Smog

Smog, a dense and often hazardous form of air pollution, is a persistent environmental problem in many industrial and urban regions. The term is a portmanteau of “smoke” and “fog”, coined in the early twentieth century to describe the thick, opaque atmospheric conditions that were historically common in London. Modern usage encompasses both winter smog driven by coal combustion and summer or photochemical smog caused by reactions between sunlight and vehicular or industrial emissions. China, like several rapidly industrialising regions, has experienced severe smog events, particularly in winter months when coal burning and meteorological inversions trap pollutants close to the ground.

Historical origins and terminology

The concept of smog emerged in the context of intense urban air pollution. In nineteenth- and early twentieth-century London, coal burning produced thick, sulphurous fogs known as London particulars or pea-soup fogs. Although Henry Antoine Des Voeux is often credited with coining the term “smog” in 1905, earlier printed uses can be traced to late nineteenth-century Californian publications. Regardless of its precise origin, the term came to describe the characteristic mixture of smoke, fog, and chemical pollutants common to industrial cities, including those in contemporary China.

Primary and secondary pollutants

Smog formation generally involves both primary and secondary pollutants. Primary pollutants include substances directly emitted into the atmosphere such as sulphur dioxide from coal combustion, nitrogen oxides from vehicles, and various hydrocarbons from industrial activities. Secondary pollutants, in contrast, arise from atmospheric chemical reactions involving primary pollutants. An example is ozone, a key component of photochemical smog, formed when nitrogen oxides and volatile organic compounds (VOCs) react under sunlight.
China’s smog episodes typically involve a mixture of primary emissions from coal burning, heavy industry, and vehicles, along with secondary pollutants formed through photochemical processes. The prevalence of these pollutants results in poor visibility, corrosive air, and a toxic environment detrimental to health.

Anthropogenic causes of smog in China

The principal human-driven causes of smog in China stem from combustion activities and industrial development.
Coal combustionCoal remains a significant energy source in China, especially for winter heating. Burning coal releases large quantities of smoke, soot, sulphur dioxide, and other pollutants. Northern cities frequently experience severe smog when winter heating systems are switched on. A notable example occurred in Harbin in 2013, when particulate levels reached extreme values, prompting widespread closures of schools, airports, and major roads.
Transportation emissionsVehicle exhaust contributes heavily to smog, especially in large metropolitan areas. Emissions include carbon monoxide, nitrogen oxides (NO and NO₂), particulate matter, and VOCs. These substances not only degrade air quality directly but also take part in photochemical reactions that generate ozone and other secondary pollutants. Growing car ownership and traffic congestion in cities such as Beijing, Shanghai, and Guangzhou exacerbate the problem.
Industrial activitiesChemical plants, oil refineries, steel mills, and power stations emit numerous pollutants. Without effective controls, these emissions combine with urban traffic exhausts to increase smog severity. In regions with dense industrial clusters, such as the Beijing–Tianjin–Hebei corridor, the cumulative effect of industrial output reinforces smog accumulation.

Photochemical smog and its chemistry

Photochemical smog—also known as summer smog—is generated when sunlight drives reactions between nitrogen oxides and VOCs, forming ozone, aldehydes, peroxyacyl nitrates (PAN), and other oxidising compounds. Cities with warm climates, heavy traffic, and abundant sunlight are particularly susceptible.
In the morning, NO and VOCs are emitted into the atmosphere, primarily from vehicles. Hydrocarbons react with hydroxyl radicals to form peroxy radicals, which convert NO to NO₂. NO₂ absorbs light and dissociates, generating oxygen atoms that combine with O₂ to form ozone. Under temperature inversion conditions, which restrict vertical mixing, these pollutants accumulate near the surface, allowing ozone concentrations to rise throughout the day.
The combination of ozone, particulates, and reactive organic compounds produces the eye-stingingly harsh conditions typical of photochemical smog. In Chinese cities, summer smog adds to the broader year-round air quality challenge.

Winter smog and meteorological influences

Winter smog in China is driven largely by combustion from heating systems, coal burning in industry, and reduced atmospheric dispersion during temperature inversions. An inversion forms when a layer of warm air overlays cooler air at the surface, preventing pollutants from rising. Without vertical mixing, emissions build up near ground level, contributing to poor visibility and extreme particulate concentrations.
Winter smog episodes may include high levels of PM₂.₅ and PM₁₀, sulphur dioxide, nitrogen oxides, and unburned hydrocarbons. The lack of sunlight limits photochemical activity, so ozone levels may be lower, but other pollutants accumulate to harmful concentrations.

Etymology and early documentation

Although commonly associated with Des Voeux’s 1905 address to the Public Health Congress, the word “smog” appears in English print earlier. Nineteenth-century reports from California used the term, and by the 1880s British publications also referred to smog in relation to London’s polluted air. The widespread use of coal for heating and manufacturing during this period made London a defining example of early anthropogenic smog.

Modern examples and public health impacts

Regions such as Beijing, Tianjin, and the North China Plain have recorded some of the world’s highest pollution levels during severe smog events. Similar episodes occur in other global megacities, including Los Angeles, Delhi, Tehran, Mexico City, and Lahore, often aggravated by local meteorological conditions and emission sources.
Smog poses serious health risks, including respiratory illness, cardiovascular disease, reduced lung function, and premature death. Long-term exposure is associated with chronic conditions such as asthma and bronchitis. Vulnerable populations, including children, the elderly, and those with pre-existing health issues, are particularly affected.

Chemical reactions and atmospheric processes

The atmospheric chemistry underlying smog involves numerous interlinked reactions. Morning emissions of NO and VOCs initiate oxidation processes that convert primary pollutants into more reactive secondary pollutants. Nitrogen dioxide participates in a photostationary equilibrium involving ozone formation and destruction, but the presence of VOCs tends to push the system towards net ozone production. Formaldehyde and other carbonyl compounds contribute additional radicals that further increase ozone levels.
Sulphur dioxide and nitrogen oxides can also be oxidised to produce sulphuric and nitric acids. When dissolved in atmospheric moisture, these acids contribute to acid rain, which has environmental impacts beyond the immediate smog conditions.

Broader significance

Smog remains one of the most visible manifestations of air pollution. Its causes reflect patterns of energy use, industrialisation, and transport behaviour. In China, the scale of economic development and reliance on coal have made smog an environmental and public health challenge of national significance. Efforts to address smog include transitioning to cleaner energy sources, improving emissions standards for vehicles and factories, expanding public transport, and improving air quality monitoring networks.