Role of urban growth, technology, and judicial interventions on vehicle exhaust emissions in Delhi for 1991–2014 and 2014–2030 periods

• Counterfactual scenarios were evaluated for their effectiveness on Delhi׳s on-road emissions
• VOC emissions improved the most by introducing 4-stroke technology in 2-wheelers
• CNG made the most significant reductions of PM2.5 emissions
• Dual standards negated the PM emission benefits achieved from improved standards

Between late 1980s and 2014, the Greater Delhi region has witnessed an increase in vehicular fleet, four sets of emission standards, and changes in engine technology and fuel usage. This paper presents and evaluate these measures on on-road vehicle exhaust emissions under four counterfactual scenarios – (a) no penetration of 4-stroke (4S) 2-wheelers (2Ws) (b) no introduction of compressed natural gas (CNG) (c) no implementation of emission standards post 2000 and (d) no dual emission standards (supply of better fuel in the metropolitan areas and a grade lower for the rest). Introduction of 4S engines reduced VOC emissions by 90%, thus being the most effective compared to the three emission standards (BS-II, III, and IV) combined. Introducing CNG reduced 50% of PM2.5 and increased 20% of NOx emissions in 2014, mostly from buses and light duty vehicles. Implementation of emission standards affected all pollutants, with 60% reduction in VOCs and 20–30% reduction for the rest. Dual emission standards increased the PM2.5 emissions from heavy duty vehicles, as much as the reductions from passenger vehicles, thus negating the benefits of the latter. Under the proposed roadmap of emission standards and vehicular technology by the Auto Fuel Policy 2025 committee, PM2.5 emissions in 2030 will be halved, CO emissions will reach three times, and VOC and NOx emissions will at least stabilize, compared to 2014 estimates. If leapfrogged to BS-VI in 2017, there will be additional reduction in NOx and VOC emissions.