On the origin of tropospheric ozone and NOx over the tropical South Pacific

The budgets of ozone and nitrogen oxides (NOx = NO + NO2) in the tropical S outh Pacific troposphere are analyzed by photochemical point modeling of ai rcraft observations at 0-12 km altitude from the Pacific Exploratory Missio n-Tropics A campaign flown in September-October 1996. The model reproduces the observed NO2/NO concentration ratio to within 30% and has similar succe ss in simulating observed concentrations of peroxides H2O2, CH3OOH), lendin g confidence in its use to investigate ozone chemistry. It is found that ch emical production of ozone balances only half of chemical loss in the tropo spheric column over the tropical South Pacific. The net loss is 1.8 x 10(11 ) molecules cm(-2) s(-1),The missing source of ozone is matched by westerly transport of continental pollution into the region. Independent analysis o f the regional ozone budget with a global three-dimensional model corrobora tes the results from the point model and reveals the importance of biomass burning emissions in South America and Africa for the ozone budget over the tropical South Pacific. In this model; biomass burning increases average o zone concentrations by 7-8 ppbv throughout the troposphere. The NOx respons ible for ozone production within the South Pacific troposphere below 4 km c an be largely explained by decomposition of peroxyacetylnitrate (PAN) trans ported into the region with biomass burning pollution at higher altitudes.