View source for Sulfur dioxide

{{Short description|Chemical compound of sulfur and oxygen}}
{{cs1 config|name-list-style=vanc|display-authors=etal}}
{{Use American English|date=August 2020}}
{{Use mdy dates|date=August 2024}}
{{Chembox
|Verifiedfields = changed
|Watchedfields = changed
|verifiedrevid = 477313199
|ImageFileL1 = Sulfur-dioxide-2D.svg
|ImageFileL1_Ref = {{chemboximage|correct|??}}
|ImageSizeL1 = 160
|ImageNameL1 = Skeletal formula sulfur dioxide with assorted dimensions
|ImageFile2 = Sulfur-dioxide-3D-vdW.png
|ImageFile2_Ref = {{chemboximage|correct|??}}
|ImageSize2 = 120
|ImageName2 = Spacefill model of sulfur dioxide
|ImageFileR1 = Sulfur-dioxide-ve-B-2D.png
|ImageFileR1_Ref = {{chemboximage|correct|??}}
|ImageSizeR1 = 160
|ImageNameR1 = The Lewis structure of sulfur dioxide (SO2), showing unshared electron pairs.
|IUPACName = Sulfur dioxide
|OtherNames = {{Unbulleted list|Sulfurous anhydride|Sulfur(IV) oxide}}
|Section1={{Chembox Identifiers
|CASNo = 7446-09-5
|CASNo_Ref = {{cascite|correct|CAS}}
|PubChem = 1119
|ChemSpiderID = 1087
|ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
|UNII = 0UZA3422Q4
|UNII_Ref = {{fdacite|correct|FDA}}
|EINECS = 231-195-2
|UNNumber = 1079, 2037
|KEGG = D05961
|KEGG_Ref = {{keggcite|correct|kegg}}
|MeSHName = Sulfur+dioxide
|ChEBI_Ref = {{ebicite|correct|EBI}}
|ChEBI = 18422
|ChEMBL = 1235997
|ChEMBL_Ref = {{ebicite|changed|EBI}}
|RTECS = WS4550000
|Beilstein = 3535237
|Gmelin = 1443
|SMILES = O=S=O
|StdInChI = 1S/O2S/c1-3-2
|StdInChI_Ref = {{stdinchicite|correct|chemspider}}
|InChI = 1/O2S/c1-3-2
|StdInChIKey = RAHZWNYVWXNFOC-UHFFFAOYSA-N
|StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
|InChIKey = RAHZWNYVWXNFOC-UHFFFAOYAT
}}
|Section2={{Chembox Properties
|Formula = {{Chem|SO|2}}
|MolarMass = 64.066 g/mol
|Appearance = Colorless gas
|Density = 2.619 kg m<sup>−3</sup><ref>{{cite web |title=PubChem Compound Summary for CID 1119, Sulfur Dioxide|url=https://pubchem.ncbi.nlm.nih.gov/compound/1119 |access-date=2025-01-15|archive-date=2023-09-24 |archive-url=https://web.archive.org/web/20230924183723/https://pubchem.ncbi.nlm.nih.gov/compound/1119 |url-status=live|via=U.S. National Library of Medicine|author=((National Center for Biotechnology Information))|year=2025}}</ref>
|Solubility = 94 g/L<ref>{{RubberBible87th}}</ref><br />forms [[sulfurous acid]]
|MeltingPtK = 201
|BoilingPtC = −10
|VaporPressure = 230 kPa at 10&nbsp;°C; 330 kPa at 20&nbsp;°C; 462 kPa at 30&nbsp;°C; 630 kPa at 40&nbsp;°C<ref>{{cite web |title=Hazardous Substances Data Bank: Sulfur Dioxide|url=https://pubchem.ncbi.nlm.nih.gov/source/hsdb/228#section=Vapor-Pressure|website=PubChem|access-date=2025-01-15|date=2018-12-18|author=((National Center for Biotechnology Information))}}</ref>
|pKa = ~1.81
|pKb = ~12.19
|Viscosity = 12.82 μPa·s<ref>{{cite journal |title=Correlation constants for chemical compounds|journal=Chemical Engineering |date=1976|last1=Miller|first1=J.W. Jr.|last2=Shah|first2=P.N.|last3=Yaws|first3=C.L.|volume=83|issue=25|pages=153–180|issn=0009-2460}}</ref>
|Odor = Pungent; similar to a just-struck match<ref>[http://toxtown.nlm.nih.gov/text_version/chemicals.php?id=29 Sulfur dioxide] {{Webarchive|url=https://web.archive.org/web/20191230172725/https://toxtown.nlm.nih.gov/text_version/chemicals.php?id=29 |date=December 30, 2019 }}, U.S. National Library of Medicine</ref>
|MagSus = −18.2·10<sup>−6</sup> cm<sup>3</sup>/mol
}}
|Section3={{Chembox Structure
|PointGroup = ''C''<sub>2''v''</sub>
|Coordination = Digonal
|MolShape = Dihedral
|Dipole = 1.62 D
}}
|Section4={{Chembox Thermochemistry
|DeltaHf = −296.81 kJ mol<sup>−1</sup>
|Entropy = 248.223 J K<sup>−1</sup> mol<sup>−1</sup>
}}
|Section5={{Chembox Hazards
|GHSPictograms = {{GHS gas cylinder}} {{GHS corrosion}} {{GHS skull and crossbones}} {{GHS health hazard}}
|GHSSignalWord = Danger
|HPhrases = {{H-phrases|314|331|370}}<ref>{{Cite web|url=https://echa.europa.eu/information-on-chemicals/cl-inventory-database/-/discli/details/115657|website=C&L Inventory|title=Summary of Classification and Labelling: Sulphur Dioxide|access-date=2025-01-15}}</ref>
|NFPA-H = 3
|NFPA-F = 0
|NFPA-R = 0
|IDLH = 100 ppm<ref name=PGCH>{{PGCH|0575}}</ref>
|REL = TWA 2 ppm (5 mg/m<sup>3</sup>) ST 5 ppm (13 mg/m<sup>3</sup>)<ref name=PGCH/>
|PEL = TWA 5 ppm (13 mg/m<sup>3</sup>)<ref name=PGCH/>
|LC50 = 3000 ppm (mouse, 30 min)<br />2520 ppm (rat, 1 hr)<ref name=IDLH>{{IDLH|7446095|Sulfur dioxide}}</ref>
|LCLo = 993 ppm (rat, 20 min)<br />611 ppm (rat, 5 hr)<br />764 ppm (mouse, 20 min)<br />1000 ppm (human, 10 min)<br />3000 ppm (human, 5 min)<ref name=IDLH/>
}}
|Section6={{Chembox Related
|OtherFunction_label = [[sulfur]] [[oxide]]s
|OtherFunction = [[Sulfur monoxide]]<br />[[Sulfur trioxide]]<br />[[Disulfur monoxide]]
|OtherCompounds = [[Ozone]]<br />[[Selenium dioxide]]<br />[[Tellurium dioxide]]<br />[[Polonium dioxide]]
}}
}}

'''Sulfur dioxide''' ([[IUPAC]]-recommended spelling) or '''sulphur dioxide''' (traditional [[Commonwealth English]]) is the [[chemical compound]] with the formula {{chem|[[sulfur|S]]|[[oxygen|O]]|2}}. It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is released naturally by [[volcanic activity]] and is produced as a by-product of metals refining and the burning of [[Sour gas|sulfur]]-[[Sour crude oil|bearing]] fossil fuels.<ref name=Greenwood/>

Sulfur dioxide is somewhat toxic to humans, although only when inhaled in relatively large quantities for a period of several minutes or more. It was known to medieval [[alchemy|alchemists]] as "volatile spirit of sulfur".<ref name="Wothers-2019">{{Cite book |last=Wothers |first=Peter |url=https://books.google.com/books?id=8Cy7DwAAQBAJ&pg=PA69 |title=Antimony, Gold, and Jupiter's Wolf: How the Elements Were Named |date=2019 |publisher=Oxford University Press |isbn=978-0-19-965272-3 |language=en}}</ref>

== Structure and bonding ==
SO<sub>2</sub> is a bent molecule with ''C''<sub>2v</sub> [[Point groups in three dimensions|symmetry point group]].
A [[valence bond theory]] approach considering just ''s'' and ''p'' orbitals would describe the bonding in terms of [[resonance (chemistry)|resonance]] between two resonance structures.
[[File:Sulfur-dioxide-resonance-2D.svg|center|upright=1.25|thumb|Two resonance structures of sulfur dioxide]]
The sulfur–oxygen bond has a [[bond order]] of 1.5. There is support for this simple approach that does not invoke ''d'' orbital participation.<ref>{{cite journal
| title = Chemical bonding in oxofluorides of hypercoordinatesulfur
|author1=Cunningham, Terence P. |author2=Cooper, David L. |author3=Gerratt, Joseph |author4=Karadakov, Peter B. |author5=Raimondi, Mario  | journal = Journal of the Chemical Society, Faraday Transactions
| year = 1997
| volume = 93
| issue = 13
| pages = 2247–2254
| doi = 10.1039/A700708F
}}</ref>
In terms of [[electron counting|electron-counting]] formalism, the sulfur atom has an [[oxidation state]] of +4 and a [[formal charge]] of +1.

==Occurrence==
[[File:Io Aurorae color.jpg|thumb|left|The blue auroral glows of Io's upper atmosphere are caused by volcanic sulfur dioxide.]]

Sulfur dioxide is found on Earth and exists in very small concentrations in the atmosphere at about 15 [[Parts-per notation|ppb]].<ref>{{Cite web |last=US EPA |first=OAR |date=May 4, 2016 |title=Sulfur Dioxide Trends |url=https://www.epa.gov/air-trends/sulfur-dioxide-trends |access-date=2023-02-16 |website=www.epa.gov |language=en}}</ref>

On other planets, sulfur dioxide can be found in various concentrations, the most significant being the [[atmosphere of Venus]], where it is the third-most abundant atmospheric gas at 150 ppm. There, it reacts with water to form clouds of sulfurous acid ({{chem2|SO2}} + {{chem2|H2O}} ⇌ {{chem2|HSO3−}} + {{chem2|H+}}), and is a key component of the planet's global atmospheric [[sulfur cycle]]. It has been implicated as a key agent in the warming of early [[Mars]], with estimates of concentrations in the lower atmosphere as high as 100 ppm,<ref name="HalevyZuber2007">{{cite journal|last1=Halevy|first1=I.|last2=Zuber|first2=M. T.|last3=Schrag|first3=D. P.|title=A Sulfur Dioxide Climate Feedback on Early Mars|journal=Science|volume=318|issue=5858|year=2007|pages=1903–1907|issn=0036-8075|doi=10.1126/science.1147039|pmid=18096802|bibcode=2007Sci...318.1903H|s2cid=7246517}}</ref> though it only exists in trace amounts. On both Venus and Mars, as on Earth, its primary source is thought to be volcanic. The [[atmosphere of Io]], a natural satellite of [[Jupiter]], is 90% sulfur dioxide<ref name="IobookChap10">{{cite book |last=Lellouch |first=E. |editor=Lopes, R. M. C. |editor1-link=Rosaly Lopes |editor2=Spencer, J. R. |title=Io after Galileo |date=2007 |publisher=Springer-Praxis |isbn=978-3-540-34681-4 |pages=231–264 |chapter=Io's atmosphere }}</ref> and trace amounts are thought to also exist in the [[atmosphere of Jupiter]]. The [[James Webb Space Telescope]] has observed the presence of sulfur dioxide on the [[exoplanet]] [[WASP-39b]], where it is formed through [[photochemistry]] in the planet's atmosphere.<ref>{{cite web | url=https://phys.org/news/2022-11-james-webb-space-telescope-reveals.html?adlt=strict&toWww=1&redig=BF6DD536430F43AF981737C5EEABD064 | title=James Webb Space Telescope reveals an exoplanet atmosphere as never seen before }}</ref>

As an ice, it is thought to exist in abundance on the [[Galilean moons]]—as subliming ice or frost on the trailing hemisphere of [[Io (moon)|Io]],<ref name="CruikshankHowell1985">{{cite book |doi=10.1007/978-94-009-5418-2_55 |chapter=Sulfur Dioxide Ice on IO |title=ICES in the Solar System |year=1985 |last1=Cruikshank |first1=D. P. |last2=Howell |first2=R. R. |last3=Geballe |first3=T. R. |last4=Fanale |first4=F. P. |pages=805–815 |isbn=978-94-010-8891-6 }}</ref> and in the crust and mantle of [[Europa (moon)|Europa]], [[Ganymede (moon)|Ganymede]], and [[Callisto (moon)|Callisto]], possibly also in liquid form and readily reacting with water.<ref>[http://www.jpl.nasa.gov/news/news.php?release=2010-319 Europa's Hidden Ice Chemistry – NASA Jet Propulsion Laboratory]. Jpl.nasa.gov (October 4, 2010). Retrieved on September 24, 2013.</ref>

==Production==
Sulfur dioxide is primarily produced for [[sulfuric acid]] manufacture (see [[contact process]], but other processes predated that at least since 16th century<ref name="Wothers-2019" />). In the United States in 1979, 23.6&nbsp;million metric tons (26&nbsp;million U.S. short tons) of sulfur dioxide were used in this way, compared with 150,000 metric tons (165,347 U.S. short tons) used for other purposes. Most sulfur dioxide is produced by the combustion of elemental [[sulfur]]. Some sulfur dioxide is also produced by roasting [[pyrite]] and other [[sulfide]] ores in air.<ref name = Ullmann>{{Ullmann| author = Müller, Hermann | title = Sulfur Dioxide | doi = 10.1002/14356007.a25_569}}</ref>
[[File:03. Горење на сулфур во атмосфера од кислород.webm|thumb|left|upright=1.25|An experiment showing burning of sulfur in [[oxygen]]. A flow-chamber joined to a gas washing bottle (filled with a solution of [[methyl orange]]) is being used. The product is sulfur dioxide (SO<sub>2</sub>) with some traces of [[sulfur trioxide]] (SO<sub>3</sub>). The "smoke" that exits the gas washing bottle is, in fact, a sulfuric acid fog generated in the reaction.]]

===Combustion routes===
Sulfur dioxide is the product of the burning of [[sulfur]] or of burning materials that contain sulfur:

:{{chem2|S8}} + 8 {{chem2|O2}} → 8 {{chem2|SO2}}, ΔH = −297 kJ/mol

To aid combustion, liquified sulfur ({{convert|140|–|150|C|F}} is sprayed through an atomizing nozzle to generate fine drops of sulfur with a large surface area. The reaction is [[exothermic]], and the combustion produces temperatures of {{convert|1000|–|1600|C|F}}. The significant amount of heat produced is recovered by steam generation that can subsequently be converted to electricity.<ref name = Ullmann/>

The combustion of [[hydrogen sulfide]] and organosulfur compounds proceeds similarly. For example:

:2 {{chem2|H2S}} + 3 {{chem2|O2}} → 2 {{chem2|SO2}} + 2 {{chem2|H2O}}

The [[Roasting (metallurgy)|roasting]] of sulfide ores such as [[pyrite]], [[sphalerite]], and [[cinnabar]] (mercury sulfide) also releases SO<sub>2</sub>:<ref>Shriver, Atkins. Inorganic Chemistry, Fifth Edition. W. H. Freeman and Company; New York, 2010; p. 414.</ref>

:4 {{chem2|FeS2}} + 11 {{chem2|O2}} → 2 {{chem2|Fe2O3}} + 8 {{chem2|SO2}}
:2 {{chem2|ZnS}} + 3 {{chem2|O2}} → 2 {{chem2|ZnO}} + 2 {{chem2|SO2}}
:{{chem2|HgS + O2 -> Hg + SO2}}
:4 FeS + 7 {{chem2|O2}} → 2 {{chem2|Fe2O3}} + 4 {{chem2|SO2}}

A combination of these reactions is responsible for the largest source of sulfur dioxide, volcanic eruptions. These events can release millions of tons of SO<sub>2</sub>.

===Reduction of higher oxides===
Sulfur dioxide can also be a byproduct in the manufacture of [[calcium silicate]] cement; [[Calcium sulfate|CaSO<sub>4</sub>]] is heated with [[coke (fuel)|coke]] and sand in this process:

:2 {{chem2|CaSO4}} + 2 {{chem2|SiO2}} + C → 2 {{chem2|CaSiO3}} + 2 {{chem2|SO2}} + {{chem2|CO2}}

Until the 1970s commercial quantities of sulfuric acid and cement were produced by this process in [[Whitehaven]], England. Upon being mixed with [[shale]] or [[marl]], and roasted, the sulfate liberated sulfur dioxide gas, used in sulfuric acid production, the reaction also produced calcium silicate, a precursor in cement production.<ref>[http://www.lakestay.co.uk/whitehavenmininghistory.html WHITEHAVEN COAST ARCHAEOLOGICAL SURVEY]. lakestay.co.uk (2007)</ref>

On a laboratory scale, the action of hot concentrated sulfuric acid on copper [[swarf|turnings]] produces sulfur dioxide.

:Cu + 2 {{chem2|H2SO4}} → {{chem2|CuSO4 + SO2 + 2 H2O}}
Tin also reacts with concentrated sulfuric acid but it produces tin(II) sulfate which can later be pyrolyzed at 360&nbsp;°C into tin dioxide and dry sulfur dioxide.

:Sn + {{chem2|H2SO4}} → {{chem2|SnSO4 + H2}}
:{{chem2|SnSO4}} → {{chem2|SnO2}} + {{chem2|SO2}}

===From sulfites===
The reverse reaction occurs upon acidification:
:{{chem2|H+ + HSO3- -> SO2 + H2O}}

==Reactions==
Sulfites result by the action of aqueous base on sulfur dioxide:
:{{chem2|SO2 + 2 NaOH → Na2SO3 + H2O}}

Sulfur dioxide is a mild but useful [[reducing agent]]. It is oxidized by [[Halogen|halogens]] to give the sulfuryl halides, such as [[sulfuryl chloride]]:
:{{chem2|SO2 + Cl2 → SO2Cl2}}

Sulfur dioxide is the [[oxidising agent]] in the [[Claus process]], which is conducted on a large scale in [[oil refineries]]. Here, sulfur dioxide is reduced by hydrogen sulfide to give elemental sulfur:

:{{chem2|SO2 + 2 H2S → 3 S + 2 H2O}}

The sequential oxidation of sulfur dioxide followed by its hydration is used in the production of sulfuric acid.
:{{chem2|SO2}} + {{chem2|H2O}} + {{frac|1|2}} {{chem2|O2}} → {{chem2|H2SO4}}
Sulfur dioxide dissolves in water to give "[[sulfurous acid]]", which cannot be isolated and is instead an acidic solution of [[bisulfite]], and possibly [[sulfite]], ions.
:{{chem2|SO2 + H2O ⇌ HSO3− + H+}}{{spaces|10}}''K''<sub>a</sub> = 1.54{{e|−2}}; p''K''<sub>a</sub> = 1.81

===Laboratory reactions===
Sulfur dioxide is one of the few common acidic yet reducing gases. It turns moist litmus pink (being acidic), then white (due to its bleaching effect). It may be identified by bubbling it through a [[dichromate]] solution, turning the solution from orange to green (Cr<sup>3+</sup> (aq)). It can also reduce ferric ions to ferrous.<ref name=Lucas >{{Cite web|url=http://publications.gc.ca/collections/collection_2017/rncan-nrcan/M34-20/M34-20-107-eng.pdf|title = Information archivée dans le Web}}</ref>

Sulfur dioxide can react with certain 1,3-[[diene]]s in a [[cheletropic reaction]] to form cyclic [[sulfone]]s. This reaction is exploited on an industrial scale for the synthesis of [[sulfolane]], which is an important solvent in the [[petrochemical industry]].

:[[File:Cheletropic reaction of butadiene with SO2.svg|frameless|upright=0.7]]

Sulfur dioxide can bind to metal ions as a [[ligand]] to form [[metal sulfur dioxide complex]]es, typically where the transition metal is in oxidation state 0 or +1. Many different bonding modes (geometries) are recognized, but in most cases, the ligand is monodentate, attached to the metal through sulfur, which can be either planar and pyramidal [[hapticity|η]]<sup>1</sup>.<ref name=Greenwood>{{Greenwood&Earnshaw2nd}}</ref>  As a η<sup>1</sup>-SO<sub>2</sub> (S-bonded planar) ligand sulfur dioxide functions as a Lewis base using the lone pair on S. SO<sub>2</sub> functions as a [[Lewis acids]] in its η<sup>1</sup>-SO<sub>2</sub> (S-bonded pyramidal) bonding mode with metals and in its 1:1 [[adducts]] with Lewis bases such as [[dimethylacetamide]] and [[trimethyl amine]]. When bonding to Lewis bases the [[ECW model|acid parameters]] of SO<sub>2</sub> are E<sub>A</sub> = 0.51 and E<sub>A</sub> = 1.56.

==Uses==
The overarching, dominant use of sulfur dioxide is in the production of [[sulfuric acid]].<ref name = Ullmann/>

===Precursor to sulfuric acid===
Sulfur dioxide is an intermediate in the production of sulfuric acid, being converted to [[sulfur trioxide]], and then to [[oleum]], which is made into sulfuric acid. Sulfur dioxide for this purpose is made when sulfur combines with oxygen. The method of converting sulfur dioxide to sulfuric acid is called the [[contact process]]. Several million tons are produced annually for this purpose.

===Food preservative===
{{See also|Food preservation}}
Sulfur dioxide is sometimes used as a preservative for dried apricots, dried figs, and other dried fruits, owing to its [[antimicrobial]] properties and ability to prevent [[oxidation]],<ref>{{cite conference |last1=Zamboni |first1=Cibele B. |last2=Medeiros |first2=Ilca M. M. A. |last3=de Medeiros |first3=José A. G. |title=Analysis of Sulfur in Dried Fruits Using NAA |url=https://www.ipen.br/biblioteca/2011/inac/17204.pdf |conference=2011 International Nuclear Atlantic Conference – INAC 2011 |isbn=978-85-99141-03-8 |date=October 2011 |access-date=2020-06-04 |archive-date=2020-06-04 |archive-url=https://web.archive.org/web/20200604193519/https://www.ipen.br/biblioteca/2011/inac/17204.pdf }}</ref> and is called [[E number|E]]220<ref>[http://www.food.gov.uk/safereating/chemsafe/additivesbranch/enumberlist#h_3 Current EU approved additives and their E Numbers], The Food Standards Agency website.</ref> when used in this way in Europe. As a preservative, it maintains the colorful appearance of the fruit and prevents [[Decomposition|rotting]]. Historically, [[molasses]] was "sulfured" as a preservative and also to lighten its color. Treatment of dried fruit was usually done outdoors, by igniting sublimed sulfur and burning in an enclosed space with the fruits.<ref name="University of Georgia">{{Citation |title=Preserving foods: Drying fruits and Vegetable |url=https://nchfp.uga.edu/publications/uga/uga_dry_fruit.pdf |publisher=University of Georgia cooperative extension service |access-date=2022-06-06 |archive-date=2022-09-27 |archive-url=https://web.archive.org/web/20220927163031/https://nchfp.uga.edu/publications/uga/uga_dry_fruit.pdf }}</ref> Fruits may be sulfured by dipping them into [[sodium bisulfite]], [[sodium sulfite]] or [[sodium metabisulfite]].<ref name="University of Georgia" />

==== Winemaking ====
Sulfur dioxide was first used in [[winemaking]] by the Romans, when they discovered that burning sulfur candles inside empty wine vessels keeps them fresh and free from vinegar smell.<ref>{{cite web|url=http://www.practicalwinery.com/janfeb09/page1.htm|publisher=www.practicalwinery.com|date=February 1, 2009|title=Practical Winery & vineyard Journal Jan/Feb 2009|archive-url=https://web.archive.org/web/20130928111625/http://www.practicalwinery.com/janfeb09/page1.htm|archive-date=2013-09-28}}</ref>

It is still an important compound in winemaking, and is measured in [[parts per million]] (''ppm'') in wine. It is present even in so-called unsulfurated wine at concentrations of up to 10&nbsp;mg/L.<ref>[http://www.morethanorganic.com/sulphur-in-the-bottle Sulphites in wine], MoreThanOrganic.com.</ref> It serves as an [[antibiotic]] and [[antioxidant]], protecting wine from spoilage by bacteria and oxidation – a phenomenon that leads to the browning of the wine and a loss of cultivar specific flavors.<ref name="Jackson">Jackson, R.S. (2008) Wine science: principles and applications, Amsterdam; Boston: Elsevier/Academic Press</ref><ref name="Guerrero">{{cite journal | doi = 10.1016/j.tifs.2014.11.004| title = Demonstrating the efficiency of sulphur dioxide replacements in wine: A parameter review| journal = Trends in Food Science & Technology| volume = 42| pages = 27–43| year = 2015| last1 = Guerrero| first1 = Raúl F| last2 = Cantos-Villar| first2 = Emma| issue = 1}}</ref> Its antimicrobial action also helps minimize volatile acidity. Wines containing sulfur dioxide are typically labeled with "containing [[sulfite]]s".

Sulfur dioxide exists in wine in free and bound forms, and the combinations are referred to as total SO<sub>2</sub>. Binding, for instance to the carbonyl group of [[acetaldehyde]], varies with the wine in question. The free form exists in equilibrium between molecular SO<sub>2</sub> (as a dissolved gas) and bisulfite ion, which is in turn in equilibrium with sulfite ion. These equilibria depend on the pH of the wine. Lower pH shifts the equilibrium towards molecular (gaseous) SO<sub>2</sub>, which is the active form, while at higher pH more SO<sub>2</sub> is found in the inactive sulfite and bisulfite forms. The molecular SO<sub>2</sub> is active as an antimicrobial and antioxidant, and this is also the form which may be perceived as a pungent odor at high levels. Wines with total SO<sub>2</sub> concentrations below 10 ppm do not require "contains sulfites" on the label by US and EU laws. The upper limit of total SO<sub>2</sub> allowed in wine in the US is 350 ppm; in the EU it is 160 ppm for red wines and 210 ppm for white and rosé wines. In low concentrations, SO<sub>2</sub> is mostly undetectable in wine, but at free SO<sub>2</sub> concentrations over 50 ppm, SO<sub>2</sub> becomes evident in the smell and taste of wine.{{Citation needed|date=May 2009}}

SO<sub>2</sub> is also a very important compound in winery sanitation. Wineries and equipment must be kept clean, and because bleach cannot be used in a winery due to the risk of [[cork taint]],<ref>[http://www.extension.purdue.edu/extmedia/FS/FS-50-W.pdf Chlorine Use in the Winery]. Purdue University</ref> a mixture of SO<sub>2</sub>, water, and citric acid is commonly used to clean and sanitize equipment. [[Ozone]] (O<sub>3</sub>) is now used extensively for sanitizing in wineries due to its efficacy, and because it does not affect the wine or most equipment.<ref>[https://www.practicalwinery.com/janfeb00/ozone.htm Use of ozone for winery and environmental sanitation] {{Webarchive|url=https://web.archive.org/web/20170912102459/https://www.practicalwinery.com/janfeb00/ozone.htm |date=September 12, 2017 }}, Practical Winery & Vineyard Journal.</ref>

===As a soaking agent===
Aqueous sulfur dioxide solution is used in [[corn wet-milling]], in the steeping stage of the process. The corn kernels are soaked in this solution in large tanks containing lactic acid and sulphur dioxide at around 53˚C (127˚F) temperature for nearly 40 hours. This is done to soften the kernel so that the oil in the germ will not contaminate other products and is easy to separate.

===As a reducing agent===
Sulfur dioxide is also a good [[Reducing agent|reductant]]. In the presence of water, sulfur dioxide is able to decolorize substances. Specifically, it is a useful reducing [[bleach]] for papers and delicate materials such as clothes. This bleaching effect normally does not last very long. Oxygen in the atmosphere reoxidizes the reduced dyes, restoring the color. In municipal wastewater treatment, sulfur dioxide is used to treat chlorinated wastewater prior to release. Sulfur dioxide reduces free and combined chlorine to [[chloride]].<ref>{{cite book |last=Tchobanoglous |first=George |title=Wastewater Engineering |edition=3rd |location=New York |publisher=McGraw Hill |year=1979 |isbn=0-07-041677-X }}</ref>

Sulfur dioxide is fairly soluble in water, and by both IR and Raman spectroscopy; the hypothetical [[sulfurous acid]], H<sub>2</sub>SO<sub>3</sub>, is not present to any extent. However, such solutions do show spectra of the hydrogen sulfite ion, HSO<sub>3</sub><sup>−</sup>, by reaction with water, and it is in fact the actual reducing agent present:
:SO<sub>2</sub> + H<sub>2</sub>O ⇌ HSO<sub>3</sub><sup>−</sup> + H<sup>+</sup>

===As a fumigant===
In the beginning of the 20th century sulfur dioxide was used in [[Buenos Aires]] as a fumigant to kill rats that carried the ''[[Yersinia pestis]]'' bacterium, which causes bubonic plague. The application was successful, and the application of this method was extended to other areas in South America. In Buenos Aires, where these apparatuses were known as [[Sulfurozador]], but later also in Rio de Janeiro, New Orleans and San Francisco, the sulfur dioxide treatment machines were brought into the streets to enable extensive disinfection campaigns, with effective results.<ref>{{cite journal |last1=Engelmann |first1=Lukas |title=Fumigating the Hygienic Model City: Bubonic Plague and the Sulfurozador in Early-Twentieth-Century Buenos Aires |journal=Medical History |date=July 2018 |volume=62 |issue=3 |pages=360–382 |doi=10.1017/mdh.2018.37 |pmid=29886876 |pmc=6113751 }}</ref>

===Biochemical and biomedical roles===
Sulfur dioxide or its conjugate base bisulfite is produced biologically as an intermediate in both sulfate-reducing organisms and in sulfur-oxidizing bacteria, as well. The role of sulfur dioxide in mammalian biology is not yet well understood.<ref>{{cite journal |last1=Liu |first1=D. |last2=Jin |first2=H. |last3=Tang |first3=C. |last4=Du |first4=J. |title=Sulfur Dioxide: a Novel Gaseous Signal in the Regulation of Cardiovascular Functions |journal=Mini-Reviews in Medicinal Chemistry |year=2010 |volume=10 |issue=11 |pages=1039–1045 |doi=10.2174/1389557511009011039 |pmid=20540708 }}</ref>  Sulfur dioxide blocks nerve signals from the [[pulmonary stretch receptors]] and abolishes the [[Hering–Breuer reflex|Hering–Breuer inflation reflex]].

It is considered that endogenous sulfur dioxide plays a significant physiological role in regulating [[cardiac]] and [[blood vessel]] function, and aberrant or deficient sulfur dioxide metabolism can contribute to several different cardiovascular diseases, such as [[arterial hypertension]], [[atherosclerosis]], [[pulmonary arterial hypertension]], and [[stenocardia]].<ref>{{cite journal |last1=Tian |first1=Hong |title=Advances in the study on endogenous sulfur dioxide in the cardiovascular system |journal=Chinese Medical Journal |date=November 5, 2014 |volume=127 |issue=21 |pages=3803–3807 |doi=10.3760/cma.j.issn.0366-6999.20133031 |pmid=25382339 |s2cid=11924999 |doi-access=free }}</ref>

It was shown that in children with pulmonary arterial hypertension due to congenital heart diseases the level of [[homocysteine]] is higher and the level of endogenous sulfur dioxide is lower than in normal control children. Moreover, these biochemical parameters strongly correlated to the severity of pulmonary arterial hypertension. Authors considered homocysteine to be one of useful biochemical markers of disease severity and sulfur dioxide metabolism to be one of potential therapeutic targets in those patients.<ref>{{cite journal|vauthors=Yang R, Yang Y, Dong X, Wu X, Wei Y |title=Correlation between endogenous sulfur dioxide and homocysteine in children with pulmonary arterial hypertension associated with congenital heart disease|language=zh|journal=Zhonghua Er Ke Za Zhi|date=Aug 2014|volume=52|issue=8|pages=625–629|pmid=25224243}}</ref>

Endogenous sulfur dioxide also has been shown to lower the [[Cell proliferation|proliferation]] rate of endothelial [[smooth muscle]] cells in blood vessels, via lowering the [[MAPK]] activity and activating [[adenylyl cyclase]] and [[protein kinase A]].<ref>{{cite journal|vauthors=Liu D, Huang Y, Bu D, Liu AD, Holmberg L, Jia Y, Tang C, Du J, Jin H |title=Sulfur dioxide inhibits vascular smooth muscle cell proliferation via suppressing the Erk/MAP kinase pathway mediated by cAMP/PKA signaling|journal=Cell Death Dis.|date=May 2014|volume=5|issue=5|pages=e1251|doi=10.1038/cddis.2014.229|pmid=24853429|pmc=4047873}}</ref> Smooth muscle cell proliferation is one of important mechanisms of hypertensive remodeling of blood vessels and their [[stenosis]], so it is an important pathogenetic mechanism in arterial hypertension and atherosclerosis.

Endogenous sulfur dioxide in low concentrations causes endothelium-dependent [[vasodilation]]. In higher concentrations it causes endothelium-independent vasodilation and has a negative inotropic effect on cardiac output function, thus effectively lowering blood pressure and myocardial oxygen consumption. The vasodilating and bronchodilating effects of sulfur dioxide are mediated via ATP-dependent [[calcium channel]]s and L-type ("dihydropyridine") calcium channels. Endogenous sulfur dioxide is also a potent antiinflammatory, antioxidant and cytoprotective agent. It lowers blood pressure and slows hypertensive remodeling of blood vessels, especially thickening of their intima. It also regulates lipid metabolism.<ref>{{cite journal|vauthors=Wang XB, Jin HF, Tang CS, Du JB |title=The biological effect of endogenous sulfur dioxide in the cardiovascular system.|journal=Eur J Pharmacol|date=November 16, 2011|volume=670|issue=1|doi=10.1016/j.ejphar.2011.08.031|pmid=21925165|pages=1–6}}</ref>

Endogenous sulfur dioxide also diminishes myocardial damage, caused by [[isoproterenol]] [[adrenergic]] hyperstimulation, and strengthens the myocardial antioxidant defense reserve.<ref>{{cite journal|vauthors=Liang Y, Liu D, Ochs T, Tang C, Chen S, Zhang S, Geng B, Jin H, Du J |title=Endogenous sulfur dioxide protects against isoproterenol-induced myocardial injury and increases myocardial antioxidant capacity in rats.|journal=Lab. Invest.|date=Jan 2011|volume=91|issue=1|pages=12–23|doi=10.1038/labinvest.2010.156|pmid=20733562|doi-access=free}}</ref>

===As a reagent and solvent in the laboratory===
Sulfur dioxide is a versatile inert solvent widely used for dissolving highly oxidizing salts. It is also used occasionally as a source of the sulfonyl group in [[organic synthesis]]. Treatment of aryl [[diazonium salt]]s with sulfur dioxide and [[cuprous chloride]] yields the corresponding aryl sulfonyl chloride, for example:<ref>{{OrgSynth | author = Hoffman, R. V. | title = m-Trifluoromethylbenzenesulfonyl Chloride | collvol = 7 | collvolpages = 508 | year = 1990| prep = CV7P0508}}</ref>

:[[File:Preparation of m-trifluoromethylbenzenesulfonyl chloride.svg|frameless|upright=2]]

As a result of its very low [[Lewis basicity]], it is often used as a low-temperature solvent/diluent for superacids like [[magic acid]] (FSO<sub>3</sub>H/SbF<sub>5</sub>), allowing for highly reactive species like ''tert''-butyl cation to be observed spectroscopically at low temperature (though tertiary carbocations do react with SO<sub>2</sub> above about −30&nbsp;°C, and even less reactive solvents like [[Sulfuryl chloride fluoride|SO<sub>2</sub>ClF]] must be used at these higher temperatures).<ref>{{Cite journal|last1=Olah|first1=George A.|last2=Lukas|first2=Joachim.|date=August 1, 1967|title=Stable carbonium ions. XLVII. Alkylcarbonium ion formation from alkanes via hydride (alkide) ion abstraction in fluorosulfonic acid-antimony pentafluoride-sulfuryl chlorofluoride solution|journal=Journal of the American Chemical Society|volume=89|issue=18|pages=4739–4744|doi=10.1021/ja00994a030|bibcode=1967JAChS..89.4739O |issn=0002-7863}}</ref>

===As a refrigerant===
Sulfur dioxide was one of the earliest [[refrigerant]]s adopted for mechanical refrigeration owing to its ease of liquefaction and high latent [[heat of vaporization]]. In 1784, [[Jean-François Clouet]] and [[Gaspard Monge]] first demonstrated that sulfur dioxide gas could be liquefied at low temperatures. In the mid-1870s, [[Raoul Pictet]] successfully employed sulfur dioxide in a prototype refrigeration system. Beginning in 1920, it saw widespread use in the "Rollator" rotary-compressor [[refrigeration#Home and consumer use|home refrigerators]] produced by [[Norge (appliance manufacturer)|Norge]]. Following the introduction of less toxic, non-flammable [[chlorofluorocarbon]] (CFC) refrigerants, the use of sulfur dioxide in refrigeration systems gradually declined.<ref name="ASHRAE">{{cite web |title=Air Conditioning and Refrigeration Timeline |website=ASHRAE |url=https://www.ashrae.org/about/mission-and-vision/ashrae-industry-history/air-conditioning-and-refrigeration-timeline |access-date=1 August 2025}}</ref><ref name="Ayman 2020">{{cite journal |author=Ayman G. A. Essa |author2=Adil A. Mohamed |title=Selection of optimal fluid for refrigeration cycles |journal=World Journal of Advanced Engineering Technology and Sciences |volume=1 |issue=2 |date=30 December 2020 |doi=10.30574/wjaets.2020.1.2.0031 |doi-access=free |pages=21–036 |url=https://wjaets.com/sites/default/files/WJAETS-2020-0031.pdf |access-date=1 August 2025}}</ref>

===As an indicator of volcanic activity===
Sulfur dioxide content in naturally-released geothermal gasses is measured by the [[Icelandic Meteorological Office]] as an indicator of possible volcanic activity.<ref>{{Cite web |date=n.d. |title=Volcanic gases |url=https://en.vedur.is/volcanoes/volcanic-hazards/volcanic-gases/ |website=Iceland Met Office}}</ref>

==Safety==
[[File:20180519 USGS Leilani Estates Hawaii Volcanic EruptionDSC 0411 medium.jpg|thumb|[[United States Geological Survey|US Geological Survey]] volunteer tests for sulfur dioxide after the [[2018 lower Puna eruption]].]]

===Ingestion===
In the United States, the [[Center for Science in the Public Interest]] lists the two food preservatives, sulfur dioxide and [[sodium bisulfite]], as being safe for human consumption except for certain asthmatic individuals who may be sensitive to them, especially in large amounts.<ref>{{cite web | title = Center for Science in the Public Interest – Chemical Cuisine | url = http://www.cspinet.org/reports/chemcuisine.htm | access-date = March 17, 2010}}</ref>  Symptoms of sensitivity to [[sulfite#Health effects|sulfiting]] agents, including sulfur dioxide, manifest as potentially life-threatening trouble breathing within minutes of ingestion.<ref>{{cite web | title = California Department of Public Health: Food and Drug Branch: Sulfites | url = http://www.cdph.ca.gov/pubsforms/Guidelines/Documents/fdb%20Sulfites.pdf | access-date = September 27, 2013 | archive-url = https://web.archive.org/web/20120723065412/http://www.cdph.ca.gov/pubsforms/Guidelines/Documents/fdb%20Sulfites.pdf | archive-date = July 23, 2012 }}</ref> Sulphites may also cause symptoms in non-asthmatic individuals, namely [[dermatitis]], [[hives|urticaria]], [[flushing (physiology)|flushing]], [[hypotension]], [[abdominal pain]] and diarrhea, and even life-threatening [[anaphylaxis]].<ref name="pmid24834193">{{cite journal |vauthors=Vally H, Misso NL |title=Adverse reactions to the sulphite additives |journal=Gastroenterol Hepatol Bed Bench |volume=5 |issue=1 |pages=16–23 |date=2012 |pmid=24834193 |pmc=4017440 |doi= |url=}}</ref>

===Inhalation===
Incidental exposure to sulfur dioxide is routine, e.g. the smoke from matches, coal, and sulfur-containing fuels like [[bunker fuel]]. Relative to other chemicals, it is only mildly toxic and requires high concentrations to be actively hazardous.<ref>[https://www.epa.gov/so2-pollution Sulfur Dioxide Basics] U.S. Environmental Protection Agency</ref> However, its ubiquity makes it a major air pollutant with significant impacts on human health.<ref name="EPA">[https://www.epa.gov/so2-pollution Sulfur Dioxide (SO2) Pollution]. [[United States Environmental Protection Agency]]</ref>

In 2008, the [[American Conference of Governmental Industrial Hygienists]] reduced the [[short-term exposure limit]] to 0.25 parts per million (ppm). In the US, the [[Occupational Safety and Health Administration|OSHA]] set the [[Permissible exposure limit|PEL]] at 5 ppm (13&nbsp;mg/m<sup>3</sup>) time-weighted average. Also in the US, [[NIOSH]] set the [[IDLH]] at 100 ppm.<ref name=NIOSH>{{cite web |url=https://www.cdc.gov/niosh/npg/npgd0575.html |title=NIOSH Pocket Guide to Chemical Hazards }}</ref>  In 2010, the [[EPA]] "revised the primary SO<sub>2</sub> [[NAAQS]] by establishing a new one-hour standard at a level of 75 [[parts per billion|parts per billion (ppb)]]. EPA revoked the two existing primary standards because they would not provide additional public health protection given a one-hour standard at 75 ppb."<ref name="EPA" />

==Environmental role==
===Air pollution===
[[File:Volcanic injection.svg|350px|thumb|The effect of major [[volcanic eruption]]s on sulfate aerosol concentrations and chemical reactions in the atmosphere]]
Major [[volcanic eruption]]s have an overwhelming effect on sulfate [[aerosol]] concentrations in the years when they occur: eruptions ranking 4 or greater on the [[Volcanic Explosivity Index]] inject {{chem2|SO2}} and water vapor directly into the [[stratosphere]], where they react to create sulfate aerosol plumes.<ref name="nasa-aerosols">{{cite web |url=http://volcanoes.usgs.gov/hazards/gas/s02aerosols.php |title=Volcanic Sulfur Aerosols Affect Climate and the Earth's Ozone Layer |access-date=February 17, 2009 |publisher=United States Geological Survey |archive-date=November 14, 2015 |archive-url=https://web.archive.org/web/20151114184944/https://volcanoes.usgs.gov/hazards/gas/s02aerosols.php }}</ref> Volcanic emissions vary significantly in composition, and have complex chemistry due to the presence of ash particulates and a wide variety of other elements in the plume. Only [[stratovolcanoes]] containing primarily [[felsic]] magmas are responsible for these fluxes, as [[mafic]] magma erupted in [[shield volcanoes]] doesn't result in plumes which reach the stratosphere.<ref>{{cite journal |doi=10.1016/j.atmosenv.2004.06.017 |journal=Atmospheric Environment |volume=38 |issue=33 |year=2004 |pages=5637–5649 |title=Aerosol chemistry of emissions from three contrasting volcanoes in Italy |vauthors=Mathera TA, Oppenheimer AG, McGonigle A|bibcode=2004AtmEn..38.5637M }}</ref> However, before the [[Industrial Revolution]], dimethyl sulfide pathway was the largest contributor to sulfate aerosol concentrations in a more average year with no major volcanic activity. According to the [[IPCC First Assessment Report]], published in 1990, volcanic emissions usually amounted to around 10&nbsp;million tons in 1980s, while dimethyl sulfide amounted to 40 million tons. Yet, by that point, the global human-caused emissions of sulfur into the atmosphere became "at least as large" as ''all'' natural emissions of sulfur-containing compounds ''combined'': they were at less than 3&nbsp;million tons per year in 1860, and then they increased to 15&nbsp;million tons in 1900, 40&nbsp;million tons in 1940 and about 80 millions in 1980. The same report noted that "in the industrialized regions of Europe and North America, anthropogenic emissions dominate over natural emissions by about a factor of ten or even more".<ref name="IPCC_FAR">IPCC, 1990: [https://www.ipcc.ch/site/assets/uploads/2018/03/ipcc_far_wg_I_chapter_01.pdf Chapter 1: Greenhouse Gases and Aerosols] [R.T. Watson, H. Rodhe, H. Oeschger and U. Siegenthaler]. In: [https://www.ipcc.ch/site/assets/uploads/2018/03/ipcc_far_wg_I_full_report.pdf Climate Change: The IPCC Scientific Assessment] [J.T.Houghton, G.J.Jenkins and J.J.Ephraums (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 31–34,</ref> In the eastern United States in the early 2000s, sulfate particles were estimated to account for 25% or more of all [[air pollution]].<ref name="EPAHealth" /> Exposure to sulfur dioxide emissions by coal power plants (coal PM<sub>2.5</sub>) in the US was associated with 2.1 times greater mortality risk than exposure to PM<sub>2.5</sub> from all sources.<ref name="science2023mortality">{{cite journal |last1=Henneman |first1=Lucas |last2=Choirat |first2=Christine |last3=Dedoussi |first3=Irene |last4=Dominici |first4=Francesca |last5=Roberts |first5=Jessica|last6=Zigler |first6=Corwin |title=Mortality risk from United States coal electricity generation |journal=[[Science (journal)|Science]] |date=November 24, 2023 |volume=382 |issue=6673 |pages=941–946|doi=10.1126/science.adf4915 |pmid=37995235 |pmc=10870829 |bibcode=2023Sci...382..941H |language=en}}</ref> 
Meanwhile, the [[Southern Hemisphere]] had much lower concentrations due to being much less densely populated, with an estimated 90% of the human population in the north. In the early 1990s, anthropogenic sulfur dominated in the [[Northern Hemisphere]], where only 16% of annual sulfur emissions were natural, yet amounted for less than half of the emissions in the Southern Hemisphere.<ref>{{Cite journal|last1=Bates|first1=T. S.|last2=Lamb|first2=B. K.|last3=Guenther|first3=A.|last4=Dignon|first4=J.|last5=Stoiber|first5=R. E.|date=April 1992|title=Sulfur emissions to the atmosphere from natural sources|url=http://link.springer.com/10.1007/BF00115242|journal=Journal of Atmospheric Chemistry|language=en|volume=14|issue=1–4|pages=315–337|doi=10.1007/BF00115242 |bibcode=1992JAtC...14..315B |s2cid=55497518|issn=0167-7764|url-access=subscription}}</ref>
[[File:Acid rain woods1.JPG|thumb|left|Acid rain-damaged forest in Europe's [[Black Triangle (region)|Black Triangle]]]]
Such an increase in sulfate aerosol emissions had a variety of effects. At the time, the most visible one was [[acid rain]], caused by [[precipitation]] from clouds carrying high concentrations of sulfate aerosols in the [[troposphere]].<ref>{{Cite journal|last1=Burns|first1= Douglas A.|last2=Aherne|first2=Julian|last3=Gay|first3=David A.|last4=Lehmann|first4=Christopher M.~B.|title =Acid rain and its environmental effects: Recent scientific advances|journal = Atmospheric Environment|language=en|year = 2016|volume=146|pages = 1–4|doi = 10.1016/j.atmosenv.2016.10.019|bibcode= 2016AtmEn.146....1B|doi-access=free}}</ref>
At its peak, acid rain has eliminated [[brook trout]] and some other fish species and insect life from lakes and streams in geographically sensitive areas, such as [[Adirondack Mountains]] in the United States.<ref name="EPASurface">{{Cite web|title=Effects of Acid Rain – Surface Waters and Aquatic Animals|url=http://www.epa.gov/acidrain/effects/surface_water.html|archive-url=https://web.archive.org/web/20090514121649/http://www.epa.gov/acidrain/effects/surface_water.html|archive-date=May 14, 2009|website=US EPA |date=September 8, 2006 }}</ref> Acid rain worsens [[soil]] function as some of its [[microbiota]] is lost and heavy metals like aluminium are mobilized (spread more easily)  while essential nutrients and minerals such as [[magnesium]] can leach away because of the same. Ultimately, plants unable to tolerate lowered [[pH]] are killed, with montane forests being some of the worst-affected [[ecosystem]]s due to their regular exposure to sulfate-carrying fog at high altitudes.<ref>{{Cite journal|last1=Rodhe|first1=Henning|last2=Dentener|first2=Frank|last3=Schulz|first3=Michael|date=October 1, 2002|title=The Global Distribution of Acidifying Wet Deposition|journal=Environmental Science & Technology|volume=36|issue=20|pages=4382–4388|doi=10.1021/es020057g|pmid=12387412|bibcode=2002EnST...36.4382R|issn=0013-936X}}</ref><ref name="EPA: Forests">US EPA: [http://www.epa.gov/acidrain/effects/forests.html Effects of Acid Rain – Forests] {{webarchive |url=https://web.archive.org/web/20080726034352/http://www.epa.gov/acidrain/effects/forests.html |date=July 26, 2008 }}</ref><ref>{{cite journal|doi=10.1126/science.272.5259.244|url=http://www.esf.edu/efb/mitchell/Class%20Readings/Sci.272.244.246.pdf|title=Long-Term Effects of Acid Rain: Response and Recovery of a Forest Ecosystem|year=1996|last1=Likens|first1=G. E.|last2=Driscoll|first2=C. T.|last3=Buso|first3=D. C.|journal=Science|volume=272|issue=5259|page=244|bibcode=1996Sci...272..244L|s2cid=178546205|access-date=February 9, 2013|archive-date=December 24, 2012|archive-url=https://web.archive.org/web/20121224203613/http://www.esf.edu/efb/mitchell/Class%20Readings/Sci.272.244.246.pdf|url-status=live}}</ref><ref>{{Cite journal|last1=Larssen|first1=T.|last2=Carmichael|first2=G. R.|date=October 1, 2000|title=Acid rain and acidification in China: the importance of base cation deposition|url=http://www.sciencedirect.com/science/article/pii/S0269749199002791|journal=Environmental Pollution|language=en|volume=110|issue=1|pages=89–102|doi=10.1016/S0269-7491(99)00279-1|pmid=15092859|issn=0269-7491|access-date=April 22, 2020|archive-date=March 30, 2015|archive-url=https://web.archive.org/web/20150330041614/http://www.sciencedirect.com/science/article/pii/S0269749199002791|url-status=live|url-access=subscription}}</ref><ref>{{Cite journal|last1=Johnson|first1=Dale W.|last2=Turner|first2=John|last3=Kelly|first3=J. M.|date=1982|title=The effects of acid rain on forest nutrient status|journal=Water Resources Research|language=en|volume=18|issue=3|pages=449–461|doi=10.1029/WR018i003p00449|bibcode=1982WRR....18..449J|issn=1944-7973}}</ref> While acid rain was too dilute to affect human health directly, breathing smog or even any air with elevated sulfate concentrations is known to contribute to [[heart]] and [[lung]] conditions, including [[asthma]] and [[bronchitis]].<ref name="EPAHealth">[https://www.epa.gov/acidrain/effects-acid-rain#health Effects of Acid Rain – Human Health] . Epa.gov (June 2, 2006). Retrieved on February 9, 2013.</ref> Further, this form of pollution is linked to [[preterm birth]] and [[low birth weight]], with a study of 74,671 pregnant women in Beijing finding that every additional 100&nbsp;μg/m<sup>3</sup> of {{SO2}} in the air reduced infants' weight by 7.3 g, making it and other forms of air pollution the largest attributable risk factor for low birth weight ever observed.<ref>{{Cite journal |last1=Wang |first1=X. |last2=Ding |first2=H. |last3=Ryan |first3=L. |last4=Xu |first4=X. |s2cid=2707126 |date=May 1, 1997 |title=Association between air pollution and low birth weight: a community-based study |journal=Environmental Health Perspectives |volume=105 |issue=5 |pages=514–20 |issn=0091-6765 |pmc=1469882 |pmid=9222137 |doi=10.1289/ehp.97105514 |bibcode=1997EnvHP.105..514W }}</ref>

====Control measures====
[[File:Estimates of past and future SO2 global anthropogenic emissions.png|thumb|upright=1.25|Early 2010s estimates of past and future anthropogenic global sulfur dioxide emissions, including the [[Representative Concentration Pathway]]s. While no [[climate change scenario]] may reach Maximum Feasible Reductions (MFRs), all assume steep declines from today's levels. By 2019, sulfate emission reductions were confirmed to proceed at a very fast rate.<ref name=XuRamanathanVictor>{{Cite journal|last1=Xu|first1=Yangyang|last2=Ramanathan|first2=Veerabhadran|last3=Victor|first3=David G.|date=December 5, 2018|title=Global warming will happen faster than we think|journal=Nature|language=en|volume=564|issue=7734|pages=30–32 |url=https://www.researchgate.net/publication/329411074 |doi=10.1038/d41586-018-07586-5|pmid=30518902|bibcode=2018Natur.564...30X|doi-access=free}}</ref>]]
Due largely to the US EPA's [[Acid Rain Program]], the U.S. has had a 33% decrease in emissions between 1983 and 2002 (see table). This improvement resulted in part from [[flue-gas desulfurization]], a technology that enables SO<sub>2</sub> to be chemically bound in [[power plant]]s burning sulfur-containing coal or petroleum.
{| class="wikitable"
|-
! Year
! SO<sub>2</sub>
|-
| 1970
| {{convert|31161000|ST|Mt|sigfig=3}}
|-
| 1980
| {{convert|25905000|ST|Mt|sigfig=3}}
|-
| 1990
|{{convert|23678000|ST|Mt|sigfig=3}}
|-
| 1996
|{{convert|18859000|ST|Mt|sigfig=3}}
|-
| 1997
|{{convert|19363000|ST|Mt|sigfig=3}}
|-
| 1998
|{{convert|19491000|ST|Mt|sigfig=3}}
|-
| 1999
|{{convert|18867000|ST|Mt|sigfig=3}}
|}

In particular, [[calcium oxide|calcium oxide (lime)]] reacts with sulfur dioxide to form [[calcium sulfite]]:

: CaO + SO<sub>2</sub> →  CaSO<sub>3</sub>

Aerobic oxidation of the CaSO<sub>3</sub> gives CaSO<sub>4</sub>, [[anhydrite]]. Most gypsum sold in Europe comes from flue-gas desulfurization.

To control sulfur emissions, dozens of methods with relatively high efficiencies have been developed for fitting of coal-fired power plants.<ref>{{Cite journal|last1=Lin|first1=Cheng-Kuan|last2=Lin|first2=Ro-Ting|last3=Chen|first3=Pi-Cheng|last4=Wang|first4=Pu|last5=De Marcellis-Warin|first5=Nathalie|last6=Zigler|first6=Corwin|last7=Christiani|first7=David C.|date=February 8, 2018|title=A Global Perspective on Sulfur Oxide Controls in Coal-Fired Power Plants and Cardiovascular Disease|journal=Scientific Reports|language=en|volume=8|issue=1|page=2611 |doi=10.1038/s41598-018-20404-2|pmid=29422539|issn=2045-2322|pmc=5805744|bibcode=2018NatSR...8.2611L}}</ref> Sulfur can be removed from coal during burning by using limestone as a bed material in [[fluidized bed combustion]].<ref>{{cite book |last=Lindeburg |first=Michael R. |title=Mechanical Engineering Reference Manual for the PE Exam |location=Belmont, C.A. |publisher=Professional Publications, Inc |year=2006 |pages=27–3 |isbn=978-1-59126-049-3}}</ref>

Sulfur can also be removed from fuels before burning, preventing formation of SO<sub>2</sub> when the fuel is burnt. The [[Claus process]] is used in refineries to produce sulfur as a byproduct. The [[Stretford process]] has also been used to remove sulfur from fuel. [[Redox]] processes using iron oxides can also be used, for example, Lo-Cat<ref>[https://web.archive.org/web/20100304170107/http://www.gtp-merichem.com/support/index.php FAQ's About Sulfur Removal and Recovery using the LO-CAT® Hydrogen Sulfide Removal System]. gtp-merichem.com</ref> or Sulferox.<ref>[http://www.netl.doe.gov/technologies/coalpower/gasification/pubs/pdf/SFA%20Pacific_Process%20Screening%20Analysis_Dec%202002.pdf Process screening analysis of alternative gas treating and sulfur removal for gasification]. (December 2002) Report by SFA Pacific, Inc. prepared for U.S. Department of Energy (PDF) Retrieved on October 31, 2011.</ref>

Fuel additives such as [[calcium]] additives and magnesium carboxylate may be used in marine engines to lower the emission of sulfur dioxide gases into the atmosphere.<ref>May, Walter R.  [http://www.fuelspec.com/library/Marine%20Emissions%20Abatement.pdf Marine Emissions Abatement] {{Webarchive|url=https://web.archive.org/web/20150402130001/http://www.fuelspec.com/library/Marine%20Emissions%20Abatement.pdf |date=April 2, 2015 }}. SFA International, Inc., p. 6.</ref>

===Effects on ozone layer===
Sulfur dioxide aerosols in the stratosphere can contribute to [[ozone depletion]] in the presence of chlorofluorocarbons and other halogenated ozone-depleting substances.<ref name=klobas2017>{{cite journal|last1=Klobas|first1=J.E.|last2=Wilmouth|first2=D.M.|last3=Weisenstein|first3=D.K.|last4=Anderson|first4=J.G.|last5=Salawitch|first5=R.J.|title=Ozone depletion following future volcanic eruptions|journal=Geophysical Research Letters|volume=44|issue=14|pages=7490–7499|year=2017|doi=10.1002/2017GL073972|bibcode=2017GeoRL..44.7490E |doi-access=free}}</ref>  The effects of volcanic eruptions containing sulfur dioxide aerosols on the ozone layer are complex, however.  In the absence of anthropogenic or biogenic halogenated compounds in the lower stratosphere, depletion of [[dinitrogen pentoxide]] in the middle stratosphere associated with its reactivity to the aerosols can promote ozone formation.<ref name=klobas2017/>  Injection of sulfur dioxide and large amounts of water vapor into the stratosphere following the [[2022 Hunga Tonga–Hunga Haʻapai eruption and tsunami|2022 eruption of Hunga Tonga-Hunga Haʻapai]] resulted in altered atmospheric circulation that promoted a decrease in ozone in the southern latitudes but an increase in the tropics.<ref>{{cite web|url=https://www.chemistry.harvard.edu/news/new-research-massive-2022-eruption-reduced-ozone-levels|title=New research: Massive 2022 eruption reduced ozone levels|website=Department of Chemistry and Chemical Biology|publisher=Harvard University|date=21 November 2023|access-date=7 January 2025}}</ref><ref>{{cite journal|last1=Wilmouth|first1=D.M.|last2=Østerstrøm|first2=F.F.|last3=Smith|first3=J.B.|last4=Anderson|first4=J.G.|last5=Salawitch|first5=R.J.|title=Impact of the Hunga Tonga volcanic eruption on stratospheric composition|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=120|issue=46|article-number=e2301994120|doi=10.1073/pnas.2301994120|doi-access=free|year=2023|pages=e2301994120 |pmid=37903247 |pmc=10655571 |bibcode=2023PNAS..12001994W }}</ref>  The additional presence of hydrochloric acid in eruptions can result in net ozone depletion.<ref name=klobas2017/>

===Impact on climate change===
{{excerpt|Global dimming#History|paragraph=2}}
{{excerpt|Global dimming#Causes|paragraph=1|hat=no|files=no}}

====Projected impacts====
[[File:Physical Drivers of climate change.svg|thumb|upright=1.35|The extent to which physical factors in the atmosphere or on land affect [[climate change]], including the cooling provided by sulfate aerosols and the dimming they cause. The large [[error bar]] shows that there are still substantial unresolved uncertainties.]]
{{excerpt|Global dimming#Future|paragraphs=1,3|hat=no|files=no}}

====Solar geoengineering====
[[File:SPICE SRM overview.jpg|thumb|upright=1.5|alt=refer to caption and image description|Proposed tethered balloon to inject [[aerosols]] into the stratosphere]]
As the real world had shown the importance of sulfate aerosol concentrations to the global climate, research into the subject accelerated.  Formation of the aerosols and their effects on the atmosphere can be studied in the lab, with methods like [[Ion chromatography|ion-chromatography]] and [[mass spectrometry]]<ref>{{Cite journal |last1=Kobayashi |first1=Yuya |last2=Ide |first2=Yu |last3=Takegawa |first3=Nobuyuki |date=3 April 2021 |title=Development of a novel particle mass spectrometer for online measurements of refractory sulfate aerosols |journal=Aerosol Science and Technology |volume=55 |issue=4 |pages=371–386 |doi=10.1080/02786826.2020.1852168 |bibcode=2021AerST..55..371K |s2cid=229506768 |issn=0278-6826 |url=https://figshare.com/articles/journal_contribution/13275230 }}</ref> Samples of actual particles can be recovered from the [[stratosphere]] using balloons or aircraft,<ref>{{cite journal |url=https://www.researchgate.net/publication/234296252 |journal=Societa Astronomica Italiana |title=The DUSTER experiment: collection and analysis of aerosol in the high stratosphere |author1=Palumbo, P. |author2= A. Rotundi |author3=V. Della Corte |author4=A. Ciucci |author5=L. Colangeli |author6=F. Esposito |author7=E. Mazzotta Epifani |author8=V. Mennella |author9=J.R. Brucato |author10=F.J.M. Rietmeijer |author11=G. J. Flynn |author12=J.-B. Renard   |author13=J.R. Stephens |author14=E. Zona |access-date=19 February 2009 }}</ref> and remote [[satellite]]s were also used for observation.<ref name="Myhre-2004">{{Cite journal |last1=Myhre |first1=Gunnar |last2=Stordal |first2=Frode |last3=Berglen |first3=Tore F. |last4=Sundet |first4=Jostein K. |last5=Isaksen |first5=Ivar S. A. |date=1 March 2004 |title=Uncertainties in the Radiative Forcing Due to Sulfate Aerosols |journal=Journal of the Atmospheric Sciences |language=EN |volume=61 |issue=5 |pages=485–498 |doi=10.1175/1520-0469(2004)061<0485:UITRFD>2.0.CO;2 |bibcode=2004JAtS...61..485M |s2cid=55623817 |issn=0022-4928|doi-access=free }}</ref> This data is fed into the [[climate model]]s,<ref>{{Cite journal |last1=Zhang |first1=Jie |last2=Furtado |first2=Kalli |last3=Turnock |first3=Steven T. |last4=Mulcahy |first4=Jane P. |last5=Wilcox |first5=Laura J. |last6=Booth |first6=Ben B. |last7=Sexton |first7=David |last8=Wu |first8=Tongwen |last9=Zhang |first9=Fang |last10=Liu |first10=Qianxia |date=22 December 2021 |title=The role of anthropogenic aerosols in the anomalous cooling from 1960 to 1990 in the CMIP6 Earth system models |url=https://acp.copernicus.org/articles/21/18609/2021/ |journal=Atmospheric Chemistry and Physics |volume=21 |issue=4 |pages=18609–18627 |language=en |doi=10.5194/acp-21-18609-2021 |bibcode=2021ACP....2118609Z |doi-access=free }}</ref> as the necessity of accounting for aerosol cooling to truly understand the rate and evolution of warming had long been apparent, with the [[IPCC Second Assessment Report]] being the first to include an estimate of their impact on climate, and every major model able to simulate them by the time [[IPCC Fourth Assessment Report]] was published in 2007.<ref>{{cite web|url=https://earthobservatory.nasa.gov/features/Aerosols/page3.php|title=Aerosols and Incoming Sunlight (Direct Effects)|publisher=[[NASA]]|date=2 November 2010}}</ref> Many scientists also see the other side of this research, which is learning how to cause the same effect artificially.<ref>{{cite web |url=https://www.sciencedaily.com/releases/2006/09/060914182715.htm |title=Stratospheric Injections Could Help Cool Earth, Computer Model Shows | access-date=19 February 2009 |publisher=ScienceDaily |date=15 September 2006 }}</ref> While discussed around the 1990s, if not earlier,<ref>{{cite journal |journal=Phil. Trans. R. Soc. A |year=1996 |volume=366 |pages=4039–56 |title=Global and Arctic climate engineering: numerical model studies |doi=10.1098/rsta.2008.0132 |author1=Launder B. |author2=J.M.T. Thompson |pmid=18757275 |issue=1882 |bibcode=2008RSPTA.366.4039C|doi-access=free }}</ref> stratospheric aerosol injection as a [[solar geoengineering]] method is best associated with [[Paul Crutzen]]'s detailed 2006 proposal.<ref name="Crutzen062">{{Cite journal|last1=Crutzen|first1=P. J.|year=2006|title=Albedo Enhancement by Stratospheric Sulfur Injections: A Contribution to Resolve a Policy Dilemma?|journal=Climatic Change|volume=77|issue=3–4|pages=211–220|article-number=211 |bibcode=2006ClCh...77..211C|doi=10.1007/s10584-006-9101-y|doi-access=free}}</ref> Deploying in the stratosphere ensures that the aerosols are at their most effective, and that the progress of clean air measures would not be reversed: more recent research estimated that even under the highest-emission scenario [[Representative Concentration Pathway|RCP 8.5]],  the addition of stratospheric sulfur required to avoid {{convert|4|C-change|F-change}} relative to now (and {{convert|5|C-change|F-change}} relative to the preindustrial) would be effectively offset by the future controls on tropospheric sulfate pollution, and the amount required would be even less for less drastic warming scenarios.<ref name="Visioni2020">{{Cite journal|last1=Visioni|first1=Daniele|last2=Slessarev|first2=Eric |last3=MacMartin|first3=Douglas G|last4=Mahowald|first4=Natalie M|last5=Goodale|first5=Christine L|last6=Xia|first6=Lili|date=1 September 2020|title=What goes up must come down: impacts of deposition in a sulfate geoengineering scenario|journal=Environmental Research Letters|volume=15|issue=9|page=094063|doi=10.1088/1748-9326/ab94eb|bibcode=2020ERL....15i4063V|issn=1748-9326|doi-access=free}}</ref> This spurred a detailed look at its costs and benefits,<ref>{{cite web |url=http://www.met.reading.ac.uk/pg-research/downloads/2009/pgr-charlton.pdf |title=Costs and benefits of geo-engineering in the Stratosphere |author1=Andrew Charlton-Perez |author2=Eleanor Highwood |access-date=17 February 2009 |archive-date=14 January 2017 |archive-url=https://web.archive.org/web/20170114032949/http://www.met.reading.ac.uk/pg-research/downloads/2009/pgr-charlton.pdf }}</ref> but even with hundreds of studies into the subject completed by the early 2020s, some notable uncertainties remain.<ref name="IPCC_WGI_SRM" >{{Cite journal |last1=Trisos |first1=Christopher H. |last2=Geden |first2=Oliver |last3=Seneviratne |first3=Sonia I. |last4=Sugiyama |first4=Masahiro |last5=van Aalst |first5=Maarten |last6=Bala |first6=Govindasamy |last7=Mach |first7=Katharine J. |last8=Ginzburg |first8=Veronika |last9=de Coninck |first9=Heleen |last10=Patt |first10=Anthony |title=Cross-Working Group Box SRM: Solar Radiation Modification |url=https://www.ipcc.ch/report/ar6/wg2/downloads/report/IPCC_AR6_WGII_Chapter16.pdf |journal=Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change |year=2021 |volume=2021 |page=1238 |doi=10.1017/9781009157896.007|bibcode=2021AGUFM.U13B..05K }}</ref>

== Properties ==
Table of thermal and physical properties of saturated liquid sulfur dioxide:<ref>{{Cite book |last=Holman |first=Jack P. |title=Heat Transfer |publisher=McGraw-Hill Companies, Inc. |year=2002 |isbn=978-0-07-240655-9 |edition=9th |location=New York, NY |pages=600–606 |language=English}}</ref><ref>{{Cite book |last1=Incropera |last2=Dewitt |last3=Bergman |last4=Lavigne |first1=rank P. |first2=David P. |first3=Theodore L. |first4=Adrienne S. |title=Fundamentals of Heat and Mass Transfer |publisher=John Wiley and Sons, Inc. |year=2007 |isbn=978-0-471-45728-2 |edition=6th |location=Hoboken, NJ |pages=941–950 |language=English}}</ref>
{|class="wikitable mw-collapsible"
|Temperature (°C)
|Density (kg/m^3)
|Specific heat (kJ/kg K)
|Kinematic viscosity (m^2/s)
|Conductivity (W/m K)
|Thermal diffusivity (m^2/s)
|Prandtl Number
|Bulk modulus (K^-1)
|-
| −50
|1560.84
|1.3595
|4.84E-07
|0.242
|1.14E-07
|4.24
| –
|-
| −40
|1536.81
|1.3607
|4.24E-07
|0.235
|1.13E-07
|3.74
| –
|-
| −30
|1520.64
|1.3616
|3.71E-07
|0.23
|1.12E-07
|3.31
| –
|-
| −20
|1488.6
|1.3624
|3.24E-07
|0.225
|1.11E-07
|2.93
| –
|-
| −10
|1463.61
|1.3628
|2.88E-07
|0.218
|1.10E-07
|2.62
| –
|-
|0
|1438.46
|1.3636
|2.57E-07
|0.211
|1.08E-07
|2.38
| –
|-
|10
|1412.51
|1.3645
|2.32E-07
|0.204
|1.07E-07
|2.18
| –
|-
|20
|1386.4
|1.3653
|2.10E-07
|0.199
|1.05E-07
|2
|1.94E-03
|-
|30
|1359.33
|1.3662
|1.90E-07
|0.192
|1.04E-07
|1.83
| –
|-
|40
|1329.22
|1.3674
|1.73E-07
|0.185
|1.02E-07
|1.7
| –
|-
|50
|1299.1
|1.3683
|1.62E-07
|0.177
|9.99E-08
|1.61
| –
|}

==See also==
* [[Bunker fuel]]
* [[National Ambient Air Quality Standards]]
* [[Sulfur trioxide]]
* [[Sulfur–iodine cycle]]

==References==
{{Reflist|30em}}

==External links==
{{Commons category|Sulfur dioxide|lcfirst=yes}}
* [https://earth.nullschool.net/#current/chem/surface/level/overlay=so2smass/winkel3 Global map of sulfur dioxide distribution]
* [https://www.epa.gov/so2-pollution United States Environmental Protection Agency Sulfur Dioxide page]
* [https://inchem.org/documents/icsc/icsc/eics0074.htm International Chemical Safety Card 0074]
* [https://web.archive.org/web/20090325155828/http://monographs.iarc.fr/ENG/Monographs/vol54/volume54.pdf IARC Monographs. "Sulfur Dioxide and some Sulfites, Bisulfites and Metabisulfites". vol. 54. 1992. p. 131.]
* [https://www.cdc.gov/niosh/npg/npgd0575.html NIOSH Pocket Guide to Chemical Hazards]
* [https://www.cdc.gov/niosh/topics/SulfurDioxide/ CDC – Sulfure Dioxide – NIOSH Workplace Safety and Health Topic]
* [http://www.chm.bris.ac.uk/motm/so2/so2h.htm Sulfur Dioxide, Molecule of the Month]

{{Oxides}}
{{Molecules detected in outer space}}
{{sulfur compounds}}
{{Authority control}}

[[Category:Acidic oxides]]
[[Category:IARC Group 3 carcinogens]]
[[Category:Industrial gases]]
[[Category:Interchalcogens]]
[[Category:Preservatives]]
[[Category:Refrigerants]]
[[Category:Airborne pollutants]]
[[Category:Sulfur oxides]]
[[Category:Gaseous signaling molecules]]
[[Category:Trace gases]]
[[Category:Triatomic molecules]]
[[Category:Reducing agents]]
[[Category:Inorganic solvents]]
[[Category:Hypervalent molecules]]
[[Category:E-number additives]]
[[Category:Sulfur(IV) compounds]]