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Context: Delhi witnesses spike in air pollution.

                                

General factors that play role in pollution spikes are:

• Extremely low temperatures and low mixing height, with moderate to dense fog in the morning.
• Low temperature and moisture in the air traps pollutants close to the surface.

 

What conditions lead to smog in late October and early November?

• Monsoon withdraws, and wind direction changes to north-westerly that bring emissions from stubble fires in Punjab and Haryana.
• These winds are colder and dryer as compared to the south-easterly winds blowing earlier, leading to a drop in temperature by the end of October. The winds also bring pollutants from stubble burning towards Delhi.
• Firecracker burning on Diwali, celebrated during late October or early November, combined with stubble emissions lead to a smog.

                                                           

Why it spikes in the end of December?

• Stubble burning has completely stopped by this time of the year.
• Pollution spikes largely becauses temperatures dip to 1-3 degrees Celsius.
• Colder air slows down dispersal of pollutants and brings down the mixing height, trapping pollutants close to the ground.
• Delhi also sees fog spells at this time, as the moisture also traps pollutants in the air.

                                              

What is the role of temperature?

• Low temperatures not only make the air heavier and slows down pollutants, but it also brings down the mixing height -- an invisible layer of the atmosphere within which pollutants mix.
• The closer this layer drops to the surface, the lesser the space for pollutants to mix freely.

                         

 

How does fog causes spike in pollution?

• Moisture in the air traps pollutants and makes them heavier, not allowing them to disperse easily.
• This can lead to a constant haze in the air that dissipates only when there is adequate sunlight leading to warmer air.

How far can stubble emissions travel?

• Satellite data shows stubble emissions can travel all the way across the Indo-Gangetic plains. However, they start dispersing after a few hundred kilometres, and generally lose their potency after crossing Delhi-NCR.
• The longer the distance they travel, the lesser their impact.

                                  

 

https://www.hindustantimes.com/cities/delhi-news/explained-delhi-s-air-pollution-spikes-yearround-101637000903417.html