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.
Bulk freezing experiments have been performed with binary and ternary HNO3/H2SO4/H2O solutions containing original micrometeorites, ground samples of representative larger meteorites and other freezing nuclei of potential stratospheric importance. The experiments enable us to determine upper bounds for the heterogeneous freezing rates of sulfuric and nitric acid hydrates. Based on an analysis of the meteoritic mass flux from space and of the modifications meteorites undergo when entering the atmosphere, the resulting morphology and surface area of extraterrestrial material in the stratosphere are estimated. From this micrometeorites gained from Antarctica are shown to be a good proxy for meteoritic surfaces in the stratosphere. In combination with this analysis the freezing experiments suggest that heterogeneous nucleation rates on micrometeorites are too low to enhance freezing of polar stratospheric clouds above the frost point.
[1] Recent laboratory measurements of the uptake of gaseous hypoiodous acid (HOI) on NaCl, NaBr, and sea-salt surfaces have been used to aid calculation of the expected liberation of chlorine atoms from sea-salt aerosol resulting from marine boundary layer (MBL) iodine chemistry. An existing model has been extended to include reactive chlorine chemistry resulting from two activation mechanisms. Measurements of the iodine monoxide (IO) and nitrate (NO 3 ) radicals from three recent field experiments have been used to constrain numerical simulations, investigating the relative importance of the iodine-and nitrogen-mediated mechanisms. These calculations show that the reactive uptake of both HOI and N 2 O 5 in the MBL can liberate Cl atoms from sea-salt aerosol at a significant rate. A characteristic pulse of Cl production leading to several thousand molecules per cubic centimeter after sunrise may prove to be a useful diagnostic for the iodine-mediated mechanism. Steady daytime production is predicted, leading to steady state [Cl] of several hundred to a few thousand molecules per cubic centimeter, perhaps over wide areas of the oceans. In addition, under certain conditions reactive bromine chemistry due to IBr release from fresh sea salt by reaction of HOI may dominate ICl production.
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