Urban Air Pollution and Acid Rain – Comprehensive Study Notes (IB ESS SL 8.3)

Urban Air Pollution

• Guiding context: Understanding how urban air pollution can be effectively managed.

Urban Air Pollution – Key Facts

• Global exposure and impact
  • 1,000,000 deaths per year are premature due to outdoor air pollution.
  • About 1 billion people are exposed to outdoor air pollution annually.
  • Economic impact: roughly 2% of GDP lost to air pollution in MEDCs and about 5% in LEDCs.
• Major pollutants in urban air
  • Nitrogen oxides (NOx)
  • Sulphur dioxide (SO2)
  • Carbon monoxide (CO)
  • Particulate matter (PM)
  • PM is categorized by size: $PM<em>{2.5}$ (diameter ≤ 2.5 μm) and $PM</em>{10}$ (diameter ≤ 10 μm).
  • These pollutants originate from both natural and anthropogenic sources.

Primary vs Secondary Pollutants

• Primary pollutants
  • Directly emitted at the source; act immediately on emission.
  • Natural sources include volcanoes, dust storms, wildfires.
  • Anthropogenic sources include burning fossil fuels (energy production, transport), agricultural burning, biomass burning, and dust from construction/roads.
• Secondary pollutants
  • Formed in the atmosphere from reactions of primary pollutants with other chemicals (often in sunlight).
  • Example: Tropospheric (ground-level) ozone formed via photochemical reactions involving NOx and other compounds.

Indicator Species and Lichens

• Indicator species are used to assess environmental conditions.
• Lichens are particularly sensitive to gaseous pollutants like SO2.
• Historical pattern: coal-fired power stations increased SO2, leading to localised extinctions of many lichen species.
• After emission reductions, lichen communities recover away from pollutant sources; only tolerant species survive downwind of heavy industry.
• Lichens are used as an indirect measure of pollution; they grow on trees and buildings and are sensitive to pollution levels.

Toxic Effects of Acid Deposition – Lichens

• Lichens: symbiotic relationship between algae and a fungus; used as bioindicators of acidity and pollution.
• Sensitive to SO2; presence/absence and diversity reflect pollution history and current levels.
• Downwind areas show fewer sensitive species; distance from source correlates with increased species richness.

Studying Lichens – Practical Skills

• Investigations designed to examine how air pollution affects lichen distribution (as per course instructions).

Combustion of Fossil Fuels – Key Pollutants and Health Impacts

• Most common urban pollutants arise from combustion of fossil fuels.
• Particulate matter (PM2.5 and PM10)
  • Largely from combustion in industry and vehicle emissions.
  • Higher PM correlates with increased respiratory illnesses and deaths (e.g., lung cancer).
• Carbon monoxide (CO)
  • From incomplete combustion of fossil fuels in motor vehicles.
  • Reduces oxygen uptake by haemoglobin in red blood cells; increases heart stress and affects the nervous system.
• Sulphur dioxide (SO2)
  • Emitted from burning coal/oil in power stations.
  • Causes respiratory problems, aggravates heart conditions, and damages plants.
• Nitrogen oxides (NO and NO2, collectively NOx)
  • Formed when nitrogen in the air reacts with oxygen at high combustion temperatures (e.g., vehicle engines).
  • Can irritate respiratory passages (coughs, sore throats).
  • Lead to photochemical reactions contributing to ozone formation.

Secondary Pollutants – Photochemical Formation

• In presence of sunlight, primary pollutants undergo reactions to form secondary pollutants.
• Tropospheric ozone (O3) is a key secondary pollutant:
  • Formed when oxygen molecules react with oxygen atoms released from NO2 in sunlight.
  • Simplified sequence: $ext{NO}<em>2 + h\nu \rightarrow \text{NO} + \text{O}$; $\text{O} + \text{O}</em>2 \rightarrow \text{O}_3$

Reducing Urban Air Pollution – Management Strategies

• A broad suite of strategies is used to reduce urban air pollution:

Improved Public Transportation

• Promote trains, trams, and buses to reduce car use and emissions.
• Low-emission or electric buses shorten pollutant exposure.
• Encourage shared mobility (carpooling, shuttle services).
• Examples: some cities offer free public transport (e.g., Metro Mover in Miami, USA).

Cycling Infrastructure

• Dedicated bike lanes reduce car use, traffic congestion, and emissions.
• Common in many European countries, especially in Scandinavia.
• Benefits include improved public health and reduced emissions.

Trees and Vegetation

• Trees and plants absorb particulate pollution from the air.
• Green cover improves air quality, provides shade, cools urban areas, and enhances well-being.

Natural Screens and Green Walls

• Green walls can reduce urban air pollution by up to around 30% in some locations.
• Climbing ivy and grass absorb particulates and NOx gases; also offer cooling and aesthetic benefits.
• Drawbacks: high maintenance costs.

Catalytic Converters

• Convert harmful emissions (CO, hydrocarbons, NOx) into less harmful gases (e.g., N2, CO2).
• Since 1993, all new cars in the EU must have catalytic converters.
• Efficiency can be up to ~90% under optimal conditions, but effectiveness is reduced in real-world urban conditions since catalysts require around $150\ ^{\circ}\mathrm{C}$ to be fully effective; urban operating temperatures may limit performance, yielding around 50% effectiveness.

Limited Car Use – Low Emission Zones (LEZ) and Other Measures

• LEZs, tolls, and odd-even license plate schemes reduce car numbers in city centers.
• Example impacts: Rome LEZ reduced NO2 by ~23% and PM10 by ~10%.
• Odd-even plate schemes piloted in cities like Mexico City, Singapore, and Jakarta; wealthier residents can sometimes circumvent by owning multiple cars.
• Goal: promote public transport and reduce congestion-related emissions.

Pedestrianised Town Centres

• Restrict vehicles from key shopping/tourist areas.
• Benefits: reduced local pollution and noise; safer, more attractive public spaces.
• Drawbacks: can make access to town centers more challenging for some residents.

Acid Rain – Formation and Impacts

Formation of Acid Rain

• Primary pollutants: $ext{SO}<em>2$ and $ext{NO}</em>x$ from fossil fuel combustion.
• Secondary pollutants form when these gases react with water and oxygen in the atmosphere, producing nitric and sulfuric acids.
• Acid deposition occurs as dry deposition (near source) and wet deposition (farther afield via rain, snow, fog).
• Acid rain can travel long distances (up to about $1500\ \text{km}$) in clouds before deposition.

The Acid Rain Cycle (Regional Focus)

• Emission from smokestacks introduces $\text{SO}<em>2$ and $\text{NO}</em>x$ into the atmosphere.
• Reactions form $\text{H}<em>2\text{SO}</em>4$ and $\text{HNO}_3$, which are deposited as rain, snow, or dry particles.
• Deposited acids cause acidification in soils and water bodies, with downstream ecological impacts.
• A regional pattern: deposition affects neighboring regions and even other countries due to wind patterns.
• Regional deposition maps show deposition in kg per hectare per year: range from 0 to around 32.8 in some areas (illustrative scale on maps).

Acid Rain – pH Scale and Impacts

• Typical acid rain has a pH around 4; the scale shows broad categories from strong acids (battery acid ~0) to more neutral values (pH ~7).
• Normal rain has a slightly acidic pH (~5.6) due to natural carbonic acid; acid rain lowers pH further.

Impacts of Acid Rain

• On ecology, humans, and buildings.
• Direct effects on soil and water: acidification lowers pH, harming aquatic organisms and weakening tree growth; metals (e.g., aluminum) can become more soluble and toxic.
• Indirect nutrient effects: leaching of essential plant nutrients (Ca, Mg, K) from soils; reduces soil fertility.
• Terrestrial habitats: foliage yellowing, leaf damage, reduced growth, nutrient leaching, easier pathogen entry; microbial symbiosis in roots (nitrogen-fixing bacteria) can be disrupted; reduced nutrient availability to trees.
• Soil health: reduced nutrient retention by soil particles; toxic aluminum ions released from soil particles; damage to root hairs; overall tree decline.
• Peat bogs: acid deposition reduces methane production by outcompeting methane-producing bacteria, altering greenhouse gas dynamics.
• Freshwater habitats: low pH affects salmonids; aluminum mobilization harms fish (e.g., impaired osmoregulation, gill damage, suffocation at higher concentrations).
• Invertebrates: calcifying organisms (e.g., crabs, lobsters, snails) experience dissolution of calcium carbonate shells in acidic waters.
• Construction materials: limestone, marble, bronze, brass, copper corrode; historical monuments and buildings are affected.
• Human health: dry deposition of particulates (sulfur/nitrate compounds) contributes to respiratory problems (pneumonia, asthma, bronchitis) and can cause tissue damage.

Regional and International Aspects

• Regional effects: acid deposition effects often cross borders; emissions from one country can impact ecosystems in another country.
• Regional deposition patterns have led to international cooperation efforts.
• The Acid Rain Cycle and deposition patterns show how pollutants travel and deposit outside their source regions.

Role of International Agreements and Policy Solutions

• Acid rain is an international issue; deposition does not equal emissions in the same country.
• Key agreements and timelines in Europe:
  • 1970s-1980s: rising evidence of acid rain impacts (German forests, lake biodiversity, weathering of buildings).
  • 1979: UN Economic Commission for Europe (UNECE) Long-Range Transboundary Air Pollution (LRTAP) framework begins.
  • 1983–1985–1988: Protocols aiming to reduce SO2 and NOx emissions; 30% reductions target relative to 1980 levels by 1993 (the so-called 30% club).
  • 1994–1999: Additional agreements to cut emissions by larger targets (e.g., 80% of 1980 levels by 2003).
  • 27 countries signed new protocols to reduce and prevent air pollution.
• European Evaluation of Success
  • By 2000, Europe achieved roughly a 50% reduction in emissions.
  • Developing Eastern European nations were industrializing; emissions tended to rise there, highlighting need for technology transfer and support.
• European Reduction in Emissions – Key Directives
  • EU Large Combustions Plants Directive: regulates emissions from large installations with a thermal capacity of 50 MW (electricity plants and large industries).
  • Kyoto Protocol: CO2-focused but also contributes to reducing acid deposition indirectly through broader emission reductions.

Pollution Management Strategies – A Model

• Management goals: reduce SO2 and NOx impacts, minimize ecological and health effects, and restore damaged systems.
• Three broad categories of strategies: 1) Altering human activity (source reduction):
  • Replace fossil fuels with alternatives (e.g., ethanol for vehicles, renewable electricity).
  • Improve energy efficiency and reduce overall electricity demand (education, insulation).
  • Promote less private transport; increase public transport, cycling, walking.
  • Shift to low-sulfur fuels or fuels burned with limestone to neutralize sulfur.
  • Develop and deploy renewable energy sources (wind, solar) and potentially nuclear.
    2) Controlling at the point of release (end-of-pipe measures):
  • Use cleaner combustion technologies; install waste gas scrubbers to remove SO2 from flue gases in power plants.
  • Catalytic converters in cars to reduce NOx and other pollutants.
    3) Restoring damaged systems (remediation):
  • Liming lakes to neutralize acidity (historical Scandinavia activity in 1950s–1990s).
  • Biodiversity restoration efforts; ecological recovery can be slow and sometimes incomplete.
• Practical considerations and trade-offs:
  • Altering human activity: beneficial for multiple pollutants and CO2, but expensive and requires behavioral changes.
  • End-of-pipe controls: effective but cost, maintenance, and logistics; catalytic converters require maintenance and proper temperatures.
  • Liming ecosystems: can raise pH quickly but is temporary and costly; treats symptoms, not causes; may require repeated application.
  • International agreements: require cooperation, monitoring, and enforcement; implementation varies by country and economic capacity.

Pollution Management Strategies – Example Actions (Summary Matrix)

• Altering the human activity producing pollution
  • Replace fossil fuels with alternatives (e.g., ethanol, renewables) and promote energy efficiency.
  • Reduce electric demand (conservation, insulation).
  • Encourage less private transport (carpool, public transport, walking, cycling).
• Regulating and reducing pollutants at the point of emission
  • Use low sulfur fuels or remove sulfur before burning; burn fuels with limestone to capture SO2.
  • Install end-of-pipe cleanup technologies (scrubbers for SO2; catalytic converters for NOx).
• Clean-up and restoration
  • Liming acidified lakes/rivers; reestablish biodiversity; limestone plantations to influence local acidity.
• International agreements and evaluation
  • Emission reductions (e.g., 30% club, later targets); difficulties in monitoring and enforcement; transfers of clean technologies to developing regions.
• Key observations
  • Each strategy has trade-offs (cost, maintenance, ecological side effects).
  • A combination of strategies is typically required for effective long-term reduction in acid deposition and related impacts.

Numerical and Formula References (Selected Highlights)

• Pollutant size definitions:
  • $PM_{2.5}$: fine particulate matter with diameter ≤ 2.5 μm
  • $PM_{10}$: particulate matter with diameter ≤ 10 μm
• Critical temperature for catalytic converter effectiveness:
  • Effective operating temperature around $150^{\circ}\mathrm{C}$; efficiency can drop if this isn’t reached.
• Emission reductions targets in Europe (historical):
  • Target reductions of sulfur dioxide by 1980 levels by 1993: 30% reductions (the "+30% club").
  • Target reductions of nitrogen oxides by 1987 levels by 1998 (Sofia Protocol). 80% of 1980 levels by 2003 (later amendments).
• Large Combustions Plants Directive threshold:
  • Plants with a thermal capacity of at least $50\ \mathrm{MW}$ (i.e., $P \ge 50\ \text{MW}$) subject to emission controls.
• Deposition scales (illustrative regional map values):
  • Deposition rates can be represented as $\frac{kg}{ha \cdot yr}; ranges observed on regional maps from 0 up to ~32.8 kg/ha/yr.
• Transport distance of acid deposition:
  • Acid deposition can travel up to about 1500\ \text{km}$$ from the source via clouds before deposition.

Quick recap of connections to real-world relevance

• Urban air pollution directly affects human health, ecosystems, and buildings; addressing it requires a mix of technology, policy, urban planning, and public behavior changes.
• Acid rain exemplifies how emissions in one region can harm distant ecosystems, reinforcing the need for international cooperation and shared technology transfer.
• Indicator species like lichens provide practical, low-cost ways to monitor air quality over time and guide policy decisions.