Microbial degradation of atmospheric halocarbons

Abstract

Halocarbons are present in the atmosphere at parts-per-trillion (ppt) mixing ratios and are represented by such substances as chlorofluorocarbons (CFCs), hydrochlolofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), and methyl halides like methyl bromide (MeBr) and their further substituted halomethane analogues (e. g., dibromomethane, bromoform). Many Halocarbons have only an anthropogenic origin (e.g., CFCs, HCFCs, HFCs) whereas others are of mixed anthropogenic and natural origin (e.g., MeBr). The molecular composition of the various CFCs, HCFCs, and HFCs can be deduced from their numerical classification by adding 90 to their designated numbers. The first digit (hundreds place) refers to the number of carbon atoms, the second to the number of hydrogens, and the third to the number of fluorines. For example, the molecular structure of CFC-12 is obtained from the resultant number of 102, and refers to a one carbon molecule containing no hydrogens and two fluorines. By inference from its CFC designation two chlorines must also be included. Similarly, the compound HFC-134 has a resultant sum of 224, which implies a two carbon molecule containing two hydrogens and four fluorines (tetrafluoroethane).