UC Irvine UC Irvine Previously Published Works Title The atmospheric chemistry of hydrogen cyanide (HCN) Permalink https://escholarship.org/uc/item/98d9x8wp Journal Journal of Geophysical Research, 88(C15) ISSN 0148-0227 Authors Cicerone, RJ Zellner, R Publication Date 1983 DOI 10.1029/JC088iC15p10689 License https://creativecommons.org/licenses/by/4.0/ 4.0 Peer reviewed eScholarship.org Powered by the California Digital Library University of California JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 88, NO. C15, PAGES 10,689-10,696, DECEMBER 20, 1983 The AtmosphericChemistry of Hydrogen Cyanide (HCN) R. J. CICERONE National Centerfor AtmosphericResearch R. ZELLNER UniversitdtGSttin•Ten, lnstitut fur PhysikalischeChemie Since 1981,three groupshave reportedspectroscopic detections and measurementsof hydrogency- anidein the atmosphere.HCN concentrations(volume mixing ratios) of (1.5-1.7)x 10-•ø appearto characterizethe stratosphereand the northernhemisphere's nonurban troposphere. In this paper we explorethe atmosphericbehavior of HCN by examiningits chemicaland photochemicalproperties. Its principalsinks are reactionswith atmosphericOH and O(•D); precipitationappears to be a negligible sink. In the stratosphere,vacuum UV photonsalso attack HCN. Atmosphericmodel calculationsshow that HCN shouldbe relativelywell mixedin the troposphereand that its concentrationdecreases slowly with altitudein the stratosphere.Its atmosphericresidence time appearsto be about 2.5 years,although 1-5 yearsis a possiblerange. To maintain the observedatmospheric burden of HCN, an annual source of about 2 x 10TM g nitrogenas HCN is required;we speculateas to the identityof thesesources. Oxidation of HCN by OH, while the major sink for atmosphericHCN, is not simpleor direct.Instead, oxidation proceedsfrom the HCN-OH adduct formed in HCN + OH reactions.These pathways and their uncertainties are outlined here. 1. INTRODUCTION be explored.It is alsoof interestto knowwhether the avail- HCN was first seenin the earth's atmosphereby Coffey et able HCN measurementsshed any light on the origin and al. [1981]. They employed infrared absorption instrumenta- behaviorof stratosphericCH3CN detectedby Arnoldet al. tion aboard an aircraft that flew between 5øN and 50øN lati- [1978],Arijs et al. [1980],and B6hrinqer and Arnold [1981]. tude. Coffey et al. measuredthe total amount of HCN above In thispaper we explore the atmospheric chemistry of HCN 12 km altitude; their vertical column was 6.5 x 10•½ cm-2. by evaluatingits propertiesand availablekinetic and thermo- Assuming that HCN is distributed uniformly with altitude dynamicdata; this discussion comprises section 2. Section3 above 12 km, they deduced a stratosphericmixing ratio of presentsour atmosphericmodel calculations and estimates of 1.55x 10-•ø by volume.These corrected HCN amountsare HCN verticalprofiles, source strengths, and atmosphericresi- slightly lower (M. T. Coffey, private communication, 1982) dence times. than those reported by Coffey et al. [1981]. Similarly, Carli et 2. ATMOSPHERICCHEMISTRY OF HCN al. [1982] discoveredHCN by examiningstratospheric micro- wave emissionspectra. They obtained data between20 and 40 HCN is a stronglybound molecule. Its bond dissociation km altitude. They state that the HCN mixing ratio was (1.3- energyD hasbeen determined by Davisand Okabe[1968]; 2.6) x 10-•ø. More recently,Rinsland et al. [1982] detected D(H-CN) = 5.20 eV or 120 kcal/mol.This D value corre- HCN in the troposphere. With IR absorption measurements spondsto a long-wavelengthcutoff of 238.4nm for photo- from Kitt Peak (altitude = 2.095 kin), they found vertical dissociation.In reality, the dissociationof HCN at wave- columnamounts of 2.73x 10•5 cm-2, yieldingan average lengthsnear 238 nm has not been observed.Instead, Hilgen- troposphericmixing ratio of 1.66x 10-•0 between2.1 and 12 dorff [1935] observeda regular band seriesabsorption be- km. They were also able to estimatestratospheric H CN con- tween 178 and 200 nm, with an onset to continuous absorp- centrations. tion below 178 nm. Herzberg and Innes [1957] observedpre- Becausethe earth's atmosphereis oxidizing,reasons for the dissociation for /l < 178.5 nm (6.93 eV), although they presenceof HCN are not entirely obvious.Questions as to the admitted that weak predissociationabsorption might have sourcesof this compound and the rate of oxidation arise im- escapedtheir detection.In their research,Herzberg and Innes mediately. Further, it is important to know if the oxidation of found that HCN representsa significant in situ source of gaseousni- HCN + hv• H(2S)+ CN(2•) trogen oxides or of NO3- and CN- in precipitation and whether atmospheric HCN can interfere with attempts to the 2n stateof CN is 1.15eV abovethe X2E groundstate. In measureNOz or peroxyacetylnitrate, for example,by conver- Davis and Okabe's [1968] work, they actually excited HCN sion to NO followed by chemiluminescentdetection of NO. At with much shorter wavelengthsand observedfluorescence of conc•ntrauons Ol I tu ppt, za•'• concentrationscan exceed CN(B2y•),an excitedstate ot c;• mat is .•.zue V aboveground. those of NO and NO2 in the background troposphere[see There are no available data on photodissociationof HCN for Kley et al., 1981], so the possibilitiesmentioned above should 175 nm</l < 238 nm, a wavelengthregion of some interest for the stratospherebut of nn con•qequencefor tropospheric HCN. The strong H-CN bond cannot be broken by photons This paper is not subjectto U.S. copyright.Published in 1983 by available in the troposphereor lower stratosphere.In our at- the AmericanGeophysical Union. mosphericmodel calculationswe calculatedHCN photolysis Paper number3C1570. rates as described in section 3. 10,689 10,690 CICERONEAND ZELLNER' ATMOSPHERICCHEMISTRY OF HYDROGENCYANIDE TABLE 1. Enthalpyof ReactionAH• for PossibleReactions of electronshell reactants exhibit relativelylarge activationener- HCN With 14 AtmosphericGases gies[see, e.g., Kneba and 'Wolfrum, 1980]. Reactants • Products AHs, kcal/mol In other casesfrom Table 2, for example,HCN + O3-• HOCN + 02, while one can imagine reasonablepathways H CN + O2--• H OCN + O +14 like • HNCO + O +2O HCN + OH--• HOCN + H >+2 HCN + 03--} HOCN + 02 HCN + O3--• OH + CN + 02 +43 HCN + HO: • H 2 + 02 + CN +66 -• H202 + CN +35 primaryattacks on a closedelectron shell molecule (HCN) are HCN + H:O: • H 2 + HO 2 + CN + 102 required.For example,the reactionof CO (isoelectronicwith HCN + NO 2--• HNO 2 + CN +43 HCN) and 03 is unmeasureablyslow (k < 10-23 cm3/ +94 HCN + HNO: • H: + NO 2 + CN molecules). In severalof the possiblereactions listed in Table HCN + CH:O--, H: + CN + CHO + 104 HCN + SO 2 • NCO + HSO +101 2, extensiverearrangements are alsorequired. Finally, thereis HCN + O• OH + CN +19 no available evidencethat any of the possiblereactions in H CN + C1--• H C1 + CN +18 Table 2 actually occur exceptfor one of the pathwayswith HCN + H • H 2 + CN +17 OH. HCN + HC1---•H 2 + CICN +23 HCN + CO• CHO + CN + 105 There are two likely atmosphericreactants of significancein the atmosphericchemistry of HCN: O(•D) and OH. Indeed, Heats of formation for these calculations are taken from Okabe the reaction of OH and HCN has been studied in the labora- [1978] and apply for 0øK except for HO 2 for which we took tory [Phillips, 1978, 1979; Fritz et al., 1983; R. Zellner and B. Howard's [1980] value of 2.5 _ 1 kcal/mol and for NCO we adopted 48 kcal/mol from Sullivan et al. [1983]. Note that all these potential Fritz, unpublishedmanuscript, 1983]. From theselaboratory reactionsare endothermic(AHs > 0); rates of these reactionsin the experiments,it appearsthat an adduct is formed; the thermo- atmospherewould be negligiblyslow. chemistryof the possiblereaction pathways is shownin Table 2. The central valuesof AH298ø for HCN, OH, CN, and H20 lead to the conclusion that HCN + OH--}CN + H20 is Contrary to extrapolations from information in several slightly endothermic (A/-/a = + 1.5 kcal/mol). Further, for a popular handbooks,HCN is not very solublein water when hydrogen abstraction reaction like this, BEBO calculations low partial pressures(for example, < 1 torr) of HCN are in indicate that there should be an activationenergy of at least 9 question.Henry's constantKn = P/X, where P is the partial kcal/mol (R. Zellner and B. Fritz, unpublishedmanuscript, pressure(torr) of HCN and X is the mole fraction of HCN in 1983). The experimentsof Fritz et al. [1983] imply that this the liquid, is Kn = 4000 at 18øCand 0.01 atm HCN [Interna- reaction path is not followed at atmospheric temperatures. tional Critical Tables of Numerical Data, 1928]. At the same Further, the exchange reaction path (OH + HCN temperaturethe Ostwald solubility coefficient(see, e.g., Wil- HOCN + H) which has been suggestedto be one of the domi- helm et al., 1977] is about 252 [Linke, 1958]. Further confirm- nant HCN oxidation routes in flames [Haynes, 1977] is en- ing data are found in the works by Lewisand Randall [1923] dothermicby 2.4 kcal/mol, and it is probably preventedby an and Landolt-B6rnstein[1962]. On the basis of this solubility additional dissociation barrier between HCN.OH and and the fact that HCN is a very weak acid HOCN + H. Formation of HNCO + H is less likely than HCNaq•- H + + CN- (K = 1.3x 10-9) HOCN due to an additional isomerizationof the heavy atom moiety. one deducesthat HCN has a very long atmosphericresidence time against rainout. If 1 m of rain falls per unit area each year and it were saturatedwith HCN (with respectto, say,200 TABLE 2. Possible Exothermic Reactions of HCN With