2021 in numbers: Delhi’s air was better than usual; weather and lockdown helped
2021 in numbers: The average AQI in Delhi for the January 1-December 23 period in 2021 is 205.5, which is still lower than the corresponding figures for 2018 (221.4) and 2019 (210.7).
NEW DELHI: The lockdown and social distancing measures put in place when Covid-19 infections first started spreading in India improved air quality. In Delhi - the national capital is among India’s most polluted cities - the average Air Quality Index (AQI) improved by 48% in April 2020 compared to April 2019.
Hard lockdowns are no longer the first response when there is an outbreak. Moreover, social distancing norms also do not allow for public transport to be used at full capacity. How has this changed Delhi’s air in 2021? Here are two charts that explain this.
The average AQI for the January 1-December 23 period in Delhi was 221.4 in 2018 and 210.7 in 2019, according to 4pm-to-4pm 24-hour bulletins issued by the Central Pollution Control Board (CPCB). In 2020, there was a substantial improvement in AQI and it decreased to an average of 181.8. The number of ‘good’ and ‘satisfactory’ days also increased in the year substantially. In 2021, the index has gone back to above 200 levels to 205.5, and the number of days of ‘very poor’ and ‘severe’ category air quality has increased.
These annual averages still suggest a gradual improvement over the years. Is it possible that this is due to measures taken for controlling pollution? A month-by-month comparison of air quality along with indicators of mobility, farm fires, and the weather does not suggest so.
Compared to 2019, for example, air quality improved in 2021 the most in May. The average AQI in Delhi was 220.6 in May 2019 and 144.4 in May 2021, a 34.6% improvement.
However, it was in May 2021 when the Delta variant of the Covid-19 virus was raging through the national capital and people stayed indoors. Non-essential mobility – excluding stay or visits to residences, groceries, and pharmacies – was 69% below pre-pandemic levels in this month.
To a lesser degree, this story was repeated in June.
Although mobility increased every month after May this year, it is still about 12% below pre-pandemic levels. Any improvement in air quality has to take this into account.
There was also a substantial improvement in AQI in 2021 compared to 2019 in the months of September and October. Both the weather and the source of the pollutants, farm fires, were responsible for this. Fires observed in these two months in Punjab and Haryana were not only less than in 2019, they were the least since 2012, the earliest year for which this data is available from the Suomi-NPP satellite’s VIIRS instrument. The air in these months was also helped by unusual rainfall, around four times higher than in 2019.
The reduction in fires in September and October may not have been due to any meaningful policy action. Data shows that the reduction was merely a delay, and the fires finally took place in November, the highest since 2012 for the month. With less rainfall in December and lower than usual daytime temperatures (https://bit.ly/3JhRW8A), air quality in December is again worse than in 2019.