Air Quality and Environmental Pollution - First Principles Study Notes

Introduction to Air Quality and First Principles

• Definition of Air pollution: The presence of substances in the atmosphere that cause adverse effects to man and the environment. It describes unwanted chemicals or materials contaminating the air, resulting in a reduction of air quality.

• Pollutant Defined: Technically, an air pollutant is any chemical species exceeding the concentrations of natural constituents of air. Strictly, it is a substance potentially harmful to the health/well-being of humans, animals, plants, or ecological systems.

• Royal Commission Definition: The Tenth Report of the Royal Commission on Environmental Pollution defines pollution as the introduction by man into the environment of substances or energy liable to cause hazard to human health, harm to living resources/ecological systems, damage to structure or amenity, or interference with legitimate use of the environment.

• Anthropogenic vs. Biogenic:

  • Anthropogenic: Pollution arising from human activities (e.g., $SO_2$ from coal-burning, $CH_4$ from rice-growing).

  • Biogenic: Natural emissions from living organisms (e.g., terpenes from forests).

  • Natural Hazards: Substances like Radon (radioactive gas in granitic areas) or $SO_2$ from volcanoes are not counted as "pollution" by the strict human-led definition unless influenced by human actions.

• Terpenes: A class of natural products (>30,000 compounds) with the formula $(C_5H_8)_n$; unsaturated hydrocarbons produced predominantly by plants like conifers.

• The Path of Pollution: Solid, liquid, or gaseous material emitted from stationary or mobile sources, traveling through an aerial path, undergoing chemical/physical transformations (e.g., ozone formation, acid rain creation), and returning to the surface.

• Interactions for Impact: Material must interact with a target to have environmental impacts:

  • Photochemical formation of $O_3$ from hydrocarbons.

  • Interaction with electromagnetic radiation (greenhouse gases).

  • Liquid water interaction (formation of acid rain from $SO_2$).

  • Direct effects on vegetation ($O_3$).

  • Soiling of mineral surfaces (particles on buildings).

  • Respiratory damage in animals (acidified aerosols).

Historical Context and Significant Pollution Events

• Ancient Observations: Hippocrates noted the harmful effects of breathing polluted air over 2,000 years ago.

• 13th Century London: Pollution was severe enough from hearths and furnaces that a commission was established.

• Meuse Valley, Belgium (1930): Over 60 deaths due to high industrial air pollution levels.

• Donora, Pennsylvania (1948): 20 deaths and nearly 6,000 related illnesses in a population of 14,000 due to industrial emissions.

• The London "Killer Fog" (December 1952): Resulted in several thousand deaths.

• World Trade Center Collapse (2001): The dust cloud contained more than 2,500 contaminants, including known carcinogens. The EPA did not determine air quality had returned to pre-attack levels until June 2002.

Contemporary Air Quality Issues and Legal Cases

• Ella Adoo-Kissi-Debrah (2020): Southwark Coroner's Court ruled air pollution "made a material contribution" to the death of the 9-year-old; recognized as perhaps the first time air pollution was listed as a cause of death.

• Awaab Ishak (2022): Rochdale Coroner’s Court ruled indoor pollution (mould spores) caused the death of the 2-year-old.

• Wildfires: January 2022 in Colorado and early 2025 in Los Angeles. These were predicted by UCLA researchers in a paper published January 9, 2025.

• UK Economic and Mortality Burden: The World Health Organisation (WHO) estimates the annual cost of air pollution in the UK is $\pounds 54$ billion, with 44,800 to 52,500 deaths annually attributable to particulates and $NO_2$.

• Covid-19 Lockdowns: Provided a data point for measured reductions in pollution due to decreased motor travel and increased active transport (walking/cycling).

• London "Crisis": Mayor Sadiq Khan stated in 2022 that London faces a crisis of "filthy air and gridlocked roads." A peak episode in January 2023 reached 10 on the government's pollution index.

• Global Response: Driven by Greta Thunberg’s school strikes, Client Earth litigation against EU governments, and severe winter pollution in Delhi, India.

Chemical Composition and Evolution of the Atmosphere

• Formation: The Earth formed approximately 4,600 million years ago via gravitational accretion of planetesimals.

• Escape Velocity: The initial atmosphere was lost because molecular speeds exceeded the escape velocity of $11.2\,km\,s^{-1}$.

• Outgassing: Radioactive decay and impact energy heated the Earth, leading to outgassing of $H_2O$ and $CO_2$ from minerals. About 100 times more gas has evolved into the atmosphere over its lifetime than remains today.

• Oxygen Evolution: Small amounts of free oxygen formed via photolysis of water. Life (single-celled) appeared 3,000 million years ago; subsequent photosynthesis and respiration balanced $CO_2$ and $O_2$.

• Lighter Atom Attrition: Lighter atoms escape more easily. At $600\,K$, a Hydrogen atom has a $10^{-16}$ chance of exceeding escape speed, while Oxygen has a $10^{-84}$ chance.

• Air as a Mixture: Robert Boyle (17th Century) described air as a mixture. Sir Humphrey Davy incorrectly thought it was a compound. Proofs that air is a mixture include:

  • Variation in the $O_2$ to $N_2$ ratio.

  • Physical properties match a mixture of the components.

  • Components can be separated.

  • No heat release or volume change occurs upon mixing $N_2$ and $O_2$.

• Standard Dry Air Composition (Sea Level, 15°C, 101325 Pa):

  • Nitrogen ($N_2$): $78.08\%$ ($780,800\,ppm$)

  • Oxygen ($O_2$): $20.95\%$ ($209,500\,ppm$)

  • Argon ($Ar$): $0.93\%$ ($9,300\,ppm$)

  • Carbon dioxide ($CO_2$): $0.0417\%$ ($424.6\,ppm$; peaked at $426.91\,ppm$ in June 2025)

  • Neon ($Ne$): $0.0018\%$ ($18.0\,ppm$)

  • Helium ($He$): $0.000524\%$ ($5.2\,ppm$)

  • Methane ($CH_4$): $0.0002\%$ ($1.7\,ppm$)

  • Krypton ($Kr$): $0.00011\%$ ($1.1\,ppm$)

  • Nitrous oxide ($N_2O$): $0.00003\%$ ($0.3\,ppm$)

  • Ozone ($O_3$): $0.000004\%$ ($0.04\,ppm$)

Water and Trace Gases

• Variable Water Vapour: Not included in Standard Dry Air because it varies with humidity/temperature. Saturated Vapour Pressure ($e_s$) varies exponentially with temperature ($T$):

  • Equation: $e_s(T) = 611 \exp\left( \frac{17.27T}{T + 237.3} \right)\,Pa$

  • At $-10\,^°C$: $289\,Pa$

  • At $0\,^°C$: $611\,Pa$

  • At $30\,^°C$: $4275\,Pa$

• Relative Humidity ($h_r$): Ratio of ambient vapour pressure ($e_a$) to saturated vapour pressure ($e_s$).

• Residence Time: Long-lived trace gases (residence time $≥ 2$ years) are uniformly mixed globally. Short-lived gases vary by local sources and sinks.

Classification of Air Pollutants

• Pollutant Types:

  • Primary Pollutants: Emitted directly from source (e.g., $CO$ from vehicles, $SO_2$ from power stations).

  • Secondary Pollutants: Formed in the air via chemical reactions (e.g., Ozone formed photochemically). Note: $NO_2$ can be both primary and secondary.

• Anthropogenic Source Categories:

  • Point Sources: High-concentration single emitters (stacks, industrial vents).

  • Area Sources: Small dispersed sources (domestic heating, agricultural emissions).

  • Mobile/Linear Sources: Sources along a line (roads, aircraft paths).

• Regulated vs. Unregulated:

  • Regulated: $SO_2$, $NO_x$, $CO$, Lead ($Pb$), Cadmium ($Cd$), Platinum ($Pt$), Benzene, PAHs, $O_3$, PAN, Particulate Matter ($PM_{10}$, $PM_{2.5}$).

  • Unregulated: Carbon dioxide ($CO_2$) and Nitrous oxide ($N_2O$).

• Major Directives:

  • Directive 2008/50/EC: Covers $SO_2$, $NO_x$, $PM$, $Pb$, Benzene, $CO$, $O_3$.

  • Fourth Daughter Directive (2004/107/EC): Covers $As$, $Cd$, $Ni$, $Hg$, and PAHs.

Measurement of Atmospheric Statistics

• Concentration Units:

  • Mass per unit volume: $\mu g\,m^{-3}$.

  • Volume per unit volume: $\mu l\,l^{-1}$.

  • Volume mixing ratio: $ppm$ ($10^6$), $ppb$ ($10^9$), $ppt$ ($10^{12}$).

• Conversion Formula:

  • $\mu g\,m^{-3} = ppb \times \frac{\text{molecular weight}}{\text{molar volume}}$

  • $molar\,volume = 22.4 \times \frac{1013}{P} \times \frac{T}{273}$

• Example Conversion (1.1): To convert $100\,ppb$ of $NO_2$ (MW = 46) at $293\,K$ and $1000\,mb$:

  • $molar\,volume = 22.4 \times \frac{1013}{1000} \times \frac{293}{273} = 24.35\,litres$

  • $\mu g\,m^{-3} = 100 \times \frac{46}{24.35} = 189\,\mu g\,m^{-3}$

• Averaging Times: Used because pollutant impact varies over time. Seasonal breaks include "Winter" (October–March) and "Summer" (April–September).

Emissions Inventories and Trends

• Emissions Inventory: A schedule of pollutant sources within a geographical area. Uses the formula: $\text{Activity rate} \times \text{Emission factor} = \text{Emission rate}$.

• UK National Atmospheric Emissions Inventory (NAEI): Tracks over 30 major pollutants.

• UK Emission Trends (2012–2022):

  • $SO_2$ and $NO_x$: Decreased by $90\%$ and $66\%$ respectively in energy industries.

  • Road Transport: $NO_x$ fell $49\%$; NMVOCs fell $60\%$.

  • Particulates: Overall $PM_{2.5}$ fell $18\%$; $PM_{10}$ fell $7\%$. However, domestic combustion $PM$ increased by $19\%$ due to wood-burning stoves.

  • Ammonia ($NH_3$): Remained stable.

• Gothenburg Protocol: International agreement (signed 1999, revised 2012) setting binding emission ceilings for $SO_2$, $NO_x$, VOCs, and $NH_3$.

Health and Environmental Impacts

• Acute vs. Chronic Health Effects:

  • Acute: Eye/nose/throat irritation, bronchitis, nausea, headaches.

  • Chronic: Heart disease, COPD, lung cancer, asthma, leukaemias, and lymphoma.

• Specific Pollutant Health Effects:

  • Benzene: Cancer; affects central nervous system.

  • Ozone: Reduces lung function, worsens asthma, inflames lung linings, permanent scarring.

  • Sulphur Dioxide: Causes respiratory problems.

• Sick Building Syndrome (SBS): Temporary ailments (headache, respiratory issues) in occupants of specific buildings (often air-conditioned offices) that disappear when leaving.

• Building Related Illness (BRI): Diagnosable diseases (e.g., Legionnaires' disease) linked to building contaminants.

• Environmental Damage:

  • Acid Rain: Formed by $SO_2$ and $NO_x$; damages aquatic life (via aluminium release), leaches soil nutrients, and corrodes buildings.

  • Plants: $O_3$ reduces growth/forest health. London Plane tree survives by shedding bark but its leaf hairs irritate human lungs.

  • Global Warming: Result of greenhouse gases ($H_2O$, $CO_2$, $CH_4$, $N_2O$, $O_3$, CFCs) trapping infrared radiation. Predicted rise of $1.5$ to $4\,^°C$ by mid-21st century.