CHEMICAL LIFETIMES OF CHLORINATED ALIPHATIC PRIORITY POLLUTANTS IN THE CANADIAN TROPOSPHERE

Estimates have been obtained for the half-lives of several chlorinated aliphatic pollutants in the Canadian troposphere. The rates of reacti on are strongly dependent on the magnitude of the second-order rate co nstant for reaction of the pollutant with OH, and on the intensity of solar radiation, which is determined both by season and geographical l ocation. Of the pollutants studied, trichloroethylene has the shortest half-life of 2-5 days in summer, depending on location. Dichlorometha ne, 1,2-dichloroethane and tetrachloroethylene are each about one orde r of magnitude longer lived than trichloroethylene; 1,1,1-trichloroeth ane and 1,1,2,2-tetrachloroethane are about two orders of magnitude lo nger lived than trichloroethylene. Under urban conditions, high rates of oxidation, and hence short half-lives, are favoured by high troposp heric concentrations of ozone and low concentrations of NO2. With the exception of 1,1,1-trichloroethane, none of these substances is predic ted to pose a significant threat to stratospheric ozone.