AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Development of a Source Oriented Version of the WRF/Chem Model and Its Application to the California Regional PM10/PM2.5 Air Quality Study
HONGLIANG ZHANG, Steven DeNero, David Joe, Hsiang-He Lee, Shu-Hua Chen, Michael Kleeman, UC Davis
Abstract Number: 226 Working Group: Aerosols, Clouds, and Climate
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 and light absorption properties rather than instantaneously combining them into an internal mixture. The source-oriented approach avoids artificially mixing light absorbing black+brown carbon particles with hygroscopic material 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 (December 15, 2000 to January 7, 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 4km and hourly time resolution. Primary particles emitted from diesel engines, wood smoke, high sulfate 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 affects the aerosol extinction coefficients, downward shortwave, and primary and secondary particulate matter concentrations. Downward shortwave radiation predicted by source-oriented model is enhanced by 1%. The extinction coefficient predicted by the source-oriented model is reduced by an average of about 5-10% in the central valley and a maximum of 20%. Particulate matter concentrations predicted by the source-oriented model are all 5-10% lower than the internally mixed version of the same model. All of these results stem from the mixing state of black carbon. The source-oriented model predicts that hydrophobic diesel engine particles remain largely uncoated during the simulation, while the internal mixture model predicts significant accumulation of secondary nitrate and water on diesel engine particles. More substantial differences in predicted concentrations generated by the source-oriented model and the internally mixed version are expected when predicted concentrations of secondary particulate matter increase to higher levels.