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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.

An Indian national was killed and four others injured in alleged firing by Nepal police personnel along the India-Nepal border in Bihar's Sitamarhi district today.

Sources said the firing took place after a clash between the Indians and personnel of Nepal police at the Lalbandi-Janki Nagar border in Pipra Parsain panchayat under Sonebarsha police station of the district.

Jitendra Kumar, the additional director general of police (headquarters), confirmed the death and injuries. The place of firing falls under Nepal jurisdiction.

Locals said Vikesh Kumar Rai, 25, died on the spot and Umesh Ram and Uday Thakur received bullet injuries when they were working in an agricultural field. Another person, Lagan Rai, is said to have been detained by the Nepali police.

Injured persons were rushed to Sitamarhi Sadar Hospital for better treatment.

Vikesh Kumar Rai’s father, Nageshwar Rai, said that his agriculture land falls under Narayanpur in Nepal where his son was working.

On May 17, Nepal police had fired blank rounds to disperse dozens of Indians trying to cross the border. It was not clear if they were also farmers.

The district magistrate and the superintendent of police of Sitamarhi have rushed to the spot.

Nepal shares a 1,850-kilometre (1,150-mile) open border with India and people travel across it for work and to visit family. It had closed its international borders on March 22 amid the coronavirus pandemic.