Organic molecular tracers in

pollution control Pollution is the introduction of contaminants into the natural environment that cause adverse change. Pollution can take the form of any substance (solid, liquid, or gas) or energy (such as radioactivity, heat, sound, or light). Pollutants, the ...

and

environmental science Environmental science is an interdisciplinary academic field that integrates physics, biology, and geography (including ecology, chemistry, plant science, zoology, mineralogy, oceanography, limnology, soil science, geology and physical geograp ...

are referred to as organic molecular markers or emission markers, and are compounds or compound classes. These tracers are of interest in the field of

air quality Air pollution is the contamination of air due to the presence of substances in the atmosphere that are harmful to the health of humans and other living beings, or cause damage to the climate or to materials. There are many different types ...

because they can help identify

particulate Particulates – also known as atmospheric aerosol particles, atmospheric particulate matter, particulate matter (PM) or suspended particulate matter (SPM) – are microscopic particles of solid or liquid matter suspended in the air. The te ...

emission sources, as they are relatively unique to those sources. This approach is generally applied to particulate matter under 2.5μm in diameter because of the formation mechanisms and the health risks associated with this size regime. With tracer compounds, the principles of mass balance are used to 'trace' emissions from the source to the receptor site where a sample is taken. Use of organic tracers has become more common as measurement quality has improved, costs have decreased, and compounds that were historically good tracers, such as

lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...

, have decreased in ambient concentrations due to various factors including government regulation.

Requirements

In order to be used as a tracer, a compound must be emitted preferentially by some sources and not by others, giving the emission source a relatively unique chemical makeup. The compound must react slowly enough in the atmosphere that it is chemically conserved from the emission source to the receptor site where an ambient sample may be taken. Additionally, a tracer species should not be formed in the atmosphere and it should not volatilize during transport so that mass balance is maintained. Tracer compounds must then be of primary origins (not formed in the atmosphere), which are created through

condensation Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor to ...

and

coagulation Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a blood clot. It potentially results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The mechanism o ...

of mainly combustion and biological sources.H Seinfeld and N. Pandis. Atmospheric chemistry and physics: from air pollution to climate change. (2006)

Examples

Samples have been analyzed from many known biogenic and anthropogenic emissions sources such as

diesel Diesel may refer to: * Diesel engine, an internal combustion engine where ignition is caused by compression * Diesel fuel, a liquid fuel used in diesel engines * Diesel locomotive, a railway locomotive in which the prime mover is a diesel engin ...

and

gasoline Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organic co ...

vehicles,

cigarette A cigarette is a narrow cylinder containing a combustible material, typically tobacco, that is rolled into thin paper for smoking. The cigarette is ignited at one end, causing it to smolder; the resulting smoke is orally inhaled via the opp ...

smoke, road dust, vegetative detritus, wood smoke, and meat cooking. Examples of some results of preferential emissions from sources include hopanes,

polycyclic aromatic hydrocarbons A polycyclic aromatic hydrocarbon (PAH) is a class of organic compounds that is composed of multiple aromatic rings. The simplest representative is naphthalene, having two aromatic rings and the three-ring compounds anthracene and phenanthrene. ...

and steranes from different types of mobile sources,

retene Retene, methyl isopropyl phenanthrene or 1-methyl-7-isopropyl phenanthrene, C18H18, is a polycyclic aromatic hydrocarbon present in the coal tar fraction, boiling above 360 °C. It occurs naturally in the tars obtained by the distillation of resin ...

and methoxyphenols from wood smoke, odd n-

alkane In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in which ...

s and even n-

alkanoic acids In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic ...

from vegetative detritus,

cholesterol Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell mem ...

and the C16 and C18 n-alkanoic acids from cooking, and lighter n-alkanes from brake wear.

Analytic use

Chemical analysis of ambient and source samples is performed using

gas chromatography-mass spectrometry Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...

, and the chemical profile of the emission sources can be compared to an ambient sample using chemical mass balance techniques to identify the ambient mass contribution from each pollution source. This approach assumes that an ambient air sample has particulate matter contributions from a linear combination of emission sources. If the chemical compositions of local sources are not available, source apportionment models such as positive matrix factorization and

principal components analysis Principal component analysis (PCA) is a popular technique for analyzing large datasets containing a high number of dimensions/features per observation, increasing the interpretability of data while preserving the maximum amount of information, and ...

can be used by employing statistical methods to identify emissions sources from time series of ambient samples.

References

External links

EPA-Chemical Mass Balance 8.2
{{Portal bar, Chemistry, Earth sciences, Environment, Weather, Science Air pollution Particulates