Aerosols

Why in news?

Contrary to the general notion that pre-monsoon aerosol loading results in decrease in seasonal rainfall, a long-term (2002-2013) satellite observational study and model-based analysis by IIT Kanpur has found that higher aerosol loading results in delayed but more rainfall over Central and Northern India.

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. Volcanoes, for example, eject huge columns of ash into the air, as well as sulfur dioxide and other gases, yielding sulfates.
• 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. Even indoors, cigarettes, cooking stoves, fireplaces, and candles are sources of aerosols.

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.
• An 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, in contrast, 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.
• In the Arctic, aerosols from wildfires and industrial pollution are likely hastening the melting of ice.

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 IIT Kanpur study say?

• It said Satellite data showed that clouds are getting taller and wider under high aerosol loading.
• As the height of clouds increases, the ice particles generated at top of the cloud come in contact with numerous water and ice particles and become bigger in size.
• This results in more ice mass in the cloud and eventually more rainfall when the ice particles fall down due to gravity.
• In the absence of cloud, aerosol particles tend to absorb solar radiation and this leads to warming or less decrease in temperature with height.
• As a result, there is suppression of convection leading to further suppression of cloud formation.
• Till now scientists have shown that presence of more aerosol in pre-monsoon season may lead to reduction in total monsoon rainfall due to aerosol-solar radiation interactions.
• But in the recent study revealed that higher aerosol loading can enhance the strength of convective rainfall and increase the frequency and intensity of extreme rainfall during Indian summer monsoon.

Source: The Hindu