Iodine Oxide Homogeneous Nucleation an Explanation for Coastal New

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GEOPHYSICAL RESEARCH LETTERS, VOL. 28, NO. 10, PAGES 1949-1952, MAY 15, 2001

Iodine oxide homogeneous nucleation' An explanation for coastal new particle production

Thorsten Hoffmann Institute of Spectrochernistry, P.O. Box 101352, 44013 Dortmund, Germany

Colin D. O'Dowd 1 University of Helsinki, Physics Department, PO Box 9, FIN 00014 Helsinki, Finland

John H. Seinfeld California Institute of Technology, Mail Code 210-41, Pasadena, CA 91125, USA

Abstract. A series of laboratory experiments on the chem- identified. Similarly, O'Dowd et at. [1999] analyzedthe ical composition of aerosol particles formed after photodis- field data from Mace Head and performed a series of model sociation of CH212, a major volatile alkyl halide released simulations, which suggested a ternary nucleation processes from macroalgae, have been performed in a laboratory scale involving H2SO4, H20 and NH3, but the authors could only reaction chamber using on-line atmospheric pressure chemi- explain the bursts when an additional unknown speciesX, cal ionizationmass spectrometry (APCI/MS). Basedon the emitted directly from the shore biota or produced from a mass spectrometric results and the molecular properties of released parent species, was assumed to be present. Moti- iodine oxides, we suggest that the self-nucleation of iodine vated by existing knowledge about release of alkyl iodides oxides provides an efficient source of natural condensable by seaweed[Laturnus, 1996; Carpenter et at., 1999]and their material in coastal environments and discuss this concept rapid photodissociationduring daytime [Roehtet at., 1997; focusing on OIO as one potential key speciesfor new particle MSssingeret at., 1998],we report the resultsof laboratory formation. The presented hypothesis not only fits the mea- experiments on the particle composition following photolysis sured enrichment of iodine speciesin submicrometer parti- of CH•I•. cles, but also can explain the frequently observed nucleation bursts in the coastal boundary layer. Experimental Measurements Ground cuvette measurements on two prevailing sea- Introduction weed species at Mace Head showed that CH•I2 is the most abundant single iodine-containing compound released by The study of the sourcesand formation pathways of new macroalgaes[O'Dowd and HSmeri, 2000]. Therefore, we particles in the troposphere is presently a subject of con- have studied particle formation and composition after pho- siderable interest. Recent field studies point to two envi- tolysis of CH•I•. Experiments were performed in a 100 L ronments as regions of significant natural new particle pro- cylindrical reaction chamber, equipped with a tubular low- duction,remote forested areas (e.g. [Kavouraset at., 1998]) pressuremercury lamp in the centerof the chamber(Philips, and coastalsites [O'Dowd et at., 1999, 1998]. While new CLEO PL-L 36W, sunlight simulation lamp between 300- particle formation events in forested areas are speculated 400 nm, Amax 350 nm, mean actinic flux (300-400 nm) to be connected with homogeneousor heterogeneousnucle- 4.6 x 1015photons cm -• s-l). CH•I2 wasadded to the ation of low volatility oxidation products of naturally re- reaction vessel from a dynamic test gas generator, which leased VOCs, the observed nucleation bursts in the coastal was based on an open tube diffusion technique. The result- boundary layer are less well understood. Nevertheless, ob- ing CH•I• concentrations were adjusted between 4 and 17 servational evidence of the underlying processesat coastal ppb(v/v). Additionally,ozone could be introducedinto the sites do exist. For example, particle measurements at 'Mace chamber, produced by UV irradiation of another synthetic Head', located at the West coast of Ireland, showed that air supply. Ozone concentrations were adjusted in the range the occurrence of nucleation events coincides with low tide of < 2 and 50 ppb(v/v). The humidity in the chamberwas and solar radiation, suggesting a tidal-biological source of controlled by adjusting the ratio of humidified to dry syn- the aerosolprecursors. Grenfett et at. [1999]focused their thetic air in the inlet region of the chamber. The overall analysis of the rapid increase in condensation nuclei con- flow throughthe chamberwas adjustedto 8.4 L min-1. A centrations at Mace Head on the role of DMS, however no small ventilator was installed inside the chamber to enable convincing mechanism for the burst phenomenon could be rapid mixing of all components. Particle number concentration wasmeasured with a condensationnucleus counter (TSI 1Alsoat National Universityof Ireland, Galway,Ireland. 8020, 50% cut-offsize 20 nm). After constantmixing ratios of all reactants were achieved, the light source was switched on and resulting particle number concentration and particle Copyright2001 by the AmericanGeophysical Union. composition were recorded. Papernumber 2000GL012399. The chamber air was introduced directly into the ion 0094-8276/01/2000GL012399505.00 source of a Finnigan LCQ ion trap mass spectrometer. De-

1949 1950 HOFFMANN ET AL.: IODINE OXIDES AND NEW PARTICLE FORMATION

CH=I=, N= cations for organic ions were found when only the particle 0 sheathgas !• .." . Activated charcoal ! !•i•i::i?i'...... '¾..- phase was allowed to enter the ion source. syntn air . . . •;-'•: '• nummß .. symn.' ..' air. filledd•ffuslon denuder {= ...... J:i::iiiiiiiJvaporizer•r::!•!•!!!:::t . "! Based on these results and available literature data, we suggest the following reaction sequence:

• •• ...... • •-• ...... •1•1• CH212+hu • I+CH2I (1) photochemical• CPC, I manifold discharge I+O3 • IO+O2 (2) ...... reaction chamber ,, O,-analyzerI glasstubeneedle IO+IO • OIO+I (3) Figure 1. Sketch of the experimentalsetup to investigatethe Reactions(1) and (2), the photodissociationof alkyl iodides particle composition after CH212 photolysis. to produce I atoms followed by the rapid reaction of I with 03 to form IO radicals,have been described previously [Vogt et al., 1999;Davis et al., 1996]. Direct observationof IO in tails of the on-line mass spectrometric technique are given the boundary layer at remote coastal locations by DOAS by Kiickelmannet al. [2000]and are thereforeonly briefly supportsthe occurrenceof reactions(1) and (2) underam- described here. Using an activated charcoal filled denuder in bient conditions[Alicke et al., 1999]. Recentinvestigations front of the ion source, only the particle phase was allowed on the IO self-reaction(3) showedthat iodinedioxide (OIO) to enter the MS (seeFig. 1). The particle phaseproducts is produced in these reactions. Based on ab initio methods formed after CH212 photolysis were introduced into the ion Misra and Marshall [1998]assessed certain thermodynamic source using a small glass tube extending into the vaporizer constants of OIO and reported its formation from the IO of the ion source. The temperature of the vaporizer was set self-reactionto be exothermicby 48.3 kJ mol-• Cox et at 550øC to allow evaporation and hence ionization of the al. [1999]studied the kineticsand mechanismsof the reac- components. tions of IO with itself and with BrO and found that OIO is a product of these reactions. Together with the results of Results and discussion the studiesof Misra and Marshall [1998],Cox and cowork- ers conclude that, due to its photochemical stability, OIO Figure 2 shows the signal intensity of the total ion cur- is a significant gas-phaseiodine reservoir. Nevertheless, the rent (TIC), the relativeabundance of m/z 175 (Fig. 2a) and authors also suggestedthat OIO is lost to particles, a pro- the particle number concentration(Fig. 2b) during a pho- cessthat might explain the high enrichment of iodine in the tolysisexperiment ([03]= 50 (ppb(v/v)). Immediatelyafter small size fraction of marine aerosol[Sturges and Barpie, the light source was switched on at t = 2rain, particles were 1988; Wimschneiderand Heumann,1995]. However,based producedin the chamberby homogeneousnucleation (Fig. on the results presented above, we suggest that OIO is not 2b). It shouldbe noted that the maximumparticle number only removed from the gas-phase by heterogeneousuptake concentration recorded by the CPC seriously exceeds the on existing particles, but also might be the source of ob- measurement range of the instrument and, as a result, CPC served nucleation events in coastal areas by self-reaction of saturationeffects have to be taken into account[O'Dowd OIO, and Hiirneri, 2000]. Therefore,the numbersshown in Fig- ure 2b certainly underestimate the actual particle number OIO + OIO • 1204(or [IO]+[IO3]-) (4) concentration in the reaction chamber. 1204+ n OIO • [-I- O - IO2 - O--]l-Fn/2 (5) At the same time when the particle number concentration increased, the total ion current of the MS increased Although knowledge about the oxides of iodine is incom- (Fig. 2a). Figure 3 showsthe backgroundcorrected mass plete, a series of investigations do exist that form the basis spectrum in the negative ion mode of the aerosol reaching of speculation on the fate of OIO and support the occurrence the ion source.The dominantion is m/z 175, indicatingthe [IO3]- ion. Also the relative abundancesof m/z 143 and m/z 159, presumably[IO]- and [IO2]- respectively,as well as, to a lesserextent, m/z 127 ([I]-) and m/z 254 ([I2]-) increased. Although to our knowledge no APCI reference spectra of iodine oxides are available, the on-line mass spectrum shown in Fig. 3 indicates that the particles formed are composedpurely of oxides of iodine. These either undergo molecular fragmentation in the ion source to yield mainly [IO3]--ionsor thermallydecompose in the vaporizerregion to compounds that subsequently give rise to the observed ß• 8E+051 iodate-ion. Changes in the relative humidity of the chamber air between < I and 20% significantly altered neither the •o 4E+051(b) resulting mass spectra nor the development of the particle • 2E+05i number concentration. In contrast, particle number concen- • 01 tration was drastically reduced when no ozone was added to 1 2 3 4 5 6 7 8 9 10 11 12 13 the chamber. Time [min] In the positive ion mode, the dominant ion observed was m/z 177, which could be characterizedby MS/MS- Figure 2. Temporal behavior of the total ion current, the rela- experimentsas a wateradduct of [IO2]+, mostlikely formed tive abundanceof m/z 175 (a) and the CPC readout (b), [CH212] in the APCI-source following the ionization step. No indi- 17ppb(v/v), r.H. 10%, T = 298 K. HOFFMANN ET AL' IODINE OXIDES AND NEW PARTICLE FORMATION !95!

ß100] 75.3 might also be describedas a mixed iodine (III/V) oxide c• 80 ([IO]+[IO3]-)(reaction(4)). Collisionswith further OIO • 60 molecules can lead to the formation of chain-like structures -• 40 in which the loss of single OIO units from the clusters is '• 20 143.3 hindered by the strong associationbetween the -I-O-IO2-O- n- 127.3 159.3 . L - ß . IlJI . units (reaction (5)). Such intermolecularassociations can o, -. i'o' 140 160' 180'...... 200"" 2•20 2hO- 260 explain not only the spectroscopic behavior of solid OIO m/z and the stability of the condensed phase but also the high particle formation potential observed in the chamber after Figure 3. Backgroundcorrected APCI mass spectrum (nega- photolysis of CH212. tive ion mode) of aerosolreaching the ion source.

Conclusions of reactions(4) and (5). In contrastto the halogen(IV) ox- We have carried out laboratory experiments focusing on ides of chlorine and bromine, which rapidly decompose at the particle formation after photolysis of CH212 in the pres- sub-ambienttemperature [Bailar et al., 1973], OIO forms ence of O3 and unequivocal rapid new particle formation was a stable iodine oxide at room temperature that can be iso- observedin the lower ppb-range of CH212. Chemical charac- lated as yellowcrystals [Fjellvag and Kjkshus,1994]. There terization of the newly formed particle phase by on-line MS exist no data on the vapor pressure of solid OIO nor on indicates that the particles are composed purely of oxides its melting point, since the compound decomposesto I205 of iodine. One plausible explanation of the experimental and I2 at temperatureshigher than about 130øC [Chase, results is the self-reaction of iodine oxides, such as OIO q- 1996]. However, iodine (IV) oxide exhibits some remark- OIO, to form inorganic polymeric structures as proposedfor able properties that are related to its potential to form new I204. Although other reactions of iodine species might be particles. The investigation of the crystal structure of OIO also involved in the particle formation process(e.g. IO q- showsthat the compound is better described as I204, since OIO or reactionswhich involveHOI or I202), the hypoth- the crystallinesolid is consideredto be [IO]+[IO3]- (iodosyl esis presented above is consistent with the most prominent iodate)(disproportionationof iodine (IV)). Besidethis in- signals observed in the mass spectra of the aerosol, since teraction of two OIO entities, recent investigations on the both undecomposedI204 ([IO]+[IO3]-) andits thermalde- I204 structure suggest that the I204 units are strongly in- compositionproduct (I205) are expected to result in the teracting with the formation of one-dimensional ...-I-O-IO2- formation of iodate ions. O-...chains, which are bonded together by weaker interchain The laboratory results presented here are highly signifi- interactions[Fjellvag and Kjkshus,1994]. This polymeric cant in light of field investigations at Mace Head. The con- structure was suggestedresponsible for the low solubility of centrationdata of alkyl iodides(CH2IBr, CH2IC1, CH212) I204 in manysolvents including water [Ellestadet al., 1981]. measured in air samples at the site, which reached their There are few other laboratory studies that report the highest concentrations in air masses influenced by the lo- formation of aerosolin iodine/O3 reaction systems. For cal shoreline and were rapidly destroyed in sunlit hours example, Cox and Coker [1983]observed an aerosolin the [Carpenter et al., 1999], together with the temporal be- photolysisof CH3I in the presenceof 03. Without chemical havior of nucleationbursts [O'Dowd et al., 1999], and the analysis of the yellow deposit at their cell walls, the authors occurrenceof iodine in submicrometerparticles [O'Dowd proposed a molecular formula of I409, based on literature and Hiimeri, 2000] support the proposednucleation mech- data of product studies of the dark reaction of I2 with O3 anism. Likewise, if simple photochemical box model cal- (see e.g. [Vikis and MacFarlane, 1985]) and on the stoi- culations are performed, based on the measured IO con- chiometry of the ozone removal for each I atom produced centrationsat Mace Head (up to 6ppt) [Carpenteret al., in their CH3I photolysis(AO3 -- 2.1 4-0.2 AI). Obviously, 1999],an OIO yieldfrom IO self-reaction(k = 8.2 x 10-zz alsoa molecularcomposition of (IO2),•, as suggestedabove, molecules-Zcm3s-z) of 0.38, a photolysisrate of OIO of would meet these observations. More recently, Harwood et or = 1.2 x 10-5 s-z, and OIO is assumedto react with NO al. [1997]observed aerosol formation when mixturesof I2 and OH [Cox et al., 1999], productionrates of OIO of the and O3 were photolyzed whereas no aerosol was observed orderof 1-6 x 105molecules cm -3 s-z result,a rangethat fits when the photolysisof N20/I2 mixtures was used as a IO well the expected source rate of condensablespecies during source. The latter observation might be used to argue that nucleationevents at the site [O'Dowd and Hiimeri, 2000]. the products of the IO self reaction, such as OIO, are not Whether the nucleation events observed at Mace Head are a directly involved in the formation of condensable species. result primarily of homomolecular homogeneousnucleation However, due to the high I atom concentrationpresent when of compounds like OIO or if iodine oxides simply lead to N20/I2 systemsare photolyzed,Harwood et al. [1997]as- rapid growth of already existing thermodynamically stable sumed that reactions like OIO q-I -+ 2IO might prevent a sulfateclusters [Kulmala et al., 2000]remains an openques- significant built-up of this speciesin the gas phase, a reaction. Certainly, more work is necessary on the chemistry tion that can be expected to be less important when ozone of iodine species(e.g. kineticsof the OIO self reaction) to is present. thoroughly understand the overall processesin the marine Based on the discussion above, we suggest that OIO boundary layer. molecules, which are formed in the gas-phase by reaction sequence(1)-(3), not only attach to pre-exiting particles, Acknowledgments. This work was supportedby the EC, but, driven by strong intermolecular interactions, read- Environment and Climate program, Contract No. ENV4-CT97- ily form OIO dimers (I204) as a first step. This dimer 0391 (NUCVOC) and ENV4-CT97-0526 (PARFORCE). 1952 HOFFMANN ET AL.: IODINE OXIDES AND NEW PARTICLE FORMATION

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