Temperature reduction to worsen AQI in days ahead

Mumbai's air quality is declining ahead of winter, with AQI levels worsening to 124. Experts warn of further deterioration as temperatures drop and pollution increases.

Mumbai: Winter, the season for poor air quality, has not yet begun, but the signs of deteriorating air quality are already in Mumbai’s air. After two whole weeks of Air Quality Index (AQI) in the satisfactory and good levels, air quality dropped to the moderate levels on Thursday to 102, only to worsen further on Friday to 124. With the beckoning of Diwali, experts foreshadow that worse is coming.

Temperature reduction to worsen AQI in days ahead

On Friday, at least 12 stations in Mumbai had their AQI in the moderate levels, including Byculla, Chembur, Kurla, Navy Nagar in Colaba, Worli, and Vasai West, spelling particularly bad news for those with asthma and respiratory illnesses. A reduction of temperatures is more commonly associated with poor AQI, and experts unanimously expect this all over the country, but high temperatures haven’t stopped Mumbai’s AQI from taking a hit. From Wednesday, Mumbai has recorded maximum temperatures - over 35 degrees Celsius - that are the highest in the state. “Air quality will get worse for the next few days now, especially if temperatures reduce,” explained Gufran Beig, founder of SAFAR. “Then it will improve slightly for four to five days, then degrade again. It’s cyclical.” Such a pattern is typical to coastal areas, where the change in wind direction and its source affects the air quality. When wind blows from the sea, air quality is better. But when it blows from the lands, it brings with it accumulated pollution from the land it has traversed, hence air quality suffers, he said.

The high temperatures, however, will not last much longer, said Sushma Nair, scientist at IMD Mumbai. “Temperatures will gradually drop from Saturday or Sunday,” she said. “Easterly winds are dominating over Mumbai, delaying the setting of sea breeze, the usual mechanism which reduces temperatures in coastal cities like Mumbai.”

“Even a one degree drop in temperatures can worsen AQI in the city,” says Sunil Dahiya, Founder and Lead Analyst, Envirocatalysts. “All over India, it is accepted that temperatures start dropping by the end of October, which leads to a degradation in air quality.” The ensuing reduction in temperatures ahead will only make AQI worse. Sunil Tripathy, dean of IIT Kanpur’s Kotak School of Sustainability, explains this is due to a process called inversion. Air is warmer the higher you go, and cool air is trapped below. This traps pollutants in the layer of cool air near the ground, mixing with the humidity, preventing its dispersal, and causing it to stay suspended for longer.

Climate change is also affecting weather patterns that dictate wind patterns and amount of rainfall. Last time, due to La Nina, there was a quicker monsoon withdrawal, then AQI dropped badly. Dahiya said, “Even if temperatures don’t decrease significantly, AQI in November will be worse, but not as bad as last year.” He emphasised that the effect of temperature dropping could be negated in only one way: reducing emissions at their source, including traffic pollution, dust, pollution from factories and bakeries, etc. If the rules instituted by the BMC to reduce air pollution last year are obeyed, Mumbai has a chance to keep air pollution in check. He brought up the need to fine tune India’s AQI standards to match global ones, as they are currently too relaxed for a coastal city.