Air Pollution: The Atmosphere, Pollutants and Standards, Meteorology and Dispersion

The atmosphere is layered by its temperature profile. In the troposphere, from the ground to about 8 km at the poles and 16–18 km at the equator, temperature falls with height at about 6.5 °C/km on average; it holds most of the mass and all the weather, and nearly all air pollution lives in its lowest kilometre or two. Above the tropopause the stratosphere warms with height to about 50 km because its ozone absorbs solar ultraviolet; the mesosphere cools again to about 85 km, and the thermosphere warms above it. Dry air is about 78% N₂, 21% O₂ and 0.93% Ar by volume, with CO₂ now above 400 ppm, and trace amounts of Ne, He, CH₄, N₂O, H₂ and O₃; water vapour varies from nearly zero to about 4%.

Natural sources include volcanoes (SO₂, ash), wind-blown dust, sea salt, forest fires, pollen and the biogenic VOCs (isoprene, terpenes) of vegetation. Anthropogenic sources are dominated by combustion — power plants and industry (SO₂, NOₓ, particulates), vehicles (CO, NOₓ, hydrocarbons, fine particles), domestic solid fuels and open burning of crop residue and waste — together with industrial processes, solvents and agriculture, which emits most of the ammonia. Once emitted, pollutants go through four atmospheric processes: emission; transformation, chiefly oxidation by the OH radical (SO₂ to sulphate, NO₂ to nitric acid and nitrate, VOCs to secondary organic aerosol, and the NOₓ–VOC photochemistry that makes ozone); transport by the mean wind and dilution by turbulence; and removal by dry deposition (settling and uptake at surfaces) and wet deposition (in-cloud rainout and below-cloud washout).

A primary pollutant is emitted as such (SO₂, NO, CO, soot, lead); a secondary pollutant forms in the air from precursors (ozone, sulphate and nitrate aerosol, peroxyacetyl nitrate, most PM₂.₅ mass in many cities). Particulate matter is classed by aerodynamic diameter: PM₁₀ (inhalable, reaching the airways) and PM₂.₅ (fine, reaching the alveoli), with coarse particles mostly mechanical in origin and fine particles mostly from combustion and secondary formation. The criteria pollutants — those regulated by ambient standards on the basis of health criteria — are classically six: particulate matter, ozone, carbon monoxide, sulphur dioxide, nitrogen dioxide and lead; India’s National Ambient Air Quality Standards (2009) cover twelve, adding PM₂.₅ separately, ammonia, benzene, benzo(a)pyrene, arsenic and nickel.

Indoor air pollution can exceed outdoor levels by far. Cooking and heating with solid fuels in poorly ventilated homes produce high PM₂.₅ and CO; building materials and furnishings release formaldehyde and other VOCs; soil gas brings radon, a radioactive decay product of uranium and a cause of lung cancer; tobacco smoke, combustion appliances (NO₂), moulds and bioaerosols add to it; and "sick building syndrome" describes complaints tied to poor ventilation. The effects on health follow from where a pollutant acts: fine particles penetrate to the alveoli and are linked to cardiovascular and respiratory disease; CO binds haemoglobin some 200 times more strongly than oxygen, forming carboxyhaemoglobin and starving tissues of oxygen; SO₂ and NO₂ irritate the airways; ozone inflames the lungs; lead is a neurotoxin, especially for children. The effects on the environment include acid deposition on soils, lakes and buildings, ozone injury to crops and forests, fluoride damage to vegetation, soiling and corrosion of materials, and haze.

An ambient standard limits the concentration in the air people breathe, for a stated averaging time; an emission standard limits what a source may release, as a concentration in the stack gas, a mass rate or a mass per unit of product. Ambient standards set the goal; emission standards, stack-height rules and fuel standards are the tools that meet it. Each pollutant is set against averaging periods that match its harm — 24 hours and a year for particulates, 8 hours for CO and ozone, 1 hour for peaks — and compliance is judged over those periods, not at an instant.

An air quality index turns several pollutants into one number for the public. Each pollutant’s concentration C_p is converted to a sub-index by linear interpolation within its band, I_p = I_lo + (I_hi − I_lo)(C_p − BP_lo)/(BP_hi − BP_lo), and the AQI is the maximum sub-index, reported with the responsible pollutant. India’s National AQI uses eight pollutants and six categories from Good (0–50) to Severe (401–500). Visibility is reduced by particles that scatter and absorb light; the visual range follows Koschmieder’s relation L_v = 3.912/b_ext, with b_ext the extinction coefficient, and fine particles near the wavelength of light (0.1–1 μm) are the most effective. The radiative effects of aerosols are direct — sulphate scatters sunlight and cools, black carbon absorbs and warms — and indirect, as particles act as cloud condensation nuclei and make clouds brighter and longer-lived.

For management, sources are grouped by geometry: point sources (stacks), line sources (highways, railways), and area sources (a city’s domestic fuel use, a landfill, a field being burnt). An emission inventory estimates each source’s release as activity × emission factor × (1 − control efficiency) — tonnes of coal burnt times kg SO₂ per tonne, vehicle-kilometres times g/km — and adds them by pollutant, sector and grid cell. It is the starting point for dispersion modelling, for apportioning ambient pollution among sources, and for choosing where control buys the most improvement.

Dispersion is governed by atmospheric stability, found by comparing the environmental lapse rate (ELR) with the dry adiabatic lapse rate Γ_d = g/c_p ≈ 9.8 °C/km, at which a rising parcel of dry air cools. If the environment cools faster than Γ_d (superadiabatic), a displaced parcel keeps rising and the air is unstable; if it cools at Γ_d it is neutral; if slower (subadiabatic), isothermal or warming with height (inversion), it is stable and vertical mixing is suppressed. Inversions form by radiation from the ground on clear, calm nights, by subsidence of air in high-pressure systems, at fronts, and by advection of warm air over a cold surface. The mixing height is the depth through which pollutants mix: the height at which the dry adiabat drawn from the afternoon maximum surface temperature meets the environmental profile. With the mean wind through that layer it gives the ventilation coefficient, mixing height × wind speed (m²/s), a single measure of how well a region disperses what it emits. Wind speed increases with height, often represented by the power law u = u₁(z/z₁)^p. A wind rose shows, for a site, how often the wind blows from each direction and at what speed — used to place industries downwind of towns and to site monitors.

The Gaussian plume model treats a continuous point source of strength Q (g/s) in a steady wind u (m/s) at the stack height, with the plume spreading as normal distributions across wind (σ_y) and vertically (σ_z), both growing with distance downwind at rates set by the Pasquill stability class (A, very unstable, to F, stable). With the ground treated as a reflecting surface by an image source, C(x, y, z) = [Q/(2πuσ_yσ_z)] exp(−y²/2σ_y²) {exp[−(z − H)²/2σ_z²] + exp[−(z + H)²/2σ_z²]}. At ground level on the plume centreline (y = 0, z = 0) this becomes C = [Q/(πuσ_yσ_z)] exp(−H²/2σ_z²), and for a ground-level source (H = 0) simply Q/(πuσ_yσ_z). The ground-level maximum occurs roughly where σ_z = H/√2, so raising H both lowers the peak and pushes it further away.

H is the effective stack height, the physical height plus the plume rise Δh from the exit momentum and buoyancy of the gas. Holland’s equation is a common estimate: Δh = (v_s d/u)[1.5 + 2.68 × 10⁻³ p d (T_s − T_a)/T_s], with v_s the exit velocity (m/s), d the stack diameter (m), p the pressure (mbar) and T_s, T_a the stack-gas and air temperatures (K); Briggs’ formulas are the more modern alternative. In India, stack height for SO₂ dispersion is set by the CPCB relation H = 14Q^0.3, with Q the SO₂ emission in kg/h. Topographic influences can defeat all of this: valleys trap cold air and inversions at night and channel the wind along their axis; slopes generate drainage (downslope) winds at night; coasts have sea breezes by day that can carry a shoreline plume inland and fumigate it; tall buildings create a turbulent wake that drags a short plume down (downwash), which is why a stack is made at least about 2.5 times the height of nearby buildings; and cities form a heat island that alters the local circulation.