Improving Aerosol Transport to the Arctic in CAM5

Of the many factors contributing to the rapid arctic climate change, arctic haze has been identified as a potentially important forcing agent. It has been well established that arctic aerosols largely originate from lower latitudes. Hence, the long-range atmospheric transport of aerosols to the Arctic is of great concern for studying arctic climate change. The treatment of aerosol and cloud processes has been substantially improved in the current version of the Community Atmosphere Model (CAM5) which is widely used in the research of aerosol effects on clouds and climate. However, like many other global models, the CAM5 produces a relatively poor simulation of arctic aerosols and clouds. For example, previous studies have shown that the standard version of CAM5 remarkably underpredicts arctic aerosol concentrations, particularly during the arctic haze season, compared to various measurements. In this study, we focus on improving processes associated with aerosol-cloud interactions, cloud microphysics and macrophysics, and aerosol emission, transformation, removal, and deposition that are key to determining the amount of aerosols reaching the Arctic. Sensitivity experiments are conducted to understand the role of each of the processes and to identify sources of uncertainties, and improvements are made to processes that are not well represented in the CAM5. The evaluation and improvement are guided by aerosol and cloud measurements together with process-oriented model results from the multi-scale aerosol-climate model (PNNL-MMF) that embeds a cloud-resolving model in each CAM5 grid column to explicitly represent convection and aerosol-cloud interactions. Results show that including black carbon (BC) aging process through a more complete 7-mode version of the aerosol module in CAM5 can substantially increase the amount of arctic BC, compared to simulations with the standard 3-mode version, but has minimal effect on other species such as dust and sulfate. Excessive mid- and high-latitude liquid cloud fraction predicted by CAM5 is a key contributor to excessive removal of aerosols during their transport to the Arctic. Improvement in the treatment of liquid cloud fraction and, therefore, in predicting arctic aerosols may have implications for improving other aspects of CAM5 climate modeling. A newly implemented unified treatment of aerosol vertical transport, activation, and removal in convective clouds reduces the artificially high BC concentrations near the tropopause. This study also has implications for arctic aerosol source attribution and BC/dust deposition onto snow/ice that generates further impacts on global climate.