Tropospheric photochemistry and ozone budget

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In situ aircraft measurements of O_3, CO, total reactive nitrogen (NO_y), NO, and non-methane hydrocarbons (NMHCs) were made over the western Pacific Ocean and Australia during the Biomass Burning and Lightning Experiment (BIBLE)-A and B conducted in August-October 1998 and 1999. The mixing ratios of these species reached minimum values over the tropical Pacific Ocean throughout the troposphere due to active convection, although occasional enhancements in reactive nitrogen occurred caused by NO production by lightning near New Guinea. The low values of O_3, CO, NO_y, and NO in the tropical upper troposphere are compared with a 3-dimensional chemistry and transport model, which reproduces the latitudinal variations of O_3, NO_y, and NO from northern midlatitudes to the equator reasonably well. Over Australia, the levels of O_3, CO, NO_y, NO, and NMHCs were elevated throughout the troposphere over those observed in the tropical Pacific both in 1998 and 1999. Biomass burning over Australia strongly enhanced the mixing ratios of these species only within the boundary layer due to subsidence. Free tropospheric air masses originated from Indonesia, the Indian Ocean, Africa, and southern midlatitudes were identified basing on 14-day back trajectories. The levels of O_3, CO, NO_y, and NMHCs in these air masses were much higher than those from the tropical Pacific due to their stronger sources from biomass burning and lightning. The median values of these species in different types of air masses were similar except for the southern midlatitude air, which showed somewhat higher values. Biomass burning and lightning are found to be important for O_3 and its precursors in these air masses.