What are Aerosols ?

Apart from trace gases, the atmosphere contains a variety of liquids and solids that exist as dispersed phases in the air. They are collectively called aerosols. An aerosol is considered a two-phase system consisting of solid or liquid particles and the gas (air) they are suspended in. Aerosols result from both natural and anthropogenic sources. Examples are dust particles generated by wind erosion of surface soils, agricultural activities, sea salt and wave breaking over oceans. Other sources include generation via chemical reactions in the atmosphere. For example, sulfate aerosols are generated by oxidation of sulfur dioxide in atmospheric moisture, particles are generated in automobile exhaust and incomplete combustion of fossil fuels in power plants. The aerosols have a direct radiative forcing effect on climate because they scatter and absorb solar and infrared radiation in the atmosphere.

Aerosols interact both directly and indirectly with the Earth’s radiation budget and climate.

Effect of Aerosols

• As a direct effect, the aerosols scatter sunlight directly back into space. 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. Evidence now exists that shows similar changes in stratospheric ozone concentrations occur after major volcanic eruptions, like Mt. Pinatubo in 1991, where tons of volcanic aerosols are blown into the atmosphere.
• Radiative forcing is the process through which about 30 per cent of sunlight reaching the Earth’s surface is reflected back into space as invisible infrared light. Aerosols generated by human activities reflect infrared light generated by reflected sunlight, thereby trapping it in the atmosphere. This alters radiative forcing, resulting in climate change. Recent studies have identified aerosol emission, particularly black carbon emission from industrializing countries like India and China, as emission control targets for mitigating climate change.
• So, the researchers have found that these small airborne particles called aerosols (for example, black carbon particles in diesel exhaust and sulfate particles produced by coal burning) in India and China may indirectly contribute to climate change. Higher black carbon levels in the atmosphere lead to warming, whereas increased sulfate levels cause cooling.

To find out the situation in India and China, the researchers examined emissions from the most important aerosol sources in the two neighboring countries and estimated the net radiative forcing from each source, both locally and globally. In this analysis, they used models developed by the NASA Goddard Institute for Space Studies.

Major emission sources of black carbon are diesel truck and bus exhaust and residential biofuel and fossil-fuel combustion. For organic carbon, residential biofuel and fossil-fuel combustion are important sources. The study found that fossil-fuel combustion in the power sector accounts for 52.3 per cent of Sulphur dioxide emission in India.

The researchers reveal that residential biofuel combustion in both India and China gave rise to significant positive direct radiative forcing through black carbon emission. They say that aerosol emission from diesel trucks and buses also makes a positive contribution to radiative forcing in India.