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, CH3I, C2H5I, CH3CH2CH2I, CH3CHICH3, CH2I2, and CH2ClI, have been measured over the wavelength range 200-380 nm and at temperatures between 298 and 210 K. The absorption spectra of the monoiodides CH3I, C2H5I, CH3CH2CH2I, and CH3CHICH3 are nearly identical in shape and magnitude and consist of single broad bands centered near 260 nn. The addition of a chlorine atom in CH2ClI shifts its spectrum to longer wavelengths (sigma(max) at 270 nm). The spectrum of CH2I2 is further red-shifted, reaching a maximum of 3.85 x 10(-18) cm(2) molecule(-1) at 288 nm and exhibiting strong absorption in the solar actinic region, lambda>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 x 10(-6)s(-1) for CH3I to 5 x 10(-3)s(-1) for CH2I2 at a solar zenith angle of 40 degrees.