AQI fluctuations solely depend on wind for now
NEW DELHI: With farm fires in Punjab and Haryana decreasing and wind speed picking up, air quality in Delhi showed a marginal improvement since this weekend, compared to the week after Diwali, data from satellites and air quality stations analysed by HT shows.
The one-hour average concentration of PM2.5 – particles of diameter less than 2.5 micrometres – in Delhi was over 200 micrograms per cubic metre (µg/m3) in 176 of the 216 onehour intervals from November 5 midnight to the end of November 13. Diwali was celebrated on the night of November 4.
This means that in 81% of the one-hour intervals after Diwali to November 13, PM2.5 concentration was above 200µg/m3. From midnight of November 14 to 4 PM on November 16, this share has dropped to 52%. In only 33 of the 64 one-hour intervals in this period PM2.5 has averaged above 200µg/m3.
To be sure, this does not mean that the air quality in Delhi is good, or remotely close to good. The analysis above only means that air pollution is on the higher end of the “very poor” or lower end of “severe” category rather than on its upper end. In 99% of the onehour intervals from November 5 to November 13, the PM2.5 concentration was in very poor or worse (concentration above 120µg/m3). This has marginally decreased to 94% of the onehour intervals in the period analysed after November 13.
A combination of lower farm fires and higher wind speed is responsible for this improvement. From November 5 to November 11, fires observed in Punjab and Haryana averaged 5,269 per day, according to Suomi-NPP satellite’s VIIRS instrument. From November 12 to November 16, they have averaged 3,178. The number of fires observed in the two states have decreased daily after November 13. The number was 4,570 on November 13 and is 2072 on November 16.
However, November 12 was about just as bad a day for air quality in Delhi as November 5 (the day after Diwali) because wind speed was lower than it was in the November 5-November 11 period. With farm fires decreasing in Punjab and Haryana and wind speed increasing back again after November 12, both the addition of pollutants in the air and their accumulation in the air has decreased. This has made a minor improvement in the air quality of the National Capital Territory.
The trends underscore the fact that none of the steps and mechanisms put in place over the last five years have made any difference in what determines how bad the pollution will be -- the only determinants are weather and the farm fires in neighbouring states.
Experts have long called for new, consequential mechanisms that could help reduce these effects.