KINETICS OF THE ATMOSPHERICALLY IMPORTANT REACTIONS OF DIMETHYL SELENIDE

The biomethylation of selenium into dimethyl selenide from soils, sediments, and plants is a major source of atmospheric selenium. The photolysis and kinetics of the gas-phase reactions of dimethyl selenide with OH and NO3 radicals and O3 were investigated at 296 ± 2 K and atmospheric pressure of air. No evidence for photolysis of dimethyl selenide was observed. Rate constants for reaction with the OH radical and O3 of (6.78 ± 1.70) × 10−11 and (6.80 ± 0.72) × 10−17 cm3 molecule−1 s−1, respectively, were obtained. The rate constant for reaction with the NO3 radical was observed to increase with increasing NO2 concentration, with an extrapolated value of 1.4 × 10−11 cm3 molecule−1 s−1 at zero NO2 concentration. Combining these rate constants with estimated ambient tropospheric concentrations of OH radicals (1.5 × 10−6 molecule cm−3), NO3 radicals (2.4 × 108 molecule cm−3) and O3 (7 × 1011 molecule cm−3) leads to the following calculated lifetimes of dimethyl selenide due to reaction with these species: OH radicals, 2.7 h; NO3 radicals, 5 min; and O3, 5.8 h. The products of these reactions of dimethyl selenide are presently unknown but are likely to be involved in gas-to-particle conversions and aerosol scavenging during the global cycling of selenium. © 1990, American Chemical Society. All rights reserved.