Beginning Sunday, November 1, non-BS-VI petrol and diesel vehicles registered outside Delhi will no longer be able to enter the capital. The restriction, which will be in effect for three months until January 31, 2027, will impact lakhs of older vehicles that do not meet the prescribed tailpipe emission norms.Approximately 3.02 crore Bharat Stages-I to IV vehicles are registered in Uttar Pradesh, Haryana, and Rajasthan. Forty-eight lakh of these vehicles are registered in National Capital Region (NCR) districts adjoining Delhi.All these vehicles frequently enter and exit Delhi, often on a daily basis. Roughly three-quarters of these pre-BS-VI vehicles are two-wheelers.Several Delhi Traffic Police officers with whom The Indian Express spoke acknowledged that ensuring effective implementation of the restrictions through the winter would present a formidable challenge.More than 60 per cent – 30.65 lakh – of the 48 lakh pre-BS-VI vehicles are registered in the NCR districts of Haryana, including Gurugram, Faridabad, Rohtak, Sonipat, Jhajjar, Nuh, Rewari, and Palwal. Smaller numbers of these older vehicles are registered in the NCR districts of UP (11.8 lakh) and Rajasthan (5.6 lakh).Must Read | Delhi pollution curbs from Nov 1: What’s banned, what’s allowed under revised GRAP rules?Across the region, two-wheelers make up the largest chunk of these vehicles – numbering around 36.5 lakh. Cars and cabs total 7.26 lakh vehicles, or 15 per cent of the active pre-BS-VI stock. Goods vehicles, buses, and tractors together number about 2.9 lakh, or 6 per cent of the total.In the heavier segments, diesel is the norm: 81 per cent of goods vehicles, 70 per cent of buses, and 93 per cent of tractors and farm vehicles run on diesel, which is more polluting than petrol as a fuel.Story continues below this adTransport-related pollution has been the dominant contributor to air pollution in the capital, according to the Decision Support System (DSS), a numerical model-based framework developed by the Indian Institute of Tropical Meteorology (IITM), Pune. The average contribution of transport-related pollution to Delhi’s air pollution in the winter months is estimated at 15-20 per cent.The traffic load on Delhi is enormous. Around 16,900 heavy-duty trucks enter the capital daily, more than half of which arrive at four primary entry points, a joint study by IIT-Delhi and The Energy and Resources Institute (TERI) reported earlier this year.While these trucks represent only a fraction of the total vehicles on Delhi’s roads, they generate 61 per cent of all transport-related PM2.5 emissions between the hours of 10 pm and 7 am, the study reported.Also Read | With pollution curbs in Delhi, labourers stare at months without wagesA draft emissions inventory for 2021 prepared by TERI estimated that vehicles accounted for around 47 per cent of Delhi’s PM2.5 emissions, 78 per cent of nitrogen oxide emissions, and 49 per cent of non-methane volatile organic compounds.Story continues below this adTwo-wheelers were identified as the largest vehicular source of PM2.5, PM10, sulphur dioxide, and carbon monoxide emissions. Buses were the largest source of nitrogen oxides, while three-wheelers contributed the most volatile organic compounds.For the wider NCR, the same inventory identified industries as the largest source of particulate pollution, accounting for 44 per cent of PM2.5, and 41 per cent of PM10. Among vehicular sources, two-wheelers were the largest contributors to PM2.5 (31 per cent) and carbon monoxide (51 per cent). Buses contributed the largest share of vehicular nitrogen oxide emissions, at 34 per cent.An earlier emissions inventory prepared by IIT Kanpur, based on data collected in 2013-14 and submitted to the Delhi government in 2016, had identified trucks as the largest contributors to vehicular PM2.5 and PM10 emissions, accounting for around 46 per cent of each, followed by two-wheelers at around 33 per cent.Buses were identified as the largest contributors to nitrogen oxides, while two-wheelers contributed the most to carbon monoxide emissions.Story continues below this adA separate study by TERI and the Automotive Research Association of India, based on monitoring and modelling carried out in 2016-17, estimated that the transport sector contributed 28 per cent of Delhi’s winter PM2.5 concentrations.Also Read | Environment Minister reviews CAQM’s new digital nerve centre to track air pollutionThe study also found that older vehicles manufactured before BS-IV standards came into force had a higher contribution to particulate matter concentrations than BS-IV vehicles.Traffic Police officers said coordination and cooperation among neighbouring districts would be crucial to the effective implementation of the restrictions, particularly in preventing pre-BS-VI vehicles from entering Delhi. Once a vehicle manages to enter the city or get on to a flyover leading into Delhi, it would be difficult to stop and turn it around, the officers said.“It is virtually impossible to ensure that every vehicle is turned around and sent back without causing massive traffic congestion. Given the available manpower, it is also not possible to deploy personnel to monitor every vehicle that enters the city,” an officer said. There are 23 entry points to Delhi, and officials estimated that more than 12 lakh vehicles enter the capital every day.