Prevalence of wide area impacts downwind of freeways under pre-sunrise stable atmospheric conditions

In urban areas, a large fraction of the population typically lives within 1.5–2 km of a freeway. In an earlier study, Hu et al. (2009) showed that in the pre-sunrise hours, with stable, nocturnal meteorological conditions, the vehicle-related pollution from a freeway in southern California extended more than 2 km downwind. This was in striking contrast to earlier studies in the U.S. and Australia showing that during the daytime pollutant concentrations extended no more than ∼300 m downwind of major roadways. The present study was designed to explore the variability of the large downwind pollutant impact zone observed by Hu et al. (2009) in the pre-sunrise hours on a wider geographic scale. An instrumented mobile platform was employed to measure vehicle-related pollutant concentrations on transects running upwind and downwind of four freeways in the coastal, central and eastern areas of the California South Coast Air Basin (SoCAB) during the pre-sunrise period (04:30–06:30). Extended freeway plumes were observed for ultrafine particles (UFP), nitric oxide and particle-bound polycyclic aromatic hydrocarbons (PAH) at all four sites during stable pre-sunrise periods. Plume lengths were measured to be ∼2 km or more with a dilution rate coefficient about a factor of ten lower than commonly observed for daytime. An average of 39 and 19% (±8–9%) of freeway plume peak concentrations of UFP remained 500 and 1500 m downwind, respectively, from the freeways for the four transects studied here. Because a large fraction of UFP and other vehicle related pollutants typically penetrate into indoor environments, and nocturnal surface inversions are widespread across the globe, our findings have significant implications for more extensive human exposures to vehicle-related pollutants than previously indicated based on daytime measurements of roadway plumes.