Daily, Seasonal, and Spatial Trends in PM_2.5 Mass and Composition in Southeast Texas
Special Issue of Aerosol Science and Technology on Findings from the Fine Particulate Matter Supersites Program

Abstract

Daily, seasonal, and spatial trends in fine particulate matter concentrations, compositions, and size distributions were examined using data collected through the regulatory fine particulate matter monitoring network in Southeast Texas and during the Gulf Coast Aerosol Research and Characterization Study (GC-ARCH or Houston Supersite). PM_2.5 mass concentrations and compositions are generally spatially homogeneous throughout Southeast Texas when averaged over annual or seasonal time periods. There is relatively little seasonality to mean total PM_2.5 mass and mean PM_2.5 composition throughout the region, although slightly higher concentrations of total mass tend to occur in the spring and late fall. High FRM PM_2.5 mass (>20 μg/m³) occurs both when there is high spatial variability among sites and low spatial variability among sites. This suggests that both local and regional emission sources contribute to PM_2.5 in Southeast Texas. Sulfate ion (32%), organic carbon (30%), and ammonium ion (9%) are the largest components on average of PM_2.5 by mass. Mean diurnal patterns for PM_2.5 mass concentrations throughout the region show a consistent morning peak and a weaker and slightly less consistent peak in the late afternoon to early evening. High hourly averaged PM_2.5 mass concentrations (>40 μg/m³) tend to be associated with daily average PM_2.5 above the annual NAAQS of 15 μg/m³. These high hourly PM_2.5 concentrations also tend to occur on days with high diurnal variation, indicative of elevated, short-lived PM_2.5 events. In contrast to mass concentrations, particle size distributions are not spatially homogeneous throughout Southeast Texas. Industrial sites have higher concentrations of freshly emitted, primary particles than more residential sites. Because the freshly emitted particles generally have diameters of 0.1 μm or less, these primary emissions do not have as large an impact on PM_2.5 mass or bulk composition as they have on the number density of fine particles.