Near road urban air pollution due to vehicular traffic: Effect of real-world driving conditions and vehicle composition

Rapid economic growth with ongoing urbanization trends has led to exponential growth in global transport demand, especially in Asia. The growing vehicle population in India is one of the major contributors to congestion and air pollution causing related health and climate risks in urban areas. In this study, we assessed the impact of motor vehicles on near-road air pollution using roadway tunnel and roadside measurements under real-world traffic conditions based on speed, type of roads, vehicle types (LDV: light duty vehicles, HDDV: heavy-duty diesel vehicles), and fuel composition (gasoline and diesel vehicles). Portable real-time and gravimetric instruments were used to measure emissions from high-speed urban traffic (at Freeway tunnel: all LDV fleet), low-speed creeping traffic (at LBS road: 5% HDDVs and rest LDV fleet), and idling traffic (at Mulund toll plaza: 8% HDDVs and rest LDV fleet) in Mumbai city, and high-speed inter-city traffic (at Kamshet-I tunnel on Mumbai-Pune expressway: 20% HDDV and 80% LDV), covering both peak and off-peak traffic hours. A very small fraction of electric vehicles was also observed in the fleet at LBS road and Toll plaza. Simultaneous measurements were also carried out at an urban background location in Powai, Mumbai. All measured gaseous (CO₂, CO, NO_2, and VOCs) and particulate (PM_2.5 and BC) pollutants at the roadsides and the tunnels were 1.2 to 3.8 times higher than in the background. Total fine carbonaceous species, comprising of elemental carbon (EC) and organic carbon (OC) accounted for up to 47%(±5%) of total PM_2.5, highest in the inter-city traffic which could be attributed to its high HDDV and super-emitter fraction followed by the urban idling traffic which had significant HDDV fraction. The OC/EC ratio was 1.8 (± 0.3), highest in the high-speed urban traffic with all light-duty vehicles (LDV) fleet, and the higher HDDV fraction in the inter-city traffic attributed to the low OC/EC ratio of 0.7 (±0.4).  The water-soluble organic carbon (WSOC) fraction that affects aerosol hygroscopicity accounted for up to 70% of the total OC. WSOC was found highest during the afternoon period at the roadsides as well as the background site indicating the contribution of other sources, including photochemical processes. Our study finds that while the near-road emission levels are inevitably higher due to the significant contribution from on-road vehicles, the emission profiles vary significantly depending on the vehicle and fuel composition. Higher HDDV fraction and super-emitters in the fleet contributed to 2.8 folds of higher EC concentrations.  These findings can help in making informed policy decisions towards urban vehicle emission control and monitoring by focusing on targeted vehicles that are polluting disproportionately more than the rest of the vehicles.