Chemical characteristics of continental outflow from Asia to the troposphere over the western Pacific Ocean during September–October 1991: Results from PEM‐West A

An important objective of the Pacific Exploratory Mission‐West A (PEM‐West A) was the chemical characterization of the outflow of tropospheric trace gases and aerosol particles from the Asian continent over the western Pacific Ocean. This paper summarizes the chemistry of this outflow during the period September – October 1991. The vertical distributions of CO, C 2 H 6 , and NO x showed regions of outflow at altitudes below 2 km and from 8 to 12 km. Mixing ratios of CO were ≈130 parts per billion by volume (ppbv), ≈1000 parts per trillion by volume (pptv) for C 2 H 6 , and ≈100 pptv for NO x in both of these regions. Direct outflow of Asian industrial materials was clearly evident at altitudes below 2 km, where halocarbon tracer compounds such as CH 3 CCl 3 and C 2 Cl 4 were enhanced about threefold compared to aged Pacific air. The source attribution of species outflowing from Asia to the Pacific at 8–12 km altitude was not straightforward. Above 10 km altitude there were substantial enhancements of NO y , O 3 , CO, CH 4 SO 2 , C 2 H 6 , C 3 H 8 , C 2 H 2 , and aerosol 210 Pb but not halocarbon industrial tracers. These air masses were rich in nitrogen relative to sulfur and contained ratios of C 2 H 2 /CO and C 3 H 8 /C 2 H 6 (≈1.5 and 0.1 respectively) indicative of several‐day‐old combustion emissions. It is unclear if these emissions were of Asian origin, or if they were rapidly transported to this region from Europe by the high wind speeds in this tropospheric region (60 – 70 m s −1 ). The significant cyclonic activity over Asia at this time could have transported to the upper troposphere emissions from biomass burning in Southeast Asia or emissions from the extensive use of various biomass materials for cooking and space heating. Apparently, the emissions in the upper troposphere were brought there by wet convective systems since water‐soluble gases and aerosols were depleted in these air masses. Near 9 km altitude there was a distinct regional outflow that appeared to be heavily influenced by biogenic processes on the Asian continent, especially from the southeastern area. These air masses contained CH 4 in excess of 1800 ppbv, while CO 2 and OCS were significantly depleted (349 – 352 ppmv and 450 – 500 pptv, respectively). This signature seemingly reflected CH 4 emissions from wetlands and rice paddies with coincident biospheric uptake of tropospheric CO 2 and OCS.