Three‐dimensional distribution of nonmenthane hydrocarbons and halocarbons over the northwestern Pacific during the 1991 Pacific Exploratory Mission (PEM‐West A)

A total of 1667 whole air samples were collected onboard the NASA DC‐8 aircraft during the 6‐week Pacific Exploratory Mission over the western Pacific (PEM‐West A) in September and October 1991. The samples were assayed for 15 C 2 ‐C 7 hydrocarbons and six halocarbons. Latitudinal (0.5°S to 59.5°N) and longitudinal (114°E to 122°W) profiles were obtained from samples collected between ground level and 12.7 km. Thirteen of the 18 missions exhibited at least one vertical profile where the hydrocarbon mixing ratios increased with altitude. Longitude‐latitude color patch plots at three altitude levels and three‐dimensional color latitude‐altitude and longitude‐altitude contour plots exhibit a significant number of middle‐upper tropospheric pollution events. These and several lower tropospheric pollution plumes were characterized by comparison with urban data from Tokyo and Hong Kong, as well as with natural gas and the products from incomplete combustion. Elevated levels of nonmethane hydrocarbons (NMHC) and other trace gases in the upper‐middle free troposphere were attributed to deep convection over the Asian continent and to typhoon‐driven convection near the western Pacific coast of Asia. In addition, NMHCs and CH 3 CCl 3 were found to be useful tracers with which to distinguish hydrocarbon and halocarbon augmented plumes emitted from coastal Asian cities into the northwestern Pacific.