Concentrations and fluxes of water soluble inorganic aerosol components above tropical rainforest

Abstract

The interaction between biosphere and atmosphere in the cycling of gas and aerosol species is of key importance in considering overall emission and deposition rates of nutrients and pollutants. Understanding of the biosphereatmosphere processes that govern these cycles is critical to modelling global concentrations of atmospheric aerosols and trace gases, which in turn is vital to developing predictions for future climate, air quality and trans‐boundary air pollution. However, to understand these processes, more measurements over a variety of different ecosystems are required, preferably measurements which are taken in real time, which are of high temporal resolution and record a variety of species simultaneously and at potentially low background concentrations. In particular, very little is known about the role of biosphere‐atmosphere exchange in the nutrient cycling in and above tropical rain forests. While aerosol concentrations can provide information on the regional transport of compounds, fluxes allow an insight into the chemical process in the forest. In this work, the concentrations and fluxes of water‐soluble aerosol species NH4 +, Cl‐, NO2 ‐, NO3 ‐ and SO4 2‐ in total suspended particulate (TSP) and their precursor gases NH3, HCl, HNO2, HNO3 and SO2 were measured using a Gradient of Aerosols and Gases Online Registrator (GRAEGOR) above a tropical rainforest site located at the Amazon Tall Tower Observatory (ATTO) in Amazonia, Brazil. Measurements were taken during the dry season from the 4 October to 10 November 2017. From the measured concentration‐gradients and ancillary meteorological measurements, fluxes for each species were derived using the hybrid Aerodynamic Gradient Method and the Modified Bowen Ratio Method. Deposition velocities for each species were calculated and compared to theoretical deposition velocities, and interpreted in relation to measurements of leaf wetness at the canopy level. The average concentrations for aerosol species at each of the measurement heights (42 m and 60 m) throughout the campaign were 0.26/0.26 μg m‐3 for NH4 +, 0.11/0.12 μg m‐3 for Cl‐, 0.39/0.48 μg m‐3 for NO3 ‐ and 0.48/0.49 μg m‐3 for SO4 2‐. The mean concentration values of measured precursor gases were 0.26/0.23 μg m‐3 for NH3, 0.10/0.11 μg m‐3 for HCl, 0.06/0.06 μg m‐3 for HNO2, 0.24/0.26 μg m‐3 for HNO3 and 0.20/0.23 μg m‐3 for SO2. The aerosol concentration values were mostly in agreement with previous measurements made at similar time of year, except for higher mass fraction contribution of chloride and nitrate in the current study. This could indicate regional or long‐distance transport of coarse chloride (sea salt) and coarse nitrate that previous measurements would not have detected due to their use of a smaller particle size cut‐off compared with the one in this study. Copyright © 2018 by the International Aerosol Conference (IAC)