The air quality this October in Delhi is not only better than last year, but also the best in at least the past four years. Covid-19 related restrictions and their impact on mobility would suggest that this improvement in air quality should have happened in 2020, not 2021. After all, transport and industry are important contributors to pollution, and both mobility and economic activity are higher this year.
Why is this comparison limited to after 2018? The Central Control Room for Air Quality Management (CCR) shows that PM2.5 data is available from 40 different stations for Delhi. However, only 35 of them have consistent data from 2018 for October. In 2017, this number falls to 15. This comparison is limited to after 2018 so that we have a sufficient number of stations to better represent Delhi’s air, and its quality also remains comparable across years. This means that it is possible that Delhi’s air quality is better this October than in a longer time period than four years.
…despite higher mobility and economic activity
The reason good air in October is surprising is because movement of people and economic activity is both higher this year than in 2020. An improvement in air quality 2020 – as it did happen during the 68-day lockdown starting March 25 – could be explained because the sources of pollution were shut. Google mobility data shows this is not the cause for improved air in October 2021. Mobility levels in Delhi, as well as in the neighbouring north-western states of Punjab and Haryana, are far higher this October than last year. An analysis published in HT on October 26 showed economic activity is also higher this October than at the same time last year.
Weather, not sources of pollution, is the likely reason for better air this October
As the above data suggests, vehicular and industrial sources of pollution are unlikely to have driven the improvement in air quality. There is a third source of pollution that becomes important at this time of the year – farm fires in Punjab and Haryana. Data shows that farm fires are also somewhat lower this year, especially in the second-half of the month when the number of fires is usually more. On eight days until October 25, farm fires were the lowest since 2017. Five of these eight days have come after October 15.
One reason for a lower number of farm fires is that paddy procurement was postponed from October 1 to October 11. This could possibly have delayed clearing of farms for the next crop, which is why the stubble is burnt. Another likely reason for lower number of fires is the rainfall this October. Until 8:30 AM on October 25, rainfall in Punjab and Haryana has been 290% and 192% more than the average rainfall for the same interval between 1960 and 2010, also called the Long Period Average, according to the India Meteorological Department’s gridded rainfall database. Such rainfall, apart from preventing fires, has also likely cleared existing pollutants in the air from other sources. In not only Punjab and Haryana, but also in Delhi, the rainfall this October has been much higher than in all years for which comparable air quality data is available.
Good air dependent on weather is not sustainable
If the farm fires have just been delayed, it is likely that farmers will be even more compelled to take the faster route of burning the stubble to clear the fields for their next crop. This can possibly bring an even bigger peak in air pollution later on. This situation can also be complicated by the trend in temperatures so far this October. The maximum temperatures in Punjab, Haryana and Delhi, especially in the past one week, have been lower than in the past four years for which air quality data is available. Maximum temperatures are daytime temperatures.
A higher temperature during the day prevents the accumulation of pollutants close to the ground. This has been prevented so far because of rainfall. Without a curb on the sources of air pollution, a quick reversal to bad air quality days will not be surprising. But for now, breathe.
