Emissions of Size-Segregated Aerosols from On-Road Vehicles in the Caldecott Tunnel

Abstract

Particulate matter emissions from the California in-use vehicle fleet were measured as 37 500 vehicles traveled through two bores of the Caldecott Tunnel located in the San Francisco Bay area. Microorifice cascade impactors and filter-based samplers were used to determine the particle chemical composition as a function of particle size. Ammonia emissions from the vehicle fleet were measured as well. Concentrations of aerosol mass, organic carbon, elemental carbon, sulfate ion, nitrate ion, and ammonium ion, as well as 13 elements are reported. The particle mass distribution peaks in the particle size range 0.1−0.18 μm aerodynamic diameter (D_a). Elemental carbon and organic matter were the largest components of particle mass in all the size ranges studied. The Caldecott Tunnel bores studied include one which carries light-duty vehicle traffic and one which carries a mixture of light- and heavy-duty vehicle traffic. From experiments conducted in both bores, estimates are made of the size distribution and chemical composition of particulate matter emissions extrapolated to the 100% light-duty and 100% heavy-duty vehicle fleets. The heavy-duty vehicle fleet emitted 1285 ± 237 mg of fine particulate matter (D_a < 1.9 μm)/kg of C contained in the fuel burned (corresponding to approximately 430 ± 79 mg/km driven). Light-duty vehicles emitted less than 85 ± 6 mg/kg of C in the fuel burned (corresponding to less than approximately 5.5 ± 0.4 mg/km driven). Emissions of gas-phase ammonia in the Caldecott Tunnel were measured to be 194 and 267 mg/L of gasoline-equivalent fuel burned in the tunnel. The ammonia emissions are attributed to automobiles that were equipped with 3-way catalysts and operating fuel rich.

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