Delhi celebrated a more polluted Diwali this year as compared to last when a complete ban on selling fireworks was in place, data shows. This year, the ban was on selling “non-green” crackers while a window of two hours — between 8 pm and 10 pm was opened to burst crackers and fireworks.
A colder Delhi with reduced wind speeds, and winds coming in from the North West that brought pollutants from stubble burning to the capital’s doorstep — contributed to a more polluted Diwali this year.
Data released by the Central Pollution Control Board (CPCB) shows that 2018 has been on par, and in some instances worse than in 2016, when the city residents were exposed to very high levels of particulate matter.
The festival falling almost a month later than last year is cited as a reason for different met conditions which play a significant role in dispersing air.
Take for instance Janakpuri in west Delhi where PM10 was recorded at 1,076 µg/m3 on Diwali, compared to 988 µg/m3 in 2016. Comparatively, in 2017 it was recorded at 638 µg/m3. At ITO and Pitampura, particulate matter was higher in 2016 compared to this year. In Parivesh Bhawan, the CPCB saw higher concentrations of PM2.5 this year, and lower concentrations of PM10 when compared to two years ago.
“It is very important to take into account that the first week of November this year was in fact better than last year. Diwali days always lead to a spike in pollution levels but comparing one Diwali to the next does not offer a clear picture since met conditions do play a significant role,” said a senior CPCB scientist. Overall, there were more number of moderate days this year, he said.
Monitoring stations run by Delhi Pollution Control Committee (DPCC) showed that the concentration of particulate matter spiked suddenly at night.
At Wazirpur in northwest Delhi the concentration of PM 2.5 was recorded as 936µg/m3 at 10 pm. It increased to an alarming 4,659 µg/m3 at 1 am. At Jawaharlal Nehru Stadium, the concentration of pollutants started increasing at around 10 pm, when the concentration of PM 2.5 was recorded as 404 µg/m3. It shot up to 1,771 µg/m3 at midnight and peaked at 1990 µg/m3 at 1 am. Similarly at RK Puram, PM2.5 levels went from 602 µg/m3 at 8 pm to 2,851 µg/m3 at 11 pm.
According to the CPCB report while mixing height and wind speed during the day were similar, the wind speed dropped at night. There was also a massive 8-degree difference in temperature.
Average mixing height on the day of Diwali in 2017 and 2018 was in the range of 400m-600 m. The average wind speed was also similar (1.6 m/sec) but the wind speed decreased to 0.8m/sec at night in 2018, the CPCB report said.
“A significant drop in temperature of about 8° Celsius, reduced wind speed, specially during night, were two important factors that might have contributed to delayed dispersion because of which higher PM concentration values are reported during night hours,” the report said.
The direction of wind this year as compared to the last was also crucial, the report said. Diwali was on October 19 last year, when the wind direction was east and southeast. This year the direction was north-west, which might have added pollutant load from stubble burning, the report said.
More active fire incidences have been reported this year on the day of Diwali (4,203) compared to Diwali in 2017 (1,702), the report added.