Every winter, Delhi disappears. A thick grey haze settles over the capital, the sun becomes a pale disc, schools close, and the air quality index climbs into categories officially described as severe. The smog is not a single pollutant but a seasonal conspiracy of sources and weather: stubble burning in neighbouring states, vehicle exhaust, construction dust, and winter meteorology that traps everything close to the ground. Understanding Delhi’s smog means understanding both chemistry and the atmosphere’s winter behaviour.

The sources: where the smoke comes from

Delhi’s pollution has year-round contributors. Vehicles, numbering in the millions, emit nitrogen oxides and fine particles; road dust and construction add coarse particles; industries and diesel generators contribute their share. But the dramatic winter spike has a seasonal signature: the burning of paddy stubble by farmers in Punjab and Haryana after the kharif harvest. In late October and November, thousands of fires light up satellite images, and the smoke drifts southeast toward Delhi. Studies attribute a large fraction, sometimes a third or more, of peak-season PM2.5 in the capital to this burning, though the exact share is debated and varies day to day.

Why winter makes it worse

The same emissions in July would disperse harmlessly; in winter they accumulate. Cold air near the ground creates temperature inversions, layers where air gets warmer with height instead of cooler, acting as a lid that prevents pollutants from rising and dispersing. Winter winds are typically slow, and the mixing height, the depth of air available for dilution, shrinks dramatically. The result is a shallow bowl of stagnant air over the Indo-Gangetic plain in which a day’s emissions accumulate instead of blowing away. Add fog, whose droplets provide surfaces for chemical reactions that create secondary particles, and the smog thickens further.

What is actually in the smog

The most dangerous component is PM2.5, particles smaller than 2.5 microns that penetrate deep into the lungs and bloodstream, linked to asthma, heart disease, strokes and reduced life expectancy. Delhi’s winter PM2.5 routinely exceeds safe limits by tenfold or more. The particles come in two kinds: primary particles emitted directly, like soot from burning, and secondary particles formed in the atmosphere when gases like sulphur dioxide, nitrogen oxides and ammonia react. This secondary chemistry is why simply banning one source never fully clears the air; the atmosphere manufactures pollution from gases too.

What has been tried

Delhi has thrown an arsenal of measures at the problem, with mixed results:

• The odd-even vehicle scheme, restricting cars by number plate on alternate days, cut traffic but showed modest air quality gains.
• Stubble management subsidies, paying for machines that clear fields without burning, have reduced fires in some districts but adoption remains patchy.
• Construction bans and GRAP, the Graded Response Action Plan, triggers escalating restrictions as pollution worsens.
• Cleaner fuels and the metro, including the shift to cleaner fuel standards and massive metro expansion, address the year-round baseline.

Why the problem persists

Smog endures because its causes span jurisdictions and economics. Stubble burning persists because the window between harvest and sowing is brutally short and alternatives cost money many farmers lack. Enforcement across state borders is weak. And the atmosphere itself is regional: Delhi’s air is the Indo-Gangetic plain’s air, shared with hundreds of millions of people and countless sources. Lasting improvement requires coordinated regional action, economic alternatives for farmers, and sustained pressure on every source, not just the most visible one.

FAQs

Is stubble burning the main cause? It is the biggest seasonal driver of the worst episodes, but vehicles, dust, industry and secondary chemistry sustain high pollution all winter. No single source explains everything.

Do masks help? Well-fitted N95 masks filter PM2.5 effectively outdoors, but they do not protect against gases, and indoor air needs purifiers or ventilation strategies.

Has air quality improved at all? Annual averages have improved modestly from their worst years thanks to cleaner fuels and metro expansion, but winter peaks remain severe.

Delhi’s winter smog is a systems problem: agriculture, transport, industry, weather and geography locked together. It will not be solved by one ban or one technology, but by treating the entire airshed, from Punjab’s fields to Delhi’s streets, as a single atmosphere, which is exactly what it is.

Source: Central Pollution Control Board