Characterization of the chemical signatures of air masses observed during the PEM experiments over the western Pacific

Extensive observations of tropospheric trace species during the second NASA Global Tropospheric Experiment Western Pacific Exploratory Mission (PEM-We st B) in February-March 1994 showed significant seasonal variability in com parison with the first mission (PEM-West A), conducted in September-October 1991. In this study we adopt a previously established analytical method, i .e., the ratio C2H2/CO as a measure of the relative degree of atmospheric p rocessing, to elucidate the key similarities and variations between the two missions, In addition, the C2H2/CO ratio scheme is combined with the back- trajectory-based and the LIDAR-based air mass classification schemes, respe ctively, to make in-depth analysis of the seasonal variation between PEM-We st A and PEM-West B (hereinafter referred to as PEM-WA and PEM-WB). A large number of compounds, including long-lived NMHCs, CH4, and CO2, are, as exp ected, well correlated with the ratio C2H2/CO. In comparison with PEM-WA, a significantly larger range of observed C2H2/CO values at the high end for the PEM-WB period indicates that the western Pacific was more impacted by " fresher" source emissions, i.e., faster or mote efficient continental outfl ow. As in the case of PEM-WA, the C2H2/CO scheme complements the back-traje ctory air mass classification scheme very well. By combining the two scheme s, we found that the atmospheric processing in the region is dominated by a tmospheric mixing for the trace species analyzed. This PEM-WB wintertime re sult is similar to that found in PEM-WA for the autumn. In both cases, phot ochemical reactions are found to play a significant role in determining the background mixing ratios of trace gases, and in this way the two processes are directly related and dependent upon each other. This analysis also ind icates that many of the upper tropospheric air masses encountered over the western Pacific during PEM-WB may have had little impact from eastern Asia' s continental surface sources. NOx mixing ratios were significantly enhance d during PEM-WB when compared with PEM-WA, in the upper troposphere's more atmospherically processed air masses. These high levels of NOx resulted in a substantial amount of photochemical production of O-3. A lack of correspo nding enhancements in surface emission tracers strongly implies that in sit u atmospheric sources such as lightning are responsible for the enhanced up per tropospheric NOx. The similarity in NOx values between the northern (hi gher air traffic) and southern continental air masses together with the ind ications of a large seasonal shift suggests that aircraft emissions are not the dominant source. However, photochemical recycling cannot be ruled out as this in situ source of NOx.