Sulfur dioxide (IUPAC-recommended spelling) orsulphur dioxide (traditionalCommonwealth English) is thechemical compound with the formulaSO 2. It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is released naturally byvolcanic activity and is produced as a by-product of metals refining and the burning ofsulfur-bearing fossil fuels.[9]
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 medievalalchemists as "volatile spirit of sulfur".[10]
SO2 is a bent molecule withC2vsymmetry point group.Avalence bond theory approach considering justs andp orbitals would describe the bonding in terms ofresonance between two resonance structures.
Two resonance structures of sulfur dioxide
The sulfur–oxygen bond has abond order of 1.5. There is support for this simple approach that does not invoked orbital participation.[11]In terms ofelectron-counting formalism, the sulfur atom has anoxidation state of +4 and aformal charge of +1.
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 15ppb.[12]
On other planets, sulfur dioxide can be found in various concentrations, the most significant being theatmosphere 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 (SO2 +H2O ⇌HSO−3 +H+), and is a key component of the planet's global atmosphericsulfur cycle. It has been implicated as a key agent in the warming of earlyMars, with estimates of concentrations in the lower atmosphere as high as 100 ppm,[13] though it only exists in trace amounts. On both Venus and Mars, as on Earth, its primary source is thought to be volcanic. Theatmosphere of Io, a natural satellite ofJupiter, is 90% sulfur dioxide[14] and trace amounts are thought to also exist in theatmosphere of Jupiter. TheJames Webb Space Telescope has observed the presence of sulfur dioxide on theexoplanetWASP-39b, where it is formed throughphotochemistry in the planet's atmosphere.[15]
As an ice, it is thought to exist in abundance on theGalilean moons—as subliming ice or frost on the trailing hemisphere ofIo,[16] and in the crust and mantle ofEuropa,Ganymede, andCallisto, possibly also in liquid form and readily reacting with water.[17]
Sulfur dioxide is primarily produced forsulfuric acid manufacture (seecontact process, but other processes predated that at least since 16th century[10]). In the United States in 1979, 23.6 million metric tons (26 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 elementalsulfur. Some sulfur dioxide is also produced by roastingpyrite and othersulfide ores in air.[18]
An experiment showing burning of sulfur inoxygen. A flow-chamber joined to a gas washing bottle (filled with a solution ofmethyl orange) is being used. The product is sulfur dioxide (SO2) with some traces ofsulfur trioxide (SO3). The "smoke" that exits the gas washing bottle is, in fact, a sulfuric acid fog generated in the reaction.
Sulfur dioxide is the product of the burning ofsulfur or of burning materials that contain sulfur:
S8 + 8O2 → 8SO2, ΔH = −297 kJ/mol
To aid combustion, liquified sulfur (140–150 °C (284–302 °F) is sprayed through an atomizing nozzle to generate fine drops of sulfur with a large surface area. The reaction isexothermic, and the combustion produces temperatures of 1,000–1,600 °C (1,830–2,910 °F). The significant amount of heat produced is recovered by steam generation that can subsequently be converted to electricity.[18]
The combustion ofhydrogen sulfide and organosulfur compounds proceeds similarly. For example:
A combination of these reactions is responsible for the largest source of sulfur dioxide, volcanic eruptions. These events can release millions of tons of SO2.
Sulfur dioxide can also be a byproduct in the manufacture ofcalcium silicate cement;CaSO4 is heated withcoke and sand in this process:
2CaSO4 + 2SiO2 + C → 2CaSiO3 + 2SO2 +CO2
Until the 1970s commercial quantities of sulfuric acid and cement were produced by this process inWhitehaven, England. Upon being mixed withshale ormarl, and roasted, the sulfate liberated sulfur dioxide gas, used in sulfuric acid production, the reaction also produced calcium silicate, a precursor in cement production.[20]
On a laboratory scale, the action of hot concentrated sulfuric acid on copperturnings produces sulfur dioxide.
Cu + 2H2SO4 →CuSO4 + SO2 + 2 H2O
Tin also reacts with concentrated sulfuric acid but it produces tin(II) sulfate which can later be pyrolyzed at 360 °C into tin dioxide and dry sulfur dioxide.
Sulfur dioxide is theoxidising agent in theClaus process, which is conducted on a large scale inoil refineries. Here, sulfur dioxide is reduced by hydrogen sulfide to give elemental sulfur:
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.
SO2 +H2O +1⁄2O2 →H2SO4
Sulfur dioxide dissolves in water to give "sulfurous acid", which cannot be isolated and is instead an acidic solution ofbisulfite, and possiblysulfite, ions.
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 adichromate solution, turning the solution from orange to green (Cr3+ (aq)). It can also reduce ferric ions to ferrous.[21]
Sulfur dioxide can bind to metal ions as aligand to formmetal sulfur dioxide complexes, 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 pyramidalη1.[9] As a η1-SO2 (S-bonded planar) ligand sulfur dioxide functions as a Lewis base using the lone pair on S. SO2 functions as aLewis acids in its η1-SO2 (S-bonded pyramidal) bonding mode with metals and in its 1:1adducts with Lewis bases such asdimethylacetamide andtrimethyl amine. When bonding to Lewis bases theacid parameters of SO2 are EA = 0.51 and EA = 1.56.
Sulfur dioxide is an intermediate in the production of sulfuric acid, being converted tosulfur trioxide, and then tooleum, 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 thecontact process. Several million tons are produced annually for this purpose.
Sulfur dioxide is sometimes used as a preservative for dried apricots, dried figs, and other dried fruits, owing to itsantimicrobial properties and ability to preventoxidation,[22] and is calledE220[23] when used in this way in Europe. As a preservative, it maintains the colorful appearance of the fruit and preventsrotting. 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.[24] Fruits may be sulfured by dipping them intosodium bisulfite,sodium sulfite orsodium metabisulfite.[24]
Sulfur dioxide was first used inwinemaking by the Romans, when they discovered that burning sulfur candles inside empty wine vessels keeps them fresh and free from vinegar smell.[25]
It is still an important compound in winemaking, and is measured inparts per million (ppm) in wine. It is present even in so-called unsulfurated wine at concentrations of up to 10 mg/L.[26] It serves as anantibiotic andantioxidant, 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.[27][28] Its antimicrobial action also helps minimize volatile acidity. Wines containing sulfur dioxide are typically labeled with "containingsulfites".
Sulfur dioxide exists in wine in free and bound forms, and the combinations are referred to as total SO2. Binding, for instance to the carbonyl group ofacetaldehyde, varies with the wine in question. The free form exists in equilibrium between molecular SO2 (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) SO2, which is the active form, while at higher pH more SO2 is found in the inactive sulfite and bisulfite forms. The molecular SO2 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 SO2 concentrations below 10 ppm do not require "contains sulfites" on the label by US and EU laws. The upper limit of total SO2 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, SO2 is mostly undetectable in wine, but at free SO2 concentrations over 50 ppm, SO2 becomes evident in the smell and taste of wine.[citation needed]
SO2 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 ofcork taint,[29] a mixture of SO2, water, and citric acid is commonly used to clean and sanitize equipment.Ozone (O3) is now used extensively for sanitizing in wineries due to its efficacy, and because it does not affect the wine or most equipment.[30]
Aqueous sulfur dioxide solution is used incorn 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.
Sulfur dioxide is also a goodreductant. In the presence of water, sulfur dioxide is able to decolorize substances. Specifically, it is a useful reducingbleach 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 tochloride.[31]
Sulfur dioxide is fairly soluble in water, and by both IR and Raman spectroscopy; the hypotheticalsulfurous acid, H2SO3, is not present to any extent. However, such solutions do show spectra of the hydrogen sulfite ion, HSO3−, by reaction with water, and it is in fact the actual reducing agent present:
In the beginning of the 20th century sulfur dioxide was used inBuenos Aires as a fumigant to kill rats that carried theYersinia 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 asSulfurozador, 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.[32]
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.[33] Sulfur dioxide blocks nerve signals from thepulmonary stretch receptors and abolishes theHering–Breuer inflation reflex.
It was shown that in children with pulmonary arterial hypertension due to congenital heart diseases the level ofhomocysteine 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.[35]
Endogenous sulfur dioxide also has been shown to lower theproliferation rate of endothelialsmooth muscle cells in blood vessels, via lowering theMAPK activity and activatingadenylyl cyclase andprotein kinase A.[36] Smooth muscle cell proliferation is one of important mechanisms of hypertensive remodeling of blood vessels and theirstenosis, so it is an important pathogenetic mechanism in arterial hypertension and atherosclerosis.
Endogenous sulfur dioxide in low concentrations causes endothelium-dependentvasodilation. 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-dependentcalcium channels 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.[37]
Endogenous sulfur dioxide also diminishes myocardial damage, caused byisoproterenoladrenergic hyperstimulation, and strengthens the myocardial antioxidant defense reserve.[38]
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 inorganic synthesis. Treatment of aryldiazonium salts with sulfur dioxide andcuprous chloride yields the corresponding aryl sulfonyl chloride, for example:[39]
As a result of its very lowLewis basicity, it is often used as a low-temperature solvent/diluent for superacids likemagic acid (FSO3H/SbF5), allowing for highly reactive species liketert-butyl cation to be observed spectroscopically at low temperature (though tertiary carbocations do react with SO2 above about −30 °C, and even less reactive solvents likeSO2ClF must be used at these higher temperatures).[40]
Sulfur dioxide was one of the earliestrefrigerants adopted for mechanical refrigeration owing to its ease of liquefaction and high latentheat of vaporization. In 1784,Jean-François Clouet andGaspard 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-compressorhome refrigerators produced byNorge. Following the introduction of less toxic, non-flammablechlorofluorocarbon (CFC) refrigerants, the use of sulfur dioxide in refrigeration systems gradually declined.[41][42]
Sulfur dioxide content in naturally-released geothermal gasses is measured by theIcelandic Meteorological Office as an indicator of possible volcanic activity.[43]
In the United States, theCenter for Science in the Public Interest lists the two food preservatives, sulfur dioxide andsodium bisulfite, as being safe for human consumption except for certain asthmatic individuals who may be sensitive to them, especially in large amounts.[44] Symptoms of sensitivity tosulfiting agents, including sulfur dioxide, manifest as potentially life-threatening trouble breathing within minutes of ingestion.[45] Sulphites may also cause symptoms in non-asthmatic individuals, namelydermatitis,urticaria,flushing,hypotension,abdominal pain and diarrhea, and even life-threateninganaphylaxis.[46]
Incidental exposure to sulfur dioxide is routine, e.g. the smoke from matches, coal, and sulfur-containing fuels likebunker fuel. Relative to other chemicals, it is only mildly toxic and requires high concentrations to be actively hazardous.[47] However, its ubiquity makes it a major air pollutant with significant impacts on human health.[48]
The effect of majorvolcanic eruptions on sulfate aerosol concentrations and chemical reactions in the atmosphere
Majorvolcanic eruptions have an overwhelming effect on sulfateaerosol concentrations in the years when they occur: eruptions ranking 4 or greater on theVolcanic Explosivity Index injectSO2 and water vapor directly into thestratosphere, where they react to create sulfate aerosol plumes.[50] 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. Onlystratovolcanoes containing primarilyfelsic magmas are responsible for these fluxes, asmafic magma erupted inshield volcanoes doesn't result in plumes which reach the stratosphere.[51] However, before theIndustrial Revolution, dimethyl sulfide pathway was the largest contributor to sulfate aerosol concentrations in a more average year with no major volcanic activity. According to theIPCC First Assessment Report, published in 1990, volcanic emissions usually amounted to around 10 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" asall natural emissions of sulfur-containing compoundscombined: they were at less than 3 million tons per year in 1860, and then they increased to 15 million tons in 1900, 40 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".[52] In the eastern United States in the early 2000s, sulfate particles were estimated to account for 25% or more of allair pollution.[53] Exposure to sulfur dioxide emissions by coal power plants (coal PM2.5) in the US was associated with 2.1 times greater mortality risk than exposure to PM2.5 from all sources.[54] Meanwhile, theSouthern 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 theNorthern Hemisphere, where only 16% of annual sulfur emissions were natural, yet amounted for less than half of the emissions in the Southern Hemisphere.[55]
Such an increase in sulfate aerosol emissions had a variety of effects. At the time, the most visible one wasacid rain, caused byprecipitation from clouds carrying high concentrations of sulfate aerosols in thetroposphere.[56]At its peak, acid rain has eliminatedbrook trout and some other fish species and insect life from lakes and streams in geographically sensitive areas, such asAdirondack Mountains in the United States.[57] Acid rain worsenssoil function as some of itsmicrobiota is lost and heavy metals like aluminium are mobilized (spread more easily) while essential nutrients and minerals such asmagnesium can leach away because of the same. Ultimately, plants unable to tolerate loweredpH are killed, with montane forests being some of the worst-affectedecosystems due to their regular exposure to sulfate-carrying fog at high altitudes.[58][59][60][61][62] 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 toheart andlung conditions, includingasthma andbronchitis.[53] Further, this form of pollution is linked topreterm birth andlow birth weight, with a study of 74,671 pregnant women in Beijing finding that every additional 100 μg/m3 ofSO2 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.[63]
Early 2010s estimates of past and future anthropogenic global sulfur dioxide emissions, including theRepresentative Concentration Pathways. While noclimate 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.[64]
Due largely to the US EPA'sAcid Rain Program, the U.S. has had a 33% decrease in emissions between 1983 and 2002 (see table). This improvement resulted in part fromflue-gas desulfurization, a technology that enables SO2 to be chemically bound inpower plants burning sulfur-containing coal or petroleum.
Aerobic oxidation of the CaSO3 gives CaSO4,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.[65] Sulfur can be removed from coal during burning by using limestone as a bed material influidized bed combustion.[66]
Sulfur can also be removed from fuels before burning, preventing formation of SO2 when the fuel is burnt. TheClaus process is used in refineries to produce sulfur as a byproduct. TheStretford process has also been used to remove sulfur from fuel.Redox processes using iron oxides can also be used, for example, Lo-Cat[67] or Sulferox.[68]
Fuel additives such ascalcium additives and magnesium carboxylate may be used in marine engines to lower the emission of sulfur dioxide gases into the atmosphere.[69]
Sulfur dioxide aerosols in the stratosphere can contribute toozone depletion in the presence of chlorofluorocarbons and other halogenated ozone-depleting substances.[70] 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 ofdinitrogen pentoxide in the middle stratosphere associated with its reactivity to the aerosols can promote ozone formation.[70] Injection of sulfur dioxide and large amounts of water vapor into the stratosphere following the2022 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.[71][72] The additional presence of hydrochloric acid in eruptions can result in net ozone depletion.[70]
The observed trends of global dimming and brightening in four major geographic regions. The dimming was greater on the average cloud-free days (red line) than on the average of all days (purple line), strongly suggesting that sulfate aerosols were the cause.[73]Subsequent research estimated an average reduction in sunlight striking the terrestrial surface of around 4–5% per decade over the late 1950s–1980s, and 2–3% per decade when 1990s were included.[74][75][76][77] Notably, solar radiation at the top of the atmosphere did not vary by more than 0.1-0.3% in all that time, strongly suggesting that the reasons for the dimming were on Earth.[78][79] Additionally, only visible light andinfrared radiation were dimmed, rather than theultraviolet part of the spectrum.[80] Further, the dimming had occurred even when the skies were clear, and it was in fact stronger than during the cloudy days, proving that it was not caused by changes in cloud cover alone.[81][79][73]
The extent to which physical factors in the atmosphere or on land affectclimate change, including the cooling provided by sulfate aerosols and the dimming they cause. The largeerror bar shows that there are still substantial unresolved uncertainties.
Since changes in aerosol concentrations already have an impact on the global climate, they would necessarily influence future projections as well. In fact, it is impossible to fully estimate the warming impact of allgreenhouse gases without accounting for the counteracting cooling from aerosols.[82][83]
Regardless of the current strength of aerosol cooling, all futureclimate change scenarios project decreases in particulates and this includes the scenarios where 1.5 °C (2.7 °F) and 2 °C (3.6 °F) targets are met: their specific emission reduction targets assume the need to make up for lower dimming.[84] Since models estimate that the cooling caused by sulfates is largely equivalent to the warming caused byatmospheric methane (and since methane is a relatively short-lived greenhouse gas), it is believed that simultaneous reductions in both would effectively cancel each other out.[85]
[86] Yet, in the recent years, methane concentrations had been increasing at rates exceeding their previous period of peak growth in the 1980s,[87][88] withwetland methane emissions driving much of the recent growth,[89][90] while air pollution is getting cleaned up aggressively.[91] These trends are some of the main reasons why 1.5 °C (2.7 °F) warming is now expected around 2030, as opposed to the mid-2010s estimates where it would not occur until 2040.[82]
Proposed tethered balloon to injectaerosols 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 likeion-chromatography andmass spectrometry[92] Samples of actual particles can be recovered from thestratosphere using balloons or aircraft,[93] and remotesatellites were also used for observation.[94] This data is fed into theclimate models,[95] as the necessity of accounting for aerosol cooling to truly understand the rate and evolution of warming had long been apparent, with theIPCC 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 timeIPCC Fourth Assessment Report was published in 2007.[96] Many scientists also see the other side of this research, which is learning how to cause the same effect artificially.[97] While discussed around the 1990s, if not earlier,[98] stratospheric aerosol injection as asolar geoengineering method is best associated withPaul Crutzen's detailed 2006 proposal.[99] 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 scenarioRCP 8.5, the addition of stratospheric sulfur required to avoid 4 °C (7.2 °F) relative to now (and 5 °C (9.0 °F) 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.[100] This spurred a detailed look at its costs and benefits,[101] but even with hundreds of studies into the subject completed by the early 2020s, some notable uncertainties remain.[102]
^Cunningham, Terence P., Cooper, David L., Gerratt, Joseph, Karadakov, Peter B., Raimondi, Mario (1997). "Chemical bonding in oxofluorides of hypercoordinatesulfur".Journal of the Chemical Society, Faraday Transactions.93 (13):2247–2254.doi:10.1039/A700708F.
^Guerrero RF, Cantos-Villar E (2015). "Demonstrating the efficiency of sulphur dioxide replacements in wine: A parameter review".Trends in Food Science & Technology.42 (1):27–43.doi:10.1016/j.tifs.2014.11.004.
^Liu D, Jin H, Tang C, Du J (2010). "Sulfur Dioxide: a Novel Gaseous Signal in the Regulation of Cardiovascular Functions".Mini-Reviews in Medicinal Chemistry.10 (11):1039–1045.doi:10.2174/1389557511009011039.PMID20540708.
^Yang R, Yang Y, Dong X, Wu X, Wei Y (August 2014). "Correlation between endogenous sulfur dioxide and homocysteine in children with pulmonary arterial hypertension associated with congenital heart disease".Zhonghua Er Ke Za Zhi (in Chinese).52 (8):625–629.PMID25224243.
^Wang XB, Jin HF, Tang CS, Du JB (November 16, 2011). "The biological effect of endogenous sulfur dioxide in the cardiovascular system".Eur J Pharmacol.670 (1):1–6.doi:10.1016/j.ejphar.2011.08.031.PMID21925165.
^Olah GA, Lukas J (August 1, 1967). "Stable carbonium ions. XLVII. Alkylcarbonium ion formation from alkanes via hydride (alkide) ion abstraction in fluorosulfonic acid-antimony pentafluoride-sulfuryl chlorofluoride solution".Journal of the American Chemical Society.89 (18):4739–4744.Bibcode:1967JAChS..89.4739O.doi:10.1021/ja00994a030.ISSN0002-7863.
^Lindeburg MR (2006).Mechanical Engineering Reference Manual for the PE Exam. Belmont, C.A.: Professional Publications, Inc. pp. 27–3.ISBN978-1-59126-049-3.
^Eddy JA, Gilliland RL, Hoyt DV (December 23, 1982). "Changes in the solar constant and climatic effects".Nature.300 (5894):689–693.Bibcode:1982Natur.300..689E.doi:10.1038/300689a0.S2CID4320853.Spacecraft measurements have established that the total radiative output of the Sun varies at the 0.1−0.3% level
^Bellouin N, Quaas J, Gryspeerdt E, Kinne S, Stier P, Watson-Parris D, Boucher O, Carslaw KS, Christensen M, Daniau AL, Dufresne JL, Feingold G, Fiedler S, Forster P, Gettelman A, Haywood JM, Lohmann U, Malavelle F, Mauritsen T, McCoy DT, Myhre G, Mülmenstädt J, Neubauer D, Possner A, Rugenstein M, Sato Y, Schulz M, Schwartz SE, Sourdeval O, Storelvmo T, Toll V, Winker D, Stevens B (November 1, 2019)."Bounding Global Aerosol Radiative Forcing of Climate Change".Reviews of Geophysics.58 (1) e2019RG000660.doi:10.1029/2019RG000660.PMC7384191.PMID32734279.
^Holman JP (2002).Heat Transfer (9th ed.). New York, NY: McGraw-Hill Companies, Inc. pp. 600–606.ISBN978-0-07-240655-9.
^Incropera rP, Dewitt DP, Bergman TL, Lavigne AS (2007).Fundamentals of Heat and Mass Transfer (6th ed.). Hoboken, NJ: John Wiley and Sons, Inc. pp. 941–950.ISBN978-0-471-45728-2.
