On-Road Particulate Matter (PM2.5 and PM10) Emissions in the Sepulveda Tunnel, Los Angeles, California

Abstract

Total and speciated particulate matter (PM_2.5 and PM₁₀) emission factors from in-use vehicles were measured for a mixed light- (97.4% LD) and heavy-duty fleet (2.6% HD) in the Sepulveda Tunnel, Los Angeles, CA. Seventeen 1-h test runs were performed between July 23, 1996, and July 27, 1996. Emission factors were calculated from mass concentration measurements taken at the tunnel entrance and exit, the volume of airflow through the tunnel, and the number of vehicles passing through the 582 m long tunnel. For the mixed LD and HD fleet, PM_2.5 emission factors in the Sepulveda Tunnel ranged from 0.016 (±0.007) to 0.115 (±0.019) g/vehicle·km traveled with an average of 0.052 (±0.027) g/vehicle·km. PM₁₀ emission factors ranged from 0.030 (±0.009) to 0.131 (±0.024) g/vehicle·km with an average of 0.069 (±0.030) g/vehicle·km. The PM_2.5 emission factor was ∼74% of the PM₁₀ factor. Speciated emission rates and chemical profiles for use in receptor modeling were also developed. PM_2.5 was dominated by organic carbon (OC) (31.0 ± 19.5%) and elemental carbon (EC) (48.5 ± 20.5%) that together account for 79% (±24%) of the total emissions. Crustal elements (Fe, Mg, Al, Si, Ca, and Mn) contribute ∼7.8%, and the ions Cl^-, NO₃^-, NH₃⁺, SO₄^2-, and K⁺ together constitute another 9.8%. In the PM₁₀ size fraction the particulate emissions were also dominated by OC (31 ± 12%) and EC (35 ± 13%). The third most prominent species was Fe (18.5 ± 9.0%), which is greater than would be expected from purely geological sources. Other geological components (Mg, Al, Si, K, Ca, and Mn) accounted for an additional 12.6%. PM₁₀ emission factors showed some dependence on vehicle speed, whereas PM_2.5 did not. For test runs in which the average vehicle speed was 42.6 km/h a 1.7 times increase in PM₁₀ emission factor was observed compared to those runs with an average vehicle speed of 72.6 km/h. Speciated emissions were similar. However, there is significantly greater mass attributable to geological material in the PM₁₀, indicative of an increased contribution from resuspended road dust. The PM_2.5 shows relatively good correlation with NO_x emissions, which indicates that even at the low percent of HD vehicles, which emit significantly more NO_x than LD vehicles, they may also have a significant impact on the PM_2.5 levels.