"We clearly observed that reduction in atmospheric pollutants resulted in a decrease in local day and night-time temperature. This is an important finding to feed into the planning for sustainable urban development," Dash said.
The first lockdown imposed in India last year due to the COVID-19 pandemic led to an improvement in air quality and a reduction in land surface temperature in major urban areas of the country, according to a study.
The findings, published in the journal Environmental Research, provide a strong evidence for potential environmental benefits through larger scale policy implementation.
The study found that travel and work restrictions imposed early in the pandemic resulted in a significant environmental improvement, due to an abrupt reduction in industrial activities and a major decrease in the use of land and air transport.
The researchers used data from a range of Earth observation sensors, including those from the European Space Agency’s Sentinel-5p and NASA’s MODIS sensors, to measure changes in surface temperature and atmospheric pollutants and aerosols.
They concentrated on six major urban areas: Delhi, Mumbai, Kolkata, Chennai, Bangalore, and Hyderabad, comparing data from the lockdown in March to May last year with pre-pandemic years.
The study revealed a significant reduction in nitrogen dioxide (NO2), a greenhouse gas emitted from the combustion of fossil fuels, equivalent to an average decrease of 12 per cent throughout India, and 31.5 per cent over the six cities.
There was a 40 per cent reduction over the national capital, the researchers said.
In India alone, about 16,000 premature deaths occur annually due to exposure to poor air quality, they said.
The study also found land surface temperature over major cities in India substantially declined in contrast with the previous five-year average (2015-2019) with day temperatures being up to 1 degree Celsius cooler, and those at night up to 2 degrees Celsius cooler.
“The lockdown provided a natural experiment to understand the coupling between urbanisation and local microclimate,” said study co- author Jadu Dash, Professor at the University of Southampton in the UK.
“We clearly observed that reduction in atmospheric pollutants resulted in a decrease in local day and night-time temperature. This is an important finding to feed into the planning for sustainable urban development,” Dash said.
The study found that along with surface temperature, the atmospheric fluxes at the surface and top of the atmosphere also significantly declined over major parts of India.
The reduction of greenhouse gas concentration, higher atmospheric water vapour content and meteorological conditions played a complex role in the land and near-surface temperature reduction.
“Aerosol optical depth (AOD) and absorption AOD showed a significant reduction which could be connected with the reduction in the emission sources across India during the lockdown,” said Bikash Parida, from Central University of Jharkhand.
AOD tells us how much direct sunlight is prevented from reaching the ground by aerosol particles.
“The aerosol type sources, such as organic carbon (OC), black carbon (BC), mineral dust, and sea salt also reduced significantly.
“Moreover, in central India, increases in AOD were attributed to the supply of dust aerosols transported from the western Thar desert region,” Parida said.