After a brief relief, the national capital’s air quality plunged to the ‘very poor’ category again because of high humidity due to light rain, officials said.
According to a report by PTI, Delhi had been breathing relatively less polluted air for three days after a bright sun and improved wind speed pulled the city out of the grip of a dense haze that had been lingering over it since October 29. But the increased humidity on Thursday pushed the pollution levels up.
On Sunday, too, a drizzle resulted in an increase in humidity, triggering a massive pollution spike.
Weather experts said high humidity and slower winds hamper dispersion of pollutants and lead to the formation of more harmful secondary particles.
Secondary particles are products of complicated atmospheric reactions between primary particles, such as particulate matter, sulphur dioxide and nitrogen dioxide directly emitted by stubble burning and vehicles, in the presence of other factors such as sunlight and moisture.
Examples of secondary particles include sulphates, nitrates, ozone and organic aerosols.
Delhi’s overall air quality index read 309 at 4pm and 342 at 9.30 pm on Thursday.
The air quality also took a hit in neighbouring Noida (366), Ghaziabad (365), Greater Noida (352), and Faridabad (342).
An AQI between 201 and 300 is ‘poor’, 301-400 ‘very poor’ and 401-500 ‘severe’. An AQI above 500 falls in the severe plus category.
Gufran Beig, the head of the government’s air quality monitoring and forecasting service SAFAR, said scanty rain in cold weather is always harmful as it leads to high humidity which, in turn, creates secondary particulate matter.
However, the weather experts said, Haryana and Punjab received good rainfall which will reduce the impact of stubble burning in Delhi’s pollution.