Impacts of biomass burning in Southeast Asia on ozone and reactive nitrogen over the western Pacific in spring

[1] Aircraft measurements of ozone (O-3) and its precursors ( reactive nitrogen, CO, nonmethane hydrocarbons) were made over the western Pacific during the Transport and Chemical Evolution Over the Pacific (TRACE-P) campaign, which was conducted during February - April 2001. Biomass burning activity was high over Southeast Asia ( SEA) during this period ( dry season), and convective activity over SEA frequently transported air from the boundary layer to the free troposphere, followed by eastward transport to the sampling region over the western Pacific south of 30 degreesN. This data set allows for systematic investigations of the chemical and physical processes in the outflow from SEA. Methyl chloride (CH3Cl) and CO are chosen as primary and secondary tracers, respectively, to gauge the degree of the impact of emissions of trace species from biomass burning. Biomass burning is found to be a major source of reactive nitrogen (NOx, PAN, HNO3, and nitrate) and O-3 in this region from correlations of these species with the tracers. Changes in the abundance of reactive nitrogen during upward transport are quantified from the altitude change of the slopes of the correlations of these species with CO. NOx decreased with altitude due to its oxidation to HNO3. On the other hand, PAN was conserved during transport from the lower to the middle troposphere, consistent with its low water solubility and chemical stability at low temperatures. Large losses of HNO3 and nitrate, which are highly water soluble, occurred in the free troposphere, most likely due to wet removal by precipitation. This has been shown to be the major pathway of NOy loss in the middle troposphere. Increases in the mixing ratios of O-3 and its precursors due to biomass burning in SEA are estimated using the tracers. Enhancements of CO and total reactive nitrogen ( NOy), which are directly emitted from biomass burning, were largest at 2 - 4 km. At this altitude the increases in NOy and O-3 were 810 parts per trillion by volume ( pptv) and 26 parts per billion by volume (ppbv) above their background values of 240 pptv and 31 ppbv, respectively. The slope of the O-3-CO correlation in biomass burning plumes was similar to those observed in fire plumes in northern Australia, Africa, and Canada. The O-3 production efficiency ( OPE) derived from the O-3-CO slope and NOx/CO emission ratio ( ER) is shown to be positively correlated with the C2H4/NOx ER, indicating that the C2H4/NOx ER is a critical parameter in determining the OPE. Comparison of the net O-3 flux across the western Pacific region and total O-3 production due to biomass burning in SEA suggests that about 70% of O-3 produced was transported to the western Pacific.