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Smog: The hazy, unhealthy polluted air which accumulates over cities and other regions under certain conditions. [e] The metadata subpage is missing. You can start it via filling in this form or by following the instructions that come up after clicking on the [show] link to the right. Do you see this on PREVIEW? then SAVE! before following a link. To find out more about the use of metadata for the management of Citizendium pages, see CZ:Metadata. You may see this box for one of two reasons. Either: A - New page has been created, or subpages template was added to an existing page B - Cluster move is in progress Click either the A or B link for further instructions. [Feel free to recommend improvements to the text or links in this template] A - For a New Cluster use the following directions Subpages format requires a metadata page. Using the following instructions will complete the process of creating this article's subpages. Click the blue "metadata template" link below to create the page. On the edit page that appears paste in the article's title across from "pagename =". You might also fill out the checklist part of the form. Ignore the rest. For background, see Using the Subpages template Don't worry--you'll get the hang of it right away. Remember to hit Save! the "metadata template". However, you can create articles without subpages. Just delete the {{subpages}} template from the top of this page and this prompt will disappear. :) Don't feel obligated to use subpages, it's more important that you write sentences, which you can always do without writing fancy code. [Feel free to recommend improvements to the text or links in this template] B - For a Cluster Move use the following directions The metadata template should be moved to the new name as the first step. Please revert this move and start by using the Move Cluster link at the top left of the talk page. The name prior to this move can be found at the following link. [Feel free to recommend improvements to the text or links in this template] (PD) Photo: Dr. Edwin P. Ewing, Jr. / Centers for Disease Control and Prevention (CDC) Smog over New York skyline in 1988 Smog is the hazy, unhealthy polluted air that accumulates over cities and other regions under certain conditions. Modern smog is derived primarily from precursor chemical pollutants, emitted to the atmosphere from vehicular internal combustion engines and industrial plants, that react in the atmosphere with sunlight to produce secondary chemical pollutants that also combine with the precursor emissions to form the components of what is called photochemical smog. Origin of the term "smog" The word smog is a blend of the words smoke and fog. Coinage of the term is generally attributed to Dr. Henry Antoine des Voeux in his 1905 paper, "Fog and Smoke" for a meeting of the Public Health Congress in London, United Kingdom. The July 26, 1905 edition of the Daily Graphic, a London newspaper, quoted des Voeux: "... he said it required no science to see that there was something produced in great cities which was not found in the country, and that was smoky fog, or what was known as 'smog'."  The next day, the Globe newspaper remarked that "Dr. des Voeux did a public service in coining a new word for the London fog."> Photochemical smog (CC) Diagram: Milton Beychok Atmospheric chemistry involved in smog formation Originally, Dr. des Voeux's term smog referred to a mixture of smoke, sulfur dioxide (SO2) and fog that was once prevalent in London when coal with a high sulphur content was widely used throughout the city as heating fuel. The dark, sulfurous London smog caused reduced visibility, respiratory problems and had a noticeable deleterious affect on human health. The Great Smog of 1952 darkened the streets of London and killed approximately 4,000 people in a 4-day period (another 8,000 died from its effects in the following months) The type of smog experienced in London many decades ago is no longer encountered since other fuels have largely replaced the wide-spread use of high-sulfur coal for heating. Photochemical smog is chemically quite different than the old London-type smog and has a long history. In 1542, when exploring what is now Southern California, Juan Rodriguez Cabrillo named San Pedro Bay the "Bay of Smokes" because of the thick haze that covered the area. Complaints of eye irritation from the polluted air in Los Angeles date back to the late 1860s. In the 1940s, photochemical smog first became apparent in Los Angeles and other large cities on sunny days, although none of those cities had any significant use of coal for heating fuel or for industrial activities. Photochemical smog is a daytime phenomenon that occurs mostly during warm, sunny summertime days and is characterized by a brown haze that reduces visibility and contains oxidants, such as ozone (O3), that cause respiratory problems, eye irritation and damage to plants. It is a mixture of variable amounts of ozone, reactive hydrocarbons, nitrogen oxide (NO), nitrogen dioxide (NO2), aldehydes, peroxyacetyl nitrate (PAN), particulate matter and other components. By contrast, the London-type smog (also referred to as "classic smog") occurs mostly during cold winter days and consists primarily of a mixture of sulfur dioxide and particulate matter (i.e., "soot") usually derived from burning coal. Photochemical smog precursor pollutants The chemistry involved in the formation of smog is highly complex and involves many different reactions (see the adjacent diagram). The three major ingredients required to form photochemical smog are solar energy (i.e., sunlight), reactive hydrocarbons and nitrogen oxide, the latter two being referred to as the precursor pollutants. As shown in the diagram, the two precursors enter the atmosphere as gases emitted for the most part from vehicular internal combustion engines fuelled by gasoline and diesel oil. To a much lesser extent, nitrogen oxide is also emitted from industrial combustion sources, and reactive hydrocarbons are emitted by evaporation from the handling and storage of volatile hydrocarbons such as gasoline, solvents, some pesticides and some paints. Biogenic sources such as pine trees and certain other trees are also a source, though of relatively minor importance, of reactive hydrocarbon emissions such as isoprene and α-pinene. Simplified chemistry of photochemical smog formation In the simplified schematic overview depicted in the adjacent diagram, the major chemical reactions that contribute to the formation of photochemical smog include the following: The gaseous precursor nitrogen oxide, emitted primarily from vehicular internal combustion engines, is oxidized to produce gaseous nitrogen dioxide. nitrogen oxide + oxygen $\rightarrow$ nitrogen dioxide 2NO(g) + O2(g) $\mathrm {\rightarrow }$ 2NO2(g) The gaseous nitrogen dioxide is broken down by solar energy from sunlight to produce gaseous nitrogen oxide and atomic oxygen (O, an oxygen radical). This process is referred to as photolysis and hence the term "photochemical smog". nitrogen dioxide +solar energy $\rightarrow$ nitrogen oxide + atomic oxygen NO2(g) + solar energy $\rightarrow$ NO (g) + O The atomic oxygen reacts with gaseous atmospheric oxygen (O2) to form gaseous ozone (O3). atomic oxygen + atmospheric oxygen $\rightarrow$ ozone O + O2(g) $\rightarrow$ O3(g) The gaseous ozone also oxidizes gaseous nitrogen oxide to form gaseous nitrogen dioxide and gaseous oxygen. ozone + nitrogen oxide $\rightarrow$ nitrogen dioxide + oxygen O3(g) + NO(g) $\rightarrow$ NO2(g) + O2(g) The gaseous precursor reactive hydrocarbons (RH), emitted primarily from vehicular internal combustion engines, reacts with atomic oxygen, atmospheric oxygen and ozone to produce various highly reactive hydrocarbon free radicals (RO•2). The hydrocarbon free radicals then react with other species such as nitrogen dioxide to form peroxyacetyl nitrate (PAN), aldehydes and other smog components. Nitrogen dioxide + oxygen + hydrocarbon free radicals $\rightarrow$ peroxyacetyl nitrate 2NO2(g) + O2(g) + 2RO•2 $\rightarrow$ 2CH3CO–OO–NO2(g) The various products produced or formed from the precursor pollutants are referred to as secondary pollutants. All of the above chemical reactions (as well as others) occur more or less simultaneously during sunny, summertime days in most large cities with a great number of automobiles and other vehicles. Health effects (CC) Photo: Carol Sawada Smog in São Paulo, Brazil in 2006. United States' AQI Air Quality Index (AQI) Air Quality Category Color Code 0 – 50 Good   51 – 100 Moderate   101 – 150 Unhealthy for Sensitive Groups   151 – 200 Unhealthy   201 – 300 Very Unhealthy   301 – 500 Hazardous   Photochemical smog constitutes a serious health problem in most large cities. It is especially harmful for senior citizens, children, and people with heart and lung conditions such as emphysema, bronchitis, and asthma. It can inflame breathing passages, decrease the lungs' working capacity, cause shortness of breath, pain when inhaling deeply, wheezing, and coughing. It can cause eye and nose irritation and it dries out the protective membranes of the nose and throat. In general, it interferes with the body's ability to fight infection, thereby increasing susceptibility to illness. The adjacent table explains the Air Quality Index (AQI) ranges used by the U.S. Environmental Protection Agency (U.S. EPA) and their corresponding health effect categories and color codes. The U.S. EPA's AQI is also known as the Pollution Standards Index (PSI). A national map of the United States of America containing daily AQI forecasts across the nation, using that same color-coding, is published online jointly by the U.S. EPA and the National Oceanic and Atmospheric Administration (NOAA). If multiple pollutants are measured at a monitoring site, then the largest or "dominant" AQI value is reported for the location. The U.S. EPA has developed conversion calculators, available online, for the conversion of AQI values to concentration values and for the reverse conversion of concentrations to AQI values. The South Coast Air Quality Management District (SCAQMD) of Southern California has published an excellent, comprehensive discussion of the undesirable health effects caused by air pollution. It also includes an extensive listing and discussion of the many medical health effect studies of pollution performed during the past decades starting around 1950. (CC) Photo: Brian Jeffrey Beggerly Smog over Beijing in 2005. Areas affected Smog can form in almost any location where the level of air pollutants is high, and ii is more likely to occur during periods of warm, sunny weather when the upper air is warm enough to inhibit vertical circulation. It often persists for extended time periods over densely populated cities, such as Athens, Beijing, Berlin, Cairo, Hong Kong, Kuala Lumpur, London, United Kingdom, Los Angeles, Milan, Moscow, New York, Paris, Rome, São Paulo, Seoul and Shanghai. Smog is especially prevalent in geologic basins encircled by hills or mountains. England In 1306, concerns over air pollution were sufficient for Edward I to briefly ban coal fires in London. In 1661, John Evelyn, the noted English diarist of his day, published Fumifugium in which he suggested the widespread planting of fragrant trees, shrubs and flowers around London so as to counteract the foul effects of the smoke from coal-burning. Two years later, in 1663, a satirical poem known as the Ballad of Gresham College commemorates Evelyn's book by this verse: "He shewes that 't is the seacoale smoake, That allways London doth Inviron, Which doth our Lungs and Spiritts choake, Our hanging spoyle, and rust our Iron." (CC) Photo: Jim Budd Smog over London in 2007 In the 1800s, more than a million London residents were burning coal, and winter "fogs" became more than a nuisance. An 1873 coal-smoke saturated fog, thicker and more persistent than natural fog, hovered over the city for days. As now known from subsequent epidemiological findings, the fog caused 268 deaths from bronchitis. Another fog in 1879 lasted from November to March, four long months darkened skies and gloom. Severe episodes of smog continued even into the 20th centuries and were nicknamed "pea-soupers". As discussed earlier above, the Great Smog of 1952 killed approximately 4,000 people in the short time of 4 days and a further 8,000 died from its effects in the following months. That finally prompted real legislative reform. In 1956, the Clean Air Act 1956, sponsored by the Ministry of Housing and Local Government in England and the Department of Health in Scotland, was enacted by the Parliament of the United Kingdom and introduced smokeless zones in London. It was in effect until 1964 and consequently reduced the sulfur dioxide levels in London's air to the point that made the intense and persistent London smog a thing of the past. It was after this that the great clean-up of London began and buildings recovered their original clean stone façades which, during two centuries, had gradually blackened. However, as clearly shown in the adjacent photograph, photochemical smog still does occur in modern London. European Union (CC) Photo: Andrea Squatrito Smog in Milan in 2006. The European Union (EU) is composed of 27 member states and was established by the Treaty of Maastricht in 1993. Its executive functions are performed by the European Commission and to a limited extent by the European Council. The large cities within the EU, such as Athens, Berlin, Milan, Paris, Rome and others, all experience episodes of photochemical smog in varying degrees of severity and especially so in the summer months. For example, see the adjacent photograph of smog in Milan. The EU uses the ground-level atmospheric ozone concentration as an indicator of air quality and has adopted these limiting ozone values: Target value (TV): 120 µg/m3 for 1 hour averaging time for 8-hour average, daily maximum. Not to be exceeded more than 25 days per year. Defined as the target value for the protection of human health. Information threshold (IT): 180 µg/m3 for 1 hour averaging time. Defined as the level beyond which there is a risk to human health from brief exposure for particularly sensitive sections of the population. Any exceedance of this threshold should be reported by the member state in which it occurs to the European Commission. Alert threshold (IT): 240 µg/m3 for 1 hour averaging time. Defined as the level beyond which there is a risk to human health from brief exposure for the general population. If exceeded in a member state, national authorities of the member state are required to warn the public and give advice. According to a recent report by the European Environmental Agency (EEA), the overall area of the EU that experiences exceedances of the ozone target value has declined by 60% during the period from 2006 − 2009. The executive summary of that report states: In contrast to previous summers, in 2009 there were no pan-European multi-day episodes. Summer 2009 was characterized by ozone episodes of two to five days followed by spells with few exceedances. However, the executive summary also states that a significant area of the EU still experienced exceedance of the ozone target value. Russia (CC) Photo: Yulia Smirnova Smog in Moscow in 2010 In early August 2010, the city of Moscow was covered by a heavy, dark smog caused by extensive wildfires across much of western Russia., turning the city's buildings into blurs, grounding air flights, and forcing the city's residents and tourists to wear surgical masks. At the beginning of August, forest fires in Russia exceeded 100, 000 hectares which by August 21 had decreased to 7,000 hectares according to the Emergencies Ministry. The head of the Moscow's health department said that the abnormal heat and smog coming from fires had seriously exacerbated the environmental conditions in Moscow and “The death rate has doubled in the city. The regular daily death rate in Moscow is 360 – 380. We had about 700 deaths today."  Some airborne pollutants were four times higher than normal, the worst concentrations seen to date according to city health officials. The concentrations appeared likely to get even worse as the state news agency, ITAR - Tass, reported that the smoke was thickening. Ruhr Valley Incident of 1985 The Ruhr district, located in the Rhine river valley of western Germany, is an urban area in the state of North Rhine-Westphalia]. With a population of about 7 million (2008) and an area of 4,435 square kilometers (1,713 square miles), it is the largest urban area in Germany. Historically, it has been a heavily industrialized area that experienced decades of polluted air containing smoke and high amounts of sulfur dioxide — the key ingredients of London-type smog. In January of 1985, the Ruhr district experienced an extended episode of London-type smog that lasted 5 days with the 24-hour averages of sulfur dioxide in the air as high as 0.8 mg/m3 and particulate matter as high as 0.6 mg/m3, both primarily derived from the extensive burning of coal by the area's steel mills and other industries. In the 5 days of the episode, the daily death rate increased by 8% and the hospital admissions for respiratory and cardiovascular problems increased by 15%. During the smog episode, a smog alarm was called, personal automobile driving was banned and many schools were closed. Iran In December 2005, schools and public offices had to close in Tehran, Iran, and 1600 people were taken to hospital, in a severe smog blamed largely on automobile exhausts. Mexico (CC) Photo: Lauren Brown Mexico City smog in 2006 Mexico City is located in the tropical latitudes on a high plain at an altitude of 2,200 meters, and is surrounded on three sides by mountains that reach elevations of 5,000 meters. The major sources of air pollutants within Mexico City's urban area include exhaust from 3.5 million vehicles, thousands of industries, and mineral dust. The ancient lakebed valley in which the city is situated became a major source of dust when it was drained in the 16th century. The area is known as the Valley of Mexico, sometimes called the Bowl of Mexico. 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, as a result, the city has one of the world's worst air pollution problems. 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 measured at twice or thrice the level of international standards. Since solar radiation does not vary significantly with season at tropical latitudes, photochemical smog is present over the city much of the year. Southeast Asia (CC) Photo: Thomas Galvez Haze and smoke in Kuala Lumpur, Malaysia in 2006 Southeast Asia consists of a mainland section and a maritime section. The mainland section includes Mayanmar, Cambodia, Laos, Thailand, Vietnam and Peninsular Malaysia and the maritime section includes Brunei, East Malaysia, East Timor, Indonesia, Philippines, and Singapore. The large cities in Southeast Asia have been plagued by smoky haze and photochemical smog ever since the late 1990s. The primary cause of the smoky haze have been the extensive forest fires occurring in Indonesia. Farmers and plantation owners are said to be responsible for the fires, which they use to clear tracts of land for further plantings. Other contributing factor are that El Niño conditions have created some very dry seasons with very little rainfall and the peat swamps that underlie the Indonesian forests and add fuel to the fires. One of the recent major occurrences of smoky haze in Malaysia, Singapore and other parts of Southeast Asia occurred in October 2006 and was caused by smoke from forest fires in Indonesia being blown across the Straits of Malacca by south-westerly winds. Since December 1997, member countries of the Association of Southeast Asia Nations (ASEAN) have been undertaking joint efforts in monitoring, preventing and mitigating transboundary haze pollution resulting from land and forest fires, guided by the Regional Haze Action Plan (RHAP) that was endorsed by the ASEAN Environment Ministers. In addition, the ASEAN Agreement on Transboundary Haze Pollution (or ASEAN Haze Agreement) was adopted in June2002 and comprehensively addresses all aspects of fire and haze including prevention, emphasizing the underlying causes, monitoring, and mitigation. In addition to the smoky haze caused by the forest fires, they have resulted in large amounts of the photochemical smog precursor, nitrogen oxide, entering the atmosphere. That, coupled with the great numbers of automobiles and other vehicles used in the large cities of Southeast Asia, has resulted in those cities being subjected to photochemical smog as well as hazy smoke. United States The U.S. EPA has designated over 300 U.S. counties in the United States to be "non-attainment areas" for one or more air pollutants, meaning that they have failed to attain and maintain the National Ambient Air Quality Standards as required by the Clean Air Act enacted by the U.S. Congress . The non-attainment areas are largely clustered around large metropolitan areas, with the largest contiguous non-attainment zones being in California and the Northeast, as can be seen in the map below. Various U.S. and Canadian government agencies collaborate to produce real-time air quality maps and forecasts. (PD) Image: U.S. Environmental Protection Agency Los Angeles and the San Joaquin Valley (PD) Photo: Jon Sullivan, www.pdphoto.org Los Angeles smog in 2003. Located in low basins surrounded by mountains, Los Angeles and the San Joaquin Valley are known for their photochemical smog. The millions of vehicles in these basins, plus the added effects of the port complex in Los Angeles, result in the accumulation of photochemical smog precursors and subsequently to the formation of photochemical smog. Strict regulations by the California Air Resources Board and other California government agencies overseeing this problem have reduced the number of smog alerts from several hundred annually to just a few. However, these geographically predisposed smog areas still have air pollution levels that are a pressing issue for the more than 25 million people who live there. (CC) Photo: Rick Kimpel Denver smog in 2006. Denver Denver, the "Mile High City" suffers from frequent smogs that are made worse by its location in the Rocky Mountains. Because of Denver's high altitude, it has frequent temperature inversions when warm polluted air is trapped underneath a layer of cold air and cannot rise and disperse more widely. As a result, photochemical smog may envelope the city for many days at a time (see adjacent photo). In the past, Denver regularly failed to meet the federal National Ambient Air Quality Standards for air quality on dozens of days each year and the city has worked diligently to solve its air pollution problem. In 2007, the U.S. EPA noted that Denver was ahead of its schedule for reducing smog. Yet, Denver still exceeds the federal standards for several days a year. Major incidents in the United States 1948, October, Donora, Pennsylvania: 20 killed by smog, 600 hospitalized, thousands more stricken. 1953, November, New York: Smog kills between 170 and 260 people. 1954, October, Los Angeles: Heavy smog shuts down schools and industry for most of the month. 1963, New York: Smog caused 200 deaths 1966, New York: Smog caused 168 deaths References  (Read more...)	Daniel Mietchen 22:12, 7 September 2010 (UTC)		1

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(PD) Photo: Dr. Edwin P. Ewing, Jr. / Centers for Disease Control and Prevention (CDC)
Smog over New York skyline in 1988

Smog is the hazy, unhealthy polluted air that accumulates over cities and other regions under certain conditions.

Modern smog is derived primarily from precursor chemical pollutants, emitted to the atmosphere from vehicular internal combustion engines and industrial plants, that react in the atmosphere with sunlight to produce secondary chemical pollutants that also combine with the precursor emissions to form the components of what is called photochemical smog.

Origin of the term "smog"

The word smog is a blend of the words smoke and fog. Coinage of the term is generally attributed to Dr. Henry Antoine des Voeux in his 1905 paper, "Fog and Smoke" for a meeting of the Public Health Congress in London, United Kingdom. The July 26, 1905 edition of the Daily Graphic, a London newspaper, quoted des Voeux: "... he said it required no science to see that there was something produced in great cities which was not found in the country, and that was smoky fog, or what was known as 'smog'."  The next day, the Globe newspaper remarked that "Dr. des Voeux did a public service in coining a new word for the London fog.">

Photochemical smog

(CC) Diagram: Milton Beychok
Atmospheric chemistry involved in smog formation

Originally, Dr. des Voeux's term smog referred to a mixture of smoke, sulfur dioxide (SO₂) and fog that was once prevalent in London when coal with a high sulphur content was widely used throughout the city as heating fuel. The dark, sulfurous London smog caused reduced visibility, respiratory problems and had a noticeable deleterious affect on human health. The Great Smog of 1952 darkened the streets of London and killed approximately 4,000 people in a 4-day period (another 8,000 died from its effects in the following months) The type of smog experienced in London many decades ago is no longer encountered since other fuels have largely replaced the wide-spread use of high-sulfur coal for heating.

Photochemical smog is chemically quite different than the old London-type smog and has a long history. In 1542, when exploring what is now Southern California, Juan Rodriguez Cabrillo named San Pedro Bay the "Bay of Smokes" because of the thick haze that covered the area. Complaints of eye irritation from the polluted air in Los Angeles date back to the late 1860s. In the 1940s, photochemical smog first became apparent in Los Angeles and other large cities on sunny days, although none of those cities had any significant use of coal for heating fuel or for industrial activities.

Photochemical smog is a daytime phenomenon that occurs mostly during warm, sunny summertime days and is characterized by a brown haze that reduces visibility and contains oxidants, such as ozone (O₃), that cause respiratory problems, eye irritation and damage to plants. It is a mixture of variable amounts of ozone, reactive hydrocarbons, nitrogen oxide (NO), nitrogen dioxide (NO₂), aldehydes, peroxyacetyl nitrate (PAN), particulate matter and other components. By contrast, the London-type smog (also referred to as "classic smog") occurs mostly during cold winter days and consists primarily of a mixture of sulfur dioxide and particulate matter (i.e., "soot") usually derived from burning coal.

Photochemical smog precursor pollutants

The chemistry involved in the formation of smog is highly complex and involves many different reactions (see the adjacent diagram). The three major ingredients required to form photochemical smog are solar energy (i.e., sunlight), reactive hydrocarbons and nitrogen oxide, the latter two being referred to as the precursor pollutants. As shown in the diagram, the two precursors enter the atmosphere as gases emitted for the most part from vehicular internal combustion engines fuelled by gasoline and diesel oil. To a much lesser extent, nitrogen oxide is also emitted from industrial combustion sources, and reactive hydrocarbons are emitted by evaporation from the handling and storage of volatile hydrocarbons such as gasoline, solvents, some pesticides and some paints. Biogenic sources such as pine trees and certain other trees are also a source, though of relatively minor importance, of reactive hydrocarbon emissions such as isoprene and α-pinene.

Simplified chemistry of photochemical smog formation

In the simplified schematic overview depicted in the adjacent diagram, the major chemical reactions that contribute to the formation of photochemical smog include the following:

• The gaseous precursor nitrogen oxide, emitted primarily from vehicular internal combustion engines, is oxidized to produce gaseous nitrogen dioxide.

nitrogen oxide + oxygen $\rightarrow$ nitrogen dioxide

2NO(g) + O₂(g) $\mathrm {\rightarrow }$ 2NO₂(g)

• The gaseous nitrogen dioxide is broken down by solar energy from sunlight to produce gaseous nitrogen oxide and atomic oxygen (O, an oxygen radical). This process is referred to as photolysis and hence the term "photochemical smog".

nitrogen dioxide +solar energy $\rightarrow$ nitrogen oxide + atomic oxygen

NO₂(g) + solar energy $\rightarrow$ NO (g) + O

• The atomic oxygen reacts with gaseous atmospheric oxygen (O₂) to form gaseous ozone (O₃).

atomic oxygen + atmospheric oxygen $\rightarrow$ ozone

O + O₂(g) $\rightarrow$ O₃(g)

• The gaseous ozone also oxidizes gaseous nitrogen oxide to form gaseous nitrogen dioxide and gaseous oxygen.

ozone + nitrogen oxide $\rightarrow$ nitrogen dioxide + oxygen

O₃(g) + NO(g) $\rightarrow$ NO₂(g) + O₂(g)

• The gaseous precursor reactive hydrocarbons (RH), emitted primarily from vehicular internal combustion engines, reacts with atomic oxygen, atmospheric oxygen and ozone to produce various highly reactive hydrocarbon free radicals (RO^•₂).

• The hydrocarbon free radicals then react with other species such as nitrogen dioxide to form peroxyacetyl nitrate (PAN), aldehydes and other smog components.

Nitrogen dioxide + oxygen + hydrocarbon free radicals $\rightarrow$ peroxyacetyl nitrate

2NO₂(g) + O₂(g) + 2RO^•₂ $\rightarrow$ 2CH₃CO–OO–NO₂(g)

The various products produced or formed from the precursor pollutants are referred to as secondary pollutants. All of the above chemical reactions (as well as others) occur more or less simultaneously during sunny, summertime days in most large cities with a great number of automobiles and other vehicles.

Health effects

(CC) Photo: Carol Sawada
Smog in São Paulo, Brazil in 2006.

United States' AQI

Air Quality Index (AQI)	Air Quality Category	Color Code
0 – 50	Good
51 – 100	Moderate
101 – 150	Unhealthy for Sensitive Groups
151 – 200	Unhealthy
201 – 300	Very Unhealthy
301 – 500	Hazardous

Photochemical smog constitutes a serious health problem in most large cities. It is especially harmful for senior citizens, children, and people with heart and lung conditions such as emphysema, bronchitis, and asthma. It can inflame breathing passages, decrease the lungs' working capacity, cause shortness of breath, pain when inhaling deeply, wheezing, and coughing. It can cause eye and nose irritation and it dries out the protective membranes of the nose and throat. In general, it interferes with the body's ability to fight infection, thereby increasing susceptibility to illness.

The adjacent table explains the Air Quality Index (AQI) ranges used by the U.S. Environmental Protection Agency (U.S. EPA) and their corresponding health effect categories and color codes. The U.S. EPA's AQI is also known as the Pollution Standards Index (PSI). A national map of the United States of America containing daily AQI forecasts across the nation, using that same color-coding, is published online jointly by the U.S. EPA and the National Oceanic and Atmospheric Administration (NOAA).

If multiple pollutants are measured at a monitoring site, then the largest or "dominant" AQI value is reported for the location.

The U.S. EPA has developed conversion calculators, available online, for the conversion of AQI values to concentration values and for the reverse conversion of concentrations to AQI values.

The South Coast Air Quality Management District (SCAQMD) of Southern California has published an excellent, comprehensive discussion of the undesirable health effects caused by air pollution. It also includes an extensive listing and discussion of the many medical health effect studies of pollution performed during the past decades starting around 1950.

(CC) Photo: Brian Jeffrey Beggerly
Smog over Beijing in 2005.

Areas affected

Smog can form in almost any location where the level of air pollutants is high, and ii is more likely to occur during periods of warm, sunny weather when the upper air is warm enough to inhibit vertical circulation. It often persists for extended time periods over densely populated cities, such as Athens, Beijing, Berlin, Cairo, Hong Kong, Kuala Lumpur, London, United Kingdom, Los Angeles, Milan, Moscow, New York, Paris, Rome, São Paulo, Seoul and Shanghai. Smog is especially prevalent in geologic basins encircled by hills or mountains.

England

In 1306, concerns over air pollution were sufficient for Edward I to briefly ban coal fires in London. In 1661, John Evelyn, the noted English diarist of his day, published Fumifugium in which he suggested the widespread planting of fragrant trees, shrubs and flowers around London so as to counteract the foul effects of the smoke from coal-burning. Two years later, in 1663, a satirical poem known as the Ballad of Gresham College commemorates Evelyn's book by this verse:

"He shewes that 't is the seacoale smoake,

That allways London doth Inviron,

Which doth our Lungs and Spiritts choake,

Our hanging spoyle, and rust our Iron."

(CC) Photo: Jim Budd
Smog over London in 2007

In the 1800s, more than a million London residents were burning coal, and winter "fogs" became more than a nuisance. An 1873 coal-smoke saturated fog, thicker and more persistent than natural fog, hovered over the city for days. As now known from subsequent epidemiological findings, the fog caused 268 deaths from bronchitis. Another fog in 1879 lasted from November to March, four long months darkened skies and gloom. Severe episodes of smog continued even into the 20th centuries and were nicknamed "pea-soupers".

As discussed earlier above, the Great Smog of 1952 killed approximately 4,000 people in the short time of 4 days and a further 8,000 died from its effects in the following months. That finally prompted real legislative reform. In 1956, the Clean Air Act 1956, sponsored by the Ministry of Housing and Local Government in England and the Department of Health in Scotland, was enacted by the Parliament of the United Kingdom and introduced smokeless zones in London. It was in effect until 1964 and consequently reduced the sulfur dioxide levels in London's air to the point that made the intense and persistent London smog a thing of the past. It was after this that the great clean-up of London began and buildings recovered their original clean stone façades which, during two centuries, had gradually blackened.

However, as clearly shown in the adjacent photograph, photochemical smog still does occur in modern London.

European Union

(CC) Photo: Andrea Squatrito
Smog in Milan in 2006.

The European Union (EU) is composed of 27 member states and was established by the Treaty of Maastricht in 1993. Its executive functions are performed by the European Commission and to a limited extent by the European Council.

The large cities within the EU, such as Athens, Berlin, Milan, Paris, Rome and others, all experience episodes of photochemical smog in varying degrees of severity and especially so in the summer months. For example, see the adjacent photograph of smog in Milan.

The EU uses the ground-level atmospheric ozone concentration as an indicator of air quality and has adopted these limiting ozone values:

Target value (TV): 120 µg/m³ for 1 hour averaging time for 8-hour average, daily maximum. Not to be exceeded more than 25 days per year. Defined as the target value for the protection of human health.

Information threshold (IT): 180 µg/m³ for 1 hour averaging time. Defined as the level beyond which there is a risk to human health from brief exposure for particularly sensitive sections of the population. Any exceedance of this threshold should be reported by the member state in which it occurs to the European Commission.

Alert threshold (IT): 240 µg/m³ for 1 hour averaging time. Defined as the level beyond which there is a risk to human health from brief exposure for the general population. If exceeded in a member state, national authorities of the member state are required to warn the public and give advice.

According to a recent report by the European Environmental Agency (EEA), the overall area of the EU that experiences exceedances of the ozone target value has declined by 60% during the period from 2006 − 2009. The executive summary of that report states: In contrast to previous summers, in 2009 there were no pan-European multi-day episodes. Summer 2009 was characterized by ozone episodes of two to five days followed by spells with few exceedances. However, the executive summary also states that a significant area of the EU still experienced exceedance of the ozone target value.

Russia

(CC) Photo: Yulia Smirnova
Smog in Moscow in 2010

In early August 2010, the city of Moscow was covered by a heavy, dark smog caused by extensive wildfires across much of western Russia., turning the city's buildings into blurs, grounding air flights, and forcing the city's residents and tourists to wear surgical masks. At the beginning of August, forest fires in Russia exceeded 100, 000 hectares which by August 21 had decreased to 7,000 hectares according to the Emergencies Ministry.

The head of the Moscow's health department said that the abnormal heat and smog coming from fires had seriously exacerbated the environmental conditions in Moscow and “The death rate has doubled in the city. The regular daily death rate in Moscow is 360 – 380. We had about 700 deaths today." 

Some airborne pollutants were four times higher than normal, the worst concentrations seen to date according to city health officials. The concentrations appeared likely to get even worse as the state news agency, ITAR - Tass, reported that the smoke was thickening.

Ruhr Valley Incident of 1985

The Ruhr district, located in the Rhine river valley of western Germany, is an urban area in the state of North Rhine-Westphalia]. With a population of about 7 million (2008) and an area of 4,435 square kilometers (1,713 square miles), it is the largest urban area in Germany. Historically, it has been a heavily industrialized area that experienced decades of polluted air containing smoke and high amounts of sulfur dioxide — the key ingredients of London-type smog.

In January of 1985, the Ruhr district experienced an extended episode of London-type smog that lasted 5 days with the 24-hour averages of sulfur dioxide in the air as high as 0.8 mg/m³ and particulate matter as high as 0.6 mg/m³, both primarily derived from the extensive burning of coal by the area's steel mills and other industries. In the 5 days of the episode, the daily death rate increased by 8% and the hospital admissions for respiratory and cardiovascular problems increased by 15%.

During the smog episode, a smog alarm was called, personal automobile driving was banned and many schools were closed.

Iran

In December 2005, schools and public offices had to close in Tehran, Iran, and 1600 people were taken to hospital, in a severe smog blamed largely on automobile exhausts.

Mexico

(CC) Photo: Lauren Brown
Mexico City smog in 2006

Mexico City is located in the tropical latitudes on a high plain at an altitude of 2,200 meters, and is surrounded on three sides by mountains that reach elevations of 5,000 meters. The major sources of air pollutants within Mexico City's urban area include exhaust from 3.5 million vehicles, thousands of industries, and mineral dust. The ancient lakebed valley in which the city is situated became a major source of dust when it was drained in the 16th century. The area is known as the Valley of Mexico, sometimes called the Bowl of Mexico.

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, as a result, the city has one of the world's worst air pollution problems. 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 measured at twice or thrice the level of international standards. Since solar radiation does not vary significantly with season at tropical latitudes, photochemical smog is present over the city much of the year.

Southeast Asia

(CC) Photo: Thomas Galvez
Haze and smoke in Kuala Lumpur, Malaysia in 2006

Southeast Asia consists of a mainland section and a maritime section. The mainland section includes Mayanmar, Cambodia, Laos, Thailand, Vietnam and Peninsular Malaysia and the maritime section includes Brunei, East Malaysia, East Timor, Indonesia, Philippines, and Singapore. The large cities in Southeast Asia have been plagued by smoky haze and photochemical smog ever since the late 1990s. The primary cause of the smoky haze have been the extensive forest fires occurring in Indonesia. Farmers and plantation owners are said to be responsible for the fires, which they use to clear tracts of land for further plantings. Other contributing factor are that El Niño conditions have created some very dry seasons with very little rainfall and the peat swamps that underlie the Indonesian forests and add fuel to the fires.

One of the recent major occurrences of smoky haze in Malaysia, Singapore and other parts of Southeast Asia occurred in October 2006 and was caused by smoke from forest fires in Indonesia being blown across the Straits of Malacca by south-westerly winds.

Since December 1997, member countries of the Association of Southeast Asia Nations (ASEAN) have been undertaking joint efforts in monitoring, preventing and mitigating transboundary haze pollution resulting from land and forest fires, guided by the Regional Haze Action Plan (RHAP) that was endorsed by the ASEAN Environment Ministers. In addition, the ASEAN Agreement on Transboundary Haze Pollution (or ASEAN Haze Agreement) was adopted in June2002 and comprehensively addresses all aspects of fire and haze including prevention, emphasizing the underlying causes, monitoring, and mitigation.

In addition to the smoky haze caused by the forest fires, they have resulted in large amounts of the photochemical smog precursor, nitrogen oxide, entering the atmosphere. That, coupled with the great numbers of automobiles and other vehicles used in the large cities of Southeast Asia, has resulted in those cities being subjected to photochemical smog as well as hazy smoke.

United States

The U.S. EPA has designated over 300 U.S. counties in the United States to be "non-attainment areas" for one or more air pollutants, meaning that they have failed to attain and maintain the National Ambient Air Quality Standards as required by the Clean Air Act enacted by the U.S. Congress . The non-attainment areas are largely clustered around large metropolitan areas, with the largest contiguous non-attainment zones being in California and the Northeast, as can be seen in the map below. Various U.S. and Canadian government agencies collaborate to produce real-time air quality maps and forecasts.

(PD) Image: U.S. Environmental Protection Agency

Los Angeles and the San Joaquin Valley

(PD) Photo: Jon Sullivan, www.pdphoto.org
Los Angeles smog in 2003.

Located in low basins surrounded by mountains, Los Angeles and the San Joaquin Valley are known for their photochemical smog. The millions of vehicles in these basins, plus the added effects of the port complex in Los Angeles, result in the accumulation of photochemical smog precursors and subsequently to the formation of photochemical smog. Strict regulations by the California Air Resources Board and other California government agencies overseeing this problem have reduced the number of smog alerts from several hundred annually to just a few. However, these geographically predisposed smog areas still have air pollution levels that are a pressing issue for the more than 25 million people who live there.

(CC) Photo: Rick Kimpel
Denver smog in 2006.

Denver

Denver, the "Mile High City" suffers from frequent smogs that are made worse by its location in the Rocky Mountains.

Because of Denver's high altitude, it has frequent temperature inversions when warm polluted air is trapped underneath a layer of cold air and cannot rise and disperse more widely. As a result, photochemical smog may envelope the city for many days at a time (see adjacent photo).

In the past, Denver regularly failed to meet the federal National Ambient Air Quality Standards for air quality on dozens of days each year and the city has worked diligently to solve its air pollution problem. In 2007, the U.S. EPA noted that Denver was ahead of its schedule for reducing smog. Yet, Denver still exceeds the federal standards for several days a year.

Major incidents in the United States

• 1948, October, Donora, Pennsylvania: 20 killed by smog, 600 hospitalized, thousands more stricken.
• 1953, November, New York: Smog kills between 170 and 260 people.
• 1954, October, Los Angeles: Heavy smog shuts down schools and industry for most of the month.
• 1963, New York: Smog caused 200 deaths
• 1966, New York: Smog caused 168 deaths

References

(Read more...)

Previous Winners

•  Space (mathematics) [r]: A set with some added structure, which often form a hierarchy, i.e., one space may inherit all the characteristics of a parent space. ^[e]
•  World of Warcraft [r]: An online video game, released by Blizzard Entertainment in 2004. ^[e]
•  Social capital [r]: Productive assets arising out of social relations, such as trust, cooperation, solidarity, social networks of relations and those beliefs, ideologies and institutions that contribute to production of goods. ^[e]
•  Richard Condon [r]: (1915 – 1996) A prolific and popular American political novelist whose satiric works were generally presented in the form of thrillers or semi-thrillers. ^[e]
•  Natural gas [r]: A gas consisting primarily of methane (CH₄) which is found as raw natural gas in underground reservoirs, as gas associated with underground reservoirs of petroleum crude oil, as undersea methane hydrates and as coalbed methane in underground coal mines. ^[e]
•  Stairway to Heaven [r]: 1971 song written and recorded by Led Zeppelin, which became their signature tune and a centrepiece for the group's live performances. ^[e]
•  Cryptography [r]: A field at the intersection of mathematics and computer science that is concerned with the security of information, typically the confidentiality, integrity and authenticity of some message. ^[e]
•  English spellings [r]: Lists of English words showing pronunciation, and articles about letters. ^[e]
•  Folk saint [r]: A deceased person or spirit that is venerated as a saint but who has not been officially canonized by the Church. ^[e]
•  Led Zeppelin [r]: English hard rock and blues group formed in 1968, known for their albums and stage shows. ^[e]
•  Locality of reference [r]: A commonly observed pattern in memory accesses by a computer program over time. ^[e]
•  Rabbit [r]: Long-eared, short-tailed, burrowing mammals of the family Leporidae of the order Lagomorpha, found in several parts of the world. ^[e]
•  Scarborough Castle [r]: Ruined stone castle on the east coast of Yorkshire, England, begun in mid-twelfth century. ^[e] (September 3)
• The Clash of Civilizations and the Remaking of World Order [r]: Add brief definition or description (August 27)
•  Mauna Kea [r]: One of the three main volcanic mountains on Hawaii, the biggest island in Hawaii (U.S. state). ^[e] (August 20)
•  Brute force attack [r]: An attempt to break a cipher by trying all possible keys; long enough keys make this impractical. ^[e] (August 13)
•  Cruiser [r]: While definitions vary with time and doctrine, a large warship capable of acting independently, as a flagship, or a major escort; capabilities include anti-air warfare, anti-surface warfare, anti-submarine warfare, land attack, and possibly ballistic missile defense ^[e] (August 5)
•  The Canterbury Tales [r]: Collection of stories in verse and prose by Geoffrey Chaucer. ^[e] (July 30)
•  Milpa agriculture [r]: A form of swidden agriculture that is practiced in Mesoamerica. Traditionally, a "milpa" plot is planted with maize, beans, and squash. ^[e] (July 23)
•  Domain Name System [r]: The Internet service which translates to and from IP addresses and domain names. ^[e] (July 16)
•  Scuticaria [r]: A genus of orchids, closely related to Bifrenaria, formed by nine showy species of cylindrical leaves, which exist in three isolated areas of South America. ^[e] (July 9)
• Torture [r]: Add brief definition or description (July 2)
•  Miltonia [r]: An orchid genus formed by nine showy epiphyte species and seven natural hybrids of Brazil, one species reaching Argentina and Paraguay. ^[e] (June 25)
•  Ancient Celtic music [r]: The music and instruments of the ancient Celts until late Antiquity. ^[e] (June 18)
•  Bifrenaria [r]: A genus of orchids formed by circa twenty species of South America, some widely cultivated because of their large and colored flowers; divided in two distinct groups, one with large flowers and short inflorescences and the other with small flowers and long inflorescences. ^[e] (June 11)
•  Halobacterium NRC-1 [r]: Add brief definition or description (June 4)
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•  Korematsu v. United States [r]: Add brief definition or description (Apr 7)
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•  Large-scale trickle filters [r]: Add brief definition or description (11 Feb)
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•  Speech Recognition [r]: Add brief definition or description (26 Nov)
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•  Korean War of 1592-1598 [r]: Add brief definition or description (21 Jan)
•  Andrew Carnegie [r]: Add brief definition or description (11 January 2008)
• Bowling [r]: Add brief definition or description (31 December 2007)
•  Architecture [r]: Add brief definition or description (December 6)
•  Civil society [r]: Add brief definition or description November 29
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•  Chemistry [r]: Add brief definition or description (November 15)
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•  U.S. Electoral College [r]: Add brief definition or description (August 28)
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Rules and Procedure

Rules

• The article's status must be 0 or 1, i.e., only "Advanced Articles" may be nominated.
• Any Citizen may nominate an article.
• No Citizen may have nominated more than one article listed under "current nominees" at a time.
• The article's nominator is indicated simply by the first name in the list of votes (see below).
• At least for now--while the project is still small--you may nominate and vote for articles of which you are a main author.
• An article can be Article of the Week only once every six months. Nominated articles that have won top honors should be removed from the list.
• Comments on nominations should be made on the article's talk page.
• The list of nominees should be kept below 20, or thereabouts. Articles with very few supporters and which have not gained any new supporters in the last two weeks or so may be deleted to make room for new nominees.
• Any editor may entirely cancel the nomination of any unapproved article in his or her area of expertise if, for example, it contains obvious and embarrassing problems.

Voting

• To vote, add your name and date in the Supporters column next to an article title, after other supporters for that article, by signing <br />~~~~. (The date is necessary so that we can determine when the last vote was added.) Your vote is alloted a score of 1.
• Add your name in the Specialist supporters column only if you are an editor who is an expert about the topic in question. Your vote is alloted a score of 1 for articles which you created and a score of 2 for articles which you did not create.
• You may vote for as many articles as you wish, and each vote counts separately, but you can only nominate one at a time; see above. You could, theoretically, vote for every nominated article on the page, but this would be pointless.

Ranking

• The list of articles is sorted by number of votes first, then alphabetically.
• Admins should make sure that the votes are correctly tallied, but anyone may do this. Note that "Specialist Votes" are worth 3 points.

Updating

• Each Thursday, one of the admins listed below should move the winning article to the Current Winner section of this page, announces the winner on Citizendium-L and updates the "previous winning articles" section accordingly.
• The winning article will be the article at the top of the list (ie the one with the most votes).
• In the event of two or more having the same number of votes :
  • The article with the most specialist supporters is used. Should this fail to produce a winner, the article appearing first by English alphabetical order is used.
    
  • The remaining winning articles are guaranteed this position in the following weeks, again in alphabetical order. No further voting would take place on these, which remain at the top of the table with notices to that effect. Further nominations and voting take place to determine future winning articles for the following weeks.

Administrators

These are people who have volunteered to run this program. Their duties are (1) to ensure that this page remains "clean," e.g., as a given article garners more votes, its tally is accurately represented and it moves up the list, and (2) to place the winning article on the front page on a weekly basis. To become an administrator, you need not apply anywhere. Simply add your name below. Administrator duties are open to editors and authors alike.

• Carl Jantzen
• Todd Coles
• David E. Volk
• Milton Beychok

See Also

• CZ:New Draft of the Week
• CZ:Markup tags for partial transclusion of selected text in an article
• CZ:Monthly Write-a-Thon

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