User:Milton Beychok/Sandbox

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

Smog is a combination of the words "smoke" and "fog", and refers to the hazy, unhealthy polluted air which 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"

Coinage of the term smog 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. 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

Atmospheric chemistry involved in smog formation

Originally, Dr. des Voeux's term smog referred to a mixture of smoke, sulfur dioxide (SO₂) and fog which was once prevalent in London when coal with a high sulfur 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 so-called 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 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..

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 adjacent 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 relatively minor source of reactive hydrocarbon emissions such as isoprene and α-pinene.

Briefly, as depicted in the adjacent diagram, this a very simplified listing of the chemical reactions that take place to form photochemical smog:

• 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, 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 containing daily AQI forecasts across the nation, using that same color-coding, is published online jointly by the U.S. EPA and 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 in about 1950.

As noted at the top of this section, see Air Quality Index for information about similar air quality indices in countries other than the United States.

(CC) Photo: Brian Jeffrey Beggerly
Smog over the Forbidden City in Beijing, China 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 London, New York, Cairo, Los Angeles, São Paulo, Mexico City, Beijing, Shanghai, Hong Kong, Kuala Lumpur and Seoul. Smog is especially prevalent in geologic basins encircled by hills or mountains.

London

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 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 of 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.

Mexico City

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 being double or even triple 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 Burma, 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.

Tehran

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..

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.

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.

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