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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.

Kolkata, May 19: The super cyclonic storm 'Amphan' in west-central Bay of Bengal is likely to weaken into an 'extremely severe cyclonic storm' by noon on Tuesday, the Met department said here.

The system, which was situated 670 km south-southwest of Digha in West Bengal, is very likely to move north-northeastwards across northwest Bay of Bengal, and cross West Bengal-Bangladesh coasts in the afternoon or evening of Wednesday as a 'very severe cyclonic storm', the Met department said.

The weatherman said that 'Amphan' is expected to cross West Bengal-Bangladesh coasts between Digha in West Bengal and Hatiya islands in Bangladesh on May 20 as a very severe cyclonic storm, after losing some steam as it approaches landfall, with a maximum sustained wind speed of 155 to 165 kmph gusting to 180 kmph.

Gale wind speeds reaching 240 to 250 kmph were prevailing over west-central and adjoining east-central Bay of Bengal, the Met office said, adding, it will gradually reduce to 200 to 210 kmph gusting to 230 kmph by Tuesday evening.

The Met department, which has issued an "orange message" for West Bengal, warned of extensive damage in Kolkata, Hooghly, Howrah, South and North 24 Parganas and East Midnapore districts.

There is likely to be disruption of rail and road link at several places, uprooting of communication and power poles and extensive damage to all types of 'kutcha' houses, the weatherman said.

There is also likelihood of massive harm to standing crops, plantations and orchards, the Met office said.

Wind speeds along and off the coastal areas of West Bengal will reach 45 to 55 kmph with gusts of 65 kmph from Tuesday afternoon, and will gradually increase becoming gale wind speeds reaching 75 to 85 kmph with gusts up to 95 kmph from May 20 morning along and off districts of North and South 24 Parganas, East and West Midnapore, Kolkata, Howrah and Hooghly, Regional Met Director G K Das said.

"It will gradually increase thereafter becoming 110 to 120 kmph gusting to 130 kmph over West Midnapore, Howrah, Hooghly, Kolkata and wind speeds of 165 to 175 kmph gusting to 195 kmph over the districts of North and South 24 Parganas and East Midnapore from the afternoon to night of May 20," Das said.

Under its impact, the coastal districts of Gangetic West Bengal, including North and South 24 Parganas, Kolkata, East and West Midnapore, Howrah and Hooghly are likely to experience light to moderate rain at many places with heavy downpour at isolated places on Tuesday, he said.

On Wednesday, rainfall will occur in many places over the districts of Gangetic West Bengal, with extremely heavy rain at one or two places in Kolkata, Howrah, East Midnapore, North and South 24 Parganas and Hooghly districts, he said.