Aerosols are everywhere around us, ranging in size from the width of a virus up to the diameter of a human hair [1]. And although we cannot see them, these tiny particles can play havoc with both our health and the climate.
Aerosols are tiny particles or droplets found throughout the Earth’s atmosphere, and are often composed of sulfate, nitrate, ammonium, or sea salt. Aerosols are classed in terms of their size, with different terms used by different scientific fields. They range in size from nanometers to micrometers, and are most commonly referred to as particulate matter e.g PM2.5 or PM10, depending on their size.
The main natural sources of aerosols are volcanic eruptions, the sea, soils, wild animals and desertification. Aerosols are predominantly formed through natural processes, while roughly 10% [2] are produced by anthropogenic sources such as fossil fuel burning, land use changes, biomass burning, fires, aircraft and ship emissions and increasing numbers of domesticated animals.
Natural aerosols are spread over both land and ocean, while man made particles such as sulphate aerosols are often found in regions downwind of industrialised areas in the Northern Hemisphere, meaning that aerosol influence on the climate can be ‘…highly variable in space and time’ [3], due to winds. Particle cover over developed countries has started to decrease in recent years due to cleaner industrial processes, but there has been a steady increase across Asia, especially China, which has lead to worsening air conditions.
Aerosols impact the climate in two ways: (i) directly, by scattering and absorbing radiation in the atmosphere, and (ii) indirectly by acting as cloud condensing nuclei and so changing the microphysical structure of clouds [4]. Aerosols can influence both incoming and outgoing radiation, though they are most effective at scattering incoming radiation [5], meaning less energy will reach and warm the Earth’s atmosphere, resulting in a global cooling effect. Some aerosols also absorb light rather than just reflecting it, which warms the surrounding atmosphere, but shades and cools the surface below.
Aerosol particles also act as cloud condensing nuclei, which can effect the lifetime and composition of clouds. An increase in cloud cover will lead to more of the incoming radiation being reflected back into space, and therefore not reaching or warming the earth’s surface. A high concentration of particulate matter in the atmosphere will lead to brighter, denser clouds. These clouds are less likely to precipitate out, and will reflect more radiation than clouds formed when there are low concentrations of aerosols.
Due to their direct and indirect effect on the Earth’s climate, the negative radiative forcing (a change in the radiation balance due to the instantaneous release of a certain quantity of a radiatively active greenhouse gas – assuming that no other components of the climate system are affected) by aerosols is to create a cooling effect. Much research has been done over the years to try and identify whether or not aerosols mask the effects of global warming. If aerosols particles such as sulphites were reduced in the future, would the decrease in their cooling effects lead to an even more rapid increase in global temperatures than we are seeing at present? Despite decades of research, the potential magnitude of aerosol cooling is hard to quantify at present due to many uncertainties such as future emissions. The Intergovernmental Panel on Climate Change (IPCC) have attempted to try and quantify it somewhat, and came to the conclusion that although these microscopic particles do cause a cooling effect, they do not completely offset global warming [6].
While anthropogenic aerosol concentrations have decreased in developed regions, the rapid increase in industrialisation in developing countries such as China has meant that global aerosol concentrations have remained constant. The cooling effect that these particles create may have in some way acted to mask the effects of global warming, but with so many uncertainties in future emissions worldwide, the part that aerosols will play in the future of climate change remains to be seen.
References
[1] Voiland, A. 2010. Aerosols: Tiny Particles, Big Impact. NASA Earth Observatory. http://earthobservatory.nasa.gov/Features/Aerosols/
[2] As above.
[3] Andreae, M.O. 1996. ‘Raising dust in the greenhouse.’ Nature 380: 389-390
[4] Penner, J.E., 2000. ‘Aerosols, Effects on Climate.’ In Encyclopedia of Global Environmental Change, edited by M.C. MacCracken and J.S. Perry, 162-167. John Wiley and Sons.
[5] Andreae, M.O. 1996. ‘Raising dust in the greenhouse.’ Nature 380: 389-390
[6] Haywood, J. 2013. Climate Change and Aerosols. The Met Office. http://www.metoffice.gov.uk/climate-change/guide/science/explained/aerosols
