Delhi meteorology
Why Delhi pollution gets worse in winter.
Winter pollution is not explained by one source. Delhi's concentrations rise when ongoing local and regional emissions interact with low wind, shallow atmospheric mixing, fog/cold conditions, regional transport and enhanced secondary aerosol formation.
Winter meteorology
A smaller volume of air is available to dilute the same emissions.
Shallow winter mixing layer
pollutants accumulate near surface
Deeper / better mixed air
more vertical dilution
Meteorology changes concentration without creating emissions. Delhi's winter problem is therefore best thought of as emissions interacting with regional transport and poor dispersion.
Low wind speed
Horizontal dispersion weakens, so emitted and transported pollutants can remain over the region longer.
Shallow mixing layer
Pollutants are mixed into a smaller vertical volume of air, raising near-surface concentrations.
Regional transport
Pollution does not respect Delhi's administrative border. Regional emissions can be transported into the city.
Secondary aerosol formation
Gases emitted from vehicles, industry, biomass burning and other sources can chemically form particulate matter in the atmosphere.
Why rain often helps
Rain can remove particles from the atmosphere—but it does not eliminate emissions.
Delhi's monsoon period generally benefits from stronger dispersion and wet removal. Rain scavenges some airborne particles and can suppress resuspended dust for a time. Once those favourable conditions end, pollution can rise again if emissions continue.
The policy implication
Weather explains variability. It does not absolve emissions.
A windy or rainy year can make annual air quality look better even without an equivalent structural emissions improvement. The reverse is also true. That is why Delhi Air Health keeps meteorology in view when interpreting year-to-year changes.
See Delhi's long-term evidence →Primary reference
CAQM — Why air pollution intensifies during winter in Delhi-NCR ↗Keep learning
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