A tropospheric ozone maximum over the equatorial Southern Indian Ocean
We examine the distribution of tropical tropospheric ozone (O 3) from the Microwave Limb Sounder (MLS) and the Tropospheric Emission Spectrometer (TES) by using a global three-dimensional model of tropospheric chemistry (GEOS-Chem). MLS and TES observations of tropospheric O 3 during 2005 to 2009 reveal a distinct, persistent O 3 maximum, both in mixing ratio and tropospheric column, in May over the Equatorial Southern Indian Ocean (ESIO). The maximum is most pronounced in 2006 and 2008 and less evident in the other three years. This feature is also consistent with the total column O 3 observations from the Ozone Mapping Instrument (OMI) and the Atmospheric Infrared Sounder (AIRS). Model results reproduce the observed May O 3 maximum and the associated interannual variability. The origin of the maximum reflects a complex interplay of chemical and dynamic factors. The O 3 maximum is dominated by the O 3 production driven by lightning nitrogen oxides (NO x) emissions, which accounts for 62% of the tropospheric column O 3 in May 2006. We find the contribution from biomass burning, soil, anthropogenic and biogenic sources to the O 3 maximum are rather small. The O 3 productions in the lightning outflow from Central Africa and South America both peak in May and are directly responsible for the O 3 maximum over the western ESIO. The lightning outflow from Equatorial Asia dominates over the eastern ESIO. The interannual variability of the O 3 maximum is driven largely by the anomalous anti-cyclones over the southern Indian Ocean in May 2006 and 2008. The lightning outflow from Central Africa and South America is effectively entrained by the anti-cyclones followed by northward transport to the ESIO.