Air Pollution

Introduction

Air pollution is the transfer or presence of harmful materials in the atmosphere as a direct or indirect consequence of human activity or natural events, producing adverse effects on human health, the environment and property.

Atmosphere

• The atmosphere is the gaseous envelope surrounding the Earth extending up to several hundred kilometres. It is a mixture of gases together with suspended water vapour and particulates.
• Typical dry-air composition by volume: about 78% nitrogen (N₂), 21% oxygen (O₂), and the remainder argon, carbon dioxide (CO₂), and trace gases; water vapour (H₂O) varies with location and weather.

Major layers of the atmosphere

• Troposphere: Extends from the surface to roughly 11-16 km. Most weather phenomena (clouds, storms) and practically all commercial aircraft traffic occur in this layer. About three-quarters of the atmosphere's mass lies in the troposphere. Temperature generally decreases with height. Typical environmental lapse rates vary with conditions.
• Stratosphere: Lies above the troposphere. Temperature in the lower stratosphere may increase with height because of absorption of incoming solar ultraviolet radiation by ozone. Ozone concentration is much higher in the stratosphere than in the troposphere.
• Mesosphere: Above the stratosphere; temperature again decreases with height and the air becomes thin and turbulent. Water vapour is negligible and cloud formation is limited.
• Thermosphere: Uppermost layer where temperature increases with altitude due to strong absorption of solar radiation; air density is extremely low.

Sources of air pollution

• Natural: Volcanoes, forest fires, dust storms, sea salt, biological emissions.
• Anthropogenic (human): Emissions from industry, power plants, residential heating, transport, agriculture and other human activities.
• Stationary sources: Power plants, manufacturing industries, brick kilns, residential heating using coal, oil or gas.
• Mobile sources: Vehicles (cars, trucks, buses, two-wheelers, aircraft, ships), construction machinery.

Types of air pollutants

• Primary pollutants: Substances emitted directly from identifiable sources in harmful form (for example: CO, SO₂, NO_x, hydrocarbons, particulate matter).
• Secondary pollutants: Pollutants formed in the atmosphere by chemical reactions among primary pollutants, sunlight and natural constituents (for example: ozone (O₃), peroxyacetyl nitrate (PAN), photochemical smog, certain aerosols).

Primary pollutants - brief descriptions

• Carbon monoxide (CO): Produced by incomplete combustion of carbonaceous fuels (automobile engines are a major source; cigarette smoke is another). CO is toxic because it binds to haemoglobin, reducing oxygen transport in blood. CO is eventually oxidised in the atmosphere to carbon dioxide (CO₂).
• Sulfur dioxide (SO₂): Produced by combustion of sulfur-containing fossil fuels (coal, oil). Major sources include coal-fired power plants and industrial boilers. SO₂ can oxidise to form sulphuric acid and is an important precursor of acid rain; it can irritate the respiratory tract and aggravate asthma.
• Nitrogen oxides (NO_x): Produced during high-temperature combustion in engines and boilers. NO_x contributes to acid rain, formation of ozone and photochemical smog. Road transport is a key source.
• Hydrocarbons (volatile organic compounds, VOC): Emitted from incomplete combustion and from evaporation of fuels, solvents and industrial processes. VOCs participate in photochemical reactions that produce ozone and other secondary pollutants.
• Particulates (particulate matter): Solid particles and liquid droplets suspended in air (examples: dust, ash, soot, metallic particles, fibres, mists). Particulates can reach different regions of the respiratory system depending on size: PM-1 (≤ 1 µm) may penetrate alveoli, PM-2.5 affects lower airways and alveoli, PM-10 deposits in upper airways and nasal passages. Some particulates are carcinogenic.

Secondary pollutants - brief descriptions

• Ozone (O₃): A highly reactive tri-atomic oxygen molecule. Ozone formed near the ground (tropospheric ozone) is produced by photochemical reactions between VOCs and NO_x in sunlight. Tropospheric ozone causes respiratory irritation and plant damage; stratospheric ozone provides protection from harmful ultraviolet radiation.
• Peroxyacetyl nitrate (PAN): A component of photochemical smog formed from reactions of hydrocarbons and NO_x under sunlight; PAN is an irritant and can damage vegetation.
• Photochemical smog: A complex mixture including ozone, NO₂, PAN, aldehydes, unreacted hydrocarbons and particulates. It often appears as a brownish haze due to NO₂ and causes eye irritation, respiratory problems and reduced visibility.
• Aerosols and mists: Fine liquid droplets or solid particles suspended in air. Some mists are produced by gas-phase reactions (for example hydrolysis of SO₃ to form sulphuric acid droplets) and can be difficult to remove by conventional scrubbers.

Health and environmental effects of common pollutants

• CO: Headache, reduced mental alertness, dizziness and at high exposures can cause loss of consciousness and death due to reduced oxygen delivery to organs.
• SO₂: Eye and throat irritation, wheezing, aggravation of respiratory diseases, and when converted to acids contributes to acid deposition damaging ecosystems and structures.
• NO₂: Respiratory irritation and increased susceptibility to respiratory infection; contributes to ozone and smog formation.
• Ozone (ground level): Eye and throat irritation, coughing, exacerbation of asthma, reduced lung function.
• Lead: Anaemia, neurological effects, kidney damage and developmental problems in children.
• Particulate matter: Eye irritation, cough, asthma exacerbation, chronic bronchitis and other cardiovascular and respiratory diseases; fine particulates (PM-2.5 and smaller) are linked to increased mortality.

Air pollution control - overview

Control of air pollution relies on source control, end-of-pipe control devices and management practices. Selection of control measures depends on pollutant type, required removal efficiency, engineering feasibility and economics.

Particulate control devices

• Source control and process modification: Reducing emissions by improving combustion efficiency, changing fuels, housekeeping and process changes.
• Settling chambers: Use gravity to remove coarse particles by reducing gas velocity so large particles settle into a hopper. Effective for large particles but low efficiency for fine particulates; often used as a pre-cleaner.
• Cyclones: Use centrifugal forces: gas is made to spiral so particles, by inertia, move to the walls and fall into a hopper. Cyclones are effective for larger particles but have limited performance for very fine particles.
• Venturi scrubbers: Use high gas velocities and liquid to capture particles by impaction and entrainment; effective for small particles with efficiencies that can approach very high values under proper design (input reference states up to 99% in favourable conditions).
• Electrostatic precipitators (ESP): Impart electrical charge to particles; charged particles migrate to collector plates and are removed by rapping. ESPs are widely used for high-efficiency removal of fine particles in large industrial applications.
• Fabric filters (baghouses): Pass gas through porous fabric bags that capture particles on the surface; can achieve very high removal efficiencies (greater than 99% for many applications) and are effective for fine particulates.

Control of gaseous pollutants

• Absorption (wet scrubbing): A gaseous pollutant is dissolved in a liquid (commonly water or alkaline solutions for acidic gases). Effective for soluble gases such as SO₂, HCl.
• Adsorption: Pollutants are retained on the surface of a solid sorbent (for example activated carbon) where vapours or gases adhere to the sorbent surface. Used for VOC removal and odour control.
• Condensation: Cooling a gas or vapour to convert it to liquid form which can be collected. Used where the pollutant has a sufficiently high vapour pressure at achievable temperatures.
• Incineration (thermal oxidation): Combustion of organic gaseous pollutants to convert them to CO₂ and H₂O; widely used for control of volatile organic compounds and other combustible organics.

Air pollution meteorology

Dispersion and concentration of air pollutants are strongly influenced by meteorological factors such as wind speed and direction, atmospheric stability, temperature profile and mixing height. Understanding these factors is essential for predicting pollutant transport and designing control strategies.

Lapse rate and stability

Lapse rate is the rate at which ambient air temperature changes with height. The environmental lapse rate (ELR) is the actual rate of temperature decrease with altitude at a specific place and time. The adiabatic lapse rate (ALR) is the theoretical rate of temperature change of an air parcel as it moves vertically without exchange of heat with its surroundings. Comparison of ELR and ALR determines atmospheric stability.

If a rising parcel of air is warmer than the surrounding air it will continue to rise (unstable); if it is cooler it will tend to sink back (stable). When ELR is greater than ALR, the atmosphere is unstable; when ELR is less than ALR, it is stable; when ELR equals ALR the atmosphere is neutral.

(A)

When the rising parcel is always warmer than the surrounding atmosphere it accelerates upward; similarly, a descending parcel cooler than surroundings accelerates downward. Such an environment is unstable and dispersion of pollutants is rapid; this condition is termed a super-adiabatic lapse rate in the input reference.

(B)

When ELR is less than ALR the environment is stable (sub-adiabatic). A stable atmosphere suppresses vertical mixing and pollutants tend to remain near their release altitude.

Negative lapse rate (temperature inversion): In unusual cases temperature increases with altitude; this is called an inversion and represents a highly stable situation that inhibits vertical mixing and leads to pollutant accumulation near the ground.

Types of inversion

• Radiation inversion: Occurs usually at night when the ground cools rapidly by radiation and the air in immediate contact with the ground becomes cooler than the air above.
• Subsidence inversion: Associated with sinking air in a high pressure area (anticyclone); sinking warms the air aloft and can produce a stable layer over a region.

Plume types

Plume shape and behaviour determine ground-level concentrations downwind of a source. Common plume types include:

• Looping plume: Wavy character, occurs in very unstable (super-adiabatic) conditions with rapid mixing.
• Neutral plume: Upward vertical rise from the stack with limited bending; occurs when ELR = ALR.
• Coning plume: The neutral plume tends to spread in a cone when wind speeds are relatively high (for example, greater than about 32 km/h in the input reference) or under slightly stable conditions.
• Fanning plume: Occurs under strong temperature inversion (large negative lapse rate); plume spreads horizontally with little vertical spread.
• Lofting plume: Occurs when the atmosphere is unstable above and stable below the plume, allowing the plume to rise and disperse aloft; generally favourable for ground-level concentrations.
• Fumigating plume: A dangerous condition where the atmosphere is stable above and unstable below the plume, causing pollutants to be brought down to ground level.
• Tapping plume: Occurs when there are stable layers both above and below the plume so the plume is trapped between inversion layers.

National ambient air quality standards (as given)

Notes: Values and units presented above are retained from the source reference. In practice, ambient air quality standards are specified by national or regional authorities and values, averaging times and units (commonly µg/m³ for particulates and gases) should be checked against the current regulatory standards in force.