Aerosols
Aerosols are a general name, defined as any solid or liquid particles suspended in the air, with a typical size between a few nanometers (10^-9 meter) to about 10 micrometers (10^-6 meter). Therefore, aerosols are usually associated with complex chemical compositions and morphology (i.e. sizes and shapes).
Aerosols of different sizes form from different pathways (Figure 7.12, above):
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Smallest aerosols can be formed from nucleation and condensation of supersaturated low volatile gases (e.g. sulfate aerosols);
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Small aerosol particles can further grow through ‘coalescence’ (i.e. combinations of smaller particles through collisions) into larger particles. Sometimes, aerosols can undergo chemical reactions and form secondary aerosol particles (e.g. secondary organic aerosols, SOA);
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Some very large aerosol particles are formed directly by mechanical processes (e.g. dust and sea salt aerosols).
Due to their different sizes, various aerosols are also removed from the atmosphere through various pathways: large aerosols can be soon removed via gravitational (or dry) deposition; small aerosols are mainly removed through wet deposition (i.e. ‘washed’ out by wet precipitation).
Aerosols influence the climate through their direct radiative effects and their interactions with radiation and clouds (Figure 7.12, above). Now we know that aerosols are in nature a mixture of various kinds of inorganic or organic compounds. The measurements of their global distributions rely mainly on remote-sensing observations, both satellite-based (e.g. CALIOP, MODIS, MISR, OMI) and ground-based (e.g. AERONET). The radiative effects of aerosols on climate are mainly evaluated using climate models. In order to reduce some complexity, aerosols are separated into seven components in CMIP experiments – namely sulfate, black carbon (BC) from fossil fuel and biofuel, organic aerosol (OA) from fossil fuel and biofuel, BC an OA combined from biomass burning (BB), nitrate, secondary organic aerosol (SOA) and mineral dust (or sea salt). In Chapter 7 of IPCC AR5, the following aspects of aerosol studies are assessed: