Update on Ozone Depleting Substances (ODSs) and Other Gases of Interest to the Montreal Protocol

Abstract

This chapter concerns atmospheric changes in ozone-depleting substances (ODSs), such as chlorofluorocarbons (CFCs), halons, chlorinated solvents (e.g., carbon tetrachloride [CCl4] and methyl chloroform [CH3CCl3]) and hydrochlorofluorocarbons (HCFCs), which are controlled under the Montreal Protocol. Furthermore, the chapter updates information about ODSs not controlled under the Protocol, such as methyl chloride (CH3Cl) and very short-lived substances (VSLSs). In addition to depleting stratospheric ozone, many ODSs are potent greenhouse gases. Mole fractions of ODSs and other species are primarily measured close to the surface by global or regional monitoring networks. The surface data can be used to approximate a mole fraction representative of the global or hemispheric tropospheric abundance. Changes in the tropospheric abundance of an ODS result from a difference between the rate of emissions into the atmosphere and the rate of removal from it. The total amount of chlorine and bromine from ODSs that were controlled under the original Montreal Protocol is continuing to decline, as the overall emissions are smaller than the rate at which these ODSs are destroyed. Abundances of many of the first-stage replacement compounds, HCFCs, are now increasing very slowly or not at all.