Treatment of clouds and the associated response of atmospheric sulfur in the Community Multiscale Air Quality (CMAQ) modeling system

An air quality modeling system—“Models-3/CMAQ”—is studied to determine the relationship between simulated cloud cover and atmospheric sulfur. Sulfur dioxide oxidation to sulfate occurs in the atmosphere through gas-phase reactions and reactions in clouds. The latter heterogeneous reactions can be rapid compared to gas-phase chemistry and a model must correctly simulate cloud cover to avoid serious bias. An evaluation of CMAQ revealed serious problems with diagnosed cloud cover and a bias in simulated sulfate production that was consistent with cloud biases. CMAQ modifications were tested to determine the sensitivity of the sulfur balance to cloud modeling assumptions. The modelʹs vertical layer structure, limits on subgrid-scale cloud base and top heights, and the ability to simulate subgrid-scale convective cloud formation were found to be important. After applying model changes, atmospheric sulfur simulations were less biased and sulfur dioxide and sulfate were in better balance.