Tuesday, November 22: Exactly one month ago, Pune enjoyed the least polluted Diwali since 2019, with its air quality index (AQI) on the festive day staying restricted to the ‘satisfactory’ category. But the change in months and seasons has also brought about a change in Pune’s pollution levels, and the weather appears to be the culprit.
According to the System of Air Quality and Weather Forecasting And Research (SAFAR), Pune’s overall AQI as of Tuesday afternoon has spiked to a ‘poor’ 221. For context, an AQI between 0 and 50 is considered “good”; 51 to 100 is “satisfactory”, 101 to 200 “moderate”, 201 to 300 “poor”, 301 to 400 “very poor” and 401 to 500 “severe”.
This drop in air quality seems to be driven primarily by the drop in local temperatures.
During the early hours of Tuesday morning, Pune’s Shivajinagar station recorded a minimum temperature of 11.2°C. While low, it was still an improvement from Monday night, when Pune shivered at 8.8°C — the second coldest this Maharashtrian city has been in November since the year 2011!
The daytime trends haven’t been any different. On Sunday (Nov 20), Pune at 9.7°C was cooler than popular Maharashtrian hill stations Mahabaleshwar, Lonavala and Matheran!
Such consistently low temperatures are often positively linked with high pollution levels — not just in India, but all around the world — due to what we call winter inversion.
What is winter inversion?
Essentially, winter inversion is a phenomenon that causes pollutants to remain close to the ground, thereby preventing them from dispersing.
In summer, the air in the planetary boundary layer (the lowest part of the atmosphere) is warmer and lighter, and rises upwards more easily. This carries pollutants away from the ground and mixes them with cleaner air in the upper layers of the atmosphere in a process called ‘vertical mixing’.
But in winters, the planetary boundary layer is thinner, as the cooler air near the Earth’s surface is dense. The cooler air is trapped under the warm air above, effectively forming a kind of atmospheric ‘lid’. Since the vertical mixing of air happens only within this layer, the pollutants released lack enough space to disperse in the atmosphere.
While this phenomenon is prevalent around the globe, it impacts landlocked cities like Pune more than their coastal counterparts like Mumbai. This is because the sea breeze and moisture along the coast help disperse the pollution.
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