Evaluation of WRF-Chem on Simulations of Agricultural Dusts over the California Central Valley

The California Central Valley is one of the most polluted areas in United States due to agricultural activities, air mass ventilation prevented by the surrounding mountains, high urban emissions, and local atmospheric conditions. The Central Valley is a nonattainment area with the PM2.5 (particulate matter with diameter \u3c 2.5 um) National Ambient Air Quality Standard (NAAQS) because measurements at several monitoring sites exceed the daily and annual NAAQS PM2.5 standards. Three-dimensional air quality models (AQMs) play an important role in guiding the development of State Implementation Plans (SIP) for compliance with the NAAQS for PM2.5. In this work, a comprehensive model evaluation has been conducted for the Weather Research and Forecasting/Chemical (WRF-Chem) model version 3 for June 2008 as a base for our future study on the air quality in the Central Valley. Simulated meteorological variables, including temperature (T), relative humidity (RH), wind speed and direction and precipitation, are evaluated through comparison with observations from the Clean Air Status and Trends Network (CASTNet), the National Atmospheric Deposition Program (NADP), and the National Center for Atmospheric Research (NCAR) Earth Observing laboratory atmospheric sounding data. Simulated air pollutants, including mixing ratios of O3 (hourly, 1-hour maximum, and 8-hour average), mass concentrations of PM10, PM2.5, and PM2.5 components (e.g., sulfate, nitrate, ammonium, black carbon, organic carbon) are evaluated using the surface observations from CASTNet, the Aerometric Information Retrieval System – Air Quality System (AIRS-AQS), the Interagency Monitoring of Protected Visual Environments (IMPROVE) and the Speciation Trends Network (STN). The predicted total tropospheric aerosol optical depths (AOD) are compared with those derived from MODIS. Our evaluation has shown that the WRF-Chem predictions agree reasonably well with T, RH, and wind speed observations but relatively poorly with the observed wind direction. While the model reproduces the observed diurnal variations for O3 and PM, it tends to underpredict 1-hour maximum O3 mixing ratios and PM10 mass concentrations. Through the above model evaluation, it is seen that WRF-Chem can be reliable for further simulating impacts of agricultural dust on air quality in the Central Valley