Delhi witnessed a sharp rise in pollution on Diwali night as firecracker smoke added to the city's existing pollution levels. PM2.5 concentrations increased significantly across several areas, with some monitoring stations recording readings above 1,000 micrograms per cubic metre. However, several stations stopped displaying real-time data after reaching their measurement limit, leaving gaps in the pollution readings.
This raised questions about why the real-time data readings were unavailable when pollution levels were at their highest.
Now, a recent analysis by the Centre for Research on Energy and Clean Air (CREA) has found that Delhi's average pollution levels could have been up to 49 per cent higher on Diwali day due to missing PM2.5 data from several monitoring stations.
If the missing readings had been recorded at 1,000 micrograms per cubic metre, Delhi's average PM2.5 level would have increased from 269 to 400 micrograms per cubic metre.
The analysis comes a day after the Central Pollution Control Board (CPCB) said that its monitoring systems were designed to measure particulate matter only up to 1,000 micrograms per cubic metre. As a result, they could not display readings beyond this limit for several hours on Diwali night, The Times of India reported.
System set to measure PM up to 1,000 micrograms: CPCB
The national daily standard for PM2.5 is 60 micrograms per cubic metre, while the World Health Organisation (WHO) safe limit is 15 micrograms per cubic metre.
The CPCB said its monitoring systems could not display readings beyond 1,000 micrograms per cubic metre. As a result, higher pollution levels were not shown for several hours on Diwali night. The board also said these readings were not included in the average AQI.
Meanwhile, CREA said that if the average PM2.5 level of 400 micrograms per cubic metre had been considered, Delhi's AQI on Monday would have increased from 344, which falls in the 'very poor' category, to nearly the 'severe' category.
According to an NDTV report, several areas in Delhi recorded much higher pollution levels than usual. RK Puram and Nehru Nagar, near Lajpat Nagar, were among the most polluted areas at 9 pm. The available data showed a sharp rise in pollution before the readings stopped appearing.
The RK Puram monitoring station stopped showing data after 9 pm on Diwali night, when the PM2.5 concentration reached 924 micrograms per cubic metre. This was almost double the reading of 504 micrograms per cubic metre recorded an hour earlier, the report said.
The Delhi Pollution Control Committee (DPCC) website also stopped showing real-time data from several monitoring stations, including RK Puram, Nehru Nagar, Mundka, Sonia Vihar and Dwarka, for the next few hours.
Meanwhile, data from the Jawaharlal Nehru University (JNU) monitoring station remained available online. It showed that PM2.5 levels reached 786 micrograms per cubic metre by 11 pm on Diwali night, almost five times the level recorded at 8 pm.
How is AQI calculated?
The Air Quality Index (AQI) is calculated based on eight pollutants, including PM2.5. Pollution peaked on Diwali night, with particulate matter levels crossing 1,000 micrograms per cubic metre at several monitoring stations. Following this, the CPCB stopped displaying data from these stations for several hours.
The DPCC operates 43 of Delhi's 60 air quality monitoring stations. Its data showed that PM2.5 levels crossed 1,000 micrograms per cubic metre on Diwali night. At least 11 locations recorded hourly PM2.5 levels above 2,000 micrograms per cubic metre, while four locations recorded levels above 3,000.
How much data does the CPCB have?
The CPCB collects data from all 60 monitoring stations to calculate the AQI. However, one of these stations has been closed for the past few months.
The latest CREA analysis examined hourly PM2.5 data from 59 stations between November 8, 4 pm, and November 9, 4 pm.
This period matches the 24-hour average used in the CPCB's AQI bulletin for November 9. A total of 1,416 hourly data points (24 hours × 59 stations) were examined.
The analysis found that 255 out of the 1,416 data points, or 18 per cent, were missing.
To understand the possible impact of the missing data, CREA replaced these readings with 1,000 micrograms per cubic metre, the maximum limit set by the CPCB.
This increased Delhi's average PM2.5 level from 269 to 400 micrograms per cubic metre, marking a 49 per cent rise.
What did the CREA report reveal?
According to a TOI report citing CREA, the organisation said, "The peak PM2.5 levels of Delhi was 584 g/m³ at 1am on Nov 9 when there was missing data. However, after replacing the missing data with 1,000 g/m³, the peak PM2.5 concentration was 825 g/m³ at 2am and 824 g/m³ at 3am," said CREA.
However, CREA clarified that this "sensitivity analysis" does not prove that all 255 missing data points were unavailable because PM2.5 levels had crossed 1,000 micrograms per cubic metre.
It explained that using 1,000 micrograms per cubic metre for the missing readings was only an example to show how Delhi's average pollution levels could change.
Meanwhile, DPCC data showed that PM2.5 levels in Mundka reached 3,744 micrograms per cubic metre at 2 am on Monday. However, the CPCB did not share PM2.5 data for Mundka between midnight and 4 am.