Aerosol
Aerosols are solid or liquid particles dispersed in the air, and include dust, soot, sea salt crystals, spores, bacteria, viruses and a plethora of other microscopic particles. Collectively, they are often regarded as air pollution, but many of the aerosols have a natural origin.
Aerosols are conventionally defined as those particles suspended in air having diameters in the region of 0.001 to 10 microns (millionth of a metre). They are formed by the dispersal of material at the surface (primary aerosols), or by reaction of gases in the atmosphere (secondary aerosols).
- Primary aerosols include volcanic dust, organic materials from biomass burning, soot from combustion and mineral dust from wind-blown processes.
- Secondary aerosols include sulphates from the oxidation of sulphur-containing gases during the burning of fossil fuels, nitrates from gaseous nitrogen species, and products from the oxidation of volatile organic compounds (VOCs).
Although making up only 1 part in a billion of the mass of the atmosphere, they have the potential to significantly influence the amount of sunlight that reaches the Earth’s surface, and therefore the Earth's climate.
Although the abundance of aerosols varies over short time scales, for example after a volcanic eruption, over the long term the atmosphere is naturally cleansed through mixing processes and rainfall. Cleansing is never complete however, and there exists a natural background level of aerosols in the atmosphere.
The average time spent in the atmosphere by aerosols is dependent upon their physical and chemical characteristics, and the time and location of their release. Natural sources of aerosols are probably 4 to 5 times larger than man-made ones on a global scale, but regional variations of man-made aerosol emissions may change this ratio significantly in certain areas, particularly in the industrialised Northern Hemisphere. At certain times of the year, the natural background level of aerosols may increase, for example, during the growing season, when large quantities of pollen are released into the atmosphere.
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[edit] Overview
Aerosol are present throughout the boundary layer, at number concentrations depending upon factors such as location, atmospheric conditions, annual and diurnal cycles and presence of local sources. The highest concentrations are usually found in urban areas, reaching up to 108 and 109 particles per cc (Seinfeld and Pandis, 1998).
The study of aerosol is interesting for a number of reasons. It is thought that aerosol may be involved in a feedback to global warming. It is certainly important in the Earth’s radiation budget. There are also concerns about the effects of aerosol on human health. Finally it is, in some cases, an important part of the chemical deposition budget for certain chemical species to Ecosystem.
The feedback to global warming would tend to cool down the Earth in the event of a rise in temperature, and acts in two ways. With increased temperature would come stronger average winds, suspending more crustal material, thus increasing average aerosol concentrations. The first mechanism is called the direct effect. This is simply aerosol reflecting back incident Solar radiation into space. The magnitude of the direct effect is simpler to estimate than that of the indirect effect.
The indirect effect again involves increased average aerosol number concentration. An increased aerosol population means that there are more cloud condensation nuclei, which would lead to more clouds forming. This situation is slightly more complicated, as the effect of the clouds on the Earth’s Radiation budget depends upon the cloud height, but increased tropospheric aerosol would also have a cooling effect on the atmosphere.
The smallest aerosol are small enough to get into the human respiratory system. British standards define the respirable fraction as those aerosol smaller than 5 m m, which as we will see, is a significant proportion of the total. Even aerosol composed of benign materials can be irritants (e.g. glass calibration microspheres used in the lab), and some aerosol are partly made of toxic materials (heavy metals, organic chemicals etc.).
The rate at which aerosol can act as a deposition pathway is an important part of total deposition rates for a number of species. Clearly this is more so for species with few other deposition pathways (e.g. heavy metals).
The size of these particles ranges from around 100 m m to a few nm. These are the largest particles which can be suspended in air for a significant amount of time and the smallest clusters of molecules which can be classified as particles respectively. Aerosol has a number of properties such as size, chemical composition, hygroscopiscity, density and shape. Size is normally used to classify aerosol because it is the most easily measured property and because inferences about the other properties can be drawn from size information.
The primary interest in this work is urban aerosol and urban airshed aerosol. These two situations normally yield much higher number concentrations than any other circumstances. For the purposes of this report, we will first look at sources and sinks of aerosol in general, before looking at typical sources and ambient concentrations for three well studied cases (marine, remote continental and urban).
Facts about Aerosols
- Aerosols were invented by Norwegian Erik Rotheim in the 1920s to help him put wax on his skis.
- The most expensive aerosol ever made was a diamond spray used to rub down precision moulds.
- The most popular aerosols are Deodorant and body sprays. They were first made in the UK in 1954.
- The first aerosols made in the UK were insecticides and air fresheners in 1949.
- Pepsi produced an aerosol so that astronauts could get a drink of cola without getting soaked.
- UK aerosols have not contained CFCs since 1989.
- Aerosols are used for many reasons, for instance on farms to encourage pigs to mate or to help lambs accept their foster mothers.
- Different types of aerosols spray at different rates and even have a certain sound! You'd have a shock if your deodorant sprayed at the same rate as your de-icer!
- Empty aerosols can be recycled.
- Food aerosols, other than the Cream we are used to, are popular in some countries; the Japanese have Coffee and chicken soup, the Americans have ketchup, Cheese and Mustard.
[edit] Emissions
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.
[edit] Transport
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). 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.
[edit] Effects
[edit] Human Health
Exposures to bioaerosols in the occupational environment are associated with a wide range of health effects with major public health impact, including infectious diseases, acute toxic effects, allergies and cancer. Respiratory symptoms and lung function impairment are the most widely studied and probably among the most important bioaerosol-associated health effects. In addition to these adverse health effects some protective effects of microbial exposure on atopy and atopic conditions has also been suggested.
[edit] Visibility
Reduction of visibility due to aerosols is a big problem in many parts of the world.
[edit] 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.
[edit] Destruction of Stratospheric Ozone
Aerosols catalyze the destruction of stratospheric ozone by chloro-radicals.
[edit] Radiative Forcing and Impact on Climatic Change
[edit] 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).
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.
[edit] 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.
[edit] 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.
