Vertical distributions of sulfur species simulated by large scale atmospheric models in COSAM: Comparison with observations

A comparison of large-scale models simulating atmospheric sulfate aerosols (COSAM) was conducted to increase Our understanding of global distributions of sulfate aerosols and precursors. Earlier model comparisons focused on w et deposition measurements and sulfate aerosol concentrations in source reg ions at the surface. They found that different models simulated the Observe d Sulfate surface concentrations mostly within a factor of two. but that th e simulated column burdens and vertical profiles were very different amongs t different models. In the COSAM exercise. one aspect is the comparison Of sulfate aerosol and precursor gases above the surface. Vertical profiles of SO2, SO42-, oxidants and cloud properties were measured by aircraft during the North Atlantic Regional Experiment (NARE) experiment in August Septemb er 1993 off the coast of Nova Scotia and during the Second Eulerian Model E valuation Field Study (EMEFSII), in central Ontario in March April 1990. Wh ile no single model stands Out as being best or worst, the general tendency is that those models simulating the full oxidant chemistry tend to agree b est with observations although differences in transport and treatment Of Cl ouds are important as well.