Heterogeneous chemistry in the smoke plume from the 1991 Kuwait oil fires

During late spring of 1991, airborne measurements in the smoke plume from the Kuwait oil fires indicated that SO2 was removed from the gas phase at rates of ∼6 to 8% h−1 and that NOx was removed at rates of ∼7 to 23% h−1. Photochemical calculations indicate that homogeneous chemical reactions were responsible for only a small fraction of this removal. Also, indirect evidence suggests that heterogeneous removal of SO2 on black carbon (or soot) and salt aerosols produced by the fires was relatively slow. The highest rates of SO2 and NOx removal were associated with high concentrations of atmospheric soil dust. This was likely due to heterogeneous oxidation of the SO2 and NOx on the surfaces of soil dust particles. The removal of SO2 and NOx on soil dust was probably accelerated by the alkaline nature of the dust. Heterogeneous reactions on soil dust particles proceeded more rapidly in regions of dispersed smoke than in the core of the plume; this was probably due to the depletion of the available surface area of the soil dust in regions of dense smoke. On the basis of smoke samples collected during the experiment we estimate that the quasi-second-order reaction rate for SO2 is (9 ± 4) × 10−8 (μg soil dust m−3)−1 s−1. For a soil dust concentration of 200 μg m−3 this implies a removal rate for SO2 on soil dust of 6.5% h−1.