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

Abstract

The UVÈVIS absorption spectra of and have been measured over the wavelength range 215È390 nm CH 2 Br 2 , CH 2 I 2 CH 2 BrI using a dual-beam diode array spectrometer. The spectra consist of broad continuous absorption bands. exhibits its CH 2 Br 2 maximum cross-section of p \ 2.71(^0.16) ] 10~18 cm2 molecule~1 at j \ 219 nm. The magnitude of the peak cross-sections for the iodine-containing molecules above j \ 210 nm are p \ 1.62(^0.10) ] 10~18 cm2 molecule~1 at j \ 248 nm and p \ 3.78(^0.23) ] 10~18 cm2 molecule~1 at j \ 288 nm for and p \ 5.67(^0.34) ] 10~18 cm2 molecule~1 at j \ 215 CH 2 I 2 , nm and p \ 2.34(^0.14) ] 10~18 cm2 molecule~1 at j \ 267 nm for The temperature dependence of the absorption CH 2 BrI. cross-sections was investigated over the temperature range 348È250 K. A decline in the cross-sections with decreasing temperature was observed in the tail of the spectra. At the peaks the opposite e †ect was observed. All three gases have been found in the atmosphere and the atmospheric photolysis rates of and were calculated as a function of altitude CH 2 Br 2 , CH 2 I 2 CH 2 BrI and solar zenith angle using the measured cross-sections. Model calculations show that, during sunlit hours, and CH 2 I 2 CH 2 BrI will be photolysed within minutes and hours, respectively. The photolysis of is much slower and reaction with the OH CH 2 Br 2 radical is the dominant atmospheric loss process.

Reactive and organic halogen species in three different European coastal environments

“…Our measurements in Brittany can also be understood as an upper limit for BrO from organic sources, since we measured CH 2 IBr as a possible source for reactive bromine in exceptionally high mixing ratios (up to 10 ppt), which have thus far not been reported. Brominated hydrocarbons have lifetimes on the order of days and weeks, with the exception of CH 2 IBr, which has a photolytic lifetime of ∼1 h (Mössinger et al, 1998). A statistical analysis of all BrO data taken during the Brittany campaign gives no indication that organic species are precursors for BrO, since there is no dependency of the BrO signal on tidal height or the appearance of brominated hydrocarbons.…”

Marine Organic Halide and Isoprene Emissions Near Mace Head, Ireland

“…Atmospheric lifetimes with respect to these processes vary with individual VOC from minutes (CH 2 I 2 , ∼4 and CH 2 IBr, ∼60 min) to days. [17] The calculated emission fluxes for CH 2 I 2 and CH 2 IBr may represent an upper limit to their fluxes, as chemical losses will act to increase the concentration gradient with altitude.…”

A Study on the Production and Emission of Marine-Derived Volatile Halocarbons

“…The mixing ratios of both CH 2 ClI and CH 2 I 2 showed high temporal variability, much higher at night than in the daytime, and CH 2 I 2 , in particular, was rarely detected at midday. This finding can be explained by their short atmospheric lifetimes (several hours for CH 2 ClI and several minutes for CH 2 I 2 ) during the day due to photolysis (e.g., Rattigan et al 1997;Mössinger et al 1998). Seasonal variations of the CH 2 ClI and CH 2 I 2 mixing ratios differed considerably between Cape Ochiishi and Hateruma Island.…”