Why is night AQI worse in Delhi?

How night aqi is higher in Delhi comparing day AQI, even though minimum vehicular emissions and parali fires can't impact only during night ?

There are clear scientific reasons why AQI in Delhi is often higher at night than during the day, even when traffic emissions are low and stubble-burning (“parali”) isn’t occurring at that exact time.

Here’s the explanation, step by step:

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🌆 1. Night-time temperature inversion traps pollutants

This is the main reason.

• After sunset, the ground cools quickly.

• The air near the surface becomes colder than the air above it.

• Cold air is heavier and cannot rise - this creates a temperature inversion layer.

• Pollutants from day sources (vehicles, industry, dust, residual biomass smoke) get trapped near the ground, causing AQI to spike.

During the day:

• Sunlight heats the ground → warm air rises → vertical mixing occurs.

• Pollutants disperse upward → AQI appears better.

➡️ Even if emissions are low at night, the inability of pollutants to disperse makes the concentration appear higher.

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🌬️ 2. Wind speed is lowest at night

• Night-time winds tend to calm significantly in plains cities like Delhi.

• With low or no wind, pollutants accumulate locally.

• Even residual pollution from the daytime stays close to the surface.

Daytime winds + sunlight = better dispersion.

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🏭 3. Continuous emissions (even at lower levels) still accumulate

Even though traffic decreases, these sources continue throughout the night:

• Waste burning (often done at night)

• Industrial emissions (not all factories shut down)

• Construction dust

• Domestic emissions (heating, cooking, generators)

• Heavy trucks enter Delhi mostly at night (unless restricted)

Lower emissions + zero dispersion still leads to higher concentration.

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🌫️ 4. Higher humidity at night forms secondary particles

At night Delhi’s humidity rises. Higher humidity promotes:

• Formation of secondary particulate matter (especially PM2.5)

• Hygroscopic growth of existing particles → they absorb moisture and get “bigger”

• Fog + particulate interaction → smog

This increases PM2.5 readings even if pollution source is unchanged.

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🔥 5. Parali (stubble burning) affects nights more due to wind patterns

Even though fires may occur in the day, their transport to Delhi often peaks at night:

• Cooler night winds from Punjab/Haryana bring smoke into Delhi.

• Unable to disperse due to inversion, the smoke accumulates.

So fires don’t have to happen “at night” to worsen night AQI.

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🌃 6. Chemical reactions differ between day & night

Daytime sunlight causes photochemical reactions that help break some pollutants (like NOx).
At night:

• No sunlight → fewer reactions to degrade pollutants.

• NO₂ builds up because it isn't photolyzed.

• Ozone decreases at night, affecting secondary particle formation dynamics.

Overall effect → pollution stays in its most concentrated form.

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✔️ In summary: Why is night AQI worse in Delhi?

Even with lower emissions, pollutants get trapped, accumulate, grow, and don’t disperse at night due to:

• Temperature inversion

• Low wind speed

• Increased humidity

• Residual emissions accumulating

• Transport of pollution from outside Delhi

• Suppressed chemical breakdown of pollutants

This phenomenon is common in polluted cities worldwide, but Delhi’s geography and winter conditions amplify it dramatically.

B S Vohra, Environment Activist, President- East Delhi RWAs Joint Front