Air quality impacts of a scheduled 36-h closure of a major highway
Graphical abstract
Highlights
► Air pollutants were examined around the time of a major planned freeway closure. ► On the closure day, particle number concentration (PNC) downwind of the freeway was reduced by 83%. ► On the closure day, PM2.5 decreased by 18–36% across the South Coast Air Basin. ► Weekday downwind PNC decreased by 60% between 2001 and 2011.
Introduction
Exposure to traffic-related air pollutants has been linked to adverse health effects by epidemiological cohort studies (e.g. Beelen et al., 2007; Laden et al., 2006). Several other epidemiological studies have specifically supported the adverse health effects of exposure to ultrafine particles (UFPs, <0.1 μm) in addition to fine and/or coarse particulate matter (PM) (Ibald-Mulli et al., 2002; Klot et al., 2002). de Kok et al. (2006) reported in a comprehensive review of toxicology studies that the mutagenicity potential and cellular toxicity of PM is highest within smaller size fractions. Donaldson et al. (2001) suggested an explanation for elevated toxicity when particle size distributions are skewed toward smaller size fractions is because total particulate surface area increases.
Many studies have characterized PM from traffic emissions. Particle number concentrations (PNC) measured on roadways can range ∼105–106 particles cm−3, depending upon meteorology and traffic configurations such as driving speed, traffic flow (vehicles hour−1), and fleet composition (e.g. Kittelson et al., 2004; Zhu et al., 2008). PNC downwind from a roadway decreases exponentially with increasing distance and particle size distribution shifts rapidly as concentration decreases (Hitchins et al., 2000; Pirjola et al., 2006; Zhu et al., 2002b).
Due to the significant contribution of air pollutants from traffic emissions, traffic calming policies such as traffic restriction could be a rational strategy to mitigate urban air pollutants. During the 1996 Olympic Games in Atlanta, peak traffic flow and ozone reduced ∼20 and ∼30% respectively (Friedman et al., 2001), but further analysis by Peel et al. (2010) indicates meteorology may have played a role since similar ozone reductions were found across the region. In July 2004 during a four-day Democratic National Convention in Boston, Massachusetts, Levy et al. (2006) reported 42% NO2 reductions in regions of decreased vehicle flow, but found no overall PM2.5 decreases. During the 2008 Olympic and Paralympic Games in Beijing, the Chinese government enacted air pollution-reducing policies for both stationary and mobile sources; near-roadway black carbon (BC) concentrations decreased 73%, sulfur dioxide 61%, carbon monoxide 25%, and nitrogen oxides 21% (Wang et al., 2009a, Wang et al., 2009b). However, these studies were unable to identify the air quality improvements due to traffic restrictions alone since parallel stationary source reductions were mandated. In August 2008, Park Avenue in New York City was closed to vehicular traffic on three consecutive Saturday mornings to promote clean air which resulted in 58% lower UFP concentrations near-roadway (Whitlow et al., 2011). This study, however, was limited to the closure region and did not consider changes to the New York City as a whole. In summary, these previous traffic restriction events were associated with improved air quality, but have the noteworthy aforementioned limitations.
In July 2011, one of the busiest freeways in United States (∼380,000 vehicles day−1), the I-405, was scheduled for a two-day closure as part of a freeway improvement project. Months in advance, Caltrans1 alerted the public to potential traffic jams. The closure event provided a valuable opportunity to conduct a natural traffic behavior and air quality experiment. The first objective of this study was to determine the impacts of this major freeway closure event, the so-called Carmageddon, on local traffic behavior and air quality. Second, we sought to compare the non-closure-weekday UFP concentrations measured during this study with measurements collected in 2001 at the same location (Zhu et al., 2002b). Since most emission control programs occur over long periods of time, and impacts are even more gradual, comparison over this ten-year period enables a strong evaluation of their efficacy. A third objective was to explore the changes of regional traffic and air quality as a result of the localized closure.
Section snippets
Study design
The I-405 closure event began midnight of Friday July 15 and remained in effect for 36 h until 12:00 on Sunday July 17. Fixed-site measurements were conducted between 10:00 and 20:00 for three consecutive Friday through Sunday periods: sampling days included Friday through Sunday July 8–10 (pre-closure period), Friday July 15, Saturday and Sunday July 16 and 17 (closure period), and Friday through Sunday July 22–24 (post-closure period). Hereafter, we refer to the pre- and post-closure periods
Meteorology
Table 2 summarizes meteorological and traffic conditions during sampling campaign. The pre-closure period average wind speed was 3.0 m s−1 and temperature was 22 °C. The during-closure period average wind speed was 4.3 m s−1 and temperature 21 °C. The post-closure period average wind speed was 5.2 m s−1 and temperature 21 °C. Daily-average wind speed and temperature were comparable within each period (±1 m s−1 wind speed and ±1 °C), but differed somewhat across periods (Table 2). During the
Conclusions
This study measured air quality and traffic patterns near I-405 in West Los Angeles before, during, and after a 36-h scheduled closure event. This study was a natural experiment to investigate the traffic behavioral response and resulting air quality impacts on local and regional scales. Local downwind reductions for the closure period were 83% PNC, 36% PM2.5, and 62% BC. Near-roadway UFP size distribution depended largely on traffic flow. During closure conditions (2500 vehicles hour−1), no
Acknowledgments
The authors thank LA National Cemetery, West LA Veterans Administration, and the Veterans Administration Police for their cooperation and coordination with our field experiments. We thank Eon Lee, Jessica Avalos, Juan de la Cruz Zavala Reyes, University and Instituto de Metalurgia, San Luis Potosí, Mexico for their assistance during data collection. We thank MichaelTsang for providing the closure photograph used in the TOC art. SEP, WC, MH and AMW gratefully acknowledge support for their
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