Emissions and environmental implications Notes

Where Do Key Air Pollutants Come From?

1. When we burn fuels, especially fossil fuels, we release energy and a range of unwanted by-products into the atmosphere.
2. The most important pollutants you need to know are:
   1. Carbon monoxide (CO)
   2. Carbon dioxide (CO₂)
   3. Sulfur dioxide (SO₂)
   4. Nitrogen oxides (NO and NO₂, often written together as NOₓ)
   5. Particulates (tiny solid or liquid particles, often called PM)

Oxides of Carbon

Carbon monoxide (CO)

Produced by incomplete combustion of hydrocarbons when there is a limited supply of oxygen, e.g.:

1. Poorly ventilated gas heaters
2. Car engines idling in enclosed spaces
3. Forest fires

Carbon dioxide (CO₂)

1. Formed by complete combustion of hydrocarbons: $$\text{hydrocarbon}+O_2 \to CO_2+H_2O$$
2. Major sources:
   1. Power stations burning coal, oil or gas
   2. Vehicle exhausts
   3. Industrial processes (e.g. cement manufacture)
   4. Domestic heating, cooking, and open fires

Oxides of Nitrogen (NOₓ)

1. Air is ~78% nitrogen (N₂) and ~21% oxygen (O₂).
2. At very high temperatures (e.g. inside car engines, power station boilers), N₂ and O₂ can react:
   1. First, nitrogen and oxygen form nitric oxide (NO)
   2. Then NO can react further with O₂ in the air to form nitrogen dioxide (NO₂)
3. Main sources:
   1. Vehicle engines (cars, lorries, buses, aircraft)
   2. Power stations and industrial furnaces
   3. Lightning (natural, smaller contribution)

Sulfur Dioxide (SO₂)

1. Many fossil fuels (especially coal and some crude oils) contain small amounts of sulfur impurities.
2. When these fuels are burned, sulfur is oxidised: $$S+O_2 \to SO_2$$
3. Main sources:
   1. Coal-fired power stations
   2. Oil refineries
   3. Some industrial processes (e.g. metal smelting)
   4. Volcanoes (natural source)
4. SO₂ is a major cause of acid rain and respiratory problems.

Particulates (Particles / PM)

1. Particulates are tiny solid or liquid particles in the air, including:
   1. Soot (carbon) from incomplete combustion of fuels
   2. Tiny droplets or particles containing:
      1. Metals (e.g. lead, mercury, iron)
      2. Compounds such as cyanides, dioxins, sulfur compounds
2. Main sources:
   1. Diesel engines
   2. Burning rubbish (plastics, packaging, cardboard)
   3. Industrial chimneys
   4. Forest and peat fires
   5. Volcanic ash (natural)
3. These are often described as PM₁₀ or PM₂.₅, depending on their size.

How Do These Pollutants Affect Health, Ecosystems and Climate?

1. Air pollutants harm people, ecosystems, and sometimes the global climate.
2. Different pollutants have different effects.

Effects on Human Health

Carbon monoxide (CO)

1. Colourless, odourless, highly toxic gas
2. Binds reversibly but strongly to haemoglobin in red blood cells, forming carboxyhaemoglobin
3. This prevents haemoglobin from carrying O₂, leading to:
   1. Headaches, dizziness, nausea
   2. Loss of consciousness
   3. Death by suffocation at high concentrations

Nitrogen oxides (NOₓ) and sulfur dioxide (SO₂)

1. Irritate lungs and airways
2. Can trigger or worsen:
   1. Asthma
   2. Bronchitis
   3. Other respiratory diseases
3. NO₂ and SO₂ can react with water in the atmosphere to form acid gases, contributing to acid rain and increasing respiratory irritation.

Particulates (PM)

1. Fine particles can be inhaled deep into the lungs
2. Very small particles (PM₂.₅) can pass into the bloodstream
3. Long-term exposure is linked to:
   1. Heart disease
   2. Lung cancer
   3. Reduced lung function
   4. Increased risk of premature death

Effects on Ecosystems

Acid Rain (from SO₂ and NOₓ)

1. SO₂ and NOₓ can react with water and oxygen to form acids (e.g. H₂SO₄, HNO₃)
2. These acids dissolve in rainwater → acid rain
3. Impacts:
   1. Lakes and rivers become more acidic → harms fish and aquatic life
   2. Soils become acidic → nutrients are leached out, harming plants and crops
   3. Trees can be damaged or killed, especially at high altitudes
   4. Buildings and statues made of limestone (CaCO₃) are slowly dissolved and eroded

Ground-Level Ozone and Photochemical Smog

1. In the lower atmosphere (troposphere), ozone (O₃) is formed when:
   1. NO₂ absorbs sunlight and splits to NO + O
   2. The free oxygen atom reacts with O₂ to form O₃
2. In clean air, O₃ and NO reach a balance (a kind of mini-equilibrium).
3. In polluted air, unburnt hydrocarbons (CₓHᵧ) from vehicles react with O₃ to produce radicals and more NO₂, disturbing the balance and building up excess ozone.
4. Effects of ground-level ozone:
   1. Irritates eyes and lungs
   2. Worsens asthma and respiratory conditions
   3. Damages plant tissue → reduces crop yields
   4. Contributes to photochemical smog (a brownish haze over cities)

Effects on Climate

1. Carbon dioxide (CO₂): Main long-lived greenhouse gas from fossil fuels; traps infrared radiation; leads to global warming and climate change.
2. Methane (CH₄): Not in the list above, but also a strong greenhouse gas from agriculture, landfills and fossil fuel extraction.
3. Nitrogen oxides (NOₓ):
   1. Contribute indirectly to climate change by influencing ozone and other atmospheric chemistry.
4. Particulates:
   1. Some particles (e.g. black carbon / soot) absorb sunlight, warming the atmosphere and speeding up melting of ice and snow.
   2. Other particles (sulphate aerosols) reflect sunlight, producing a cooling effect.

How Can We Reduce Harmful Emissions and Their Impacts?

Reducing air pollution is a shared responsibility between individuals, communities and governments, supported by technology and green chemistry.

Individual Actions

Even small changes in daily life can help lower emissions:

1. Use less energy at home
   1. Turn off unused lights and appliances
   2. Improve insulation and use efficient heating
2. Choose cleaner transport
   1. Walk, cycle or use public transport where possible
   2. Car-share to reduce the number of vehicles on the road
3. Avoid burning waste
   1. Do not burn plastics, packaging or garden waste – this releases particulates and toxic gases
4. Support cleaner technologies
   1. Choose energy from renewable sources if available (e.g. through your energy provider)
   2. Consider lower-emission or electric vehicles when possible

Community and City-Level Actions

Communities and local authorities can:

1. Improve public transport systems, making them more reliable and affordable
2. Create low-emission zones where highly polluting vehicles are restricted
3. Plant trees and green spaces:
   1. Absorb CO₂
   2. Help trap particulates
   3. Cool urban areas
4. Improve waste management:
   1. Reduce open burning of waste
   2. Increase recycling and composting

Government and Industry Actions

Governments and industries have the greatest power to reduce emissions at scale.

Cleaner Combustion and Treatment Technologies

Catalytic converters (vehicles)

1. Fitted to car exhaust systems to reduce:
   1. Unburnt hydrocarbons (CₓHᵧ)
   2. Carbon monoxide (CO)
   3. Nitrogen oxides (NOₓ)
2. Typical reactions inside a catalytic converter:
   1. Oxidation of $CO$ and hydrocarbons: $$CO+\frac{1}{2}O_2→CO_2$$ $$​C_xH_y+O_2 \to CO_2+H_2O$$
   2. Reduction of $NO$: $$2NO \to N_2+O_2$$
3. The catalyst is usually a thin coating of platinum, palladium and rhodium on a ceramic support with a very large surface area.
4. Flue-gas desulfurisation (power stations)
   1. Industrial chimneys can use systems to remove SO₂ from exhaust gases, e.g. by reacting it with calcium carbonate (limestone) to form calcium sulphite/sulphate.
5. Particulate filters
   1. Diesel vehicles and some industrial plants use filters or electrostatic precipitators to trap particulates before gases are released.

Changing the Fuel Mix

1. Low-sulfur fuels: Removing sulfur from fuels before burning reduces SO₂ emission.
2. Biofuels: Fuels derived from recently living biomass:
   1. CO₂ released when they burn is (partly) balanced by CO₂ absorbed when the plants grew.
   2. This does not make them completely “carbon neutral”, but it can reduce net emissions if managed well.
3. Renewable energy (solar, wind, hydro, geothermal):
   1. Reduce reliance on fossil fuels
   2. Produce electricity without CO₂, NOₓ and SO₂ emissions at the point of use.

Laws, Regulations and Monitoring

Governments can:

1. Set emission limits for power stations, factories and vehicles
2. Require vehicle emission testing and standards (e.g. Euro emission standards)
3. Monitor air quality and publish air pollution indexes
4. Support research and development into cleaner technologies

Green Chemistry – Designing Cleaner Processes

Key principles include:

1. Preventing waste rather than treating it afterwards
2. Using safer, less hazardous chemicals
3. Maximizing energy efficiency (e.g. lower-temperature reactions)
4. Designing products that:
   1. Last for multiple cycles (reused or recycled)
   2. Break down into harmless substances at the end of their life
5. Reducing unnecessary derivatives and steps in synthesis to use fewer chemicals and less energy