UV absorption cross-sections and atmospheric photolysis rates of CF3I, CH3I, C2H5I and CH2ICl

Abstract

The absorption cross-sections of trifluoromethyl iodide (CF ₃ I), methyl iodide (CH ₃ I), ethyl iodide (C ₂ H ₅ I) and chloroiodomethane (CH ₂ ICl) have been determined over the wavelength range 235–400 nm, with a spectral resolution of 0.6 nm (FWHM), using a diode array spectrometer. The spectra consist of a broad continuous absorption band with maximum cross-sections of (6.0±0.1)×10 ^-19 cm ² molecule ^-1 for CF ₃ I at 267 nm, (1.09±0.02)×10 ^-18 cm ² molecule ^-1 for CH ₃ I at 258 nm, (1.18±0.04)×10 ^-18 cm ² molecule ^-1 for C ₂ H ₅ I at 258 nm and (1.21±0.07)×10 ^-18 for CH ₂ ICl at 270 nm. The temperature dependence of the cross-section was investigated over the range 333–243 K. A decline in the cross-section with decreasing temperature at wavelengths longer than ca. 280 nm was observed in all cases, the decrease being most pronounced in the long wavelength tail of the absorption band. At wavelengths shorter than ca. 270 nm the cross-section increased with decreasing temperatures; the effect, however, was significantly smaller (5–10%), being most pronounced around the band maximum. The temperature dependence was parametrised in order to calculate the atmospheric photolysis rate as a function of altitude, latitude and season. Model calculations show that during sunlit hours the iodides will be rapidly photolysed with tropospheric photodissociation lifetimes of ca. 1 day for CF ₃ I, several days for CH ₃ I and C ₂ H ₅ I and several hours for CH ₂ ICl.