Aerosols, small particles suspended in air with a lifetime of at least minutes, are either emitted as primary aerosols (dust or particle emissions of diesel cars) or formed by the conversion of sulfur dioxide, nitrogen oxides, ammonia and organic compounds in atmospheric chemical reactions to sulfates, nitrates and ammonium compounds, and non-volatile organics (secondary aerosol). The total mass of aerosols per unit of volume is called "Particulate Matter." PM-10 is the mass of aerosol particles with a diameter of 10 micrometers or smaller. PM-2.5 is the mass of aerosol particles with a diameter of 2.5 micrometers or smaller.
Particles with an aerodynamic diameter below 0.05 micrometers disappear quickly because they diffuse to other particles and are lost by agglomeration. Particles greater than 15 micrometer in diameter are relatively quickly deposited by gravitational settling. Particles between 0.1 and 2 micrometer live quite long, up to 1 or 2 weeks. Particles of this size act as nuclei for the formation of cloud droplets and this process is their main sink. An overview of particles per cubic centimeter, mass per cubic meter, and lifetime in seconds of aerosols as function of size in polluted areas is given in Figure 1.
The large number of particles with a diameter below 0.1?m is caused by combustion processes including vehicle emissions and secondary particle formation; all Otto and Diesel engines cause large amounts of emissions of very small particles with very little mass per particle and short lifetimes as they coagulate very quickly. The total mass of these very small particles represents a small fraction of ambient aerosol. Aerosol particles between 0.2 and 2?m represent a large fraction of the ambient aerosol even though their number is much smaller than the small particles. The mass per particle is much higher than the very small ones and their lifetime is relatively long, so most of the mass accumulates in that size range. Large particles are characterized by low number, relative large mass per particle, and short life times; they “fall out” of the atmosphere. Large particles represent generally only a small fraction of aerosols, except in conditions such as dust and sand storms.
It has become clear that aerosols have an important role in most air pollution problems, ranging from very local effects and human health problems, see Impact of local air pollution, to regional problems, such as acid deposition and eutrophication, to continental and global problems, such as stratospheric ozone loss and climatic change. An overview of all aerosol-related air pollution problems is given in Figure 2.
Epidemiological research has made it clear that aerosols have a large impact on human health. Aerosols increase the mortality rate due to cardiopulmonary diseases (heart and lung diseases including asthma and bronchitis). Deposition of ammonium, sulfate, and nitrate in aerosols contribute significantly to acid deposition and eutrophication (damage to ecosystems due to a large input of nutrients such as nitrate). This contribution can be 30% or more in Western Europe.
Heterogeneous reactions on the surface of aerosols most likely contribute significantly to ozone formation in the troposphere while reactions on the surface of aerosols in the stratosphere are a main factor in the destruction of stratospheric ozone by CFC’s (chlorofluorocarbon compounds).
Aerosols have a large impact on the radiative balance of the Earth and subsequently on climatic change through the scattering of incoming sunlight, cloud formation, or enhanced absorption of sunlight by soot.
This complex situation means that any measures taken to influence the concentration of ambient aerosols will have an impact on many processes simultaneously. For instance, an abatement policy to reduce the impact of aerosols on human health, a local issue, will have consequences for the regional radiative balance and hence on regional climatic change. Local air quality and regional climatic change are connected by a common factor, aerosols.
Aerosols are either emitted as particles in the atmosphere (primary aerosol), or formed as secondary products of atmospheric reactions (secondary aerosol). For more information, see Air pollution emissions. Despite uncertainty, estimates indicate that natural sources are much more important on a global scale. The local situation can, on the other hand, be dominated by anthropogenic (human-made) sources. Dust and sand storms can introduce large masses of particles in the atmosphere. Other natural sources include the sea as seasalt particles are formed by evaporation of small seawater droplets and emissions from volcanoes.
The detection of sulfate in ice cores in Greenland (obtained by drilling holes in ice and analyzing the sulfur content) indicates that particle transport is indeed possible over several thousand kilometers. Measurements of ice sulfate from Antarctica have found that no anthropogenic sulfate from the northern hemisphere is deposited in Antarctica; only sulfate from large volcano eruptions is found. This proves that particles are not transported on a global scale.
Transport is dependent on the losses by deposition and cloud formation. Dry deposition is, in general, quite small for particles where most of the mass is found (around 1 micrometer, see Figure 1). Cloud droplets are also formed around aerosol particles; in general, the loss of particles due to the formation of clouds is in many areas the limiting factor for transport.
Impact on Human Health
For information on the impact of aerosols on human health, see Impact of local air pollution.
Impact on Visibility
Reduction of visibility due to aerosols is a big problem in many parts of the world. For more information, see Impact of local air pollution.
Impact on Acid Deposition
Acid compounds and ammonium in aerosol contribute to acid deposition. In fairly clean "background" areas, gas concentrations are low and aerosol concentrations are relative higher. The result is that the contribution of aerosols is relatively large in these areas. Aerosols can, on the other hand, contain basic substances like carbonate and reduce the impact of acid deposition.
Impact on Destruction of Stratospheric Ozone
Radiative Forcing and Impact on Climatic Change
Direct Aerosol Effect
Aerosol particles in the atmosphere reflect radiation differently depending on their size distribution. The size of the particles determine whether shorter-wave radiation is reflected more effectively compared to infrared. Two types of scattering, “Mie" and "Rayleigh" scattering are observed. Rayleigh scattering is scattering to all directions and is caused by all molecules and particles in the atmosphere. If the wavelength of the incoming light and the size of the particle are about the same, Mie scattering occurs and some of the light will be scattered back in the direction from which the light came (back scattering) (see Figure 3).
Mie scattering is much more intense than Rayleigh scattering. Incoming solar radiation (where most energy is present at wavelengths between 0.4 and 1 micrometer) is effectively scattered or reflected by particles in the size range of 0.1 to 2 micrometer. Particles of this size do not intercept the outgoing infrared radiation of the Earth. Particles much smaller than the wavelength of light have little influence. Very large particles (unless they are colored and absorb light) also have minimal impact. Beyond a critical angle, light will not be diffracted, but will rather be reflected. This phenomenon leads to strong back-reflection of light, especially if aerosols consist of liquid droplets of solution, as is often the case. If aerosols consist of soot or other light-absorbing materials, then light is directly absorbed which leads to heating.
Indirect Aerosol Effect
Cloud formation is dependent on aerosols. If no aerosols are present, large super-saturation (relative humidity over 100%) can be observed without droplet formation. But small particles, in different concentrations, are present everywhere in the atmosphere. Cloud droplets condense on the aerosols. If few particles (less than 200 per cm3) are available as cloud condensation nuclei, large droplets are formed. If a large number of aerosols is present, smaller droplets are formed.
Clouds with large droplets reflect sunlight less effectively compared to clouds with small droplets. Clouds effectively reflect solar radiation and low clouds contribute particularly to a cooling effect. However, high, wispy cirrus clouds, that can have relatively large droplets, do not reflect solar light very effectively, but will almost completely reflect longwave infrared radiation.
The reduction of cloud droplet size with increasing number of aerosol particles is not linear. It is quite important at low particle numbers (under clean conditions, 100 particles per cm3) but increasing particle number over 1,000 particles per cm3 no longer has any effect (see Figure 4).
Impact of Direct and Indirect Aerosol Effect
The Intergovernmental Panel on Climate Change (IPCC) estimates the total impact of aerosols to be about 30% of the forcing function of greenhouse gases. This is expressed as changes in the balance of incoming and outgoing radiation in watt·m-2 (watt per square meter).
According to the IPCC, the warming effect of all greenhouse gases together is 2.5 watt·m-2, while the cooling effect of aerosols would be 0.7 watt·m-2. New calculations and models indicate that the cooling effect could be quite a bit larger, on the order of 1.5 watt·m-2. Regionally, it could even be much larger than the warming effects of greenhouse gases, cooling up to 5 watt·m-2. So, some areas (the Netherlands and northern Italy for instance) have experienced cooling rather than warming during industrial development.
On the other hand, concentrations of black carbon (soot) in East Asia can be so high, that heating due to the absorption of incoming solar light can more than offset the cooling by reflection of solar light. The net result is that the atmosphere is heated.
These new insights have quite an impact on the predictions of climate change.