Abstract
Abstract. Ultrafine particle (UFP) number concentrations vary significantly on small spatial and temporal scales due to their short atmospheric lifetimes and multiplicity of sources. To determine UFP exposure gradients within a community, simultaneous particle number concentration measurements at a network of sites are necessary. Concurrent particle number size distribution measurements aid in identifying UFP sources, while providing data to investigate local scale effects of both photochemical and physical processes on UFP. From April to December 2007, we monitored particle number size distributions at 13 sites within 350 m–11 km of each other in the vicinity of the Ports of Los Angeles and Long Beach using Scanning Mobility Particle Sizers (SMPS). Typically, three SMPS units were simultaneously deployed and rotated among sites at 1–2 week intervals. Total particle number concentration measurements were conducted continuously at all sites. Seasonal and diurnal number size distribution patterns are complex, highly dependent on local meteorology, nearby PM sources, and times of day, and cannot be generalized over the study area nor inferred from one or two sampling locations. Spatial variation in particle number size distributions was assessed by calculating the coefficient of divergence (COD) and correlation coefficients (r) between site pairs. Results show an overall inverse relationship between particle size and CODs, implying that number concentrations of smaller particles (<40 nm) differ from site to site, whereas larger particles tend to have similar concentrations at various sampling locations. In addition, variations in r values as a function of particle size are not necessarily consistent with corresponding COD values, indicating that using results from correlation analysis alone may not accurately assess spatial variability.
Highlights
Vehicular traffic constitutes one of the most significant urban sources of ultrafine particle number concentrations (Geller et al, 2005; Ketzel et al, 2004)
Results from concurrently sampled LB4 and LB5 show that sites separated by about 600 m can experience differences in particle number size distributions
Very high traffic emissions increase particle number concentrations in the 20–40 nm size range by two-fold at LB5 compared to LB4, consistent with previous observations on the I-710 (Westerdahl et al, 2005; Zhu et al, 2002b)
Summary
Vehicular traffic constitutes one of the most significant urban sources of ultrafine particle number concentrations (Geller et al, 2005; Ketzel et al, 2004). Ultrafine particles (UFPs) make up the majority of ambient particle number concentrations but only a small fraction of ambient PM mass. Studies in various locations throughout the world have shown substantial spatial variability in particle number concentrations (Puustinen et al, 2007; Turner, 2008). Vehicular emissions and the location in the center of the city have been shown to be significant predictors of spatial variation, with larger effects for particle numbers than for fine particle mass (Lianou et al, 2007). A recent investigation in urban Basle showed that daytime profiles for UFP number concentrations were more closely related to the number of heavy-duty vehicles than to the number of light-duty vehicles, implying that diesel exhaust is a strong source of ultrafine PM (Junker et al, 2000)
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