This study presents a CO source contribution analysis for the atmosphere of South Asia during January–February 2008. The approach includes into the Weather Research and Forecasting Model with Chemistry 11 CO tracers, which track CO from different source types and regions. The comparison of model results with Measurement of Pollution in the Troposphere CO retrievals shows that the model reproduces the spatial, vertical, and temporal distributions of Measurement of Pollution in the Troposphere retrievals fairly well, but generally overestimates CO retrievals in the lower troposphere. CO mixing ratios averaged over the model domain at the surface, in the planetary boundary layer, and the free troposphere are estimated as 321 ± 291, 280 ± 208, and 125 ± 27 ppbv, respectively. Model results show that wintertime CO in the boundary layer and free troposphere over India is mostly due to anthropogenic emissions and to CO inflow. In the boundary layer, the contribution from anthropogenic sources dominates (40–90%), while in the free troposphere the main contribution is due to CO inflow from the lateral boundaries (50–90%). Over the Arabian Sea and the Bay of Bengal, 43–51% of surface CO mixing ratios come from the Indian subcontinent and 49–57% from regions outside of South Asia. The anthropogenic sources in the Indo-Gangetic Plain region are found to contribute, on average, 42% and 76% to anthropogenic surface CO over the Arabian Sea and the Bay of Bengal, respectively. The anthropogenic emissions from western and southern India contribute 49% to anthropogenic surface CO over the Arabian Sea. Anthropogenic emissions contribute only up to 40% over Burma where biomass burning plays a more important role. Regional transport contributes significantly to total anthropogenic CO over southern India (41%), Burma (49%), and even exceeds the contribution from local sources in western India (58%).