Aerosol levels at 20-year-low in north India: NASA satellite data

Scientists compare aerosol optical depth measurements over the country between March 31 to April 5 for five years

By Shagun
Published: Wednesday 22 April 2020
Average aerosol optical depth measurements in north India (from left to right: Levels above 0.75 in several areas of north India in 2019, decrease when lockdown was announced in 2020 and levels showing an ‘anomaly’ in April 2020)

Aerosol levels in north India or the Indo-Gangetic Plains are at a 20-year-low for this time of the year, according to satellite data published by the US space agency National Aeronautics and Space Administration (NASA) on April 21, 2020.

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

“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,” he added.

Scientists compared aerosol optical depth (AOD) measurements over India between March 31 to April 5 for five years (2016-2020).

AOD 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 AOD in 2020 was an ‘anomaly’, compared to previous years. On the day of the lockdown on March 25, 2020, it was 0.3 over north India. The AOD fell to 0.2 around April 1 and was found to be 0.1 on April 5.

The data was retrieved by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's satellite. There was an aerosol decrease during the last week of March, but that was because of a combination of the rain and the lockdown, said Gupta.

The air was clear of aerosols as a result of rainfall over areas across north India, around March 27. Aerosol concentrations usually increase again after such heavy precipitation.

The concentrations stayed at levels expected without anthropogenic emissions, said Gupta. “After the rainfall, I was really impressed aerosol levels did not go up and return to normal,” he added.

Aerosols may result from both natural sources — dust storms, volcanic eruptions, forest fires — and anthropogenic causes — burning of fossil fuels and croplands.

Man-made aerosols tend to contribute to a significant number of smaller particles that have greater potential for damaging human health.

Human activities — driving vehicles, operating coal-fired power plants and factories, etc — produce nitrates and sulphates that contribute to heavy concentration of aerosols across the Indo-Gangetic Plains, every year.

AOD levels in north India at the beginning of April were significantly below the norm for this time of year and the lowest in 20 years of MODIS observations, Gupta said.

Aerosol levels did not yet decrease to the same extent in south India, however, according to satellite data. One of the reasons could be forest fires reported in many areas in the last few weeks.

Scientists also expect aerosol levels to increase slightly in the coming weeks in parts of India as seasonal dust storms begin.

“The hard part with understanding aerosols is that particles can move based on wind patterns and other meteorology,” said Robert Levy, the program lead for NASA’s MODIS aerosol products. Meteorological factors need to be disentangled from man-made ones, Levy added.

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