On-line simulations of passive chemical tracers in the University of California, Los Angeles, atmospheric general circulation model 1. CFC-11 and CFC-12

Long-term simulations of the response of atmospheric CFC-11 and CFC-12 to s tandard emission scenarios have been carried out using the University of Ca lifornia, Los Angeles (UCLA) atmospheric general circulation model (AGCM) c oupled on-line with the UCLA atmospheric chemistry model. For both compound s, photochemical loss rates are computed interactively over the entire mode l domain at each time step of the integration. Using industrial-based emiss ion estimates, the simulations for CFC-12 closely track the longterm trends recorded in both hemispheres by the Atmospheric Lifetime Experiment/Global Atmospheric Gases Experiment/Advanced Global Atmospheric Gases Experiment (AGA) and Climate Monitoring and Diagnostics Laboratory monitoring networks . The agreement between simulations and observations is best when AGA-deduc ed emissions are employed. The predicted surface mixing ratios of CFC-11, o n the other hand, are somewhat overestimated by the model. Because the tran sport and loss processes, as well as source distributions, are roughly simi lar for these halocarbons, the divergence in surface concentrations points to the possibility that emissions of CFC-11 may be overestimated for the pe riod extending from the late 1980s through the early 1990s, and perhaps eve n at earlier times. As for CFC-12, the best agreement is achieved using AGA emissions. The simulated interhemispheric exchange time constant for these CFCs is about 0.6 year. In the annual cycle, maximum transport occurs from the Northern to Southern Hemisphere within the lowest atmospheric layers d uring northern winter. Our best estimates of the annually averaged mean glo bal lifetimes of CFC-11 and CFC-12 are about 55 and 100 years, respectively . The simulations indicate that both the mean residence time and interhemis pheric exchange rate depend on the assumed model vertical domain. For the m ass balance analysis, when the upper boundary of the AGCM is artificially f ixed below similar to 35 km for CFC-11, or similar to 43 km for CFC-12, the re is a tendency for the timescales (lifetimes and interhemispheric exchang e times) to be overestimated. Comparisons between CFC distributions and tre nds calculated using low and high spatial resolution show relatively small differences in the present case. These results, especially regarding CFC pe rsistence and interhemispheric exchange, suggest that the present model acc urately represents the global dispersion of long-lived chemical tracers.