Reactive Halogen Compounds in the Atmosphere

Abstract

The defining characteristic of reactive chlorine gases is that they are removed by tropospheric processes such as oxidation,hydrolysis in clouds and oceans, rain,or by deposition.Chlorine- containing gases may be classified according to their lifetimes in the troposphere, which is often a reflection of their chemical reactivities. If the lifetime is less than about one year, the gases are considered to be “reactive”;if it is between 1–10 years, the gases are “moderately reactive”;and if the lifetimes are longer, the gases are “unreactive”and are not removed in the lower atmosphere. There are some 23 Tg of gaseous chlorine in the atmosphere or about 4 parts per billion of the troposphere. Of this, only about 3 Tg can be identified to come from natural sources, and all of it is in the form of reactive chlorine gases.Moderately reactive and unreactive gases are entirely man-made. This chapter is a review of the global budgets and trends of chlorine-containing gases with emphasis on the reactive group.All the known gases in each group are included here if concentrations are above a few parts per trillion and the existence of the gas is mostly due to direct emissions. The group of reactive gases consists of chloromethane or methyl chloride (CH3Cl), chloroform (CHCl3),phosgene (COCl2), dichloro- methane (CH2Cl2), chlorinated ethylenes (C2HCl3,C2Cl4), chlorinated ethanes (CH4Cl2, C2H2Cl4), some other rarer gases, and inorganic chlorine gases such as hydrogen chloride (HCl) and molecular and atomic chlorine (Cl2 ,Cl).Natural sources are identified for most of these gases.Methylchloride, for instance, comes from the oceans and fungal action on rotting wood.Although methylchloride is the most abundant chlorine-containing gas in the atmos- phere, its budget is not understood at present. The class of moderately reactive gases consists of methylchloroform (CH3CCl3), chlorodifluoromethane (CHClF2), dichlorofluoromethane (CHCl2F), and other less abundant chlorinated gases with concentrations below a few parts per trillion. The long-lived or unreactive gases are the chlorofluorocarbons (CCl2F2,CCl3F, C2Cl3F3,C2Cl2F4,C2ClF5,CCl3F) and carbon tetrachloride (CCl4). The long-lived gases are discussed here for comparison and because of their important role in the global cycle of gaseous chlorine. The most important gases in these last two groups have well documented anthropogenic emissions that are consistent with the observed concentrations, and no natu- ral sources have been unequivocally identified. Since these gases will no longer be produced because of the Montreal Protocol and its amendments (international agreements to phase out global production of compounds that can deplete the stratospheric ozone layer), new com- pounds are being manufactured to take their place. The first data on some of the new chlori- nated replacement compounds, namely HCFC-141b and HCFC-142b, are discussed.