Influence of Fuel Standards on Vehicular Emissions: Assessing the Impact of Bharat Stage Regulations in Urban Idling Conditions (Black Carbon and Carbon Monoxide)

Accurately constraining real-world vehicular emissions remains a major challenge for megacities where certification values often fail to represent on-road behaviour. To address this gap for Delhi, we conducted near-tailpipe measurements of Black Carbon (BC), Carbon Monoxide (CO), and Carbon Dioxide (CO₂) from a representative fleet of 42 gasoline Two-Wheelers (2Ws) and Light-Duty Vehicles (LDVs) across Bharat Stage (BS) III, IV, and VI categories. Emissions were quantified under controlled idling and high-idling conditions using an Aethalometer AE33 for BC and Horiba/LI-850 analysers for CO and CO₂, with fuel-based emission factors derived through carbon-balance calculations. To incorporate real-world usage, active in-use fleet fractions (45% for 2Ws and 60% for LDVs) were applied to estimate idling-related fuel demand. Annual idling fuel consumption was 24.93 × 10³ ton for Two-Wheelers and 79.75 × 10³ ton for LDVs, corresponding to 5.7% and 14% of their respective total fuel use. These mode-specific contributions were used as weighting factors for composite BC, CO, and CO₂ emission factors. Measured idle emission factors averaged 1.24 mg km^-1 (BC) and 5.11 g km^-1 (CO) for Two-Wheelers, and 0.10 mg km^-1 (BC) and 0.72 g km^-1 (CO) for Light-Duty Vehicles. BS VI vehicles exhibited more than an order-of-magnitude reduction in BC compared with BS III–IV, confirming the efficiency of newer emission-control technologies. However, strong heavy-tailed behaviour was observed: approximately 30% of the fleet contributed nearly 80% of total BC emissions, indicating a pronounced super-emitter segment. Fuel-scaled annual emissions for Delhi’s gasoline fleet were estimated as 0.029 Gg yr^-1 (BC), 100.86 Gg yr^-1(CO), and 2.86 Mt yr^-1 (CO₂). The findings underscore the substantial impact of ageing and poorly maintained vehicles on urban pollution burdens and provide high-resolution, measurement-based emission factors essential for improved inventories and targeted mitigation strategies.