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

Abstract

Long‐term simulations of the response of atmospheric CFC‐11 and CFC‐12 to standard emission scenarios have been carried out using the University of California, Los Angeles (UCLA) atmospheric general circulation model (AGCM) coupled on‐line with the UCLA atmospheric chemistry model. For both compounds, photochemical loss rates are computed interactively over the entire model domain at each time step of the integration. Using industrial‐based emission estimates, the simulations for CFC‐12 closely track the long‐term 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‐deduced emissions are employed. The predicted surface mixing ratios of CFC‐11, on the other hand, are somewhat overestimated by the model. Because the transport and loss processes, as well as source distributions, are roughly similar for these halocarbons, the divergence in surface concentrations points to the possibility that emissions of CFC‐11 may be overestimated for the period extending from the late 1980s through the early 1990s, and perhaps even 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 during northern winter. Our best estimates of the annually averaged mean global lifetimes of CFC‐11 and CFC‐12 are about 55 and 100 years, respectively. The simulations indicate that both the mean residence time and interhemispheric exchange rate depend on the assumed model vertical domain. For the mass balance analysis, when the upper boundary of the AGCM is artificially fixed below ∼35 km for CFC‐11, or ∼43 km for CFC‐12, there is a tendency for the timescales (lifetimes and interhemispheric exchange times) to be overestimated. Comparisons between CFC distributions and trends calculated using low and high spatial resolution show relatively small differences in the present case. These results, especially regarding CFC persistence and interhemispheric exchange, suggest that the present model accurately represents the global dispersion of long‐lived chemical tracers.