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Atmospheric Aerosols: Small Particles, Big Impact

Atmospheric Aerosols: Small Particles, Big Impact

When we breathe, even if the atmosphere looks clear we are inhaling millions of aerosols. As we all have studied in our science classes that the air is composed of a mixture of gases including NO2, O2, Ar, CO2, H2Oand several others. In addition to gases, the atmosphere also contains tiny liquid droplets and solid particles. These suspended solid and liquid particles are known as atmospheric aerosols. Aerosols are highly variable in their concentration, composition, and particle size distribution on a temporal and spatial scale.

These are so small in size that usually invisible to the naked human eyes. People who monitor air quality and meteorologists call them particulate and measure as PM2.5 and PM10 depending on their aerodynamic size while climatologists generally studied them on the basis of chemical components such as nitrate, sulfates, black carbon, etc. Aerosol can generate from both anthropogenic as well as natural sources. Aerosol generated from anthropogenic sources such as fossil fuel burning and biomass burning is smaller in size as compared to natural aerosols such as dust particles, sea sulfates, etc.

Despite their less concentration and small size, they have a significant impact on human health, earth’s radiative budget, cloud formation and properties, climate, and atmospheric visibility. This also affects the photolysis rates of important processes in the atmosphere and satellite observation on trace gases.

Aerosols and climate

Aerosols affect the climate directly and indirectly by interacting with the incoming and outgoing solar radiations. Most of the aerosol scatter the incoming solar radiation and causing the cooling effect while some can absorb the radiation and contribute to global warming. Black and brown carbon causes global warming by absorbing radiations while sea sulfates and dust particles masking the warming effect by scattering solar radiation. The scientific community believes that aerosols have a net cooling effect that has counteracted the global warming effect caused by the increase in the concentration of greenhouse gases since industrialization. Despite great progress in the modeling and science, accurate estimation of the aerosol’s effects on climate is highly uncertain due to variability in concentration and composition of the aerosol.

Aerosol-cloud interactions

Clouds play a key role in Earth’s radiation budget, and aerosols serve as the seeds upon which cloud droplets form which are known as cloud condensation nuclei (CCN) and ice nucleating particles (INP). An increased amount of aerosols may increase the CCN concentration and lead to more, but smaller, cloud droplets that affect the cloud lifetime and albedo. If absorbing type aerosols are present they can modify the cloud properties by heating the surrounding air which enhances the evaporation and results in the reduction in cloud cover. Thus the net effect of aerosol on clouds in uncertain.

Earth’s Albedo and aerosol

Aerosols’ have the ability to change the albedo of the planet depending on their color. Bright surfaces like ice reflect radiation and lead to a cooling effect on the climate, while dark surfaces like the ocean absorb the radiation and warm the climate. Aerosols like black or brown carbon can deposit as a layer of dark residue on ice, which reduces the albedo and enhance the rate of melting by absorbing radiation.

Aerosols and health effects

Atmospheric aerosols are one of the major air pollutants leading to millions of premature death every year. Health effects of aerosols’ depending upon the particle size and their chemical composition.  Smaller the particle size and worst will be the health effects because fine particles can enter and deposits into the respiratory tract and cause various kinds of health effects while some can pass into the bloodstream.

As we have discussed aerosol plays a crucial role in several climatic processes and they have great uncertainty in there composition and concentration over time and space. Therefore, to improve the understanding of climate change and the effects of aerosols on the environment, we need to reduce the uncertainties in aerosols concentration and composition. Only newer instruments and the more sophisticated modeling can make such measurements possible which will provide the critical information that scientists need to fully integrate aerosol impacts into climate.

As we know the air quality of urban areas is seriously affected by the high concentration of aerosols. In most Indian metro cities PM10 and PM2.5 concentrations are higher than the National Ambient Air Quality Standards (NAAQS) prescribed limits. Which is harmful to people’s health in many ways. Therefore, it is the responsibility of us all to keep the air clean.

Dr. Vikram Mor
Assistant Professor
Department of Environmental Science
Faculty of Science, SGTU