Heterogeneous processes in the Po Valley radiation fog

Abstract

The heterogeneous processes responsible for fog water chemical composition are examined in this paper. SO 2 /SO 4 = and NO x /HNO 3 /NO 3 − systems have been particularly investigated for their role in the production of acidity within the dispersed liquid phase. A program is presented, involving several European and American institutes, which has thus far provided the following main results: (1) nucleation scavenging represents a major source of sulphate in fog, but a potential also exists for SO 2 conversion in the liquid phase; (2) excess nitrate which was found in fog, with respect to the prefog aerosol concentration, appears to derive from gas phase HNO 3 incorporation into the droplets. Chemical processes seem to be active at the stage of fog formation, with microphysical condensation/evaporation/deposition/entrainment processes being primarily responsible for changes in the chemical composition of the system during fog evolution. Additional and improved data requirements indicated by our results are discussed.