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

Concentrations of gaseous ammonia ([NH3]) and aerosol ammonium ([NH+4 ]) 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 NH+4 . 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 mg m−3) values were larger downwind than upwind, indicating the importance of island sources. In contrast, at high [NH+4 ] (>0.5 mg m−3), [NH+4 ] was significantly smaller downwind than upwind, while at low [NH+4 ] (0.2 mg m−3), the opposite was observed. The decrease in [NH+4 ] suggests that cloud processing in high [NH+4 ] conditions may enhance the evaporation of NH3 from NH+4 in cloud, while NH+4 aerosol formation could occur at low [NH+4 ]. Analysis of the average diurnal variability in [NH3] and [NH+4 ] at the different sites suggests that both NH3 emissions and post-cloud evaporation of NH+4 to NH3 are largest during the day, coupled with increased temperatures and reduced relative humidities. Although the marine NH+4 aerosol is mostly present as nonvolatile ammonium sulphate, evaporation of NH+4 at high [NH+4 ] 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.