Observations of Ozone and related species in the northeast Pacific during the PHOBEA campaigns 2. Airborne observations

During late March and April of 1999 the University of Wyoming's King Air re search aircraft measured atmospheric concentrations of NO, O-3, peroxyacety l nitrate (PAN), CO, CH4, VOCs, aerosols, and J(NO2) off the west coast df the United States. During 14 flights, measurements were made between 39 deg rees -48 degreesN latitude, 125 degrees -129 degreesW longitude, and at alt itudes from 0-8 km. These flights were part of the Photochemical Ozone Budg et of the Eastern North Pacific Atmosphere (PHOBEA) experiment, which inclu ded both groundbased and airborne measurements. Flights were scheduled when meteorological conditions minimized the impact of local pollution sources. The resulting measurements were segregated by air mass source region as in dicated by back isentropic trajectory analysis. The chemical composition of marine air masses whose 5-day back isentropic trajectories originated nort h df 40 degreesN latitude or west of 186 degreesW longitude (WNW) differed significantly from marine air masses whose 5-day back isentropic trajectori es originated south of 40 degreesN latitude and east of 180 degreesW longit ude (SW). Trajectory and chemical analyses indicated that the majority of a ll encountered air masses, both WNW and SW, likely originated from the nort hwestern Pacific and have characteristics of emissions from the East Asian continental region. However, air masses with WNW back trajectories containe d higher mixing ratios of NO, NOx, O-3, PAN, CO, CH4, various VOC pollution tracers, and aerosol number concentration, compared to those air masses wi th SW back trajectories. Calculations of air mass age using two separate me thods, photochemical and back trajectory, are consistent with transport fro m the northwestern Pacific in 8-10 days for air masses with WNW back trajec tories and 16-20 days for air masses with SW back trajectories. Correlation s, trajectory analysis, and comparisons with measurements made in the north western Pacific during NASA's Pacific Exploritory Mission-West Phase B (PEM -West B) experiment in 1994 are used to investigate the data. These analyse s provide evidence that anthropogenically influenced air masses from the no rthwestern Pacific affect the overall chemical composition of the northeast ern Pacific troposphere.