Vehicular volatile organic compounds (VOCs)-NO_x-CO emissions in a tunnel study in northern China: emission factors, profiles, and source apportionment

Abstract. Vehicular emission is a key contributor to ambient volatile organic compounds (VOCs) and NO_x in Chinese megacities. However, the information of real-world emission factors (EFs) for a typical urban fleet is still limited, hindering the development of a more reliable emission inventory in China. Based on a more-than-two-week (August 8–24, 2017) tunnel test in urban Tianjin in northern China, and on the use of a statistical regression model, the Positive Matrix Factorization (PMF) receptor model, and the Calculate Emissions from Road Transport (COPERT) IV model, characteristics of vehicular VOCs-NO_x-CO emissions were analyzed systematically. The fleet-average EFs (pollutant: downslope, upslope, and overall in mg km⁻¹ veh⁻¹) were estimated respectively as follows: (NO: 61.92 ± 72.46, 158.58 ± 73.48, 97.52 ± 69.84), (NO₂: 16.52 ± 11.49, 23.98 ± 20.14, 15.86 ± 9.38), (NO_x: 79.45 ± 78.43, 181.22 ± 88.29, 116.56 ± 77.61), and (CO: 269.96 ± 342.38, 577.76 ± 382.22, 344.67 ± 250.01). The EFs of NO-NO₂-NO_x and CO from heavy-duty vehicles (or diesel vehicles) were differentiated from light-duty vehicles (or gasoline vehicles). The ratios (v / v) of NO₂ to NO_x in the primary vehicular exhaust were approximately 0.18 ± 0.09, 0.10 ± 0.22 and 0.10 ± 0.05 for downslope, upslope, and the entire tunnel, respectively. The fleet-average EF of the 99-target non-methane VOCs (NMVOCs) was 40.56 ± 12.18 mg km⁻¹ veh⁻¹, lower than the previous studies in China. The BTEX (benzene, toluene, ethylbenzene, p-xylene, m-xylene and o-xylene) levels decreased by approximately 79 % when emission standards increased from China I to China V. The source profiles of NMVOCs from the tailpipe and evaporative emissions were resolved by the PMF model. The evaporative emissions accounted for nearly one-half of the total vehicular VOC emissions, indicating that evaporative and tailpipe emissions contributed equally to VOC emissions. The relative contributions of evaporative NMVOC emissions to total vehicular NMVOC emissions are temperature-dependent with the average increasing ratio of 7.55 % °C⁻¹. The primary emission ratio (ER, m / m) of VOCs / NO_x was approximately 2.04, suggesting that vehicular NO_x and VOCs can be co-emitted with a proper ER. According to the vehicular ERs of VOCs / NO_x in Tianjin (2000–2016) and China (2010–2030), as even more stringent emission standards are implemented in the future, the O₃ chemical regimes were likely to be VOCs-limited (i.e., 8 : 1 threshold) for cities or regions where VOCs and NO_x emissions are dominated by vehicular exhaust. Our study enriched the database on the fleet-average emission factors of on-road vehicles for emission inventory, air quality modeling, and health effects studies, provided implications for following O₃ control in China from the view of primary emission, and highlighted the importance of further control of evaporative emissions.