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aerosol: a suspension of particles in gas. Atmospheric particulate matter is an aerosol.

air quality: a term used to describe the levels of substances in the air such as ozone and particulate matter which may affect your health or visibility.

air quality index: a measure of air quality.

anthropogenic: caused by human activity.

greenhouse gas: gas that absorbs infrared (thermal) radiation due to its chemical structure. Greenhouse gases absorb Earth’s thermal energy as it radiates from the ground, through the atmosphere, out towards space. Examples of greenhouse gases include carbon dioxide, methane, and water vapor.

nanoparticle: particle with diameter ≤ 100 nanometers ( 1/250,000 inches). see ultrafine particle

NOx: nitrogen oxide and nitrogen dioxide. NOx is produced by combustion, for example, in the engine of a car.  NOx can react with VOCs in the presence of sunlight to produce ozone.

ozone: molecule containing three oxygen atoms.  Ozone is formed in the stratosphere by the reaction of oxygen with UV light or in the troposphere by reactions of NOx and VOCs in the presence sunlight.  At ground level ozone is harmful to human health in high concentrations.  In the stratosphere the “ozone layer”  absorbs harmful UV radiation.

particulate matter: very small particles in the atmosphere, sometimes called “atmospheric aerosols“. They impact human health, visibility, atmospheric chemistry, and climate. Can be emitted directly from a natural or anthropogenic process, or formed by gases in the atmosphere.

photochemistry: the study of chemical reactions which are initiated by sunlight.

PM2.5: atmospheric particles with diameter ≤ 2.5 micrometers (1/10,000 inches).

PM10: atmospheric particles with diameter ≤ 10 micrometers (1/2500 inches).

smog: high levels of ozone and particulate matter formed by reactions of NOx, VOCs, and sunlight.  Smog is associated with poor air quality.

stratosphere: atmospheric layer above the troposphere, extending from about 17 km-50 km above the ground.

troposphere: bottom layer of the atmosphere, extending from the ground up to about 17 kilometers above the earth.

ultrafine particle: particle with diameter ≤ 100 nanometers ( 1/250,000 inches). See nanoparticle.

VOC: Volatile Organic Compound. Carbon-containing gases which can affect smog and particulate matter formation.

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