AN ASSESSMENT OF OZONE PHOTOCHEMISTRY IN THE EXTRATROPICAL WESTERN NORTH PACIFIC - IMPACT OF CONTINENTAL OUTFLOW DURING THE LATE WINTER EARLY SPRING

This study examines the influence of photochemical processes on tropos pheric ozone distributions over the extratropical western North Pacifi c. The analysis presented here is based on data collected during the P acific Exploratory Mission-West Phase B (PEM-West B) field study condu cted in February-March 1994. Sampling in the study region involved alt itudes of 0-12 km and latitudes of 10 degrees S to 50 degrees N. The e xtratropical component of the data set (i.e., 20-50 degrees N) was def ined by markedly different photochemical environments north and south of 30 degrees N. This separation was clearly defined by an abrupt decr ease in the tropopause height near 30 degrees N and a concomitant incr ease in total O-3 Column density. This shift in overhead O-3 led to hi ghly reduced rates of O-3 formation and destruction for the 30-50 degr ees N latitude regime. Both latitude ranges, however, still exhibited net O-3 production at all altitudes. Of special significance was the f inding that net O-3 production prevailed even at boundary layer and lo wer free tropospheric altitudes (e.g., less than or equal to 4 km), a condition uncommon to Pacific marine environments. These results refle ct the strong impact of continental outflow of O-3 precursors (e.g., N O and NMHCs) into the northwestern Pacific Basin. Comparisons with PEM -West A, which sampled the same region in a different season (Septembe r-October), revealed major differences at altitudes below 4 km, the al titude range most influenced by continental outflow. The resulting net rate of increase in the tropospheric O-3 column for PEM-West B was 1- 3% per day, while for PEM-West A it was approximately zero. Unique to the PEM-West B study is the finding that even under wintertime conditi ons substantial column production of tropospheric O-3 can occur at sub tropical and mid-latitudes. While such impacts may not be totally unex pected at near coast locations, the present study suggests that the im pact from continental outflow on the marine BL could extend out to dis tances of more than 2000 km from the Asian Pacific Rim.