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Delhi: Bad days retreat, but the next spell may not be far off


Delhi has seen a drastic improvement in air quality in recent days. The PM2.5 concentration was in the moderate and poor categories on November 16 and 17. This was a welcome relief after 25 days in a row of severe and very poor PM2.5 concentration. What explains this sudden improvement? Does it mean that the rest of the winter season will be pollution free?

An HT analysis of historical trends suggests that Delhi’s pollution status is dependent on a combination of meteorological, external (such as agricultural fires) and local factors. Since the importance of agricultural fires will decrease during the rest of the season (the stubble burning process is almost over), the contamination situation will depend on the first and third factors. As temperatures drop further, air quality is expected to worsen again after what is a temporary respite at best.

1. How bad was the pollution this year?

In terms of the seven-day average of PM2.5 concentration, the first two weeks of November were much worse this year than November 2019 and slightly worse than November 2018. A long-term comparison shows that air pollution generally begins to rise in October, peaks sometime in late October or early November, then declines before rising again in December. .

Delhi: Bad days retreat, but the next spell may not be far off

To be sure, the average PM2.5 concentrations used in this analysis are based on only three of the 38 air quality stations for which the Central Pollution Control Board (CPCB) provides historical data on its website. This is because only these three stations; IHBAS Dilshad Garden, RK Puram, and Shaidpur have consistent data available for the period September to November 17, 2016 onwards.

2. Is there a direct correlation between agricultural fires and pollution in Delhi?

The number of fires detected in Punjab and Haryana this year between September and November 17 was 77,703, according to satellite data. This number was 46,649 in 2019 and 60,536 in 2018. The 2020 number is the third highest since 2012, the first period for which comparable data is available. The number of fires detected in the same period was 97,985 in 2016 and 79,511 in 2014. However, the fires started earlier this year. The 1,111 fires detected in September are the highest for the month since 2012. However, the role agricultural fires play in Delhi’s air quality can be assessed when we have comparable data on air quality, which is available from 2016. Seven-day moving average for agricultural fires since 2016 shows that in 2016 alone agricultural fires were almost consistently higher than in 2020, except for a brief period in late October, when the 2017 figures were also higher than those of 2020.

Also Read: Air Quality Falls To Poor As Stubble Fires Rise

Although there is no long-term data on this, the System for Weather and Air Quality Research and Forecasting (SAFAR) publishes a chart on the contribution of agricultural fires to Delhi’s air pollution during the stubble burning period. This graph shows that the biggest contribution agricultural fires have made to Delhi’s air pollution (42%) this season was on November 5. HT analysis shows 5,239 fires on the day in Punjab and Haryana, the second highest number on a day this season. . On the day with the highest number of fires (5,316 on November 7), the contribution from agricultural fires was 32%.

Delhi: Bad days retreat, but the next spell may not be far off

3. Meteorology and local factors

Delhi’s air improved to a moderate quality after two days of Diwali this year. A comparison from 2016 shows that this is an extremely rare event. What explains this? The answer is rain and wind. Delhi received 1.2 millimeters of rain a day after Diwali, something that has never happened since 2016. Although the wind speed was lower than in previous years, it was blowing from east to west (Delhi to Punjab) rather than west. To the East. This means that Delhi was spared the limited impact of even the reduced agricultural fires in Punjab and Haryana.

And one of the main reasons why air quality worsened relatively earlier this year was the lack of rainfall and low wind speeds in Delhi starting in September. The 20.4mm precipitation recorded in Delhi in September is less than every year since 2016.

Delhi: Bad days retreat, but the next spell may not be far off

Unless the weather continues to favor Delhi, the improvement in AQI will not be sustained. Then there are the local factors: biomass burning, traffic, industrial pollution, and dust from construction activities. The SAFAR (System of Weather and Air Quality Research and Forecasting System) app predicts deterioration of air quality from 20-21 November in Delhi from “poor” to the lower end of the “very poor” category.

This is where the challenge of controlling pollution persists, even after agricultural fires end.

Hindustan Times