Urban Aerosol Direct Effect on Surface Temperature

The Urban Heat Island effect (UHI) refers to urban skin or air temperature exceed-ing the temperatures in surrounding non-urban regions. The UHI is a well-established example of an anthropogenic perturbation to the weather-climate-system. In a warming climate, the UHI may intensify extreme heat waves and consequently cause significant health and energy problems. Therefore, reducing urban surface temperature could have implications for mitigating some key public health or energy issues. The aerosol direct effect, namely, scattering and absorbing sunlight in the atmosphere can lower surface temperature by reducing surface insolation. By combining National Aeronau-tics and Space Administration (NASA) AERONET (AErosol RObotic NETwork) obser-vations over large cities with Weather Research and Forecasting (WRF) model simula-tions, we find that the aerosol direct reduction of surface insolation ranges from 40-100 Wm-2, depending on aerosol loads and land-atmosphere conditions. These values are calculated using a radiative transfer model based on the top 25% of the multi-year in-stantaneous aerosol data observed by AERONET sites. As a result, surface skin tem-perature can be reduced by 1-2ºC while 2-m surface air temperature reductions are gen-erally on the order of 0.5-1°C. This study suggests that the aerosol direct effect is a competing mechanism for the urban heat island effect. More importantly, both aerosol and urban land cover effects must be adequately represented in meteorological and climate modeling systems in order to properly characterize urban surface energy budg-ets and UHI.