A comparison of large-scale atmospheric sulphate aerosol models (COSAM): overview and highlights

The comparison of large-scale sulphate aerosol models study (COSAM) compare d the performance of atmospheric models with each other and observations. I t involved: (i) design of a standard model experiment for the world wide we b, (ii) 10 model simulations of the cycles of sulphur and Rn-222 Pb-210 con forming to the experimental design, (iii) assemblage of the best available observations of atmospheric SO4=, SO2 and MSA and (iv) a workshop in Halifa x, Canada to analyze model performance and future model development needs. The analysis presented in this paper and two companion papers by Roelofs, a nd Lohmann and co-workers examines the variance between models and observat ions. discusses the sources of that variance and suggests ways to improve m odels. Variations between models in the export of SOx from Europe or North America are not sufficient to explain an order of magnitude variation in sp atial distributions of SOx downwind in the northern hemisphere. On average, models predicted surface level seasonal mean SO4= aerosol mixing ratios be tter (most within 20%) than SO2 mixing ratios (over-prediction by factors o f 2 or more). Results suggest that vertical mixing from the planetary bound ary layer into the free troposphere in source regions is a major source of uncertainty in predicting the global distribution of SO aerosols in climate models today. For improvement. it is essential that globally coordinated r esearch efforts continue to address emissions of all atmospheric species th at affect the distribution and optical properties of ambient aerosols in mo dels and that a global network of observations be established that will ult imately produce a world aerosol chemistry climatology.