Particulate

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Particulates, alternately referred to as particulate matter (PM), aerosols or fine particles are tiny particles of solid or liquid suspended in a gas. They range in size from less than 10 nanometres to more than 100 micrometres in diameter. This range of sizes represent scales from a gathering of a few molecules to the size where the particles no longer can be carried by the gas.

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Atmospheric aerosols

Some aerosols occur naturally, originating from volcanoes, dust storms, forest and grassland fires, living vegetation, and sea spray. Human activities, such as the burning of fossil fuels also generate aerosols. Averaged over the globe, anthropogenic aerosols—those made by human activities—currently account for about 10 percent of the total amount of aerosols in our atmosphere.

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Sources

There are both natural and human sources of atmospheric particulates. The biggest natural sources are wind-blown dust, volcanoes, and forest fires. Sea spray is also a large source of particles though most of these fall back to the ocean close to where they were emitted. The biggest human sources of particles are combustion sources, mainly the burning of fossil fuel in internal combustion engines in automobiles and power plants, and wind blown dust from construction sites and other land areas where the water or vegetation has been removed. Some of these particles are emitted directly to the atmosphere (primary emissions) and some are emitted as gases and form particles in the atmosphere (secondary emissions).

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Composition

The composition of fine particles depends on the source. Wind-blown dust tends to be made of mineral salts and other crustal earth material. Primary emissions from combustion sources are made primarily of unburned fuel (hydrocarbons), elemental carbon (soot), elemental sulfur, mineral salts, and often contain traces of toxic metals. Secondary emissions are a combination of ammonia with either sulfuric acid or nitric acid and water, together with a complex mixture of the organic oxidation products of VOCs. Several classes of chemical compounds are known or believe to contribute to radiative forcing. These are sulfate, sea-salt, carbonaceous, and mineral dust aerosols. Sulfate aerosols [1] consists of the sulfate anion existing in various chemical states: sulfuric acid, ammonium bisulfate, ammonium sulfate, or as a dissociated anion in aqueous solution. Sea-salt aerosol [2], the second largest contributor the global aerosol budget, consists mainly of sodium chloride salt from seawater. Other components of seawater include magnesium chloride and organic compounds, which may influence its chemistry. The range of non-mineral, carbon-containing compounds that constitute the carbonaceous aerosol fraction are usually divided into two subclasses: organic carbon and elemental carbon. Elemental carbon is often referred to as black carbon, which is a major consituent of soot. The elemental portion of carbon aerosols are strongly light-absorbing while organic carbon consists of both light-scattering and light-absorbing compounds.[3] Mineral dust refers to soil, dust and other windblown material from the earth's surface.[4] This makes up the majority of particulate matter less than 10 micrometers in size across the globe.

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Removal Processes

In general, the smaller and lighter a particle is, the longer it will stay in the air. Larger particles (greater than 10 micrometres in diameter) tend to settle to the ground by gravity in a matter of hours whereas the smallest particles (less than 1 micrometre) can stay in the atmosphere for weeks and are mostly removed by precipitation.

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Radiative forcing from aerosols

Image:Mauna Loa atmospheric transmission.png Aerosols, natural and anthropogenic, can affect the climate by changing the way radiation is transmitted through the atmosphere. Direct observations of the effects of aerosols are quite limited so any attempt to estimate their global effect necessarily involves the use of computer models. The Intergovernmental Panel on Climate Change, IPCC, says: While the radiative forcing due to greenhouse gases may be determined to a reasonably high degree of accuracy... the uncertainties relating to aerosol radiative forcings remain large, and rely to a large extent on the estimates from global modelling studies that are difficult to verify at the present time [5].

A graphic showing the contributions (at 2000, relative to pre-industrial) and uncertainties of various forcings is available here.

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Sulfate aerosol

Sulfate aerosol has two main effects, direct and indirect. The direct effect, via albedo, is to cool the planet: the IPCC's best estimate of the radiative forcing is -0.4 watts per square meter with a range of -0.2 to -0.8 W/m² [6] but there are substantial uncertainties. The effect varies strongly geographically, with most cooling believed to be at and downwind of major industrial centres. Modern climate models attempting to deal with the attribution of recent climate change need to include sulfate forcing, which appears to account (at least partly) for the slight drop in global temperature in the middle of the 20th century. The indirect effect (via the aerosol acting as cloud condensation nuclei, CCN, and thereby modifying the cloud properties) is more uncertain but is believed to be a cooling.

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Black carbon

Black carbon (BC), also called soot rather loosely is one of the most important absorbing aerosol species in the Atmosphere. BC from fossil fuels is estimated by the IPCC in the Third Assessment Report of the IPCC, TAR, to contribute a global mean radiative forcing of +0.2 W/m² (was +0.1 W/m² in the Second Assessment Report of the IPCC, SAR), with a range +0.1 to +0.4 W/m². All aerosols both absorb and scatter solar (Sun's) and terrestrial (Earth) radiation. When we say, a species is absorbing, it only means that it dominantly absorbs than scatters radiation. A term called Single Scattering Albedo (SSA) is rather used to explain this. SSA is the ratio of scattering to extinction (Extinction includes both scattering and absorption) of radiation by a particle. High SSA implies the aerosol species of interest mainly scatters radiation. Lower SSA implied absorbing aerosols. [Eg: Sea-salt aerosol has an SSA of 1, implying that a sea-salt particle only scatters, whereas Soot has an SSA of 0.23, which is one of the most important and dominant aerosol absorber in the Atmosphere.]


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Health effects

Image:Luftguete messstation.jpg The effects of inhaling particulate matter has been widely studied in humans and animals and include asthma, lung cancercardiovascular issues, and premature death. The size of the particle determines where in the body the particle will come to rest if inhaled. Larger particles are generally filtered by small hairs in the nose and throat and do not cause problems, but particulate matter smaller than about 10 micrometres, referred to as PM10, can settle in the bronchies and lungs and cause health problems. Particles smaller than 2.5 micrometres, PM2.5, can penetrate directly into the lung, whereas particles smaller than 1 micrometer PM1 can penetrate into the alveolar region of the lung and tend to be the most hazardous when inhaled. In particular, a study published in the Journal of the American Medical Association (Pope et. al, 2002), indicates that PM2.5 leads to high plaque deposits in arteries, causing vascular inflammation and atherosclerosis — a hardening of the arteries that reduces elasticity, which can lead to heart attacks and other cardiovascular problems. Researchers suggest that even short-term exposure at elevated concentrations could significantly contribute to heart disease.

There is evidence that particles smaller than 100 nanometres can pass through cell membranes. For example, particles can migrate into the brain. It has been suggested that particulate matter can cause similar brain damage as that found in Alzheimer patients. Particles emitted from modern diesel engines (commonly referred to as Diesel Particulate Matter, or DPM) are typically in the size range of 100 nanometres (0.1 micrometres). In addition, these soot particles also carry carcinogenic components like benzopyrenes adsorbed on their surface. In view of these deposition mechanisms, it is becoming increasingly clear that the legislative limits for engines, which are in terms of emitted mass, are not a proper measure of the health hazard. One particle of 10 µm diameter has approximately the same mass as 1 million particles of 100 nm diameter, but it is clearly much less hazardous, as it probably never enters the human body - and if it does, it is quickly removed. Proposals for new regulations exist in some countries, with suggestions to limit the particle surface area or the particle number.

The large number of deaths and other health problems associated with particulate pollution was first demonstrated in the early 1970s (Lave et. al, 1973) and has been reproduced many times since. PM pollution is estimated to cause 20,000-50,000 deaths per year in the United States (Mokdad et. al, 2004) and 200,000 deaths per year in Europe). For this reason, the US Environmental Protection Agency (EPA) sets standards for PM10 and PM2.5 concentrations in urban air. EPA regulates primary particulate emissions and precursors to secondary emissions (NOx, sulfur, and ammonia). Many urban areas in the US and Europe still frequently violate the particulate standards, though urban air has gotten cleaner, on average, with respect to particulates over the last quarter of the 20th century.

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EU legislation

In directives 1999/30/EC and 96/62/EC, the European Commission has set limits for PM10 in the air:

Phase 1

from 1 January 2005

Phase 2¹

from 1 January 2010

Yearly average 40 µg/m³ 20 µg/m³
Daily average (24-hour)

allowed number of exceedences per year.

50 µg/m³

35

50 µg/m³

7

¹ indicative value.

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References

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See also

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External links

de:Feinstaub ko:미세먼지 it:Particolato nl:Fijn stof pl:Aerozole atmosferyczne zh:气溶胶

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