Air Pollution in Mumbai: Its AQI Worse than Delhi’s

GS Paper III Environment and Ecology

Context:

According to data from the Central Pollution Control Board, the Air Quality Index (AQI) in Mumbai was recorded as ‘poor’ at various stations with an AQI above 200, way worse than Delhi’s.

Mumbai is a coastal city that enjoys a natural cleansing advantage where winds move from land to sea for a few days, then from sea to land. This cyclic pattern usually repeats every three to four days.
66 of the 92 days between November 2022 and January 2023 saw Mumbai air quality in poor or very poor cateogry—compared to an average of 28 during those months over the previous three years.

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In comparison, during the same 92 days, Delhi had 79 days of Poor and Very Poor AQI and as many as eight days of Severe. (According to Data from the System of Air Quality and Weather Forecast and Research (SAFAR) — under the Ministry of Earth Sciences).

What is the National Air Quality Index?

It transforms complex air quality data of various pollutants into a single number (index value), nomenclature, and color.
There are six AQI categories (Refer Image): Each category is decided based on ambient concentration values of air pollutants and their likely health impacts (known as health breakpoints).
AQ sub-index and health breakpoints are evolved for eight pollutants namely PM10, PM2.5, NO2, SO2, CO, O3, NH3, and Pb

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Climate change, the dip in La Nina, the ocean surface cooling, and changing wind patterns have all played a role in air pollution in Mumbai.
According to Experts, change in surface wind patterns is mainly due to a sudden drop in ocean surface temperature below normal.
Role of La Nina: During La Nina, colder surfaces allow oceans to absorb more atmospheric heat. Consequently, air temperature tends to go down producing a cooling effect.
- As a result, there is no strong wind reversal from across the sea, which would otherwise occur every 2-3 days and disperse pollutants in the air. Instead, wind reversal happens every 10-15 days, keeping pollutants hanging longer in the lower atmosphere.
- Delayed wind reversal: Due to delay the suspended particulate matter lingers in the lower atmosphere for a longer period of time thus worsening AQI.
- Role of developmental projects: Also, over the past few years several infrastructure and redevelopment projects have come up, this has increased the proportion of particulate matter in the air, which in turn has worsened the AQI.