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‐West B) in February‐March 1994 showed significant seasonal variability in comparison 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 C 2 H 2 /CO as a measure of the relative degree of atmospheric processing, to elucidate the key similarities and variations between the two missions. In addition, the C 2 H 2 /CO ratio scheme is combined with the back‐trajectory‐based and the LIDAR‐based air mass classification schemes, respectively, to make in‐depth analysis of the seasonal variation between PEM‐West A and PEM‐West B (hereinafter referred to as PEM‐WA and PEM‐WB). A large number of compounds, including long‐lived NMHCs, CH 4 , and CO 2 , are, as expected, well correlated with the ratio C 2 H 2 /CO. In comparison with PEM‐WA, a significantly larger range of observed C 2 H 2 /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 more efficient continental outflow. As in the case of PEM‐WA, the C 2 H 2 /CO scheme complements the back‐trajectory air mass classification scheme very well. By combining the two schemes, we found that the atmospheric processing in the region is dominated by atmospheric mixing for the trace species analyzed. This PEM‐WB wintertime result is similar to that found in PEM‐WA for the autumn. In both cases, photochemical 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 indicates 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. NO x mixing ratios were significantly enhanced during PEM‐WB when compared with PEM‐WA, in the upper troposphere's more atmospherically processed air masses. These high levels of NO x resulted in a substantial amount of photochemical production of O 3 . A lack of corresponding enhancements in surface emission tracers strongly implies that in situ atmospheric sources such as lightning are responsible for the enhanced upper tropospheric NO x . The similarity in NO x values between the northern (higher air traffic) and southern continental air masses together with the indications 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 NO x .