After recording satisfactory Air Quality Index (AQI) readings for two weeks, Mumbai’s overall AQI plummeted to the ‘moderate’ category yet again this week. According to the Central Pollution Control Board (CPCB) dashboard, Mumbai’s AQI reading stood at 106 on Saturday afternoon. On Friday, Mumbai recorded an AQI of 157 and prior to that – between October 24 and November 6 – Mumbai’s AQI remained satisfactory.
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Mumbai’s AQI had recorded a dip following the monsoon withdrawal, which took place on October 10. Following this, several pockets like Deonar, Bandra Kurla Complex (BKC) and Colaba started recording poor AQI readings. A day after Diwali, on October 21, Mumbai’s overall AQI had touched the poor category. But soon after this, the unseasonal rain followed, and the AQI improved.
“At present, we are witnessing an early onset of La-Niña conditions. For a coastal city like Mumbai, La Niña usually slows down the overall wind speed, which forces the AQI to go down. This phenomenon was earlier seen during the 2021-2022 period, which had resulted in a serious dip in Mumbai’s air quality,” Gufran Beig, Chair Professor, National Institute of Advanced Sciences (NIAS), told The Indian Express.
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La Niña is a climate pattern characterised by the cooling of surface-ocean waters in the equatorial Pacific Ocean. During La Niña, atmospheric circulation changes, resulting in reduced wind speeds that prevent pollutants from dispersing quickly, causing them to linger longer in the air, particularly in coastal cities like Mumbai.
Experts stated that when the wind starts to cool, it becomes heavy, and the pollutants remain suspended in the lower atmosphere for a prolonged period of time, creating a smog and mist-like condition, which mainly comprises suspended particulate matter.
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“At present, the onset of La Niña is still very slow. This may intensify further in November, which will create a phenomenon of cooler temperatures mixed with slower wind speeds. Therefore, it is very likely that Mumbai’s overall AQI may worsen further as the winter season progresses,” Beig said.
On Saturday, the CPCB dashboard showed that out of the 26 AQI monitoring stations in Mumbai, 15 have recorded AQI figures in three digits, with BKC at 192, Deonar at 172, Sewri at 157, Ghatkopar at 155, Colaba at 148, Malad West at 129, and Shivaji Nagar at 122. Borivali showed a satisfactory AQI of 55, followed by Sion at 64 and Vile Parle at 73.
AQI readings between 0-50 are regarded as good, 51-100 are regarded as satisfactory, 101-200 are regarded as moderate, 201-300 are regarded as poor, 301-400 are regarded as very poor and above 400 are considered to be severe.
“Once the temperature starts dropping, the wind pattern slows down, due to which the suspended particulate matter stays trapped in the atmosphere for a prolonged period of time. The city’s air quality has already started to dip, after the monsoon withdrawal took place… because when the rains are there, a natural cleansing of the atmosphere takes place and the atmosphere stays free of pollution. That is why the AQI improved after the unseasonal rain, and now, since the rain is not there anymore, the overall AQI is plummeting,” Mahesh Palawat, Vice President, Meteorology, Skymet Weather, said.
