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Carmageddon or Carmaheaven? Air Quality Results of a Freeway Closure

FSPH Professors Arthur Winer and Yifang Zhu and Professor Suzanne Paulson from the UCLA Department of Atmospheric and Oceanic Sciences found a significant improvement in air quality during the 2011 closure of the 405 freeway in Los Angeles.

Monday, June 16, 2014

Air quality researchers have recently shifted their focus from regional smog, which has been reduced dramatically over the past 40 years, to the more localized impacts of vehicle emissions near roadways. Numerous studies have linked traffic-related air pollution to a broad range of adverse health outcomes. Concern has focused on black carbon, particles less than 2.5 microns in diameter (PM2.5), and ultrafine particles less than 100 nanometers in diameter, small enough to penetrate cell walls and cross the blood-brain barrier. These pollutants may be carried up to 300 meters downwind of major roadways during the day, and more than 2,000 meters downwind in the early morning hours, affecting large populations in major urban centers. By addressing these pollutants, policies to reduce traffic, congestion, and emissions can improve air quality and health.

Opportunities to directly quantify the relationship between vehicle emissions and air quality by investigating effects of large scale, rapid reductions in traffic are rare. Typically, measures to reduce vehicle emissions and traffic management policies are phased in over years or decades. For example, during the 1996 Olympic Games in Atlanta, peak traffic decreased by around 20 percent and ozone fell by nearly 30 percent, although meteorology may have played a role. During the 2008 Olympic and Paralympic Games in Beijing, the Chinese government enacted air pollution-reducing policies, including traffic restrictions, that resulted in significant reductions in near-roadway emissions. Because emissions from stationary sources were also reduced, traffic restrictions do not fully explain these air quality improvements. In August 2008, New York City closed Park Avenue to vehicular traffic on three consecutive Saturday mornings to promote clean air. This resulted in 58 percent lower ultrafine particle concentrations in the near-roadway environment during the closures.

Overall, there have been few opportunities to test whether reducing traffic (or adopting zero-emitting vehicles) will lead to dramatic improvements in air quality. Although the relationship between traffic and air quality may seem self-evident, it is important to quantify this relationship for key pollutants such as ultrafine particles.

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