Carbonyl sulfide and carbon disulfide: Large-scale distributions over the western Pacific and emissions from Asia during TRACE-P

An extensive set of carbonyl sulfide (OCS) and carbon disulfide (CS₂) observations were made as part of the NASA Transport and Chemical Evolution over the Pacific (TRACE-P) project, which took place in the early spring 2001. TRACE-P sampling focused on the western Pacific region but in total included the geographic region 110°E to 290°E longitude, 5°N to 50°N latitude, and 0-12 km altitude. Substantial OCS and CS₂ enhancements were observed for a great many air masses of Chinese and Japanese origin during TRACE-P. Over the western Pacific, mean mixing ratios of long-lived OCS and shorter-lived CS₂ showed a gradual decrease by about 10% and a factor of 5-10, respectively, from the surface to 8-10 km altitude, presumably because land-based sources dominated their distribution during February through April 2001. The highest mean OCS and CS₂ levels (580 and 20 pptv, respectively, based on 2.5° × 2.5° latitude bins) were observed below 2 km near the coast of Asia, at latitudes between 25°N and 35°N, where urban Asian outflow was strongest. Ratios of OCS versus CO for continental SE Asia were much lower compared to Chinese and Japanese signatures and were strongly associated with biomass burning/biofuel emissions. We present a new inventory of anthropogenic Asian emissions (including biomass burning) for OCS and CS₂ and compare it to emission estimates based on regional relationships of OCS and CS₂ to CO and CO₂. The OCS and CS₂ results for the two methods compare well for continental SE Asia and Japan plus Korea and also for Chinese CS₂ emissions. However, it appears that the inventory underestimates Chinese emissions of OCS by about 30-100%. This difference may be related to the fact that we did not include natural sources such as wetland emissions in our inventory, although the contributions from such sources are believed to be at a seasonal low during the study period. Uncertainties in OCS emissions from Chinese coal burning, which are poorly characterized, likely contribute to the discrepancy.