Lightning Impact on Tropospheric Ozone over the Tropical Southern Indian Ocean

Significant enhancements of middle and upper tropospheric ozone including tropospheric column ozone (TCO) were observed over the tropical southern Indian Ocean in May of 2006 and 2008 from the Aura Microwave Limb Sounder (MLS) and Tropospheric Emission Spectrometer (TES) instruments and the Aqua Atmospheric Infrared Sounder (AIRS). By contrast, no significant enhancement was observed in 2007. We applied the GEOS-Chem global chemical transport model to investigate the factors contributing to these enhancements. GEOS-Chem driven by GEOS-5 meteorological data, with a lightning NOx emission parameterized based upon convective cloud top height, was able to capture the ozone enhancements and the associated interannual variability, both in MLS and TES ozone mixing ratios and TES and AIRS TCO. Results from model sensitivity simulations indicated that lightning NOx emissions contributed over 60% of the enhancements in TCO. Lighting NOx emissions from Indonesia accounted for 27% of the total TCO consistently throughout the year, and were thus not responsible for the relative ozone enhancements. Lightning NOx over Central Africa and South America contributed 12% and 11% respectively to the TCO in May. These contributions peaked in May and were directly responsible for the ozone enhancements. Our model results indicted that biomass burning had a rather small contribution to the ozone enhancements.