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 spect ra of the monoiodides CH3I, C2H5I, CH3CH2CH2I, and CH3CHICH3 are nearl y identical in shape and magnitude and consist of single broad bands c entered near 260 nn. The addition of a chlorine atom in CH2ClI shifts its spectrum to longer wavelengths (sigma(max) at 270 nm). The spectru m 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 cons tants, J values, have been calculated assuming quantum efficiencies of unity for different solar zenith angles as a function of altitude usi ng the newly measured cross sections. Surface photolysis rate constant s. calculated from the absorption cross sections measured at 298 K, ra nge from 3 x 10(-6)s(-1) for CH3I to 5 x 10(-3)s(-1) for CH2I2 at a so lar zenith angle of 40 degrees.