Modelling the fate of chemicals in the atmosphere

Atmosphere is an important component of the whole ecosystem because it directly interacts with all the other media, i.e. soil, surface waters, vegetation and biota. This chapter describes the processes that should be considered in models simulating the fate of chemicals in the atmosphere. The first section describes model approaches able to simulate the long-range transport of chemicals in the atmosphere. The second section describes the partition of chemicals between gaseous and particulate phases in the atmosphere. Two approaches, respectively, based on liquid vapour pressure and octanol-air partition coefficient are presented. The third section describes chemical reactions occurring in the atmosphere, driven by photolysis and reactions with photooxidants like the hydroxyl radical OH. The forth section describes dry deposition of both gaseous and particulate chemicals on the earth surface. Dry deposition is driven by aerodynamic, quasi-laminar sublayer and canopy resistances. The calculation of these latter is presented here in detail. The fifth section describes wet deposition of both gaseous and particulate chemicals on the earth surface, driven by rainout (in-cloud) and washout (below-cloud) scavenging.