DISTRIBUTION AND SEASONALITY OF SELECTED HYDROCARBONS AND HALOCARBONSOVER THE WESTERN PACIFIC BASIN DURING PEM-WEST-A AND PEM-WEST-B

Nonmethane hydrocarbons (NMHCs) and halocarbons were measured in the t roposphere over the northwestern Pacific as part of the airborne compo nent of NASA's Pacific Exploratory Mission-West Phase B (PEM-West B). This study took place in late winter of 1994, a period characterized b y maximum outflow from the Asian continent. The results are compared t o those from Pacific Exploratory Mission-West Phase A (PEM-West A), wh ich was flown in the same region during late summer of 1991, when flow from the subtropical western Pacific dominated the lower troposphere. Mixing ratios of NMHCs, tetrachloroethene (C2Cl4), and methyl bromide (CH3Br) were significantly higher during PEM-West B than during PEM-W est A, particularly at latitudes north of 25 degrees N and altitudes l ower than 6 lan. The primary reasons for these higher ambient concentr ations were the seasonal increase in the atmospheric lifetimes of trac e gases controlled by HO radical reactions, and the more frequent inpu t of continental air masses. During PEM-West B, air masses of continen tal origin observed north of 25 degrees N latitude were augmented with urban signature gases such as C2Cl4. By contrast, more southerly cont inental outflow had characteristics associated with combustion sources such as biomass burning, including wood fuel burning. During the summ er PEM-West A period, the spatial distribution of methyl iodide (CH3I) was consistent with effective oceanic sources at all latitudes, being especially strong in tropical and subtropical regions. At low latitud es, PEM-West B CH3I mixing ratios in the lower troposphere were simila r to PEM-West A, but at latitudes greater than about 25 degrees N PEM- West B concentrations were significantly reduced. Equatorial regions e xhibited enhanced CH3I mixing ratios extending into the upper tropical troposphere, consistent with fast vertical transport of air from the tropical marine boundary layer.