Delhi’s air turns ‘severe’; data anomalies at NSIT Dwarka stn raise questions

Delhi's air quality hit a season high of 391 AQI, with 24 stations reporting 'severe' levels; inconsistencies raise concerns over data accuracy.

Several stations in the Capital on Sunday recorded their worst readings of the season, even as t0he city’s 24-hour average air quality index (AQI) stood at 391 at 10am, the highest this season so far, according to data from the Central Pollution Control Board (CPCB).

A thick layer of smog engulfs Rashtrapati Bhavan in New Delhi on Sunday. (PTI)

Of the 39 active ambient air quality monitoring stations across Delhi, 24 reported ‘severe’ air quality, with readings above 400. Bawana was the most polluted, registering an AQI of 438, followed by Jahangirpuri at 436 — both industrial hubs. Rohini recorded an AQI of 435, while Wazirpur (430), Burari Crossing (428), Mundka (428), Nehru Nagar (425), Patparganj (424), and Vivek Vihar (423) also recorded AQI in the ‘severe’ category.

However, the NSIT Dwarka monitoring station remained an outlier, recording readings in the “moderate” category throughout the day with an AQI of 198, well below the city average.The station’s data showed inconsistent fluctuations in PM 2.5 values , ranging from as low as 70–80 to as high as 350 at different times of the day, raising suspicions of possible data anomalies. To be sure, these are not PM 2.5 values but the PM 2.5 sub-index values, used to calculate the AQI. Delhi’s AQI is based on sub-indices from 39 stations using 24-hour averages for six pollutants and 8-hour averages for two. Data from 16 hours and at least three pollutants, including PM2.5 or PM10, suffice.

An HT analysis of CPCB data showed that if the NSIT Dwarka station’s readings were excluded, Delhi’s average AQI would have been 396 — much closer to the “severe” threshold of 401. Another station, IHBAS Dilshad Garden, registered an AQI of 263, falling in the “poor” range.

Experts said that while stations located in greener areas can record lower readings, the extent of deviation seen at NSIT Dwarka requires examination. “Trees to an extent block dust and prevent high readings. That is why the CPCB’s guidelines prohibit trees and canopy from being within a 20-metre radius of stations. At these two locations, the stations are inside green institutes, surrounded by trees. Even then, extremely low readings indicate there could be external factors at play, which need to be assessed, for instance sprinkling,” said Sunil Dahiya, founder and lead analyst at the think-tank Envirocatalysts.

24 of Delhi’s stations are managed by the Delhi Pollution Control Committee (DPCC), while the remaining come under CPCB, India Meteorological Department (IMD) and the Indian Institute of Tropical Meteorology (IITM). The NSIT Dwarka station is under the CPCB.

Despite multiple attempts from HT, CPCB did not respond to queries about data inconsistencies.

A day earlier, the DPCC held a press conference denying allegations of data manipulation or incorrect reporting at its monitoring stations.

HT’s own analysis of air quality data from November 5 across all 39 stations had previously revealed missing data sets, suspicious measurement patterns and algorithmic loopholes in the way Delhi’s average AQI is calculated. It flagged that these inconsistencies could result in reported AQI values that do not accurately reflect ground-level conditions — meaning the actual air quality could be significantly worse.