Aerosols and Environmental Pollution: A Rising Concern

Introduction:

An aerosol refers to a mixture of small liquid or solid particles suspended in a gas, such as air. It comes from the prefix ‘aero’, meaning air, and the suffix ‘sol’, which is sort of for solution. Hence, the term refers to an airborne solution. Aerosols can be natural or anthropogenic. Natural aerosols include fog, dust, forest vapor, and geyser steam. Anthropogenic aerosols, on the other hand, include haze, particulate air pollutants, and smoke. Aerosol particles can be as small as 0.001 micrometers and as large as 100 to 200 micrometers.

What Are the Examples of Aerosols?

There are various examples of aerosols that affect people’s lives every day, such as -

• Volcanic Ash - Ash can be delivered into the air by volcanic eruptions, and it has a significant impact on the global climate, producing a net cooling effect.

• Pollen - The pollen produced by plants is also a natural aerosol that enables genetic recombination. It also produces allergies in humans.

• Sea Salt - The spray from the sea ocean waves that impact the shoreline can suspend fine salt particles in the air.

• Sulfates - Coal-fired power plants are also a major source of sulfates. These artificial aerosols pollute the air, but ironically, they have a cooling effect on the earth’s climate.

What Are the Types of Aerosols?

1. Primary Aerosols - Primary aerosols are present in our atmosphere directly from short natural sources. For example, smoke and soot from wildlife and desert dust.

2. Secondary Aerosol - Secondary aerosols arise when gaseous substances are converted into particulate matter in the atmosphere. For example, sulfur dioxide gas is emitted from a volcanic eruption.

What Are the Effects of Aerosols?

1. Direct Effect - In this type, the aerosols scatter and absorb incoming solar radiation. The result of this would primarily be a cooling of the surface, as the scattered solar radiation is reflected back into space. Another effect of it may be an increase in surface temperature, which can occur due to the absorption of incoming solar energy.

2. Indirect Effect - In indirect effect, the aerosols influence the formation of clouds. They increase the number of cloud droplets and make clouds more reflective. These substances prolong the lifespans of massive storm clouds by postponing precipitation, resulting in the growth and persistence of clouds and ultimately causing more intense storms.

3. Other Effects - Under conditions when the aerosols absorb the pollutants, it promotes the deposition of these pollutants on the earth’s surface and other water bodies. This has the potential to damage both the environment and human health. Aerosol particles having a diameter that is less than 10 micrometers can penetrate into the bronchi. Aerosol particles that have an effective diameter of less than 2.5 micrometers can reach the gas exchange region in the lungs, posing a potential risk to human health.

What Are the Impacts of Aerosols on the Environment and Human Health?

Climate:

• Both natural and anthropogenic aerosols have the potential to change the global radiation balance.

• Aerosols can influence the atmosphere in two ways, direct and indirect effects. Direct effects mean scattering and absorption of radiation with influence on the climatic system and planetary albedo (fraction of the incoming solar energy scattered by Earth back to space). Indirect effects mean an increase in the available cloud condensation nuclei due to an increased anthropogenic aerosol concentration.

• The role of anthropogenic aerosol is still a major uncertain factor in understanding and quantifying both the present and future global climate change. In order to quantify the impact of aerosols on climate and assess the feedback of climate change on aerosols, it needs a thorough understanding of the physicochemical processes on the microscale level and aerosol evolution in the context of regional-scale and global-scale circulation.

• The elevated aerosol concentrations shift the cloud’s droplet size distribution to a higher number of densities with smaller effective radii, which results in more side-scattering and back-scattering in comparison to forward-scattering. This leads to an increase in cloud albedo (indirect effect).

Health:

• Various documents claim that ultrafine particles are important as they are present in the atmosphere in huge numbers compared to their mass and because they can penetrate into the bloodstream causing systemic effects.

• Recent cohort studies have shown significant effects of PM levels on mortality and morbidity. Long-term average mortality rates were 17 to 26 percent higher for people living in communities with elevated levels of PM 2.5.

• The Health Effects Institute US-wide National Morbidity, Mortality and Air conducted a study that showed a 0.5 percent increase in total non-accidental mortality associated with a 10 microgram per meter cube increase in the 90 largest United States cities where daily average PM10 ranged from 15 to 53 microgram per meter cube.

How Can Aerosol Levels Be Reduced?

The exhaust pipes of diesel vehicles are a major source of soot in many countries, and the installation of filters can aid in reducing the amount of soot emitted into the air. Power plants in the United States and Europe have effectively reduced emissions of sulfate aerosols, produced by sulfur dioxide, by using scrubbers. Similar solutions can be adopted by other countries to address this issue. By reducing the aerosol (soot) emissions, the climate changes due to aerosols can be reduced, and the emissions of greenhouse gasses such as carbon dioxide (CO2) should also be reduced in parallel. The longevity of CO2 in the atmosphere can range from decades to centuries, which means that even if measures are enacted to reduce its concentration, it will take a considerable amount of time for any noticeable impact to be observed. However, aerosols have much shorter lifetimes, and if actions are taken to reduce soot emissions, it will enhance the global warming effect of carbon dioxide by 20 to 50 percent, and the climate impacts will be felt more rapidly.

Conclusion:

The dynamics of particulate matter in the atmosphere involve numerous physical and chemical processes and different time scales. Aerosol dynamics encompass particles of varying sizes, ranging from a few nanometers to several hundred micrometers. Aerosols come from natural and anthropogenic sources, and they are a mixture of primary emissions and secondary species. There is an urgent need to provide the policy-making community in different countries with the characteristics of particulate matter. The quality assurance of PM 10 and PM 2.5 monitoring data should be improved, and the different measurement methodologies used should be assessed.