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

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 (NHx). 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 NHx 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.