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

Abstract

Abstract. The regional concentrations of airborne ultrafine particulate matter mass (Dp<0.1 µm; PM0.1) 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 PM0.1. Predicted PM0.1 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. PM0.1 spatial gradients were sharper than PM2.5 spatial gradients due to the dominance of primary aerosol in PM0.1. Artificial source tags were used to track contributions to primary PM0.1 and PM2.5 from 15 source categories. On-road gasoline and diesel vehicles made significant contributions to regional PM0.1 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 PM0.1 in all cities but biomass combustion was only important in locations impacted by summer wildfires. Aviation was a significant source of PM0.1 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 PM0.1 concentrations in any of the target 39 cities. Natural gas combustion made significant contributions to PM0.1 concentrations due to the widespread use of this fuel for electricity generation, industrial applications, residential use, and commercial use. The major sources of primary PM0.1 and PM2.5 were notably different in many cities. Future epidemiological studies may be able to differentiate PM0.1 and PM2.5 health effects by contrasting cities with different ratios of PM0.1∕PM2.5. In the current study, cities with higher PM0.1∕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 PM0.1 to PM2.5 ratios (ratio lower than 0.05) include Lake Charles, LA, Baton Rouge, LA, St. Louis, MO, Baltimore, MD, and Washington, D.C.

Trends in PM2.5 transition metals in urban areas across the United States

“…This has increased the Cr/PM 2.5 ratio by more than a factor of three from 2001 to 2016 (figures S2-S21). Differences in the PM 2.5 trends compared to those of the individual metals are consistent with predictions of ultrafine particle concentrations (PM 0.1 ) in urban areas across the US [55]. Similar to PM metals, ultrafine particles are dominated by primary emissions in urban areas, especially during pollution episodes, whereas major contributors PM 2.5 (e.g.…”

“…Similar to PM metals, ultrafine particles are dominated by primary emissions in urban areas, especially during pollution episodes, whereas major contributors PM 2.5 (e.g. organics, sulfate, nitrate, ammonium) are predominantly secondary in origin [55]. A separate study similarly concluded that metals exhibit much greater spatial variability than PM 2.5 mass across the US due to the high contribution and regional nature of secondary PM formation [56].…”

Regional sources of airborne ultrafine particle number and mass concentrations in California

“…The current study focuses on outdoor exposure to UFPs that may be useful in future epidemiological studies. Indoor or in-vehicle exposure to UFPs can also be significant (Wallace and Ott, 2011;Rim et al, 2010;Bhangar et al, 2011;Weichenthal et al, 2015;Fruin et al, 2008) but characterizing these micro-environments is beyond the scope of the current paper.…”

Ultrafine particles: unique physicochemical properties relevant to health and disease

“…Nonresidential natural gas combustion was deemed the chief source of UFPs across the major cities therein. On-road gasoline and diesel vehicles contributed, on average, 14% to regional UFP emissions, which could have been underestimated due to measurement limitations 42 .…”