Impact of fair-weather cumulus clouds and the Chesapeake Bay breeze on pollutant transport and transformation

Two fine-scale meteorological processes, fair-weather cumulus cloud development and a bay breeze, are examined along with their impacts on air chemistry. The impact of model resolution on fair-weather cumulus cloud development, transport of pollutants through clouds, sulfur dioxide to sulfate conversion in clouds, and the development of the Chesapeake Bay breeze are examined via 13.5, 4.5, 1.5, and 0.5 km resolution simulations covering the Washington – Baltimore area. Results show that as the resolution increases, more pollutants are transported aloft through fair-weather cumulus clouds causing an increase in the rate of oxidation of sulfur dioxide to sulfate aerosols. The high resolution model runs more nearly match observations of a local pollutant maximum near the top of the boundary layer and produce an increase in boundary layer venting with subsequent pollutant export. The sensitivity of sulfur dioxide to sulfate conversion rates to cloud processing is examined by comparing sulfur dioxide and sulfate concentrations from simulations that use two different methods to diagnose clouds. For this particular event, a diagnostic method produces the most clouds and the most realistic cloud cover, has the highest oxidation rates, and generates sulfur dioxide and sulfate concentrations that agree best with observations. The differences between the simulations show the importance of accurately simulating clouds in sulfate simulations. The fidelity of the model’s representation of the bay breeze is examined as a function of resolution. As the model resolution increases, a larger temperature gradient develops along the shoreline of the Chesapeake Bay causing the bay breeze to form sooner, push farther inland, and loft more pollutants upward. This stronger bay breeze results in low-level convergence, a buildup of near surface ozone over land and a decrease in the land-to-sea flux of ozone and ozone precursors as seen in measurements. The resulting 8 h maximum ozone concentration over the Bay is 10 ppbv lower in the 0.5 km simulation than in the 13.5 km simulation.