pollution hangs over Goregaon on the morning of November 9, a day after Diwali. Mumbai’s air stayed in the ‘moderate’ category at 172, helped by winds that Chennai did not get. (Express Photo by Anurag Ahire)
Two coastal cities, the same festival, two very different readings. A day after Diwali, Mumbai stayed in the ‘moderate’ category with an Air Quality Index (AQI) of 172. Chennai recorded ‘poor’ air through Monday morning, with four of its seven operational monitoring stations above 200.
The difference, scientists say, came down largely to which way the wind was blowing. In both cities, firecrackers released PM2.5, fine particles 2.5 micrometres or less that rise into the air and stay suspended in the lower atmosphere. What happened next depended on the weather.
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What helped Mumbai?
Mumbai is surrounded by the sea on three sides, which gives it the advantage of wind reversal, with breezes moving between land and sea and dispersing pollutants held in the lower atmosphere.
An anticyclonic circulation in the Himalayas has also brought easterly winds, which blow from the mainland towards the sea, to coastal regions of western India, including Mumbai.
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These winds have brought down the city’s temperature and dispersed the pollutants suspended in the air. Their continued onset over the past two days has helped Mumbai and eight other cities in the Mumbai Metropolitan Region stay in the ‘moderate’ category despite heavy cracker bursting.
What went wrong in Chennai?
Mumbai looks out on the Arabian Sea to its west. Chennai, on the south-eastern coast, faces the Bay of Bengal to its east, so the same wind has the opposite effect. Easterlies would bring Chennai cleaner air from the sea, but the city is not getting them.
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