<p>Regional concentrations and source contributions are calculated for airborne particle number concentration (PNC) and ultrafine particle mass concentration (PM<sub>0.1</sub>) in the San Francisco Bay Area (SFBA) and the South Coast Air Basin (SoCAB) surrounding Los Angeles with 4 km spatial resolution and daily time resolution for selected months in the years 2012, 2015, and 2016. Performance statistics for daily predictions of PNC concentrations meet the threshold normally required for regulatory modeling of PM<sub>2.5</sub> (MFB < ± 0.5 and MFE < 0.75). Predicted source contributions to PM<sub>0.1</sub> are in good agreement with results from receptor-based studies that use molecular markers for source apportionment at four locations in California. Different sources dominated regional concentrations of PNC and PM<sub>0.1</sub> because of the different emitted particle size distributions and different choices for heating fuels. Non-residential natural gas combustion (38–74 %) made the largest single contribution to PNC concentrations at the ten regional monitoring locations, followed by nucleation (6–14 %), wood smoke (1–8 %), food cooking (1–9 %), and mobile sources (4–8 %). In contrast, wood smoke (25–49 %) was the largest source of PM<sub>0.1</sub> in the SFBA followed by mobile sources (15–33 %), non-residential natural gas combustion (13–28 %), and food cooking (4 %–14 %). Non-residential natural gas combustion (42–57 %) was the largest PM<sub>0.1</sub> source at the SoCAB sites, followed by traffic sources (16–35 %) and food cooking (6–14 %). The study region encompassed in this project is home to more than 25M residents, which should provide sufficient power for future epidemiological studies on the health effects of airborne ultrafine particles. Correlations between PM<sub>2.5</sub> and PNC are low (R<sup>2</sup> = 0.35) suggesting that the health effects of these metrics may be assessed independently. All of the PM<sub>0.1</sub> and PNC outdoor exposure fields produced in the current study are available free of charge at <a href=" http://webwolf.engr.ucdavis.edu/data/soa_v2/monthly_avg2"target="_blank">http://webwolf.engr.ucdavis.edu/data/soa_v2/monthly_avg2</a>.</p>