Aerodynamic size
the size of a particle as it moves in the air.
Air Quality Health Index
real-time information system that provides the public with an indication of air quality in cities, towns and in rural areas across Ontario. The AQHI derives a value based on the cumulative health effects of three pollutants – O3, PM2.5 and NO2.
AQHI station
continuous monitoring station used to inform the public of general ambient air quality levels over an entire region (not a localized area) on a real-time basis; station reports on criteria pollutant levels that are not unduly influenced by a single emission source, but rather are the result of emissions from multiple sources, including those in neighbouring provinces and states.
Ambient air
outdoor or open air.
Annual mean
the average value of hourly data for a given year.
Anthropogenic
derived from human activities.
Carbon monoxide
a colourless, odourless, tasteless, and at high concentrations, poisonous gas.
Continuous pollutants
pollutants for which a continuous measurement record exists; effectively, pollutants that have hourly data (maximum 8,760 values per year except leap year – e.g. 2004 where maximum values for the year are 8,784).
Continuous station  
where pollutants are measured on a real-time basis and data determined hourly (for example ozone, sulphur dioxide).
Criterion
maximum concentration or level (based on potential effects) of pollutant that is desirable or considered acceptable in ambient air.
Exceedance
above the air pollutant concentration levels established by environmental protection criteria or other environmental standards.
Fine Particulate Matter
also referred as respirable particles: particles smaller than 2.5 micrometres in aerodynamic diameter, which arise mainly from fuel combustion, condensation of hot vapours and chemically-driven gas-to-particle conversion processes; also referred to as PM2.5 or respirable particles. These are fine enough to penetrate deep into the lungs.
Fossil fuels
natural gas, petroleum, coal and any form of solid, liquid or gaseous fuel derived from organic materials for the purpose of generating heat.
Fly ash
generated as a by-product of coal combustion and is used as a replacement for cement in concrete, among other uses.
Ground-level ozone
colourless gas formed from chemical reactions between nitrogen oxides and volatile organic compounds (VOCs) in the presence of sunlight near the Earth’s surface.
Micrometre
a millionth of a metre.
Nitrogen dioxide
a reddish-brown gas with a pungent and irritating odour.
Oxidation
a chemical reaction where a substance gains an oxygen; for example, in the atmosphere, sulphur dioxide is oxidized by hydroxyl radicals to form sulphate.
Particulate matter
the general term used to describe a mixture of microscopic solid particles and liquid droplets suspended in air.
Point sources
sources that have a fixed location and are identified individually by name and location.
Primary pollutant
pollutant emitted directly to the atmosphere.
Residence time
the average length of time during which a particle is in a given location or condition.
Respirable particles
see definition for fine particulate matter.
Secondary pollutant
pollutant formed from other pollutants in the atmosphere.
Smog  
a contraction of smoke and fog; colloquial term used for photochemical smog, which includes ozone, fine particulate matter and other contaminants; tends to be a brownish haze.
Stratosphere  
atmosphere 10 to 40 kilometres above the Earth’s surface.
Stratospheric ozone
ozone formed in the stratosphere from the conversion of oxygen molecules by solar radiation; ozone found there absorbs some of the sun’s ultraviolet radiation and prevents it from reaching the Earth.
Styrene
primarily a synthetic chemical that is used extensively in the manufacture of plastics, rubber, and resins. It is also known as vinylbenzene, ethenylbenzene, cinnamene, or phenylethylene.
Sulphur dioxide
a colourless gas that smells like burnt matches.
Troposphere  
atmospheric layer extending from the surface up to about 10 kilometres above the Earth’s surface.