7.1 Acid deposition

• Acid deposition: mix of air pollutants that deposit from the atmosphere as acidic wet deposition (with a pH <5.6) or acidic dry deposition

  • Wet deposition/acid rain (e.g. snow, rain, hail, fog) — secondary air pollutants (smog)

  • Dry deposition (e.g. dust & gases) — primary air pollutants (e.g. SO2 & NOx)

    • Washed off by rain → acidic water that harms plants + wildlife

• Wind can blow pollutants long distances → air pollution = global problem

• Acid deposition formation:

  1. fossil fuels contain sulfur compounds

  2. combustion of fossil fuels releases sulfur dioxide gas

  3. sulfur dioxide gas reacts with water and oxygen in the atmosphere to form sulfuric acid

  4. nitrogen from the atmosphere reacts with oxygen in the high temperatures of vehicle engines to form nitrogen monoxide gas

  5. nitrogen monoxide gas is released into the atmosphere in vehicle emissions

  6. nitrogen monoxide gas reacts with oxygen and water in the atmosphere to form nitric acid

• Coal-burning power stations created the most acid deposition

Impacts of acid deposition on…

• Aquatic environments: acidic water leaches aluminum from clay & is transported into water + clogs fish gills → decline in fish populations

  • Low pH level → kills fish + their larvae → disrupts food chain

  • Acidification of seawater → dissolves coral (impacts food web + biodiversity)

  • Nitrogen in deposition → eutrophication

• Vegetation and crops: increased acidity → aluminum seeps out of soil — removes beneficial nutrients + minerals

  • Acidic fog coats vegetation → nutrients are stripped from plants → loss of leaves → prevents photosynthesis → reduced crop yield

  • Defoliation

• Stone & brick buildings: enhanced chemical weathering (e.g. can dissolve limestone)

7.2 Photochemical smog

• Photochemical smog: mixture of air pollutants and particulates, including ground level ozone, that is formed when oxides of nitrogen and VOCs react in the presence of sunlight

  • More often in sunlight

  • Ex. of secondary pollutant: ground-level ozone → formed b/t chemical reactions with NOx and VOCs

• Particulates: solid particles and liquid droplets in the air — often from burning/dust-generating activities

• VOCs: high vapor pressure + low water solubility; emitted as gases from certain solids or liquids (e.g. paints, paint strippers, cleaning supplies, & pesticides)

  • Can cause: headaches, cancer, damage to the liver & kidneys

Impacts of photochemical smog

• eye and respiratory irritation: can harm vision (inflammation + dry eyes) + lead to illness (e.g. lung cancer + asthma)

• decreased crop yields: reduces photosynthesis → blocks sunlight + pollutant can damage/yellow plant leaves

• deterioration of plastics and rubber → particles + dust stick to these surfaces

  • Acidic atmospheric accelerate the decay of these materials (esp. in humid environments)

7.3 Managing air pollution

• Reduced use of fossil fuels + renewable energy: lack of emissions (e.g. SO2) means production of clean energy doesn’t add to the acid deposition problem

• Reducing emissions of SO2 by:

  • Flue-gas desulfurisation (FGD): remove SO2 from exhaust emissions of fossil-fuel-powered stations

  • Fuel desulfurisation: remove sulfur from a fuel source before it’s burnt

    • e.g) Coal washing & low-sulfur fuels

• Reduce emissions of NOx by catalytic converters: lower emissions from exhaust systems in vehicles; reduce air pollution

• Reduce emissions of particulates using electrostatic precipitators: use electric charge to remove particulates from gases emitted in industrial smoke

  • e.g) removing oil mist in machine shops, removing acid mist in chemical plants, etc

• Reduce emissions of VOCs: safe usage (e.g. increased ventilation when using products with VOCs), not storing open containers of VOCs, disposing of empty containers safely

• Restricting vehicle use in urban areas: fewer vehicles → reduced air pollution

  • Congestion charging: people using public transportation because it’s cheaper

• Legislation: manage the production and impact of atmospheric pollution

  • “Polluter pays principle”: polluter is responsible for reducing/preventing it

  • e.g) 1979 Geneva Convention that created framework for reducing acid deposition + air pollution in Europe

  • e.g) Clean Air Act of 1970: permanent limits on SO2 and NOx emission in the U.S.

7.4 Ozone depletion

• Ozone concentration is measured using the Dobson unit

• Ozone hole: an area where the average concentration of ozone is below 100 Dobson Units

• CFCs: chemical compounds that speed up the breakdown of ozone

  • e.g) refrigerants & solvents

• Ozone naturally regenerates - depletion rate from CFCs can be faster than regeneration rate

Occurs by…

• CFCs from aerosols & refrigerants are unreactive compounds and aren’t broken down in the troposphere

• CFCs move into the stratosphere and break down in the presence of UV light to release a chlorine atom

• rapid reactions between chlorine atoms and ozone breaks down ozone (O3) to oxygen (O2), causing ozone depletion

• chlorine atoms remain in the stratosphere and can continue to destroy ozone

Ozone depletion over Antarctica

• Due to atmospheric conditions in Antarctica

• Ozone-depleting substances are transported great distances by wind

• Very low temperatures for a long period of time → PSCs - stratospheric clouds that form over the poles in winter

• Chemical reactions on liquids + solids increase abundance of chlorine, which reacts with ozone & creates the ozone hole over Antarctica

• Polar vortex: large, long-lasting rotating low-pressure system located over the North and South Poles; strengthens winters

  • Chlorine gases persist for a long time & damage the ozone layer

Impacts of ozone depletion due to increased UV radiation

• Human health: skin cancer, formation of cataracts, & immune system suppression

• Decreased crop yields: impacts how plants form + utilize nutrients → damages growth, which can harm biodiversity

• Biodiversity of terrestrial and aquatic ecosystems: decreased #s of phytoplankton & can harm early developmental stages of many marine organisms

• degradation of materials used in clothing and construction: lose strength, crack, & disintegrate

Impacts of alternatives to CFCs

• HCFCs: less stable than CFCs & break down more quickly in the atmosphere

  • Less global warming potential + impact on ozone

• F-gases/HFCs: don’t harm ozone layer, BUT are a powerful GHG

  • 23000x greater global warming effect than CO2

Rowland-Molina Hypothesis

• Suggested CFCs could reach the stratosphere where they’d release chlorine atoms due to UV radiation

• Initially wasn’t accepted — didn’t carry out experiments; based on existing information

• Some of the auxiliary hypotheses were not backed up by experimental evidence

• Led to further research and data collection by other scientists, which confirmed that CFCs are ozone depleting

International agreements

• Montreal Protocol (1987): 50% reduction of CFC use by 2000; negotiated by 24 countries & resulted in heavy financial burdens if failed

  • Further expanded in recent years (e.g. limiting the amount of F-gases)