Glossary of Key Terms
Glossary of Key Terms
| Term | Definition |
| Acid Deposition | A significant environmental problem resulting from strong inorganic acids, primarily nitric acid (HNO₃) and sulfuric acid (H₂SO₄), being present in the atmosphere and subsequently deposited. |
| Airshed Model | A type of mathematical air-quality model used on an urban scale (approx. 100 km²) to predict concentrations of primary and secondary pollutants from a set of stationary and mobile sources. |
| Atmospheric Chemistry | The scientific field focused on determining the quantities, origins, and roles of various chemicals in the atmosphere, including the study of reaction kinetics, measurement techniques, and chemical modeling. |
| Chapman Mechanism | The set of four chemical reactions involving only oxygen species that describe the natural formation and destruction of stratospheric ozone. |
| Chlorofluorocarbons (CFCs) | Very stable compounds, such as CFCl₃ (CFC-11) and CF₂Cl₂ (CFC-12), formerly used as propellants and refrigerants, which can diffuse into the stratosphere and release chlorine atoms that catalytically destroy ozone. |
| Elemental Carbon (EC) | Also known as black carbon or graphitic carbon, it is a component of carbonaceous particulate matter that is emitted directly into the atmosphere during combustion processes. |
| Free Radicals | Reactive intermediates, such as an atom or a fragment of a molecule with an unpaired electron, that play a key role in atmospheric chemical reactions. The hydroxyl radical (OH) is a prominent example. |
| Hydroxyl Radical (OH) | A highly reactive free radical that acts as a catalyst in the troposphere, initiating the chain oxidation of hydrocarbons which leads to the formation of ozone and other secondary pollutants. |
| Incremental Reactivity | A measure of the change in ozone production that occurs when a small amount of a specific Volatile Organic Compound (VOC) is added to or subtracted from a base mixture of VOCs at a fixed NOₓ concentration. |
| Leighton Relationship | Also known as the photostationary state, it is a mathematical relationship showing that the ozone concentration depends on the ratio of NO₂ to NO concentrations and the rate constants for NO₂ photolysis and the NO + O₃ reaction. |
| Long-Range Transport (LRT) Model | A mathematical model used to predict acid deposition over large geographical areas (on the order of 1000 km or more), accounting for emissions, meteorology, and chemistry over long distances. |
| Maximum Incremental Reactivities (MIR) | A specific scale of incremental reactivity used to evaluate the effect of changing the components of a VOC mixture on ozone production. |
| National Ambient Air Quality Standards (NAAQS) | A set of standards established in the United States for several key air pollutants. Primary standards are set to protect public health, while secondary standards protect public welfare (e.g., plants, materials, visibility). |
| Nitrate Radical (NO₃) | A radical formed at night from the reaction of ozone with NO₂. It is important in nighttime atmospheric chemistry, reacting with certain organics and forming N₂O₅. |
| Organic Carbon (OC) | A component of carbonaceous particulate matter that can be either emitted directly (primary OC) or formed in the atmosphere by the condensation of low-volatility products from hydrocarbon photooxidation (secondary OC). |
| Ozone Isopleths | Contour diagrams that illustrate how the maximum ozone concentration reached over a fixed period depends on the initial concentrations of nitrogen oxides (NOₓ) and Volatile Organic Compounds (VOCs). |
| Peroxyacetyl Nitrate (PAN) | A photochemical oxidant (CH₃C(O)O₂NO₂) that is an eye irritant, causes plant damage, and serves as an important reservoir for NOₓ in the upper troposphere due to its thermal stability at cold temperatures. |
| Photochemical Oxidants | A class of secondary pollutants, including ozone (O₃) and PAN, formed through chemical reactions involving organic compounds, nitrogen oxides, and sunlight. |
| Polar Stratospheric Clouds (PSCs) | Clouds that form in the extremely cold polar stratosphere (below 195 K) and provide surfaces for chemical reactions that convert stable chlorine reservoir species into forms that can catalytically destroy ozone. |
| Primary Pollutants | Pollutants that are emitted directly into the atmosphere from a source, such as VOC, NOₓ, SO₂, and CO. |
| Secondary Organic Aerosols (SOAs) | Fine particulate matter formed in the atmosphere through the condensation of low-vapor-pressure products generated by the atmospheric oxidation of organic gases. |
| Secondary Pollutants | Pollutants that are not emitted directly but are formed in the atmosphere through chemical and physical transformations of primary pollutants. Ozone is a key example. |
| Smog Chamber | A large photochemical-reaction vessel used in laboratories to simulate the chemistry occurring in the urban atmosphere and study pollutant formation. |
| Stratosphere | The region of the atmosphere above the troposphere (from ~10-17 km up to ~50 km altitude) where temperature increases with altitude, leading to poor vertical mixing. It contains the ozone layer. |
| Troposphere | The lowest layer of the atmosphere, extending from the Earth’s surface up to about 10-15 km, where most weather occurs and temperature generally decreases with altitude. |