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Photochemical effects on NOx and CO emissions in a Brazilian savanna.
KISSELLE, KEITH1, ZEPP, RICHARD1, BURKE, ROGER1, PINTO, ALEXANDRE2, BUSTAMANTE, MERCEDES2, 1 2
ABSTRACT- Land clearing and burning in the tropics often results in increased solar irradiation of soil and surface organic matter. This increased light exposure may impact the emissions of nitrogen oxides (NOx) and carbon monoxide (CO), trace gases that play an important role in tropospheric chemistry. Our objective in this study was to quantify the effect of light on these trace gas emissions at sites located in the Cerrado (savanna) in central Brazil. Two native vegetation types (cerrado sensu stricto and campo sujo) with or without recent burning, and a pasture site were studied. Gas measurements were made in the field using either clear, or covered (opaque), Pyrex chambers sealed on the soil surface. Laboratory studies of surface litter allowed gas emissions measurements while controlling the light wavelength and intensity and the temperature. Field NOx flux measurements using clear chambers were higher than when using opaque chambers (approximately 4-7 times higher in burned sites; 2 times higher in the unburned native grassland and pasture). Immediately after burning, CO emissions from soils and charred surface organic matter increased in the cerrado, where 30 days after the fire, daytime CO emissions were over 10-fold higher than those from the unburned cerrado (812.8 x 109 molecules cm-2 s-1 vs. 76.8 x 109 molecules cm-2 s-1). The increase in CO production occurred both in light and dark chambers, suggesting that the fire created thermally-reactive precursors.
KEY WORDS: Trace gases, Ultraviolet Radiation