Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 5 3 2005 10.5194/acpd-5-3131-2005 http://www.atmos-chem-phys-discuss.net/5/3131/2005/ http://www.atmos-chem-phys-discuss.net/5/3131/2005/acpd-5-3131-2005.html http://www.atmos-chem-phys-discuss.net/5/3131/2005/acpd-5-3131-2005.pdf 3131 3189 2005-05-19 Dry and wet deposition of inorganic nitrogen compounds to a tropical pasture site (Rondônia, Brazil) I. Trebs L. L. Lara L. M. M. Zeri L. V. Gatti P. Artaxo R. Dlugi J. Slanina M. O. Andreae F. X. Meixner Max Planck Institute for Chemistry, Biogeochemistry Department, Mainz, Germany Centro de Energia Nuclear na Agricultura (CENA), Laboratorio de Ecologia Isotópica, Universidade de São Paulo, Piracicaba, São Paulo, SP, Brazil Max Planck Institute for Biogeochemistry, Jena, Germany Instituto de Pesquisas Energéticas e Nucleares, CQMA, Atmospheric Chemistry Laboratory, Cidade Universitaria, São Paulo, SP, Brazil Instituto de Física, Universidade de São Paulo (USP), São Paulo, SP, Brazil Working Group Atmospheric Processes (WAP), Munich, Germany University of Wageningen, Department of Environmental Sciences, Wageningen, AP, The Netherlands The input of nitrogen (N) to ecosystems has increased dramatically over the past decades. While total N deposition (wet + dry) has been extensively determined in temperate regions, only very few data sets exist about wet N deposition in tropical ecosystems, and moreover, experimental information about dry N deposition in tropical environments is lacking. In this study we estimate dry and wet deposition of inorganic N for a remote pasture site in the Amazon Basin based on in-situ measurements. The measurements covered the late dry (biomass burning) season, a transition period and the onset of the wet season (clean conditions) (12 September to 14 November 2002, LBA-SMOCC). Ammonia (NH₃), nitric acid (HNO₃), nitrous acid (HONO), nitrogen dioxide (NO₂), nitric oxide (NO), ozone (O₃), aerosol ammonium (NH₄⁺) and aerosol nitrate (NO₃^-) were measured in real-time, accompanied by simultaneous (micro-)meteorological measurements. Dry deposition fluxes of NO₂ and HNO₃ are inferred using the ''big leaf multiple resistance approach'' and particle deposition fluxes are derived using an established empirical parameterization. Bi-directional surface-atmosphere exchange fluxes of NH₃ and HONO are estimated by applying a ''canopy compensation point model''. Dry and wet N deposition is dominated by NH₃ and NH₄⁺, which is largely the consequence of biomass burning during the dry season. The grass surface appeared to have a strong potential for daytime NH₃ (re-)emission, owing to high canopy compensation points, which are related to high surface temperatures and to direct NH₃ emissions from cattle excreta. NO₂ also significantly accounted for dry N deposition, whereas HNO₃, HONO and N-containing aerosol species were only minor contributors. We estimated a total (dry + wet) N deposition of 7.3–9.8 kgN ha^-1 yr^-1 to the tropical pasture site, whereof 2–4.5 kgN ha^-1 yr^-1 are attributed to dry N deposition and ~5.3 kgN ha^-1 yr^-1 to wet N deposition. Our estimate exceeds total (wet + dry) N deposition to tropical ecosystems predicted by global chemistry and transport models by at least factor of two.