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. The regional concentrations of airborne ultrafine particulate matter mass( Dp µm ; PM 0.1 ) were predicted in 39 cities acrossthe United States (US) during summertime air pollution episodes.Calculations were performed using a regional source-oriented chemicaltransport model with 4  km spatial resolution operating on theNational Emissions Inventory created by the U.S. Environmental Protection Agency (EPA). Measured sourceprofiles for particle size and composition between 0.01 and 10  µm wereused to translate PM total mass to PM 0.1 . Predicted PM 0.1 concentrations exceeded 2  µg m−3 during summer pollution episodesin major urban regions across the US including Los Angeles, the SanFrancisco Bay Area, Houston, Miami, and New York. PM 0.1 spatialgradients were sharper than PM 2.5 spatial gradients due to thedominance of primary aerosol in PM 0.1 . Artificial source tags were usedto track contributions to primary PM 0.1 and PM 2.5 from 15source categories. On-road gasoline and diesel vehicles made significantcontributions to regional PM 0.1 in all 39 cities even though peakcontributions 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 locationsimpacted by summer wildfires. Aviation was a significant source ofPM 0.1 in cities that had airports within their urban footprints.Industrial sources, including cement manufacturing, process heating, steelfoundries, and paper and pulp processing, impacted their immediate vicinitybut did not significantly contribute to PM 0.1 concentrations in any ofthe target 39 cities. Natural gas combustion made significant contributionsto PM 0.1 concentrations due to the widespread use of this fuel forelectricity generation, industrial applications, residential use, and commercialuse. The major sources of primary PM 0.1 and PM 2.5 were notablydifferent in many cities. Future epidemiological studies may be able todifferentiate PM 0.1 and PM 2.5 health effects by contrastingcities with different ratios of PM0.1∕PM2.5 . In the currentstudy, cities with higher PM0.1∕PM2.5 ratios (ratio greaterthan 0.10) include Houston, TX, Los Angeles, CA, Bakersfield, CA, Salt LakeCity, 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.

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