On-Road Emission Factors and Ratios of NO x , CO, Aromatic VOCs, and Black Carbon from Mobile Measurements in Two Major Cities in Texas

Abstract In urban areas, on-road vehicles are major sources of nitrogen oxides (NOx), volatile organic compounds (VOCs), and black carbon (BC), yet mobile-source emission inventories remain incompletely constrained by direct measurements. During the Corpus Christi and San Antonio (CCSA) field study, we conducted mobile measurements of NO, NO2, aromatic VOCs (benzene, toluene, xylene), CO, CO2, and BC along major roadways in San Antonio (SA) and Corpus Christi (CC), two Texas metropolitan areas with recurrent air quality issues. Fleet-average pollutant emission ratios and emission factors (EFs) were derived from on-road emissions and evaluated against the Fuel-based Inventory for Vehicle Emissions (FIVE). SA exhibited higher fuel-normalized NOx and BC emissions than CC, whereas aromatic VOC EFs were consistently higher in CC, indicating distinct fleet emission characteristics between the two cities. Compared with mobile observations, FIVE overestimated fleet-average NOx and BC EFs with larger discrepancies in CC. For aromatic VOCs, FIVE reproduced benzene EFs in SA but underestimated benzene in CC, while xylene EFs were underestimated by approximately a factor of 2 in both cities. These results show that inventory performance varies by pollutant and city and that city-specific mobile measurements improve the representation of on-road emissions, particularly for NOx, BC, and aromatic VOCs that contribute to ozone and particulate matter formation and exposure assessments.