Air Pollution and Air Quality: Pollutants, AQI and Laws

Air pollution is the presence in the atmosphere of substances in concentrations high enough to harm human health, other living beings, crops or materials. For an examination the subject breaks into four blocks that can be revised separately: which pollutant is which, what each one does, how air quality is measured, and what the law and the technology do about it.

Primary and secondary pollutants

A primary pollutant is released into the air directly from its source. A secondary pollutant is not emitted at all; it is formed in the atmosphere when primary pollutants react with one another or with sunlight. The distinction is the single most examined idea in this topic.

The two kinds of smog

The word smog was coined from smoke and fog. Classical or London smog forms in cold, humid weather when sulphur dioxide and smoke from coal burning mix with fog; it is chemically reducing and is sometimes called sulphurous smog. Photochemical or Los Angeles smog forms in warm, dry, sunny weather from oxides of nitrogen and unburnt hydrocarbons; it is chemically oxidising, has a brown haze, and its chief components are ozone, peroxyacetyl nitrate and aldehydes, which irritate the eyes and the lungs and crack rubber.

What each pollutant does

• Carbon monoxide binds to haemoglobin to form carboxyhaemoglobin, roughly two hundred times more readily than oxygen does, so the blood carries less oxygen; the result is headache, giddiness and, at high levels, death.
• Sulphur dioxide irritates the respiratory tract and, oxidised in the air, returns as sulphuric acid in rain. It is also the agent of the yellowing and pitting of the Taj Mahal's marble, popularly called marble cancer.
• Oxides of nitrogen return as nitric acid in rain and drive the formation of photochemical smog.
• Particulate matter is graded by size. PM10 is particles of ten micrometres or less in diameter and PM2.5 is 2.5 micrometres or less. The finer fraction is the more dangerous because it travels past the nose and throat and reaches the alveoli of the lungs, and can pass into the bloodstream.
• Lead from leaded petrol and some industries damages the nervous system, particularly in children, which is why leaded petrol was withdrawn.

Rain is called acid rain when its pH falls below about 5.6, which is the value of normal rain made mildly acidic by dissolved carbon dioxide. Acid rain lowers the pH of lakes and soils, releases toxic aluminium from soil minerals, damages leaves and corrodes limestone and marble monuments.

Measuring air quality

The Central Pollution Control Board notifies the National Ambient Air Quality Standards, which set the permitted concentration of each pollutant, and it also publishes the National Air Quality Index, which turns many readings into one number with one colour and one description so that a citizen can read it at a glance. The index is calculated for eight pollutants: PM10, PM2.5, sulphur dioxide, oxides of nitrogen, carbon monoxide, ozone, ammonia and lead. A sub-index is worked out for each pollutant for which enough data exist, and the worst sub-index becomes the day's AQI, with the pollutant responsible named as the prominent pollutant.

Law and institutions in India

The Air (Prevention and Control of Pollution) Act was passed in 1981 and is the law that deals specifically with air pollution; the Environment (Protection) Act of 1986, passed after the Bhopal gas tragedy of December 1984, is the umbrella legislation. The Central Pollution Control Board, created in 1974 under the Water Act, and the State Pollution Control Boards enforce standards, and a Commission for Air Quality Management in the National Capital Region and adjoining areas was set up by an Act of Parliament in 2021. The National Clean Air Programme, launched in 2019 by the Ministry of Environment, Forest and Climate Change, works to cut particulate pollution in cities that do not meet the standards, and the Graded Response Action Plan ties emergency restrictions in Delhi and its neighbourhood to the AQI category of the day. Bharat Stage VI emission norms for vehicles came into force across the country from 1 April 2020, moving straight from Bharat Stage IV.

Controlling emissions

Particulate matter in the flue gas of a thermal power station is removed by an electrostatic precipitator, which charges the particles and collects them on oppositely charged plates, or by a bag filter or a cyclone separator. Gases such as sulphur dioxide are removed by a scrubber, in which the gas is washed with water or a lime slurry. In a motor vehicle a catalytic converter, using platinum, palladium and rhodium, changes carbon monoxide and unburnt hydrocarbons into carbon dioxide and water and reduces oxides of nitrogen to nitrogen; it needs unleaded petrol, because lead poisons the catalyst. Indoor air pollution from burning wood and dung is fought by supplying clean cooking fuel, and lichens, which are killed by even low levels of sulphur dioxide, are used as living indicators of air quality.