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Aerosols
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Aerosols are liquid or solid particles suspended in a gas that range in size from nanometers to micrometers in diameter.1 Aerosols can have natural or anthropogenic sources. Natural sources of aerosols include desert dust, volcanic dust, pollen, sea spray, and soot, while anthropogenic sources include transportation, slash-and-burn agriculture, industrial activities etc. Aerosol particles are also classified as primary or secondary aerosols where primary aerosols are said to be the ones that emitted directly and secondary aerosols are those that are formed by gas-to-particle conversion. Concentration and composition of aerosol particles is highly variable and they remain in atmosphere for very short period of time. Averaged over the globe, aerosols made by human activities currently account for about 10 percent of the total amount of aerosols in our atmosphere. They are said to cause local as well as global impacts, including on climate change. Reduction in atmospheric visibility is often attributed to aerosols and it is well-known that aerosols scatter light and hence, also cause reduction in the amount of light that reaches the earth.
What are aerosols?
Credit: You Tube
Health repercussions
At a local level, aerosols are known to cause a variety of health problems. Emissions of aerosols are known to cause direct and immediate negative health impacts to people in the surrounding areas2 . Since they are small in size, they tend to deposit in the airways and alveoli in human lungs. High concentrations of aerosols have been associated with aggravated asthma, bronchitis, decreased lung function, breathing aberrations, allergic reactions etc on the basis of epidemiological as well as toxicological studies.3 However, the causal chains of how aerosols cause these health problems are not yet properly understood.
Aerosols and climate change
The Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report has concluded that the effect of aerosols on climate is approximately 20% of the effect of greenhouse gases since the industrial era4 . It is believed that the aerosols are the reason why Earth is still at about 380 ppm CO2-equivalent.5 Some aerosols have a cooling effect on the global climate. Aerosol particles are known to modify the energy balance6 and thus, they can cause cooling of the atmosphere in two ways; directly, under clear sky conditions through reflection or scattering of incoming radiation or indirectly by increasing cloud reflectivity7 8 . They cause a cooling effect on the surface just below them. Thus, it is expected that aerosols can actually help offset the global warming phenomena, though only partially. This is due to the fact that aerosol particles can effectively scatter/ absorb light and hence, bring about a change in the temperature. Also, they can alter the cloud size and cloud number, thereby altering the atmospheric temperature. However, on the flip side, there have studies that have concluded that aerosol clouds over large regions of Asia actually cause as much warming as GHGs cause which contradicts the understanding that aerosols have a cooling effect on climate.9 Thus, there is a need for more focused research on how exactly aerosols are contributing to the climate change phenomenon.
Footnotes
1. Pöschl, U., 2005. Atmospheric aerosols: Composition, Transformation, Climate and Health Effects. Angewandte Chemie International Edition, vol.44, p. 7520-7540.
2. Hausfather, Z., 2008. Why reducing sulfate aerosol emissions complicates efforts to moderate climate change. [Online]
3. Pöschl, U., 2005. Atmospheric aerosols: Composition, Transformation, Climate and Health Effects. Angewandte Chemie International Edition, vol.44, p. 7520-7540.
4. Pilewskie, P., 2007. Climate change: Aerosols heat up. [Online]
5. Hausfather, Z., 2008. Why reducing sulfate aerosol emissions complicates efforts to moderate climate change. [Online]
6. Wyslouzil, B., 2007. Aerosols and global warming. [Online]
7. Hausfather, Z., 2008. Why reducing sulfate aerosol emissions complicates efforts to moderate climate change. [Online]
8. Illman, D.L., n.d. Sulfate aerosol and global warming. [Online]
9. Pilewskie, P., 2007. Climate change: Aerosols heat up. [Online]
Resources
- Victoria Gill, How Aerosols Mask Climate Change, BBC News (online version), 19 Jun 2009. Accessed 19 Jun 2009. A new study published in the journal Science by Norwegian scientist Gunnar Myhre (Center for International Climate and Environmental Research, Oslo) shows aerosol particles' ability to scatter and reflect the sun's rays has less of a cooling effect on the planet (and less of a "masking" effect of climate change) than previously estimated.
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