Number theory: Is clean air now a warning of worse air quality days later?

Delhi has experienced some rare days of good air this week. The good air on two days was the result of unprecedented rain – the fourth highest since 1901 for the first 11 days of October – but it also serves as a warning of a coming peak of air pollution, an HT analysis of past rain, air quality, and active fires data shows.

Delhi has experienced some rare days of good air this week. The air quality index (AQI) was 44 on October 10, the best since August 31, 2020, and followed another low AQI of 48 experienced on October 9 (AQI was ‘satisfactory’ at 66 on October 11, but rose to 143 on October 12). The good air on two days was the result of unprecedented rain – the fourth highest since 1901 for the first 11 days of October – but it also serves as a warning of a coming peak of air pollution, an HT analysis of past rain, air quality, and active fires data shows. The following four charts explain this argument in detail and show which districts need the most effort in preventing farm fires.

A layer of smog over India Gate during the early hours of the morning on Kartavya Path in New Delhi, India. (HT Photo)

Good air quality was a result of unprecedented rain

The air in Delhi was not clear when October started. The average AQI on the first five days of October was 171, much worse than the average of 109 in 2021 and in line with the 2018-2020 average of 157. This changed drastically in the next week. The average AQI from October 6 to October 12 was just 70.1, compared to 156 last year and an average of 199 in 2018-2020. Since there was no 2020-like lockdown imposed and most year-round sources of pollution such as vehicles and factories were functioning, there is no doubt that this rapid cleaning was the result of the weather. Rainfall data bears this out. It rained on two days in 2018-2020 in this 12-day period, on three days in 2021, and it has rained on seven days (continuously from October 6 to October 12) in 2022.

Clean air in early October can mean more fires and worse air pollution later

Apart from cleaning the pollutants from year-round sources, the rain has also helped the air in Delhi in the past week in another way. Satellite data shows that this has prevented farmers in Punjab and Haryana from burning stubble in early October (it has been raining there too). Unfortunately, because this decision has been forced by the weather, it is not a sustainable one. Data from last year shows what this can mean later in October or in November. It rained on six days in the first half of October in both Punjab and Haryana last year, compared to four days on average in the 2018-2020 period. This led to a much cleaner air in October in 2021 than in 2018-20. However, this only meant that farm fires were concentrated in a few days in late October and early November (it rained on one and two days in Haryana and Punjab last year compared to around seven days in 2018-20 average). Moreover, their total count was also the highest in the five years up to 2021. This led to a bigger peak of air pollution than seen in the recent past. Given this trend, unless it continues to rain throughout the entire rabi season – which will have an adverse impact on crop output – a repeat of 2021 in terms of farm fires and air quality will not be surprising.

Where do these fires take place in Punjab and Haryana?

As is clear from the data above, Delhi cannot depend on the weather to keep its air clean in coming days and must tackle all sources of pollution to keep it clean year-round. However, knowing where farm fires take place can help in decreasing the peak of the worst AQI Delhi experiences this year. As Punjab has reported about seven times the number of fires Haryana has in the past decade in October and November, it is natural that the districts with the most fires are located in that state. The top five districts in terms of the October-November average of farm fires for 2012-2021 are Firozpur, Sangrur, Bathinda, Moga, and Ludhiana. In Haryana, most fires took place in Fatehabad, Sirsa, and Karnal, ranked 12th, 14th, and 20th among the 41 districts in the two states. With little reshuffling, these trends were also true last year. It might help the region’s air if the governments of these two states can focus their efforts in these districts.

Why we can’t wait for rain, and must take a different route to tackle farm fires

To be sure, both the Union government and the state governments involved do seem to be aware of the problem. For example, the Punjab government has been distributing machines for managing paddy straw for a few years now. Why this does not seem to show an impact on air quality is a question that researchers or government surveys can perhaps answer better. However, one reason that makes itself evident is that the pressure on farmers to clear stubble quickly might have become greater in recent years. Rainfall data from IMD shows that in Punjab and Haryana, either the total amount of rain or the number of days of rain in the first half of October has increased in 52% and 40% of the area respectively in the last decade. This is a situation when farmers are likely to resort to stubble burning more. At the very least, this data and last year’s example shows that future measures for reducing farm fires should also take into account the changing weather patterns post-monsoon, something that will become increasingly frequent because of the climate crisis.