Editing Smog (section)

== Photochemical smog ==
[[File:Photochemical_smog_formation.png|thumb|The photochemical smog formation diagram. (Based on U 6.3.3 in mrgsciences.com<ref>{{Cite web|url=https://www.mrgscience.com/ess-topic-63-photochemical-smog.html|title=ESS Topic Smog|website=Amazing World of Science With Mr. Green|language=en|access-date=19 September 2019}}</ref>)]]

Photochemical smog, often referred to as "summer smog", is the chemical reaction of sunlight, [[nitrogen oxides]] and [[volatile organic compound]]s in the atmosphere, which leaves [[Atmospheric particulate matter|airborne particles]] and [[Tropospheric ozone|ground-level ozone]].<ref>{{Cite web|title=Nox/VOC Smog Fact Sheet|url=http://www.ccme.ca/assets/pdf/pn_1257_e.pdf|url-status=dead|publisher=Canadian Council of Ministers of the Environment|archive-url=https://web.archive.org/web/20110928160543/http://www.ccme.ca/assets/pdf/pn_1257_e.pdf|archive-date=28 September 2011}}</ref> Photochemical smog depends on primary pollutants as well as the formation of secondary pollutants. These primary pollutants include [[nitrogen oxide]]s, particularly [[nitric oxide]] (NO) and [[nitrogen dioxide]] (NO<sub>2</sub>), and [[volatile organic compound]]s. The relevant secondary pollutants include [[Peroxyacyl nitrates|peroxylacyl nitrates]] (PAN), [[tropospheric ozone]], and [[aldehyde]]s. An important secondary pollutant for photochemical smog is ozone, which is formed when hydrocarbons (HC) and nitrogen oxides (NO<sub>x</sub>) combine in the presence of sunlight; nitrogen dioxide (NO<sub>2</sub>), which is formed as nitric oxide (NO) combines with oxygen (O<sub>2</sub>) in the air.<ref>{{Cite web|url=https://www.pmfias.com/smog-sulfurous-smog-photochemical-smog-london-smog-los-angeles-smog-effects-geography-upsc-ias/|title=Smog: Photochemical smog & Sulfurous smog|date=4 January 2016}}</ref> In addition, when SO<sub>2</sub> and NO<sub>x</sub> are emitted they eventually are oxidized in the troposphere to [[nitric acid]] and [[sulfuric acid]], which, when mixed with water, form the main components of acid rain.<ref>{{Cite web|url=https://www.englishnotes4all.com/|title=Educate about Smog: What causes acid rain?|website=www.englishnotes4all.com|access-date=5 November 2018|archive-date=2 November 2018|archive-url=https://web.archive.org/web/20181102203022/http://englishnotes4all.com/|url-status=dead}}</ref> All of these harsh chemicals are usually highly reactive and oxidizing. Photochemical smog is therefore considered to be a problem of modern industrialization. It is present in all modern cities, but it is more common in cities with sunny, warm, dry climates and a large number of motor vehicles.<ref>{{cite book|title=Living in the Environment: Principles, Connections, and Solutions|last=Miller|first=George Tyler Jr.|publisher=[[The Thomson Corporation]]|year=2018|isbn=978-0-534-37697-0|location=Belmont|page=423|edition=12th}}</ref> Because it travels with the wind, it can affect sparsely populated areas as well.

[[File:PLANE (SUPPLIED BY NASA) USES SPACE TECHNIQUES IN SMOG RESEARCH. SCIENTISTS FROM STATEWIDE AIR - NARA - 542670.jpg|thumb|right|Airplane used to collect airborne hydrocarbons, May 1972]]
The composition and chemical reactions involved in photochemical smog were not understood until the 1950s. In 1948, flavor chemist [[Arie Jan Haagen-Smit|Arie Haagen-Smit]] adapted some of his equipment to collect chemicals from polluted air, and identified ozone as a component of Los Angeles smog. Haagen-Smit went on to discover that nitrogen oxides from automotive exhausts and gaseous hydrocarbons from cars and oil refineries, exposed to sunlight, were key ingredients in the formation of ozone and photochemical smog.<ref name="hundred2">{{cite book|url=https://books.google.com/books?id=edtL_hIi8M0C&pg=PA219|title=Arnold O. Beckman : one hundred years of excellence|author-last1=Thackray|author-first1=Arnold|author-last2=Myers Jr.|author-first2=Minor|publisher=Chemical Heritage Foundation|year=2000|isbn=978-0-941901-23-9|location=Philadelphia, Pa.|name-list-style=amp}}</ref>{{rp|219–224}}<ref name="Gardner2">{{cite web|url=http://www.englishnotes4all.com/|title=Smog: the battle against air pollution|last=Gardner|first=Sarah|date=14 July 2018|website=Marketplace.org|publisher=American Public Media|access-date=6 November 2015|archive-date=24 January 2018|archive-url=https://web.archive.org/web/20180124115059/http://englishnotes4all.com/|url-status=dead}}</ref><ref name="Kean2">{{cite journal|last1=Kean|first1=Sam|date=2015|title=The Flavor of Smog|url=https://www.sciencehistory.org/distillations/magazine/the-flavor-of-smog|journal=Distillations|volume=2|issue=3|page=5|access-date=22 March 2018}}</ref> Haagen-Smit worked with [[Arnold Beckman]], who developed various equipment for detecting smog, ranging from an "Apparatus for recording gas concentrations in the atmosphere" patented on 7 October 1952, to "air quality monitoring vans" for use by government and industry.<ref name="hundred2" />{{rp|224–226}}

=== Formation and reactions ===
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 (NO<sub>2</sub>).

:(1) {{chem2 | R• + O2 + M -> RO2• + M }}

:(2) {{chem2 | RO2• + NO -> NO2 + RO• }}

:(3) {{chem2 | HO2• + NO -> NO2 + OH• }}

Nitrogen dioxide (NO<sub>2</sub>) and nitric oxide (NO) further react with ozone (O<sub>3</sub>) in a series of chemical reactions:

:(4) {{chem2 | NO2 + ''hν'' -> O(^{3}P) + NO }}{{spaces|4}}λ < 400 nm

:(5) {{chem2 | O(^{3}P) + O2 + M -> O3 + M(heat) }}

:(6) {{chem2 | 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<sub>x</sub> and ozone are not in a perfectly steady state. By replacing Reaction 6 with Reaction 2 and Reaction 3, the O<sub>3</sub> 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 NO<sub>2</sub>. 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 O<sub>3</sub> in smog. The main limiting reaction in polluted areas is:

:(7) {{chem2 | NO2 + HO• + M -> HNO3 + M }}

This reaction removes NO<sub>2</sub> which limits the amount of O<sub>3</sub> that can be produced from its photolysis (reaction 4). HNO<sub>3</sub>, nitric acid, 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.

[[File:Lightmatter_Golden_gate_bridge.jpg|center|thumb|600x600px|The presence of smog in [[California]] is shown near the [[Golden Gate Bridge]]. The brown coloration is due to the NO<sub>2</sub> formed from photochemical smog reactions.<nowiki/>]]

===Natural causes===
====Volcanoes====
An erupting volcano can emit high levels of [[sulfur dioxide]] along with a large quantity of particulate 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====
Plants are a natural source of hydrocarbons that can 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 [[terpene]]s.<ref>{{Cite book|url=https://www.elsevier.com/books/chemistry-of-the-natural-atmosphere/warneck/978-0-12-735632-7|title=Chemistry of the Natural Atmosphere, Volume 71 - 2nd Edition|last=Elsevier|website=www.elsevier.com|date=29 October 1999 |isbn=978-0-12-735632-7 |language=en|access-date=15 November 2018}}</ref> 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.<ref>{{Cite journal|last1=Sharkey|first1=T. D.|last2=Wiberley|first2=A. E.|last3=Donohue|first3=A. R.|date=17 October 2007|title=Isoprene Emission from Plants: Why and How|journal=Annals of Botany|language=en|volume=101|issue=1|pages=5–18|doi=10.1093/aob/mcm240|pmid=17921528|pmc=2701830|issn=0305-7364}}</ref>