Farm fires and weather behind choke; respite likely in 4 days

Subject : Geography

Section: Physical geography

Worsening air quality in Delhi:

The combined effect of stubble burning in neighbouring States, local pollutants, and unfavourable meteorological conditions resulted in Delhi’s air quality slipping into the worst- ‘severe plus’– category for the first time this season.

Details:

The Graded Response Action Plan (GRAP), a set of emergency measures implemented in Delhi-NCR during the winter, has four sets of responses depending on the severity of air pollution — Stage 1, ‘poor’ (AQI 201-300); Stage 2, ‘very poor’ (AQI 301-400); Stage 3, ‘severe’ (AQI 401-450); Stage 4, ‘severe plus’ (AQI over 450).

Why does air pollution increase in winter in Delhi?

Subsidence of air and continental effect: The atmospheric circulation over this region is usually anti-cyclonic in nature. This makes the air move downwards thus preventing the pollutant particles from escaping vertically, and keeping them near the surface. Delhi is also a landlocked territory, thus worsening the air pollution.
Temperature Inversion: As you may have learned in school, cold air sinks and warm air rises. The reason for this is that cold air is denser and has less space between the molecules of gas. When the temperature drops and cold air blankets the ground, any warm air is forced to pass over it. In this way, the cold air can form a kind of cap. Pollutants aren’t as free to escape and disperse in the dense colder air. You can think of this as a sort of pollutant catching blanket that covers the ground when it is winter. Cold air is denser and moves slower than warm air. This density means that cold air traps the pollution but also doesn’t whisk it away. Air pollution in winter remains in place for much longer and therefore is breathed in at a higher rate than during the summer. The U.S. has made efforts to reduce air pollution

Wind pattern: Winds flow from Pakistan and Punjab, crossing Haryana and reaching Delhi. These winds have been pushing pollutants from Punjab and Haryana into Delhi.
Boundary layer of troposphere: The boundary layer (the layer of the troposphere in contact with the surface of the earth) has been lower. The boundary layer generally becomes shallower from November onwards, suppressing the atmospheric ability to push pollutants upwards. As the temperature falls further, the boundary layer falls further.
- The wind has changed direction from southeast to northwest. This is very adverse meteorology. The ventilation, which is the dispersion capacity of the atmosphere, and depends on boundary layer height and vertical wind velocity, is poor.
Spike in agricultural waste burning in the Punjab-Haryana region: This is the harvesting season for paddy crops in the Punjab-Haryana region, which leads to crop waste burning for the sowing of other crops. Polluting particles accumulate over the delhi-NCR region making the air polluted.

How will it be improved?

Better wind speed or rainfall can improve the situation. Western disturbance can improve wind speed and change the anti-cyclonic conditions to cyclonic conditions, with the dispersion of polluting particles away from Delhi.

Source of this article: Indian Express