Modeling of air toxics from urban and industrial sources within complex terrain

Charleston Jo Roque Ramos

The MM5/SMOKE/CMAQ modeling suite was used to investigate the spatial and temporal distributions of air toxics in the Nez Perce Reservation in Idaho and the Clarkston, WA-Lewiston, ID urban area for two time periods: August-September, 2006 and December, 2006-January, 2007. Model performance was evaluated using data collected during a one year comprehensive air toxics monitoring program in the area. Statistical comparison of modeled to observed temperature, wind speed, and wind direction were within the typical range seen in previous applications of the MM5 meteorological model. The magnitudes of temperature, wind speed, and wind direction mean biases were equal to or less than 0 K, 0.1 m/s, and 50 degrees, respectively, and mean absolute gross errors were less than 3 K, 2 m/s, and 80 degrees, respectively. Comparison of the modeled to observed air toxic concentrations (24-hr averages) showed that formaldehyde, acetaldehyde, and benzene were underestimated by as much as a factor of five. Further experiments regarding these three air toxics showed that the modeled concentrations were less sensitive to uncertainties in anthropogenic emissions than to uncertainties in chemical boundary conditions. Concentrations of toluene and ethyl benzene were underestimated most of the time, but the predicted levels were always within a factor of two of the observations. Concentrations of chloroform and the xylene isomers were overestimated most of the time and were within a factor of two for the AugustSeptember period. CMAQ modeling results showed that pollutant spatial distributions depended upon chemistry, pollutant source, and terrain. Air toxics with secondary formation, such as formaldehyde, had broad coverage across the model domain. Primary pollutants emitted from a point source tended to remain in the valley because of the terrain. Elevated concentrations of primary mobile pollutants tended to occur along major roadways within the valley. Diurnal patterns of the air toxics were explained by the mixing height and chemical formation rates. Primary pollutant concentrations were lower during the day due to dilution throughout a deeper mixing layer. Formaldehyde and acetaldehyde had much higher concentrations during the August-September period due to higher chemical production rates compared to the DecemberJanuary period.

Modeling of air toxics from urban and industrial sources within complex terrain

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