Surface boundary layer exchange of nitric oxide, nitrogen dioxide, and ozone over a Brazilian pasture

G.A. Kirkman¹, C. Ammann², and F.X. Meixner ¹

¹⁾ Max Planck Institute for Chemistry, P.O. Box 3060, 55020 Mainz, Germany

²⁾ Eidgenössische Forschungsanstalt für Agrarökologie und Landbau (FAL), Bern, Switzerland

Abstract

Measurements of NO–NO₂–O₃ trace gas exchange were performed for two-transition season periods during the La Nińa year 1999 (30 April to 17 May, "wet–dry", and 24 September to 27 October, "dry–wet") over a cattle pasture in Rondōnia. A dynamic chamber system (applied during the dry–wet season) was used to directly measure emission fluxes of NO and surface resistances for NO₂ and O₃ deposition. In order to determine ecosystem-representative NO₂ and O₃ deposition fluxes for both measurement periods, an inferential method (multi-resistance model) was applied to measured ambient NO₂ and O₃ concentrations using observed quantities of turbulent transport. The observed NO soil emission fluxes were nine times lower than old-growth rainforest 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 (a = 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 This was due to 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 NO_x sink during 1999, removing seven times more NO₂ from the atmosphere than was emitted as NO.