Why Air Pollution in Delhi Increases During Winters?

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Air pollution is a health hazard in most of the large cities in Indian sub-continent. However, the air pollution in Delhi during winter season is most notorious amongst them all. However, the air quality degrades in most of the areas of north India during the winter season. Why does it happen? To understand that, we have to know the concept of air quality and understand the vertical air movement in the atmosphere.

What is good air quality?

World Health Organization has set the standards for air quality for a good health and living. The WHO suggests measuring six air pollutants for estimating air quality i.e. Particulate Matter 2.5 (PM 2.5), Particulate Matter 10 (PM 10), Carbon Monoxide (CO), Nitrogen Dioxides (NO2), Sulphur Dioxides (SO2) and Ozone (O3). However, the Pollution Control Board of India measures the quantity of eight pollutants i.e. PM 10, PM 2.5, SO2, NO2, CO, O3, Ammonia (NH3) and Lead (Pb) for arriving at National Air Quality Index (AQI). The table 1 shows the different standards for these eight pollutants. When the quantity of pollution remains below the permissible level as shown in Table 1, the air quality is considered good.

Table 1: Air Quality Standards by National Air Quality Index of India

As the amount of different pollutants rises, the air quality is categorized into many categories such as good, satisfactory, moderate, poor, very poor and severe etc. Since, the different categories for different pollutants become very confusing for common people, CPCB has constructed a composite index from all these pollutants as visible in Table 2.

Table 2: National Air Quality Index Categories in India

Reason For Bad Air Quality in Winters?

For understanding the reason for air quality degradation in winters, one must understand different types of lapse rates of temperature such as normal lapse rate and dry adiabatic lapse rate. After understanding lapse rates, you will know that during the normal climatic conditions, the air temperature at earth surface is highest. It starts declining as you go higher. Hence, the warm air from the earth surface is lighter than the air at the higher altitude. Therefore, the lighter air rises upwards and the denser air subsides downwards. During winter season, inversion of temperature causes disturbance in abovementioned vertical temperature distribution.

Inversion of Temperature During Winters

When the atmospheric temperature starts to rise with increasing altitude, we call such weather situation as inversion of temperature. It is complete opposite of the normal lapse rate as mentioned in previous section.

Reason for Inversion of Temperature

  • During winter season, the nights are long and cold along with clear skies.
  • Consequently, earth surface loses heat through outgoing long-wave earth radiation.
  • In the absence of clouds, the earth radiation easily leaves the atmosphere.
  • Ultimately, the earth surface becomes very cold.
  • Subsequently, the air near earth surface loses heat to cold earth and becomes very cold.
  • Since the cold air is heavy, it stays near the earth surface whereas the air in the higher altitude remains warm and light.

Effect of Temperature Inversion on Pollution

  • The atmospheric conditions, as mentioned in previous section, leads to very stable atmosphere.
  • The stable atmosphere denotes that the atmospheric pressure is high, sky is clear and there is negligible movement of air either horizontal or vertical.
  • In such conditions, there is no mixing of air.
  • Hence, any pollutant which is released in the atmosphere near earth surface, remains near the earth surface.
  • Further, the absence of vertical or horizontal movement of air  causes high concentration of pollutants near earth surface.
  • This is why, the air quality is poor during cooler time of the day i.e. evening to the morning.

Conclusion

From the above discussion, we can conclude that it is not that the pollution only occurs in winters. It is just that the pollutants disperse during summer season due to convective currents of air, whereas, pollutants do not disperse during winters season. Additionally, the burning of paddy straw near NCR Delhi region exacerbates the air quality.