A modelling study of the interaction of ammonia with cloud

A modelling study is used to gain insight into the role of reduced nitrogen in orographic cloud chemistry. As the major basic gas in the atmosphere, ammonia is critical in determining the influence of cloud chemistry on particulate aerosol loadings and the magnitude of pollutant gaseous species in the lee of an orographic cloud. During the evaporation of wet cloud droplets to dry aerosol particles below their deliquescence point, the non-ideal solutions present in this microphysical regime “outgas” reduced nitrogen as gaseous ammonia. In polluted conditions (high SO2(g), NH3(g)) ammonia is fixed as particulate ammonium to neutralize sulphate production, and there is a net loss of ammonia in the airstream due to cloud processing. Conversely, in low pollutant conditions there may be a loss of particulate ammonium as gaseous ammonia. Cloud chemistry acts as a mechanism to re-partition reduced nitrogen from the larger aerosol particles to small aerosol particles, and evaporating cloud droplets outgas particulate ammonium as gaseous ammonia. Additionally, nucleation of a new ultrafine aerosol particle mode is likely to occur in the presence of evaporating cloud droplets if acidic gases such as nitric acid, and hydrochloric acid undergo similar “outgassing” from large aerosol particles.