UV ABSORPTION CROSS-SECTIONS AND ATMOSPHERIC PHOTOLYSIS RATES OF CF3I, CH3I, C2H5I AND CH2ICL

The absorption cross-sections of trifluoromethyl iodide (CF3I), methyl iodide (CH3I), ethyl iodide (C2H5I) and chloroiodomethane (CH2ICI) ha ve been determined over the wavelength range 235-400 nm, with a spectr al resolution of 0.6 nm (FWHM), using a diode array spectrometer. The spectra consist of a broad continuous absorption band with maximum cro ss-sections of (6.0 +/- 0.1) x 10(-19) cm(2) molecule(-1) for CF3I at 267 nm, (1.09 +/- 0.02) x 10(-18) cm(2) molecule(-1) for CH3I at 258 n m, (1.18 +/- 0.04) x 10(-18) cm(2) molecule(-1) for C2H5I at 258 nm an d (1.21 +/- 0.07) x 10(-18) for CH2ICI at 270 nm. The temperature depe ndence of the cross-section was investigated over the range 333-243 K. A decline in the cross-section with decreasing temperature at wavelen gths longer than ca. 280 nm was observed in all cases, the decrease be ing most pronounced in the long wavelength tail of the absorption band . At wavelengths shorter than ca. 270 nm the cross-section increased w ith decreasing temperatures; the effect, however, was significantly sm aller (5-10%), being most pronounced around the band maximum. The temp erature dependence was parametrised in order to calculate the atmosphe ric photolysis rate as a function of altitude, latitude and season. Mo del calculations show that during sunlit hours the iodides will be rap idly photolysed with tropospheric photodissociation lifetimes of ca. 1 day for CF3I, several days for CH3I and C2H5I and several hours for C H2ICI.