Marine and land-based influences on atmospheric ammonia and ammonium over Tenerife

Abstract

Concentrations of gaseous ammonia ([NH₃]) and aerosol ammonium ([NH₄⁺]) were measured across Tenerife as part of the ACE-2 “HILLCLOUD” experiment to assess the effect of cloud processing on the marine budget of reduced nitrogen (NH_x). Several methods for measuring NH₃ were applied: continuous rotating annular denuder, diffusion scrubber and multi-stage filter packs, with the latter also measuring NH₄⁺. The measurement sites were located both upwind and downwind of the hill-cloud. Terrestrial NH₃ sources provide a major constraint in addressing marine NH_x from land-based studies, and the measurements showed local NH₃ emissions from both decomposing potato fields and livestock. [NH₃] was correlated between upwind and downwind sites; at high [NH₃](>0.5 μg m-3) values were larger downwind than upwind, indicating the importance of island sources. In contrast, at high [NH₄⁺](>0.5 μg m^-3), [NH₄⁺] was significantly smaller downwind than upwind, while at low [NH₄⁺](0.2μg m^-3), the opposite was observed. The decrease in [NH₄⁺] suggests that cloud processing in high [NH₄⁺] conditions may enhance the evaporation of NH₃ from NH₄⁺ in cloud, while NH₄⁺ aerosol formation could occur at low [NH₄⁺]. Analysis of the average diurnal variability in [NH₃] and [NH₄⁺] at the different sites suggests that both NH₃ emissions and post-cloud evaporation of NH₄⁺ to NH₃ are largest during the day, coupled with increased temperatures and reduced relative humidities. Although the marine NH₄⁺ aerosol is mostly present as non-volatile ammonium sulphate, evaporation of NH₄⁺ at high [NH₄⁺] may be explained by in-cloud mixing with nitrate and chloride leading to the production of NH₄NO₃ and NH₄Cl which are subsequently volatilized on leaving the cloud.