THE FATE OF ATMOSPHERIC PHOSGENE AND THE STRATOSPHERIC CHLORINE LOADINGS OF ITS PARENT COMPOUNDS - CCL4, C2CL4, C2HCL3,CH3CCL3, AND CHCL3

A study of the tropospheric and stratospheric cycles of phosgene is ca rried out to determine its fate and ultimate role in controlling the o zone depletion potentials of its parent compounds (CCl4, C2Cl4, CH3CCl 3, CHCl3, and C2HCl3). Tropospheric phosgene is produced from the OH-i nitiated oxidation of C2Cl4, CH3CCl3, CHCl3, and C2HCl3. Simulations u sing a two-dimensional model indicate that these processes produce abo ut 90 pptv/yr of tropospheric phosgene with an average concentration o f about 18 pptv, in reasonable agreement with observations. We estimat e a residence time of about 70 days for tropospheric phosgene, with th e vast majority being removed by hydrolysis in cloudwater. Only about 0.4% of the phosgene produced in the troposphere avoids wet removal an d is transported to the stratosphere, where its chlorine can be releas ed to participate in the catalytic destruction of ozone. Stratospheric phosgene is produced from the photochemical degradation of CCl4, C2Cl 4, CHCl3, and CH3CCl3 and is removed by photolysis and downward transp ort to the troposphere. Model calculations, in good agreement with obs ervations, indicate that these processes produce a peak stratospheric concentration of about 25-30 pptv at an altitude of about 25 km. In co ntrast to tropospheric phosgene, stratospheric phosgene is found to ha ve a lifetime against photochemical removal of the order of years. As a result, we find that a significant portion of the phosgene that is p roduced in the stratosphere is ultimately returned to the troposphere, where it is rapidly removed by clouds. This phenomenon effectively de creases the amount of reactive chlorine injected into the stratosphere and available for ozone depletion from phosgene's parent compounds; w e estimate approximate decreases of 14, 3, 15, and 25% for the stratos pheric chlorine loadings of CCl4, CH3CCl3, C2Cl4, and CHCl3, respectiv ely. A similar phenomenon due to the downward transport of stratospher ic COFCl produced from CFC-11 is estimated to cause a 7% decrease in t he amount of reactive chlorine injected into the stratosphere from thi s compound. Our results are potentially sensitive to a variety of para meters, most notably the rate of reaction of phosgene with sulfate aer osols. However, on the basis of the observed vertical distribution of COCl2, we estimate that the reaction of COCl2 with sulfate aerosol mos t likely has a gamma < 5x10(-5) and, as a result, has a negligible imp act on the stratospheric chlorine loadings of the phosgene parent comp ounds.