Surface exchange of nitric oxide, nitrogen dioxide, and ozone at a cattle pasture in Rondonia, Brazil

Measurements of NO-NO2-O-3 trace gas exchange were performed for two transition season periods during the La Nina a year 1999 (30 April to 17 May, "wet-dry," and 24 September to 27 October, "dry-wet") over a cattle pasture in Rondonia. A dynamic chamber system (applied during the dry-wet season) was used to directly measure emission fluxes of nitric oxide (NO) and surface resistances for nitrogen dioxide (NO2) and ozone (O-3) deposition. A companion study was simultaneously performed in an old-growth forest. In order to determine ecosystem-representative NO2 and O-3 deposition fluxes for both measurement periods, an inferential method (multiresistance model) was applied to measure ambient NO2 and O-3 concentrations using observed quantities of turbulent transport. Supplementary measurements included soil NO diffusivity and soil nutrient analysis. The observed NO soil emission fluxes were nine times lower than old-growth rain forest emissions under similar soil moisture and temperature conditions and were attributed to the combination of a reduced soil N cycle and lower effective soil NO diffusion at the pasture. Canopy resistances (R-c) of both gases controlled the deposition processes during the day for both measurement periods. Day and night NO2 canopy resistances were significantly similar (alpha = 0.05) during the dry-wet period. Ozone canopy resistances revealed significantly higher daytime resistances of 106 s m(-1) versus 65 s m(-1) at night because of plant, soil, and wet skin uptake processes, enhanced by stomatal activity at night and aqueous phase chemistry on vegetative and soil surfaces. The surface of the pasture was a net NOx sink during 1999, removing seven times more NO2 from the atmosphere than was emitted as NO.