2 years since Delhi saw ‘good’ air: Alarming data amid pollution woes
The last time the capital saw ‘good’ air was in September 2023 — when a combination of rain and strong winds flushed out pollutants.
As Delhi remained shrouded under the usual November haze, an analysis of Central Pollution Control Board (CPCB) data showed that the city has not recorded a ‘good’ air day, defined as an air quality index (AQI) of 50 or less, in over two years. The last time the capital saw ‘good’ air was in September 2023 — when a combination of rain and strong winds flushed out pollutants.
On Monday, the average AQI at 4pm, when the CPCB releases its national bulletin, was 309 (very poor). This comes a day after Delhi officially recorded its worst air this season, with a reading of 388 at 10am on Sunday. Forecasts say the AQI will dip to ‘severe’ on Tuesday.
The CPCB classifies air as ‘good’ when the AQI is 50 or lower, ‘satisfactory’ when it is between 51 and 100, ‘moderate’ when it is between 101 and 200, ‘poor’ when it is between 201 and 300, ‘very poor’ when it is between 301 and 400, and ‘severe’ when it is over 400.
On September 10, 2023, the AQI was 45, helped by rains and winds over 30km/h. Experts say, with meteorological conditions staying unfavourable, despite an excess of 41% rain during monsoon months, Delhi is likely to end 2025 also with no ‘good’ air days. This is similar to what happened in 2024, when, despite ample rain, the year ended without a single ‘good’ air day. Rain typically has a washout effect, helping settle particulate matter (PM) and dust.
A lack of ‘good’ air days , experts say, possibly indicates high background emissions in Delhi.
To calculate AQI, CPCB looks at sub-indices for eight individual pollutants (PM10, PM2. 5, NO2, SO2, CO, O3, NH3, and Pb), with the worst sub-index or highest value out of the eight considered as the day’s lead pollutant. The 24-hour average AQI is then calculated for the city, based on data from Delhi’s 40 ambient air quality monitoring stations. This means even if PM 2.5 or PM10 see a reduction, gases such as ozone or nitrogen dioxide can become lead pollutants of the day. And, if their concentration is high, the AQI is unlikely to see much of an improvement.
“We have to remember that past good air days have generally been on account of rain. However, we have rain over a limited window in the year, which is the monsoon season, as compared to close to 300 such days in European countries ,” said Dipankar Saha, former head of CPCB’s air laboratory.
Moreover, he said, rain only settles particulate matter, and has no impact on gases, making them the lead pollutants. “This thus shows that you need both rain and winds. It also shows our background emissions, particularly gases, is high.”
For this year, the lowest AQI was recorded on July 15, when it was 51. On that day, ozone in addition to PM10 were recorded as lead pollutants. Last year, the lowest AQI recorded was 53 on August 8, with even rains failing to have a washout effect on particulate matter (PM), which was observed to be the primary pollutant.
Mohammad Rafiuddin, Programme Lead at the Council on Energy, Environment and Water (CEEW), had a similar view. “The total rainfall (this year) has so far exceeded 1,000 mm, which is significantly higher than the normal annual average of around 774mm. Delhi’s AQI has not been entering the good category, likely due to the presence of other pollutants which are dominating the area, such as oOzone, carbon monoxide, and nitrogen dioxide.”
“The presence of these gases, even when it rains, is likely above the prescribed limits due to excess emissions from vehicles, industries and open burning,” Rafiuddin said, calling for a larger focus towards tackling emissions at source, including a shift towards clean mobility.
CPCB launched the AQI in April 2015. Complete annual data from 2016 onwards shows Delhi has recorded only 14 good air days so far. In 2016 and 2018, there were no such days. In 2017 and 2019, Delhi had two each. In 2020, a lockdown-impacted year, restricted activities saw Delhi witness a record five good air days. The count was one in 2021, three in 2022 and one in 2023.
This has typically come in the monsoon months of June until September. A late withdrawal of the monsoon, in certain years, has also given the city ‘good’ air days in early October.
