thesis basic definitions

The ambient concentration level reflecting actual air quality as monitored or modeled ... [http://www.arb.ca.gov/DRDB/TUO/CURHTML/R102.HTM]

Ambient : Surrounding (for example, ambient air). [http://www.atsdr.cdc.gov/glossary.html]

Concentration : The amount of a substance present in a certain amount of soil, water, air, food, blood, hair, urine, breath, or any other media.
[http://en.mimi.hu/environment/concentration.html]

Concentration : The relative amount of a substance mixed with another substance. An example is five parts per million of carbon monoxide in air or 1 milligram/liter of iron in water.
[http://en.mimi.hu/environment/concentration.html]

Contaminant : A substance that is either present in an environment where it does not belong or is present at levels that might cause harmful (adverse) health effects.
[http://www.atsdr.cdc.gov/glossary.html#Exposure%20Pathway]
[http://www.atsdr.cdc.gov/glossary.html#G-A-]

“Background concentration” means the ambient concentration of a
given parameter upstream or upgradient from a facility, practice or activity
which has not been affected by that facility, practice or activity.
[http://www.owrb.ok.gov/util/rules/pdf_rul/background_def.pdf]

Exposure pathway
The route a substance takes from its source (where it began) to its end point (where it ends), and how people can come into contact with (or get exposed to) it. An exposure pathway has five parts: a source of contamination (such as an abandoned business); an environmental media and transport mechanism (such as movement through groundwater); a point of exposure (such as a private well); a route of exposure (eating, drinking, breathing, or touching), and a receptor population (people potentially or actually exposed). When all five parts are present, the exposure pathway is termed a completed exposure pathway.