Particulate

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Aerosol pollution over Northern India and Bangladesh - Photo: NASA
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Aerosol pollution over Northern India and Bangladesh - Photo: NASA

Particulates, alternatively referred to as particulate matter (PM), aerosols or fine particles, are tiny particles of solid (a smoke) or liquid (an aerosol) 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.

Contents

[edit] 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.

[edit] 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).

"In Europe and the United States, particulate emissions from vehicles are expected to decline over the next decade. For example, by 2005, the European Union will introduce more stringent standards for particulate emissions from light duty vehicles of 0.025 grams per kilometer [0.04 grams per mile]. Even under these standards, diesel-powered cars may still warm the climate more over the next 100 years than may gasoline-powered cars, according to the study.
The state of California is implementing an even more restrictive standard in 2004, allowing only 0.006 grams per kilometer [0.01 grams per mile] of particulate emissions. Even if the California standard were introduced worldwide, says Jacobson, diesel cars may still warm the climate more than gasoline cars over 13 to 54 years." Diesel cars may promote more global warming than gasoline cars

[edit] Composition

The composition of aerosol particles depends on their source. Wind-blown mineral dust [1] tends to be made of mineral oxides and other material blown from the Earth's crust; this aerosol is light-absorbing. Sea salt [2] is considered the second largest contributor in the global aerosol budget, and consists mainly of sodium chloride originated from sea spray; other consituents of atmospheric sea salt reflect the composition of sea water, and thus include magnesium, sulfate, calcium, potassium, etc. In addition, sea spray aerosols may contains organic compounds, which influence their chemistry. Sea salt does not absorb.

Secondary particles derive from the oxidation of primary gases such as sulfur and nitrogen oxides into sulfuric acid (liquid) and nitric acid (gaseous). The precursors for these aerosols, i.e. the gases from which they originate, may have an anthropogenic origin (from fossil fuel combustion) and a natural biogenic origin. In the presence of ammonia, secondary aerosols often take the form of ammonium salts, i.e. ammonium sulfate and ammonium nitrate (both can be dry or in aqueous solution); in the absence of ammonia, secondary compounds take an acidic form as sulfuric acid (liquid aerosol droplets) and nitric acid (atmospheric gas). Secondary sulfate and nitrate aerosols are strong light-scatterers. [3]

Organic matter (OM) can be either primary or secondary, the latter part deriving from the oxidation of VOCs; organic material in the atmosphere may either be biogenic or anthropogenic. Organic matter influences the atmospheric radiation field by both scattering and absorption. Another important aerosol type is constitude of elemental carbon (EC, also known as black carbon, BC): this aerosol type includes strongly light-absorbing material and is thought to yield large positive radiative forcing. Organic matter and elemental carbon together constitute the carbonaceous fraction of aerosols. [4]

[edit] 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.

[edit] Radiative forcing from aerosols

Solar radiation reduction due to volcanic eruptions
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Solar radiation reduction due to volcanic eruptions

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.

[edit] Sulphate aerosol

Sulphate 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.

[edit] 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 Fourth 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.]

[edit] Health effects

Air pollution measurement station in Emden, Germany
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Air pollution measurement station in Emden, Germany

The effects of inhaling particulate matter has been widely studied in humans and animals and include asthma, lung cancer, cardiovascular issues, and premature death. The size of the particle is a main determinant of where in the respiratory tract the particle will come to rest when inhaled. Larger particles are generally filtered 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 bronchi and lungs and cause health problems. The 10 micrometer size does not represent a strict boundary between respirable and non-respirable particles, but has been agreed upon for monitoring of airborne particulate matter by most regulatory agencies. Similarly, particles smaller than 2.5 micrometres, PM2.5, tend to penetrate into the gas-exchange regions of the lung, and very small particles (< 100 nanometers) may pass through the lungs to affect other organs. 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 also evidence that particles smaller than 100 nanometres can pass through cell membranes. For example, particles may 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. 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.

[edit] 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.

[edit] Aerosol science

The field of aerosol science and technology has grown in response to the need to understand and control natural and manmade aerosols.

[edit] References

[edit] See also

[edit] External links

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