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

Regional concentrations and source contributions are calculated for airborne particle number concentration (N-x) and ultrafine particle mass concentration (PM0.1) 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 N-10 concentrations meet the goals typically used for modeling of PM2.5 (mean fractional bias (MFB) < +/- 0.5 and mean fractional error (MFE) < 0.75). The relative ranking and concentration range of source contributions to PM0.1 predicted by regional calculations agree with results from receptor-based studies that use molecular markers for source apportionment at four locations in California. Different sources dominated regional concentrations of N-10 and PM0.1 because of the different emitted particle size distributions and different choices for heating fuels. Nucleation (24%-57%) made the largest single contribution to N-10 concentrations at the 10 regional monitoring locations, followed by natural gas combustion (28%-45%), aircraft (2%-10%), mobile sources (1%-5%), food cooking (1%-2%), and wood smoke (0%-1%). In contrast, natural gas combustion (22%-52%) was the largest source of PM0.1 followed by mobile sources (15%-42%), food cooking (4%-14%), wood combustion (1%-12%), and aircraft (2%-6%). The study region encompassed in this project is home to more than 25 million residents, which should provide sufficient power for future epidemiological studies on the health effects of airborne ultrafine particles. All of the PM0.1 and N-10 outdoor exposure fields produced in the current study are available free of charge at http://webwolf.engr.ucdavis.edu/data/soa_v3/hourly_avg/ (last access: 20 November 2019).