Best post-Diwali air in Delhi in 3 years? Not quite
The number of monitoring stations in Delhi has grown from seven to 36 in last five years. This means that Delhi’s average AQI the day after Diwali in 2015, for instance, represents data from only seven localities, while the average AQI the day after Diwali this year represents about five times more localities, making a comparison between the two defective.
Despite a ban on firecrackers, Delhi’s air quality a day after Diwali this year was nearly as bad as the day after Diwali last year, shows a Hindustan Times analysis of Air Quality Index (AQI) figures from multiple air quality monitoring stations in the city.
Several reports have claimed that the city saw the least polluted Diwali in four years. The data cited by many such reports is the 24-hour average AQI figure published by the Central Pollution Control Board (CPCB). The AQI figure of 368, given by the CPCB, as on October 28 is indeed the lowest in four years. However, this does not necessarily mean that Delhi’s air was also the least polluted in this period. This is because a comparison of these figures over time is fundamentally incorrect because the number of air quality monitoring stations taken into account to calculate these figures has varied over time.
The number of monitoring stations in Delhi has grown from seven to 36 in last five years. This means that Delhi’s average AQI the day after Diwali in 2015, for instance, represents data from only seven localities, while the average AQI the day after Diwali this year represents about five times more localities, making a comparison between the two defective.
To better compare air quality over last five years, Hindustan Times has looked at hourly AQI figures from two monitoring stations, data for which is available for all five years for the day after Diwali. These stations are located at the Institute of Human Behaviour and Allied Sciences, Dilshad Garden, and Netaji Subhas University of Technology, Dwarka. Conveniently, these two stations are almost 40 km apart; one is towards the eastern end of the city, and the other, the western end; and a circle drawn with the midpoint between the two as the centre and the distance to either as the radius would encompass a significant part of Delhi.
The average AQI from these two stations this year, the day after Diwali was 350, just three points less than that in 2018 and 28 points more than that in 2017. In the past five years, 2015 saw the least polluted air in Delhi a day after Diwali while it was the most polluted in 2016. (See Chart 1)
Even a comparison of AQI figures from 25 monitoring stations, complete data for which is available for 2018 and 2019, confirms that the air quality in Delhi a day after Diwali this year was not remarkably better than the last year. The average figure from these 25 stations was 357 this year, compared to 368 last year.
Moreover, a comparison of the hourly data from these 25 stations shows that the air quality in Delhi in the morning after Diwali this year was in fact worse than the morning after Diwali last year. (See Chart 2)
(Abhishek Jha contributed to this story)
