Controlling Vehicular Air Pollution: A Case Study of Delhi

Why Delhi became a case study

Delhi has one of the largest vehicle populations of any Indian city — at one point it had more registered vehicles than the states of Gujarat and West Bengal combined — and by the 1990s it ranked among the four most polluted cities in the world. The crisis became serious enough that a public interest litigation (PIL) was filed in the Supreme Court of India over Delhi's air quality.

The court-driven fix: switching to CNG

Acting on the Supreme Court's directions, the government was ordered to clean up the city's public transport within a fixed time frame, and by the end of 2002 every bus in Delhi's public fleet had been converted from diesel to compressed natural gas (CNG). CNG burns far more completely than petrol or diesel, so very little of it escapes unburnt from the engine; it is also cheaper than these fuels, and — unlike petrol or diesel — it cannot be siphoned off by thieves or adulterated. The main practical challenge in the switch was building the pipeline network and pump/distribution infrastructure needed to supply CNG reliably across the city.

Alongside the CNG switch, Delhi also phased out old, high-polluting vehicles, moved to unleaded and low-sulphur petrol and diesel, mandated catalytic converters, and tightened permissible pollution limits for vehicles.

Tightening fuel and emission standards nationally

The Government of India's auto fuel policy set out a roadmap for progressively reducing the sulphur and aromatic-hydrocarbon content of vehicle fuels — for instance, Euro III norms of the time capped sulphur at 350 parts per million (ppm) in diesel and 150 ppm in petrol, with aromatics capped at 42 per cent, and the long-term goal was to bring sulphur down further, to 50 ppm, while vehicle engines were upgraded to match the cleaner fuel.

The older Bharat Stage II emission norms (roughly equivalent to Euro-II) are no longer used anywhere in the country; the table below summarises the mass emission standards now applicable to different classes of vehicles.