Interpretation of TOMS observations of tropical tropospheric ozone with a global model and in-situ observations

Martin, R.V., D.J. Jacob, J.A. Logan, I. Bey, R.M. Yantosca, A.C. Staudt, Q. Li, A.M. Fiore, B.N. Duncan, H. Liu, P. Ginoux, and V. Thouret

J. Geophys. Res., 107(D18), 4351, 10.1029/2001JD001480, 2002. [Full Text (pdf)]

The efficiency correction is available for September 1996 through November 1997. [Data Format] [Sep96] [Oct96] [Nov96] [Dec96] [Jan97] [Feb97] [Mar97] [Apr97] [May97] [Jun97] [Jul97] [Aug97] [Sep97] [Oct97] [Nov97]

We have developed an efficiency correction to the TOMS retrieval algorithm to account for the variability of ozone in the lower troposphere. The correction is motivated by the fact that tropical tropospheric ozone columns (TTOCs) reported by TOMS over northern Africa and South Asia do not capture the seasonal enhancements found in aircraft observations and in the model. A characteristic feature of these northern tropical enhancements, in contrast to southern tropical enhancements, is that they are driven by the lower troposphere where the sensitivity of TOMS is relatively poor due to Rayleigh scattering.

We used a radiative transfer model (LIDORT), local surface reflectivities from TOMS, and the GEOS-CHEM model to derive the efficiency correction. We calculated the efficiency correction as the difference between the standard ozone profiles in the TOMS retrieval algorithm and monthly mean GEOS-CHEM fields, weighted by the TOMS sensitivity. The efficiency correction increases TTOCs over biomass burning regions by 3-5 Dobson Units (DU) and decreases them by 2-5 DU over oceanic regions, improving the agreement between TTOCs from the Convective Cloud Differential (CCD) method and in-situ observations, and explaining 13% of the variance between GEOS-CHEM and CCD TTOCs. The correction is applicable to TTOCs developed from any method that uses the standard ozone profiles including the CCD [Ziemke et al., 1998], Modified Residual [Thompson and Hudson, 1999], and Clear-Cloudy Pair [Newchurch et al., 2003] methods.

The lower panel shows the CCD TTOCs after the correction has been applied. The correction reduces the magnitude of the "tropical Atlantic paradox" [Thompson et al., 2000], i.e. the north-south gradient reversal in ozone over the tropical Atlantic during the northern African biomass burning season in December-February (DJF) as observed by TTOCs and ozonesondes from ship cruises. The CCD TTOCs over the southern tropical Atlantic are reduced by 10 DU relative to northern Africa during DJF. Even after this correction, a TTOC enhancement during DJF is still present over the southern tropical Atlantic. We reproduce this enhancement in the model and explain it by the combination of upper tropospheric ozone production from lightning NOx, strong subsidence over the southern tropical Atlantic as part of the Walker circulation, and cross-equatorial transport of upper tropospheric ozone from northern mid-latitudes in the African "westerly duct".