Characterization of organic, metal and trace element PM2.5 species and derivation of freeway-based emission rates in Los Angeles, CA.

Abstract On–road particulate matter (PM) was collected during a sampling campaign in March-April of 2011 on two major Los Angeles freeways, I-710 and Route 110. I-710 is a major route for heavy-duty vehicles (HDVs) traveling to and from the Ports of Long Beach and Los Angeles, while Route 110 has a much lower HDV fraction – 3.9% versus 11.4%. Two sets of samples were collected for each roadway, each set representing approximately 50° h of on-road sampling. Concurrent sampling at a fixed site at the University of Southern California's (USC) downtown Los Angeles campus provided estimates of urban background levels. Chemical analysis was performed for elemental carbon (EC), organic carbon (OC), polycyclic aromatic hydrocarbons (PAHs), hopanes and steranes, and metals and trace elements. Freeway-based emission rates (ERs) – mass per kilometer of freeway per hour – were calculated using mass concentrations, fuel characteristics, and traffic flow rates. These ERs are presented such that freeways could be treated as a line source of emissions for use in predictive models of population exposure for nearby communities. This data could also be used to assess the exposure of commuters to traffic-related PM2.5 emissions. ERs are compared to data from a previous fixed-site roadside study of I-710 as well as to reconstructed values from a tunnel study. ERs were generally lower (or comparable) on the gasoline-vehicle dominated freeway (Route 110) than the freeway with more diesel trucks (I-710), with EC and pyrene being notably lower on Route 110, findings consistent with the Route 110's lower HDV fraction. We found EC emission rates decreased over time suggesting that efforts to reduce diesel emissions from HDVs at the Ports of Los Angeles and Long Beach have been successful. While ERs for most of the organic species were within the range of values reported by previous studies, the present study found much higher ERs for metals and trace elements. This suggests that the sampling methods employed in this campaign are more efficient at capturing particles from sources such as resuspended road dust and wear from tires and brakes, which are usually not included in traditional sampling methodologies for assessing vehicular emissions (e.g. dynamometer studies).