Deriving robust regional estimates of the sources of chemically and radiatively important gases and aerosols to the atmosphere is challenging. Here, we focus on carbon monoxide. Using an inverse modeling methodology, we find that the source of carbon monoxide from fossil-fuel and biofuel combustion in Asia during 1994 was 350-380 Tg yr-1, which is 110-140 Tg yr-1 higher than bottom-up estimates derived using traditional inventory-based approaches. This discrepancy points to an important gap in our understanding of the human impact on atmospheric chemical composition.