Understanding Air Quality: Key Concepts

Air Quality Basics

• Air pollution is the introduction of contaminants like particulate matter, dust, fumes, gas, mist, smoke or vapor into the atmosphere, especially from man-made processes.
• Ambient air refers to the outdoor air or atmosphere surrounding the Earth.
• Emission is the release of air contaminants into the outdoor atmosphere.
• An emission inventory is a detailed list of the types, amounts, quantities, and sources of emissions.
• Fossil fuels include natural gas, petroleum, coal, and any solid, liquid, or gaseous fuel derived from these materials.
• Fugitive dust involves solid, airborne particulate matter emitted from any source other than a stack.
• Particulate matter consists of airborne finely divided solid or liquid material with an aerodynamic diameter less than 100 micrometers (one micron = one millionth of a meter).
• Aerodynamic diameter is how the aerodynamic behavior of airborne particles with irregular shapes is expressed.
• Aerodynamic diameter refers to the diameter of an idealized spherical particle.
• Particles sharing the same aerodynamic diameter can differ in dimensions and shapes, but are often referred to as particle size.
• PM10 refers to particulate matter with an aerodynamic diameter of 10 micrometers or less.
• PM2.5 refers to particulate matter with an aerodynamic diameter of 2.5 micrometers or less.
• Photochemical oxidants are products of chemical reactions triggered by sunlight between hydrocarbons or organic compounds and nitrogen oxides.
• Volatile Organic Compounds (VOCs) are any organic compound that participates in atmospheric photochemical reactions.
• VOC sources include paints, wood preservatives, aerosol sprays, disinfectants, pesticides, air fresheners, moth repellents, copiers, printers, glues, permanent markers, and petroleum, as well as chemicals such as hexane, propane, butane, toluene, ethane, and pentane.

Sources of Air Pollution

• Air pollution sources can be mobile, stationary, area/still, or natural.
• Mobile sources include cars, buses, planes, trucks, and trains.
• Stationary sources include power plants, oil refineries, industrial facilities, and factories.
• Area/still sources include agricultural areas, cities, and wood-burning fireplaces.
• Natural sources include wind-blown dust, wildfires, and volcanoes.
• Anthropogenic (human-caused) sources of air pollution include the combustion of fossil fuels, emissions from industries and factories, agricultural activities, and waste production.
• Fossil fuels are combusted to produce electricity and for road transport, releasing pollutants such as nitrogen, carbon monoxide, and sulfur dioxide.
• Industries and factories release large amounts of carbon monoxide, hydrocarbons, chemicals, and organic compounds.
• Agricultural activities involve the use of pesticides, insecticides, and fertilizers that emit harmful chemicals.
• Waste production primarily generates methane from landfills.

Common Air Pollutants and Their Effects

• Carbon monoxide (CO) is a colorless, odorless gas emitted by cars, trucks, and other vehicles that reduces oxygen transport to critical organs like the heart and brain.
• Lead emissions come from ore and metal processing, waste incineration, and lead-acid battery manufacturing, can accumulate in bones, harm the nervous system, cause kidney failure, affect the immune and reproductive systems, and damage the cardiovascular system.
• Nitrogen dioxide (NO2), also known as oxide of nitrogen (Nox), nitrous acid, and nitric acid, comes from fuel burning and vehicle emissions, it irritates the human respiratory system, causing asthma, coughing, and wheezing.
• NO2 reacts in the air to form ozone (O3).
• Ozone (O3) is the main ingredient in smog, is created by chemical reactions between oxides of nitrogen and VOCs, and is not directly emitted into the air.
• Pollutants from cars, power plants, and chemical plants react in sunlight to form ozone, causing breathing problems and asthma, particularly in older adults and outdoor workers; those with lower levels of Vitamins C and E are at greater risk.
• Health effects from ozone include chest pain, coughing, throat irritation, and airway inflammation.
• Particulate matter (PM) includes solid or liquid droplets like dust, dirt, soot, or smoke found in the air from construction sites, unpaved roads, fields, smokestacks, or fires.
• PM reduces visibility, can penetrate deep into the lungs, and may even enter the bloodstream.
• Sulfur dioxide (SO2), a group of sulfur oxides (Sox), comes from fossil fuels used by power plants, industries, metal ore extraction, volcanoes, and vehicles using fuel with high sulfur content.
• SO2 contributes to haze, acid rain, and foliage damage, harms the human respiratory system, causes breathing difficulties, and asthma, and can react with other compounds to form PM.

Atmospheric Composition and Properties

• Earth's atmosphere is composed of gases and particulate phase substances.
• Nitrogen (N2) and oxygen (O2) are the most abundant atmospheric gases.
• Nitrous oxide, once thought to be consistent, now increases due to human activities.
• Carbon dioxide is vital as a raw material for photosynthesis by green plants, it is a major greenhouse gas responsible for maintaining heat balance due to its thermal absorptivity.
• Water vapor is the atmospheric constituent with the highest degree of variability (0.1 to 30,000 ppmv), is a major greenhouse gas that absorbs thermal energy from the Earth's surface and readily changes phase, condensing into liquid water when cooled.
• Noble gases include Helium (He), Argon (Ar), Neon (Ne), Krypton (Kr), and Xenon (Xe), these are inert and have minimal impact on the atmosphere.

Trace Gases and Atmospheric Water

• Variable trace gases are produced through biogenic (biological) or geogenic (geological) sources.
• Ammonia (NH3) is produced by biological decomposition.
• Methane (CH4) absorbs thermal energy, it is a greenhouse gas, and is also formed through biological decomposition.
• Hydrogen sulfide (H2S) is emitted during biological decomposition.
• Carbon monoxide (CO) and sulfur dioxide (SO2) are also variable trace gases.
• Biogenic organic substances, such as isoprenes and pinenes, are released by plants, microorganisms, and some animals (especially during growing seasons).
• Biogenic organic substances are quickly removed, and are not considered "normal" atmospheric constituents.
• Water covers around 70% of the Earth's surface in liquid form, it exists as solid, liquid, and gas in the atmosphere, varying from trace levels to ~3%
• Water phase changes influence weather processes.
• Vapor pressure is the partial pressure of H2O vapor, which increases with higher H2O vapor concentration.
• Absolute humidity is measured as mass/volume or vapor pressure.
• Relative humidity is the percentage of H2O vapor air holds at a given temperature; it decreases with higher temperature and reaches 100% at saturation.
• Dew point is the temperature at which condensation occurs when air is cooled below saturation.
• Clouds form when warm, moist air rises and cools below the dew point, leading to condensation. -Initial cloud droplets are incredibly small, about 1/1,000,000th the size of rain droplets.
• Clouds contain tiny H2O droplets or ice particles too small to fall, precipitation occurs as droplets grow 1,000,000 times larger.
• Clouds reflect sunlight, increasing Earth's albedo, and absorb heat, slowing heat loss to space.
• H2O vapor and CO2 absorb infrared radiation.
• Ozone (O3) absorbs in UV and visible light.
• Oxygen (O2) absorbs in visible and infrared radiation.
• Thermal energy is transported as warm currents move poleward and cold currents move equatorward.

Atmospheric Layers and Properties

• Troposphere: The lowest layer, extending 8-18 km, with temperatures decreasing from ~15°C (59°F) to -60°C (-76°F) at 15 km; It's unstable, and where weather occurs, divided into planetary boundary layer and free troposphere.
• Tropopause: The boundary between the troposphere (decreasing temperature) and stratosphere (increasing temperature).
• Stratosphere: Temperature increases with height due to UV absorption by O3 (ozone), it is stable with little vertical mixing.
• Mesosphere: Above the stratopause, temperature decreases to -90°C (-130°F) at 85 km.
• Thermosphere: Above 90 km, temperatures rise due to UV absorption by N2 and O2, reaching ~1200°C (2128°F).
• Ionosphere: Part of the thermosphere, contains ionized particles responsible for auroras (N2+, O2+, N+ and O+ ions) and reflects AM radio signals at night, its ion density decreases without sunlight.
• Gravity holds atmospheric gases near Earth's surface: cool air sinks (dense), warm air rises (less dense).
• Atmospheric pressure is the force exerted by air, maximal at sea level (1013.25 mbars or 760 mmHg).
• Warm air expands (lower pressure), cool air contracts (higher pressure).
• Standard pressure units: 1 atm = 1013.25 mbars, 1.013×10^5 Pa, or 14.7 PSI; atmospheric density and pressure decrease with height.
• Wind is horizontal air movement caused by pressure differences.
• Wind is caused by unequal heating of Earth's surface which creates pressure variations.
• Wind airflow is controlled by pressure gradient force, Coriolis effect, and friction.
• Air flows directly from high to low pressure without friction/rotation.
• Pressure Gradient Force (PGF) is the force caused by horizontal pressure differences in the lower atmosphere.
• Weather maps use isobars (lines of equal pressure) to show pressure gradients, close isobars mean a steep pressure gradient (strong winds) and wide isobars mean a small pressure gradient (light winds)
• PGF direction is perpendicular to isobars, from high to low pressure.
• Coriolis Effect occurs as air flows from the tropics (55 degrees latitude), it appears to be deflected from its expected path north- or southward.
• This deflection is east in the northern hemisphere and west in flows toward the equator due to Earth's rotation, as the Earth moves from west to east, air appears to lag behind.
• The Coriolis Effect causes air to deflect east in poleward flow and west toward the equator due to Earth's rotation.
• Friction opposes the Pressure Gradient Force (PGF) near the Earth's surface, moderating wind speed and reducing Coriolis Effect (CE) deflection.