Predicted ultrafine particulate matter source contribution across the continental United States during summertime air pollution events

Abstract

The regional concentrations of airborne ultrafine particulate matter mass ( D p<0.1 μm; PM 0.1) were predicted in 39 cities across the United States (US) during summertime air pollution episodes. Calculations were performed using a regional source-oriented chemical transport model with 4 km spatial resolution operating on the National Emissions Inventory created by the U.S. Environmental Protection Agency (EPA). Measured source profiles for particle size and composition between 0.01 and 10 μm were used to translate PM total mass to PM 0.1. Predicted PM 0.1 concentrations exceeded 2 μg m-3 during summer pollution episodes in major urban regions across the US including Los Angeles, the San Francisco Bay Area, Houston, Miami, and New York. PM 0.1 spatial gradients were sharper than PM 2.5 spatial gradients due to the dominance of primary aerosol in PM 0.1. Artificial source tags were used to track contributions to primary PM 0.1 and PM 2.5 from 15 source categories. On-road gasoline and diesel vehicles made significant contributions to regional PM 0.1 in all 39 cities even though peak contributions within 0.3 km of the roadway were not resolved by the 4 km grid cells. Cooking also made significant contributions to PM 0.1 in all cities but biomass combustion was only important in locations impacted by summer wildfires. Aviation was a significant source of PM 0.1 in cities that had airports within their urban footprints. Industrial sources, including cement manufacturing, process heating, steel foundries, and paper and pulp processing, impacted their immediate vicinity but did not significantly contribute to PM 0.1 concentrations in any of the target 39 cities. Natural gas combustion made significant contributions to PM 0.1 concentrations due to the widespread use of this fuel for electricity generation, industrial applications, residential use, and commercial use. The major sources of primary PM 0.1 and PM 2.5 were notably different in many cities. Future epidemiological studies may be able to differentiate PM 0.1 and PM 2.5 health effects by contrasting cities with different ratios of PM0.1ĝ•PM2.5. In the current study, cities with higher PM0.1ĝ•PM2.5 ratios (ratio greater than 0.10) include Houston, TX, Los Angeles, CA, Bakersfield, CA, Salt Lake City, UT, and Cleveland, OH. Cities with lower PM 0.1 to PM 2.5 ratios (ratio lower than 0.05) include Lake Charles, LA, Baton Rouge, LA, St. Louis, MO, Baltimore, MD, and Washington, D.C.