Atmospheric Chemistry and Global Environmental Governance Study Notes

Introduction to Atmospheric Chemistry

• Definition: Atmospheric chemistry is the scientific study of complex chemical reactions occurring within the Earth’s atmosphere. This field has grown in importance due to the increasing release of human-derived chemicals from fossil fuel combustion, industrial processes, air conditioning, paints, solvents, and domestic aerosols (deodorants, air fresheners).

• Anecdotal Context: Heathcote Williams in "Autogeddon" (1991) highlighted the crisis of air quality: in Rome, traffic police struck because they could not breathe, while in Japan, department stores sold flavored oxygen (lime, lemon, coffee, or mushroom) in specialized bars as a remedy for Tokyo’s suffocating conditions.

• Core Objectives:

  • Identify physical and chemical properties of the Troposphere and Stratosphere.

  • Explain photochemical principles and distinguish reaction types.

  • Analyze key reactions in atmospheric layers and their environmental impacts.

  • Assess the role of the ozone layer and related international protection agreements.

Physical Structure of the Atmosphere

• Mass Comparison: James Lovelock (1991) notes the atmosphere is the least massive part of the physical earth, weighing roughly five times more than a billion megatons (though the transcript states "not much more than a billion megatons"), whereas oceans are a thousand times more massive and the earth a million times more.

• Vertical Extent: The atmosphere extends approximately $400\,km$ above the surface and consists of four regions: the Troposphere, Stratosphere, Mesosphere, and Thermosphere.

• Mixing and Movement: Circulation is driven by solar energy and modified by the earth’s rotation. Winds, such as the trade winds or doldrums, facilitate the transport of pollutants (e.g., desert dust from the Arabian Peninsula across the Atlantic).

• Temperature and Density Profiles:

  • Troposphere: Temperature decreases with altitude (positive lapse rate), promoting rapid mixing.

  • Stratosphere: Temperature increases with altitude (negative lapse rate), inhibiting vertical mixing except via diffusion and turbulence. Ozone absorbs solar heat directly here.

  • Tropopause: The boundary between the Troposphere and Stratosphere, ranging from $10-17\,km$ in depth depending on latitude and season. Transport of gases between regions is very slow here because the rate of vertical mixing falls sharply.

The Troposphere: Inner Layer Dynamics

• Etymology: The name comes from the Greek "trophos," meaning "change," reflecting the role of turbulent mixing.

• Planetary Boundary Layer (Atmospheric Boundary Layer): The lowest segment of the Troposphere where air flow is strongly influenced by friction with the Earth's surface.

• Physical Properties:

  • Contains approximately three-quarters ($75\%$) of the atmosphere’s total mass.

  • Contains $99\%$ of the atmosphere's aerosols and water vapor.

  • Altitudinal Range: From the surface up to $6-20\,km$ ($4-12$ miles), averaging $17\,km$ in middle latitudes. It is highest at the equator ($18-20\,km$) and lowest at the poles (<4\,km).

  • Temperature Gradient: Drops from approx. $17^{\circ}C$ at the surface to $-51^{\circ}C$ at the top. Virtually all weather occurs in this region.

• Transition Points:

  • Tropopause: The point where air ceases to cool with height and becomes completely dry. Technically defined as the point where the lapse rate changes from positive to negative (the Equilibrium Level or EL).

  • WMO Definition: The lowest level where the lapse rate decreases to $2^{\circ}C/km$ or less, provided the average lapse rate between this and higher levels within $2\,km$ does not exceed $2^{\circ}C/km$.

  • Aviation: Most commercial aircraft fly below the Tropopause to take advantage of low temperatures, which improve jet engine efficiency. Supersonic flight occurs at approx. $15\,km$ (lower Stratosphere).

Chemical Composition of the Troposphere

• Composition Table (Dry, Unpolluted Air at Sea Level - 2025):

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

  • Oxygen ($O_2$): $20.94\%$ ($209,476\,ppm$)

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

  • Carbon dioxide ($CO_2$): $0.042\%$ ($424.7\,ppm$)

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

  • Helium ($He$): $0.0005\%$ ($5.2\,ppm$)

  • Methane ($CH_4$): $0.0002\%$ ($2.0\,ppm$)

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

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

  • Hydrogen ($H_2$): $0.00005\%$ ($0.5\,ppm$)

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

• Water Vapor Variance: Proportion is greatest at the surface and decreases with height due to temperature drops and saturation limits.

The Stratosphere and the Ozone Layer

• Vertical Range: Extends from approx. $10-13\,km$ (latitudinal variance) up to $50\,km$. At the equator, it begins around $17\,km$ and reaches $48\,km$.

• Key Differences from Troposphere:

  • $1,000$ times less water vapor.

  • $1,000$ times more ozone ($O_3$).

• The Ozone Shield (Global Sunscreen):

  • Absorbs lethal UV-C radiation and reduces harmful UV-B.

  • Middle section: Most natural ozone produced where atomic oxygen ($O$) and diatomic oxygen ($O_2$) combine, releasing heat and creating a temperature inversion.

  • Upper section: Receives high UV, causing dissociation.

  • Lower section: Very low UV-C, hence no atomic oxygen is present and ozone does not form.

• Chemical Stability: Conditions are very stable; foreign material introduced here persists for long periods, compounding environmental issues like ozone depletion.

Stratospheric Pollution and Chemical Classes

• Historical Concern: Initially associated with supersonic aircraft (e.g., Concorde, Boeing SST) injecting exhaust into the lower Stratosphere. Later shifted to CFCs.

• Ozone Depletion Mechanisms:

  • Mario Molina and F. Sherwood Rowland (1974): Suggested chlorine atoms from CFCs catalytically destroy ozone.

  • Joe Farman (British Antarctic Survey, 1982): Reported first damage over Antarctica.

  • Phenomena: $4\%$ decadal decline globally and larger springtime polar "holes" ($O_3$ concentrations down to $100\,ppb$).

• Anthropogenic Compounds:

  • CFCs (Chlorofluorocarbons): Organic compounds containing Carbon, Chlorine, and Fluorine (e.g., Freon-12 or dichlorodifluoromethane; formula $CCl_2F_2$). Formulas: $CCl_mF_{4-m}$ and $C_2Cl_mF_{6-m}$.

  • HCFCs (Hydrochlorofluorocarbons): Contain Carbon, Chlorine, Fluorine, and Hydrogen. Formulas: $CCl_mF_nH_{4-m-n}$ and $C_2Cl_xF_yH_{6-x-y}$.

  • HFCs (Hydrofluorocarbons): Contain Carbon, Hydrogen, and Fluorine. Ozone-safe but potent greenhouse gases.

  • Halons: Similar to CFCs but contain at least one Bromine atom. Used as unreactive, non-toxic fire suppressants.

• Naming System: Prefixed with Freon-, R-, CFC-, or HCFC-. Values from right to left: 1. Number of Fluorine atoms. 2. Number of Hydrogen atoms plus 1. 3. Number of Carbon atoms minus 1 (omitted if zero). All remaining positions are Chlorine.

International Agreements and Recovery Timeline

• Montreal Protocol (1989): Phased out $99\%$ of ozone-depleting substances. Most successful environmental treaty.

• Projected Recovery Dates:

  • Global (most regions): 2040.

  • Arctic: 2045.

  • Antarctica: 2066.

• London Amendment (1990): Committed to complete CFC phase-out by 2000.

• Copenhagen Agreements (1992): Accelerated phase-out to 1996.

• Kyoto Protocol (1997): Targeted greenhouse gas reduction ($5.2\%$ below 1990 levels).

• Kigali Amendment (2016): Mandated phase-out of HFCs to avoid up to $0.5^{\circ}C$ of global warming by 2100.

Principles of Photochemistry

• Definition: The study of chemical reactions resulting from light absorption ($Far\,UV\,200\,nm$ to $Near\,IR\,800\,nm$).

• Laws of Photochemistry:

  • First Law (Grotthaus): Only light absorbed by a molecule can produce photochemical change.

  • Second Law (Einstein): Only one quantum (photon) is absorbed per molecule for the primary process.

• The Photon: A packet of energy with no charge/mass, possessing energy ($E$) and momentum.

• Planck’s Equation:     $E = h\nu = \frac{hc}{\lambda} = hc\bar{\nu}$

  • $h = 6.6256 \times 10^{-34}\,J\,s\,photon^{-1}$

  • $c = 2.9979 \times 10^{8}\,m\,s^{-1}$

  • $\nu = frequency\,(s^{-1})$

  • $\lambda = wavelength\,(m)$

  • $\bar{\nu} = wavenumber\,(m^{-1})$

• Energy Calculations (per mole of photons uses Avogadro’s Number $N = 6.022 \times 10^{23}$):

  • $\lambda = 200\,nm \rightarrow E = 598\,kJ\,mol^{-1}$

  • $\lambda = 400\,nm \rightarrow E = 300\,kJ\,mol^{-1}$

  • $\lambda = 600\,nm \rightarrow E = 200\,kJ\,mol^{-1}$

  • $\lambda = 800\,nm \rightarrow E = 150\,kJ\,mol^{-1}$

• Bond Energies for Comparison:

  • $H_2: 436\,kJ\,mol^{-1}$

  • $N_2: 946\,kJ\,mol^{-1}$

  • $O_2: 493\,kJ\,mol^{-1}$

  • $O_3: 101\,kJ\,mol^{-1}$

  • $CO: 1070\,kJ\,mol^{-1}$

  • $NO_2: 305\,kJ\,mol^{-1}$

• Quantum Yield (\Phi): Defined as the number of molecules reacting divided by the number of photons absorbed. For the reaction $HI + h\nu \rightarrow H + I$, the subsequent chain can result in a quantum yield of 2.

Electronic Excited States (EES)

• Molecular Orbitals: Combination of atomic orbitals creates bonding (lower energy) and anti-bonding (higher energy) orbitals.

• Ground State: Electrons are in the lowest energy orbitals, paired with opposite spin.

• Excited State: One or more electrons promoted to higher orbitals (LUMO) after absorbing a photon ($h\nu = \Delta E = E_{LUMO} - E_{HOMO}$).

• Singlet vs. Triplet States:

  • Singlet State: Usual EES formed by direct absorption; unpaired electrons have opposite spin (total zero spin).

  • Triplet State: Unpaired electrons have parallel spin (non-zero spin); not formed readily by direct absorption but often more stable (e.g., Triple $O_2$ is more stable than Singlet $O_2$).

Types of Photochemical Processes

• Photosynthesis: Trapping light to form carbohydrates.

  • Light reaction: $12H_2O + light \rightarrow 6O_2 + 24H^+ + 24e^-$

  • Dark reaction: $6CO_2 + 24H^+ + 24e^- \rightarrow C_6H_{12}O_6 + 6H_2O$

• Photosensitisation: Indirect initiation (e.g., $Hg$ initiating formaldehyde formation).

• Photodissociation (Photolysis): Splitting of a molecule. Basis of the Chapman Mechanism (1930):

  1. O_2 + h\nu (\lambda < 240\,nm) \rightarrow O + O

  2. $O + O_2 + M \rightarrow O_3 + M$

  3. O_3 + h\nu (\lambda < 320\,nm) \rightarrow O_2 + O

  4. $O_3 + O \rightarrow 2O_2$

• Photo-ionisation: Radiation removing electrons ($O_2 + h\nu \rightarrow O_2^+ + e^-$).

• Intramolecular re-arrangement: Producing a new electronic state via collision ($XY + XY^* \rightarrow XY + XY^{\delta}$).

• Intermolecular energy transfer: absorbed energy transferred to another molecule ($XY + AB \rightarrow XY + AB^*$).

• Quenching: Excited state ($AB^*$) returns to ground state via collision with unreactive molecule $M$.

• Luminescence: Re-emission of energy at different wavelengths. Includes short-lived Fluorescence and longer-lived Phosphorescence.

Chemistry of the Troposphere

• Complex VOC Environment: Millions of reactions occur. Natural sources (trees, molds) contribute significantly. The Great Smoky Mountains (USA) are named for tree emissions (isoprenes/terpenes).

• Oxygen Allotropes:

  • Atomic oxygen ($O(3P)$): Highly reactive ground state.

  • Singlet oxygen ($O(1D)$): Metastable, highly reactive excited state.

  • Tetraoxygen ($O_4$): Discovered 2001.

• Free Radicals: Atoms/groups with an odd number of electrons (denoted by a dot $\bullet$). Highly reactive intermediates.

  • Initiation: $Non\text{-}radical + h\nu \rightarrow radical + radical$

  • Propagation: $Non\text{-}radical + radical \rightarrow radical + non\text{-}radical$

  • Termination: $Radical + radical \rightarrow non\text{-}radical$

• Hydroxyl Radical (\bullet OH): The "cleanser" of the atmosphere. Formed via:

  1. Ozone photolysis: O_3 + h\nu (\lambda < 320\,nm) \rightarrow O(1D) + O_2

  2. Reaction with moisture: $O(1D) + H_2O \rightarrow 2\bullet OH$

  3. Carbonyl photolysis: Photodissociation of aldehydes (e.g., formaldehyde $HCHO$).

Tropospheric Removal and Smog

• Methane Removal: Complex path: $CH_4 \rightarrow CH_3 \rightarrow CH_3O_2 \rightarrow HCHO \rightarrow HCO \rightarrow CO \rightarrow CO_2$.

• Carbon Monoxide Removal: Reacts with $\bullet OH$ to form $CO_2$ and $H\bullet$. $H\bullet$ then forms hydroperoxyl radicals ($HO_2 \bullet$).

• Sulphur Dioxide Removal:

  • Wet removal: Solubility in raindrops as sulphurous acid.

  • Dry removal: Mechanism involving $\bullet OH + SO_2 \rightarrow HOSO_2 \bullet \rightarrow SO_3 \bullet \rightarrow H_2SO_4$.

• Photochemical Smog: Oxidizing mixture of $NO_x$, $O_3$, $VOCs$, and $PANs$. Requires strong sunlight and stable air. Common in sunny cities (Los Angeles, Tokyo, Beijing).

• Nitrogen Oxide Cycles:

  • Stationary State: $[NO][O_3] / [NO_2] = k_2$.

  • Hydrocarbons ($RH$) disturb this, leading to net ozone formation: $RH + 2NO + 2O_2 \rightarrow Carbonyl + 2NO_2 + H_2O$.

• Peroxyacetyl Nitrates (PAN): Formed from aldehydes and $NO_2$. Stable in cold upper Troposphere; acts as a reservoir species transportable over long distances.

UK Environmental Policy and Case Studies

• Climate Change Act 2008: Statutory commitment to $80\%$ net UK GHG emission reduction by 2050 (based on 1990 baseline).

• Local Government Authorities (LGAs): No explicit statutory reduction mandates, but many have voluntary targets.

• Southampton City Council (SCC):

  • Goal: World-leading low carbon city.

  • Target: $34\%$ reduction in $CO_2$ by 2020 on 1990 levels.

  • Status: $14.8\%$ achieved by 2012.

• Transport Initiatives: White Paper (2011) "Creating Growth, Cutting Carbon." Two-thirds of UK trips are <5\,miles, targeted for walking/cycling.

• LSTF (Local Sustainable Transport Fund) Successes in Southampton:

  • "Southampton Sustainable Travel City": $\pounds 5.72\,million$ ($\pounds 3.96\,million$ from LSTF).

  • "A Better Connected South Hampshire": $\pounds 31.16\,million$ ($\pounds 17.84\,million$ from LSTF).

• Other Initiatives: Nottingham Declaration on Climate Change (2000), Climate Local (2012).