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 Dart 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 mu g m(-3)) values were larger downwind than upwind, indicating the importance of island sources. In contrast, at high [NH4+] (>0.5 mu g m(-3)), [NH4+] was significantly smaller downwind than upwind, while at low [NH4+] (0.2 mu g 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 NH: aerosol is mostly present as nonvolatile 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.