New Delhi: After a brief relief, the national capital's air quality plunged to the "very poor" category again on Thursday due to high humidity because of light rains, officials said.
Weather experts said high humidity and light winds affect dispersion of pollutants and lead to the formation of more potent 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 4.45 pm. It was 217 at 9 pm on Wednesday.
The air quality also took a hit in neighbouring Noida (328), Ghaziabad (325), Greater Noida (318), and Faridabad (312).
An AQI between 0-50 is considered 'good', 51-100 'satisfactory', 101-200 'moderate', 201-300 '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 rains which will reduce the impact of stubble burning in Delhi's pollution.
The wind speed will increase again Friday evening onwards, flushing out pollutants, they said.
Mahesh Pawalat, a senior scientist at Skymet Weather - a private forecaster, said, "We are expecting cold, dry winds to blow from Jammu and Kashmir and Himachal Pradesh due to snowfall. Dry winds will reduce humidity. The air quality will start improving again after 24 hours."
Light rains on Sunday resulted in an increase in humidity, triggering
a massive pollution spike.