8.3 Urban Air Pollution

Guiding Question

  • How can urban air pollution be effectively managed?

Definition and Sources

  • Urban air pollution is caused by inputs from human activities to atmospheric systems, including:

    • Nitrogen oxides (\text{NO}_x)

    • Sulfur dioxide (\text{SO}_2)

    • Carbon monoxide (CO)

    • Particulate matter (PM)

  • These human activities range from industrial processes and energy generation to transportation and household activities.

  • Sources of primary pollutants are both natural and anthropogenic.

  • The most common air pollutants in urban environments are derived from combustion of fossil fuels, primarily from vehicular exhaust, industrial chimneys, and power plants.

  • Air pollution is defined as the contamination of the indoor or outdoor environment by any chemical, physical, or biological agent that modifies the natural characteristics of the atmosphere.

    • Particulate Matter (PM): Very small particles of solids or liquids, such as dust, soot, and smoke, suspended in the air. PM2.5 (particles with a diameter less than 2.5 micrometers) are particularly dangerous as they can penetrate deep into the lungs and bloodstream.

    • Air pollution can come from:

      • Natural sources: Volcanoes (releasing sulfur compounds and ash), dust storms (fine soil particles), wildfires (soot, carbon monoxide, hydrocarbons), and biological sources like pollen and mould spores.

      • Anthropogenic sources: Vehicle exhausts (releasing \text{NO}_x, CO, hydrocarbons, PM), industrial emissions (various organic and inorganic compounds), power generation, agricultural activities (ammonia, pesticides), burning of solid fuels, and indoor activities like use of spray cleaning products, cooking, and heating.

Types of Pollutants
Primary Pollutants

  • Emitted directly from processes. These pollutants retain their chemical form from their source.

  • Most common anthropogenic air pollutants from fossil fuel combustion include:

    • Carbon monoxide (CO): A colorless, odorless, and toxic gas formed from the incomplete combustion of fossil fuels, common in vehicle exhaust and faulty heating systems.

    • Carbon dioxide (\text{CO}_2): A major greenhouse gas from complete combustion of fossil fuels, contributing to climate change.

    • Unburned hydrocarbons (VOCs): Volatile organic compounds that contribute to ground-level ozone formation and can be toxic.

    • Nitrogen oxides (\text{NO}x): Primarily nitrogen dioxide (\text{NO}2), formed at high temperatures during the combustion of fuels in vehicle engines and power plants. NO_x contributes to acid rain and smog.

    • Sulfur dioxide (\text{SO}_2): Released primarily from the burning of coal and oil with high sulfur content in power plants and industrial facilities. It is a major precursor to acid rain.

    • Particulate matter (PM): Direct emissions from combustion processes, vehicle wear and tear, and industrial operations.

    • Mercury: A heavy metal emitted from coal-fired power stations, which can bioaccumulate in the environment and food chains.

Secondary Pollutants

  • Formed when primary pollutants react with other atmospheric chemicals, often in the presence of sunlight.

  • Examples include:

    • Tropospheric ozone (\text{O}3): Formed by photochemical reactions involving nitrogen oxides (\text{NO}x) and volatile organic compounds (VOCs) in sunlight. Unlike stratospheric ozone, tropospheric ozone is a harmful air pollutant and a component of smog.

    • Particulates from gaseous primary pollutants: Fine secondary particulate matter can form from the condensation and reaction of gaseous pollutants like \text{SO}2 and \text{NO}x into sulfates and nitrates.

    • Peroxyacetyl nitrate (PAN): A component of photochemical smog, formed from reactions involving hydrocarbons and \text{NO}_x. It is a phytotoxic pollutant (damaging to plants).

    • Sulfuric acid (\text{H}2\text{SO}4) and nitric acid (\text{HNO}3): Formed when \text{SO}2 and \text{NO}_x react with water vapor, oxygen, and other chemicals in the atmosphere, leading to acid deposition (acid rain, snow, fog, or dry particles).

Health and Environmental Impact

  • Air pollution is a significant risk factor for many leading causes of death, by damaging the respiratory and cardiovascular systems and exacerbating chronic conditions:

    • Heart disease, stroke, lower respiratory infections, lung cancer, diabetes, and chronic obstructive pulmonary disease (COPD).

    • Exposure to PM2.5 can lead to premature deaths and reduced life expectancy.

  • In 2019, it contributed to 11.65% of all deaths globally, highlighting its widespread impact.

  • Greater air pollution prevalence in low-income countries (LICs) and middle-income countries (MICs) due to:

    • Reliance on solid fuels (wood, dung, crop residues, coal) for cooking and heating in poorly ventilated homes (indoor pollution) in many LICs, leading to high exposure to PM and CO.

    • Rapid industrialization, urbanization, and less stringent environmental regulations contributing to severe outdoor pollution in MICs.

  • According to the World Health Organization (WHO), 2.5 billion people are exposed to air pollution levels seven times higher than recommended guidelines, indicating a global health crisis.

  • In LICs, over 90% of urban air pollution often comes from poorly maintained and older vehicles lacking modern emission controls.

Photochemical Smog

  • Formed when sunlight acts on primary pollutants, particularly \text{NO}_x and VOCs, transforming them into a complex mixture of secondary pollutants (like ozone and PAN).

  • It appears as a yellowish-brown haze, reducing visibility and is characteristic of urban areas with high vehicle traffic.

  • Meteorological (e.g., thermal inversions trapping pollutants close to the ground, light winds) and topographical factors (e.g., valleys) can intensify processes that cause photochemical smog formation by preventing the dispersal of pollutants.

  • Direct impacts: Biological (respiratory problems, eye irritation in humans; damage to crops and forests) and physical (material degradation, reduced visibility, altered atmospheric chemistry), affecting human health and ecosystems.

  • Indirect impacts: Societal costs (increased healthcare expenses, lost productivity, reduced tourism) and lost economic output due to adverse health effects and environmental damage.

Wildfires

  • While natural occurrences (e.g., lightning), human activities contribute to 90% of wildfires, often through accidental ignitions or arson. Climate change also increases their frequency and intensity.

  • Produce a wide array of pollutants including soot (black carbon), dust, carbon monoxide, nitrogen oxides, and volatile organic compounds.

  • These pollutants can linger in the air for days or weeks and travel thousands of kilometers, contributing to thermal pollution, increasing fire risk in susceptible areas, and widely impacting air quality. This also results in significant CO_2 emissions.

Additional Sources of Air Pollution
Chemical Industries

  • Release various organic (e.g., benzene, toluene, formaldehyde) and inorganic chemical compounds (e.g., chlorine, hydrogen sulfide), and heavy metals into the atmosphere. These can be toxic, carcinogenic, or contribute to acid rain and smog formation.

Construction and Demolition

  • A major source of particulate matter pollution due to airborne dust (from concrete, wood, silica), as well as emissions of \text{NO}_x and PM from diesel machinery and vehicles. Asbestos exposure during demolition of older buildings is also a concern.

Indoor Air Pollution

  • Caused by inefficiencies in fuel use (e.g., wood, charcoal, kerosene) in poorly ventilated homes, leading to high concentrations of emissions from wood-burning stoves, tobacco smoke, volatile organic compounds (VOCs) from building materials and furnishings, and cleaning products. Other sources include radon gas and biological allergens.

Statistics on Indoor Air Pollution

  • One-third of the global population still relies on unsafe cooking fuels and technologies.

  • In 2020, household air pollution caused approximately 3.2 million deaths, primarily due to respiratory and cardiovascular diseases.

  • An alarming 650 children under age five die daily from household air pollution, mainly from pneumonia and other acute respiratory infections.

  • Combined, ambient (outdoor) and household (indoor) air pollution causes 6.7 million premature deaths per year, disproportionately affecting women and children who spend more time indoors or are more physiologically vulnerable.

Strategies for Urban Air Pollution Management
Management Approaches

  1. Altering Human Activity

    • A preventative approach that is often the most cost-effective. It involves fundamental changes in behavior, technology, and policy:

      • Promoting renewable energy sources (solar, wind) over fossil fuels.

      • Implementing urban planning that reduces travel distances and encourages mixed-use developments.

      • Educational campaigns to raise public awareness about emission sources and health impacts.

      • Government legislation and economic incentives (e.g., subsidies for electric vehicles, carbon taxes).

  2. Controlling Pollutant Release

    • Regulatory measures imposed by legislation to control emissions at the source or end-of-pipe. This typically involves pollution extraction and abatement technologies:

      • Installing scrubbers in industrial chimneys to remove \text{SO}_2 and other gases.

      • Using electrostatic precipitators or baghouse filters to capture particulate matter from industrial exhausts.

      • Implementing improved industrial processes to minimize waste and emissions.

      • Requiring catalytic converters in vehicles.

Specific Strategies to Reduce Urban Air Pollution

  1. Improve Public Transportation

    • Investing in and promoting efficient, extensive, and affordable public transport networks (e.g., electric buses, trams, trains).

    • Using alternative fuels (e.g., natural gas, electric) for public transport vehicles.

    • Reducing road congestion by encouraging modal shifts away from private cars, resulting in significantly lower overall emissions per passenger-mile.

  2. Enhance Cycling Infrastructure

    • Install dedicated, safe cycleways and bike-sharing programs to promote cycling as a viable mode of transport.

    • This reduces reliance on fossil fuel vehicles, cuts carbon emissions, and improves physical health.

  3. Increase Urban Vegetation

    • Planting trees, creating green roofs, and establishing urban parks absorb gaseous pollutants like ozone and nitrogen dioxide through stomatal uptake.

    • Their leaf surfaces also intercept particulate matter, reduce urban heat island effects, and provide aesthetic and psychological benefits.

  4. Legislate Catalytic Converters

    • Mandating the installation and maintenance of three-way catalytic converters in all gasoline vehicles. These devices convert toxic pollutants (carbon monoxide, nitrogen oxides, and unburned hydrocarbons) into less harmful substances (carbon dioxide, nitrogen, and water vapor).

  5. Pedestrianize Town Centers

    • Banning non-essential vehicle traffic in central urban areas to reduce local pollution, noise, and traffic congestion.

    • This creates safer, cleaner, and more pleasant environments that promote walking, cycling, and community engagement.

Additional Strategies

  • Encourage the buying of efficient, low-polluting vehicles, including electric and hybrid models, through incentives and charging infrastructure development.

  • Promote carpooling and reduced car usage through ride-sharing apps, public awareness campaigns, and employer incentives.

  • Implement strict fuel quality and emissions standards for vehicles and industrial facilities.

  • Increase green infrastructure such as green walls, noise barriers with vegetation, and street trees along roads to act as buffer zones and absorb pollutants.

  • Develop and implement smart city technologies for traffic management and environmental monitoring.

Acid Deposition and Its Management

  • Acid Deposition: Includes both wet deposition (acid rain, snow, fog, cloudwater with a pH less than 5.6) and dry deposition (acidic gases and particles like sulfur dioxide and nitrogen oxides that fall directly to the ground).

  • The primary precursors are \text{SO}2 and \text{NO}x emitted from fossil fuel combustion. These react in the atmosphere with water (\text{H}2\text{O}), oxygen (\text{O}2), and other chemicals to form sulfuric acid (\text{H}2\text{SO}4) and nitric acid (\text{HNO}_3).

  • Effects: Directly impacts aquatic ecosystems (acidification of lakes and streams, harming fish and other aquatic life), soil chemistry (leaching essential nutrients, releasing toxic metals), plant growth (damaging leaves, weakening trees), and biodiversity, as well as corroding buildings, statues, and infrastructure.

  • Management: Liming lakes and forests (adding calcium carbonate) to neutralize acidity, though this is a temporary and localized solution.

  • Broader strategies include reducing emissions at the source through:

    • Flue gas desulfurization (FGD) or