The atmospheric chemistry of hydrogen cyanide (HCN)

R J Cicerone,R Zellner

doi:10.1029/jc088ic15p10689

Abstract

Since 1981, three groups have reported spectroscopic detections and measurements of hydrogen cyanide in the atmosphere. HCN concentrations (volume mixing ratios) of (1.5–1.7) × 10−10 appear to characterize the stratosphere and the northern hemisphere's nonurban troposphere. In this paper we explore the atmospheric behavior of HCN by examining its chemical and photochemical properties. Its principal sinks are reactions with atmospheric OH and O(1D); precipitation appears to be a negligible sink. In the stratosphere, vacuum UV photons also attack HCN. Atmospheric model calculations show that HCN should be relatively well mixed in the troposphere and that its concentration decreases slowly with altitude in the stratosphere. Its atmospheric residence time appears to be about 2.5 years, although 1–5 years is a possible range. To maintain the observed atmospheric burden of HCN, an annual source of about 2 × 10−11 g nitrogen as HCN is required; we speculate as to the identity of these sources. Oxidation of HCN by OH, while the major sink for atmospheric HCN, is not simple or direct. Instead, oxidation proceeds from the HCN·OH adduct formed in HCN + OH reactions. These pathways and their uncertainties are outlined here.

Highlights

HCN was first seenin the earth'satmosphereby Coffeyet al. [1981]
HCN is a stronglyboundmoleculeI.ts bonddissociation km altitude.They state that the HCN mixing ratio was (1.3- energyD hasbeendeterminebdy DavisandOkabe[1968]; 2.6)x 10-ø
Becausethe earth'satmosphereis oxidizing,reasonsfor the dissociation for /l < 178.5 nm (6.93 eV), they presenceof HCN are not entirelyobvious.Questionsas to the admitted that weak predissociationabsorption might have sourcesof this compoundand the rate of oxidation arise im- escapedtheir detection.In their research,Herzbergand Innes mediately.Further, it is important to know if the oxidation of found that

Summary

National Centerfor AtmosphericResearch

Since1981,threegroupshavereportedspectroscopidcetectionsand measurementosf hydrogencyanidein the atmosphereH.CN concentration(svolumemixingratios)of (1.5-1.7)x 10-ø appearto characterizethe stratospheraend the northernhemisphere'nsonurbantroposphereI.n this paperwe explorethe atmosphericbehaviorof HCN by examiningits chemicaland photochemicapl ropertiesI.ts principasl inksarereactionws ithatmospheriOcH andO(D);precipitatioanppearsto bea negligible sink.In the stratospherev,acuumUV photonsalsoattack HCN. Oxidationof HCN by OH, whilethe major sinkfor atmosphericHCN, is not simpleor direct.Instead, oxidationproceedsfrom the HCN-OH adductformedin HCN + OH reactions.Thesepathwaysand their uncertainties are outlined here

INTRODUCTION

Let usproceednow to examinepossiblegasphasereactions

HO ß

HCN MOLE FRACTION

Findings

CICERONEAND ZELLNER:ATMOSPHERICCHEMISTRYOF HYDROGENCYANIDE