Dry and wet deposition of inorganic nitrogen compounds to a tropical pasture site (Rondônia, Brazil)

Abstract. 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.