ARB Research Seminar

This page updated May 12, 2014

Characterizing Spatially Inhomogeneous Non-Criteria Pollutants in the Los Angeles Air Basin

Photo of Suzanne Paulson, Ph.D.

Suzanne Paulson, Ph.D., Vice Chair, Department of Atmospheric and Oceanic Sciences and Director, Center for Clean Air, University of California, Los Angeles

May 30, 2013
Cal EPA Headquarters, 1001 "I" Street, Sacramento, CA

Research Project


A number of epidemiological studies have shown that exposure to elevated levels of fresh vehicular emissions causes a wide range of adverse human health effects.  Fresh vehicular emissions contain a wide range of particle- and gas-phase species.  Because such emissions are emitted and diluted together, their individual impacts are difficult to separate.  Ultrafine particles (UFP), a focus of this presentation, might contribute to the degradation of health associated with exposure to elevated levels of fresh vehicular emissions, and they are also an excellent tracer for fresh vehicle emissions.
An earlier mobile measurement study by our group demonstrated a large pollutant impact zone, extending beyond 2.5 km downwind of a freeway in Santa Monica, California during pre-sunrise hours. The current study explores the variability of extended freeway plumes at several locations in Southern California.  The ARB mobile measurement platform (MMP) was employed to measure vehicle-related pollutant concentrations on transects running upwind and downwind perpendicular to four freeway segments in the coastal, central and eastern areas of the South Coast Air Basin during the pre-sunrise period.  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.  Factors controlling pollutant plume length downwind of freeways under stable conditions were background-subtracted peak concentration (which is a function of traffic flows and temperature) as well as meteorological parameters, such as wind direction and speed. Vertical stability plays a minor role in dispersion coefficient variations within stable boundary layer conditions.  A curve fit using a Gaussian dispersion model solution describes the pollutant concentrations very well.

With an aim toward making comparisons of measurements made at different locations at different times quantitative, we have developed an objective and systematic classification scheme of meteorological conditions affecting atmospheric primary pollutant levels in Southern California.  The method used is a classification and regression tree (CART) modeling approach.  The resulting regressions provide excellent correlations between the regression classifications developed for different primary pollutant metrics, such as daily CO and NO maxima, as well as between monitoring sites.  The meteorological parameters that determine the variability in primary pollutant concentrations, in approximate order of importance, are the mean surface wind speed, geopotential heights at 925 mbar, the upper air north-south pressure gradient, the daily minimum temperature, relative humidity at 1000 mbar, and vertical stability.

Here we apply the CART analysis to an inter-comparison of mobile measurements collected in several locations and times within Southern California. Daytime UFP concentrations in neighborhoods showed strong inter-community variations between West Los Angeles, downtown Los Angeles and Boyle Heights in 2008. Intra-community pollutant variations were less intense but significant as an air mass experiences emissions from major freeways (I-405 and I-10). Pollutant concentrations including UFP were highly elevated in close proximity to major freeways, as well as Santa Monica Airport.  Impacts of high emitting vehicles on UFP distributions both on arterial roadways and in neighborhoods were also significant. About 70% reductions of UFP and PM2.5 were observed during the I-405 closure event (so called "Carmageddon") in 2011 with 20 - 85% decreases in nearby traffic flows in West Los Angeles. Evidence that in-use motor vehicle emissions of ultrafine particles in general have declined will be also be presented.

Speaker Biography

Suzanne Paulson, Ph.D., is Professor and Vice Chair of the Department of Atmospheric & Oceanic Sciences and the Director of the UCLA Center for Clean Air, at the University of California, Los Angeles . Professor Paulson's current research involve the impact of tiny naturally occurring and human-made particles on human health and the Earth's climate.  Dr. Paulson teaches climate change and air pollution to undergraduate and graduate students and has given more than twenty invited public lectures and expert testimony for elected officials on air pollution and climate change.  Dr. Paulson and has also been featured in numerous radio, print and video interviews.  Currently Dr. Paulson serves on advisory boards for the California Air Resources Board and the Georgia Tech/Environmental Protection Agency Supersite. Professor Paulson is the recipient of a National Science Foundation CAREER award for her research.  Professor Paulson earned a B.A. in Chemistry from the University of Colorado, and a PhD in Environmental Engineering Science from the California Institute of Technology.

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