Environmental Pollution and Control I: Air Pollution — Sources and Types, Criteria Pollutants, Sampling and Monitoring of Ambient Air and Stack Emissions, the National Ambient Air Quality Standards, Impacts on Health, Plants and Materials, Acid Rain, and Dispersion: Mixing Height, Lapse Rates and the Gaussian Plume, Line and Area Source Models

The period, frequency and duration of sampling must match the averaging times of the standards. Under the National Air Quality Monitoring Programme, manual stations sample for 24 hours, gases in 4-hourly and particulate matter in 8-hourly periods, twice a week, giving 104 measurements a year; the annual standard is the arithmetic mean of at least 104 such measurements taken at uniform intervals, and the 24-hour standard must be met 98% of the time, so it may be exceeded on no more than 2% of days and not on two consecutive days. Continuous ambient air quality monitoring stations report in real time. Sites are chosen by land use, residential, industrial, commercial and sensitive, with the inlet at a height of about 3 to 10 m, away from obstructions and direct local sources, and placed using the wind rose.

Stack emissions are measured at a sampling port located several stack diameters downstream of any bend or obstruction, where flow is uniform. Gas velocity is measured with a Pitot tube, temperature with a thermocouple and moisture by condensation. Particulate matter must be sampled isokinetically: the velocity of gas entering the sampling nozzle must equal the stack gas velocity at that point. If sampling is faster than the stack gas (super-isokinetic), the streamlines converge into the nozzle but heavy particles cannot follow them, so large particles are under-sampled and the concentration is underestimated; if slower (sub-isokinetic), large particles are over-sampled. The Orsat apparatus analyses carbon dioxide, oxygen and carbon monoxide in flue gas by sequential absorption, and continuous emission monitoring systems are required of the most polluting industrial categories in India, reporting online to the pollution control boards.

Carbon monoxide is the only one of the twelve specified in mg/m³; benzo(a)pyrene, arsenic and nickel are in ng/m³ and the rest in µg/m³. The national Air Quality Index, launched in 2014, converts the concentrations of up to eight pollutants (PM₁₀, PM₂.₅, NO₂, SO₂, CO, O₃, NH₃ and Pb) into sub-indices, and the overall index is the highest sub-index, reported with the responsible pollutant. It has six categories: good (0 to 50), satisfactory (51 to 100), moderately polluted (101 to 200), poor (201 to 300), very poor (301 to 400) and severe (401 to 500), each with its health message; in Delhi and the National Capital Region the Graded Response Action Plan triggers emergency measures as the index worsens.

Fine particulate matter is the most harmful common air pollutant: PM₂.₅ penetrates to the alveoli and ultrafine particles can enter the bloodstream, causing respiratory and cardiovascular disease, stroke and lung cancer, and it is the leading environmental risk factor for premature death worldwide. Sulphur dioxide constricts the airways and aggravates asthma; nitrogen dioxide inflames the lungs and increases susceptibility to infection; ozone reduces lung function; carbon monoxide forms carboxyhaemoglobin; and lead damages the nervous system, especially in children. The elderly, children and people with heart and lung disease are most vulnerable.

Acid rain, more exactly acid deposition, is precipitation with a pH below about 5.6, together with the dry deposition of acidic gases and particles, caused by sulphur dioxide and nitrogen oxides that are oxidised to sulphuric and nitric acids, often hundreds of kilometres from their sources. It acidifies lakes and streams on poorly buffered granitic terrain, as in Scandinavia and north-eastern North America, where aluminium mobilised from soils damages fish gills; it leaches calcium and magnesium from forest soils and contributed to forest decline in central Europe; and it attacks buildings and monuments, dissolving limestone and marble. Where bedrock and soils contain carbonate, they buffer the acid. The transboundary nature of the problem led to the Convention on Long-range Transboundary Air Pollution (1979). In India rainfall is largely buffered by alkaline dust, but acidic rain has been reported near large emission sources, and the protection of the Taj Mahal from air pollution led the Supreme Court to create the Taj Trapezium Zone.

How far a pollutant is diluted depends on the wind, which carries and stretches the plume, and on atmospheric stability, which controls vertical mixing. The mixing height (mixing depth) is the height to which pollutants are mixed by convection and turbulence; it is estimated by drawing the dry adiabatic line up from the day's maximum surface temperature until it meets the morning temperature profile, so it is deep on sunny afternoons and shallow at night and in winter. The ventilation coefficient, mixing height multiplied by the mean wind speed through the mixed layer, measures the capacity of the air to disperse pollutants: with a mixing height of 400 m and a wind of 2 m/s it is 800 m²/s, a poor value typical of winter nights over the Indo-Gangetic plain, while values above about 6,000 m²/s indicate good dispersion.

The Gaussian plume model treats a continuous point source as spreading into a plume whose concentration falls off from the centreline as a normal (Gaussian) distribution in the crosswind (y) and vertical (z) directions. For a source of emission rate Q (g/s) with effective height H (the physical stack height plus the plume rise from buoyancy and momentum) in a wind of speed u, with reflection from the ground, the ground-level concentration on the plume centreline is C = [Q ÷ (π u σy σz)] exp(−H² ÷ 2σz²). The dispersion coefficients σy and σz grow with distance downwind and are read from Pasquill-Gifford curves for stability classes A (very unstable) to F (moderately stable). With Q = 100 g/s, u = 5 m/s, σy = 100 m, σz = 50 m and H = 50 m, C = [100 ÷ (π × 5 × 100 × 50)] × exp(−2500 ÷ 5000) = 1.273 × 10⁻³ × 0.607 ≈ 7.7 × 10⁻⁴ g/m³, about 772 µg/m³. Concentration is inversely proportional to wind speed, which is why calm conditions are the most dangerous, and a taller effective stack reduces ground-level concentrations through the exponential term.

A busy highway is better treated as a line source. For an infinitely long line source of strength q (g/s per metre) with the wind blowing perpendicular to it, the ground-level concentration downwind is C = [2q ÷ (√(2π) σz u)] exp(−H² ÷ 2σz²), which falls off more slowly with distance than that from a point source because there is no crosswind spreading. A city, with emissions from innumerable small sources, is treated as an area source, most simply with a box model: if emissions per unit area are q (g/m²/s) over a city of length L along the wind, and they mix uniformly through the mixing height Hm, the steady concentration is C = qL ÷ (u Hm), showing directly why low mixing heights and light winds bring urban pollution episodes.