Abstract
Concentrations of gaseous ammonia ([NH
3
]) 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 (NH
x
). Several methods for measuring NH
3
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 NH
3
sources provide a major constraint in addressing marine NH
x
from land-based studies, and the measurements showed local NH
3
emissions from both decomposing potato fields and livestock. [NH
3
] was correlated between upwind and downwind sites; at high [NH
3
](>0.5 μg m-3) values were larger downwind than upwind, indicating the importance of island sources. In contrast, at high [NH
4
+
](>0.5 μg m
-3
), [NH
4
+
] was significantly smaller downwind than upwind, while at low [NH
4
+
](0.2μg 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 NH
3
from NH
4
+
in cloud, while NH
4
+
aerosol formation could occur at low [NH
4
+
]. Analysis of the average diurnal variability in [NH
3
] and [NH
4
+
] at the different sites suggests that both NH
3
emissions and post-cloud evaporation of NH
4
+
to NH
3
are largest during the day, coupled with increased temperatures and reduced relative humidities. Although the marine NH
4
+
aerosol is mostly present as non-volatile 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 NH
4
NO
3
and NH
4
Cl which are subsequently volatilized on leaving the cloud.