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Effective densities of atmospheric aerosols in various locations 53-4507-0482 and 53-4507-7721 to the University of Southern California. The research described herein has not been subjected to the agency's required peer and policy review and therefore does not necessarily reflect the views of the agency, and no official endorsement should be inferred. Mention of trade names or commercial products does not constitute an endorsement or recommendation for use.Address correspondence to Constantinos Sioutas, University of Southern California, Department of Civil and Environmental Engineering, 3620 S. Vermont Avenue, Los Angeles, CA 90089, USA. E-mail: sioutas@usc.edu CA-110 (D f = 2.54) aerosols, presumably due to the influence of vehicular combustion emission on these sites. By contrast, average fractal dimensions at USC, Riverside and Coast were found to be 2.79, 2.83, and 2.92, respectively. High fractal dimensions at these sites may be the effects of aging, moisture absorption and/or organic vapor condensation on the particles, which fills void space and makes particles more spherical.

Particulate matter emissions were measured in two bores of the Caldecott Tunnel in Northern California during August and September 2004. One bore (Bore 1) is open to both heavy- and light-duty vehicles while heavy-duty vehicles are prohibited from entering the second bore (Bore 2). Particulate matter number and mass size distributions, chemical composition, and gaseous copollutants were recorded for four consecutive days near the entrance and exit of each bore. Size-resolved emission factors were determined for particle number, particle mass, elemental carbon, organic carbon (OC), sulfate, nitrate, and selected elements. The size distributions in both the bores showed a single large mode at roughly 15-20 nm in mobility diameter, with occasional smaller modes around 100 nm. The PM10 mass emission factor for heavy-duty vehicles was 14.5 times higher than that of light-duty vehicles. The particles derived from diesel are more abundant in elemental carbon, 70.9% of PM10 emissions, as compared to the light-duty vehicles. Conversely, a greater percentage of OC was found in light-duty emissions than heavy-duty emissions. In comparison to previous studies at the Caldecott Tunnel, less particle mass but more particle numbers are emitted by vehicles than was the case 7 years ago.

A summer air quality monitoring campaign focusing on daily variation of ultrafine (<180 nm in diameter) particle chemical characteristics was conducted in a typical urban site in Los Angeles during June-July 2006. Ultrafine particles (UFP) were collected weekly for two 3 h periods each day, one to capture the morning commute (06:00-09:00 PDT) (Pacific Daylight Time) and one to investigate photochemically altered particles (13:00-16:00 PDT). Samples were analyzed for ionic compounds, metals, trace elements, elemental carbon, and organic carbon. In addition, measurements of individual organic species and their variation with time of day at the urban site were conducted. The relative abundances of alkanes, PAH, and hopanes in the morning denote a strong influence of commute traffic emissions on ultrafine particle concentrations. By contrast, afternoon concentrations of oxygenated organic acids and sulfate rose, while other species were diluted by increased mixing height or lost due to increasing temperature. These are clear indicators that secondary photochemical reactions are a major formation mechanism of ultrafine aerosols in the afternoon. The concentrations of organic species originating from vehicular emissions measured in this study compare favorably to those from freeway-adjacent measurements by using CO2 concentrations to adjust for dilution, demonstrating the effectiveness of this tool for relating sites affected by vehicular emissions.

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This study presents the number, surface and volume concentrations, and size distribution of particles next to the 1-710 freeway during February through April 2006. 1-710 has the highest ratio (up to 25%) of heavy-duty diesel vehicles in the Los Angeles highway network. Particle concentration measurements were accompanied by measurements of black carbon, elemental and organic carbon, and gaseous species (CO, CO2). Using the incremental increase of CO2 over the background to calculate the dilution ratio, this study makes it possible to compare particle concentrations measured next to the freeway to concentrations measured in roadway tunnels and in vehicle exhaust. In addition to the effect of the dilution ratio on the measured particle concentrations, multivariate linear regressions showed that light and heavy organic carbon concentrations are positively correlated with the particle volume in the nucleation and accumulation modes, respectively. Solar radiation was also positively correlated with the particle surface concentration and the particle volume in the accumulation (40-638 nm) mode, presumably as a result of secondary particle formation. The methods developed in this study may be used to decouple the effect of sampling position, meteorology, and fleet operation on particle concentrations in the proximity of freeways, roadway tunnels, and in street canyons.

Individual organic compounds found in particulate emissions from vehicles have proven useful in source apportionment of ambient particulate matter. Species of interest include the hopanes, originating in lube oil, and selected PAHs generated via combustion. Most efforts to date have focused on emissions and apportionment PM10 or PM2.5 However, examining how these compounds are segregated by particle size in both emissions and ambient samples will help efforts to apportion size-resolved PM, especially ultrafine particles which have been shown to be more potent toxicologically. To this end, high volume size-resolved (coarse, accumulation, and ultrafine) PM samples were collected inside the Caldecott tunnel in Orinda, California to determine the relative emission factors for these compounds in different size ranges. Sampling occurred in two bores, one off-limits to heavy-duty diesel vehicles, which allows determination of the different emissions profiles for diesel and gasoline vehicles. Although tunnel measurements do not measure emissions over a full engine duty cycle, they do provide an average emissions profile over thousands of vehicles that can be considered characteristic of "freeway" emissions. Results include size-fractionated emission rates for hopanes, PAHs, elemental carbon, and other potential organic markers apportioned to diesel and gasoline vehicles. The results are compared to previously conducted PM2.5 emissions testing using dynamometer facilities and othertunnel environments.

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