A general model of how fire emissions and chemistry produce African/oceanic plumes (O3, CO, PAN, smoke) in TRACE A


  • Robert B. Chatfield,

  • John A. Vastano,

  • H. B. Singh,

  • G. Sachse


A full-chemistry simulation of the Great African Plume gives one example of a broad conceptual model of the intercontinental pollution of the tropical middle troposphere by lofted biomass burning plumes. This two-dimensional idealization “calibrated” by carbon monoxide distributions links conventional estimates of burning emissions to oceanic concentrations of pollutants. This paper makes use of GRACES, a modular photochemical simulation system, in two forms. The results of the chemically intensive two-dimensional form, using idealized winds, mixing, deposition, and rainout, match the general concentration patterns of a three-dimensional GRACES model study of CO during the TRACE A/SAFARI period of October 1992 (reported separately). The study highlights the importance of simulating the vertical and diurnal variation of the planetary boundary layer and cloud activity. These correlate temporally with the intensity of tropical agricultural burning. We emphasize one situation, the drift northward and eastward of pollution into the interocean convergence region, where it rises by small-scale motions and rides out westward in the lower midtroposphere (<5 km). These effects help set in place large strata of enhanced CO, ozone, and other pollution over the equatorial Atlantic Ocean. Overall, our comparisons of simulations with the TRACE A data on the cycling of CO, NOx, and O3 in the tropical atmosphere suggest substantial agreement of current emission estimates and atmospheric concentrations. In certain regions, ozone is simulated slightly below observed levels. The striking major disagreements are in NOy, (total reactive nitrogen) and HNO3, which are intimately related to CO and O3; this suggests that current theory omits at least one fundamental process.