UV–VIS absorption cross-sections and atmospheric lifetimes of CH2Br2, CH2I2 and CH2BrI

Abstract

The UV–VIS absorption spectra of CH₂Br₂, CH₂I₂ and CH₂BrI have been measured over the wavelength range 215–390 nm using a dual-beam diode array spectrometer. The spectra consist of broad continuous absorption bands. CH₂Br₂ exhibits its maximum cross-section of σ=2.71(±0.16)×10^-18 cm² molecule^-1 at λ=219 nm. The magnitude of the peak cross-sections for the iodine-containing molecules above λ=210 nm are σ=1.62(±0.10)×10^-18 cm² molecule^-1 at λ=248 nm and σ=3.78(±0.23)×10^-18 cm² molecule^-1 at λ=288 nm for CH₂I₂, and σ=5.67(±0.34)×10^-18 cm² molecule^-1 at λ=215 nm and σ=2.34(±0.14)×10^-18 cm² molecule^-1 at λ=267 nm for CH₂BrI. The temperature dependence of the absorption cross-sections was investigated over the temperature range 348–250 K. A decline in the cross-sections with decreasing temperature was observed in the tail of the spectra. At the peaks the opposite effect was observed. All three gases have been found in the atmosphere and the atmospheric photolysis rates of CH₂Br₂, CH₂I₂ and CH₂BrI were calculated as a function of altitude and solar zenith angle using the measured cross-sections. Model calculations show that, during sunlit hours, CH₂I₂ and CH₂BrI will be photolysed within minutes and hours, respectively. The photolysis of CH₂Br₂ is much slower and reaction with the OH radical is the dominant atmospheric loss process.