Study Notes on Chemistry of the Atmosphere and Pollution

Chemistry for Engineers

Objectives

  • By the end of this section, students will be able to:

    • Learn about the chemistry of the atmosphere.

    • Identify common atmospheric pollutants.

Composition and Evolution of the Earth’s Atmosphere

Proportions of Different Gases

  • For approximately 200 million years, the composition of gases in the atmosphere has been largely unchanged:

    • Nitrogen (N₂): about 80%

    • Oxygen (O₂): about 20%

    • Others: small proportions of various gases including carbon dioxide (CO₂), water vapor (H₂O), and noble gases.

    • Summary:

    • 80% N₂

    • 20% O₂

    • <1% (includes CO₂, H₂O, Noble Gases)

Theories of Earth's Early Atmosphere

  • Theories regarding the early atmosphere have evolved due to the limited evidence over a time scale of 4.6 billion years. Theories include:

    • Volcanic Activity: During the first billion years, volcanic eruptions released gases that were predominantly carbon dioxide with minimal oxygen.

    • Formation of Oceans: Water vapor from volcanic activity condensed to form the oceans.

    • Nitrogen Production: Volcanoes contributed nitrogen, building up in the atmosphere.

    • Presence of Other Gases: Small amounts of methane and ammonia were likely present.

    • Sedimentation: When oceans formed, CO₂ dissolved in water, leading to the precipitation of carbonates which reduced atmospheric CO₂ levels.

Summary of Early Atmosphere Formation:

  • Predominantly CO₂ → minimal O₂

  • Formation of H₂O oceans

  • Build-up of N₂ and trace gases (methane, ammonia)

  • Decrease in CO₂ via dissolution and sedimentary processes.

Role of Photosynthesis

Oxygen Production

  • Photosynthesis Equation:
    6CO2 + 6H2O \xrightarrow{light} C6H{12}O6 + 6O2

  • Algae and plants produced oxygen through photosynthesis, significantly modifying the atmosphere.

  • Oxygen began appearing about 2.7 billion years ago, gradually increasing due to the evolution of plants that further fueled photosynthesis.

    • Consequences: Increased oxygen levels enabled the evolution of animals.

    • Reduction of CO₂: Accomplished through photosynthesis and geological storage in sedimentary rocks and fossil fuels.

Greenhouse Gases

Importance and Types

  • Greenhouse gases trap heat in the atmosphere, supporting life on Earth. Key greenhouse gases include:

    • Carbon Dioxide (CO₂)

    • Water Vapor

    • Methane (CH₄)

  • Mechanism of Heat Retention:

    • Visible light passes through the atmosphere, is absorbed by the Earth, and is re-radiated as longer wavelength infra-red radiation.

    • Greenhouse gases (e.g., CO₂, CH₄) prevent this heat from escaping, warming the atmosphere.

Human Activities and Climate Change

Impact on Greenhouse Gas Emissions

  • Certain human activities exacerbate greenhouse gas emissions:

    • Carbon Dioxide:

    • Burning fossil fuels

    • Deforestation

    • Methane:

    • Livestock farming (e.g., cows, pigs)

    • Landfills

  • Scientific Consensus: Many scientists warn that these actions lead to an increase in Earth’s average temperature, creating a risk for climate change.

  • Challenges in predicting climate change due to the complexity of the systems involved can lead to oversimplified models and media representations that may be biased.

Effects of Climate Change

  • Potential effects include:

    • Rising sea levels causing flooding

    • Increased incidence of droughts

    • Desertification

    • Melting ice caps

  • Carbon Footprint: Defined as the total emissions of CO₂ and other greenhouse gases produced throughout a product’s life cycle. Methods to reduce the footprint include minimizing greenhouse gas emissions.

Atmospheric Pollutants

Common Sources and Effects

  • Combustion of fuels (coal, oil, gas) is a primary source of atmospheric pollutants. Key points:

    • Most fuels contain carbon (C) and/or hydrogen (H) and might include sulfur (S).

    • Incomplete Combustion Products: Produces carbon monoxide (CO) and soot (carbon particles).

    • Complete Combustion Products: Creates CO₂ and H₂O.

    • Oxidation of Trace Elements: Small amounts of sulfur and nitrogen produce sulfur dioxide (SO₂) and nitrogen oxides (NOₓ).

Gases Resulting from Fuel Combustion

  • The combustion of fuels releases:

    • Carbon dioxide (CO₂)

    • Water vapor (H₂O)

    • Carbon monoxide (CO)

    • Sulfur dioxide (SO₂)

    • Nitrogen oxides (NOₓ)

    • Solid particles and unburned hydrocarbons form particulates in the atmosphere.

Health and Environmental Impacts

  • Carbon Monoxide (CO):

    • Toxic, colorless, and odorless; difficult to detect.

  • Sulfur Dioxide (SO₂) and NOₓ:

    • Cause respiratory issues.

    • Contribute to acid rain.

  • Particulates:

    • Lead to global dimming and health complications.

Air Pollution: An Ongoing Issue

Statistics and Trends

  • Air pollution presents a significant public health issue, often compared to pandemics (notably COVID-19) in terms of mortality.

  • Data illustrated by time-series graphs can show fatalities attributable to air pollution versus COVID-19.

  • Importance of understanding the chemistry of air pollution for mitigation strategies.

Historical Incidents and Impact

  • Historical air pollution episodes that resulted in health crises include:

    • 1930 Meuse Valley: SO₂ concentrations led to 63 deaths.

    • Donora, Pennsylvania (1948): 20 deaths and widespread illness due to acid aerosols.

    • London Smog Events (1952): Resulted in significant fatalities due to pollution.

Stratospheric Ozone Layer and Ozone Depletion

Impacts of Ozone-Depleting Substances (ODS)

  • Beginning in the 1960s, the use of various chemicals led to profound impacts on the ozone layer, highlighting the problem of stratospheric air pollution.

  • Historical Trends:

    • 1960-1980: Baseline levels with minimal human activity impacts.

    • 1980-2000: Marked depletion due to ODS usage.

    • 2000-2020: Recovery phase begins as ODS levels decrease.

    • Projections for 2040-2066 indicate full recovery of the ozone layer.

Importance of the Ozone Layer

  • The ozone layer provides protection against harmful UVB radiation.

  • Consequences of ozone depletion include:

    • Increased cases of skin cancer and cataracts.

    • Ecological impacts, particularly to phytoplankton and agricultural yields.

    • Contributing factor to climate change due to some ODS also acting as greenhouse gases.

  • The effectiveness of the Montreal Protocol in managing ODS has averted significant warming impacts, estimated at up to 2.5°C by the end of the century.

Conclusion

  • Understanding atmospheric chemistry is crucial to addressing both health and environmental challenges related to air pollution and climate change. Addressing these issues requires both scientific understanding and legislative action.