Morning after Diwali was cleaner, felt many. But why did AQI data differ?

A day after Diwali, on October 21, many Delhi–NCR residents believed that the air was less polluted compared to last year's. Then they saw the Air Quality Index (AQI) data, and were left shocked. Pollution levels this year were at a five-year-high, with Delhi air ranked as India's most polluted that morning.

But not all felt the same. Accounts from people across the National Capital Region (NCR) pointed at varied perceptions of post-Diwali air quality compared to last year's. The opinions were primarily based on visibility and the smog intensity.

But if pollution was at its peak, how did some believe it wasn't as bad?

This is likely because of several factors, such as temperature, wind speed, and humidity, that can influence how air pollution appears to the naked eye. Nonetheless, this perception is still subjective.

Here's what people from across Delhi–NCR told India Today Digital what they thought about the pollution levels. And then an expert explains why there is a difference between the perceived and the real air quality.

BETTER OR WORSE: DELHI-NCR RESIDENTS WEIGH IN ON POLLUTION

Gaurav, a person from Rajouri Garden, West Delhi, said it felt less polluted than last year, mentioning clearer visibility the next morning.

But in East Delhi near Akshardham, a resident described severe pollution the morning after Diwali, even as he noted, "Residents are accustomed to such levels of pollution here due to constant fumes from the nearby Ghazipur landfill, which is as bad as Diwali smoke."

A resident of Greater Kailash 1, South Delhi, said the pollution levels were better compared to last year. However, a person from nearby Kalkaji described the air quality as "too bad", but "less severe than what was last year", though she noted that the air quality deteriorated a day later, on Wednesday. The source from Kalkaji also added she could "taste the dust in the air, and her eyes burned due to the air pollution."

Meanwhile, in South Delhi's Jangpura, the air quality was pretty much the same as in previous years, and Devdutt, a resident, said there was consistent haze, and he had throat irritation.

In North Delhi's Keshav Puram, two residents told India Today Digital that the air pollution levels felt as bad as every year.

In Noida Extension's Gaur City, the visibility was better, three sources told India Today Digital. In Noida, residents said firecracker use was also seemingly less compared to last year. But the air the morning after Diwali was heavy with pollution, they said.

A resident of Indirapuram in Ghaziabad felt that air quality was worse last year due to heavy use of firecrackers. "This year, the air pollution seemed lesser," he said.

WHAT WERE THE ACTUAL POLLUTION LEVELS

On October 21 at 6 am, Delhi's PM2.5 levels reached 228 micrograms per cubic meter, 15 times the World Health Organisation's 24-hour limit of 15 micrograms per cubic meter, according to Central Pollution Control Board (CPCB) data.

By 9 am, the 24-hour average AQI was 356, in the "very poor" category, with stations in Bawana, Burari, Jahangirpuri, and Wazirpur reporting "severe" levels.

According to CPCB data, PM2.5 levels spiked from 150 micrograms per cubic meter at 4 pm on October 20 to nearly 650 by 11 pm, which aligns with peak firecracker activity.

Delhi's average AQI of 429 between October 20–21 surpassed some of the worst AQIs globally, putting it in the list of cities such as Lahore (260) and Karachi (182), according to a Swiss air quality technology company's live data for over 120 cities.

Despite a Supreme Court order restricting firecracker use to 8–10 pm and mandating "green" firecrackers with 30% less smoke, violations were widespread across Delhi-NCR.

A day after Diwali, 16 North Indian cities reported "very poor" or "severe" AQI.

WHAT AIR POLLUTION WAS PERCEIVED NOT AS BAD?

Though surprising, some people did feel that the air quality wasn't as bad in Delhi-NCR. But what made them think or feel so?

The gap between perception and reality seemingly stemmed from many factors.

Firstly, firecracker use varies by area. Habituation, as noted by an East Delhi resident, dulls sensitivity to pollution spikes, especially near ongoing air pollution sources like landfills.

Weather also plays a major role in air pollution with humidity and air speed being crucial factors.

On Diwali night, the temperature in Delhi-NCR was between 23–25 degrees Celsius, the warmest in five years.

Scientist and meteorologist Gufran Beig, told India Today Digital, "This Diwali came much earlier, when we are still in a warmer period, and winter is not yet setting in. When the temperature is warmer, air movement is faster, and there is relatively less moisture in the air. So dispersion is very fast."

Higher temperatures cause air to become less dense, and promotes vertical mixing in the atmosphere. This allows pollutants like PM2.5 from firecrackers to rise and disperse upward, or mix with cleaner air at higher altitudes, reducing ground-level concentrations.

This might have aided slight smoke dispersal a day after Diwali.

"Air quality was very bad until morning, about 5 or 6 o'clock. The moment the sun came out, wind picked up immediately because of the warm temperature, and it (particulate matter) dispersed like anything. By afternoon, as I also predicted, it is going to be very clear by 4 or something," said Beig, chair professor at the National Institute of Advanced Studies, IISc.

But the wind speed was low in Delhi too—around 8 to 10 km/hr, according to the Air Quality Early Warning System for Delhi, which might have trapped pollutants and created a dense haze over the city.

Humidity also plays a significant role in air pollution dynamics, particularly in how it affects the persistence and dispersal of smoke and particulate matter.

"When there is relatively less moisture in the air, dispersion is very fast," said Beig.

When relative humidity is high (above 60-70%), water vapour in the air can cause PM2.5 particles from firecracker smoke to absorb moisture, increasing their size and weight, which eventually blankets the city in a dense haze.

FIREWORKS, NOT STUBBLE-BURNING, POLLUTED DELHI AIR: EXPERT

When chronic exposure to pollutants—common for Delhi-NCR residents—is combined with other environmental factors, it can impair the sense of smell, making people underestimate pollution because they can no longer detect it as strongly. A 2019 study by scientist and physician Dr Lilian Calderon-Garciduenas supports this, and an East Delhi resident's experience aligned with the findings.

Meanwhile, according to Gufran Beig, the real impact of stubble burning in Punjab and Haryana is yet to reach Delhi, which might make air pollution worse. "When stubble smoke intrudes, it's rich in black carbon. This time, the contribution [of stubble burning] was negligible, just 1 to 2%," he said. The dip in air quality was because of fireworks.

This pattern is typical of the Indo-Gangetic plains, where Diwali is celebrated with fervour but also coincides with annual winter pollution spikes.

Residents may have perceived clearer air this time, but "better" still means hazardous.

- Ends