Context: According to the data published by the National Aeronautics and Space Administration (NASA), aerosol levels in northern India at the beginning of April were significantly below the normal for this time of year.

• The levels were the lowest in 20 years of Moderate Resolution Imaging Spectroradiometer (MODIS) observations.
• MODIS is a key instrument on NASA’s satellites designed to monitor the Earth's atmosphere, ocean, and land surface. Data provided by it assists policymakers in making sound decisions concerning the protection of the environment.

Key Points

• Use of Aerosol Optical Depth (AOD): The data published with maps show Aerosol Optical Depth (AOD) in 2020 compared to the average for 2016-2019. 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.
• 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.

Possible Reasons behind it

• Every year, aerosols from anthropogenic (human-made) sources contribute to unhealthy levels of air pollution in many Indian cities.
• 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.
• On March 25,2020 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. This has contributed to the decreased levels of aerosols.

Similar Conditions are not there in southern India:

• 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.

About Aerosols

• An aerosol is a colloid of fine solid particles or liquid droplets, in air or another gas.
• They are minute particles suspended in the atmosphere. When these particles are sufficiently large, we notice their presence as they scatter and absorb sunlight. Their scattering of sunlight can reduce visibility (haze) and redden sunrises and sunsets.
• Aerosols can be natural (fog, forest exudates and geyser steam) or artificial (haze, dust, particulate air pollutants and smoke).
• Aerosols interact both directly and indirectly with the Earth's radiation budget and climate.
• As a direct effect, the aerosols scatter sunlight directly back into space.While as an indirect effect, aerosols in the lower atmosphere can modify the size of cloud particles, changing how the clouds reflect and absorb sunlight, thereby affecting the Earth's energy budget.
• Aerosols also can act as sites for chemical reactions to take place (heterogeneous chemistry). The most significant of these reactions are those that lead to the destruction of stratospheric ozone.
• During winter in the Polar Regions, aerosols grow to form polar stratospheric clouds. The large surface areas of these cloud particles provide sites for chemical reactions to take place. These reactions lead to the formation of large amounts of reactive chlorine and, ultimately, to the destruction of ozone in the stratosphere.

Effects of aerosols:

• They affect the atmospheric chemical composition.
• They can reduce visibility.
• They have important impacts on air quality and human health (e.g. aerosols can cause damage to heart and lungs).
• They serve as nuclei for cloud droplets or ice crystals in ice clouds.

Three types of atmospheric aerosol have a significant effect on Earth's climate: volcanic; desert dust; and human-made.

• Volcanic aerosol forms in the stratosphere after an eruption as droplets of sulfuric acid that can last up to two years, and reflect sunlight, lowering temperature.
• Desert dusts, mineral particles blown to high altitudes, absorb heat and may be responsible for inhibiting storm cloud formation.
• Human-made sulfate aerosols, primarily from burning oil and coal, affect the behavior of clouds.