Table of Contents
Main Air Pollutants and Their Sources
Air pollution refers to the presence of substances in the atmosphere that, in the concentrations reached, harm organisms, materials, or climate, or disturb natural processes. The basic physics and chemistry of the atmosphere and the general role of humans as environmental factors are assumed; here we concentrate on air as a specific environmental compartment.
Air pollutants are often divided into:
- Primary pollutants – directly emitted.
- Secondary pollutants – formed from primary pollutants by chemical reactions in the air (often driven by sunlight).
Important pollutant groups and typical human-made sources:
Particulate Matter (PM)
Particulate matter consists of tiny solid and liquid particles suspended in the air (aerosols).
- PM₁₀: particles with diameter $\leq 10\,\mu\text{m}$
- PM₂.₅: particles with diameter $\leq 2.5\,\mu\text{m}$ (fine particles)
- Ultrafine particles: $< 0.1\,\mu\text{m}$
Sources
- Combustion of fossil fuels (coal power plants, diesel and gasoline engines, domestic heating with coal, oil, wood).
- Industrial processes (metal smelting, cement production, waste incineration).
- Abrasion (brake and tire wear, dust resuspension from roads and construction sites).
- Agriculture (ammonia from manure and fertilizers forms secondary particles with acids).
- Natural sources (volcanic ash, sea salt, desert dust, pollen) – relevant regionally, but current concern focuses on the human‑driven increase.
Properties and Transport
- Fine and ultrafine particles remain airborne for days to weeks and can be transported over hundreds to thousands of kilometers.
- Larger particles settle faster and mostly affect local air quality.
Sulfur Oxides (SOₓ)
The main sulfur oxide in air pollution is sulfur dioxide, SO₂; in the atmosphere it is partly oxidized to sulfur trioxide, SO₃, and further to sulfuric acid.
Sources
- Burning of sulfur‑containing fuels (coal, heavy fuel oil, some crude oils) in power plants, industry, and shipping.
- Metal smelting from sulfur‑rich ores.
- Volcanoes and geothermal areas (natural).
Chemical Behavior
- SO₂ dissolves in water droplets to form sulfurous acid and is further oxidized to sulfuric acid:
$$\text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3 \quad\text{and}\quad
\text{SO}_2 + \text{O}_2 \rightarrow \text{SO}_3 \rightarrow \text{H}_2\text{SO}_4$$ - These acids become part of acid rain (discussed more fully with soils and waters) and contribute to secondary particulate matter (sulfate aerosols).
Nitrogen Oxides (NOₓ)
The most important nitrogen oxides as air pollutants are nitric oxide (NO) and nitrogen dioxide (NO₂); together they are called NOₓ.
Sources
- High‑temperature combustion:
- Car and truck engines (especially diesel).
- Power plants and industrial burners.
- Agriculture:
- Emission of nitrous oxide (N₂O) from fertilized soils and manure (a greenhouse gas; less relevant for local smog, but important globally).
- Natural:
- Lightning (forms NO).
- Soil microbes (N₂O, NO).
Chemical Behavior
- NO is oxidized to NO₂ in the air.
- NO₂ participates in the formation of:
- Ozone (O₃) in the lower atmosphere.
- Nitric acid (HNO₃), which contributes to acid rain and nitrate aerosols.
Carbon Oxides: CO and CO₂
Carbon Monoxide (CO)
- Formed by incomplete combustion (insufficient oxygen): vehicle engines, defective heaters, charcoal grills.
- Binds strongly to hemoglobin, impairing oxygen transport in blood; important for health, less for global climate.
Carbon Dioxide (CO₂)
- End product of complete combustion and cellular respiration.
- Major human sources:
- Burning of fossil fuels (energy, transport, industry).
- Cement production (release from limestone).
- Land‑use change, especially deforestation.
CO₂ is not toxic at current ambient concentrations, but it is the most important long‑lived greenhouse gas from human activities.
Volatile Organic Compounds (VOCs)
VOCs are organic substances that easily evaporate.
Sources
- Solvents in paints, cleaning agents, adhesives.
- Fuel evaporation during storage and refueling.
- Incomplete combustion (vehicle exhaust, biomass burning).
- Natural: emissions from vegetation (isoprene, terpenes).
In the presence of NOₓ and sunlight, reactive VOCs drive the formation of photochemical smog and ground‑level ozone.
A subset, persistent organic pollutants (POPs), are chemically stable, fat‑soluble, and can travel long distances and bioaccumulate, but these are more often discussed in relation to soil and water contamination.
Ground‑Level Ozone (O₃)
Ozone in the stratosphere protects life from ultraviolet radiation and is beneficial. In the lower atmosphere (troposphere) it is a harmful secondary air pollutant.
Formation
- Ozone forms from NOₓ and VOCs under sunlight in a chain of photochemical reactions.
- This process is most intense in warm, sunny weather, especially in urban and downwind rural areas.
Ground‑level ozone is a major component of summer smog.
Ammonia (NH₃)
Ammonia is a basic gas that plays a central role linking agriculture and air quality.
Sources
- Livestock farming (manure, slurry, urine in stables).
- Mineral nitrogen fertilizers (volatilization).
- Some industrial processes.
Ammonia neutralizes acidic components in the air (e.g., from SO₂, NOₓ) and forms ammonium salts (e.g., ammonium sulfate, ammonium nitrate), which are important constituents of fine particulate matter.
Heavy Metals and Other Inorganic Pollutants
Several metals and trace elements can be transported by air as particles or vapor.
Important Examples
- Lead (Pb): formerly from leaded gasoline; today mainly from industry, waste incineration, and some mining and smelting operations.
- Mercury (Hg): emitted as vapor from coal combustion, metal processing, and waste incineration.
- Cadmium (Cd), arsenic (As), nickel (Ni): from metal smelting and certain industrial processes.
These substances often deposit from air onto soils and water surfaces and then enter food chains.
Local, Regional, and Global Air Pollution
The same emitted substance can have effects on multiple spatial scales:
- Local – e.g., high NO₂ concentrations on busy roads, indoor CO from faulty heaters.
- Regional – e.g., acid rain affecting forests and lakes downwind; particulate matter transported between neighboring regions or countries.
- Global – e.g., CO₂ and N₂O accumulation in the entire atmosphere; long‑lived halogenated gases that deplete the stratospheric ozone layer; widespread dispersion of some POPs and mercury.
This spatial differentiation is important for understanding where measures must be taken and how international cooperation becomes necessary.
Effects of Air Pollution on Organisms
Effects on Humans and Animals
Respiratory Tract and Cardiovascular System
- Fine particulate matter (PM₂.₅) penetrates deep into the lungs, and some ultrafine particles can enter the bloodstream.
- Short‑term effects:
- Irritation of eyes and respiratory tract.
- Worsening of asthma and other lung diseases.
- Increased hospital admissions during smog episodes.
- Long‑term effects:
- Chronic bronchitis, reduced lung function, increased risk of cardiovascular disease.
- Increased mortality from heart and lung diseases.
Toxic Gases and Substances
- CO: impairs oxygen transport; high concentrations can be fatal.
- NO₂ and ozone: irritate and damage lung tissue, reduce resistance to infections.
- Heavy metals and certain organic pollutants: can cause nerve damage, developmental disorders, and are partly carcinogenic.
Animals, especially those in dense urban areas or near industrial plants, experience similar health risks; additionally, changes in vegetation quality due to air pollution can indirectly affect them.
Effects on Plants
Direct Leaf Injury
- Ozone and other reactive gases can oxidize pigments and membrane components in leaf cells:
- Chlorosis (yellowing).
- Necrosis (dead spots).
- Reduced photosynthesis and growth.
Sensitive plant species or cultivars (e.g., some crops) can show visible damage already at concentrations common in polluted regions.
Acidic Deposition and Nutrient Imbalances
Although acid rain and nutrient cycles are treated in detail elsewhere, air is the transport route for:
- Acids (from SO₂ and NOₓ).
- Ammonia and nitrogen oxides, which act as airborne fertilizers.
Excess inputs of nitrogen and acidity alter soil chemistry and thus indirectly affect plant nutrition and root health.
Effects on Ecosystems and Materials
Ecosystems
- Combination of air pollutants can shift species composition:
- Sensitive lichens and mosses disappear from polluted air zones.
- Nitrogen‑loving species spread with high nitrogen deposition.
- Forests may suffer from damaged foliage, weakened roots, and increased susceptibility to pests and drought when exposed to high pollutant and acid loads.
Materials and Cultural Heritage
Air pollutants also damage non‑living structures:
- Acidic gases and particulate matter corrode metals and concrete, and chemically attack stone (especially limestone and marble).
- Soot and dust soil building facades and monuments.
This leads to economic costs and the loss of cultural assets.
Air Pollution and Climate
Air pollutants affect the climate system in different, sometimes opposing ways.
Greenhouse Gases
Several gases important for air quality are also greenhouse gases:
- CO₂ – longest‑lasting and quantitatively most important anthropogenic greenhouse gas.
- Methane (CH₄) – from livestock, rice cultivation, landfills, fossil fuel extraction.
- N₂O – from agriculture and industry.
- Some industrial gases (halogenated compounds) – potent greenhouse gases and, in some cases, ozone‑depleting substances in the stratosphere.
They absorb long‑wave (infrared) radiation emitted by the Earth and thus lead to warming of the atmosphere.
Aerosols
Particles in the air (aerosols) influence:
- Solar radiation:
- Some reflect sunlight and cool the surface (e.g., sulfate particles from SO₂).
- Others absorb sunlight and warm the atmosphere (e.g., black carbon, a component of soot).
- Cloud formation:
- Aerosols serve as condensation nuclei for water droplets, affecting cloud brightness, lifetime, and precipitation patterns.
Thus, the total climate effect of air pollution is a combination of warming (greenhouse gases, black carbon) and cooling (reflective aerosols); however, the long‑term warming effect of greenhouse gases dominates.
Measurement and Assessment of Air Quality
To evaluate air pollution and its effects, monitoring networks and standardized indicators are used.
Measuring Pollutants
Typical measured parameters:
- Concentrations of:
- NO₂, SO₂, CO, O₃.
- PM₁₀ and PM₂.₅.
- Benzene and other VOCs.
- Heavy metals associated with particulate matter.
- Meteorological data (wind, temperature, humidity, solar radiation), which influence pollutant dispersion and formation.
Measurement stations are placed in:
- Urban background (general city levels).
- Traffic hot spots (busy roads).
- Industrial areas.
- Rural and remote areas (background levels, transboundary pollution).
Air Quality Indices
Many countries summarize current pollutant levels into a simple index (e.g., “Air Quality Index”), which classifies air quality from “good” to “very unhealthy” based on thresholds designed to protect human health.
These indices are especially important for sensitive groups, such as people with lung or heart diseases, the elderly, and children.
Approaches to Reducing Air Pollution
Measures against air pollution operate on several levels: technical, regulatory, and behavioral. Detailed policy and legal aspects are treated elsewhere; here we outline principles specific to air.
Technical Measures
- Emission reduction at the source:
- Flue gas desulfurization in power plants (removal of SO₂).
- Catalytic converters and particulate filters in vehicles (reduction of NOₓ, CO, hydrocarbons, and particles).
- Low‑NOₓ burners and optimization of combustion systems.
- Fuel and process changes:
- Switching from high‑sulfur coal and heavy oil to low‑sulfur fuels or renewable energy.
- Closed industrial processes and better dust collection systems.
- Agricultural measures:
- Covered manure storage, injection of slurry into soil.
- Improved feeding strategies to reduce nitrogen excretion.
Regulatory and Economic Instruments
- Emission limits and quality standards for air pollutants.
- Ban or phase‑out of particularly harmful substances (e.g., lead in gasoline, certain halogenated hydrocarbons).
- Zoning and traffic measures:
- Low‑emission zones in cities.
- Speed limits or driving bans during smog episodes.
- Economic incentives:
- Taxes on fuels or CO₂.
- Emissions trading systems.
Behavioral and Structural Changes
- Reduction of fossil fuel use through:
- Public transport, cycling, and walking instead of car use.
- Improved building insulation and efficient heating/cooling.
- Preference for regional products and reduced meat consumption, which can lower transport and agricultural emissions.
- Urban planning that reduces traffic and promotes green spaces can both lower emissions and mitigate some effects of air pollution (e.g., through altered air flow and deposition).
Interconnections Between Air, Water, and Soil Pollution
Although this chapter focuses on air, air pollution is tightly coupled to contamination of other compartments:
- Gaseous pollutants become acid rain, affecting lakes and soils.
- Deposition of nitrogen compounds from the air alters nutrient cycles and contributes to eutrophication.
- Heavy metals and persistent organic compounds transported in the air deposit onto soils and water surfaces and then enter food webs.
Understanding air not as an isolated “medium” but as part of a cycle of transport and transformation is crucial for assessing the full ecological impact of human activities.