Source analysis of carbon monoxide pollution during INDOEX 1999

A source analysis of carbon monoxide (CO) over the Indian Ocean is presente d using marked tracers in a chemistry general circulation model. The model includes a nonmethane hydrocarbon chemistry scheme and has been used at two different resolutions (3.75 degrees x 3.75 degrees and 1.9 degrees x 1.9 d egrees). European Centre for Medium-Range Weather Forecasts meteorological analyses have been assimilated into the model to represent actual meteorolo gy during February and March of 1999. A comparison with measurements indica tes that the model simulates realistic CO distributions. In general, the mo del performance is more realistic at higher resolution. Discrepancies exist close to the Indian coasts, possibly related to a sea breeze circulation a t the Indian west coast, not resolved by the model. Discrepancies are also found in the vicinity of convection at the Intertropical Convergence Zone ( ITCZ). The marked tracer study suggests that biofuel use and agricultural w aste burning in India are major CO sources for the Indian Ocean north of th e ITCZ, with minor contributions from Middle East, China, and Southeast Asi a. In the much cleaner boundary layer over the southern Indian Ocean, CO fr om hydrocarbon oxidation is a dominant source. There are no other regions a round the globe where biofuel use and biomass burning contribute so much to the CO mixing ratios. In general, most of the Asian CO over the Indian Oce an remains north of the ITCZ, although some of the CO is transported to the southern hemisphere in the free troposphere near the African east coast.