Speciation, photosensitivity, and reactions of transition metal ions in atmospheric droplets

Abstract

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.

A numerical investigation of the destruction of peroxy radical by Cu ion catalysed reactions on atmospheric particles

“…Weschler et al (1986) considered the chelating ability of various ions to Cu, Fe and Mn in raindrops. The authors found that for Cu, SOi-and SOi-are probably the most important ligands after water.…”

Atmospheric transport of iron and its deposition in the ocean

“…The soluble Fe(III) may then be photoreduced to Fe(II). Graedel et al (1986), Weschler et al (1986), and Faust and Hoigne (1990) suggested that at a pH of -2.5-5, typical of cloud and rain droplets, the following reaction may produce Fe(II): hv [Fe(OH) (H20)J2+ H20 -+…”

Speciation of Metals in the Atmosphere

“…Thus it has been suggested that dissolved metals in wetted aerosols or rainwater occur as the aquated free ions and in complexes with chloride, sulfite, sulfate and hydroxide ions. Weschler et al (1986) have shown, using critical stability constants, that whereas Mn II and Ni II occur almost entirely as the hexaqua ion, Fe III can also occur in atmospheric droplets as [Fe(OH)(H 2 O) 5 ] 2ϩ , [Fe(OH) 2 (H 2 O) 4 ] ϩ and [Fe(SO 3 )(H 2 O) 4 ] ϩ with the partitioning being a function of pH. More recent studies have pointed to the importance of specific low molecular weight organic compounds in both aerosols and rain in controlling the speciation of iron.…”