An evaluation of the optical effects of stratospheric aerosol layers on total ozone retrieval from space by the TOMS/SBUV type instruments is presented here. Using the Dave radiative transfer model we estimate the magnitude of the errors in the retrieved ozone when polar stratospheric clouds (PSC's) or volcanic aerosol layers interfere with the measurements. The largest errors are produced by optically thick water ice PSC's. Results of simulation experiments on the effect of the Pinatubo aerosol cloud on the Nimbus-7 and Meteor-3 TOMS products are presented

Ahmad, Zia

Bhartia, P. K.

Herman, J. R.

Mcpeters, R.

Pan, L.

Torres, O.

English

NASA Technical Reports Server

N95- 11121
Abstract
An evaluation of the optical effects of stratospheric
aerosol layers on total ozone retrieval from space
by the TOMS/SBUV type instruments is presented
here. Using the Dave radiative transfer model we
estimate the magnitude of the errors in the
retrieved ozone when polar stratospheric clouds
(PSCs) or volcanic aerosol layers interfere with the
measurements. The largest errors are produced by
optically thick water ice PSCs. Results of simulation
experiments on the effect of the Pinatubo aerosol
cloud on the Nimbus7 and Meteor3 TOMS products
are presented.
Introduction
EFFECT OF STRATOSPHERIC AEROSOL LAYERS ON THE
TOMS/SBUV OZONE RETRIEVAL
O. Torres, Z. Ahmad, L. Pan
Hughes STX Corporation
4400 Forbes BIvd, Lanham, Md, 20776
J.R. Herman, P.K. Bhartia, R. McPeters
NASA Goddard Space Flight Center
Greenbelt, Md, 20771
microphysical properties and on the geometry of
the viewing conditions. In this paper we summarize
the results of simulation experiments using the
Dave aerosol radiative transfer model to quantify
the artifact introduced in the TOMS retrieval in the
presence of stratospheric aerosol layers.
Optically thick stratospheric aerosol layers have
important effects on the measurements of the
properties of the earth's atmosphere. In particular,
they affect satellite borne sensors that derive
atmospheric ozone content based on the
measurement of the backscattered ultraviolet (buv)
radiation. This kind of instrumentation includes total
ozone content mappers (Nimbus-7 TOMS, Meteor-3
TOMS) as well as instruments retrieving both total
ozone and its vertical distribution (Nimbus-7 SBUV,
NOAA9 and NOAA11 SBUV/2).
A non-absorbing stratospheric aerosol layer
backscatters much of the incoming UV radiation
before it reaches the ozone underneath the aerosol
cloud, and, therefore, an error is introduced in the
retrieved ozone. The size and direction of the error
depend on the aerosol layer altitude, its optical and
TOMS Total Ozone Algorithm
To compute total ozone, radiance measurements at
a pair of wavelengths, an ozone-sensitive
wavelength X, and a longer wavelength, relatively
insensitive to ozone, X', are paired together. A
quantity called Npa+,,defined as
=-m0Iog,0[ ]
is computed. In the above expression I and F are
respectively the measured radiance and irradiance
at the top of the atmosphere.
Three wavelength pairs are used: .3125H and
.3312/.+(A-pair); .3175/J and .3398/.1 (B-pair); .3312/J
and .3398H (C-pair).
The NA, Na, and Nc quantities are used to compute
the total ozone amounts aA, as and _c, by an
interpolation procedure from a set of lookup tables.
These three ozone amounts are weighted and
linearly combined to produce the final total ozone
amount _.
Aerosol Effects
There are mainly two kinds of stratospheric
aerosols that may affect the TOMS/SBUV ozone
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retrieval: nitric acid trihydrate (NAT) and water ice
polar stratospheric clouds (PSCs), and sulfuric acid
aerosol layers formed in the aftermath of strong
volcanic eruptions.
To estimate the magnitude of the errors introduced
by stratospheric aerosol layers, simulation
experiments were performed using the Dave
radiative transfer model (multiple scattering with
aerosols). Ozone and aerosol content profiles are
prescribed, and the emerging UV radiation at the
top of the atmosphere is computed. The obtained
radiances are used to retrieve the ozone content as
in the SBUV_TOMS algorithm. The difference
between the retrieved and the input ozone is the
error introduced by the intervening aerosol layer.
Errors introduced by PSCs
A detailed analysis of the PSC effects on the TOMS
retrieval is presented in Torres et. al. (1992). PSCs
effects were modeled for a high latitude ozone
profile (275 D.U.) with peak concentration
at 17 km for nadir view condition. Figure 1
illustrates the error introduced as a function of solar
zenith angle for PSCs of different optical
0
-10
Z
-20
'_ -30
-40
-50
O
-70
EF'_CTCFP'_ ONIUIE'_ OZON[
X----------._+_____ o.o4
I"YP[ I |AU=O.O1
PfP[ I lk,U':0.04
.-,N-
TYR" I TAU=0,¢0
_JOi OZO_ III[YJL
O.CUIIIE_: 14rod(IG010ZN _)
-8O 6s 7_ 7'5 8_ 8_
SOLAR Z[I, ITH AN(;L[ (_G)
Figure 1. Error introduced in the TOMS ozone
retrieval by PSCs below the ozone peak,
9O
thicknesses (tau), when the cloud is situated at 14
km, below the level of maximum ozone. The largest
error (30 DU) is produced by the thickest cloud at
the highest solar zenith angle.
0 _ul
a,,= uo_: 20r, 1_=== n_() I \
• -B-
N -60 _ i Tk_0.04
- \-70 r_ i IA_.¢0
I.¢1
-80 ....65 7; 7_ _ _ 90
SOLAR Z[HITH ANO.[ ([}[G)
Figure 2. As in fig. 1 for PSCs above the level of
maximum ozone concentration.
As shown in figure 2, much larger errors are
produced by PSCs located above the ozone
maximum (20 km). For the modeled conditions, a
water ice PSC of optical depth 0.4 produces an
ozone underestimation of 75 D.U. when the solar
zenith angle is 88 degrees. Some of these ozone
under-estimations appear in the TOMS maps as
areas of reduced ozone, commonly referred to as
ozone mini.holes. Not all of the TOMS retrieved
ozone mini-holes, however, are artifacts created by
PSCs. On occasions, mini-holes are largely real
ozone depressions resulting from dynamical
processes in the lower stratosphere (Rood et. al.,
1992).
Volcanic Aerosol Effects
An across-scan bias characterized by peak ozone
values near the ends of the scan and minimum
values around the nadir, is commonly observed in
the nimbus 7 TOMS ozone retrieval in the aftermath
of strong volcanic eruptions as in the cases of El
Chichon (1982) and Pinatubo (1991). The effect of
the Pinatubo volcanic cloud was simulated using as
916
input a 275 D.U. low latitude ozone profile (peak at
26 km). The aerosol cloud was set at 26 km
consistent with lidar profiles over Mauna Loa
(Defoor et, al 1992) and over Sao Jose Dos
Campos, Brazil (Clemesha and Simonich, 1992). A
Iognormal particle size distribution was assumed.
Figure 3 shows a comparison of the observed and
the modeled across-scan bias for an aerosol cloud
of optical thickness 0.2 (at 360 nm).
8-
G
N
O
-4
-6
-Nr=-V=d_MI _ N_- T014 W
272
270
'268
Z66 :_
264
262
260 _
25,8
t.--
_ Till ,[ l[ ,, ITITII,IIIIIIlIII' ''''rllllll '' '' '''' #' II [] TT fIfT If 2_6
-64-56-48-40-32-24-16-8 0 8 16 Z4 32 40 48 56 64
SATELLITE ZENITH ANGLE (DEG)
Figure 3. Observed and simulated Nimbus7
TOMS across-scan bias for an aerosol layer of
optical thickness 0.2 at 26 km.
Best results are obtained for a mean radius of 0.4
microns and standard deviation of 1.5. The TOMS
measurement is the average scan for a ten degree
wide latitude belt centered at the equator during
the 9/10-9/16 1991 time period. Absolute errors of
1% to 2% in the retrieved ozone are introduced by
the aerosol cloud. The structure observed around
the nadir position is related to the aerosol
scattering phase function.
The Meteor3/TOMS instrument measured an
anomalous effective reflectivity during the first few
months after its launch in August 15 (Pan et. al.,
1992) Simulation experiments indicate that a
combination of high solar and satellite zenith
angles and relative azimuth angle in the vicinity of
0 and 180 degrees when an optically thick volcanic
Iz0[
m
-6(] ...... , , .... ,
-70 40 -50 -40 -30 -20 -to 0 to 20 30 40 50 60 70
$ATELUTE ZENITH ANGLE (DFG.)
Figure4.Observed and modeled Meteor3TOMS
across-scanreflectivitybiasfora volcaniccloud
ofopticalthickness0.2 at26 kin.
cloud is present, produces an effective reflectivity
field similar to the observed by the Meteor3/TOMS
sensor, characterized by extremely high values at
one end of the scan and extremely low, even
negative, values at the other end. Figure 4 displays
a comparison of the Meteor3 TOMS retrieved
reflectivity for a scan near the equator on
September 26 1991, to the results of the
simulatedreflectivity when an optically thick aerosol
layer interferes with the measurements. The
magnitude of the error of the Meteor3/TOMS total
ozone retrieval is being investigated.
References
Clemesha, B.R. and D.M. Simonich, Comparison
between the El Chichon and Pinatubo aerosol
clouds seen at 23° S., AGU Chapman Conference
on Climate, Volcanism and Global Change, March
23-27, 1992, Hilo, Hawaii.
917
DeFoor T., E. Robinson and S. Ryan, Early Lidar
observations of the June 1991 Pinatubo eruption
plume at Mauna Loa observatory, Hawaii, Geophys.
Res. Lett., 19, 187-190, 1992
Pan L., C. Wellemeyer, C. Seftor, Z. Ahmad, D.
Larko, C. Scott, J. Herman, P.K. Bhartia and A.
Krueger, Study of effective surface reflectivity
derived from Meteor3/TOMS measurement, AGU,
1992 Spring Meeting, Montreal, Canada
Rood, R.B., J.E. Nielsen, R.S. Stolarski, A. R.
Douglass, J. A. Kaye and D.J. Allen, Episodic total
ozone minima and associated effects on
heterogeneous chemistry and lower stratospheric
transport, J. Geophys. Res., 97, 7979-7996, 1992.
Torres O., Z Ahmad and J.R. Herman, Optical
Effects of Polar Stratospheric Clouds on the
Retrieval of TOMS total ozone, J. Geophys. Res.,
97, 13015-13024, 1992.
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Effect of stratospheric aerosol layers on the TOMS/SBUV ozone retrieval

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