Impact of Radiative Absorption by Black Carbon on Meteorology over the Eastern United States in July 2004

The increase in atmospheric abundance of aerosols since the preindustrial period has perturbed the radiative balance of the Earth-atmosphere system and may be contributing significantly to anthropogenic climate change. The radiative energy perturbation referred to as direct radiative forcing is due to aerosols' ability to scatter and absorb radiation. Global climate studies indicate that light-absorbing aerosol such as soot, often called black carbon (BC), exerts a warming influence that may be second only to that of carbon dioxide and may cause even larger perturbations on a regional scale. Atmospheric heating due to the absorption of solar radiation by BC is coincident with a reduction of solar radiation reaching the surface. This vertical redistribution of radiation directly affects static stability, boundary layer dynamics, and cloud evaporation. Each of these is an important factor in the transport and atmospheric distribution of aerosols and other chemical species, potentially resulting in complex feedbacks that occur at spatial scales smaller than typical resolutions of global climate models. This study examines the impact on meteorology of radiative absorption by BC. The mesoscale Weather Research and Forecast/Chemistry model (WRF/Chem) is used to simulate meteorology and air quality in the eastern United States for July 14-30, 2004. The Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) is coupled to WRF/Chem so that all aerosol processes and radiative calculations are simulated online with meteorological calculations. Simulation results show that a reduction of solar radiation due to scattering of aerosols has relatively small impact on surface temperature, whereas absorption of solar radiation by BC can cancel the cooling effect of scattering aerosols and induce a surface warming that is often correlated with reduction in low level clouds.