NASA Satellite Reveals Huge Drop in Air Pollution Over Parts of India Since COVID-19 Lockdown

NASA satellite data reveals how aerosol levels in India have dropped dramatically since March 25, when the government imposed the world's largest lockdown on its 1.3 billion-strong population amid the COVID-19 pandemic.

This lockdown, like in other parts of the planet, has significantly reduced industrial activities and caused large falls in road vehicle and plane traffic in the country. Subsequently, aerosol levels dropped to a 20-year-low for the beginning of April in northern India—just one week after the lockdown was introduced—according to measurements taken by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite.

Aerosols are tiny solid particles or liquid droplets in the air that come from both natural and man-made sources—such as the burning of fossil fuels or cropland. They can range in size from a few nanometers (less than the width of the smallest viruses) to several tens of micrometers (about the same width as a human hair.) But despite their minuscule size, they can have a significant impact on our climate and human health.

In India, aerosols produced by human activities contribute to unhealthy levels of air pollution in many areas of the country, while also causing poor visibility. Exposure to these kinds of man-made aerosols—which tend to be on the smaller end of the scale—have been linked to lung and heart problems in humans.

While the majority of aerosols around the world are produced by natural phenomena, such as volcanic eruptions, forest fires and dust storms, in urban areas—and regions which are downwind from these areas—aerosols created by humans tend to dominate.

In the Ganges Valley of northern India, for example, the majority of the aerosols generated during early spring come from human sources, such as vehicle traffic, industrial activities, cooking fires, and controlled burns on farmland. However, all these activities appear to have been limited by the lockdown.

The maps below—compiled using MODIS data—demonstrate the extent to which aerosol levels subsequently dropped in northern India following the introduction of the lockdown. They show an indication of aerosol levels for the period March 31-April 5 in every year between 2016 and 2020.

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

Scientists observed a decrease in aerosol levels during the first week of the shutdown, but meteorological factors also had influence in this initial period—with large parts of northern India drenched by heavy rain around March 27, which helped to clear the air.

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

The chart above shows an indication of aerosol levels above northern India for the period January 1 to April 5 of this year. At the beginning of April, aerosol readings fell to particularly low levels—their lowest point in 20 years of MODIS data. The spike visible at the end of February can likely be attributed to fires in the northern Indian state of Punjab and Pakistan.

Ground-based monitoring stations have also reported a fall in pollutants since the lockdown began, and residents in the region also appear to be noticing an improvement in air quality. For example, some people in the city of Jalandhar—located in Punjab—have posted images on social media showing parts of the Himalayas that haven't been visible from this area for decades.

The skies also appear to have cleared in India's notoriously polluted capital, New Delhi, in the wake of the lockdown measures.

"I have not seen such blue skies in Delhi for the past 10 years," Jyoti Pande Lavakare, co-founder of Indian environmental organization Care for Air, told CNN. "It is a silver lining in terms of this awful crisis that we can step outside and breathe."

However, the picture is not the same all across the country. The MODIS data reveals aerosol levels in some parts of southern India have not fallen as far as they have in the north. At present, it is not clear why, however, recent weather, agricultural fires, and other factors, may help to explain the phenomenon, scientists say.