The opposite timing of these two smogs perfectly illustrates their completely different chemistry: Photochemical smog peaks afternoon: The key driver is sunlight intensity, which is maximum at solar noon (12–2 PM). The photochemical reaction chain (NO₂ + UV → NO + O· → O₃) only works with UV radiation. By afternoon, morning rush hour emissions (NOₓ, VOCs) have accumulated for hours AND the sun is at its most intense → peak ozone and PAN. Clear, sunny skies make it worse; cloudy or rainy days dramatically reduce it. London/sulphurous smog peaks early morning: This depends on temperature inversion (coldest in pre-dawn hours when the ground radiates maximum heat away), minimum wind speeds (most stagnant in early morning before thermal mixing begins), and peak coal/heating activity in London’s case (people lighting fires to heat homes in the morning). As the sun rises → air begins mixing vertically → inversions weaken → pollution disperses. By midday, London smog often partially clears. By evening, if temperatures drop again, a new inversion can reform. Delhi winters: Delhi suffers from sulphurous-type smog in the early morning (temperature inversion, cold) which partially disperses by late morning, then a mild photochemical component (from vehicles) builds in the afternoon. The two types can overlap in the same city.

This surprises most people — air pollution as a heart attack cause. The mechanism: (1) PM2.5 enters the bloodstream: Particles ≤2.5 µm can cross the alveolar membrane in the lungs and enter the bloodstream directly. Once in the blood, they trigger systemic inflammation. (2) Inflammation damages blood vessel walls: The chronic low-grade inflammation from circulating particles promotes atherosclerosis — the buildup of plaques in artery walls. This narrows arteries and reduces blood flow. (3) Autonomic nervous system disruption: Pollution exposure disrupts the heart’s electrical regulation — altering heart rate variability and increasing risk of arrhythmias (irregular heartbeat). (4) Oxygen stress: Carbon monoxide (CO) from pollution binds to haemoglobin 250× more strongly than oxygen → reduces oxygen delivery to cardiac muscle → heart works harder. (5) Coagulation changes: PM2.5 exposure makes blood platelets more “sticky” → promotes clot formation → clots in coronary arteries = heart attack. This is why studies consistently show that air pollution peaks (like after Diwali in Delhi, or during the Great Smog) are followed by increased hospital admissions for heart attacks and strokes 24-48 hours later.

Delhi is unique in experiencing both types of smog, often simultaneously, because it has both key drivers: (1) Sulphurous (London-type) in Delhi: Despite declining coal use in Delhi itself, coal-based thermal power plants in the surrounding region (UP, Haryana), brick kilns, and industrial units around Delhi emit SO₂. Additionally, biomass burning (stubble, wood, dung cakes) emits particulate matter and SO₂ precursors. Combined with winter temperature inversions (Delhi gets very cold winters) and fog → classic sulphurous smog conditions. (2) Photochemical (LA-type) in Delhi: Delhi has one of the world’s highest vehicle densities — millions of cars, trucks, auto-rickshaws emit massive NOₓ and VOCs. On summer and spring afternoons with strong sunlight, photochemical reactions form ground-level ozone and PAN. Delhi’s summer ozone levels are a growing concern for both health and crop damage. (3) The winter cocktail: In November-January, Delhi faces its worst air quality when BOTH types converge — temperature inversions trap PM, SO₂ from surrounding industries AND the accumulated NOₓ/VOC from vehicle exhaust, PLUS stubble burning smoke from Punjab-Haryana, PLUS low wind speeds, PLUS fog. The AQI doesn’t care which “type” of smog it is — all these pollutants add up. This is why Delhi’s winter smog is so catastrophically bad — it’s a multi-component pollution disaster, not a single-cause event.