AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Factors and Particle Dynamics Controlling Pollutant Plume Length Downwind of Major Roadways in Nocturnal Surface Inversions
WONSIK CHOI, Meilu He, Vincent Barbesant, Kathleen Kozawa, Steve Mara, Arthur Winer, Suzanne Paulson, University of California Los Angeles
Abstract Number: 520 Working Group: Urban Aerosols
Abstract Recent epidemiological and toxicological studies have shown evidence for significant impacts on human health from exposure to fresh vehicular emissions. The pollutant plumes from major roadways generally can impact near roadway neighborhoods over a much wider area under stable atmospheric conditions (> 2 km) than unstable convective boundary layers (< 300 – 500 m). However, factors controlling pollutant plume length downwind of line sources still need to be quantitatively understood for the precise prediction of plume impacts. This study focuses on understanding (1) what are the major contributors to the extent of freeway plumes for the pre-sunrise periods, and (2) how freshly-emitted ultrafine particles (UFP) evolve in their characteristics during downwind transport periods.
We have measured spatial concentration profiles of pollutants emitted from major roadways for stable pre-sunrise periods (4:00 – 6:30 A.M.) using an electric vehicle mobile monitoring platform with no emissions of its own in the California's South Coast Air Basin (SoCAB). Curve fits using a Gaussian dispersion model solution were successfully applied to obtain both the dispersion coefficients and particle number emission factor (PNEF) directly from the observed UFP concentration profiles. The dispersion coefficients ranged more widely than conventionally used Briggs' values with strong correlations with meteorological parameters as well as with differences between the background and plume peak concentrations. These relationships were applied to predict freeway plume transport using a multivariate regression method. The mean PNEF for a mixed vehicle fleet on the four freeways studied here was estimated as 1.2×10$^(14) particlesmi$^(-1)vehicle$^(-1), which is 15% of a previously estimated value for the I-405 freeway in 2001, implying significant reductions in UFP emissions over the past decade in the SoCAB.
Preliminary results of observed particle dynamics from size-segregated UFP concentration profiles, including dilution, evaporation/condensation, coagulation, and production rates with distance downwind of the freeways, are also presented.