Aerosols

Aerosols refer to tiny liquid or solid particles suspended in the atmosphere that can scatter sunlight and impact visibility. Generated naturally and by human activities, they interact with climate change, ozone depletion, and even public health. An important way of measuring aerosols is through Aerosol Optical Depth (AOD), which has a critical role in environmental safety assessments. In this article, you will learn about meaning and definition of aerosol, its impact on climate and definition of Aerosol optical depth (AOD). To explore more interesting UPSC GS Paper-3 Environment & Ecology like Aerosols of NCERT Class 9, check out other articles and IAS Notes of IASToppers.   

Table of Content

• What are Aerosols?
• Origin of Aerosol
• Types of Aerosols
• Impact of Aerosol
• Climatic Effects of Aerosols
• What is Aerosol Optical Depth (AOD) 
• FAQs on Aerosol

What are Aerosols?

• Aerosols are minute liquid or solid particles suspended in the gaseous or liquid atmosphere.
• When these minuscule particles grow larger, their presence becomes noticeable as they scatter and absorb sunlight.
• Sunlight scattering by aerosols results in reduced visibility (forming a haze) and intensifying the red colors in sunrises and sunsets.
• Mainly, aerosols are found in the lower atmospheric layers (below 1.5 km) because their sources are terrestrial.
• Yet, specific aerosols, particularly those originating from volcanic eruptions, can ascend into the stratosphere.

Origin of Aerosol

Aerosols in the atmosphere are created through both natural processes and human activities.

Natural sources of aerosols 

• Sea salt generated from breaking waves
• Wind carrying mineral dust from the surface
• Volcanic eruptions
• Forest fires that send partially burned organic carbon aloft
• Certain plants that produce gases that react with other substances in the air to yield aerosols
• Microalgae in the ocean that produce a sulfurous gas called dimethyl sulfide that can be converted into sulfates in the atmosphere

Anthropogenic sources of aerosols 

• Sulfates, nitrates, and carbon-based particles, primarily from the burning of fossil fuels.
• Urban ecosystems: Dust, cigarette smoke, Mist from aerosol spray cans, automobile exhaust soot or fumes.
• Combustion processes and biomass burning

Types of Aerosols

Primary aerosols

• Primary aerosols are particles directly dispersed into the atmosphere.
• Examples include elements like sea spray, mineral dust, volcanic ash, and smoke.

Secondary aerosols

• Aerosols that originate in a different form (for example, gases) and transform into aerosol particles following atmospheric chemical reactions are classified as ‘secondary aerosols’.
• These include sulfate aerosols from industrial emissions or volcanic activity.

All aerosols, regardless of their origin, are capable of experiencing additional chemical alterations, a process known as ‘aging effects’.

Impact of Aerosol

• Role of Aerosols in Chemical Reactions: Aerosols offer a platform for specific chemical reactions, particularly those contributing to the depletion of stratospheric ozone. During the polar winter, aerosols accumulate, forming polar stratospheric clouds. The large surface areas of these clouds create the perfect environment for chemical reactions that result in the creation of reactive chlorine, which ultimately leads to the stratospheric ozone depletion.
• Volcanic Eruptions and Aerosols: The sulfuric acid, hydrogen sulfide, and hydrochloric acid are released during volcanic eruptions represent. These gases have aerosols and eventually return to the earth as acid rain, potentially causing harmful effects on the environment and human lives.
• Aerosols and the Earth’s Energy Budget: Aerosols have a dual impact on the earth’s energy budget – a direct and an indirect influence.
  • A direct impact is the ability of aerosols to scatter and absorb solar radiation. This results in surface cooling as the solar radiation is reflected back to space but can also lead to surface warming due to the absorption of solar energy. This is in addition of the greenhouse effect and subsequently contributes to global climate change.
  • They indirectly affect radiation-interacting formations, such as altering cloud particles’ size, thereby changing how clouds reflect and absorb light and modifying the energy balance.
• Aerosols’ Mitigating Effect on Greenhouse Gases: Man-made aerosols can mitigate greenhouse gas effects in some regions, slowing surface warming in the Northern Hemisphere. However, this heat is eventually absorbed through warmer ocean currents originating from the South. On a global scale, the cooling effect of aerosols does not entirely negate the warming caused by greenhouse gases.
• Aerosols’ Role in Pollutant Deposition: Aerosols can absorb pollutants and deposit them on Earth and water, potentially damaging the environment and human health.
• Aerosols and Indoor Air Quality: Aerosols around 20 μm can remain longer in air-conditioned rooms due to their ability to move with air currents, potentially aiding in the spread of diseases like COVID-19.  
• Aerosols and Respiratory Health: Aerosol particles smaller than 10 μm can infiltrate the bronchi, while smaller than 2.5 μm can reach the lungs’ gas exchange area, posing risks to respiratory health.
• Other Impacts of Aerosols: Aerosols can alter the chemical makeup of the atmosphere, reduce visibility, negatively affect air quality and human health (for instance, causing damage to the heart and lungs), and serve as nuclei for cloud droplets or ice crystals in ice clouds.

What is Aerosol Optical Depth (AOD)?

• Aerosol Optical Depth (AOD) is the measure of aerosols distributed within a column of air from Earth’s surface to the top of the atmosphere.
• This technique also enables an indirect estimation of PM2.5.
• The estimation of AOD relies on the amount of light that gets attenuated due to particulate matter or aerosols.
• A higher concentration of particulates results in a greater absorption of light and correspondingly, a higher AOD.
• It is measured by remote sensing using satellites.
• AOD values can span from 0 to 1.
  • A reading of 0 means the sky is clear with maximum visibility, while a reading of 1 suggests heavily fogged conditions.

AOD < 0.3: Considered to be in the green zone, which is safe,

0.3 < AOD < 0.4: Blue zone, denoting less vulnerability,

0.4 < AOD < 0.5: Orange zone, indicating vulnerability, and

AOD > 0.5: Red zone, which indicates high vulnerability.

conclusion

erosols play various roles in our environment and climate system. They can influence climate by scattering and absorbing sunlight and by modifying the properties and lifetimes of clouds. Additionally, aerosols affect air quality and have health implications for living organisms. They can serve as carriers for pollutants, including microbes and toxins, which can lead to respiratory and other health issues upon inhalation.

In terms of practical applications, aerosols are widely used in household and industrial products such as spray paints, insecticides, and aerosol propellants. Understanding the behavior of aerosols is crucial for addressing environmental and public health issues, and for developing strategies to mitigate their adverse impacts on climate and health.

Ref: Source-1

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