Delhi gets its baseline for pollution due to impact of Covid-19 lockdown
Delhi’s baseline pollution level --- the minimum pollution when major polluting sources are shut --- is about a tenth of the peak pollution the capital witnesses, especially during the October-January period, a new study based on real time pollution data during Covid-19 lockdown this April, has found.
Air quality of a place depends on overall pollution load from sources such as industries and vehicles and the local weather conditions, which impact dispersion of pollutants.
Scientists from the Delhi Pollution Control Committee (DPCC) and the Indian Institute of Tropical Meteorology (IITM) have published a paper in peer reviewed science journal, Current Science, providing baseline pollution level for six different pollutants for Delhi, after studying air quality levels at 34 locations in the capital, where real time pollution is monitored.
“This is the first baseline pollution level data of Delhi. While background pollution is pollution which comes from outside or which would have been naturally present, baseline pollution is the minimum pollution level to which the population is chronically exposed and hence greatly relevant to epidemiological research. Such emission scenario was nearly impractical in a highly populated megacity like Delhi,” said MP George, a scientist with the Delhi Pollution Control Committee and one of the team members who did the research.
The study is based on average pollution levels for the city before and during the Covid-19 lockdown that kicked in pan-India from March 25, halting factories, construction sites, almost all vehicular movement and some other local sources that pollute Delhi’s air.
While emission sources dropped by 85% - 90% resulting in sharp decline in pollution levels, the weather also favoured Delhi and pushed down the city’s pollution to rock bottom.
“The baseline levels of PM10 and PM2.5 were found to be 38μg/m3 (± 8) and 22μg/m3 (± 6) respectively. The baseline value of NO2 was found to be 8ppb (± 3), which is considered to be very high,” states the research paper, published on October 10.
The research was led by Gufran Beig, a senior scientist with the Indian Institute of Tropical Meteorology, in Pune. Eleven researchers from IITM, DPCC, Utkal University and the India Meteorological Department (IMD) analyzed data from 34 automatic air quality monitoring stations in Delhi from February 20 to April 14 and prepared base level of seven pollutants including PM10, PM2.5, NO2 and SO2 among others.
The data were compared with pollution levels of 2019 during the same period for reference purpose. The level of PM10 was found to shoot up occasionally in May despite lockdown because of dust storms bringing in pollutants from the sandy western parts of India.
The findings show that even though PM10 levels remained mostly above the permissible limit of 100μg/m3 during the pre-lockdown period occasionally touching and crossing the 200μg/m3 mark, in the post lockdown period it remained much below the 100 μg/m3 mark. On Saturday at 5 pm, it was 232μg/m3, almost six time the pollution level of the baseline.
The level of PM2.5 remained mostly above the permissible limit of 60μg/m3 in the pre-lockdown period and on one occasion it shot up to nearly 140μg/m3. Compared to this in the lockdown period it remained below the 40μg/m3 mark on most days and even touched 20μg/m3. On Saturday at 5 pm it was 95μg/m3, almost five times of the baseline.
In winter, PM2.5 and PM10 breach their respective severe+ marks of 300ug/m3 and 500ug/m3, primarily because of poor dispersion and higher pollution load because of stubble burning in the farm rich states of Punjab and Haryana. It starts coming down from February as weather starts becoming warmer and burning also reduces.
“Pollution level dropped sharply immediately after the lockdown and concentration of pollutants touched the saturation level within a few days, beyond which further decline was not possible. In the present case, each pollutant of Delhi reached the saturation level within 4–6 days after lockdown, beyond which a steady level was maintained,” the authors said in the paper.
Experts have welcomed the study saying that it is this baseline pollution which the population is chronically exposed and hence greatly relevant to epidemiological research.
“This is the first baseline data. Some pollution will always be there because of kitchen pollution and pollution triggered to operate emergency services, say an ambulance on the road or a thermal power plant. Such baseline-pollution scenario is practically impossible to achieve in a megacity like Delhi but the lockdown helped us to prepare the first set of baseline pollution data. We should now utilize this to redefine our air quality standards and also plan our pollution mitigation strategies,” said D Saha, former chief of the Central Pollution Control Board’s air quality laboratory.