Solar photolysis of CH2I2, CH2ICI, and CH2IBr in water, saltwater, and seawater

Ultraviolet-visible absorption spectroscopy and purge-and-trap GC-MS were used to determine the rates and products of the photodissociation of low concentrations of CH(2)l(2), CH(2)lBr, and CH(2)lCl in water, saltwater (0.5 M NaCl), and seawater in natural sunlight. Photoproducts of these reactions include iodide (l(-)) and, in salt- and seawater environments, CH2XCl (where X = Cl, Br, or 1) Thus, CH2ICl was produced during CH(2)l(2) photolysis (with a molar yield of 35 20%), CH2BrCl from CH2IBr photolysis, and CH2Cl2 from CH(2)lCl photolysis (in lower yields of 6-10%). Formation of these chlorine- atom-substituted products may be via direct reaction of Cl- with either (A) the isopolyhalomethane photoisomer or associated ion pair (e.g., CH(2)l(+)-l(-)) or (B) the initially produced CH(2)l (.) photofragment. Estimated quantum yields for photodissociation were 0.62 +/- 0,09, 0.17 +/- 0.03, and 0.26 +/- 0.06 for CH(2)l(2), CH(2)lBr, and CH(2)lCl, respectively, in 0.5 M NaCl, with only small differences from these values in water and seawater. The much higher quantum yield of CH(2)l(2) photolysis compared to CH(2)l(2), and CH(2)lCl photolysis may be explained by the higher yield of the isodiiodomethane photoisomer of CH(2)l(2), resulting in reduced geminate recombination of the initially produced radical photofragments back to the parent molecule. We use a radiative transfer model with measured absorption cross-sections in saltwater to calculate seasonal values of CH(2)l(2), CH(2)l(2), and CH(2)lCl photodissociation in surface seawater at midlatitudes (50 degrees N) and show that a significant proportion of CH(2)lCl in surface seawater may arise from CH(2)l(2) photodecomposition. We also suggest that surface seawater photolysis of CH(2)l(2) over an 8 h period may contribute up to similar to 10% of the surface seawater l(-) levels, with implications for the increased deposition of O-3 to the surface ocean.