Chemical characteristics of continental outflow from Asia to the troposphere over the western Pacific Ocean during February‐March 1994: Results from PEM‐West B

We present here the chemical composition of outflow from the Asian continent to the atmosphere over the western Pacific basin during the Pacific Exploratory Mission‐West (PEM‐West B) in February–March 1994. Comprehensive measurements of important tropospheric trace gases and aerosol particulate matter were performed from the NASA DC‐8 airborne laboratory. Backward 5 day isentropic trajectories were used to partition the outflow from two major source regions: continental north (>20°N) and continental south (<20°N). Air parcels that had not passed over continental areas for the previous 5 days were classified as originating from an aged marine source. The trajectories and the chemistry together indicated that there was extensive rapid outflow of air parcels at altitudes below 5 km, while aged marine air was rarely encountered and only at <20°N latitude. The outflow at low altitudes had enhancements in common industrial solvent vapors such as C 2 Cl 4 , CH 3 CCl 3 , and C 6 H 6 , intermixed with the combustion emission products C 2 H 2 , C 2 H 6 , CO, and NO. The mixing ratios of all species were up to tenfold greater in outflow from the continental north compared to the continental south source region, with 210 Pb concentrations reaching 38 fCi (10 −15 curies) per standard cubic meter. In the upper troposphere we again observed significant enhancements in combustion‐derived species in the 8–10 km altitude range, but water‐soluble trace gases and aerosol species were depleted. These observations suggest that ground level emissions were lofted to the upper troposphere by wet convective systems which stripped water‐soluble components from these air parcels. There were good correlations between C 2 H 2 and CO and C 2 H 6 ( r 2 =0.70–0.97) in these air parcels and much weaker ones between C 2 H 2 and H 2 O 2 or CH 3 OOH ( r 2 ≈0.50). These correlations were the strongest in the continental north outflow where combustion inputs appeared to be recent (1–2 days old). Ozone and PAN showed general correlation in these same air parcels but not with the combustion products. It thus appears that several source inputs were intermixed in these upper tropospheric air masses, with possible contributions from European or Middle Eastern source regions. In aged marine air mixing ratios of O 3 (≈20 parts per billion by volume) and PAN (≤10 parts per trillion by volume) were nearly identical at <2 km and 10–12 km altitudes due to extensive convective uplifting of marine boundary layer air over the equatorial Pacific even in wintertime. Comparison of the Pacific Exploratory Mission‐West A and PEM‐West B data sets shows significantly larger mixing ratios of SO 2 and H 2 O 2 during PEM‐West A. Emissions from eruption of Mount Pinatubo are a likely cause for the former, while suppressed photochemical activity in winter was probably responsible for the latter. This comparison also highlighted the twofold enhancement in C 2 H 2 , C 2 H 6 , and C 3 H 8 in the continental north outflow during PEM‐West B. Although this could be due to reduced OH oxidation rates of these species in wintertime, we argue that increased source emissions are primarily responsible.