Topic 7.1 Notes – Introduction to Air Pollution
What Air Pollution Is
Air pollution means gases, particles, or other substances in the atmosphere at concentrations high enough to harm people, other organisms, or the environment.
The core idea here is a chain:
- Source = the activity or process releasing a pollutant or a precursor
- Pollutant = the harmful substance itself
- Effect = what that substance does to health or ecosystems
In this topic, the biggest source is combustion, especially burning coal and other fossil fuels.
One detail students miss a lot is that emissions are not the same as air concentration. A smokestack can release a pollutant, but what builds up in ambient air depends on:
- wind and atmospheric mixing
- sunlight driving reactions
- precipitation removing pollutants
- chemical changes in the air
So what comes out of the source is not always what you measure later.
Primary and Secondary Pollutants
The difference is about how the pollutant forms.
Primary pollutants
Primary pollutants are emitted directly in harmful form.
Examples you need to know:
- Carbon monoxide (CO) from incomplete combustion
- Sulfur dioxide (SO₂) from sulfur-containing fuels
- Nitrogen oxides (NOx) from high-temperature combustion
- Hydrocarbons and VOCs from exhaust and fuel evaporation
- Soot, smoke, fly ash, metal particles
- Lead and mercury from fuels or industrial processes
Secondary pollutants
Secondary pollutants form in the atmosphere from reactions involving primary pollutants or precursors.
Examples:
- Ground-level ozone from NOx + VOCs + sunlight
- Nitric acid from nitrogen oxides
- Sulfuric acid from sulfur dioxide
- Sulfate and nitrate particles
- Photochemical smog, which is a mixture with several secondary pollutants
Ground-level ozone is the classic example. Sunlight drives reactions between NOx and VOCs released from sources like vehicles, industry, and fuel vapors.
Ground-level ozone formation
The particulate matter exception
Particulate matter (PM) can be either one.
- Primary PM = emitted directly, like soot, smoke, fly ash
- Secondary PM = forms in air, like sulfate, nitrate, and ammonium particles
That means you classify PM by its formation pathway, not by the name “particulate matter” alone.
Major Pollutants, Their Sources, and Their Effects
| Pollutant | Common source | Main effect | Type |
|---|---|---|---|
| Carbon dioxide | Complete combustion of coal and other fossil fuels | Enhanced greenhouse effect, climate change | Primary |
| Carbon monoxide | Incomplete combustion in cars, heaters, fires, generators | Binds to hemoglobin, lowers oxygen transport, can be deadly | Primary |
| Nitrogen oxides | High-temperature combustion in vehicles and power plants | Respiratory irritation; precursor to ozone, smog, nitric acid, nitrate PM | Primary when emitted |
| Sulfur dioxide | Burning coal and sulfur-containing diesel | Respiratory irritation, plant damage, acid rain precursor | Primary |
| Hydrocarbons/VOCs | Exhaust, fuel evaporation, solvents, paints, industry | Toxic/carcinogenic; ozone and smog precursor | Primary |
| Particulate matter | Coal, diesel, biomass burning, industry, dust | Asthma, lung and heart disease, premature death, reduced visibility | Primary or secondary |
| Lead, mercury | Coal combustion, metal processing, waste combustion; historically leaded gasoline | Neurotoxicity; lead harms child development | Primary |
For PM size, smaller particles penetrate farther into the respiratory system.
- PM10 enters the respiratory tract
- PM2.5 reaches deep into the lungs and is usually more dangerous
Particulate matter by size
Combustion Sources and What They Commonly Release
Different fuels make different pollutant mixes.
- Coal releases CO₂, SO₂, particulates/fly ash, toxic metals, and NOx
- Diesel releases NOx, particulate matter, and SO₂ if sulfur is present
- Gasoline vehicles commonly release CO, NOx, and hydrocarbons/VOCs
- Natural gas releases CO₂ and NOx, has less sulfur and little ash, and can release CO if combustion is incomplete
That’s why “fossil fuels cause pollution” is too vague for APES. You want the specific fuel and the specific pollutant.
Why This Matters and the Lead Regulation Example
Pollutants can cause harm directly, and they can also act as precursors that create later problems like photochemical smog and acid rain.
The Clean Air Act gave the EPA power to regulate air pollutants. A classic example is lead. Lead in fuel was regulated, atmospheric lead dropped sharply, and human exposure decreased.
The six criteria air pollutants to recognize are:
- carbon monoxide
- lead
- nitrogen dioxide
- sulfur dioxide
- particulate matter
- ground-level ozone
When you get a scenario, run this sequence:
- Identify the source
- Name the emitted pollutant
- Decide primary or secondary
- State the effect
Key Takeaways
Air Pollution
Gases, particles, or other atmospheric substances present at concentrations that harm health, organisms, materials, or environmental systems
Coal Combustion
Burning coal releases CO₂, SO₂, particulate matter such as fly ash, toxic metals such as mercury and lead, and nitrogen oxides
Fossil-Fuel Combustion
Burning coal, oil, natural gas, gasoline, or diesel can release CO₂, CO, NOₓ, SO₂, hydrocarbons, VOCs, particulate matter, and toxic metals
Carbon Dioxide (CO₂)
A product of complete carbon combustion that contributes to the enhanced greenhouse effect and climate change
Carbon Monoxide (CO)
A colorless, odorless primary pollutant from incomplete combustion that binds to hemoglobin and reduces the blood’s oxygen-carrying capacity
Nitrogen Oxides (NOₓ)
Primarily NO and NO₂ formed during high-temperature combustion; they irritate the respiratory system and form ozone, nitric acid, nitrate particles, and photochemical smog
Sulfur Dioxide (SO₂)
A primary pollutant from burning sulfur-containing coal or oil, including diesel, that irritates lungs and forms sulfuric acid and sulfate particles
Hydrocarbons
Compounds containing hydrogen and carbon, often released as unburned fuel, that can react with NOₓ in sunlight to form ground-level ozone and photochemical smog
Volatile Organic Compounds (VOCs)
Organic compounds that readily enter the gas phase; emissions from fuels, solvents, paints, and industry help form ozone and photochemical smog with NOₓ and sunlight
Particulate Matter (PM)
Solid particles and liquid droplets suspended in air; primary PM is emitted directly, while secondary PM forms through atmospheric reactions
PM₁₀
Particulate matter 10 micrometers or less in aerodynamic diameter that can enter the respiratory tract
PM₂.₅
Fine particulate matter 2.5 micrometers or less in diameter that can penetrate deep into the lungs and is associated with respiratory and cardiovascular harm and premature death
Lead (Pb)
A persistent neurotoxic metal formerly emitted widely from leaded gasoline; it especially harms childhood nervous-system development and cognitive function
Mercury (Hg)
A neurotoxic metal released by sources including coal combustion and transported through the atmosphere before deposition
Ground-Level Ozone (O₃)
A secondary pollutant formed when NOₓ and VOCs react in sunlight; it damages respiratory tissues and plants and is a major component of photochemical smog
Primary Pollutants vs. Secondary Pollutants
Primary pollutants are emitted directly from sources; secondary pollutants form in the atmosphere through reactions involving primary pollutants or precursors
Clean Air Act
The federal law, especially the 1970 act and later amendments, that authorizes EPA air-quality standards and emissions regulation; under it, the EPA phased out leaded gasoline, dramatically reducing atmospheric lead
Criteria Air Pollutants
The six common pollutants regulated by national ambient air-quality standards: CO, lead, NO₂, SO₂, particulate matter, and ground-level ozone
Notes
Air Pollution
Gases, particles, or other atmospheric substances present at concentrations that harm health, organisms, materials, or environmental systems
Coal Combustion
Burning coal releases CO₂, SO₂, particulate matter such as fly ash, toxic metals such as mercury and lead, and nitrogen oxides
Fossil-Fuel Combustion
Burning coal, oil, natural gas, gasoline, or diesel can release CO₂, CO, NOₓ, SO₂, hydrocarbons, VOCs, particulate matter, and toxic metals
Carbon Dioxide (CO₂)
A product of complete carbon combustion that contributes to the enhanced greenhouse effect and climate change
Carbon Monoxide (CO)
A colorless, odorless primary pollutant from incomplete combustion that binds to hemoglobin and reduces the blood’s oxygen-carrying capacity
Nitrogen Oxides (NOₓ)
Primarily NO and NO₂ formed during high-temperature combustion; they irritate the respiratory system and form ozone, nitric acid, nitrate particles, and photochemical smog
Sulfur Dioxide (SO₂)
A primary pollutant from burning sulfur-containing coal or oil, including diesel, that irritates lungs and forms sulfuric acid and sulfate particles
Hydrocarbons
Compounds containing hydrogen and carbon, often released as unburned fuel, that can react with NOₓ in sunlight to form ground-level ozone and photochemical smog
Volatile Organic Compounds (VOCs)
Organic compounds that readily enter the gas phase; emissions from fuels, solvents, paints, and industry help form ozone and photochemical smog with NOₓ and sunlight
Particulate Matter (PM)
Solid particles and liquid droplets suspended in air; primary PM is emitted directly, while secondary PM forms through atmospheric reactions
PM₁₀
Particulate matter 10 micrometers or less in aerodynamic diameter that can enter the respiratory tract
PM₂.₅
Fine particulate matter 2.5 micrometers or less in diameter that can penetrate deep into the lungs and is associated with respiratory and cardiovascular harm and premature death
Lead (Pb)
A persistent neurotoxic metal formerly emitted widely from leaded gasoline; it especially harms childhood nervous-system development and cognitive function
Mercury (Hg)
A neurotoxic metal released by sources including coal combustion and transported through the atmosphere before deposition
Ground-Level Ozone (O₃)
A secondary pollutant formed when NOₓ and VOCs react in sunlight; it damages respiratory tissues and plants and is a major component of photochemical smog
Primary Pollutants vs. Secondary Pollutants
Primary pollutants are emitted directly from sources; secondary pollutants form in the atmosphere through reactions involving primary pollutants or precursors
Clean Air Act
The federal law, especially the 1970 act and later amendments, that authorizes EPA air-quality standards and emissions regulation; under it, the EPA phased out leaded gasoline, dramatically reducing atmospheric lead
Criteria Air Pollutants
The six common pollutants regulated by national ambient air-quality standards: CO, lead, NO₂, SO₂, particulate matter, and ground-level ozone
