The figures are telling. Mumbai’s air quality has seen a noticeable decline since the withdrawal of the monsoon on October 10. Until October 9, the city’s Air Quality Index (AQI) was in the double digits, indicating relatively clean air. But from October 10 onwards, AQI readings surged into triple digits, marking a consistent deterioration in air quality.
On Wednesday afternoon, the Central Pollution Control Board (CPCB) dashboard showed Mumbai’s overall AQI at 139, which is regarded as ‘moderate’. While the city’s overall AQI reading has been hovering near the same category, several pockets have already started to witness worse air.
For example, the Bandra Kurla Complex (BKC) and Deonar have recorded AQI readings of 221 and 235, which are regarded as ‘poor’. The data further shows that between October 10 and 15, while BKC recorded poor AQI for two days, Deonar did the same for three consecutive days.
AQI readings between 0-50 are regarded as ‘good’, 51-100 as ‘satisfactory’, 101-200 as ‘moderate’, 201-300 as ‘poor’, 301-400 as ‘very poor’ and above 400 as ‘severe’.
Why is the AQI deteriorating rapidly?
Meteorologists and atmospheric scientists have attributed Mumbai’s current weather phenomenon to the effect of the La-Niña climatic pattern.
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.
“The major component affecting the overall AQI situation of Mumbai is that, 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 downside. This phenomenon was earlier seen during the 2021-2022 period, which had resulted in a
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serious dip in Mumbai’s air quality,” Gufran Beig, chair professor of National Institute of Atmospheric Sciences (NIAS), told The Indian Express.
Experts stated that when the wind starts to cool down, it becomes heavy and due to the combined effect of slow wind speed, the pollutants remain suspended in the lower atmosphere for a prolonged period of time, creating smog and mist-like conditions, which mainly comprise suspended particulate matter.
“At present, the onset of La-Niña is still very slow. This may intensify starting November, which will create a phenomenon of cooler temperatures mixed with slower wind speed. Therefore, it is very likely that Mumbai’s overall AQI may worsen further as winter progresses,” Beig explained.
AQI and the winter connection
The India Meteorological Department (IMD) had earlier predicted that India may witness a prolonged winter this year. While Mumbai is not known for experiencing harsher cold temperatures during the winter months, weather experts have stated that this year the city may experience comparatively cooler days between November and January.
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Speaking to Express, Mahesh Palawat, vice-president of meteorology, Skymet Weather, said that the western disturbances have already led to good snowfall in the northern region of India.
“All the global models are indicating that winter will be harsher this time. While Mumbai is a coastal city, we are not expecting the temperatures to go very low, but to stay below the normal levels consistently for a prolonged period of time,” Palawat said.
“Once the temperature starts dropping, the wind pattern slows down, so the prolonged winter condition will definitely affect the overall AQI of Mumbai. The city’s air quality has already started to dip after the monsoon withdrawal took place, mainly because when the rains are there, a natural cleansing of the atmosphere takes place and the atmosphere stays free of pollution,” he added.
What the administration can do
In its Mumbai Air Pollution Mitigation Plan (MAPMP), the Brihanmumbai Municipal Corporation (BMC) had identified vehicular emissions, dust displacement from roads and construction sites, and garbage burning to be some of the top contributors to Mumbai’s worsening AQI.
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According to Beig, considering there have been early warning signs about deteriorating air quality, the administration should take proactive measures to ensure that the situation does not worsen further.
“Two of the major man-made contributing factors are emissions and waste burning. These two things need to be taken care of. The authorities should take adequate measures to ensure there is a smooth traffic flow and minimum congestion. At the same time, activities like open burning of garbage in public spaces and landfills need to be strictly checked,” Beig said. He also asserted that local-level AQI monitoring needs to be done to make sure the overall pattern of AQI can be studied.
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