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Using a mass balance approach, an atmospheric selenium budget for the region 30° N−90° N has been constructed. The atmosphere was assigned four different aerosol burdens corresponding to mean selenium aerosol concentrations in urban, intermediate (rural), remote/continental, and oceanic air. The budget calculations indicate that nearly all atmospheric selenium originates from anthropogenic emissions and biological activity in terrestrial and marine ecosystems. After comparing the sources of selenium to those of sulphur it is concluded that there are many similarities in their atmospheric cycling, but that there are also important differences.

To determine if Cu mediated reactions on atmospheric particles are important to HO,* chemistry in the ambient atmosphere, Cu molalities were calculated from measured Cu aerosol particle concentrations, mass and number size distribution data from a site in central Sweden. A comparison of characteristic times indicates that at low relative humidities the reaction is limited by the mass transport of gas phase HO,* to the particle surface and not by the chemical kinetics of the aqueous reaction. Comparison of half-lives for particle reactions and the gas phase destruction of HO,* to form H,O, indicate that heterogeneous reactions on aerosol particles may have important consequences on the chemistry of HO,*and H202 in the troposphere.