As Delhi battles its annual pollution crisis, the city&rsquo;s air composition has undergone a sharp transformation this October. 
What began as a month dominated by photochemical pollutants like ozone (O₃) and carbon monoxide (CO) has now shifted to particulate matter &mdash; PM2.5 and PM10 &mdash; following Diwali celebrations.
According to experts, this shift signals a surge in combustion-based emissions caused by firecrackers, vehicles, industries, and local waste burning, which now define Delhi&rsquo;s toxic air.
Data from the Central Pollution Control Board&rsquo;s (CPCB) Sameer app shows that between October 1 and 19, ground-level ozone was the main pollutant on 12 of those days, with CO and nitrogen dioxide (NO₂) also contributing to poor air quality.
Ozone, a secondary pollutant, forms when nitrogen oxides (NOx) and volatile organic compounds (VOCs) from vehicles and industries react under sunlight. 
In contrast, CO results directly from incomplete combustion. 
Favorable weather conditions earlier in the month &mdash; clear skies, occasional rain, and ample sunshine &mdash; supported ozone formation. 
Delhi even recorded &ldquo;satisfactory&rdquo; AQI levels on October 3 (88), October 7 (73), October 8 (81), and October 9 (100).
However, the chemistry of Delhi&rsquo;s air changed drastically after Diwali. 
Starting October 20, ozone and CO disappeared from AQI charts, replaced by PM10 and PM2.5 &mdash; fine particles that can penetrate deep into the lungs and cause serious health issues.
PM10 became the dominant pollutant on October 20, 22, and 23. 
Since October 20, the Capital&rsquo;s air quality index has stayed in the &ldquo;very poor&rdquo; category for four consecutive days, with Thursday&rsquo;s AQI at 305 &mdash; slightly better than Wednesday&rsquo;s 353 but still hazardous.
CPCB data indicates a nearly 300-point AQI swing this month &mdash; from a &ldquo;satisfactory&rdquo; 73 at the start of October to a &ldquo;very poor&rdquo; 353 after Diwali, as firecracker emissions and stagnant weather trapped pollutants over the city.
The CPCB calculates sub-indices for eight pollutants (PM10, PM2.5, NO₂, SO₂, CO, O₃, NH₃, and Pb) based on 24-hour average concentrations, except CO and O₃, which use eight-hour averages. 
The highest sub-index becomes the AQI for that area, and the citywide AQI is the average across stations with valid data for at least three pollutants, including PM2.5 or PM10.
The AQI scale categorizes pollution levels as: &ldquo;good&rdquo; (0&ndash;50), &ldquo;satisfactory&rdquo; (51&ndash;100), &ldquo;moderate&rdquo; (101&ndash;200), &ldquo;poor&rdquo; (201&ndash;300), &ldquo;very poor&rdquo; (301&ndash;400), and &ldquo;severe&rdquo; (above 400).
Sunil Dahiya, an air quality expert, said, &ldquo;The first half of the month had light rain and sunshine, leading to ozone formation and minimal external combustion sources.&nbsp;But with dry weather and reduced wind speed, pollutants from vehicles, industries, and waste burning started accumulating, raising PM2.5 and PM10 levels.&nbsp;Since October 20, firecrackers have become the primary combustion source.&rdquo;
Anumita Roychowdhury, executive director of research and advocacy at the Centre for Science and Environment (CSE), said the shift was predictable. 
&ldquo;Combustion sources dominate Delhi&rsquo;s air most of the year, except in peak summer when dust levels are higher,&rdquo; she explained. 
&ldquo;In early October, rainfall settled particulate matter, allowing gases like ozone and CO to dominate. But as the weather turned dry and calm, particulates surged. Add vehicles, industries, and firecrackers &mdash; and you have the current pollution mix.&rdquo;

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Post-Diwali, Delhi’s air turns toxic as PM2.5 and PM10 replace ozone as key pollutants

Ozone Fades, PM2.5 Rises Post-Diwali In Delhi

Post-Diwali, Delhi’s air turns toxic as PM2.5 and PM10 replace ozone as key pollutants

October 24, 2025 | 09:19 IST