Dissolved tranistion metal ions (TMI) are common constituents of atmospheric droplets. They are known to catalyze sulfur oxidation in droplets and are suspected of being involved in other chemical proesses as well. We have reviewed the relevant equilibrium constants and chemical reactions of the major TMI (iron, manganese, copper, and nickel), their ability to form complexes in aqueous solution, and their potential involvement in photochemical process in atmospheric droplets. Among the results are the following: (1) The major Fe (III) species in atmospheric water droplets are Fe(OH)(H2O)5>2+, 2(H2O)4>+, and 3)(H2O)5>+; the partitioning among these complexes is a function of pH. In contrast, Cu(II),Mn(II), and Ni(II) exist almost entirely in the droplets as hexaquo complexes. (2) Within the tropospheric solar spectrum, some of the complexes of Fe(III) have large absorption cross sections. In this work we report cross-section data for several of the complexes. Absorption of solar photons by such complexes is generally followed by cleavage, which in the same process reduces the iron (III) atom and produces a reactive free radical.

This mechanism has the potential to be a significant and heretofore unappreciated source of free radical in atmospheric droplets. (3) TMI participate in redox reactions with H2O2 and its associated species HO2. and O2-. These reactions furnish the potential for catalytic cycles involving TMI in atmospheric droplets under a variety of illumination and acidity conditions. (4) A number of organic processed in atmospheric droplets may involve TMI. Among these processes are the production and destruction of alkyhydroperoxides, the chemical chains linking RO2. radicals to stable alcohols and acids, and the oxidation of aliphatic aldehydes to organic acids.