Unit 7: Atmospheric Pollution

Air pollution

The introduction of harmful or excessive substances into Earth’s atmosphere; pollutants can be emitted, transported, chemically transformed (often by sunlight), and removed by deposition.

Parts per million (ppm)

A common unit for expressing air pollutant concentration; many exam questions link a ppm value to sources, conditions (wind/sun/inversions), impacts, and controls.

Point source pollution

Pollution from an obvious, identifiable single source (e.g., a specific smokestack).

Non-point source pollution

Pollution from many spread-out sources that are not easily traced to one location (e.g., citywide vehicle emissions).

Criteria air pollutants

The six widespread pollutants regulated with national ambient standards under the U.S. Clean Air Act: CO, NO2, SO2, PM, tropospheric O3, and Pb.

Troposphere

The lowest atmospheric layer where weather occurs; most air pollution and virtually all smog happens here.

Stratosphere

The atmospheric layer above the troposphere that contains the ozone layer; some long-lived pollutants can reach it and contribute to ozone depletion.

Tropospheric (ground-level) ozone (O3)

A harmful secondary pollutant in smog, formed when NOx and VOCs react in sunlight; a strong oxidant that irritates lungs and damages plants.

Stratospheric ozone (ozone layer)

Protective ozone in the stratosphere that absorbs harmful UV radiation; depletion increases UV exposure at Earth’s surface.

Wet deposition

Removal of pollutants by precipitation (rain/snow/fog) that carries dissolved or captured pollutants to the ground.

Dry deposition

Acidic particles and gases settling onto surfaces; later rainstorms can wash them off, increasing acidic runoff.

Thermal (temperature) inversion

A condition where warmer air overlies cooler surface air, suppressing vertical mixing and trapping pollutants near the ground.

Residence time

How long a pollutant remains in the atmosphere before being removed or transformed; longer residence time allows regional/global spread.

Urban heat island

A metropolitan area that is significantly warmer than surrounding areas due to heat release and heat-absorbing, impervious surfaces; can worsen smog chemistry and heat-health risks.

Street canyon

A street flanked by buildings that reduces air circulation and can increase localized pollutant concentrations.

Primary pollutant

A pollutant emitted directly from a source into the atmosphere (e.g., CO, SO2, NOx, many PM, VOCs, Pb).

Secondary pollutant

A pollutant formed in the atmosphere through chemical reactions (often sunlight-driven), so control focuses on reducing precursors (e.g., O3, PANs, sulfuric/nitric acids, secondary PM).

Precursor

A chemical that reacts in the atmosphere to form a secondary pollutant (e.g., NOx and VOCs are ozone precursors; SO2 and NOx are acid deposition precursors).

Carbon monoxide (CO)

A colorless, odorless gas from incomplete combustion; binds to hemoglobin and reduces blood’s oxygen-carrying capacity, making it dangerous indoors and near traffic.

Nitrogen oxides (NOx)

Mainly NO and NO2 formed during high-temperature combustion (vehicles, power plants); contribute to ozone formation and nitric acid (acid deposition) and can irritate the respiratory system.

Sulfur dioxide (SO2)

A colorless gas with a choking odor from burning sulfur-containing coal/oil and smelting; irritates lungs and contributes to sulfuric acid, sulfate PM, and acid deposition.

Particulate matter (PM)

Microscopic solid or liquid particles suspended in air; can be primary or secondary, reduces visibility (haze), and causes respiratory/cardiovascular harm.

PM10

Particulate matter ≤10 micrometers; can reach the upper respiratory tract.

PM2.5

Fine particulate matter ≤2.5 micrometers; can penetrate deep into lungs and may enter the bloodstream, increasing health risk.

Volatile organic compounds (VOCs)

Organic chemicals with high vapor pressure (evaporate easily) from fuels, solvents, paints, etc.; key precursors to tropospheric ozone and photochemical smog, and some are toxic/carcinogenic.

Photochemical smog

Brownish, sunlight-driven smog in car-dependent cities; forms when NOx and VOCs react under UV radiation to produce secondary pollutants like O3 and PANs (often worst on hot, sunny, stagnant days).

Industrial (gray) smog

Sulfur- and soot-based smog historically linked to coal burning and heavy industry; associated with SO2, sulfuric acid, and particulate matter (haze/health impacts).

Peroxyacyl nitrates (PANs)

Secondary pollutants in photochemical smog that irritate eyes and damage plants; break apart slowly, allowing them to travel far from their source.

Acid deposition

Regional pollution where acids formed from SO2 and NOx reactions return to Earth as wet deposition (rain/snow/fog) or dry deposition (particles/gases), altering soil and water chemistry.

Acid shock

A sudden pulse of high acidity in lakes/streams during rapid snowmelt when accumulated acidic deposits are released; concentrations can be much higher than acidic rainfall.

Buffering capacity

An ecosystem’s ability to neutralize added acids; carbonate-rich (limestone) geology buffers well, while granite/thin soils buffer poorly and are more vulnerable to acidification.

Liming

Adding crushed limestone to lakes/soils to temporarily raise pH as a remediation for acidified ecosystems; does not remove the emission cause.

Radon

An invisible radioactive gas from decay of radium in rocks beneath buildings; can seep into basements/lower floors and increase lung cancer risk.

Formaldehyde

A common indoor air pollutant (often from pressed-wood products and new materials); a carcinogen linked to lung cancer.

Asbestos

A durable, fireproof insulating material; when disturbed, inhaled fibers can cause serious health problems.

Sick building syndrome (SBS)

A set of symptoms (e.g., headaches, irritation, fatigue) associated with time spent in a building without a single specifically identified illness or cause.

Building-related illness

A diagnosable illness traced to a specific building contaminant (e.g., certain mold-related illnesses).

Ozone-depleting substances (ODS)

Long-lived chemicals (e.g., CFCs, halons) that reach the stratosphere and release chlorine/bromine that catalytically destroys ozone, increasing UV at the surface.

Chlorofluorocarbons (CFCs)

Stable ODS once used in refrigerants, aerosol propellants, and foams; UV in the stratosphere breaks them down, releasing chlorine that destroys ozone.

Halons

ODS used in fire suppression; release bromine that contributes to stratospheric ozone depletion.

Montreal Protocol

International agreement to phase out many ozone-depleting substances; key example of global cooperation to reduce long-lived pollutants and allow atmospheric recovery.

Catalytic converter

Vehicle exhaust device using catalysts to convert harmful gases into less harmful ones (commonly reduces CO, NOx, and unburned hydrocarbons); tradeoff: oxidizes CO to CO2.

Scrubber (flue-gas desulfurization)

End-of-pipe control that removes SO2 from power plant exhaust (often using limestone-based sorbents), reducing acid deposition but producing solid byproducts.

Electrostatic precipitator

Particulate control device that charges particles and collects them on plates to remove PM from exhaust streams.

Baghouse filter

Particulate control technology that filters exhaust through fabric bags to capture PM (used on smokestacks/industrial emissions).

Low-NOx burner

Combustion modification that reduces nitrogen oxide formation by controlling combustion conditions/temperature in power plants and industrial boilers.

Cap-and-trade

A policy tool that sets a total emissions cap and allows trading of allowances; used in the U.S. to reduce SO2 contributing to acid deposition.

Nitrous oxide (N2O)

A long-lived pollutant whose atmospheric levels have risen since 1750; contributes to stratospheric ozone depletion and is produced largely by microbial nitrification and denitrification.

Sensory hearing loss

Hearing loss caused by damage to the inner ear; the most common health effect associated with excessive noise exposure.

Noise pollution

Unwanted, human-created sound (often from transportation) that disrupts the environment and can cause health effects such as hearing loss, anxiety, and cardiovascular stress.