Delhi has recorded a drastic improvement in air quality in the last two days. The PM2.5 concentration was in the moderate and weak categories on 16 and 17 November. This was a welcome relief after 25 days of heavy and very weak PM2.5 concentration in a row. What explains this sudden improvement? Does that mean the rest of the winter season will be pollution-free?
An HT analysis of historical trends suggests that Delhi’s pollution status depends on a mix of meteorological, external (such as farm fires) and local factors. While the importance of farm fires will decrease for the rest of the season – the reed burning process is almost over – the pollution situation will depend on the first and third factors. As temperatures drop further, air quality is expected to deteriorate again after what is at best a temporary break.
1. How bad was the pollution this year?
In terms of the seven-day average 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 usually starts to increase in October , reaches peaks sometime in late October or early November, and then falls before rising again in December. .
To be sure, the average PM2.5 concentrations used in this analysis are based on just 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 – November 17 from 2016 onwards.
2. Is there a direct correlation between farm 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. That number was 46,649 in 2019, and 60,536 in 2018. The 2020 number is the third highest since 2012, the earliest period for which there are comparable data. The number of fires detected in the same period was 97,985 in 2016 and 79,511 in 2014. The fires, however, started earlier this year. The 1,111 fires detected in September are the highest for the month since 2012. The role played by farm fires in air quality in Delhi can, however, be assessed when we have comparable air quality data, which are available from 2016. A seven-day average farm fire since 2016 shows that in 2016 alone farm fires were almost consistently higher than in 2020, except for a short period at the end of October when the year figures 2017 were also higher than those in 2020.
Read also: Air quality drops to poor levels as straw fires rise again
Although there is no long-term data on this, the Air Quality and Weather Forecasting System (SAFAR) publishes a table on the contribution of farm fires to Delhi air pollution during the straw burning period. This table shows that the highest contribution that farm fires have made to Delhi air pollution (42%) this season was on 5 November. HT analysis shows 5,239 fires a day in Punjab and Haryana, the second highest number for a day this season. On the day the highest number of fires was recorded (5,316 on November 7), the contribution of farm fires was 32%.
3. Meteorology and local factors
Delhi air improved to moderate quality after two Diwali days 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 a rainfall of 1.2 millimeters per day after Diwali, something that has never happened since 2016. Although the wind speed was lower than it was in previous years, it was blowing from east to west (Delhi in the Punjab) than in the west to the east. This means that Delhi was spared from the limited impact of even the reduced farm fires in Punjab and Haryana.
And one of the biggest reasons why air quality deteriorated relatively early this year was the lack of rain and the low wind speed in Delhi from September onwards. The 20.4 mm rainfall recorded in Delhi in September is the lowest of all years since 2016.
If the meteorology does not continue to favor Delhi, the improvement of AQI will not stand. Then there are local factors – biomass burning, traffic, industrial pollution and dust from construction activities. The SAFAR (Air Quality and Weather Forecast and Search System) app predicts deteriorating air quality from November 20 to 21 for Delhi from “poor” to the bottom of the “very poor” category.
This is where the challenge of pollution control continues, even after farm fires are over.
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