UV ABSORPTION CROSS-SECTIONS AND ATMOSPHERIC PHOTOLYSIS LIFETIMES OF HALOGENATED ALDEHYDES

UV absorption cross-sections for CCl3CHO, CCl2FCHO and CCIF2CHO have b een determined over the wavelength range 200-370 nm and at temperature s in the range 298-243 K using a dual beam diode array spectrometer. T he spectra show characteristic absorption due to the n --> pi transit ion of the C=O group with absorption maxima around 300 nm. On substitu tion of Cl by F in the CX3 group the absorption maxima showed a shift to longer wavelengths and a corresponding increase in the intensity of the absorption in the region of atmospheric photolysis. With decreasi ng temperature, all three aldehydes showed a small non-negligible decr ease in the cross-section in the long wavelength tail of the absorptio n band, and an increase in the cross-section around the absorption max ima. A two-dimensional photochemical model has been used to calculate atmospheric lifetimes due to photodissociation and OH radical loss. (C ) 1998 Elsevier Science S.A.