Fishman and Larsen have proposed a new algorithm, called 'tropospheric residual method,' which retrieves the climatological tropospheric ozone by using SAGE (Stratospheric Aerosol and Gas Experiment) and TOMS (Total Ozone Mapping Spectrometer) data. In this paper, we will examine the feasibility of detection for tropospheric ozone using only TOMS data. From a case study over the Atlantic Ocean off the coast of west Africa, it has been found that total ozone in the archived TOMS data has been overestimated over a region of marine-stratocumulus clouds

Hudson, Robert D.

Kim, Jae-Hwan

English

NASA Technical Reports Server

N95- 10618
DIRECT MEASUREMENTS OF TROPOSPHERIC OZONE USING TOMS DATA
Robert D. Hudson and Jae-Hwan Kim
Department of Meteorology, University of Maryland
College Park, Maryland 20742, USA
ABSTRACT
Fishman and Larsen (1987) have proposed a
new algorithm, called "tropospheric residual method",
which retrieves the climatological tropospheric nznne
by using SAGE (Stratospheric Aerosol and Gas
Experiment) and TOMS (Total Ozone Mapping Spec-
trometer) data. In this paper, we will examine the
feasibility of detection for Iropospheric ozone using
only TOMS data. From acase study over the Atlantic
Ocean off coast of west Africa, it has been found that
total ozone in the archived TOMS data has been
overestimated over a region of marine-stratocumulus
clouds.
INTR OD U CTION
The TOMS total ozone data is derived from the
b,'ickscattered ultraviolet radiances measured by the
TOMS instrument on Nimbus-7 using an algorithm
thai was developed principally for measurements of
stratospheric variability. In 1986, Fishman et. al., used
IhQ.TOMS total ozone data to determine tropospheric
ozone fields in the tropics. They assumed that hor-
izontal stratospheric gradients in the tropics are small,
and used the TOMS relative total ozone field as a sur-
rogate for the tropospheric ozone field. In 1987, Fish-
man and Larsen, extended this method, by using the
SAGE profiles to determine the stratospheric ozone
field, and then subtracted this quantity from the
TOMS data. In this paper we examine the physics of
the TOMS algorithm as it applies to the measurement
of tropospheric ozone.
PRESENT ALGORITHM
A simplified schematic of the principle of the
TOMS measurement is shown in Figure 1. This sim-
ple case assumes that the atmosphere does not
scatter, and that the layer of ozone is confined to a
narrow strip in the stratosphere. If the reflectivit), of
the ground is R, then the measured albedo A is gwen
by:
A = R exp( -_f2s - 13As) (1)
where ¢t is the ozone absorption cross section, O. is
the column content of the ozone layer, 13 is the Ray-
leigh scattering coefficient for the atmosphere, and A
is the column content of the atmosphere, s, the path
length, is given by :
s = 1/cos(O) (2)
0 is given in Figure 1. If we can measure the
reflectivity, R, then the only unknown in equation (I)
is f2. In the TOMS algorithm, R is measured at
another wavelength where the absorption due to
ozone is negligible, the assumption being that in the
ultraviolet R varies only slowly with wavelength.
r ._' TOT/_L OZONK MEASUREMENT
\.I 2s Io_
___,•:,-;' %"T /_ , _. *_ :_'_" _ : _'_; ,_,_::__'r..',__,_r'_''
It... 4 - Fo. exp. (mecO.fl.ot)
X..t.ltit . - /g.,,.,,,0.11.exp(_Q.a)
" Fo. R. oxp- ((l÷IecO) .Q.a)
Fig. I. Schematic diagram that shows how TOMS
measures the backscattered radiation from
reflecting surface. Fo is the solar irradiance,
l_,ou,,a is the intensity reaching to ground, l,t is
the measured intensity by TOMS, f2 is total
ozone, and ot is the ozone absorption
coefficient.
Unfortunately for the simple method described
above, the atmosphere does scatter, and thus the
measured radiance at the instrument is the sum of the
reflected sunlight plus the scattered sunlight from the
atmosphere. However because the density of the
atmosphere increases exponentially as the altitude
decreases, ahnost all of the scattered radiation comes
from the troposphere. Thus, if the ozone were
confined to the stratosphere, as shown in the illustra-
tion, then one can derive an equation similar to (1),
where R is replaced with the effective reflectivity of
the ground plus the atmosphere. In reality, about 90%
of the column ozone is in the stratosphere.
119
https://ntrs.nasa.gov/search.jsp?R=19950004206 2020-06-16T11:28:25+00:00Z
1 le+
j-!
_ m
m
w
m
However it is the 10% in the troposphere which
is of interest to us now. In the illustrations above, thc
total radiance, both reflected and scattered, passes
through the entire ozone column, and thus the
efficiency of detection of the ozone column is 100%.
However if we now consider ozone in the tropo-
sphere, the outgoing scattered radiation from a partic-
ular altitude will not pass through any ozone below it,
thus the efficiency of detection for tropospheric ozone
will in general become less than 100%, and will get
worse the lower the altitude of the ozone layer. Figure
2 gives the results of calculations of this efficiency as
a function of the altitude of the ozone, and the
reflectivity of the surface. Note that the efficiency
becomes larger as the reflectivity of the surface
increases, i.e. as the reflected sunlight becomes larger
than the scattered sunlight. It should be noted that
the efficiency can become larger than 100%. This is
because multiple scattering at the lower boundary can
increase the effective path length, and hence enhance
the effect of the absorption due to ozone.
tr.c_--v o_01O_.e,,=eVAL
FO_ lmPIIEIK; INJECtiON
I : ..........
.............. ::I:1:1,'l'
_:If m= r'+_ .m _" D. =._ ....Ii
++ +Iie"= in I1+'-I I- t. I ++_1 tl- "_t
. i. , .. ,,.i ....
v-'+ ,'_"I "'I_ ,_<. I • iii m '+
, i .....
i.
, _,O Im 141_
"k,
m
...... "_ ,
.... • ...... "_,\
pl_ AL'mUI_., M8
m
"-'='-- REFL=O,I m nEFL-O.4 m, FIEFLwO.I -I_- ItlEFL=o,I "]
Fig. 2. The efficiency of detection for tropospheric
_. ozone as a function of the altitude and the
reflectivity of the surface (1000 rob).. X-axis is
the altitude in atmosphere, y-axts is the
retrieval efficiency (sensitivity) of tropospheric
ozone in percentage, and R is the reflectivity of
the surface. The values are calculated at zero
solar zenith angle, azimuthal angle, and scan
angle.
There is another factor however which effects
the derivation of tropospheric ozone, the height of the
rellecting boundary within the TOMS footprint. The
present algorithm assumes a cloud base if the derived
reflectivity is 0.6 or greater (snow and ice is discrim-
inated against by reference to other satellite data).
The lower boundary is then obtained from a climato-
logical table of mean cloud height versus latitude. If
the reflcctivity is less than 0.2, then the lower boun-
dary is obtained from a table of terrain height versus
latitude and longitude. If the reflectivity is between
0.2 and 0.6, then broken clouds are assumed, and the
lower boundary is obtained by interpolation between
the terrain height and the mean cloud height.
In general, of course, the cloud height is not at
the mean. If the real cloud height is below the clima-
tological mean, then the observed albedo will be
larger than the expected albedo, and the algorithm
will interpret this as additional ozone. The reverse is
true for a real cloud height greater than the climato-
logical mean.
R ESU LTS
Figure 3-a shows a plot of the total ozone and
Figure 3-b shows a plot of lower boundary rellectivity
as retrieved by TOMS (archived TOMS data) for a
region off the west coast of Africa, on October 14,
1989.
65
8S
lOS
12S
14S
6W 4W 2W 0 2£ 4_ 6Z
Fig. 3-a. Total ozonc from the archivcd TOMS data
on October 14, 1989. The unit is Dobson
(D.U) and the interval is 5 D.U. X-axis is lati-
tudc and y-axis is longitude.
(is
BS
lOS
12S
14S
6W 4W 2'd 0 21_ 41_ 6E
Fig. 3-b. Reflectivity from the archived TOMS data
on October 14, 1989. The unit is percentage
(%) and the interval is 10%. X-axis is latitude
and y-axis is longitude.
In the region near 8S and between 0 to 6E, and near
120
between 9S and 14S and 3W and 7W the total ozone
contours follow closely those of the reflectivity. This
is a region of marine-stratocumulus clouds. The
climatological cloud height is set at about 300 mb if
reflectivity is greater than 0.6. whereas the measured
height is about 810 rob. The algorithm is expected to
rctrievc higher ozone values over this region, and, for
the reasons discussed above, it is also expected that
the higher the reflectivity the greater the retrieved
value because of the effect of multi-scattering at the
reflecting boundary. If the cloud is at or near the
climatological height, and the contribution from scat-
tered radiation is small, hence little correlation is seen
between the measured total ozone and the reflectivity.
In general the regions over low reflectivity show lower
total ozone values.
The data for the area near 8S and 0 to 6E has
been reanalysed using the measured radiances, assum-
ing the following:
(a) The real cloud height
(b) That the measured reflectivities represent a
real change in cloud reflectivities (Satellite pictures do
not indicate broken clouds over this region)
(c) The standard altitude profiles used in the
TOMS retrievals.
The results of this analysis are shown in Figure
4. As can bc seen, the correlation between the
reflectivity and the total ozone field has been
removed. From a comparision between Fig 3-a and
Fig 4, we can see that total ozone in the archived
TOMS data has been overestimated over marine-
stratocum ulus cloud.
6s
8s
]0s
12S
14S
I.
.....(5i ..... ....".....: ....i ....;i ......... ..... ...
6H 4N 2W 0 2E 4R 6B
CONCLUSIONS
This preliminary analysis has shown that there
is indeed information in the TOMS data on levels of
tropospheric ozone. However. in the region of
clouds, great caution should be exercised in the use of
the archivcd total ozone data from TOMS.
ACKNOWLEDG EM ENTS
This research was supported by grant number
NAGW-2696 from the National Aeronautics and
Space Administration.
REFERENCES
Dave, J.V., Meaning of successive iteration of the
auxiliary equation in the theory of radiative
transfer, Astrophys. J., 140, 1292-1303, 1964.
Dave, J.V., Effect of aerosols on the estimation of
total ozone in an atmospheric column from the
measurement of its ultraviolet radiance, J. Almos.
Sci., 35, 899-911, 1978.
Dave, J.V., and C.L. Mateer, A preliminary study on
the possibility of estimating total atmospheric ozone
from satellite measurements, J. Atmos. Sci., 24,
414-427, 1967.
Fishman, J., K. Fakhruzzaman, B. Cros, and D.
Nganga, Identification of widespread pollution in
the Southern Hemisphere deduced from satellite
analyses, Science, 252, 1693-1696, 1991.
Fishman, 2., P. Minis, and H.G. Reichle, Jr., The use
of satellite data to study tropospheric ozone in the
tropics,./. Geophys. Res., 91 14451-14465, 1986
Fishman, J., and J.C. Larsen, Distribution of total
ozone and stratokpheric ozone in the tropics: impli-
cation for the distribution of tropospheric ozone, ./.
Geophys. R e$., 91, 14451-14465, 1987.
Fishman, .I., C.E. Watson, 1.C. Larsen, and J.A.
Logan, The distribution of tro, j_ospheric ozone
determined from satellite data, J. oeophys. Res., 95,
3599-3618, 1990.
Mateer, C.L., D.F. Heath, and A.J. Krueger, Estima-
tion of total ozone from satellite measurements of
backscattered ultraviolet earth radiance, J. Atmos.
Sci., 28, 1307-1311, 1971.
Fig. 4. Corrected total ozone over cloud whose
reflectivity is greater than 45% on October 14,
1989. The unit is D.U and the interval is 5
D.U. X-axis is latitude and y-axis is longitude.
Perhaps what is more important, is that the apparent
high ozone levels shown in Figure 3-a, which would
be interpreted as high tropospheric levels if one sub-
tracts a uniform stratospheric level, are removed.
121


Direct measurements of tropospheric ozone using TOMS data

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