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

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.