Delhi AQI crosses 500: Why does pollution spike at night and early morning?

Delhi is hurtling towards Diwali wrapped in a blanket of toxic air. The science behind its worsening morning haze reveals why the problem is so persistent.

In Short

• Delhi's AQI hit a hazardous 510 on November 5 morning
• Pollution worsens early morning due to stable, cool air
• Soon to come Diwali firecrackers may further deteriorate air quality

Delhi woke up to another morning of toxic air on Thursday, with pollution levels soaring to an alarming AQI of 510 at 10:01 am, according to AQI.in.

After days of steadily worsening air quality, the capital is now choking on a concoction of smoke, dust and trapped emissions, with Diwali still days away.

The AQI has climbed from a daily average of 410 on November 3 to 493 on November 4, and now 510 on November 5. But there is another troubling pattern Delhi residents have unwillingly grown accustomed to.

At night and early in the morning, the air gets far more polluted than it is in the day, revealing itself through the haze that covers everything and often leaves people waking up to thicker smog than the evening before. Makes one wonder why pollution builds up while the city sleeps.

The answer to the question is not just about what Delhi is spewing into the air but how the atmosphere conspires after sunset.

WHEN THE AIR STOPS MOVING

During the day, sunlight heats the ground, which in turn warms the air above it.

This warm air rises, allowing pollutants to spread upwards and mix with a larger volume of air. Adding to that are the stronger daytime winds that also help carry pollution away.

But after sunset, the ground begins to cool. The air close to the surface becomes colder than the air above it, creating a condition known as a temperature inversion. Instead of rising and mixing freely, the colder air remains trapped near the ground.

This creates a lid-like effect, preventing pollutants from dispersing upwards.

Emissions from vehicles, industries, construction and other sources accumulate in a much smaller volume of air, causing pollution levels to rise.

WHY IS POLLUTION WORSE IN THE MORNING?

The problem becomes more pronounced during the early morning hours, when the atmosphere is still cool and stable and winds are often weak.

Delhi's atmospheric mixing layer, the portion of air closest to the ground where pollutants normally spread, becomes much shallower at night.

With less air available for pollutants to mix into, concentrations near the surface can rise sharply.

The early morning hours are also when the temperature inversion is often strongest, just before sunlight begins warming the ground. This explains why pollution readings can peak around sunrise even when traffic is lighter than during the day.

DELHI'S POLLUTION WOES CONTINUE

This daily cycle is now playing out during Delhi's annual pollution season.

Vehicle emissions, road dust, construction, industries and stubble burning across neighbouring states contribute to the overall pollution load.

Daily recorded data has shown how Delhi's AQI has recently risen in mere days, going from 124 on October 30 to nearly 400 by November 2, following a brief improvement after rainfall and stronger winds.

And now, with Diwali approaching on November 8, firecracker smoke could add another layer of pollutants to an atmosphere already struggling to clear them.

The result would be a mix of factors that are bound to make the situation worse – rising emissions, the winter cold and harmful pollutants trapped close to the ground.

- Ends