REVIEWS OF GEOPHYSICS AND SPACE PHYSICS, VOL. 19, NO. 1, PAGES 123-139, FEBRUARY 1981 Halogensin the Atmosphere RALPH J. CICERONE National Centerfor AtmosphericResearch, Boulder, Colorado 80307 ScrippsInstitution of Oceanography,University of California,San Diego,La Jolla, CaliJornia92093 Extant data from measurementsof halogensin the atmosphereare reviewed in the following cate- gories:gaseous chlorine compounds (inorganic and organic), particulate chloride and chloridein precipi- tation,gaseous bromine compounds (inorganic and organic),particulate bromide and bromidein precip- itation, gaseousiodine compounds(inorganic and organic), iodine in particlesand in precipitation, gaseousfluorine compounds (inorganic and organic),and fluoridein particlesand precipitation.The roles that thesedata and other unavailabledata play in defining global cyclesof the halogensare dis- cussed.Speciation of the halogengases in the troposphereis very uncertain:the only inorganicspecies detectedby species-specificmethods are HC1 and SF6. More specificdata are availableon organicforms that containhalogens. Key speciesof gaseoushalogens, either establishedor suspectedto be important, are listedalong with key processesthat needinvestigation. Heterogeneous reactions, both gas-to-particle and particle-to-gasprocesses, precipitation removal, and sea-saltaerosol generation and fractionation processesneed quantitative investigationto allow progressin estimating halogen sourcesand sinks. Where practical, as with stratosphericinorganic chlorine gases,quantitative comparisonsare made be- tween measuredand predictedconcentrations. CONTENTS Several practical concernsover halogensin the atmosphere Overviewof researchon halogensin the atmosphere...................... 123 are now demanding quantitative information: stratospheric Thepresent data base ......................................................................... 123 ozone layer photochemicaleffects of C1 and Br (direct 03 de- Introduction .................................................................................... 123 struction and interruption of other reaction chains), infrared Chlorinein the atmosphere............................................................ 123 effects of halocarbons, contributions of HC1 to regionally Brominein the atmosphere............................................................ 127 Iodinein the atmosphere................................................................ 128 acidic precipitation, atmospherictransport of persistentand Fluorinein the atmosphere............................................................ 128 often toxic pesticides(e.g., DDT, 2, 4, 5-T, dioxin), and ques- Keyhalogen species, reactions, and transformations ...................... 129 tions on chemicalintermediates in the troposphericdecompo- Key processesand species: Chlorine ............................................. 129 sition of chlorinated solvents(e.g., chloroethylenes).There is Key processesand species: Bromine ............................................. 132 also interest in the use of man-made halogen compoundsas Key processesand species: Iodine ................................................. 133 Keyprocesses and species: Fluorine ............................................. 134 potent tracersof atmosphericand hydrosphericmotions. Globalcycles of atmospherichalogens ............................................. 134 Pathwaysof chlorinecompounds .................................................. 134 B. THE PRESENT DATA BASE Pathwaysof brominecompounds ................................................. 135 1. Introduction Pathwaysof iodinecompounds ..................................................... 136 Pathwaysof fluorinecompounds ................................................... 136 Our knowledgeof the atmosphericchemistry and the geo- Interactionsbetween halogens and otherchemical cycles ........... 137 chemical cycles of the halogenshas come from researchers A. OVERVIEW OF RESEARCH ON HALOGENS IN THE with varied interestsand investigativetechniques. Thus the ATMOSPHERE scopeand depth of understandingof atmospherichalogen be- havior is highly unevenamongst the individual halogensand Questionsthat stimulateresearch on atmospherichalogens from phenomenonto phenomenon.In sectionsB2-B5 I re- includevery basicones on nature'scycles and transformations view presentlyavailable data for chlorine,fluorine, bromine, of F, C1, Br, and I and very practicalconcerns over halogen- and iodine. In doing so I hopeto elucidatethe questionsthat containingcompounds. The mostelemental observations, for arise from, or persistin spite of, the extant data. Becauseof example,the presenceof troposphericgaseous halogens and the methodsthat have been used to gather data it is natural to the ratio of their concentrations,have not been explained group the data for each halogenas organicgases, inorganic quantitativelyor evenqualitatively in the sensethat the prin- gases,and particles.The fifth halogen,astatine, is not dis- cipal sourcesof gaseoushalogens are not certain.Similarly, cussed.Its most stable isotope has only an 8-hour haft-life, the processesthat enrich marine aerosolparticles in iodine and its natural concentrationin the atmosphereis probably with respectto seawateriodine and the mechanismsthat re- vanishingly small. lease chlorine-containinggases from marine aerosolsare not understood.Thus the major pathwaysof nature'shalogen cy- 2. Chlorinein the Atmosphere cles are seenonly in generalterms; key transformationproc- a. Inorganicgases (Clg 9. Although it had been known essesand rates are not known with any accuracy.Most of the for many years that chlorineis a constituentof the atmo- suspectedkey troposphericspecies have nevereven been de- sphereboth in gaseousand particulateform, modem investi- tectedin the atmosphere,thus precludingthe qualitativeand gations appear to have begun only after Junge [1956] ad- quantitativetesting of varioushypotheses of halogencycle be- vanced several criteria for measurements. He asserted that havior and evaluationsof sourcestrengths and distributions. advancesin understandingwould require that the chloride contentof particlesand gasesbe measuredsimultaneously but Copyright¸ 1981by the AmericanGeophysical Union. separatelyand that the particlesbe separatedwith respectto Paper number 80R1042. 123 0034-6853/81/080R- 1042506.00 124 CICERONE: ATMOSPHERIC HALOGENS TABLE 1. Summary of Available Data on Halogen-ContainingGases in the Troposphere Bubblers,Charcoal Traps: Halogen Form EC-GC Infrared Absorption ElementalAnalysis, ng/m 3 STP Clgø, organicCI 2.5 ppbv(4000 ng/m 3 STP) 3050 Clgi, inorganicC1 1-2 ppbv (by IR absorption) 1000-2000 (marine air) Brgø, organicBr 10-25pptv (36-90 ng/m 3 STP) 14-68 Brgi, inorganicBr ... 10-25 (xnarineair); 0.5-11 (continentalair) Igø, organic I 1-5 pptv(5.7-28 ng/m 3 STP) 3-28 Igi, inorganicI ... 0.4-17 Fg.ø, organic F I ppbv(855 ng/m 3 STP) polluted air only Fg',inorganic F 0.1-0.4 ppbv (stœatospheric 0.01-0.1 ppbv (stratosphericfilter data by IR absorption) collections) EC-GCrepresents electron capture-gas chromatography. EC-GC value of 4000ng/m 3 for Clgø is for late1977, while the elemental analysis value,3050 ng/m 3, is from 1976(see text, section B2b). Identities and concentrations of individual organic halogen species are in Table3. Of the inorganicgases, only HC1 and SF6 have beenidentified unequivocally in the troposphere.Conversion factors for monohalogenatedcom- poundsare as follows at standardtemperature and pressure: for CI, I ppbv= 1600ng(C1)/m3; for F, I ppbv= 855ng(F)/m3; for Br, 10pptv = 36 ng(Br)/m3;and for I, I pptv = 5.6 ng(I)/m•. size. A further useful distinction is that betweeninorganic and Stedmanet aL, 1975],but this averagedfigure is hardlymean- organic gases.Until about 1971, attention was limited to the ingful for the interpretation of individual measurements.In inorganic gaseousfraction of chlorine and other halogens, the marine atmosphericboundary layer the residencetime of partly becauseonly the inorganicspecies (e.g., HC1) were ex- HC1 might be only 2 days [Kritz and Rancher,1980]. pected to arise in any quantity from the two conjectured In thestratosphere several forms of Clgi have been detected: sources:sea-salt spray releaseof HC1 and volcanic and fuma- HC1 by infrared absorption[Farmer et al., 1976; Williamset rolic emissionsof, for example, HC1, C12,and HF. Typically, al., 1976; Ackerman et al., 1976; Eyre and Roscoe, 1977; the Cl-containing gases were collected by bubbling an air Farmer et al., 1980].Also, Lazrus et al. [1975] have detected stream through K2CO3, KOH, or LiOH in aqueous solution acidicC1- (gaseouschlorine compounds that hydrolyzeto C1- after filtering out all particles [Junge, 1957; Duce et al., 1965; on a basic-impregnatedfilter). Generafly,these IR and filter Chesseletet al., 1972], a processthat was over 80% efficient for measurementsshow HC1 mole fractionsgrowing from less HC1. More recently, Rahn et al. [1976] and Berg and Winches- than 10-lø at the tropopauseto 1-2.10 -9 between30 and 40 ter [ 1977]separated the inorganicand organicfractions of gas- kin, approximatelyas predicted [Ciceroneet al., 1975; Cice- eous C1 by carbon trapping the stream after it passedthrough roneand Liu, 1976;Crutzen et al., 1978;Wofsy, 1978; Logan et LiOH-impregnated filters. Table 1 summarizesthese data; the al., 1978]. Inorganic chlorine gas in the stratospherearises total inorganicC1 gasesClg i exhibit variability. Concentra- from decompositionof organic chlorine compoundsin the tionsbetween