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

I. Trebs (1) , L. L. Lara (2) , L. M. M. Zeri (3) , L. V. Gatti (4) , P. Artaxo (5) , R. Dlugi (6) , J. Slanina (7) , M. O. Andreae (1) , F. X. Meixner (1)

1 MPIC - Max Planck Institute for Chemistry
2 Laboratorio de Ecologia Isotopica
3 MPI-BGC - Max Planck Institute for Biogeochemistry
4 IPEN/CNEN-SP - Instituto de Pesquisas Energéticas e Nucleares [São Paulo]
5 Instituto de Fisica
6 Working Group Atmospheric Processes (WAP)
7 Department of Environmental Sciences

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.

Domaines: Océan, Atmosphère

I. Trebs, L. L. Lara, L. M. M. Zeri, L. V. Gatti, P. Artaxo, et al.. Dry and wet deposition of inorganic nitrogen compounds to a tropical pasture site (Rondônia, Brazil). Atmospheric Chemistry and Physics Discussions, 2005, 5 (3), pp.3131-3189. ⟨hal-00303916⟩