Concentrations and biosphere–atmosphere fluxes of inorganic trace gases and associated ionic aerosol counterparts over the Amazon rainforest

Abstract. The Amazon rainforest presents a unique, natural laboratory for the study of surface-atmosphere interactions. Its alternation between a near-pristine, marine-influenced atmosphere during the wet season, and a vulnerable system affected by periodic intrusions of anthropogenic pollution during the dry season, provides an opportunity to investigate some fundamental aspects of boundary-layer chemical processes. This study presents the first simultaneous hourly measurements of concentrations, fluxes and deposition velocities of the inorganic trace gases NH3, HCl, HONO, HNO3 and SO2 and their water-soluble aerosol counterparts NH4+, Cl−, NO2−, NO3− and SO42− over the Amazon. Species concentrations were measured in the dry season (from 6 October to 5 November 2017), at the Amazon Tall Tower Observatory (ATTO) in Brazil, using a two-point gradient, wet-chemistry instrument (Gradient of Aerosols and Gases Online Registration, GRAEGOR) sampling at 42 m and 60 m. Fluxes and deposition velocities were derived from the concentration gradients using a modified form of the aerodynamic gradient method corrected for measurement within the roughness sub-layer. Findings from this campaign include observations of elevated concentrations of NH3 and SO2 partially driven by long-range transport (LRT) episodes of pollution, and the substantial influence of coarse Cl− and NO3− particulate on overall aerosol mass burdens. From the flux measurements, the dry season budget of total reactive nitrogen dry deposition at the ATTO site was estimated as −2.9 kg N ha−1 a−1. HNO3 and HCl were deposited continuously at a rate close to the aerodynamic limit. SO2 was deposited with an average daytime surface resistance (Rc) of 28 s m−1, whilst aerosol components showed average surface deposition velocities of 2.8 and 2.7 mm s−1 for SO42− and NH4+. Deposition rates of NO3