Radiative forcing calculations for CH3Cl and CH3Br

Methyl chloride, CH3Cl, and methyl bromide, CH3Br, are particularly important in the global atmosphere as major natural sources of chlorine and bromine to the stratosphere. The production of these gases is dominated by natural sources, but smaller, important anthropogenic sources, such as agricultural fumigation and/or biomass burning, also exist. As absorbers of infrared radiation these gases are of interest for their potential effect on the tropospheric energy balance as well as for chemical interactions. In this study we estimate the radiative forcing and Global Warming Potentials (GWPs) of CH3Cl and CH3Br. Our calculations use an infrared radiative transfer model based on the correlated k-distribution algorithm for band absorption. Radiative forcing values of 0.0047 W/m(2) per part per billion by volume (ppbv) for CH3Cl in the troposphere and 0.0049 W/m(2) per ppbv for CH3Br in the troposphere were obtained. On a per molecule basis the radiative forcing values are about 2% of the forcing of CFC II and about 270 times the forcing of CO2. GWPs for these gases are about 8 for CH3Cl and about 4 for CH3Br (100 year time integration, CO2 = 1). These results indicate that while CH3Cl and CH3Br have direct GWPs similar to that of CH4, the current emission rates are too low to contribute meaningfully to atmospheric greenhouse heating effects.