A new method of deriving time-averaged tropospheric column ozone over the tropics using total ozone mapping spectrometer (TOMS) radiances: Intercomparison and analysis using TRACE A data

Abstract

Error analysis of archived total O₃ from total ozone mapping spectrometer (TOMS) (version 6) presented in earlier studies [Hudson and Kim, 1994; Hudson et al., 1995] is extended to include scan angle effects. Daily total O₃ maps for the tropics, from the period October 6–21, 1992, are derived from TOMS radiances following correction for these errors. These daily maps, averaged together, show a wavelike feature, which is observed in all latitude bands (10°N to 14°S), underlying sharp peaks which occur at different longitudes depending on the latitude. The wave pattern is used to derive both time-averaged stratospheric and tropospheric O₃ fields. The nature of the wave pattern (stratospheric or tropospheric) cannot be determined with certainty due to missing data (no Pacific sondes, no lower stratospheric Stratospheric Aerosol and Gas Experiment (SAGE) ozone for 18 months after the Mt. Pinatubo eruption) and significant uncertainties in the corroborative satellite record in the lower stratosphere (solar backscattered ultraviolet (SBUV), microwave limb sounder (MLS)). However, the time-averaged tropospheric ozone field, based on the assumption that the wave feature is stratospheric, agrees within 10% with ultraviolet differential absorption laser Transport and Atmospheric Chemistry near the Equator-Atlantic) (TRACE A) O₃ measurements from the DC-8 [Browell et al., this issue] and with ozonesonde measurements over Brazzaville, Congo (4°S, 15°W;), Ascension Island (8°S, 15°W), and Natal, Brazil (5.5°S, 35°W), for the period October 6–21, 1992. The derived background (nonpolluted) Indian Ocean tropospheric ozone amount, 26 Dobson units (DU), agrees with the cleanest African ozonesonde profiles for September–October 1992. The assumption of a totally tropospheric wave (flat stratosphere) gives 38 DU above the western Indian Ocean and 15–40% disagreements with the sondes. Tropospheric column O₃ is high from South America to Africa, owing to interaction of dynamics with biomass burning emissions [Thompson et al., this issue (a, b)]. Comparison with fire distributions from advanced very high resolution radiometer (AVHHR) during October 1992 suggests that tropospheric O₃ produced from biomass burning in South America and Africa dominates the O₃ budget in the tropical southern hemisphere during the study period.