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Centre for Atmospheric Science


What are Aerosols?

Aerosols are tiny solid or liquid particles suspended in the atmosphere.  These particles range in size from less than 0.01μm to greater then 10μm, and have significant impacts upon climate, weather and air quality.  While there has been much work on the effects of greenhouse gases on global climate, the effects of aerosols are much less fully understood.

There are many different types of atmospheric aerosol, such as sulphate, nitrate, organics, black carbon (soot), sea salt and mineral dust.  These all have different chemical and physical properties, which are influenced by their origin, method of formation and subsequent transformations they undergo.  It is these properties and processes which control the impacts of aerosols in the atmosphere.


Aerosol Sources and Sinks

There are many different sources of atmospheric aerosol; some are natural, while others are man-made.  Natural sources include volcanic dust, sea salt sprayed by breaking waves and smoke from wildfires.  Anthropogenic sources include vehicle emissions, industrial processes and `slash and burn' deforestation.  Urban areas tend to be particularly affected by aerosol pollution, as can be seen in smog clouds in cities such as Los Angeles or Beijing.

The length of time aerosols spend in the atmosphere before being removed is called the `residence time'.  Typical residence times are days or weeks. In comparison, greenhouse gases can have residence times of many years.  This is significant because any major change in aerosol emissions would have a much more immediate effect than a corresponding change in greenhouse gases.

Processes that remove aerosols from the atmosphere are divided into two categories - wet and dry deposition.  Dry deposition is the removal of particles by gravitational settling, and is more important for larger particles. Smaller aerosols tend to undergo wet deposition, which involves the incorporation of aerosols into clouds, and transport to the ground by precipitation.


Direct and Indirect Effects

Aerosols create a perturbation on Earth’s radiative balance through one of two ways.  The first of these is known as the direct effect, and relates to the alteration of radiative fluxes by aerosol particles.  The nature of the forcing induced by the presence of these particles in the atmosphere is determined by their physical and chemical composition and subsequent optical properties.  Black carbon predominantly absorbs both incoming solar radiation and transmitted terrestrial radiation, producing a warming effect within the atmosphere.  Other aerosol species, such as organics and sulphates, scatter or reflect incoming solar radiation, reducing the amount which is incident at the Earth’s surface and consequently cooling the Earth system.  However, the interaction of different species can affect the radiative properties of aerosols, and ultimately the direction of the forcing they produce.

The second method by which aerosols influence radiative forcing in the atmosphere is known as the indirect effect. This relates to the role of aerosols as cloud condensation nuclei (CCN), and their influence on cloud microphysical properties. CCN provide surfaces onto which water can condense, promoting the formation of cloud droplets. As well as increasing the total number of cloud droplets, the greater availability of condensation nuclei also results in the production of smaller droplets, increasing the albedo of clouds. There are also implications for the lifetime of clouds, with smaller droplets taking longer to coalesce to form droplets large enough to precipitate. This reduces the rate at which droplets are lost from clouds, making clouds more persistent and extending the duration of their radiative effects.


The Role of Aerosol Research

The exact extent of the climatic effects of atmospheric aerosols is currently subject to considerable uncertainty, and as such is one of the most poorly quantified areas of climate science.  More precise constraint of aerosol radiative forcings therefore represents one of the key challenges in climate research.  Any assessment of the effects of aerosols on a regional or global scale first requires a highly developed understanding of the underlying processes.  It is this level of research upon which the work of the Centre for Atmospheric Science at Manchester is primarily focused.  Measurements of aerosol chemical and physical properties are obtained through a wide range of field and laboratory experiments, and from process-based modelling activities. These are used to inform and develop investigations of aerosol impacts on a wider scale.  Ultimately, an improved understanding of atmospheric aerosols and their influence on climate will assist the development of effective mitigation strategies for climate change, whilst enabling improvement of urban air quality and a reduction in the detrimental effects on human health.