1Department of Civil and Environmental Engineering, University of California, Davis. One Shields Avenue, Davis CA, USA
2Department of Land, Air, and Water Resources, University of California, Davis. One Shields Avenue, Davis CA, USA
3National Renewable Energy Laboratory, Golden CO, USA
Abstract. A source-oriented representation of airborne particulate matter was added to the Weather Research & Forecasting (WRF) model with chemistry (WRF/Chem). The source-oriented aerosol separately tracks primary particles with different hygroscopic properties rather than instantaneously combining them into an internal mixture. The source-oriented approach avoids artificially mixing light absorbing black + brown carbon particles with materials such as sulfate that would encourage the formation of additional coatings. Source-oriented particles undergo coagulation and gas-particle conversion, but these processes are considered in a dynamic framework that realistically "ages" primary particles over hours and days in the atmosphere. The source-oriented WRF/Chem model more accurately predicts radiative feedbacks from anthropogenic aerosols compared to models that make internal mixing or other artificial mixing assumptions.
A three-week stagnation episode (15 December 2000 to 6 January 2001) during the California Regional PM10/PM2.5 Air Quality Study (CRPAQS) was chosen for the initial application of the new modeling system. Emissions were obtained from the California Air Resources Board. Gas-phase reactions were modeled with the SAPRC90 photochemical mechanism. Gas-particle conversion was modeled as a dynamic process with semi-volatile vapor pressures at the particle surface calculated using ISORROPIA. Source oriented calculations were performed for 8 particle size fractions ranging from 0.01–10 μm particle diameters with a spatial resolution of 4 km and hourly time resolution. Primary particles emitted from diesel engines, wood smoke, high sulfur fuel combustion, food cooking, and other anthropogenic sources were tracked separately throughout the simulation as they aged in the atmosphere. Results show that the source-oriented representation of particles with meteorological feedbacks in WRF/Chem changes the aerosol extinction coefficients, downward shortwave flux, and primary and secondary particulate matter concentrations relative to the internally mixed version of the model. Downward shortwave radiation predicted by source-oriented model is enhanced by 1% at ground level chiefly because diesel engine particles in the source-oriented mixture are not artificially coated with material that increases their absorption efficiency. The extinction coefficient predicted by the source-oriented WRF/Chem model is reduced by an average of ∼ 5–10% in the central valley with a maximum reduction of ∼ 20%. Particulate matter concentrations predicted by the source-oriented WRF/Chem model are ∼ 5–10% lower than the internally mixed version of the same model because increased solar radiation at the ground increases atmospheric mixing. All of these results stem from the mixing state of black carbon. The source-oriented model representation with realistic aging processes predicts that hydrophobic diesel engine particles remain largely uncoated over the +7 day simulation period, while the internal mixture model representation predicts significant accumulation of secondary nitrate and water on diesel engine particles. Similar results will likely be found in any air pollution stagnation episode that is characterized by significant particulate nitrate production.