Quantification of the factors controlling tropical tropospheric ozone and the South Atlantic maximum

We quantify the processes controlling the tropical tropospheric ozone burden with particular attention to the tropical Atlantic, using a global chemical transport model (GEOS-Chem) constrained by satellite and in situ observations of O₃, NO₂, and HCHO. Lightning is the dominant contributor to tropical tropospheric O₃, accounting for more than 37% of the O₃ burden over the Atlantic on annual average. The contributions from biomass burning, soils, and fossil fuels are 4 to 6 times smaller, despite comparable source strengths. This discrepancy can be explained by the tropical ozone production efficiency of lightning (32 mol/mol), soils (14 mol/mol), biomass burning (10 mol/mol), and fossil fuel (13 mol/mol) sources, as calculated using sensitivity simulations with a 1% perturbation. The role of volatile organic compound emissions on the tropical Atlantic ozone burden is negligible (<2.5%). Stratosphere-troposphere exchange accounts for less than 5% of the regional O₃ burden. The tropical Atlantic O₃ burden is more strongly influenced by nitrogen oxides from Africa (>30%) than from South America (>18%) or the eastern tropics (>11%). Lightning is responsible for more than 39% of the atmospheric oxidation capacity, higher than other sources. The dominant sources of uncertainty in the tropical oxidation rate are the lightning magnitude and the cloud convective parameterization.