Importance of atmospheric aerosols

Heterogeneous chemistry

Aerosols serve as media upon which chemical reactions can occur.
Example: Volcanic stratospheric aerosols.
(i) Volcanoes inject gaseous SO2 and HCl directly into the stratosphere.
(ii) Because of its large solubility, HCl is rapidly removed by liquid water.
(iii) SO2 remains and is converted to H2SO4 aerosol (typically, an aqueous sulfuric acid solution of 60 to 80%).
    Thus, volcanic eruptions lead to an increase in the amount of the stratospheric aerosol => increase in the available aerosol surface area concentrations => make the chlorine present more effective at ozone depletion, even if no increases in chlorine are occurring.

Air quality and human health

Visibility reduction

Aerosol particles can scatter or/and absorb electromagnetic radiation at different wavelengths.

Acid deposition

    It is caused by emissions of sulphur dioxide and nitrogen oxides. These primary air pollutants arise from the use of coal in the production of electricity, from base-metal smelting, and from fuel combustion in vehicles. Once released into the atmosphere, they can be converted chemically into such secondary pollutants as nitric acid and sulfuric acid, both of which dissolve easily in water. The resulting acidic water droplets can be carried long distances by prevailing winds, returning to Earth as acid rain, snow, or fog.

Cloud formation

    Aerosols are also believed to have an "indirect" effect on climate by changing the properties of clouds. Indeed, if there were no aerosols in the atmosphere, there would be no clouds. It is very difficult to form cloud droplets without small aerosol particles acting as "seeds" to start the formation of cloud droplets. As aerosol concentration increases within a cloud, the water in the cloud gets spread over many more particles, each of which is correspondingly smaller. Smaller particles fall more slowly in the atmosphere and decrease the amount of rainfall. In this way, changing aerosols in the atmosphere can change the frequency of cloud occurrence, cloud thickness, and rainfall amounts.
Climate and climate change
Since aerosols can scatter, absorb and emit electromagnetic radiation and serve as cloud nuclei, they can influence global temperature, rainfall and dynamical circulations.

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