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DIRECT MEASUREMENTS OF TROPOSPHERIC OZONE 

FROM TOMS DATA

NAGW - 2696

PROGRESS REPORT 

Principle Investigator - Robert D. Hudson

January 1, 1993 

(NASA-CS-191884) DIRECT N93-16645 

MEASUREMENTS OF TROPOSPHERIC OZONE 

FROM TOMS DATA Progress Report 

(Maryland Univ.) 12 p Unclas 

G3/45 0141139
I 

Introduction Nifl the past year we have made measurements of the tropospheric 

total column of ozone during the biomass burning season in Africa. 

(August to October) .Fishman et. al. had reported previously that 

by taking a time average of the low spatial resolution data from 

TONS (Total Ozone Mapping Spectrometer) on Nlmbus-7 (referred to as 

the Grid-T data set), that during the Biomass burning season in 

Africa (August to October), a plume of ozone extends from the East 

coast of Africa into the Atlantic. In this report we present an 

analysis that we have made using the measured TOMS radiances taken 

from the High Density TOMS data set (referred as the HDT data set), 

which examines this plume in more detail. 

Analysis technique 

We have re-analyzed the measured radiances for two specific 

cases, (1) those measured above the marine stratocumulus cloud 

bank that is a persistent feature off the East coast of Africa 

during the period of biomass burning, (2) those measured above noncloudy regions over the African continent 

Figure la shows a plot of the total ozone taken from the 

archived HDT data as a function of latitude and longitude near the 

east coast of Africa for October 15 1989. Figure lb is a plot over 

the same region of the measured surface reflectivity from the same 

data set. There is an obvious correlation between the two 

quantities, especially above the cloud base. There are three 

factors that account for this correlation. The first is that the 

cloud height assumed in the TONS algorithm is a mean climatological 

height . Figure 2 shows a plot of this climatological cloud height 

versus latitude. At the latitudes shown in Figure 1 this mean 

height is near 300 mb, whereas the measured height of the marine 

stratocumulus cloud is about 800 mb. In Figure 3 we have 

calculated the effect of this assumption on the retrieved total 

ozone. It will cause the archived results over the cloud to be too 

high.

The second factor is more subtle. In general TONS tends to 

underestimate the amount of tropospheric ozone over and above the 

climatological amount assumed in the standard profiles which are 

used to generate the look-up tables for the algorithm. This is 

especially true over regions of low reflectivity. This 

underestimate arises because the scattered radiance observed by the 

instrument is the sum of the scattered radiance from the atmosphere 

and the reflected radiance from the surface. For low reflectivity 

surfaces, e.g the earth's surface or over the ocean, the 

contribution of the scattered radiance dominates. If the additional 

tropospheric ozone is distributed at low altitudes then some of the 

measured radiance will not have passed through the total column of 

ozone and the algorithm will underestimate the additional ozone. 

However, over high reflectivity clouds the reflected radiance 

dominates over the scattered radiance, and the derived total ozone 

amount is close to the actual amount. We say close, because in 

general, the algorithm will tend to overestimate the column amount 

due to multiple scattering just above the cloud.. 

The third factor is not as universal as the other two. The
present TOMS algorithm assumes that any surface that has a measured 

reflectivity between 0.2 and 0.6, is the result of partial cloud 

cover. The algorithm uses a formula that expresses this 

reflectivity as an actual reflectivity of 0.6 but assumes a lower 

cloud altitude, i.e. in the case that we have chosen, at a cloud 

height corresponding to a higher pressure that 300 nib. It then uses 

this cloud height in the look-up table procedure. However when we 

examine the visible cloud images for this day we cannot detect 

broken clouds, and it is our conclusion that clouds can have 

reflectivities down to at least 0.4. 

Figure lc shows the results of an analysis of the TOMS 

radiances for the same region as in Figure !a, for that data taken 

above the cloud base. We have limited the analysis to 

ref lectivities above 0.4, and have assumed that the cloud has a 

solid base. We also used a look-up table approach, but the tables 

used were generated for the particular measured cloud height. The 

resultant total ozone map differs substantially from that in the 

archives, but shows a latitude dependence which is similar to the 

archived total ozone for the same day for a region over the Indian 

Ocean, where no clouds were observed. From this similarity we 

conclude that the new analysis of the data appears to give a more 

realistic result. 

In order to derive the tropospheric total ozone amount, we now 

have to subtract the amount of ozone in the stratosphere. We have 

used two methods to do this. In the first we used the fact that the 

results over the Indian Ocean were similar in general structure to 

those over the cloud base. If we make the assumption that any ozone 

resulting from the biomass burning will travel East, then we can 

argue that the total ozone measured above the Indian Ocean, will be 

a surrogate for the stratosphere ozone amount if we take into 

account the small amount of climatological tropospheric ozone. In 

essence we are assuming that the stratospheric ozone is constant 

along any given latitude band. 

In the second method, we assumed that the variation of total 

ozone along a latitude band was due only to the displacement of the 

tropopause. The variation of the tropopause height was obtained 

from the NMC analysis, and the relation between the tropopause 

height and total ozone was obtained from the data over the Indian 

Ocean. 

The next region we examined was that over the African 

continent. In this case we did not have accurate cloud height data, 

and there was some evidence that many of the clouds were at heights 

above 500 nib, i.e. above the ozone from the biomass burning. We 

therefore decided to examine data only from cloud free regions 

(reflectivities less than 0.2). As discussed earlier, for data 

obtained over regions of low reflectivity, the TOMS algorithm will 

underestimate tropospheric ozone over and above the climatological 

value used in the tables. Figure 4 shows the efficiency of 

retrieval for tropospheric ozone as a function of the height of the 

additional ozone for observations in the nadir direction. 

TOMS achieves it's contiguous coverage by scanning in a plane 

perpendicular to the orbital plane,. The scan angle varies from 

+53 degrees to -53 degrees. We have found that the efficiency for 

detection of troposphere ozone is not only a function of altitude
but also of the scan angle. Figure 5 shows the efficiency for 

ozone at altitudes of 500 and 700 mb as a function of the scan 

angle.. The actual value of the scan angle is a function of the 

attitude of the spacecraft, and this information is not given on 

the HDT tapes. However this information can be found on the TONS 

Raw Unit data tapes (RUT Tapes). Herman et. al. point out that 

there is an error in the derived altitude angles given on these 

tapes, which introduces an effective roll error in the scan 

angles. Unfortunately this correction is not given on the RUT 

Tapes. We are grateful to C. Wallermeyer for supplying us with the 

correction for the days that we have analyzed. 

The final data we need to correct the derived tropospheric 

ozone from the archived data, are profiles for the ozone resulting 

from the biomass burning. We have used the profiles recently 

published by Fishman et. al. These authors give several profiles, 

but the calculated efficiencies for each profile of ozone retrieval 

vary by only a few per cent.. 

The preliminary results of this analysis show that rather than 

a continuous plume as deduced by Fishman et. al., the plume 

consists of clouds of ozone that move with the wind profiles at 

about 500 mb. We supplied the TRACE-A mission this year with an 

analysis of the TOMS real-time HDT data (Provided for the Antarctic 

ozone hole survey) using these new techniques, and preliminary 

results from the aircraft mission agree well with our analysis. 

Future Work 

During the next year we intend to continue our analysis of the 

tropospheric ozone fields over the African continent. Given the 

time, we intend to supply the TRACE-A investigator team with 

better tropospheric ozone fields for the times of their mission. 

Our goal is to provide a continuous field over the continent and 

the Indian and Atlantic Oceans. With this data we can then follow 

the formation of the ozone clouds, and hopefully provide an 

estimate of the contribution of the biomass burning to the ozone 

budget of the troposphere. 

The analysis discussed above has general significance, and we 

will also examine HDT data off the coast of South America during 

the biomass burning in the Amazon basin, and over the Los Angeles 

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PROPOSED BUDGET 

PERIOD: 02/01/93-01/31/91

07—Jan-93 

I. PERSONNEL 

RESEARCH ASSOCIATE (6 MOs) $18,375 

0 

GRA LEVEL II(MOs) 12,814 

SECRETARIAL 0 

PROGRAMMER 0 

UNDERGRADUATE 1,064 

TOTAL SALARY AND WAGES 32,253 

II. FRINGE BENEFITS 11,131 

III. TRAVEL 

DOMESTIC 3,150 

FOREIGN 2,500 

TOTAL TRAVEL 5,650 

IV; EQUIPMENT 

DEC W/S (PARTIAL) 4,500 

1.3 GB DEC DISK STORAGE 2,500 

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TOTAL EQUIPMENT 7,000 

V. OTHER DIRECT COSTS 

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COMPUTER MAINT. 750 

PUBLICATION COSTS 2,500 

RESEARCH MATERIALS 1,000 

OFFICE SUPPLIES 532 

TOTAL OTHER DIR COSTS 5,782

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TOTAL DIRECT COSTS 25.106 

TOTAL REQUESTED SUPPORT $86,922