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Washington, Apr 23: Air pollution over northern India has plummeted to a 20-year-low for this time of the year, according to satellite data published by US space agency National Aeronautics and Space Administration (NASA).
The US space agency's satellite sensors observed aerosol levels at a 20-year low post the countrywide lockdown, implemented to slow the spread of the novel coronavirus.

"We knew we would see changes in atmospheric composition in many places during the lockdown," said Pawan Gupta, a Universities Space Research Association (USRA) scientist at NASA''s Marshall Space Flight Center. "But I have never seen aerosol values so low in the Indo-Gangetic Plain at this time of year," added Mr Gupta.

Acting Assistant Secretary of State for South and Central Asia Alice G Wells tweeted, "These images from NASA were taken each spring starting in 2016 and show a 20-year low in airborne particle levels over India. When India and the world are ready to work and travel again, let's not forget that collaborative action can result in cleaner air."

The data published with maps show aerosol optical depth (AOD) in 2020 compared to the average for 2016-2019. Aerosol optical depth is a measure of how light is absorbed or reflected by airborne particles as it travels through the atmosphere.

If aerosols are concentrated near the surface, an optical depth of 1 or above indicates very hazy conditions. An optical depth, or thickness, of less than 0.1 over the entire atmospheric vertical column is considered "clean." The data were retrieved by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite.

In the first few days of the lockdown, it was difficult to observe a change in the pollution signature. "We saw an aerosol decrease in the first week of the shutdown, but that was due to a combination of rain and the lockdown," said Mr Gupta.

Around March 27, heavy rain poured over vast areas of northern India and helped clear the air of aerosols. Aerosol concentrations usually increase again after such heavy precipitation.

"After the rainfall, I was really impressed that aerosol levels did not go up and return to normal. We saw a gradual decrease and things have been staying at the level we might expect without anthropogenic emissions," Mr Gupta said.

On March 25, the Indian government placed its 1.3 billion citizens under a strict lockdown to reduce the spread of COVID-19. The countrywide mandate decreased activity at factories and severely reduced car, bus, truck and airplane traffic. Every year, aerosols from anthropogenic (human-made) sources contribute to unhealthy levels of air pollution in many Indian cities.

Aerosols are tiny solid and liquid particles suspended in the air that reduce visibility and can damage the human lungs and heart.

In southern India though, the story is a little hazier. Satellite data show aerosol levels have not yet decreased to the same extent. In fact, levels seem to be slightly higher than in the past four years. The reasons are unclear but could be related to recent weather patterns, agricultural fires, winds or other factors.

"This a model scientific experiment," Robert Levy, program leader for NASA's MODIS aerosol products, said about the lockdown and its effects on pollution.

"We have a unique opportunity to learn how the atmosphere reacts to sharp and sudden reductions in emissions from certain sectors. This can help us separate how natural and human sources of aerosols affect the atmosphere," Mr Levy added.

New Delhi, May 10: The Delhi government has asked district magistrates to release 2,446 Tablighi Jamaat members from quarantine centres and ensure that they do not stay in any other place except their homes.

The district magistrates will explore the possibility of sending those Tablighi members, who belong to other states, in buses to their designated places in accordance with social distancing norms and other protocols, DDMA Special CEO K S Meena said in a letter to deputy commissioners (administration).

As man as 567 foreign attendees of the congregation held in Delhi's Nizamuddin area in March, will be handed over to the police, Meena said.

"They (foreign Jamaat attendees) will be handed over to police in connection with several violations like visa violation," a government official said on Saturday.

Delhi Home Minister Satyendar Jain had recently ordered the release of Tablighi members who have completed their required quarantine period in centres and tested negative for COVID-19.

"Out of such people belonging to Delhi, who could be released as per prescribed guidelines should be issued passes to travel from the quarantine centres.

"Under no circumstances, the aforesaid persons should be allowed to stay in any other places including mosques," Meena said in the letter.

In respect of those Tablighi members belonging to other states, it should be ensured by the nodal officer and the area ACP that such people reach their place of residence, he also said.

"The DC should also inform the respective resident commissioner of their states in respect of each and every movement of such persons from Delhi," the Delhi Disaster Management Authority (DDMA) Special CEO said.

Thousands of Tablighi Jamaat members had been taken out of its Markaz (centre) in Nizamuddin, where they had gathered for a religious congregation, and quarantined as the area became a major hotspot after a number of members tested positive for coronavirus.

On March 31, the Delhi Police's Crime Branch had lodged an FIR against seven people, including Maulana Saad Kandhalvi, on a complaint by Station House Officer, Nizamuddin, for holding the congregation.