Intra-Community Variability in Total Particle Number Concentrations in the San Pedro Harbor Area (Los Angeles, California)

Recent evidence links elevated ultrafine particle (UFP) concentrations with adverse health effects, but exposure assessments based upon PM 2.5 mass concentrations may be misleading. In order to better understand and quantify intra-community variability in UFP concentrations, a dense network of 14 monitoring sites was set-up in Los Angeles in two clusters—San Pedro/Wilmington and West Long Beach—in communities surrounding the Ports of Los Angeles and Long Beach. The network measured total particle number concentrations greater than 7 nm in diameter. In this range, UFP comprise approximately 90% of the total. Port-related activities—particularly goods movement associated with high volumes of heavy-duty diesel vehicle (HDDV) traffic—represent significant UFP sources. The field study was conducted from mid-February through mid-December 2007 to assess diurnal, seasonal, and spatial patterns and intra-community variability in total particle number concentrations. For sites within a few km of each other, simultaneous particle number concentrations can vary up to a factor of 10 (< 10,000 cm−3 up to 90,000 cm−3 for hourly averages calculated by month). The median hourly correlation coefficient (r) across all sites was modest and varied from 0.3 to 0.56. Specific site locations, particularly proximity to roadways used for goods movement, strongly affect observations. Clear diurnal and seasonal patterns are evident in the data. A diurnal pattern associated with high HDDV volumes and goods movement was identified. Coefficients of Divergence calculated for the site pairs suggest moderate heterogeneity overall (median study COD ≈ 0.35). The intra-urban variability observed in this study is comparable to and exceeds the inter-urban variability observed in a previous study in Los Angeles. UFP concentrations can vary considerably on short spatial scales in source-rich environments strongly influencing the accuracy of exposure assessments.