New Delhi, Nov 12 (PTI) The air quality index at Delhi Cantonment remained in the ‘moderate’ category even as the rest of the national capital gasped for breath amid very high pollution levels, leading experts to suspect a data gap at the monitoring station.
A thick, sepia-coloured haze shrouded the city on Thursday, blurring its skyline, with residents complaining of coughs, burning eyes and breathing issues as Delhi’s air quality worsened. The city’s 24-hour average Air Quality Index (AQI) climbed to 350, in the ‘very poor’ category, from 313 recorded on Wednesday.
Of the 57 air quality monitoring stations that recorded data on Thursday, 10 were in the ‘severe’ category, 42 in the ‘very poor’ category, 4 in the ‘poor’ category, and only 1 in the ‘moderate’ category.
The Delhi Cantonment monitoring station, however, recorded the city’s best air quality, with a 24-hour average AQI of 138, in the ‘moderate’ category. The station’s 24-hour average AQI stood at 117 on Wednesday and 135 on Tuesday. Before that, it was 297 on Monday, then improved drastically.
According to the Central Pollution Control Board (CPCB), an AQI between 0 and 50 is classified as ‘good’, 51 and 100 as ‘satisfactory’, 101 and 200 as ‘moderate’, 201 and 300 as ‘poor’, 301 and 400 as ‘very poor’, and 401 and 500 as ‘severe’.
Experts, however, said the comparatively low readings at Delhi Cantonment may not reflect cleaner air and could point to an issue with the station’s PM2.5 monitoring instrument. “Green cover cannot explain what we are seeing at the Delhi Cantonment station. This year its PM2.5 reads at only about a quarter of the city level, the second-lowest of 57 stations in Delhi,” Sunil Dahiya, founder and lead analyst at Envirocatalysts, said.
“The station’s PM10 readings, meanwhile, remain at around 60 to 80 per cent of the city level. In the smog season, fine particles normally make up about half of PM10 across Delhi. At Cantonment, they are coming in at just 17 per cent,” he said.
Dahiya said trees and open spaces can reduce local dust, but they cannot remove three-quarters of the fine particles while leaving coarse dust almost untouched. “Winter PM2.5 in Delhi is regional smoke and secondary pollution, and it spreads across the whole city,” he said.
“This pattern points to a likely fault in the station’s PM2.5 instrument, not cleaner air. IGNOU Maidan Garhi shows the same signature. The data alone cannot tell us why it is happening, and we are not suggesting anything deliberate,” Dahiya said.
He said the Delhi Pollution Control Committee (DPCC) should release the PM2.5 analysis calibration and service records from March. “Until then, a ‘moderate’ AQI from this station should be read with caution,” Dahiya said.
At the same time, biomass burning and secondary organic aerosols emerged as the biggest contributors to Delhi’s pollution, accounting for 25.4 per cent each of the pollution load, according to source-apportionment estimates by the Indian Institute of Tropical Meteorology (IITM).
Vehicular emissions contributed 17 per cent, while secondary inorganic aerosols accounted for 11.8 per cent. Mixed combustion contributed 10.7 per cent, firecrackers 7.1 per cent, waste burning 1.5 per cent, mineral dust and ash 0.9 per cent, and industrial emissions 0.2 per cent. PTI AHD ASD
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