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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, Dec 24: The Delhi Police said that a head constable was killed during clashes over the Citizenship Amendment Act (CAA) in northeast Delhi today.

Deputy Commissioner of Police, Shahdara, Amit Sharma was also injured during clashes, a senior official said. Sharma suffered injuries to his head and hand. He was hospitalised and is doing fine, the official added.

Tension escalated in northeast Delhi with protesters torching at least two houses and a fire tender in the Jaffrabad and Maujpur areas, where pro and anti-CAA groups clashed for the second consecutive day on Monday and hurled stones at each other.

Violence was also reported from the Chandbagh area in Jaffrabad.

Police fired tear gas shells and also resorted to lathicharge to disperse the protesters.

According to officials, a fire tender was damaged by the protesters after it responded to a fire call in the area.

The Delhi Metro closed entry and exit at the Jaffrabad and Maujpur-Babarpur stations as an uneasy calm prevailed in the area.

"Entry & exit of Jaffrabad and Maujpur-Babarpur are closed. Trains will not be halting at these stations," the Delhi Metro Rail Corporation (DMRC) said in a tweet.

Entry and exit were closed at the Jaffrabad metro station for over 24 hours.

Clashes broke out on Sunday evening between pro and anti-CAA groups near Jaffrabad after a large number of people protesting against the Citizenship (Amendment) Act (CAA) blocked a road, while similar sit-ins were launched in several other parts of the national capital.

Delhi chief minister Arvind Kejriwal has urged lieutenant governor of Delhi and the Union home minister to restore order.

Nobody should be allowed to orchestrate violence, he said.

Delhi Lt Governor Anil Baijal instructed the police commissioner to maintain law and order in the national capital in the wake of violence.

"Instructed @DelhiPolice and @CPDelhi to ensure that law and order is maintained in North East Delhi. The situation is being closely monitored. I urge everyone to exercise restraint for maintenance of peace and harmony," Baijal tweeted.

Delhi minister and Babarpur MLA Gopal Rai on Monday urged the people in the area to maintain peace, saying some people were purposely trying to disturb the atmosphere.

"With folded hands, I request people in Babarpur Assembly to maintain peace. Some people are purposely trying to spoil the atmosphere. I have spoken to Delhi LG and he has assured me that more policemen will be deployed to bring the situation under control," Rai said in a tweet.