Why in News?

• As per the recently published report of 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 Spectro-radiometer (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.

What is Aerosols?

• Aerosols are defined as a combination of liquid or solid particles suspended in a gaseous or liquid environment.
• In the atmosphere, these particles are mainly situated in the low layers of the atmosphere (< 1.5 km) since aerosol sources are located on the terrestrial surface.
• However, certain aerosols can still be found in the stratosphere, especially volcanic aerosols ejected into the high altitude layers.
• The origin of atmospheric aerosols is either natural or the result of anthropogenic activities.
• Natural sources of aerosols include sea salt generated from breaking waves, mineral dust blown from the surface by wind, and volcanoes.
• Anthropogenic aerosols include sulfate, nitrate, and carbonaceous aerosols, and are mainly from Fossil Fuel Combustion Sources.