On Tuesday, the number of active fires in the fields of Punjab and Haryana hit the season’s high, with official records suggesting over 6,600 such incidents in a single day. This is more than twice the number of farm fires on Sunday, when the two states recorded roughly 2,900 farm fires.
Sunday was also when the capital’s air quality touched the season’s high, with the AQI touching 594. Tuesday saw pollution reduce and the AQI settle at 324, or ‘very poor’ category.
According to Central pollution monitoring body SAFAR, even as the number of active fires climbed, the share of external biomass in Delhi’s PM2.5 levels, which was 25% on November 3, came down to 14% on November 4 and 12% on November 5.
According to experts, this seemingly paradoxical situation is the outcome of a “delicate combination” of a number of meteorological factors.
“This is why attempts to establish direct links between levels of pollution in Delhi and number of fires in the fields of Punjab and Haryana, while ignoring things that can be controlled locally, can be a futile exercise,” a senior scientist of the Union Ministry of Earth Sciences said.
Anumita Roychowdhury, executive director of the Centre for Science and Environment, said the contribution of farm fires in polluting Delhi’s air remains variable, as it depends on the speed of long-distance transport winds in the upper layer of the atmosphere.
She explained that had the quality of Delhi’s air been inversely proportional to stubble burning, AQI would have plunged to the season’s worst over the last two days. “But that is not happening. In fact, last year, Delhi had experienced a bad air episode around Christmas when there is no stubble burning happening,” she said.
A Union government scientist explained that the speed of upper winds — at around 80 km/hr — was such that smoke from the fields blew over the capital, and ended up in the skies of the Varanasi region. And the winds will continue for some time, see a temporary drop till November 7, and then again pick up.
“The smog episode was a result of extremely adverse meteorological conditions after Diwali. It trapped pollutants from crackers, vehicular and industrial emissions, and smoke from stubble, resulting in the spiralling of the graph. Things may have improved as pollutants are not getting trapped, but it can happen again. That is why controlling sources of pollution is important,” Roychowdhury said.
Asked about the impact of the odd-even drive, she said: “There has been a drop in the level of NOx, which is a good indicator in terms of assessing vehicular emissions. But it is too early to isolate contribution of only odd-even as other steps such as construction ban, closure of hot mixer plants, ban on DG sets will also have to be factored in.”