Pollution bounds back -- within 24 hours of rainfall stopping, PM2.5 levels increase by 10.7 per cent. After 48 hours, levels surged by 37.9 per cent
Study says relying on temporary, disruptive winter emergency measures might prove to be ineffective. What is needed is permanent, synchronised, year-round emission control measures across the entire NCR airshed
Access the CSE study paper click here:
NEW DELHI, SEPTEMBER 30, 2026: A new assessment released here today by Centre for Science and Environment (CSE) shows 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.
The monsoon period represents the absolute lower bound of Delhi’s annual pollution. With the official withdrawal of the southwest monsoon on September 23, 2026, and the imminent arrival of adverse post-monsoon winter meteorology, ongoing urban emissions will inevitably trigger seasonal pollution spikes.
“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,” says Anumita Roychowdhury, Executive Director, Research and Advocacy, CSE.
She adds that “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”.
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.”
This assessment of summer (April-May 2026) and monsoon months (June-August 2026) shows that round-the-year emission reductions must be dramatically strengthened. While monsoon washout is a temporary nature-induced relief, relying on reactive, highly disruptive and temporary emergency measures during winter can impact economic activities, daily livelihoods, informal sector wages and overall industrial productivity in the region. Systemic, cross-sectoral and airshed-wide solutions across the National Capital Region (NCR) must gain immediate momentum, say CSE researchers.
The methodology
This assessment has 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 represents the combined arithmetic mean of official measurements recorded at the IMD’s Safdarjung and Palam monitoring stations. An isolated event comprises 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.
Out of 12 isolated rainfall events identified across the study period, six occurred during the monsoon. The study identifies 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.
The key findings
Only one good air quality day despite heavy rains: During the monsoon season, rainfall leads to atmospheric cleansing through wet deposition, where raindrops scavenge suspended particulate matter. Spikes in daily mean rainfall correspond directly to downward dips in daily PM2.5 concentrations. The CSE assessment notes 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.
Rebound dynamics of particulate matter after rainfall: As soon as precipitation stops, the PM2.5 trajectory curves 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.
Persistent rainy spells and air quality: To evaluate the persistence of atmospheric cleaning induced by wet scavenging, the assessment tracked isolated rainfall events during monsoon. An isolated rainfall event is defined as one or more consecutive rainy days preceded by at least one dry day and followed by at least three consecutive dry days (allowing uninhibited tracking of pollution accumulation). 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.
Trends in daily Air Quality Index: Comparing the distribution of daily AQI categories between summer and monsoon reveals a shift away from high-pollution days toward intermediate categories, though true ‘Good’ air quality remained almost entirely elusive. ‘Moderate’ air quality (AQI range 101-200) remained dominant, representing nearly six out of every 10 monsoon days. A ‘Moderate’ AQI designation corresponds to 24-hour average PM2.5 concentrations between 61 μg/m3 and 90 μg/m3 -- substantially exceeding the prescribed 24-hour national safe limit of 60 μg/m3. Consequently, even when the monsoon suppresses severe pollution events, the baseline atmosphere remains chronically polluted.
Multi-pollutant dynamics – gaseous pollutants lead: While wet precipitation efficiently scavenges suspended physical particulates (PM10 and PM2.5), gaseous pollutants behave differently. As particulate levels drop, gases step in as the lead driver of the official daily AQI.
Explains Kaur: “Thus, combustion-derived gaseous emissions (NOx, CO and volatile organic compounds [VOCs] from vehicular transport, diesel generator sets and industrial facilities) do not disappear during rain. Sunlight breaking through the monsoon cloud cover triggers photochemical reactions between accumulated NOx and VOCs, forming toxic ground-level ozone. This multi-pollutant complexity proves that addressing particulates alone is insufficient for regional air quality management.”
The next steps
According to the India Meteorological Department (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.
Roychowdhury says: “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.”
What needs to be done?
For more information, interviews etc, please contact Sukanya Nair of The CSE Media Resource Centre – sukanya.nair@cseindia.org, 8816818864.
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