Comprehensive Notes on Atmospheric Pollutants and Environmental Pollution Management

Introduction to Atmospheric Pollutants

• Detailed examination of air quality begins by reviewing the two primary divisions of atmospheric pollutants: gaseous air pollutants and particulate matter.

• Study focuses on the physical and chemical properties of individual pollutants, emission levels and sources, effects on human health and the environment, and legislative strategies for air quality management.

• Knowledge of organic chemistry types of hydrocarbons is a recommended prerequisite for understanding volatile organic compounds (VOCs).

• Core Learning Objectives:

  • Identify the nature, sources, emissions, and effects of gaseous and particulate pollutants.

  • Accurately use terminology regarding particulate matter.

  • Differentiate modal distributions of atmospheric particles.

  • Discuss physical and chemical processes affecting particle lifetime, transport, and deposition.

  • Outline methods for estimating pollutant emissions.

  • Identify regulatory processes for emissions control and management.

  • Locate and interpret historical and current air quality information.

  • Evaluate deposition velocities, particulate mass, and density data.

Gaseous Pollutants: Nitrogen Oxides $(NO_x)$

• Nomenclature and Chemistry:

  • The most significant nitrogen compounds in urban areas are nitric oxide ($NO$) and nitrogen dioxide ($NO_2$).

  • Compounds like dinitrogen pentoxide ($N_2O_5$) and nitrogen trioxide ($NO_3$) are important in specific pollution episodes.

  • Nitrous oxide ($N_2O$), dinitrogen trioxide ($N_2O_3$), and dinitrogen tetroxide ($N_2O_4$) do not contribute significantly to urban pollution.

  • Nitric oxide ($NO$) accounts for $90\text{-}95\%$ by volume of total $NO_x$ emissions at the source but is oxidized to the more toxic secondary pollutant $NO_2$ in ambient air.

  • The equilibrium between $NO$ and $NO_2$ is influenced by ultraviolet light (which decomposes $NO_2$ back to $NO$) and accelerated by reactive hydrocarbons and ozone.

• Sources and Emissions:

  • Natural sources (lightning, volcanic eruptions, soil bacteria) outweigh global anthropogenic emissions. $N_2O$ is the most abundant natural emission but is ignored as a pollutant except for its role in stratospheric ozone ($O_3$).

  • Anthropogenic sources are dominated by fossil fuel combustion (vehicles and power stations) at temperatures of $1800^{\circ}C$ or higher.

  • UK Trend: Emissions decreased by $78\%$ between 1970 and 2022 ($643$ thousand tonnes in 2022). Coal decline and transport modernization are catalysts.

  • Road transport contributed $30\%$ of $NO_x$ emissions in 2022. Non-road transport (aviation, shipping, rail) contributed $15\%$.

  • Energy industries saw a $75\%$ reduction between 2005 and 2022 due to shifting from coal/oil to natural gas and renewables.

• Regulatory Control and Standards:

  • EC Large Combustion Plants Directive ($88/609$) required reductions (relative to 1980) of $15\%$ by 1993 and $30\%$ by 1998 for plants over $50\,MW$ thermal.

  • Consolidated Directive ($91/441/EEC$) mandated three-way catalysts on new petrol cars from 1993, potentially reducing $NO_x$ by $75\%$ per vehicle.

  • The UNECE $NO_x$ Protocol (1994) utilized the "critical load" concept to cap emissions at 1987 levels.

• Ambient Concentrations and Health Effects:

  • Urban range: $10\text{-}50\,ppb$; Urban peak: $800\,ppb$; Indoor peak: $500\,ppb$.

  • Health: Pulmonary irritant, increases flow resistance, potential for oedema or emphysema. Linked to increased hay fever, asthma, and eczema susceptibility.

  • Odour perception: Approximately $200\,\mu g\,m^{-3}$ ($0.11\,ppm$).

  • Environment: Causes plant damage (synergistic with $SO_2$), degrades textile dyes, forms pollutant haze, and contributes to acid deposition.

Gaseous Pollutants: Sulphur Oxides $(SO_x)$

• Sources and Emissions:

  • $SO_2$ is a corrosive, acidic gas produced by coal and crude oil combustion.

  • Natural sources ($SO_x$ from volcanoes and sea aerosols) roughly equal anthropogenic sources globally.

  • Oxidation: During combustion, sulfur becomes $SO_2$. Small amounts further oxidize to $SO_3$, which forms sulfuric acid aerosol in water vapor.

  • UK Trend: Emissions fell by $98\%$ from 1970 to 2020. This is attributed to the decline in coal and adoption of low-sulphur fuels and abatement equipment.

  • Low-sulphurous fuels like natural gas replace coal/oil (which traditionally contain $0.5\text{-}4.0\%\,S$).

• Regulatory Control:

  • Convention on Long Range Transboundary Air Pollution (CLRTAP) and National Emissions Ceiling Regulations (NECR) targets: reduction by $59\%$ by 2020 and $88\%$ by 2030 (referenced to 2005).

• Ambient Concentrations and Ambient Discrepancies:

  • Urban range: $3\text{-}20\,ppb$; Urban peak: $300\,ppb$; Indoor peak: $20\,ppb$.

  • Discrepancy: A $25\%$ emission drop (1976-1986) resulted in a $46\%$ concentration drop because of the shift from low-level sources (domestic chimneys) to high-level sources (power station tall stacks).

• Health and Environmental Effects:

  • Aggravates respiratory diseases (bronchitis, emphysema). Sulfuric acid mists affect function at $0.35\,mg\,m^{-3}$.

  • Environment: Key factor in acid rain/precipitation, harming forests and freshwater habitats (e.g., UK emissions damaging Scandinavia in the 70s-80s).

Carbon Oxides $(CO)$

• Properties and Sources:

  • Colourless, odourless, tasteless, slightly lighter than air.

  • Primary source: Incomplete combustion of carbon-containing fuels (mainly road vehicles).

  • Efficiency: Petrol engines are $15\text{-}20\%$ efficient compared to diesel's $30\%$, causing petrol vehicles to use more fuel and emit more $CO$. $CO$ accounts for over $90\%$ by weight of all vehicle emissions.

  • Indoor sources: Gas cookers, refrigerators (indoor levels can reach $70\text{-}100\,mg\,m^{-3}$), and cigarette smoke (inhaled concentration ~$1000\,\mu g\,m^{-3}$).

• Health Implications and Guidelines:

  • Mechanism: Forms a strong coordination bond with the iron in haemoglobin to create carboxyhaemoglobin ($COHb$), reducing oxygen capacity.

  • WHO Guideline standard (to keep $COHb$ at $2.5\text{-}3\%$):

    • $100\,mg\,m^{-3}$ ($15\,min$)

    • $60\,mg\,m^{-3}$ ($30\,min$)

    • $30\,mg\,m^{-3}$ ($1\,hour$)

    • $10\,mg\,m^{-3}$ ($8\,hours$)

  • Effect Severity:

    • $10\%\,COHb$: Few people affected.

    • $10\text{-}60\%\,COHb$: Headache, nausea, convulsions.

    • >60\%\,COHb: Coma.

    • >80\text{-}90\%\,COHb: Death.

Volatile Organic Compounds (VOCs)

• Definition: Umbrella term for organic chemicals that evaporate easily. Includes hydrocarbons (alkanes, alkenes, aromatics), oxygenates (alcohols, aldehydes, ketones, acids, ethers), and halogen-containing species.

• NMVOC Emissions Trends:

  • UK Trend: Emissions fell $67\%$ from 1970 to 2020.

  • Major shifts: Road transport fell from $33\%$ (1990) to $3\%$ (2021).

  • Emerging sources: Scotch Whisky maturation emissions increased by $89\%$ since 1990.

  • Solvents: Industrial use fell significantly ($599\,k\,tonnes$ in 1990 to $117\,k\,tonnes$ in 2020), while domestic solvent use increased with population growth ($23\%$ of emissions in 2020).

• Sub-categories of VOCs:

  • Hydrocarbons (HCs): Include Benzene (carcinogen, UK standard $5\,ppb$ rolling annual mean) and 1,3-butadiene (standard $1\,ppb$).

  • Aldehydes ($RHC=O$): Over 100 atmospheric types. Formaldehyde is the most abundant (~$70\text{-}80\%$ of total). Acetaldehyde is the second most common.

  • Ketones ($RR^1C=O$): Common constituents (e.g., acetone). Contributes to diesel exhaust odour.

  • Alcohols ($ROH$): $C1\text{-}C5$ aliphatic alcohols come from industrial use and solvents. Typical urban concentration: $5\text{-}10\,ppb$.

  • Toxic Organic Micropollutants (TOMPS): Polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls (PCB), and dioxins (very potent carcinogens measured in $pg\,m^{-3}$).

Secondary Pollutants: Ozone and PAN

• Ground Level Ozone ($O_3$):

  • Formed in the troposphere via sunlight-induced reactions between $NO_x$ and VOCs.

  • Background levels: $10\text{-}80\,\mu g\,m^{-3}$. Anthropogenic sources have doubled background levels in 100 years.

  • Urban paradox: Ozone is often lower in city centres because it reacts with freshly emitted $NO$ (scavenging).

  • Health: Pulmonary irritant, causing coughing and respiratory infection susceptibility.

  • Environment: Degrades rubber/polymers, fades dyes, and damages vegetation.

• Peroxyacetyl Nitrate (PAN):

  • Formed via reactions of HCs and $NO_x$. Powerful eye irritant (threshold ~$0.1\,ppm$).

  • Concentrations: Southern California ($5\text{-}50\,ppb$), London peak ($16\,ppb$).

Atmospheric Particulate Matter (PM)

• Basic Terminology:

  • Aerosol: Suspension of solid/liquid particles in a gas.

  • $PM_{10}$: Particles passing through an inlet with $50\%$ efficiency at $10\,\mu m$ aerodynamic diameter.

  • Inhalable: Enter nose/mouth. Respirable: Reach deep lung (alveolar region).

  • Smoke: Combustion-derived particles <15\,\mu m. Categorized as dark smoke (reflectance) or gravimetric smoke (mass).

• Physical Properties:

  • Aerodynamic Diameter (AD): Diameter of a unit-density sphere ($\rho_0 = 1 \times 10^3\,kg\,m^{-3}$) with the same settling velocity as the particle.

  • Modal Distributions:

    1. Nuclei Mode (<0.05\,\mu m\,AD): Condensation from gas, short lifetime.

    2. Accumulation Mode ($0.05\text{-}2\,\mu m\,AD$): Coagulation/condensation, lifetime of $7\text{-}30\,days$, long-range transport.

    3. Coarse Particle Mode (>2\,\mu m\,AD): Mechanical generation (dust, spray), short lifetime (hours), rapid sedimentation.

• Removal Mechanisms:

  • Dry Deposition: Continuous transfer via sedimentation, impaction, and Brownian motion.

    • Formula: $V_d (m/s) = \frac{-\text{Flux density towards surface (g m}^{-2}\text{ s}^{-1}\text{)}}{\text{Concentration at reference height (g m}^{-3}\text{)}}$

  • Wet Deposition: Episodic; transport via aqueous form (rain-in/in-cloud scavenging vs. wash-out/below-cloud scavenging).

Chemical Composition and Regulation of PM

• Composition Breakdown:

  • Particulate Elemental Carbon (PEC): Black component, absorbs light, catalytic site for acid formation. Accountable for $25\text{-}45\%$ of visibility reduction.

  • Minerals: Insoluble crustal material (iron/manganese oxides, quartz, clay).

  • Aqueous Fraction: Adsorbed water can account for $50\%$ of particle weight at $70\text{-}80\%$ relative humidity.

  • Ionic components: Sulphates (~$30\text{-}35\%$ of soluble fraction), Nitrates (~$25\%$), and Ammonium salts (~$18\text{-}25\%$).

  • Pollen: Multi-cellular grains ($10\text{-}100\,\mu m$). Bioindicator for pollution; germination rates decrease with high air acidity.

• Regulatory Controls:

  • Euro Standards: Progressive standards (Euro 1-6) for vehicle emissions.

  • Testing Procedures: Historically NEDC (laboratory). Now WLTP (more dynamic laboratory) and RDE (Real-world Driving Emissions using PEMS - Portable Emissions Measurement System).

  • RDE Tiers: RDE1 allowed $2.1\times$ the $NO_x$ limit; RDE2 allowed $1.5\times$ (mandatory for new types in Jan 2020).

UK Air Quality Strategy and Permitting

• Strategy Highlights: The UK Clean Air Strategy (updated 2023) sets the framework for local authority delivery.

• Environmental Permitting (EP): Risk-based regime for regulating industrial impact.

  • Part A1: Regulated by Environment Agency (large/complex industry).

  • Part A2: Local Authority Integrated Pollution Prevention and Control (LA-IPPC) (medium impact, integrated approach for air, land, and water).

  • Part B: Local Authority Pollution Prevention and Control (LAPPC) (regulated for air emissions only, e.g., petrol stations, cement works).

• Best Available Techniques (BAT): Regulatory standard balancing environmental benefit against operator cost.

• Management Zones:

  • Air Quality Management Areas (AQMA): Declared if national standards are unlikely to be achieved.

  • Low Emission Zones (LEZ): Restrict access for polluting vehicles (e.g., London LEZ).

  • Clean Air Zones: Introduced in Birmingham, Leeds, Nottingham, Derby, and Southampton to aggressively target $NO_2$ levels.