Winter Smog over the Indo-Gangetic Plain

Winter smog over the Indo-Gangetic Plain is a build-up of particulate pollution near the ground when weather conditions prevent polluted air from dispersing. The key mechanisms are temperature inversions and a shallow atmospheric mixing layer, which concentrate emissions within a small volume of air.

Winter cooling suppresses vertical mixing

Normally, daytime heating produces rising air currents that carry surface pollutants upward. In winter, weaker solar heating and long nights favour a colder, more stable layer near the ground.

• On clear nights, the ground loses heat through radiation and cools the air immediately above it. This can produce a temperature inversion, in which temperature increases with height rather than decreasing.
• Air displaced upward within an inversion becomes colder and denser than its surroundings, so its upward motion is suppressed. The inversion therefore acts as a lid on vertical dispersion.
• Weak daytime heating may erode the inversion slowly and produce a low mixing height, the vertical depth through which turbulence distributes pollutants. The same emissions then become concentrated in a smaller air volume.

Weak ventilation allows regional accumulation

Vertical trapping becomes especially effective when horizontal movement of air is also weak. The Indo-Gangetic Plain contains numerous emission sources, so stagnant conditions can produce a regional pollution layer rather than an isolated urban plume.

• During weak-wind conditions, pollutants are transported away slowly, allowing successive emissions to accumulate near their sources.
• The Himalayan barrier restricts cross-mountain exchange along the northern margin of the plain, while prevailing winds can redistribute pollution between locations within the region.
• Particles from combustion, industry, vehicles and dust supply the pollution; unfavourable meteorology primarily controls its dilution and persistence. Winter conditions do not themselves create all the particulate matter.

Humidity and fog can reinforce the haze

Cold conditions can bring near-surface air close to saturation. Fog and particulate pollution may then coexist, although fog is not necessary for pollution to accumulate.

• Some particles absorb water at high humidity and grow larger, increasing light scattering and reducing visibility.
• Water in fog droplets or moist particles can support chemical reactions that form secondary particulate matter from gaseous precursors.
• Persistent fog and haze can reduce sunlight reaching the ground, weakening daytime heating and delaying vertical mixing.
• Stronger winds, deeper daytime mixing or precipitation can interrupt accumulation by dispersing pollutants or removing particles from the atmosphere.