Ultrafine Particles in California – Weekly Trends, Source Apportionment, and Sampling Implications
Ultrafine particulate matter (UFP; airborne particles with diameter < 0.1 µm) is an emerging air quality concern because particles in this size range are potentially more toxic than larger airborne particles. Measurement networks that can support source apportionment calculations for UFPs have been limited in the past by the high costs for equipment, supplies, and labor. The lack of UFP sampling and chemical analysis limits the understanding of spatial and temporal trends, source contributions and health effects of ambient UFP. In the current study, two co-located cascade impactors collected 3-day averaged UFP mass (PM0.1) for a full year at four sites in California, United States for the measurement of carbonaceous components and elements, respectively. Monthly samples of particles in the diameter range 0.1 – 1.8 µm were also collected. Statistical analyses of the day-of-week trends for PM0.1 components revealed location-specific patterns along with important general trends for UFP concentrations. Source apportionment calculations by Positive Matrix Factorization (PMF) using PM0.1 elements as unique tracers identified seven sources. The major source contributions to total PM0.1 mass were consistent with results generated by Chemical Mass Balance (CMB) using molecular markers as tracers. The feasibility of employing a single cascade impactor loaded with foil substrates for future PM0.1 sampling networks was explored by modifying the extraction method for elements to work on foil substrates. The results of PMF source apportionment calculations carried out with two impactors vs. one impactor were compared. The analyses performed in this study will provide information for understanding PM0.1 emission patterns, investigating their health effects and reducing the costs of sampling to enable larger scale PM0.1 studies in the future.