New Delhi, June 23 : India rejoiced in the silver lining of clear skies during the Covid-19 resultant lockdown in 2020 and 2021 but an analysis by New Delhi-based climate communications initiative, Climate Trends on Wednesday showed pollution levels remained above permissible limits in Delhi and Lucknow. However, Mumbai's PM 2.5 concentration only increased year on year from March to May from 2019 to 2021. Also part of the study was Kolkata, the only city to show improvement in the air quality in these months from 2019 to 2021. Researchers compared the Central Pollution Control Board (CPCB) air quality data for Delhi, Lucknow, Mumbai and Kolkata over the three months from March in 2019 -- when there was no lockdown -- and 2020 and 2021 --when there was complete lockdown in these cities. The study revealed that except Mumbai, all cities showed a dip in the average PM 2.5 levels during the three months in 2020. Mumbai's average concentration of PM 2.5 between March and May in 2019 was 21.6 micrograms per cubic meter of air, or ug/m3, which increased to 31.3 ug/m3 in 2020 and then to 40.3 ug/m3. The safe limit for PM 2.5 (particulate matter measuring less than 2.5 microns) as prescribed by the CPCB is 40 ug/m3. "Mumbai being a coastal city has a mixed effect of local meteorology and prevalent conditions of large-scale motions, including that of cyclones. While cyclones such as Tauktae act as a washout or cleaning effect on the atmosphere; slow wind conditions, favourable conditions of long transport of particulate matters from neighbouring states act as accumulation, suggesting marginal increase of the pollutants," said S.K. Dhaka of Rajdhani College, Delhi University. On the other hand, Delhi's average PM 2.5 concentration for the three months dipped from 95.6 ug/m3 in 2019 to 69 ug/m3 in 2020 but was quickly back to 95 ug/m3 in 2021. Similarly, Kolkata's PM 2.5 concentration swayed from 41.8 ug/m3 in 2019 to 27.9 ug/m3 in 2020 and 37.3 ug/m3 in 2021. While there was a complete lockdown in 2020, the 2021 lockdown saw high movement of people seeking healthcare facilities due to increased Covid-19 cases and the state elections in West Bengal. Lucknow saw its PM 2.5 concentration in the three months decrease consistently from 2019 but it still remained above permissible limits. Its average PM 2.5 concentration in 2019 for March, April and May was 103 ug/m3 which dipped to 92 ug/m3 in 2020 during lockdown and further to 79.6 ug/m3 in 2021. G.C. Kisku, Chief Scientist, Environmental Toxicology, CSIR-Indian Institute of Toxicology Research, said, "The partial or complete lockdowns during 2020 and 2021 lowered vehicular movement and subsequently led to reduced consumption of fossil fuels. "Closure of industrial establishments during lockdown periods also added to it. But the levels are still relatively higher this year. The good thing is that there has been a decreasing trend in PM10 levels from 2017 onwards, however, this year, the observed levels of PM 2.5, PM 10, SO2, and NO2 at all locations were found to be relatively higher as compared with monitoring data of previous year." The CSIR also recently released a report on the assessment of ambient air quality of Lucknow which showed that the mean levels of PM10 (127.1 ig/m3) and PM2.5 (64.5 ig/m3) at all monitoring locations of residential, commercial and industrial areas from April to May 2021 were higher than permissible limits. Any relief from lockdown resultant pollution levels cannot be rejoiced, feel experts. "Lockdown related reduction in air pollution is neither consistent nor uniform. The contribution of anthropogenic activities does not totally explain the high pollution level. Thus we should stay alert about the continuing health hazards of high pollutant levels, especially in metropolitan cities. And this is not the right time to let the guards down," said Arun Sharma, President of the Society for Indoor Environment. Since the unprecedented nationwide lockdown in 2020, experts have claimed that it gave them a first-ever opportunity to understand background pollution levels in India when the majority of the polluting sources were not at play. In 2020, out of the eight primary polluting sources in India, four were completely closed during the lockdown period namely construction and industrial activity, brick kilns and vehicles. Meanwhile, sources like household emissions, open burning, diesel generators and dust remained operational along with coal-fired thermal power plants at reduced capacity. The 2021 lockdown was not as absolute, although it allowed a comparative analysis between the two years. Dhaka of Rajdhani College added, "Lockdown has provided an opportunity to examine the background pollution in the atmosphere when everything was shut down in 2020; atmosphere was fairly clean. However, we did not meet the condition of 40 ug/m3 prescribed by the CPCB. "We need to redefine the natural conditions in India, for which particulate matter concentrations are more than 50-60 ug/m3 even in the clean atmosphere in northern India." /IANS