Temperature dependence of UV absorption cross sections and atmospheric implications of several alkyl iodides

The ultraviolet absorption spectra of a number of alkyl iodides which have been found in the troposphere, CH 3 I, C 2 H 5 I, CH 3 CH 2 CH 2 I, CH 3 CHICH 3 , CH 2 I 2 , and CH 2 ClI, have been measured over the wavelength range 200–380 nm and at temperatures between 298 and 210 K. The absorption spectra of the monoiodides CH 3 I, C 2 H 5 I, CH 3 CH 2 CH 2 I, and CH 3 CHICH 3 are nearly identical in shape and magnitude and consist of single broad bands centered near 260 nm. The addition of a chlorine atom in CH 2 ClI shifts its spectrum to longer wavelengths (σ max at 270 nm). The spectrum of CH 2 I 2 is further red‐shifted, reaching a maximum of 3.85×10 −18 cm 2 molecule −1 at 288 nm and exhibiting strong absorption in the solar actinic region, λ>290 nm. Atmospheric photolysis rate constants, J values, have been calculated assuming quantum efficiencies of unity for different solar zenith angles as a function of altitude using the newly measured cross sections. Surface photolysis rate constants, calculated from the absorption cross sections measured at 298 K, range from 3×10 −6 s −1 for CH 3 I to 5×10 −3 s −1 for CH 2 I 2 at a solar zenith angle of 40°.