Considerable evidence indicates that dissolved transition metal ions (TMI) are capable of catalyzing oxidations in atmospheric water droplets, at least in certain circumstances. Wide variations in the importance of TMI chemistry are expected in these systems because concentrations of transition metals in water droplets range over at least 4 orders of magnitude. In the present work we perform an extensive series of model calculations for TMI chemistry in raindrops. The specific TMI discussed are iron, manganese, and copper. The present treatment is restricted to homogeneous processes, that is, those involving dissolved molecules and ions. Results are presented for studies at pH 3 and pH 4, for both daytime and nighttime conditions. Among the results are the following: (1) At pH 3 and pH 4, Fe(III) is present largely as photosensitive hydroxide complexes. Our model results indicate that under atmospheric conditions the photolysis of these complexes is the primary daytime source of reactive free radicals within the droplets, even at quite low TMI concentrations. (2) TMI complex photolysis is not, of course, operative at night. At those times the presence of TMI continues to control the concentration of free radicals in raindrops through “Fenton type” reactions with hydrogen peroxide, for example, Fe(II) + H2O2 → Fe(III) + OH + OH−. (3) The oxidation of S(IV) to S(VI) by H2O2 is the most important daytime sulfite oxidation process in raindrops, but S(IV) oxidation catalyzed by Mn(II) can be significant under certain conditions, particularly at night or in the winter months. (4) Solution nitrogen chemistry is relatively unaffected by TMI. Its most important daytime chemical process is the (rather inefficient) photodissociation of the nitrate ion, a process which generates ozone (and hence HOx radicals). (5) Organic chemistry in atmospheric water droplets is very sensitive to the presence of hydroxyl radicals. Since OH· production is strongly influenced by catalysis involving iron complexes, the presence of soluble iron is a major stimulus for organic chemical processes (such as the conversion of alkyl aldehydes to carboxylic acids). (6) For cases where S(IV) concentrations exceed those of H2O2 the S(IV) effectively “titrates out” the H2O2. If the H2O2 concentration dominates, however, residual H2O2 remains to initiate a variety of solution oxidation chains, particularly those producing organic acids. The presence of H2O2 in the gas phase thus implies acid production in the aqueous phase, the form of the acid depending upon the particular oxidizable species available. (7) The chemical reactions and rate constants used in the calculations are relatively well determined, but our results are quite sensitive to the assumed concentrations of TMI and other species. Increased attention to measurement of species concentrations in fog, clouds, and rain is therefore indicated.

Dissolved transition 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 processes 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 processes 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+, [Fe(OH)2(H2O)4]+, and [Fe(SO3)(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 radicals 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 processes in atmospheric droplets may involve TMI. Among these processes are the production and destruction of alkylhydroperoxides, the chemical chains linking RO2· radicals to stable alcohols and acids, and the oxidation of aliphatic aldehydes to organic acids.

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