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Atmospheric oxidation processes for possible chlorofluorocarbon replacement compounds


It is the purpose of this project to provide detailed rate and mechanistic data for the oxidation of halogenated compounds in the troposphere. This data will provide estimates of chlorine and bromine fluxes into the stratosphere and an understanding of their tropospheric sinks. Structure reactivity relationships for these species will be derived in order that reliable predictions of the behaviour of any released halogenated compound and its oxidation products can be made and its importance as an ozone depletion agent evaluated.

The laser flash photolysis technique with ultraviolet (UV) absorption detection has been used to determine the rate coefficients for the reactions of peroxy radicals, RO2, formed from hydrochlorofluorocarbon (HCFC)-22, -14lb and -142b with the important atmospheric radical, HO2. The peroxy radicals were formed following the 248 nm laser photolysis of ozone, HCFC, oxygen, nitrogen mixtures. Analysis of the transient absorption profiles recorded between 220 and 240 nm was undertaken using numerical integration of assumed chemical mechanisms. The following rate coefficients were obtained for the reaction, RO2 + HO2 forming ROOH + oxygen:
k = (3.4 +/- 1.0)E-12 cm{3} molecule{-1} s{-1} (HCFC-22, R = CF2Cl);
k = (9.2 +/- 2.6)E-12 cm{3} molecule{-1} s{-1} (HCFC-14lb, R = CFCl2CH2);
k = (6.8 +/- 2.0)E-12 cm{3} molecule{-1} s{-1} (HCFC-142b, R = CF2ClCH2).
Over the past few years there have been significant advances in our understanding of the impact of human activities on the Earth's stratospheric ozone layer. Large reduction in amount of ozone over Antarctica, with consequent increases in surface ultraviolet radiation, have been observed over the last five years. While ozone loss over the Arctic is not as pronounced, localized Arctic ozone losses have been recorded in winter.

It is now apparent from ground-based and satellite measurements that significant decreases of total column ozone have also occurred in both the northern and southern hemispheres at middle and high latitudes and that these decreases are largely in the lower stratosphere. Recent laboratory and field measurement data have provided evidence to suggest that stratospheric ozone loss is predominantly the result of chlorine and bromine containing compounds of anthropogenic origin.

The above mentioned data are urgently required to improve the predictive ability of global atmospheric models. The dominant loss process in the troposphere for hydrogen-containing chlorinated and brominated compounds is through reaction with the hydroxyl radical. Initial studies have provided some insight into the oxidation mechanisms for the haloalkyl radical formed by reaction with OH radicals. It is now apparent that oxidation of these species is rather complex with the product distributions being temperature and pressure dependent and it is therefore necessary to elucidate the mechanisms of such oxidation process, task of this project.


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CSC - Cost-sharing contracts


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Stillorgan Road Belfield
4 Dublin

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Participantes (8)