Day-of-week patterns for ultrafine particulate matter components at four sites in California

Abstract

Ultrafine particulate matter (PM0.1) samples were collected during three-day averaging periods over an annual cycle at four sites across California: Los Angeles, East Oakland, San Pablo, and Fresno. PM0.1 samples were analyzed for thermal carbon fractions using a thermal-optical carbon analyzer and trace elements using ICP-MS. A statistical analysis of the day-of-week trends for 15 PM0.1 components reveals location-specific patterns along with important general trends for UFP concentrations. PM0.1 elemental carbon (EC) concentrations are elevated in the middle of the week consistent with increased activity for diesel engines during this time period. Just as significantly, PM0.1 organic carbon (OC) concentrations do not exhibit a day-of-week pattern. Since OC accounts for the majority of the PM0.1 total mass, the lack of a day-of-week pattern for PM0.1 OC suggests that diesel engines do not dominate total PM0.1 mass in California. A paired t-test constructed using measurements on weekends compared to measurements immediately preceding or immediately following the weekend showed that PM0.1 potassium (K) and rubidium (Rb) concentrations were elevated on weekends. This pattern is consistent with increased biomass combustion on weekends (a previously unknown exposure pattern for UFPs). The two closest sampling locations (East Oakland and San Pablo) had the greatest number of UFP components with identical weekly trends, but even at these locations only 4 out of 15 components displayed the exact same day-of-week profiles. Less similarity was observed in measured day-of-week profiles for UFP components at other sites. The heterogeneity between sites suggests that the details of UFP concentrations at each location reflect the mixture of sources immediately adjacent to that site. By extension, individual neighborhoods across California will each experience unique day-of-week concentration profiles for UFP components that reflect the surrounding sources. These factors must be considered when estimating population exposure to ultrafine particles during future health effects studies.