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Aerosol Particle Types and Characteristics

General Background

Particles larger than about 0.1 micrometer in diameter scatter most of their incident solar radiation in the forward direction, and less back to space. The larger the particle, the greater percentage of the light is scattered forward. The larger particles thus contribute less to tropospheric cooling, as less light is scattered back to space. Some types of particles, especially black carbon, are primarily absorbers while other aerosols, especially sulfates and nitrates, are primarily scatterers. Cloud condensation nuclei (CCN) are particles that can serve as condensation points for water vapor, and thus lead to the formation of clouds which reflect sunlight. The following material is drawn mostly from Chapter 7 of the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC, 2013). See especially Table 7.2 an Figure 7.18.

Black Carbon

Black carbon and other absorbers of incoming solar radiation act to warm the troposphere. These particles can also deposit on snow and ice, causing absorption of incoming solar radiation, and re-radiation as heat, thereby melting the snow/ice, leading to reduced albedo and further tropospheric warming. Some additional material on black carbon is available from the United States Environmental Protection Agency. For a comprehensive summary of black carbon in the climate system, see Bond et al. 2013.

Biomass Burning Aerosols

A variety of particle types, including black carbon, organic aerosols, sulfates and nitrates, are subsumed in this category. These particles arise from fires, some of which are natural and some are anthropogenic. Drought leads to increased effects of both kinds, so that a climate effect can affect climate forcing. In general these particles contribute to cooling the troposphere via scattering, or by becoming CCN. However some of these particles (primarily black carbon) absorb incoming solar radiation, thereby contributing to tropospheric warming.

Mineral Dust

Mineral dust can absorb and/or scatter incoming solar radiation. The scattering seems to be dominant, so that the net effect on the troposphere is cooling. Especially when the dust is blown out over an ocean so the reflection far exceeds the background albedo of the underlying ocean surface.

Sea Salt

These particles are introduced to the atmosphere via breaking of air bubbles in sea spray and by wind erosion of deposited salt particles. They are particularly effective CCN and their effect is cooling of the troposphere.

Organic Aerosols

Organic aerosols are a complex mixture of chemical compounds produced from fossil fuel and biofuel burning and natural biogenic emissions. Organic aerosols are emitted as primary aerosol particles or formed as secondary aerosol particles from condensation of organic gases considered semi-volatile or having low volatility. Although organic aerosols probably scatter more light than they absorb (cooling effect), they can lead to the formation of ozone which is a greenhouse gas (warming effect).

Sulfate/Nitrate Aerosols

These strongly scattering particles enhance tropospheric cooling. They may also interact with naturally occurring sea-salt particles.

Volcanic Aerosols

These aerosols are a mix of absorbing and scattering particles, with the absorbers usually being large enough to fall out quickly so that the smaller ones dominate. Sulfate aerosols are common and the scattering properties outweigh the absorbing properties to produce a net cooling effect on the troposphere. Particles from volcanoes can also be cloud condensation nuclei, enhancing their cooling effect.

Cloud Condensation Nucleii

While listed last here, this may be the most important category. Many are naturally occurring but some anthropogenic aerosols may also act as condensation points for water vapor. This may lead to the condensation of a given amount of water on more nuclei, smaller droplets, and longer-lived clouds as the smaller droplets will not fall out as fast as the larger ones. In all cases CCN lead to tropospheric cooling that may be as important as all other aerosol effects combined (IPCC, 2013).

References

  • Bond, T.C. et al. 2013. Bounding the role of black carbon in the climate system: A scientific assessment. J. Geophys. Res. - Atmospheres 118 (11) 5380-5552. DOI: 10.1002/jgrd.50171.
  • IPCC (Intergovernmental Panel on Climate Change), 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T. F., D. Qin, G.-K. Plattner, M. Tignor, S. K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P. M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. 1535 pp.