Search

Ten-day backward trajectories are used to determine the origins of air parcels arriving at airborne DC-8 chemical measurement sites during NASA's Pacific Exploratory Mission-Tropics A (PEM-T) that was conducted during August-October 1996. Those sites at which the air had a common geographical origin and transport history are grouped together, and statistical measures of chemical characteristics are computed. Temporal changes in potential temperature are used to determine whether trajectories experience a significant convective influence during the 10-day period. Those trajectories that do not experience a significant convective influence are divided into four geographical categories depending on their origins and paths. Air parcels originating over Africa and South America are characterized by enhanced mixing ratios of O3, CO, HNO3, and PAN. The backward trajectories travel at high altitudes (∼10-11 km), covering long distances due to strong upper-tropospheric westerly winds. The observed enhancement of combustion-related species is attributed to biomass burning from distant sources to the west, extending even to South America. The relatively large value of Be-7 probably is due either to less efficient removal of aerosols from upper tropospheric air or to small stratospheric contributions. Aged marine parcels are found to have relatively small concentrations of burning-related species. Although these trajectories arrive at a wide range of aircraft altitudes, they do not pass over a land mass during the preceding 10-day period. Air passing over Australia but no other land mass exhibits a combustion signature; however, photochemical product species such as O3 and PAN are less enhanced than in the long-range transport category. These trajectories travel shorter distances and are at lower altitudes (∼5-8 km) than those reaching Africa and/or South America. The combustion influence on these parcels is attributed to biomass burning emissions injected over Australia. That burning is less widespread than in Africa and South America. Finally, trajectories originating over Southeast Asia appear to receive a weak combustion influence. However, compared to Africa and South America, Southeast Asia has a relatively small incidence of biomass burning. There is little combustion input from Australia due to the high transport altitudes compared to the lower heights of the convection. The Southeast Asian parcels exhibit the greatest NOx to ΣNOi ratio of any category, perhaps due to lightning. Parcels experiencing a significant convective influence also are examined. Most of these parcels pass through widespread, persistent convection along either the South Pacific Convergence Zone or Intertropical Convergence Zone approximately 5 days prior to arriving at the aircraft locations. Thus the category mostly represents marine convection. Mixing ratios of peroxides and acids in the convective category are found to be smaller than in parcels not experiencing convection. Small mixing ratios of Be-7 and Pb-210 suggest particle removal by precipitation. Copyright 1999 by the American Geophysical Union.

The budgets of ozone and nitrogen oxides (NOx = NO + NO2) in the tropical South Pacific troposphere are analyzed by photochemical point modeling of aircraft observations at 0-12 km altitude from the Pacific Exploratory Mission-Tropics A campaign flown in September-October 1996. The model reproduces the observed NO2/NO concentration ratio to within 30% and has similar success in simulating observed concentrations of peroxides (H2O2, CH3OOH), lending confidence in its use to investigate ozone chemistry. It is found that chemical production of ozone balances only half of chemical loss in the tropospheric column over the tropical South Pacific. The net loss is 1.8 x 1011 molecules cm-2 s-1. The missing source of ozone is matched by westerly transport of continental pollution into the region. Independent analysis of the regional ozone budget with a global three-dimensional model corroborates 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 ozone concentrations by 7-8 ppbv throughout the troposphere. The NOx responsible for ozone production within the South Pacific troposphere below 4 km can be largely explained by decomposition of peroxyacetylnitrate (PAN) transported into the region with biomass burning pollution at higher altitudes. Copyright 1999 by the American Geophysical Union.

The convective injection and subsequent fate of the peroxides H2O2 and CH3OOH in the upper troposphere is investigated using aircraft observations from the NASA Pacific Exploratory Mission-Tropics A (PEM-Tropics A) over the South Pacific up to 12 km altitude. Fresh convective outflow is identified by high CH3I concentrations; CH3I is an excellent tracer of marine convection because of its relatively uniform marine boundary layer concentration, relatively well-defined atmospheric lifetime against photolysis, and high sensitivity of measurement. We find that mixing ratios of CH3OOH in convective outflow at 8-12 km altitude are enhanced on average by a factor of 6 relative to background, while mixing ratios of H2O2 are enhanced by less than a factor of 2. The scavenging efficiency of H2O2 in the precipitation associated with deep convection is estimated to be 55-70%. Scavenging of CH3OOH is negligible. Photolysis of convected peroxides is a major source of the HOx radical family (OH + peroxy radicals) in convective outflow. The timescale for decay of the convective enhancement of peroxides in the upper troposphere is determined using CH3I as a chemical clock and is interpreted using photochemical model calculations. Decline of CH3OOH takes place on a timescale of a 1-2 days, but the resulting HOx converts to H2O2, so H2O2 mixing ratios show no decline for ∼5 days following a convective event. The perturbation to HOx at 8-12 km altitude from deep convective injection of peroxides decays on a timescale of 2-3 days for the PEM-Tropics A conditions. Copyright 1999 by the American Geophysical Union.