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

Abstract

The absorption cross-sections of trifluoromethyl iodide (CF₃I), methyl iodide (CH₃I), ethyl iodide (C₂H₅I) and chloroiodomethane (CH₂ICl) 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 cm² molecule^-1 for CF₃I at 267 nm, (1.09±0.02)×10^-18 cm² molecule^-1 for CH₃I at 258 nm, (1.18±0.04)×10^-18 cm² molecule^-1 for C₂H₅I at 258 nm and (1.21±0.07)×10^-18 for CH₂ICl 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 CF₃I, several days for CH₃I and C₂H₅I and several hours for CH₂ICl.

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