7.5    Characterization of Speciated Aerosol Radiative Forcing over California

Zhao, Chun,  L. Ruby Leung, Richard Easter, Pacific Northwest National Laboratory, Richland, WA, USA, Jenny Hand, Colorado State University, CO, USA, Jeremy Avise, California Air Resources Board, CA, USA

A fully coupled meteorology-chemistry-aerosol model (WRF-Chem) is improved with the capability of diagnosing the spatial and seasonal distribution of radiative forcings for individual aerosol species over California. Model simulations for the year of 2005 are evaluated with various observations including the meteorological fields from California Irrigation Management Information System (CIMIS), the aerosol mass concentrations from US EPA Chemical Speciation Network (CSN) and Interagency Monitoring of Protected Visual Environments (IMPROVE), and the aerosol optical depth from AErosol RObotic NETwork (AERONET) and satellites. The model well captures the observed seasonal meteorological conditions over California. Overall, the simulation is able to reproduce the observed spatial and seasonal distribution of mass concentration of total PM2.5 and the contribution from each aerosol species. The simulation reveals high anthropogenic aerosol loading over the central valley of California and the metropolitan regions surrounding Los Angeles and high natural aerosol (dust) loading over southeast California. The seasonality of surface aerosol concentration is mainly determined by vertical turbulent mixing, ventilation, and photochemical activity, with distinct characteristics for each aerosol species and between urban and rural areas. The model significantly underestimates the surface concentrations of organic matter (OM) and elemental carbon (EC), likely due to underestimation of the anthropogenic emissions of OM and EC and the outdated secondary organic aerosol mechanism used in the model. A sensitivity simulation with doubled anthropogenic EC emission significantly reduces the model low bias. The simulations show that anthropogenic aerosols dominate the aerosol optical depth (AOD). The ratio of AOD to AAOD (aerosol absorption optical depth) shows distinct seasonality with winter maximum and summer minimum. Aerosol radiative forcing is presented along with the contribution from each aerosol species from the simulation with doubled anthropogenic EC emission. On statewide average over California, aerosol reduces the seasonal-average surface radiation fluxes by up to about 5 W m-2 with a maximum in spring. In the atmosphere, aerosol introduces a warming effect of up to about 4 W m-2 with a maximum also in spring. EC and dust contribute about 65% and 15% of the total warming throughout the seasons, respectively. At the top of atmosphere (TOA), the overall aerosol radiative effect is cooling with a maximum of 1.5 W m-2 in winter. EC is the exclusive aerosol species contributing to the TOA warming of up to about 1 W m-2. The encouraging performance of WRF-Chem in simulating aerosols and their radiative forcing suggests that the model is suitable for further investigation of the impact of emission control on radiative forcing and regional climate over California.