Primary and Secondary Pollutants
Primary pollutants enter the air directly from a source: an exhaust pipe, stack, fire, dusty activity, or solvent evaporation. They include soot, carbon monoxide, sulfur dioxide, nitrogen oxides, ammonia, and some volatile organic compounds. Secondary pollutants form through atmospheric reactions — ground-level ozone develops from nitrogen oxides and organic compounds in sunlight, while secondary particles form from precursor gases including SO₂, NOx, and ammonia. The highest concentration may therefore occur away from the original emission source.
Particulate Matter (PM)
PM is a mixture of solid particles and liquid droplets — soot, mineral dust, salts, organic compounds, and metals. PM10 includes particles up to 10 µm in diameter; PM2.5 is up to 2.5 µm. Size affects how long particles remain airborne and where they deposit in the respiratory system. Sources include fuel combustion, heating, transport, industry, construction, quarries, road dust, fires, and natural dust. Crucially, some PM forms secondarily — controlling construction dust does not address particles forming from gaseous emissions by energy or agriculture.
Nitrogen Oxides (NOₓ)
NOₓ forms mainly during high-temperature combustion in engines, power plants, boilers, and industrial furnaces. Nitrogen dioxide (NO₂) receives particular attention because it irritates airways and contributes to ozone and secondary nitrate particle formation. Roadside NO₂ can change sharply over short distances. Electric vehicles reduce exhaust NOₓ, but do not eliminate tyre, brake, and road-wear particles — transport policy must therefore address both engine technology and total traffic volume.
Sulfur Dioxide (SO₂)
SO₂ is associated with sulfur-containing fuels, ore processing, oil refining, and certain chemical processes. In the atmosphere it forms sulfate — a component of fine particles — and contributes to acid deposition. Low-sulfur fuels and flue-gas treatment have substantially reduced SO₂ in many countries, but risks may remain near industrial complexes, ports, or poor-quality fuel use.
Ground-Level Ozone (O₃)
Unlike the protective stratospheric ozone layer, ground-level ozone is a harmful pollutant. It forms in sunny conditions from NOₓ and volatile organic compounds. The chemistry is non-linear — reducing one precursor in a specific urban area does not always produce a proportional reduction in ozone. The highest levels sometimes occur downwind of a city, where photochemical reactions have had time to develop.
Carbon Monoxide (CO) and VOCs
Carbon monoxide is produced by incomplete combustion. It is colourless and odourless, making it especially dangerous in enclosed or poorly ventilated spaces, where it interferes with oxygen transport in the blood. Volatile organic compounds (VOCs) evaporate from fuels, paints, solvents, and household products. They are a large group with widely varying toxicity, and some participate in ozone and secondary aerosol formation. Benzene is an important individual indicator due to its hazardous properties.
Ammonia and Pollutant Interactions
Ammonia mainly comes from livestock, manure storage, and fertiliser application. It reacts with acidic atmospheric components to form ammonium nitrate and sulfate — major components of secondary PM2.5. Urban air quality can therefore depend significantly on agricultural sources far outside a city. Pollutants interact, react, move, and transform — effective policy must address the mixture and its precursors, not just one indicator in isolation.
Effects on Health and the Environment
Particles and gases cause irritation, worsen respiratory disease, and increase cardiovascular stress. Children, older people, pregnant women, outdoor workers, and those with chronic illness are most vulnerable, but long-term exposure affects the whole population. Ozone damages vegetation; nitrogen compounds alter ecosystem nutrient balance; sulfur and nitrogen contribute to acidification; particles contaminate surfaces, water, and soil.
Common pollutants have different sources, behaviour, and impacts. Some are emitted directly, while others form in the atmosphere. Air-quality management must combine fuel and technology controls, transport and agricultural policy, monitoring of pollutant mixtures, and regional cooperation — no single sector can solve the problem alone.