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Radiative Forcing of Aerosols and Its Impact on Surface Air Temperature on Synoptic Scale in Eastern China

Using the WRF-Chem(Weather Research and Forecasting—Chemistry) model, we investigate aerosol radiative forcing and its impacts on surface air temperature in Eastern China on the synoptic scale. Simulations of five selected periods without precipitation(August 23–25, 2006; November 10–12, 2008; December 16–18, 2008; January 15–17, 2009; and April 27–29, 2009) indicate that aerosol concentrations are generally lower in the daytime and higher at night. Simulated surface–layer concentrations of PM2.5 averaged over Northern China(29.8°–42.6°N, 110.2°–120.3°E,) are 40–80 μg m-3, higher than the simulated values of 30–47 μg m-3 in Southern China(22.3°–29.9°N,109.7°–120.2°E). Aerosols exerted a large cooling effect on surface air temperature(SAT, 2 m above ground level), and the effect is the strongest at 08:00 and 17:00 with decreases in temperature by 0.2–1 K. We also find that including aerosols in simulations improves the simulation of SAT. Based on the simulation of a weather event during August 23 to 25 in 2006, we quantitatively analyze the total(direct + indirect), direct, and indirect radiative effects of aerosols on SAT. Shortwave radiative forcing at the surface induced by direct effects of aerosols is stronger in Northern China than that in Southern China, with average forcings of-11.3 W m-2 and-5.8 W m-2, respectively, and corresponding coolings of SAT by 0.074 K and 0.039 K. However, shortwave radiative forcing induced by indirect effects of aerosols is weaker in Northern China than that in Southern China, with average forcings of-12.4 W/m2 and-14.4 W m-2, respectively, and corresponding coolings of SAT by 0.035 K and 0.094 K. The direct and indirect effects of sulfate are of similar magnitude. The total effect of sulfate on shortwave radiative forcing in Northern and Southern China is-7. 0 W m-2 and-10.5 W m-2, respectively, causing a cooling of SAT by 0.062 K and 0.074 K, while the effect of nitrate is relative weak. Due to black carbon aerosols, the solar shortwave radiation reaching the surface is decreased by 6.5 W m-2 and 5.8 W m-2 and the SAT increases by 0.053 K and 0.017 K averaged over Northern and Southern China, respectively. The direct effect of black carbon on shortwave radiation is much larger than the indirect effect.