Role of Aerosols in Indian Monsoon

Why in news?

Researchers from Indian Institute of Tropical Meteorology, Pune, think that aerosols may be weakening the rainy season.

What is an Aerosol?

• Aerosols 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 are short-lived, unlike greenhouse gases that persist and accumulate in the atmosphere for longer period.
• The bulk of aerosols — about 90% by mass have natural origins. Ex: Volcanoes.
• The remaining 10% of aerosols are considered anthropogenic, or human-made, and they come from a variety of sources.
• Automobiles, incinerators, smelters, and power plants are prolific producers of sulfates, nitrates, black carbon, and other particles.
• Deforestation, overgrazing, drought, and excessive irrigation can alter the land surface, increasing the rate at which dust aerosols enter the atmosphere.

What are the direct effects of Aerosols?

• Aerosols interact both directly and indirectly with the Earth's radiation budget and climate.
• Different aerosols scatter or absorb sunlight to varying degrees, depending on their physical properties. Although most aerosols reflect sunlight, some also absorb it.
• Aerosol’s effect on light depends primarily on the composition and color of the particles.
• Pure sulfates and nitrates reflect nearly all radiation they encounter, cooling the atmosphere.
• Black carbon absorbs radiation readily, warming the atmosphere but also shading the surface.
• Brown carbon or organic matter has a warming influence on the atmosphere depending on the brightness of the underlying ground.
• Salt particles tend to reflect all the sunlight they encounter.
• In addition to scattering or absorbing radiation, aerosols can alter the reflectivity, or albedo, of the planet.

What are the indirect effects of Aerosols?

• 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.
• The most significant of these reactions are those that lead to the destruction of stratospheric ozone.
• On a global scale, these aerosol “indirect effects” typically work in opposition to greenhouse gases and cause cooling.
• Broadly speaking, aerosols are thought to suppress precipitation because the particles decrease the size of water droplets in clouds.

What did the IITM Pune study say?

• A study by IIT Kanpur that came out in April, previously found that higher aerosol loading results in delayed but more rainfall over Central and Northern India.
• But, this recent research by IITM Pune focusses on effect of aerosols on Indian monsoon.
• Monsoon is weakening over the last 50 years. The recent reseach report that a mix of GHGs, aerosols and changes in forest and agricultural cover was affecting the strength of the monsoon.
• Their computer simulations suggest that aerosols may be a far more important factor than GHGs and it is the major cause of weakening of the monsoon.
• A good monsoon is produced by the difference in temperature between land and sea.
• But, the dust clouds shield the earth from the sun’s rays, depressing land and sea temperatures and reducing the variation between the two.
• Because of this, the Indian monsoon is getting weakened by aerosol accumulation.

Source: The Hindu