Thursday offered a rare sight for a city that spends much of the year battling hazardous air pollution, and a rarer statistic. Delhi’s air quality slipped into the “good” category for the first time in nearly three years, giving residents a fleeting sense of what it actually feels like to breathe clean air.The continuous monsoon rains washed the pollutants away from the atmosphere and the Air Quality Index (AQI) in the national capital came down to 48 from 59 a day earlier. An AQI between 0 and 50 is classified as “good” by the Central Pollution Control Board (CPCB), the cleanest category on India’s air quality scale.
It may be a small numeric change but crossing the barrier from 51 to 48 is symbolically significant for Delhi. This is the city’s first “good” air day since September 10, 2023, when heavy rains and G20-related restrictions had also helped lower pollution levels.More importantly, it is a reminder of two contrasting realities: Delhi’s air can become clean remarkably quickly under the right weather conditions, but keeping that improvement remains one of the city’s biggest environmental challenges.
Why a couple of days of rain was significantMonsoon rain is nature’s giant air purifier. Scientists call this process “wet deposition,” or the washout effect. While moving through the atmosphere, raindrops collect suspended particles like PM2. 5, PM10, dust, soot and other pollutants and bring them to the ground.The immediate effect is that the airborne particulate matter drops. Heavy rains also cut down on dust from roads and construction sites, two of Delhi’s biggest sources of pollution during dry months. Then, also, strong monsoon winds help, dispersing pollutants rather than allowing them to build up over the city.
The recent improvement again shows how powerful the cleansing effect of the monsoon can be, says Anumita Roychowdhury, executive director, research and advocacy, Centre for Science and Environment. But she warns that the weather alone will not solve Delhi’s pollution crisis. Continuous improvements require year-round action to curb emissions from transport, industries, construction, waste burning and other major pollution sources.A city that seldom breathes easySeen in historical perspective, the latest milestone is even more remarkable. In the 11 years since the CPCB launched the AQI system in 2015, Delhi has seen only 15 “good” air days. That averages out to just one or two days a year.There were several years, including 2015, 2016 and 2018, with no “good” air quality days. The pattern is disturbingly the same. Almost all of these rare clean-air days occurred after heavy monsoon rains or during times of greatly reduced human activity.
In the monsoon of 2017, Delhi saw two “good” air days in late July. Two more followed in 2019, after heavy rains in August. The city’s cleanest spell was in 2020 when it had five “good” days of air, one during the nationwide lockdown due to Covid-19 and four during the monsoon season. On October 18, 2021, continuous rain brought down the AQI to 46.Three such days were recorded during September and October in 2022. Only one day in 2023 met the “good” standard, helped by rain and temporary restrictions tied to the G20 Summit. The pattern suggests a sobering truth: Delhi rarely gets to clean air via emission reductions alone.
Weather and emissionsThe problem with pollution in Delhi is that the weather plays a dominant role and that is not understood often. Low wind speeds and temperature inversion in winter result in cold temperatures and the accumulation of pollutants near the ground. Cars, industry and biomass burning also regularly emit emissions that can accumulate quickly and form harmful smog.During monsoon the opposite happensRain washes particles out of the atmosphere, humidity suppresses dust and stronger winds disperse pollutants more widely. In other words, week-to-week emissions in Delhi may not change much but the ambient capacity to dilute or remove those emissions varies enormously.
This is why pollution levels can fluctuate wildly in just 24 to 48 hours. The latest improvement is therefore a combination of cleaner atmospheric conditions rather than a permanent reduction in sources of pollution.Delhi was not equally clean throughoutEven on one of the best air quality days the city has seen in years, local differences were still apparent. Of the 38 functional Continuous Ambient Air Quality Monitoring Stations in Delhi, air at Dr Karni Singh Shooting Range was the cleanest with an AQI of just 23, well within the “good” zone.At the other end of the spectrum, Jahangirpuri’s AQI was 102 which falls in the “moderate” category. These variations are caused by differences in traffic density, industrial activity, construction activity, local meteorology and urban design. So the air pollution is not homogeneous over Delhi. Residents in different neighbourhoods will often notice significantly different air quality on the same day.
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Will Delhi maintain this clean air?According to experts, the answer is less about the weather and more about policy. But monsoon months are the cleanest naturally as rains wash away dust and pollutants from the atmosphere, says Dipankar Saha, former head of CPCB air laboratory. But once the rains pass, pollution starts rising again.Delhi’s wintertime pollution episodes have gone world famous, caused by a combination of emissions from transport, construction, industries, diesel generators, waste burning and regional crop-residue burning, which combined with adverse meteorological conditions.That is why environmental experts say rainfall should be considered a temporary relief, not a long term solution. The challenge for the city is to reduce emissions enough so that cleaner air remains when the atmosphere is less forgiving.A reminder of what may beFor millions of Delhi residents, Thursday’s air was something increasingly scarce: a day when stepping outside didn’t come with worries of pollution. The milestone has a message of significance too.When the pollutants are removed and the weather conditions cooperate, Delhi’s atmosphere is capable of becoming remarkably clean within a short period. The real challenge is to make sure that clean air is no longer dependent on a downpour or extraordinary restrictions.Still, if there’s no steady decline in emissions year-round, then such days will probably be rare exceptions, not the rule, a brief 48-hour miracle that reminds the city of what it’s been missing.
