This study examines the influence of photochemical processes on ozone distributions in the western North Pacific. The analysis is based on data generated during NASA's western Pacific Exploratory Mission (PEM-West A) during the fall of 1991. Ozone trends were best described in terms of two geographical domains: the western North Pacific rim (WNPR) and the western tropical North Pacific (WTNP). For both geographical regions, ozone photochemical destruction, D(O3), decreased more rapidly with altitude than did photochemical formation, F(O3). Thus the ozone tendency, P(O3), was typically found to be negative for z < 6 km and positive for z > 6-8 km. For nearly all altitudes and latitudes, observed nonmethane hydrocarbon (NMHC) levels were shown to be of minor importance as ozone precursor species. Air parcel types producing the largest positive values of P(O3) included fresh continental boundary layer (BL) air and high-altitude (z > 7 km) parcels influenced by deep convection/lightning. Significant negative P(O3) values were found when encountering clean marine BL air or relatively clean lower free-tropospheric air. Photochemical destruction and formation fluxes for the Pacific rim region were found to exceed average values cited for marine dry deposition and stratospheric injection in the northern hemisphere by nearly a factor of 6. This region was also found to be in near balance with respect to column-integrated 03 photochemical production and destruction. By contrast, for the tropical regime column-integrated 03 showed photochemical destruction exceeding production by nearly 80%. Both transport of 03 rich midlatitude air into the tropics as well as very highaltitude (10-17 km) photochemical 03 production were proposed as possible additional sources that might explain this estimated deficit. Results from this study further suggest that during the fall time period, deep convection over Asia and Malaysia/Indonesia provided a significant source of high-altitude NOx to the western Pacific. Given that the high-altitude NOx lifetime is estimated at between 3 and 9 days, one would predict that this source added significantly to high altitude photochemical 03 formation over large areas of the western Pacific. When viewed in terms of strong seasonal westerly flow, its influence would potentially span a large part of the Pacific. 1980; Mahlman et al., 1980; Chameides and Tan, 1981; Logan et al., 1981]. The preponderance of evidence now suggests that both transport and photochemical factors play an important role in controlling the tropospheric distribution of ozone (see previous list of references). Of the two factors, however, the contribution from photochemical processes is generally viewed as having the larger uncertainty. This partially reflects the fact that the photochemical 2111 2112 DAVIS ET AL.: OZONE PHOTOCHEMISTRY IN THE WESTERN NORTH PACIFIC models from which global photochemical rates have been assessed are based on globally "estimated" distributions of the critical precursor species such as NO, CO, H20 , and ...

Measurements of the reactive odd nitrogen compounds NO, NO2, peroxyacetyl nitrate (PAN), and NOy are presented for the summertime middle/lower troposphere (6.1–0.15 km) over northern high latitudes. In addition, the chemical signatures revealed from concurrent measurements of O3, CO, C2H2, C2H6, C3H8, C2Cl4, and H2O are used to further characterize factors affecting the budget and distribution of NxOy, in the Arctic and sub‐Arctic tropospheric air masses sampled over Alaska during the NASA Arctic Boundary Layer Expedition (ABLE 3A) field campaign. Many of the compounds listed above exhibited a general trend of median mixing ratios increasing in proportion with altitude within the lower 6‐km column. However, median mixing ratios of NO and NOx (NO + NO2) were nearly independent of altitude, having values of about 8.5 and 25 pptv, respectively. Median mixing ratios of NOy varied from about 350 pptv within the lowest altitudes to about 600 pptv within the highest altitudes sampled. PAN constituted the largest fraction of NOy (∼50%) at the highest altitudes. In addition, PAN mixing ratios accounted for all of the approximate 60 pptv/km altitudinal dependency in NOy. The analyses presented implicate biomass burning in Siberia as the probable source of about one‐third of the NOy abundance within the middle/lower troposphere over Alaska. These analyses also implicate the downward transport of air from altitudes in the vicinity of the tropopause as a major contributor to the abundance of NOy, (∼30–50%) within the lower 6‐km column over Alaska. However, the exact origin of this high‐altitude NOy remains uncertain. The impact of lower latitude industrial/urban pollution also remains largely uncertain, although various chemical signatures imply inputs from these regions would have been relatively well aged (15–30 days).