Air quality impacts of a scheduled 36-h closure of a major highway

Abstract

Elevated concentrations of ultrafine particles (UFPs, <0.1 μm) are commonly found near roadways. On the July 16–17, 2011 weekend, a section of a major Los Angeles freeway, the I-405, was closed for 36 h. We measured UFPs and other pollutants at two fixed locations, one upwind and one downwind, and at various distances from I-405 using a mobile measurement platform (MMP) on Fridays, Saturdays, and Sundays before, during, and after closure. On the closure Saturday on July 16, I-405 traffic flow was reduced by ∼90% relative to non-closure Saturday observations. Downwind of I-405, fixed-site measurements showed the following reductions: 83% of particle number concentration (PNC), 36% of PM_2.5, and 62% of black carbon. Fixed-site measurements showed daily average UFP size distributions were bimodal for non-closure conditions (nucleation modes ∼20 nm, accumulation modes ∼60 nm), but only showed an accumulation mode ∼50 nm during closure. Spatial measurements from the MMP confirmed no nucleation mode was detected at any location 0–300 m downwind during closure. In 2011, non-closure particle emission factors were 5.0, 2.7, and 3.4 × 10¹³ particles vehicle⁻¹ km⁻¹ for Friday through Sunday respectively. After accounting for instrumental and traffic flow differences, weekday PNC in 2011 was 60% lower than 2001 at the same study location. During the closure event, regional freeway traffic was reduced compared to four selected control Saturdays. Eight stationary monitoring stations throughout the South Coast Air Basin showed PM_2.5 was reduced between 18 and 36% relative to the same control days. The outcome of this natural experiment during the I-405 closure confirms that substantial traffic reduction can improve local and regional air quality in sprawled urban regions such as Los Angeles, CA.

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 PM_2.5 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.