NEW DELHI: Delhi's monsoon this year turned out to be its most polluted in seven years, with the average AQI between July and September climbing to 105, the highest for the period since 2019, according to an analysis of official data.
The monsoon lasted for about 84 days between July and September, and during the period the city recorded one “good” AQI day, 49 “satisfactory” days, 39 “moderate” days and two “poor” days.
The average AQI during July-September stood at 105 this year, just below the 2019 average of 106, according to the analysis of official data by the Centre for Research on Energy and Clean Air (CREA) accessed by PTI.
Among the two major particulate pollutants, PM10 was the more prominent pollutant this year. Its average concentration stood at 108 micrograms per cubic metre, the highest since 2019, when it was 109 micrograms per cubic metre.
PM2.5 averaged 37 micrograms per cubic metre this year, the same as in 2024 and the highest since then, though it remained below the 39 recorded in 2023 and 40 in 2019.
Manoj Kumar from CREA told PTI that the rise in PM10 despite above-normal rainfall pointed to dust and coarse particles playing an important role in the season’s pollution.
“Delhi experienced its worst monsoon air quality since 2019, with both average AQI and PM10 levels at their highest since then. What stands out is that this happened despite slightly above-normal rainfall,” he said.
“The sharp rise in PM10 points to dust and other coarse particles as a major factor, while dry spells and local weather conditions likely allowed pollution to build up between rain events,” Kumar said.
The average AQI during July-September was 91 in both 2024 and 2025, 103 in 2023, 95 in 2022, 99 in 2021 and 88 in 2020.
The rise in PM10 was particularly notable compared with recent years.
The average stood at 88 micrograms per cubic metre in 2025, 87 in 2024, 105 in 2023, 92 in 2022, 98 in 2021 and 85 in 2020. The PM2.5 average, meanwhile, was 32 micrograms per cubic metre in 2025, 37 in 2024, 39 in 2023, 36 in 2022, 37 in 2021 and 35 in 2020.
The monsoon arrived over Delhi on July 2, five days after its normal onset date of June 27, marking the first July onset in five years. It withdrew on September 23, two days earlier than its normal withdrawal date of September 25.
In 2019, the monsoon had arrived on July 5 and withdrawn on October 10, lasting 97 days, considerably longer than this year’s spell. In 2019, Delhi recorded two “good” days, 51 “satisfactory” days, 34 “moderate” days, four “poor” days and one “very poor” day during the July to September period.
Despite the shorter monsoon, Delhi received 728.6 mm of rainfall during the season against the normal 629.7 mm, an excess of nearly 16 per cent.
An IMD official had earlier told PTI that no major rainfall record was broken this year despite the rainfall being above normal, noting that Delhi had received more than 900 mm of rainfall in each of the previous two years.
A separate study by the Centre for Science and Environment (CSE), which analysed this year’s monsoon from June to August, found that the relief from pollution during rainy spells was often short-lived, with PM2.5 levels rising sharply again once the rain stopped.
During six isolated rainy spells studied by CSE, average PM2.5 levels fell from 40.6 micrograms per cubic metre before rain to 32.7 micrograms per cubic metre during the rain. But the levels climbed to 36.2 micrograms per cubic metre a day later and 45.1 micrograms per cubic metre two days later -- higher than the pre-rain level.
The study found that even during the monsoon, Delhi recorded just one “good” AQI day in the 92 days it analysed. As many as 53 days were “moderate”, 35 were “satisfactory” and three were “poor”.
Interestingly, even the season’s heaviest rainfall spell did not translate into “good” air. On August 8, when the average rainfall recorded at Safdarjung and Palam was 101.05 mm, Delhi’s AQI was only in the “satisfactory” category.
The CSE analysis found that average PM2.5 levels during the June-August monsoon were 40.2 micrograms per cubic metre, down about 33 per cent from 60.2 micrograms per cubic metre during April-May.
But the improvement did not last once the rain stopped, as emissions from vehicles, industry, waste burning and power generation continued to add to the pollution load.
The study also found that the monsoon pollution story was not limited to particulate matter. As PM2.5 levels fell, gaseous pollutants emerged as prominent contributors to the daily AQI on several days.
Ground-level ozone was the prominent pollutant on 20 per cent of monsoon days, nitrogen dioxide on 10 per cent of the days and carbon monoxide on seven per cent.
The findings suggest that while the monsoon can temporarily wash particulate pollution out of Delhi’s air, it does not stop the emissions that keep rebuilding the pollution load between rainy spells.
With the monsoon withdrawing on September 23, the city has now entered the post-monsoon period, when the regular cleansing effect of rainfall fades and pollution can begin to accumulate more easily.
