Smog

Coal fire can emit significant clouds of smoke that contribute to the formation of winter smog. Coal fires can be used to heat individual buildings or to provide energy in a power-producing plant. Air pollution from this source has been reported in England since the Middle Ages.[7][8] London, in particular, was notorious up through the mid-20th century for its coal-caused smogs, which were nicknamed "pea-soupers". Air pollution of this type is still a problem in areas that generate significant smoke from burning coal. The emissions from coal combustion are one of the main causes of air pollution in China.[9] Especially during autumn and winter when coal-fired heating ramps up, the amount of produced smoke at times forces some Chinese cities to close down roads, schools or airports. One prominent example for this was China's Northeastern city of Harbin in 2013.

Traffic emissions – such as from trucks, buses, and automobiles – also contribute to the formation of smog.[10] Airborne by-products from vehicle exhaust systems cause air pollution and are a major ingredient in the creation of smog in some large cities.[11][12][13][14]

The major culprits from transportation sources are carbon monoxide (CO),[15][16] nitrogen oxides (NO and NO₂),[17][18][19] volatile organic compounds,[16][17] and hydrocarbons (hydrocarbons are the main component of petroleum fuels such as gasoline and diesel fuel).[16] Transportation emissions also include sulfur dioxides and particulate matter but in much smaller quantities than the pollutants mentioned previously. The nitrogen oxides and volatile organic compounds can undergo a series of chemical reactions with sunlight, heat, ammonia, moisture, and other compounds to form the noxious vapors, ground level ozone, and particles that comprise smog.[16][17]

During the morning rush hour, a high concentration of nitric oxide and hydrocarbons are emitted to the atmosphere, mostly via on-road traffic but also from industrial sources. Some hydrocarbons are rapidly oxidized by OH· and form peroxy radicals, which convert nitric oxide (NO) to nitrogen dioxide (NO₂).

(1) ${\displaystyle {\ce {R{.}+ O2 + M -> RO2{.}+ M}}}$

(2) ${\displaystyle {\ce {RO2{.}+ NO -> NO2 + RO{.}}}}$

(3) ${\displaystyle {\ce {HO2{.}+ NO -> NO2 + OH{.}}}}$

Nitrogen dioxide (NO₂) and nitric oxide (NO) further react with ozone (O₃) in a series of chemical reactions:

(4) ${\displaystyle {\ce {NO2 + hv -> O(^3P) + NO}}}$, ${\displaystyle \lambda <400nm}$

(5) ${\displaystyle {\ce {O(^3P) + O2 + M-> O3 + M(heat)}}}$

(6) ${\displaystyle {\ce {O3 + NO -> NO2 + O2}}}$

This series of equations is referred to as the photostationary state (PSS). However, because of the presence of Reaction 2 and 3, NO_x and ozone are not in a perfectly steady state. By replacing Reaction 6 with Reaction 2 and Reaction 3, the O₃ molecule is no longer destroyed. Therefore, the concentration of ozone keeps increasing throughout the day. This mechanism can escalate the formation of ozone in smog. Other reactions such as the photooxidation of formaldehyde (HCHO), a common secondary pollutant, can also contribute to the increased concentration of ozone and NO₂. Photochemical smog is more prevalent during summer days since incident solar radiation fluxes are high, which favors the formation of ozone (reactions 4 and 5). The presence of a temperature inversion layer is another important factor. That is because it prevents the vertical convective mixing of the air and thus allows the pollutants, including ozone, to accumulate near the ground level, which again favors the formation of photochemical smog.

There are certain reactions that can limit the formation of O₃ in smog. The main limiting reaction in polluted areas is:

(7) ${\displaystyle {\ce {NO2 + OH{.}+ M -> HNO3 + M}}}$

This reaction removes NO₂ which limits the amount of O₃ that can be produced from its photolysis (reaction 4). HNO₃ is a sticky compound that can easily be removed onto surfaces (dry deposition) or dissolved in water and be rained out (wet deposition). Both ways are common in the atmosphere and can efficiently remove radicals and nitrogen dioxide.

The presence of smog in California is shown near the Golden Gate Bridge. The brown coloration is due to the NO₂ formed from photochemical smog reactions.

An erupting volcano can emit high levels of sulfur dioxide along with a large quantity of particulates matter; two key components to the creation of smog. However, the smog created as a result of a volcanic eruption is often known as vog to distinguish it as a natural occurrence. The chemical reactions that form smog following a volcanic eruption are different than the reactions that form photochemical smog. The term smog encompasses the effect when a large number of gas-phase molecules and particulate matter are emitted to the atmosphere, creating a visible haze. The event causing a large number of emissions can vary but still result in the formation of smog.

Plants are another natural source of hydrocarbons that could undergo reactions in the atmosphere and produce smog. Globally both plants and soil contribute a substantial amount to the production of hydrocarbons, mainly by producing isoprene and terpenes.[28] Hydrocarbons released by plants can often be more reactive than man-made hydrocarbons. For example when plants release isoprene, the isoprene reacts very quickly in the atmosphere with hydroxyl radicals. These reactions produce hydroperoxides which increase ozone formation.[29]

The U.S. EPA has developed an air quality index to help explain air pollution levels to the general public. 8 hour average ozone concentrations of 85 to 104 ppbv are described as "Unhealthy for Sensitive Groups", 105 ppbv to 124 ppbv as "unhealthy" and 125 ppb to 404 ppb as "very unhealthy".[14] The "very unhealthy" range for some other pollutants are: 355 μg m⁻³ – 424 μg m⁻³ for PM10; 15.5 ppm – 30.4ppm for CO and 0.65 ppm – 1.24 ppm for NO₂.[38]

In 2016, the Ontario Medical Association announced that smog is responsible for an estimated 9,500 premature deaths in the province each year.[39]

A 20-year American Cancer Society study found that cumulative exposure also increases the likelihood of premature death from respiratory disease, implying the 8-hour standard may be insufficient.[40]

Tiny magnetic particles from air pollution have for the first time been discovered to be lodged in human brains– and researchers think they could be a possible cause of Alzheimer's disease. Researchers at Lancaster University found abundant magnetite nanoparticles in the brain tissue from 37 individuals aged three to 92-years-old who lived in Mexico City and Manchester. This strongly magnetic mineral is toxic and has been implicated in the production of reactive oxygen species (free radicals) in the human brain, which is associated with neurodegenerative diseases including Alzheimer's disease.[41][42]

A study examining 806 women who had babies with birth defects between 1997 and 2006, and 849 women who had healthy babies, found that smog in the San Joaquin Valley area of California was linked to two types of neural tube defects: spina bifida (a condition involving, among other manifestations, certain malformations of the spinal column), and anencephaly (the underdevelopment or absence of part or all of the brain, which if not fatal usually results in profound impairment).[43] An emerging cohort study in China linked early-life smog exposure to an increased risk for adverse pregnancy outcomes, in particular oxidative stress.[44]

According to a study published in The Lancet, even a very small (5 μg) change in PM2.5 exposure was associated with an increase (18%) in risk of a low birth weight at delivery, and this relationship held even below the current accepted safe levels.[45]

According to the Canadian Science Smog Assessment published in 2012, smog is responsible for detrimental effects on human and ecosystem health, as well as socioeconomic well-being across the country. It was estimated that the province of Ontario sustains $201 million in damages annually for selected crops, and an estimated tourism revenue degradation of $7.5 million in Vancouver and $1.32 million in The Fraser Valley due to decreased visibility. Air pollution in British Columbia is of particular concern, especially in the Fraser Valley, because of a meteorological effect called inversion which decreases air dispersion and leads to smog concentration.[46]

During the autumn and spring months, some 500 million tons of rice and wheat crop residues are burnt, and winds blow from India's north and northwest towards east.[47][48][49] This aerial view shows India's annual crop burning, resulting in smoke and air pollution over Delhi and adjoining areas.

For the past few years, cities in northern India have been covered in a thick layer of winter smog. The situation has turned quite drastic in the National Capital, Delhi. This smog is caused by the collection of Particulate Matter (a very fine type of dust and toxic gases) in the air due to stagnant movement of air during winters.[50]

Delhi is the most polluted[51] city in the world and according to one estimate, air pollution causes the death of about 10,500 people in Delhi every year.[52][53][54] During 2013–14, peak levels of fine particulate matter (PM) in Delhi increased by about 44%, primarily due to high vehicular and industrial emissions, construction work and crop burning in adjoining states.[52][55][56][57] Delhi has the highest level of the airborne particulate matter, PM2.5 considered most harmful to health, with 153 micrograms.[58] Rising air pollution level has significantly increased lung-related ailments (especially asthma and lung cancer) among Delhi's children and women.[59][60] The dense smog in Delhi during winter season results in major air and rail traffic disruptions every year.[61] According to Indian meteorologists, the average maximum temperature in Delhi during winters has declined notably since 1998 due to rising air pollution.[62]

Dense smog blankets Connaught Place, New Delhi

Environmentalists have criticized the Delhi government for not doing enough to curb air pollution and to inform people about air quality issues.[53] Most of Delhi's residents are unaware of alarming levels of air pollution in the city and the health risks associated with it.[56] Since the mid-1990s, Delhi has undertaken some measures to curb air pollution – Delhi has the third highest quantity of trees among Indian cities[63] and the Delhi Transport Corporation operates the world's largest fleet of environmentally friendly compressed natural gas (CNG) buses.[64] In 1996, the Centre for Science and Environment (CSE) started a public interest litigation in the Supreme Court of India that ordered the conversion of Delhi's fleet of buses and taxis to run on CNG and banned the use of leaded petrol in 1998. In 2003, Delhi won the United States Department of Energy's first 'Clean Cities International Partner of the Year' award for its "bold efforts to curb air pollution and support alternative fuel initiatives".[64] The Delhi Metro has also been credited for significantly reducing air pollutants in the city.[65]

However, according to several authors, most of these gains have been lost, especially due to stubble burning, rise in market share of diesel cars and a considerable decline in bus ridership.[66][67] According to CUE and System of Air Quality Weather Forecasting and Research (SAFER), burning of agricultural waste in nearby Punjab, Haryana and Uttar Pradesh regions results in severe intensification of smog over Delhi.[68][69] The state government of adjoining Uttar Pradesh is considering imposing a ban on crop burning to reduce pollution in Delhi NCR and an environmental panel has appealed to India's Supreme Court to impose a 30% cess on diesel cars.[70][71]

Joint research between American and Chinese researchers in 2006 concluded that much of the city's pollution comes from surrounding cities and provinces. On average 35–60% of the ozone can be traced to sources outside the city. Shandong Province and Tianjin Municipality have a "significant influence on Beijing's air quality",[72] partly due to the prevailing south/southeasterly flow during the summer and the mountains to the north and northwest.

Victorian London was notorious for its thick smogs, or "pea-soupers", a fact that is often recreated (as here) to add an air of mystery to a period costume drama

In 1306, concerns over air pollution were sufficient for Edward I to (briefly) ban coal fires in London.[7] In 1661, John Evelyn's Fumifugium suggested burning fragrant wood instead of mineral coal, which he believed would reduce coughing. The "Ballad of Gresham College" the same year describes how the smoke "does our lungs and spirits choke, Our hanging spoil, and rust our iron."

Severe episodes of smog continued in the 19th and 20th centuries, mainly in the winter, and were nicknamed "pea-soupers," from the phrase "as thick as pea soup". The Great Smog of 1952 darkened the streets of London and killed approximately 4,000 people in the short time of four days (a further 8,000[73] died from its effects in the following weeks and months). Initially, a flu epidemic was blamed for the loss of life.

In 1956 the Clean Air Act started legally enforcing smokeless zones in the capital. There were areas where no soft coal was allowed to be burned in homes or in businesses, only coke, which produces no smoke. Because of the smokeless zones, reduced levels of sooty particulates eliminated the intense and persistent London smog.

It was after this that the great clean-up of London began. One by one, historical buildings which, during the previous two centuries had gradually completely blackened externally, had their stone facades cleaned and restored to their original appearance. Victorian buildings whose appearance changed dramatically after cleaning included the British Museum of Natural History. A more recent example was the Palace of Westminster, which was cleaned in the 1980s. A notable exception to the restoration trend was 10 Downing Street, whose bricks upon cleaning in the late 1950s proved to be naturally yellow; the smog-derived black color of the façade was considered so iconic that the bricks were painted black to preserve the image.[74][75] Smog caused by traffic pollution, however, does still occur in modern London.

Grease filter hood after 4 days at Italian city polluted air in winter (all surface was white)

Other areas of the United Kingdom were affected by smog, especially heavily industrialised areas.

The cities of Glasgow and Edinburgh, in Scotland, suffered smoke-laden fogs in 1909. Des Voeux, commonly credited with creating the "smog" moniker, presented a paper in 1911 to the Manchester Conference of the Smoke Abatement League of Great Britain about the fogs and resulting deaths.[76]

One Birmingham resident described near black-out conditions in the 1900s before the Clean Air Act, with visibility so poor that cyclists had to dismount and walk in order to stay on the road.[77]

On 29 April 2015, the UK Supreme Court ruled that the government must take immediate action to cut air pollution,[78] following a case brought by environmental lawyers at ClientEarth.[79]

Situated in a valley, and relying heavily on automobiles, Mexico City often suffers from poor air quality.

Due to its location in a highland "bowl", cold air sinks down onto the urban area of Mexico City, trapping industrial and vehicle pollution underneath, and turning it into the most infamously smog-plagued city of Latin America. Within one generation, the city has changed from being known for some of the cleanest air of the world into one with some of the worst pollution, with pollutants like nitrogen dioxide being double or even triple international standards.[80]

Photochemical smog over Mexico City, December 2010

Similar to Mexico City, the air pollution of Santiago valley, located between the Andes and the Chilean Coast Range, turn it into the most infamously smog-plagued city of South America. Other aggravates of the situation reside in its high latitude (31 degrees South) and dry weather during most of the year.

In December 2005, schools and public offices had to close in Tehran and 1600 people were taken to hospital, in a severe smog blamed largely on unfiltered car exhaust.[81]

A NASA astronaut's photograph of a smog layer over central New York

View of smog south from Los Angeles City Hall, September 2011

Counties in the United States where one or more National Ambient Air Quality Standards are not met, as of October 2015

Smog was brought to the attention of the general U.S. public in 1933 with the publication of the book "Stop That Smoke", by Henry Obermeyer, a New York public utility official, in which he pointed out the effect on human life and even the destruction of 3,000 acres (12 km²) of a farmer's spinach crop.[82] Since then, the United States Environmental Protection Agency has designated over 300 U.S. counties to be non-attainment areas for one or more pollutants tracked as part of the National Ambient Air Quality Standards.[83] These areas are largely clustered around large metropolitan areas, with the largest contiguous non-attainment zones in California and the Northeast. Various U.S. and Canadian government agencies collaborate to produce real-time air quality maps and forecasts.[84] To combat smog conditions, localities may declare "smog alert" days, such as in the Spare the Air program in the San Francisco Bay Area. By 1970, Congress enacted the Clean Air Act to regulate air pollutant emissions.[85]

In the United States, smog pollution kills 24,000 Americans every year. The U.S. is among the dirtier countries in terms of smog, ranked 123 out of 195 countries measured, where 1 is cleanest and 195 is most smog polluted.[86]

Because of their locations in low basins surrounded by mountains, Los Angeles and the San Joaquin Valley are notorious for their smog. Heavy automobile traffic, combined with the additional effects of the San Francisco Bay and Los Angeles/Long Beach port complexes, frequently contribute to further air pollution.

Los Angeles, in particular, is strongly predisposed to the accumulation of smog, because of the peculiarities of its geography and weather patterns. Los Angeles is situated in a flat basin with the ocean on one side and mountain ranges on three sides. A nearby cold ocean current depresses surface air temperatures in the area, resulting in an inversion layer: a phenomenon where air temperature increases, instead of decreasing, with altitude, suppressing thermals and restricting vertical convection. All taken together, this results in a relatively thin, enclosed layer of air above the city that cannot easily escape out of the basin and tends to accumulate pollution.

Los Angeles was one of the best-known cities suffering from transportation smog for much of the 20th century, so much so that it was sometimes said that Los Angeles was a synonym for smog.[87] In 1970, when the Clean Air Act was passed, Los Angeles was the most polluted basin in the country, and California was unable to create a State Implementation Plan that would enable it to meet the new air quality standards.[88] However, ensuing strict regulations by state and federal government agencies overseeing this problem (such as the California Air Resources Board and the United States Environmental Protection Agency), including tight restrictions on allowed emissions levels for all new cars sold in California and mandatory regular emission tests of older vehicles, resulted in significant improvements in air quality.[89] For example, air concentrations of volatile organic compounds declined by a factor of 50 between 1962 and 2012.[90] Concentrations of air pollutants such as nitrous oxides and ozone declined by 70% to 80% over the same period of time.[91]

• 26 July 1943, Los Angeles, California: A smog so sudden and severe that "Los Angeles residents believe the Japanese are attacking them with chemical warfare."[92][93]
• 30-31 October 1948, Donora, Pennsylvania: 20 died, 600 hospitalized, thousands more stricken. Lawsuits were not settled until 1951.[94]
• 24 November 1966, New York City, New York: Smog kills at least 169[95] people.

In the late 1990s, massive immigration to Ulaanbaatar from the countryside began. An estimated 150,000 households, mainly living in traditional Mongolian gers on the outskirts of Ulaanbaatar, burn wood and coal (some poor families burn even car tires and trash) to heat themselves during the harsh winter, which lasts from October to April, since these outskirts are not connected to the city's central heating system. A temporary solution to decrease smog was proposed in the form of stoves with improved efficiency, although with no visible results. Coal-fired ger stoves release high levels of ash and other particulate matter (PM). When inhaled, these particles can settle in the lungs and respiratory tract and cause health problems. At two to 10 times above Mongolian and international air quality standards, Ulaanbaatar's PM rates are among the worst in the world, according to a December 2009 World Bank report. The Asian Development Bank (ADB) estimates that health costs related to this air pollution account for as much as 4 percent of Mongolia's GDP.[96]

Singapore's Downtown Core on 7 October 2006, when it was affected by forest fires in Sumatra, Indonesia

Smog is a regular problem in Southeast Asia caused by land and forest fires in Indonesia, especially Sumatra and Kalimantan, although the term haze is preferred in describing the problem. Farmers and plantation owners are usually responsible for the fires, which they use to clear tracts of land for further plantings. Those fires mainly affect Brunei, Indonesia, Philippines, Malaysia, Singapore and Thailand, and occasionally Guam and Saipan.[97][98] The economic losses of the fires in 1997 have been estimated at more than US$9 billion.[99] This includes damages in agriculture production, destruction of forest lands, health, transportation, tourism, and other economic endeavours. Not included are social, environmental, and psychological problems and long-term health effects. The second-latest bout of haze to occur in Malaysia, Singapore and the Malacca Straits is in October 2006, and was caused by smoke from fires in Indonesia being blown across the Straits of Malacca by south-westerly winds. A similar haze has occurred in June 2013, with the PSI setting a new record in Singapore on 21 June at 12pm with a reading of 401, which is in the "Hazardous" range.[100]

The Association of Southeast Asian Nations (ASEAN) reacted. In 2002, the Agreement on Transboundary Haze Pollution was signed between all ASEAN nations.[101] ASEAN formed a Regional Haze Action Plan (RHAP) and established a co-ordination and support unit (CSU).[102] RHAP, with the help of Canada, established a monitoring and warning system for forest/vegetation fires and implemented a Fire Danger Rating System (FDRS). The Malaysian Meteorological Department (MMD) has issued a daily rating of fire danger since September 2003.[103] Indonesia has been ineffective at enforcing legal policies on errant farmers.

Since the start of the winter season, heavy smog loaded with pollutants covered major parts of Punjab, especially the city of Lahore,[104] causing breathing problems and disrupting normal traffic.[105] A recent study from 2022 shows that the primary cause of pollution in Lahore is from traffic-related PM (both exhausts and non exhaust sources)[106]

Doctors advised residents to stay indoors and wear facemasks outside.[107]