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A woman wrapped in a shawl walks on the road during a cold winter evening in New Delhi. (Photo: Reuters)
A woman wrapped in a shawl walks on the road during a cold winter evening in New Delhi. (Photo: Reuters)

Mapped: Bad air days across cities

While pollution is a manmade problem, geographical and meteorological constraints can exacerbate its impact

As in the past few years, the great North Indian Smog left its mark on the country this year, with winter air pollution in northern India worse than most other parts of the world. Air pollution also could become an electoral issue for the first time in the national capital, Delhi, as it prepares for the upcoming state assembly elections.

The pollution in Delhi and across the Gangetic Plains is usually blamed on smoke from cars, factories, and crop burning. These are major factors but are made worse by topography. Unlike other parts of the country, the region, which sits at the foothills of the Himalayas, traps cold air during winters. This means lower wind speeds and lower temperatures but greater pollution. In his recent book, The Great Smog of India, researcher Siddharth Singh, terms this problem as a ‘meteorological misfortune’.

How big a role does this misfortune play in Delhi compared to other cities? To visualize and understand the effects of meteorology on pollution, Mint analysed PM 2.5 levels for a year between 1st December 2018 and 30th November 2019 in India’s six largest metropolitan cities (Delhi, Mumbai, Kolkata, Chennai, Hyderabad and Bengaluru) alongside wind speeds and temperature. Data for each monitoring station for each city was culled from the Central Pollution Control Board website. Delhi has the maximum number of monitoring stations (38) whereas Chennai and Kolkata have the least --- 4 and 5 respectively. PM2.5 refers to atmospheric particulate matter smaller than 2.5 micrometers and are among the most dangerous of pollutants.

Delhi has the maximum number of monitoring stations (38) whereas Chennai and Kolkata have the least.
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Delhi has the maximum number of monitoring stations (38) whereas Chennai and Kolkata have the least.

The analysis suggests that windy days have a visible impact on lowering pollution levels. In general, Indian cities see higher wind speeds during the monsoon season and the lowest wind speeds during winter. The impact of heavy winds during monsoon in alleviating polluted air is evident in the case of Bengaluru, Mumbai and Hyderabad, when wind speeds are three to five times their annual mean and pollution levels are half the average annual levels.

In addition to seasonality, the geography of a place plays a role. Coastal cities such as Chennai and Mumbai enjoy lower pollution levels due to the constant sea breeze. Inland cities such as Delhi are at a geographic disadvantage because the average wind speeds tend to be lower than others throughout the year. Temperatures, on the other hand, do not seem to have any direct impact on pollution levels. Even sharp changes in temperature are not associated with changes in PM2.5 levels.

Meteorological conditions are also not amenable to change.
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Meteorological conditions are also not amenable to change.

While meteorological or geographical conditions play a big part in driving pollution readings, neither can be entirely blamed for poor air quality. Cities such as Los Angeles and Mexico City have similar topography as Delhi but enjoy much better air quality. Meteorological conditions are also not amenable to change. The key to addressing air pollution then is to look at the local causes of pollution in each of these cities. In each metro, a different combination of factors such as transport, industry and dust contribute to pollution. According to UrbanEmissions.info, a research portal that monitors pollution data in Indian cities, transportation is the largest source of PM 2.5 emissions in Delhi and Chennai. In Mumbai and Kolkata, industries contribute the most to the problem. The UrbanEmissions data is for the 2015-17 period.

The data suggests that dust from various natural sources are the largest contributors to pollution in Bengaluru and Hyderabad. Lasting solutions will need to address these causes of pollution but take cognizance of the geographical and meteorological constraints of each city.

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