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emissions of OM and EC and the outdated secondary organic aerosol mechanism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.