Mask up, Delhi: AQI slips into ‘poor’ zone for first time since July, crosses 200

Delhi recorded its first poor air quality reading of the season on Wednesday morning (Oct 21), with the Air Quality Index (AQI) at 227 at 7.05 am, according to official data. Analysis of Central Pollution Control Board (CPCB) figures by the HT AQI Dashboard showed that air quality turned poor overnight. The deterioration followed Dussehra celebrations on Tuesday evening (Oct 20), when effigies were burnt and firecrackers were set off across several parts of the city, adding to emissions from Durga Puja festivities.

The CPCB's hourly AQI stood at 227 at 7 am, up 48 points from 179 in Tuesday's 4 pm bulletin. It crossed 200, the threshold for the "poor" category, at 2 am and has not dropped below it in any hour since. Delhi last recorded poor air on July 12 and 13, when the AQI was 261 and 246, respectively.

Under CPCB classification, an AQI of 0-50 is "good", 51-100 "satisfactory", 101-200 "moderate", 201-300 "poor", 301-400 "very poor" and 401-500 "severe". For most of October, the city's air stayed in the moderate category, apart from brief improvements on October 12 and 16 after rainfall. The AQI was 187 on October 18, 139 on October 19 and 179 on October 20.

Of Delhi's 59 monitoring stations, data from 56 was available on Wednesday (Oct 21). Two of them, Jahangirpuri and Pusa, recorded "very poor" air, with readings of 308 and 314 respectively. Sixteen stations, including Anand Vihar, remained in the moderate category.

The dip also coincided with a rise in stubble burning. NASA's satellite counted 182 farm fires in Punjab and Haryana in the three days to October 20, taking the tally since October 1 to 397, compared with 523 over the same period last season. However, the satellite passes over at about 1.30 pm, while most burning now takes place later in the day, so its count falls well short of the actual number. A lower count, therefore, does not necessarily mean less burning.

Falling temperatures and changes in wind speed have also contributed to the deterioration, as they affect the dispersion of pollutants and allow them to accumulate in the atmosphere.