Offset – A method used in the 1990 Clean Air Act to give companies which own or operate large (major) source in a nonattainment area flexibility in meeting overall pollution reduction requirements when changing production processes. If the owner or operator of the source wishes to increase release of a criteria air pollutant, an offset (reduction of a somewhat greater amount of the same pollutant) must be obtained either at the same plant or by purchasing offsets from another company.

Opacity – The amount of light obscured by particulate pollution in the air; clear window glass has zero opacity, a brick wall is 100 percent opaque. Opacity is an indicator of changes in performance of particulate control systems.

Open Burning – means any fire from which the products of combustion are emitted directly into the open air, and are not by design directed through a stack or chimney.

Organic Chemicals/Compounds – Naturally occurring (animal or plant-produced or synthetic) substances containing mainly carbon, hydrogen, nitrogen, and oxygen.

OTC – means Ozone Transport Commission. The OTC is a multi-state organization created under the Clean Air Act (CAA), responsible for advising the USEPA on transport issues and for developing and implementing regional solutions to the ground-level ozone problem in the Northeast and Mid-Atlantic regions.

OTR – means Ozone Transport Region. The ozone transport region established pursuant to 42 U.S.C. 7511c(a), comprised of the States of Connecticut, Delaware, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, Rhode Island, Pennsylvania, Vermont, the Consolidated Metropolitan Statistical Area that includes northern portions of Virginia and the District of Columbia.

Oxidant – A collective term for some of the primary constituents of photochemical smog.

Oxygenated Fuel (oxyfuel) – A special type of gasoline, which burns more completely than regular gasoline in cold start conditions; more complete burning results in reduced production of carbon monoxide, a criteria air pollutant. In some parts of the country, where carbon monoxide release from cars starting up in cold weather makes a major contribution to pollution, gasoline refiners must market oxygenated fuels, which contain higher oxygen content than regular gasoline.

Ozone – a gas which is a variety of oxygen. The oxygen gas found in the air consists of two oxygen atoms stuck together; this is molecular oxygen. Ozone consists of three oxygen atoms stuck together into an ozone molecule. Ozone occurs in nature; it produces the sharp smell you notice near a lightening strike. High concentrations of ozone gas are found in a layer of the atmosphere – the stratosphere – high above the Earth. Stratospheric ozone shields the Earth against harmful rays from the sun, particularly ultraviolet B.  Smog’s main component is ozone; this ground-level ozone is a product of reactions among chemicals produced by burning coal, gasoline and other fuels, and chemicals found in products including solvents, paints, hairsprays, etc.

Ozone Season (Entire) – means May 1 through September 30, inclusive.

Ozone Transport Commission (OTC) – means the organization established pursuant to § 184(a) of the Clean Air Act Amendments of 1990. The members of this commission include an air pollution control official from each of the following jurisdictions: Connecticut, Delaware, the District of Columbia, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont, and Virginia.

Ozone Transport Regions – The ozone transport region established pursuant to 42 U.S.C. 7511c(a), comprised of the States of Connecticut, Delaware, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, Rhode Island, Pennsylvania, Vermont, the Consolidated Metropolitan Statistical Area that includes northern portions of Virginia and the District of Columbia.