measurements of ground features, and from radiance measurements. Theoretical relationships between aerosol content and contrast reduction and radiance have been derived for ideal model atmospheres. The preliminary data analysis of the MSS transparencies has shown promising results for the contrast-aerosol content relationship in the Salton Sea/desert region

Griggs, M.

English

NASA Technical Reports Server

Paper I 2 
DETERMINATION OF AEROSOL CONTENT IN THE ATMOSPHERE 
Michael Griggs, Science Applications, lnc., P. 0. Box 2351, La Jolla, California 92037 
ABSTRACT 
The objective of this investigation is to demonstrate 
the feasibility of determining the aerosol content in the 
atmosphere from contrast measurements of ground 
features, and from radiance measurements. Theoretical 
relationships between aerosol content and contrast 
reduction and radiance have been derived for ideal model 
atmospheres. The preliminaz'y data analysis of the MSS 
transparencies has shown promising results for the 
contrast-aerosol content relationship in the Salton Sea/ 
desert region. 
1. INTRODUCTION 
The apparent increase of particles in the atmosphere is 
believed to affect the climate of the earth, and has led to the desire 
to monitor atmospheric aerosols on a global basis. The ERTS-1 
data present an opportunity to investigate two possible methods of 
measuring the aerosol content from satellite observations of contrast 
reduction and of radiance. Demonstration of the feasibility of these 
techniques will lead to routine satellite monitoring of the atmospheric 
aerosol content to provide information important in predicting the 
effect of man's activities on climate. In addition, it will provide input 
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parameters to atmospheric scattering models in algorithms being used 
to investigate surface properties of the earth's surface from space 
observations. 
2. THEORY OF CONTRAST REDUCTION 
Duntley3 showed that the contrast reduction by the atmosphere 
of an object relative to the background is given by 
where 
CR is the apparent contrast, 
Co is the inherent contrast, 
/u B' is the "sky/ground ratio", and Ba(o) o o 
T is the optical depth of the scattering atmosphere. 
To further investigate this relationship, a perfectly arbitrary 
generalization of Eq. 1 is made: 
cR - 1 c - mj 
0 
From values published by Plass and Kattawar4 for reflected radiance 
versus albedo, it is concluded that the reflected radiance is well repre- 
sented by a linear function of A: 
1106 
The apparent contrast is, consequently: 
Rrefl (7,  A) - Rrefl(T, A') - 
' R -  Rriefl" 9 A') 
Since 
A - A '  
co = T 
we have 
By identifying with Eq. 2 we find that the unspecified function g(r, A) 
may be separated into 7 - and A-dependent parts: 
where 
Values of f(7) using Eq. 8 have been cdculated for the center wave- 
lengths of the four MSS channels, based on interpolation of values of 
R(T, 0) and R(T, 1) calculated by Plass and K a t t a ~ a r , ~ ~  5* for the 
wavelengths 0.4 pm, 0.7 pm, 0.9 pm and 1.67 prn. The results, for 
three sun angles, for MSS 6 are shown in Fig, 1. The function f(7) 
is related to the experimentally observable quantities Co, CR and 
A' by Eq. 2 and 7: 
f(7) = A'(Co/CR - 1) . (9) 
1107 
Thus, a measurement of the contrast ratio and knowledge of albedo and 
sun angle yields a value of f(7) that determines 
3. RELATIONSHIP OF RADIANCE Ap3D AEROSOL CONTENT 
Ca3culations of the radiance backscattered from the earth- 
atmosphere system, as seen from space have been published by Plass 
and Kattawar. 4 7  These calculations, using Monte Carlo techniques, 
consider multiple scattering of dl1 orders, and take into account aerosol 
scattering and ozone absorption. Examination of the results of Plass 
and Kattawar shows that the outgoing radiance varies with aerosol 
content, and is most sensitive when the underlying surface albedo is 
low. The ocean, which covers much of the earth, has a low albedo at 
high sun angles and provides a suitable Underlying surface for aerosol 
measurements. The calculations also indicate that the longer wave- 
lengths (2 0.7 pm) are more sensitive to aerosol changes. At shorter 
wavelengths the Rayleigh optical thickness is comparable to, or greater 
than, the aerosol optical thickness, so that changes in the aerosol 
content have less effect. 
In order to further investigate the effect of aerosols on the 
upward radiance over a calm water surface, Plass and Kattawar6 
made some cdculations for us for several aerosol vertical distributions, 
and showed a result of great importance for satellite observations of 
the upward radiance: the upward radiance depends strongly on the 
total number of aerosols, but not on their vertical distributions. Thus 
measurements of the upward radiance can be directly related to the 
total vertical aerosol content and hence the global loading. 
The radiance in the MSS channel 6, centered at 0.75 pm, 
may be computed by interpolation of the results of Plass and Kattawar 
at wavelengths 0.4 pm, 0. 7 pm, 0.9 pm and 1.67 pm, and by assuming 
a rectangular spectral response 0.1 pm wide. The results are plotted 
in Fig. 2 to show the relationship between the upward radiance and the 
aerosol content of the atmosphere for various sun angles. A simple 
linear relationship is shown to exist between radiance and the aerosol 
content. These straight lines are based on only two values of aerosol 
content, but a linear relationship may be established by considering 
the four data points for zero albedo. From these curves a knowledge 
of the sun angle and the absolute radiance allows the aerosol content 
of the atmosphere to be determined. It is seen that a 1 percent change 
in radiance is equivalent to about 1.5 percent change in aerosol content. 
1108 
4. PROBLEMAREAS 
The discussions in Sections 2 and 3 are based on theoretical 
calculations which use a model atmosphere, a model aerosol distribution, 
and assume a smooth water surface or a Lambertian surface. In prac- 
tice, of course, these model conditions are never realized, so that 
deviations from the theoretical relationships are to be expected. Hence 
empirical relationships between the satellite data and the aerosol content 
will be investigated. However, the problem of sun glitter on the ocean 
still remains. 
If the ocean were perfectly smooth, as assumed in the calcu- 
lations, an image of the sun would be seen at the specular reflection 
angle, and the only upwelling surface radiation observable at other look 
angles from space would be the diffuse sky radiation reflected from the 
ocean surface and the radiation scattered up from below the ocean 
surface. As the smooth ocean surface is increasingly disturbed, a 
glitter pattern becomes increasingly larger about the specular point, 
At sun zenith angles greater than about 30" the glitter effect has been 
considered negligible (except for very rough seas) at the nadir point. 
However, recent measurements by Hovis (private communication from 
R. Fraser) suggest thak this assumption is not correct, so that the 
ocean surface radiance at the nadir is not known accurately. 
This problem might be overcome by making observations at 
two wavelengths, assuming that the spectral variation of the surface 
radiance is known. The choice of wavelengths must be made carefully 
since the spectral distribution of the radiance does vary due to ocean 
properties such as chlorophyll content, suspended matter and depth. 
If the ratio of reflectivities at two wavelengths remains constant for 
various surface conditions and sun angles, then the sun glitter problem 
can be eliminated. 
5, PRELIMINARY RESULTS 
Two test sites have been used for this investigation. One is 
the Salton Seddesert region in the Borrego Desert in Southern California, 
and the other includes San Diego city and the ocean. Measurements of the 
vertical aerosol optical thickness have been made with a Volz sun photo- 
meter at the times of the satellite overpass when weather conditions 
permitted. The results of these measurements given, in Fig, 3, show thak 
on all these occasions the aerosol content was lower than the Elterman 
1964 model value.' 
1109 
The ERTS-1 data received to date for these test sites have 
been only in the photographic format, and have been analyzed with a 
densitometer. Digital data have been requested and will be analyzed 
on a CDC 6400 computer. 
The preliminary analysis of the transparency data received 
to date has initially shown promising results for the contrast-aerosol 
content relationship in the Salton Seddesert region. The measured 
transparency densities were converted to radiances using the grey 
scale and preflight calibration data. The radiance of the water across 
the Salton Sea was found to be almost constant on a given frame, but, 
as expected, the desert surface was quite variable in radiance. The 
apparent contrast, CR, was computed using the brightest portion of 
the adjacent desert, The inherent contrast, Co, was calculated by 
assuming a reflectivity of ,30 for the desert and .02 for the Salton Sea 
(based on albedos measured by Griggs ), giviw a value of 14.0 for 
C . For this preliminary examination, the same value was assumed 
fo'E all sets of data, neglecting the variation with wavelength and sun 
angle. Values of f(T) were calculated from Eq. 9, and values of T 
determined from plots of f(7) vs T for MSS channels 4, 5 and 6 (e.g. 
see Fig. 1). MSS 7 is not considered due to the influence of water 
vapor in this channel. These values of T were converted to aerosol 
content assuming that T is proportional to the number of aeroS01s.~ 
The values of aerosol content in terms of N (Elterman's 1964 model 
value), obtained in our initial analysis, are given in Table 1; The 
results, except for one, are reasonable in terms of possible aerosol 
content, although no ground-truth data are avajlable for comparison. 
The results for 10-1-72 show excellent agreement among the different 
channels. 
MSS 
Channel 
TABLE 1 
Aerosol Content I 
4 5 6 .  
8 -26 -72 
10-1-72 
11-6 -72 
2.5N 1.7N 
.9N .8N 1. ON 
1.8N .07N* 1. 1N 
* Low value attributed to calibration or processing 
problems with this frame. 
1110 
Analysis of subsequent photographic data has not been so 
successful due to an apparent calibration problem, at least at the low 
radiance end of the grey scale. The density of the transparencies for 
the water surfaces w a s  found to be greater than that of the blackest 
step of the grey scale, which corresponds to zero radiance. Due to 
this problem, further analysis of the contrast -aerosol content relation- 
ship has been delayed pending the receipt of the digital data which 
provide more accurate radiance measurements. These tapes are also 
necessary to investigate the radiance-aerosol content relationship. 
6. CONCLUSIONS 
The preliminary analysis of the transparency data has shown 
promising results for the contrast -aerosol content relationship. Further 
analyses with the digital data, and for more satellite overpasses, are 
required to clearly demonstrate the feasibility of measuring the atmo- 
spheric aerosol content from ERTS measurements. 
1. 
2. 
3. 
4. 
5. 
6. 
7. 
8. 
7. REFERENCES 
"Man's Impact on the Global Environment", MIT Press, 
"Remote Measurement of Pollution", NASA SP-285, 1971. 
S. Q. Duntley, J. Opt. SOC. Am. 28, 179 (1948). 
G. N. Plass and G. W. Kattawar, Appl. Opt. 7, 1129 (1968). 
G. N. Plass and G. W. Kattawar, Appl. Opt. 9, 1122 (1970). 
G. N. Plass and G. W. Kattawar, Appl. Opt. 11, 1598 (1972). 
L. Elterman, AFCRL Report No. AFCRL-64-740 (1964). 
M. Griggs, J. Appl. Meteor. 7, 1012 (1968). 
Cambridge, Mass., 1970. 
1111 
3 
2 
3 
4 
4 
3 
3 . 
4 
4 
0 
0 
2 
0 
0 
a; 
0 
0 
1112 
. 01 
p =  .25 
p =  .45 
p =  1.0 
1 I I I I 1 1 1 1 1  1 I L 1 1 1 1 1 1  
. l  1.0 
T 
Fig. 2 f ( T )  YS T for three sun zenith angles, 6, ( p  = cos 0) 
0.3 
- 
h 
f! 
E 
Q c 
N 
0.2 - 
0)  
P 
A = 0.7-0.8 pm (MS86) 
ocean 
0.1 
0 N SN 
Aerosol ConetMt (N = Elterman 1864 Model) 
Fig. 3 Radiance vs. Aerosol Content for MSS6 
1113 


Determination of aerosol content in the atmosphere

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