IMPACTS OF FAIR-WEATHER CUMULUS CLOUDS, BAY BREEZES, AND LAND USE ON URBAN AIR QUALITY AND CLIMATE

Abstract

Fair-weather cumulus clouds, bay breezes, and land use influence air quality and climate. The impacts of urban land surface changes and model resolution on fair-weather cumulus clouds, bay breezes, air quality, and climate are examined. As model resolution increases, more pollutants are transported aloft through fair-weather cumulus clouds causing an increase in the rate of sulfur dioxide conversion to sulfate aerosols and an increase in boundary layer venting. As 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. Also, an examination of the sensitivity of sulfur dioxide to sulfate conversion to different model cloud parameters shows the importance of accurately simulating clouds to obtain accurate sulfate concentrations. To analyze the impact of urbanization on the atmosphere, an urban tree parameterization is developed for the Weather Research and Forecasting model coupled with an urban canopy model (WRF-UCM) to determine how urban trees can dampen the urban heat island (UHI). Adding vegetation decreases the (subgrid-scale) surface air temperature due to tree shading and evapotranspiration. The impact of building height on the UHI shows that shorter urban buildings have higher daytime surface temperatures due to less shading and lower nighttime temperatures due to less longwave radiative trapping in urban street canyons. The WRF-UCM with urban trees is utilized with an air quality model to investigate how urban vegetation changes impact air quality. Cooling due to planting urban trees is expected to improve air quality. However, for one case study that does not include anthropogenic emissions reductions due to cooling from increased vegetation, adding trees in the model results in higher ground level ozone concentrations due to a shallower planetary boundary layer and more pollutants converging near a stronger bay breeze near Baltimore, MD. Future work incorporating changes in anthropogenic emissions with changes in urban vegetation will help quantify how urban trees impact air quality.