Satellite Observations of Tropospheric Chemistry: Retrievals and Interpretation

R.V. Martin
Ph.D. thesis


This thesis exploits nascent capabilities for observation of tropospheric chemistry from space to gain insight into the factors controlling tropospheric O3 and its precursors. Biomass burning and lightning respectively appear to explain 54% and 20% of the seasonal variance of a 14-year record of tropical tropospheric O3 columns (TTOCs) determined from the Total Ozone Mapping Spectrometer (TOMS). The spatial and temporal variation in biomass burning is constrained with firecounts from the Along Track Scanning Radiometer and with the TOMS aerosol index. A global 3-D model of tropospheric chemistry (GEOS-CHEM) is used to show that the observed 13-17 Dobson Unit wave-1 pattern in TTOCs, with a maximum over the tropical Atlantic and a minimum over the tropical Pacific, can be explained by the combination of upper tropospheric O3 production from lightning NOx, persistent subsidence over the southern tropical Atlantic as part of the Walker circulation, and cross-equatorial transport of upper tropospheric O3 from northern mid-latitudes in the African "westerly duct". These processes in the model and a correction for the TOMS retrieval also explain the "tropical Atlantic paradox", a TTOC enhancement over the southern tropical Atlantic during the northern African biomass burning season. A global lightning NOx source of 6 Tg N yr-1 in the model is most consistent with TOMS and in-situ observations.

Retrievals of tropospheric HCHO and NO2 columns from the Global Ozone Monitoring Experiment (GOME) are presented. In the eastern United States in summer, HCHO column measurements provide a measure of isoprene emission. Comparison of retrieved tropospheric NO2 columns for 1996-97 with corresponding GEOS-CHEM values tests both the retrieval and the NOx emission inventory used in GEOS-CHEM. Monthly mean retrieved NO2 columns over the United States, where NOx emissions are particularly well known, are 20% higher than GEOS-CHEM columns and strongly spatially correlated (r=0.79, n=3337, p<0.005). Retrieved NO2 columns are persistently higher than modeled columns by up to a factor of 2 over South Africa, Spain, the Po Valley, and central Saudi Arabia, and up to a factor of 2 lower over northeast India, suggesting corresponding biases in the NOx emission inventory. Retrieved columns are up to a factor of 2 lower than modeled columns over tropical biomass burning regions except northern Africa, reflecting a combination of errors in the model NOx emissions.

I evaluate the sensitivity of tropospheric OH, O3, and O3 precursors to photochemical effects of aerosols not usually included in global models: (1) aerosol scattering and absorption of ultraviolet radiation, and (2) reactive uptake of HO2, NO2, and NO3 by aerosols. Annual mean OH concentrations decrease by 9% globally. Simulated CO increases by 5-15 ppbv, improving agreement with observations. Simulated boundary-layer O3 decreases by 15-45 ppbv over India during the biomass burning season in March, and by 5-9 ppbv over northern Europe in August, improving comparison with observations. It appears that particulate matter controls would increase surface O3 over Europe and other industrial regions.

The full text of this paper is available as a [pdf].