New Delhi: As Delhi grapples with its annual pollution crisis, a subtle but significant shift has occurred in the cocktail of pollutants that cloud its skies. Through most of October, the Capital’s air has transformed – from being dominated by photochemical pollutants like ozone (O3) and carbon monoxide (CO) in the first 19 days to particulate matter (PM10 and PM2.5) in the past four days.

The change, experts said, points to a sharp rise in combustion-driven emissions – primarily firecrackers, and aided by emissions from vehicles, industries, and local burning – that now define the city’s toxic air.

Between October 1 and 19, data from Central Pollution Control Board’s (CPCB) Sameer app showed that ground-level ozone was the primary pollutant on 12 of the 19 days, with CO and NO2 also contributing significantly to the overall air quality index (AQI).

Ozone, a secondary pollutant, is not directly emitted but formed when nitrogen oxides (NOx) and volatile organic compounds (VOCs) from vehicles, factories, and power plants react in bright sunlight. CO, meanwhile, is a direct byproduct of incomplete combustion from similar sources.

In the first 19 days, Delhi’s skies were clearer, the air was occasionally washed by light rain, and sunlight was abundant – ideal conditions for ozone formation. The city even recorded AQI reading of “satisfactory” on four days in this spell – on October 3 (88), October 7 (73), October 8 (81) and October 9 (100).

But the chemistry of Delhi’s air changed dramatically – particularly with the onset of Diwali.

From October 20 onward, ozone and CO disappeared from the AQI charts, replaced by particulate matter – tiny, hazardous particles that penetrate deep into the lungs.

Since then, PM10 was the lead pollutant on October 20, 22 and 23. To be sure, the Capital’s AQI has remained in the “very poor” category for four consecutive days since October 20. On Thursday, it stood at 305, marginally lower than Wednesday’s 353 but still chokingly high.

Overall, CPCB data shows a nearly 300-point swing this month – from 73 at the start of October to a peak of 353 as firecrackers and stagnant air combined to trap pollutants over the city.

CPCB calculates sub-indices for eight individual pollutants (PM10, PM2.5, NO2, SO2, CO, O3, NH3, and Pb) at a monitoring location using its 24-hourly average concentration value (8-hourly in case of CO and O3). The worst sub-index is the AQI for that location.

The overall average AQI is calculated by considering data for all stations, with a minimum data of three pollutants, out of which one should necessarily be either PM2.5 or PM10.

CPCB classifies AQI between 0-50 as “good”, between 51 and 100 as “satisfactory”, between 101 and 200 as “moderate”, between 201 and 300 as “poor”, between 301 and 400 as “very poor”, and over 400 as “severe”.

“The first half of the month was dominated with some rain, but also ample sunshine. Gases like ozone form when combustion sources like vehicles and emissions from industries combine with volatile organic compounds under sunlight. If O3 and CO were lead pollutants, it means that the impact of external combustion sources was minimal,” said Sunil Dahiya, stating with rains stopping, the air has become drier and a drop in wind speed had led to accumulation of both local sources and cross-boundary emissions.

“Emissions from vehicles, industries and waste burning have begun to shoot up, leading to PM 2.5 and PM 10 rising. There is also dust, which leads to a spike in PM 10. However, from October 20, we have been seeing regular bursting of firecrackers, which is the primary combustion source at the moment,” Dahiya said.

Anumita Roychowdhury, executive director of research and advocacy at the Centre for Science and Environment (CSE), said the transition was inevitable. “Combustion sources always dominate Delhi’s air, except in peak summer when dust plays a bigger role,” she said. “In the first half of October, rainfall helped settle particulate matter, allowing gases like ozone and CO to dominate. But once the weather turned dry and calm, particulates rose sharply. Add to that vehicles, industries, and the early burst of firecrackers, and you get the pollution mix we’re seeing now.”