The national capital recorded only one ‘Good’ air day despite receiving plentiful rainfall this monsoon, an assessment of summer (April-May 2026) and monsoon months (June-August 2026) this year by the Centre for Science and Environment (CSE), New Delhi, has shown.
The analysis, released on September 30, 2026, showed that even during the monsoon season — when natural atmospheric “washout” and wet deposition effects are at their annual peak — Delhi’s ambient air quality fails to achieve ‘Good’ air days. Despite heavy precipitation, baseline pollution remains elevated, and particulate matter rebounds rapidly within 48 hours of rainfall.
“While monsoon rains bring temporary relief, they only mask the underlying crisis. The fact that even heavy rainfall failed to bring consistent ‘Good’ air quality, and that pollution rebounded within 48 hours during dry spells, shows that our baseline emissions remain dangerously high. As we enter winter, we are starting from a baseline where continuous, elevated emissions will inevitably trigger severe seasonal spikes,” Anumita Roychowdhury, Executive Director, Research and Advocacy, CSE, said in a statement by the think tank.
“Delhi cannot rely on nature’s washout in the monsoon or emergency bans in winter to manage its air. What we need is ambitious, year-round, airshed-wide action to permanently cut emissions at their source across all sectors in the NCR,” Roychowdhury added.
CSE researchers considered ‘Summer’ as the days from April 1 to May 31, 2026 (61 days), and ‘Monsoon’ as the days from June 1 to August 31, 2026 (92 days).
‘Daily precipitation’ represented the combined arithmetic mean of official measurements recorded at the India Meteorological Department (IMD)’s Safdarjung and Palam monitoring stations.
An ‘isolated event’ comprised one or more consecutive rainy days bounded by at least one prior dry day and followed by at least three consecutive dry days. “This three-day post-rain tracking window isolates PM2.5 recovery dynamics from secondary rain interventions. For rainfall events spanning multiple consecutive days, the “during rain” value reflects the mean PM2.5 level across the entire rainy period,” the CSE statement noted.
“Out of 12 isolated rainfall events identified across the study period, six occurred during the monsoon. The study identified statistical associations between rainfall and ambient PM2.5 changes; it does not isolate rainfall from other concurrent meteorological variables (such as mixing height, wind speed, wind direction and relative humidity) or localised changes in baseline emission fluxes,” it added.
The researchers found that Delhi recorded only one ‘Good’ AQI day during the entire 92-day monsoon season — on July 9, 2026 — when the combined mean rainfall across the Safdarjung and Palam meteorological stations reached 67.75 mm.
Conversely, the season’s single highest daily rainfall event (101.05 mm on August 8, 2026) yielded only a ‘Satisfactory’ AQI designation. Despite the drop, the monsoon seasonal average of 40.2 μg/m3 barely dropped further to reach the ‘Good’ air quality day that principally, as per the AQI, is half of the National Ambient Air Quality Standard (NAAQS).
CSE researchers say this proves that even extreme, localised precipitation events are not cleaning adequately due to underlying baseline emissions.
Moreover, as soon as precipitation stopped, the PM2.5 trajectory curved upward. Even during the peak of the monsoon, daily PM2.5 spiked to a high of 64.5 μg/m3, while the daily AQI peaked at 261 (classified in the ‘Poor’ category). Between summer (April-May 2026) and monsoon (June-August 2026) periods, Delhi’s average PM2.5 concentration dropped from 60.2 μg/m3 to 40.2 μg/m3, representing a seasonal decline of approximately 33.2 per cent.
To evaluate the persistence of atmospheric cleaning induced by wet scavenging, the assessment tracked isolated rainfall events during the monsoon. Across the six isolated monsoon rainfall events identified, PM2.5 dynamics exhibited a distinct “washout and quick rebound” pattern.
Prior to the onset of rain, the average baseline PM2.5 level stood at 40.6 μg/m3. During active precipitation, wet scavenging drove PM2.5 concentrations down to 32.7 μg/m3, marking an immediate reduction of approximately 19.5 per cent.
However, this relief was short-lived: 24 hours post-rain PM2.5 concentrations rebounded to 36.2 μg/m3 (a 10.7 per cent increase from the washout phase); 48 hours post-rain PM2.5 levels climbed rapidly to 45.1 μg/m3 — a 37.9 per cent surge above the washout concentration: it stood 11.1 per cent higher than the pre-rain baseline (40.6 μg/m3).
“This rapid bounce-back proves that while rainfall temporarily washes out particulate matter present in the lower atmosphere, it leaves background anthropogenic emissions completely unchecked. The moment ambient surfaces dry and atmospheric mixing normalises, ongoing emissions from vehicles, industry, waste burning and power generation rapidly rebuild ambient pollution loads,” according to the CSE statement.
During summer, of 61 analysed days, 38 (62.3 per cent) fell into the ‘Moderate’ air quality category (AQI range 101-200). Seventeen (27.9 per cent) were ‘Poor’, 6 (9.8 per cent) were ‘Satisfactory’, and zero days (0.0 per cent) were ‘Good’.
During the monsoon, of 92 analysed days, ‘Poor’ days plummeted to just three (3.3 per cent). ‘Satisfactory’ days rose to 35 (38.0 per cent), while ‘Moderate’ days remained the single largest share at 53 (57.6 per cent). The season registered only one ‘Good’ day (1.1 per cent). Neither season recorded ‘Very Poor’ or ‘Severe’ days.
According to Sharanjeet Kaur, Deputy Programme Manager, Urban Lab, CSE: “This points to a clear ‘washout and quick rebound’ dynamic where PM2.5 levels surge back during dry spell, keeping the AQI in the ‘Moderate’ band. As wet deposition scavenges particulates, gaseous pollutants — notably ozone, nitrogen dioxide and carbon monoxide — become the dominant lead of daily AQI. With winter meteorology approaching, marked by shallow boundary layers, temperature inversions and stagnant air, these unmitigated baseline emissions can create the starting point for winter’s extreme pollution spikes.”
According to the IMD, the southwest monsoon officially withdrew from Delhi on September 23, 2026 — two days earlier than the normal climatological withdrawal date of September 25 (based on 1971-2019 historical data). As the region transitions into the post-monsoon phase, Delhi loses the recurring atmospheric washing by monsoon rains.
“Clearly, relying on seasonal weather or temporary emergency interventions during winter is no longer a viable strategy. Only sustained, year-round airshed-wide emission reductions can secure clean, safe air for Delhi and the broader National Capital Region,” said Roychowdhury.
What needs to be done?
Accelerate regional EV adoption and establish zero-emission freight corridors. Expand high-capacity public transit and bridge last-mile gaps across the regional airshed.
Industrial sector needs clean fuel transition and electrification of low-medium temperature industrial processes across all formal and informal industrial hubs in the NCR. Stop non-compliant fuel use, informal waste burning, and unaddressed fugitive industrial emissions.
Enforce ward-level, 100 per cent segregated municipal waste collection and processing and remediation of legacy waste to eliminate open fires. Implement digital tracking for 100 per cent construction waste recycling and mandate the uptake of recycled materials in local infrastructure.
Ensure uninterrupted grid power and clean battery storage to eliminate DG set dependency and uptake of clean cooking across Delhi-NCR.
Need airshed governance and move from disruptive winter emergency bans to permanent, synchronised, year-round implementation framework across all sectors in the NCR districts.
