Delhi Air Quality: Why Does Air Pollution Rise In October Every Year?

When the entire nation was locked down, the air in Delhi was clear, clean, and perhaps the freshest. And yet, as the temperature has dipped, the air quality has started to deteriorate and we are once again fearing a cloud of smog all over the city during the winter months.

Now, it should be explained here that air pollution in it is a complex phenomenon that is dependent on a variety of factors. The biggest reason is the impact of pollutants and it is then followed by weather and local conditions.

Reasons behind the rise in air pollution in October each year

This is the month where monsoons withdraw from Northwest India. During monsoons, the prevalent direction of the wind is easterly – and these winds travel over the Bay of Bengal and they carry moisture and bring rains to this part of the country.

So, hence, as soon as the monsoon withdraws, the predominant direction of winds changes to northwesterly. During the summer months, the direction of the wind is northwesterly and there are storms carrying dust from Rajasthan and sometimes from Pakistan and Afghanistan. As per a peer-reviewed study conducted by scientists at the National Physical Laboratory, 72 percent of Delhi’s wind in winters comes from the northwest.

Also, during these months, there is a dip in temperatures and this is the reason behind the increased pollution levels. As temperature dips, the inversion height — this is the layer beyond which pollutants cannot escape into the upper layer of the atmosphere – is lowered. This concentration of pollutants in the air increases whenever this happens.

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Combined with high-speed winds – these are very effective at dispersing pollutants in winters as opposed to summers. Hence, when these factors - farm fires and dust storms combine, they are adding to the high base pollution levels in the city.

We now come to the stubble burning, another major factor that has contributed to deteriorating air quality over Delhi. Last year, when stubble burning incidents were at its peak, it contributed to 40 percent of the factors which led to pollution. As November draws closer, the percentage contribution will also increase.