The reduced nitrogen budget of an orographic cloud

Field data collected during the GDF 93 project indicated that during polluted conditions (SO2(g) > 2 ppbv, NH3(g) > 0.5 ppbv), sulphate and ammonium concentrations in air increased through cloud chemistry by as much as 25%. Similarly, ammonia was seen to be consumed by cloud processing and decreased by up to 20%. In comparatively clean conditions (SO2(g) < 0.5 ppbv, NH3(g) < 0.5 ppbv), the sulphate loading of the aerosol was seen to remain constant, and ammonium was lost from the aerosol and outgassed as ammonia, increasing ambient ammonia concentrations by as much as 0.5 ppbv. An ideal cloud chemistry model predicted up to 20% more sulphate production than is implied by the bulk aerosol data set. A non-ideal cloud chemistry model was used to estimate the final ammonium loading of the aerosol, which is determined by the transformation from wet cloud droplet to dry aerosol particles below their deliquescence point. The non-ideal model showed that in three of the four cases ammonia outgassed from evaporating cloud droplets, consistent with field observations, but at variance with the ideal chemistry model. The results indicate that in low pollution conditions clouds act to re-equilibrate reduced nitrogen in the aerosol phase with gaseous ammonia. The outgassed ammonia will then be rapidly deposited to semi-natural ecosystems downwind of such clouds.