Size-resolved source apportionment of carbonaceous particulate matter in urban and rural sites in central California

Very little is currently known about the relationship between exposure to different sources of ambient ultrafine particles (PM0.1) and human health effects. If human health effects are enhanced by PM0.1’s ability to cross cell membranes, then more information is needed describing the sources of ultrafine particles that are deposited in the human respiratory system. The current study presents results for the source apportionment of airborne particulate matter in six size fractions smaller than 1.8 μm particle diameter including ultrafine particles (PM0.1) in one of the most polluted air basins in the United States. Size-resolved source apportionment results are presented at an urban site and rural site in central California’s heavily polluted San Joaquin Valley during the winter and summer months using a molecular marker chemical mass balance (MM–CMB) method. Respiratory deposition calculations for the size-resolved source apportionment results are carried out with the Multiple Path Particle Dosimetry Model (MPPD v 2.0), including calculations for ultrafine (PM0.1) source deposition.Diesel engines accounted for the majority of PM0.1 and PM1.8 EC at both the urban and rural sampling locations during both summer and winter seasons. Meat cooking accounted for 33–67% and diesel engines accounted for 15–21% of the PM0.1 OC at Fresno. Meat cooking accounted for 22–26% of the PM0.1 OC at the rural Westside location, while diesel engines accounted for 8–9%. Wood burning contributions to PM0.1 OC increased to as much as 12% of PM0.1 OC during the wintertime. The modest contribution of wood smoke reflects the success of emissions control programs over the past decade. In contrast to PM0.1, PM1.8 OC had a higher fraction of unidentified source contributions (68–85%) suggesting that this material is composed of secondary organic aerosol (SOA) or primary organic aerosol (POA) that has been processed by atmospheric chemical reactions. Meat cooking was the largest identified source of PM1.8 organic carbon (OC) at the Fresno site (12–13%) while diesel engines were the largest identified PM1.8 OC source at the rural site (5–8%). Wood burning contributions to PM1.8 OC increased during the wintertime at both sites (6–9%) but were relatively small during the summertime (∼1%).As expected, diesel engines were the dominant source of PM0.1 EC respiratory deposition at both the urban and rural site in both summer and winter (0.01–0.03 μg PM0.1 EC deposited per m3 air inhaled). Meat cooking accounted for 0.01–0.025 μg PM0.1 OC deposited per m3 air inhaled while diesel fuel accounted for 0.005–0.013 μg PM0.1 OC deposited per m3 air inhaled. Minor contributions from wood burning, motor oil, and gasoline fuel were calculated at levels <0.005 μg PM0.1 OC deposited per m3 air inhaled at both urban and rural locations during winter and summer seasons. If the burden of PM0.1 deposited in the respiratory system is relevant for human health effects, then future toxicology studies should be carried out at PM0.1 concentrations and source mixtures equivalent to those measured in the current study.

► We performed PM0.1 source apportionment using molecular markers.
► Source contributions by season/location are presented for the San Joaquin Valley.
► We calculated PM source respiratory deposition using size-resolved apportionment.
► PM0.1 EC deposition is dominated by diesel engines at all locations in all seasons.
► PM0.1 OC deposition has large contributions from SOA, meat cooking, and diesel.