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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. D1, PAGES 2135-2147, JANUARY 20, 1996Potential impact of iodine on tropospheric levels of ozoneand other critical oxidantsD. Davis, • J. Crawford, • S. Liu, 2 S. McKeen, 2 A. Bandy, 3 D. Thomton, 3F. Rowland, 4 and D. Blake 4Abstract. A new analysis of tropospheric iodine chemistry suggests that under certainconditions this chemistry could have a significant impact on the rate of destruction oftropospheric ozone. In addition, it suggests that modest shifts could result in the criticalradical ratio HO2/OH. This analysis is based on the first ever observations of CH3I in themiddle and upper free troposphere as recorded during the NASA Pacific ExploratoryMission in the western Pacific. Improved evaluations of several critical gas kinetic andphotochemical rate coefficients have also been used. Three iodine source scenarios wereexplored in arriving at the above conclusions. These include' (1) the assumption thatthe release of CH3I from the marine environment was the only iodine source withboundary layer levels reflecting a low-productivity source region, (2) same as scenario 1but with an additional marine iodine source in the form of higher molecular weightiodocarbons, and (3) source scenario 2 but with the release of all iodocarbons occurringin a region of high biological productivity. Based on one-dimensional model simulations,these three source scenarios resulted in estimated I x (I x =I + IO + HI + HOI + 21202+INOx) yields for the upper troposphere of 0.5, 1.5, and 7 parts per trillion by volume(pptv), respectively. Of these, only at the 1.5 and 7 pptv level were meaningfulenhancements in 03 destruction estimated. Total column 03 destruction for these casesaveraged 6 and 30%, respectively. At present we believe the 1.5 pptv I x source scenarioto be more typical of the tropical marine environment; however, for specific regions ofthe Pacific (i.e., marine Upwelling regions)and for specific seasons of the year, muchhigher levels might be experienced. Even so, significant uncertaintie still remain in theproposed iodine chemistry. In particular, much uncertainty remains in the magnitude ofthe marine iodine source. In addition, several rate coefficients for gas phase processesneed further investigating, as does the efficiency for removal of iodine due to aerosolscavenging processes.Introductionleads to the release of an O atom which, via reaction with02 , results in the formation of one net 03 molecule.Of the trace gases in the troposphere, ozone, together withthe free radicals generated by its photolysis, is mostPhotochemical destruction occurs when the 03 photolysisresponsible for defining the oxidizing capacity of theproduct O(•D) reacts with H20 to produce two hydroxyltroposphere. Within the troposphere, the mixing ratio of thisradicals, OH, or when hydroperoxyl HO 2 and OH radicalstrace gas is influenced by both transport and photochemicalreact with 03 . Less certain is the suggestion that halogenspecies might also provide a photochemical destructionprocesses [e.g., Fabian and Pruchniewicz, 1977; Mahlman etal., 1980; Chameides and Walker, 1973; Fishman andchannel for tropospheric 03 [e.g., Chameides and Davis,1980; Singh and Kasting, 1988; Bartie et al., 1988;Crutzen, 1977; Liu et al.,1980]. Conventional thinkingBottenheim et al., 1990; Cha•field and Crutzen, 1990; Keenesuggests that it is the reaction of peroxy radicals, (e.g.et al., 1990; Jenkin, 1993; Jobson et al., 1994; Le Bras andHO 2 CH302, and RO2, where "R" is any organic grouping)Platt, 1995].with NO to produce the product species NO 2 that forms theBeing the most abundant of the halogens, chlorine hasbasis of photochemical 03 formation. Photolysis of NO 2undergone the most intense investigation. Both the OH-•School of Earth and Atmospheric Sciences, Georgia Institute of initiated oxidation of natural and man-made chlorocarbonsTechnology, Atlanta.and the heterogeneous sea-salt reactions have been explored2Environmental Research Labs, NOAA Aeronomy Laboratory, as possible sources of reactive chlorine [e.g., $ingh andBoulder, Colorado.3Depamnent of Chemistry, Drexel University, Pennsylvania. Kasting, 1988; Keene et al., 1990; Pszenny et al., 1993;4Department of Chemistry, University of California, Irvine. $ingh, 1995, and references therein]. However, because ofthe absence of documented high-altitude sources of chlorine,its high affinity for hydrogen abstraction, and the lowCopyright 1996 by the American Geophysical Union.reaction probability for converting the product HC1 back toreactive chlorine, it now seems unlikely that this halogen byPaper number 95JD02727.itself represents a significant loss of 03. A possible0148-0227/96/95 JD-02727505.00exception to this conclusion might involve chemistry within2135

2138 DAVIS ET AL.' POTENTIAL IMPACT OF IODINE ON TROPOSPHERIC OZONE LEVELS10a)12b)10• 6oo.oo0.5 1 .o 1.5 2.0 o.o 0.5 1 .o1.52.0CH31 (pptv)CH31 (pptv)Figure 2. Altitude distributions for CH31 as measureduring PEM-West A for the latitude ranges' (a) 0 ø-18øN and (b) 18ø-42øN. Circles indicate the median values for altitude bins' 0-1 km, 1-2 km, 2-4 km, 6-8km, 8-10 km, and 10-12 km. Shaded regions indicate the range of CH3I observations within each bin aftertrimming the upper and lower 5% of the data within each bin.resulting hour glass shaped profile is thus seen to be similar representation might adequately simulate deep convectiveto those for several other PEM-West A trace gases whose processes that obviously are a major contributor to thedistributions were strongly influenced by deep convection relatively high mixing ratios of CH3I in the upper[e.g., Blake et at., this issue]. By contrast, in the tropics, troposphere. In this regard we have assumed that thealthough BL values start lower (i.e., 0.6 pptv), a rather steady entrainment and detrainment of air by convective processesroll-off in CH3I median values occurs from the BL to has been effective at nearly all altitudes. In other words, thealtitudes of 8-10 km, resulting in a high-altitude median combination of deep and shallow convection effectively mixvalue of -0.15 pptv.boundary layer air to various altitudes. Moreover, weassume that the ensemble of air massesampleduring PEM-2.2. Vertical Transport and I x DistributionsWest A were representative of the ambient environment. Asdiscussed in the above text, for the case of the tropics anAs discussed below in the text with reference to theexamination of the median profile of CH3I (e.g., Figure 2a)photochemistry of tropospheric iodine, the abundance of total as well as that of DMS does show a gradual decrease withreactive iodine Ix (I•= I +IO + HOI + HI + 21202 + INOx) in altitude. Thus this is consistent with the above assumptions,the middle and upper troposphere has been assessed in terms and suggests that the median profile can be simulated withof a photochemical source, e.g., photolysis of CH31 and/or the eddy diffusion formulation.other iodocarbons, and losses in the form of surface For this investigation a single value of the diffusiondeposition and heterogeneous uptake by cloud droplets, coefficient (K,) was used from 50 meters above the surfaceprecipitation, and possibly aerosols. Thus given the to the highest altitudes of the model at 12 km. Below 50 mstochastic nature of these loss processes in the middle and a very stable layer was assumed with I x being immediatelyupper troposphere, one can estimate that the timescales deposited at the surface upon contact. The initialinvolved could be significantly longer than that for vertical concentrations of Ix and CH3I were assumed to be zero. Antransport. We have investigated this question in this study by integration time of 20 days was chosen so as to permit CH3 Iusing a simple one-dimensional model [McKeen et. at., 1989] to reach a steady state condition at high-altitudes.that incorporates a first-order diffusion process to describe The key features of this one-dimensional transportvertical transport.simulation are illustrated in Figure 3. Also shown are theA particular concern in employing the one-dimensional median values estimated for CH31 as given in Figure 2a.formulation is that it is difficult to envision how such a With photolysis acting as the only loss mechanism for CH3I,

DAVIS ET AL.' POTENTIAL IMPACT OF IODINE ON TROPOSPHERIC OZONE LEVELS 21391211lOI;Reference:I;;Low Washoutoo.o0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0CH3I or I x (pptv)Figure 3. Altitude profiles of CH3I and Ix based on one-dimensional model simulations. Asterisks indicatethe median values of CH3I for the PEM-West A tropical zone 0ø-18øN. The solid line indicates thesimulated profile for CH3I assuming an emission flux of 2x10 • molecules/m2/s and an eddy diffusioncoefficient Kz of 80 m 2/s. Dotted lines indicate Ix levels for: (1) Kz value of 80 m 2/s and washout ratesaccording to Table 1, ("reference" curve), and (2) K z value of 80 m2/s with washout rates from Table 1decreased by one half below 5 km and zero for altitudes above 5 km ("low washout").a reasonable fit to the CH3I experimental observations wasfound when K z was assigned a value of 80 m2/s and thesurface emission rate was given a value of 2x10 •molecules/m2/s. In this case, the value of Kz, relative to thephotolysis loss rate, defines the vertical gradient for CH3I,while the flux value simply shifts the vertical profile alongthe abscissa. We note that our value for K z of 80 m2/s isscenarios. The low scenario, involving 0.5 pptv of Ix at 10km, assumes CH3I to be the only source of iodine and usesthe "reference" value of K z of 80 m2/s and a CH3I flux of2x10 • molecule/m2/s, e.g. low productivity marine water. Asomewhat more realistic scenario, we believe, is oneinvolving 1.5 pptv of Ix which reflects the inclusion of sourcecompounds like CH2I 2 and C2H5I that could more than doubleonly slightly larger than the value used by Liu et al. [1984] the iodine flux to the atmosphere. The final scenario explored(i.e., 50-70 m2/s ). These authors based their value on here involves a high-altitude value of Ix of 7 pptv. Thisobservations of 222Rn over the continental United Statesduring summertime. Like CH3I, the tracer 222Rn has only asurface source and during the summer season is probably alsodynamically forced by convection to high-altitudes.The Ix "reference" profile shown in Figure 3 was obtainedscenario considers possibilities such as CH3I defining only26% of the marine iodine flux to the troposphere (see earlierdiscussion of iodine sources), lower loss rates for Ix, andhigher marine primary productivity relative to that observedduring PEM-West A. We note that vertical profiles of Ix forusing the same K z value together with an altitude dependent scenarios 2 and 3 were obtained here by scaling the abovewashout rate for all soluble species as defined by Logan etal. [1981] (see Table 1). The resulting Ix profile shows acited 10-km values according to the Ix reference profile inFigure 3.maximum of- 0.5 pptv for altitudes approaching 12 km. Toexplore the magnitude of influence of the assumed value for 3. Tropospheric Iodine Chemistrythe washout rate on Ix, a simulation was also carried out in and Model Resultswhich this rate was reduced by a factor of « for altitudesbelow 5 km and to zero above 5 km. As shown in Figure 3,Ix is enhanced by only 0.2 pptv, yielding a maximum valueof 0.7 pptv. The effect resulting from assuming a much3.1 Description of ModelThe basic photochemical model used in this study issimilar to that described by Davis et al. [this issue],faster Ix removal rate than that employed for the "reference" Crawford et al. [this issue], and Davis et al. [1993]. Iodineprofile is discussed below in the text.For purposes of exploring the tropospheric impact ofiodine in this investigation, we have selected three Ix sourcechemistry was added to this basic model as per the reactionsshown in Table 1. Once the level of Ix was established, asper the discussion in the. above text, the partitioning of Ix and

2140 DAVIS ET AL.: POTENTIAL IMPACT OF IODINE ON TROPOSPHE•C OZONE LEVELSTable 1. Tropospheric Iodine Reactions in the Evaluation of Iodine PhochemistryReaction J or K Values Reference(R1)(R2)(R3)(R4)(R5)(R6)CH3I+hv---•CH3+II+O3---•IO+O 2I+HO2---•HI+O 2I+NO2+M---}INO2+MINO2+hv---•. 5 (I+IO+NO+NO2)IO+hv---•I+O(R7) IO+NO--}I+NO2(R8) IO+HO2--•HOI+O 2(R9) IO+IO--•.2(2I+O2)+. 8(I202)(R10) IO+NO2+M--•IONO2+M(R11) IONO2+hv--•.5(I+IO+NO2+NO3)(R12) IONO2---•IO+NO 2(R13) HI+OH-•I+H20(R14) HOI+hu--}I+OH(R15) HOI+OH--•IO+H20(R16) I202+hv---•2I+O 2(R17) HI---•deposition(R18) HOI---•deposition(R19) I202---•deposition(R20) HI---•rainout/washout(R21) HOI---•rainout/washout(R22) I202---•rainout/washout(1t23) HI---•aerosol(R24) HOI---•aerosol(R25) I202--•aerosol7.9x 10 '6•2.0x10 'll exp (-890/T)1.5xl0 'll exp (-1090/T)ko=3.0xl 0'31(T/300)'1 [N2] blq.•=6.6xl 0 'llFc=[exp(-T/650)+exp(-2600/T)]2.9x 10 '32.2xl 0 'l a7.3xl 0'12exp(330/T)8.4x10 -111.7xl 0'12exp(1020/T)ko=7.7xl 0-31(T/300)'5[N2] blq.•=l.6xl 0 'llFc=0.42.9xl 0 '3 a2.07xl 0-15exp(- 11859/T)3.0X10 'll5.9x 10 '3 •2.0X10 ©1.3x10 -2 ,1.16x10 -51.16x10 -51.16x10 -•see footnote csee footnote csee footnote csee textsee textsee textcross sections from Calvert and Pitts [1963]Atkinson et al. [ 1993]Atkinson et al. [ 1993]Atkinson et al. [ 1993]assumed same as IONO2+hvcross sections from Stickel et al. [1990]and Laszlo et al. [1995]Atkinson et al. [ 1993]DeMore et al. [1994]Atkinson et al. [1993]/products denkin et al. [1985]Atkinson et al. [1993]cross sections from Atkinson et al. [1993]products assumed, this workChatfield and Crutzen. [1990]Atkinson et al. [ 1993]cross sections from Atkinson et al. [1993]Chameides and Davis [1980]assumed 9x(C1202+hv )cross sections from DeMote et al. [1994]estimatedestimatedestimatedLogan et al. [1981]Logan et al. [1981 ]Logan et al. [1981 ]estimatedestimatedestimated•Photolysis rates have been calculated for a solar zenith angle of 30 ø at the surface.bk=(kokinJ(ko+kin•))F•.•2.31x10 -6(0-4 km), 2.31x104exp(1.6-4z) (4-12 km).the interaction of iodine species with 03 and other HOx Quite noteworthy here is thathe rate-controlling step in eachspecies was defined by our iodine-modified box model. sequence involves the reaction of IO, either with itself orFigure 4 shows our current iodine chemistry in schematic with HO2. In this regard, the proposed iodine chemistryform, thus illustrating the extensive cycling among the differs from that outlined for the lower stratosphere byindividual iodine species I, IO, I202, HOI, and INOx. Model- Solomon et al. [1995]. These investigators have suggestedestimated vertical profiles for each of these species are that interhalogen/IO reactions involving BrO and C10shown in Figure 5. These profiles are based on a high- radicals were probably the dominant 03 loss processes in thealtitude I x level of 1.5 pptv. They tend to show that at all lower stratosphere. This reflects the fact that Solomon et al.altitudes but 8-10 km, HOI is the dominant iodine species. [1995] estimate much higher levels of bromine and chlorineFor the latter altitude range, the concentration levels of I, HI,IO, and HOI are seen to all lie within a factor of 2.5 of eachother. By comparison, the species I202, INO2, and IONO2are 1 to 2 orders of magnitude lower in concentration.From a detailed analysis of the chemistry presented inFigure 4, two 03 destruction sequences can be identified:for the lower stratosphere than those predicted by ourstandard model for the upper troposphere. In our standardmodel only gas phase speciesuch as CH3C1 , CH3C13, andC2C14 have been considered as sources of C1 x. This results inonly a few pptv of Clx, most of which is tied up in the formof HC1 and therefore of little chemical consequence.To further explore the possible importance of interhalogen2(1+O3 '-> IO+O2)(R2) chemistry in the upper troposphere, we have examined twoIO+IO ---> 21+O 2(R9) other possible C1 x sources. The first of these considered203 •> 30 2transport of a Clx source from the stratosphere. An upperlimit to the amount of C1 x from this source can be estimatedby assuming that both 03 and C1 x are inert tracers in theI + 03 ---> IO+O2(R2) lower stratosphere and upper troposphere. We believe thisIO + HO 2 •> HOI + 0 2should be a reasonably good assumption for 03 as itsHOI+hv ---> I+OH(R14) production and loss rates are very small [e.g., Davis et al.,03 + HO 2 •> 202 + OHthis issue]. For Clx, one would intuitively expecthat soluble

DAVIS ET AL.: POTENTIAL IMPACT OF IODINE ON TROPOSPHERIC OZONE LEVELS 2141Tropospheric Iodine Photochemical Reaction SchemeW•o• W I / hv / W•o• W•o•I // /••onSe••onhvOH HO•I ,W•ho••o•W•o•R•o•Se••on•igure 4. Tropospheric iodine photochemical reaction scheme. The relative Eux strcn•h for each pathwayis indicated by the width of each connecting linc. Stable species arc those shown as rectangles. Sho•-livcdspecies arc indicated as circles and ellipses.species like HC1 •ould bc cffcctNcly scavenged by high- resulting C10 level gives a rate for the IO + C10 ozone sin•altitude clouds. However, c•tcnsivc obsc•ations of similar reaction that is still only a fc• percent of that for (RS).species ( i.e., NOy) suggests that it mightend to behave as Thus wc conclude that if interhalogen O• sin• reactions arcif itwcrc nearly inc• [c.g., •y•h• • •[., ]993; •[• • •[., to bc impo•ant in the troposphere, a significant yetthis issue; and •h • •[., this issue]. GNcn that the unidcnti•cd source of rcactNc chlorine •ill nccd toassumption of incaness of CI• and O• is co•cct, the ratio found. This could potentially bc some fo• of heterogeneousCI•/O• should bc approximately constant for the lower process •hich might not only release chlorine but alsostratosphere and upper troposphere. Thus using values near bromine.20 • [•oH• •o•o[o•[•[ Oy•[z•t[o• •O), ] 989], Although some uncc•ainty still remains conceding aspectswc have estimated a value for C]•/O• of •Sx]0-•. For the of the gas phase chcmist• of iodine, the issuetropical zone 0%] 8•N, where the median level of O• was 32 heterogeneous reactions encompasses perhaps an cvcn greaterppbv, this gives an estimated value for the mixin• ratio of uncc•ainty. As indicated in Table ], washouVrainout andCI• of 16 pp•. Similarly, for the Pacific rim region (]8% surface deposition arc major components of the cu•cnt42•) where the median value of O• •as 4S ppbv, the model. • the marine •L this results in an approximate I-estimated mixing ratio for CI• would bc 23 ppW. Thus wc day lifetime for •. •y comparison, for middle •ccestimate that these CI• levels could provide no more than a tropospheric altitudes these processes lead to an • lifetime]0• molcculcs/cm • source of CIO, assuming only gas phase that ranges •om S to ]0 days, thus being comparable to thatchcmist• to bc impo•ant. At this level the concentration is for reasonably fast vc•ical mi•ing. For the uppermore than 2 orders of magnitude less than that for HO• and troposphere, as evaluated by equations 20-23,hence would render any interhalogen O• sink reaction washouVrainoutratcs arc factors ors to ]S times slower thanunimpo•ant. vc•ical mixing. Thus •hcr enhancements in the removalThe other high-altitude source of CI• considered hcrc rate duc to microphysical processes operating within dccpinvolves the reaction of sulktic or nitric acid •ith sea-salt convection or, altc•ativcly, to other noncloud related aerosolaerosols, the la•cr being transposed to the upper troposphere heterogeneous losses could potentially lead to significantvia dccp convection. This source, together •ith H•I released decreases in the high-altitude levels of I•. To explore this•om the gas phase oxidation of chlorocarbons (scc above possibility, simulations •crc ca•icd out using the previouslytext), has bccn suggested as a possible contributor to the 400 described one-dimensional model in which removal timespp• levels of HCI reposed for the tropical tropopausc by 7 and ]4 days wcrc assumed. The result showed that • wasT• • •[. [199S]. Our assessment of this altc•atc source decreased at the highest altitudes by S0% and 30%,suggests that if only gas phase chcmist• is invoked, the respectively.

2142 DAVIS ET AL.' POTENTIAL IMPACT OF IODINE ON TROPOSPHE•C OZONE LEVELSlOß I• IO-•- INO•8I•o•• HOI• HI42o10 4 10 10 10 7 10 8molec/cmFigure 5. Altitude profiles of reactive iodine species for 0ø-18øN based on the "reference" altitude trendgiven in Figure 2, scaled to give 1.5 pptv of I• at high-altitude.3Solomon et al. examined the issue of aerosol sinks for (RS), (R14), and (R2), and (R9). Ozone destruction due toiodine as related to the lower stratosphere. The chemical the IO self-reaction (R9) is found to be strongly dependentnature of these aerosols was taken to be predominantly on both the altitude and the absolute Ix level selected. Thussulfate and/or sulfuric acid, as are probably also aerosols forthe upper troposphere. Although very little information isonly for Ix levels at 7 pptv are IO levels found to besufficiently high that this reaction contributes greater thannow available on heterogeneous reactions of iodine, Solomon 10% to the total iodine-related 03 destruction; for example,and co-workers have argued from analogy with chlorine see Table 2. In addition, even at 7 pptv of Ix, (R9) does notchemistry [e.g., Hanson et al., 1994] that because of the become significant until reaching altitudes >6 km. Forhigh reactivity of iodine, it would most likely not be example, from Table 2 it can be seen that only at 10 to 12permanently removed but would simply be recycled. km is approximately 30% of the iodine-related 03 destructionFurthermore, these investigatorsuggest that the partitioning due to the IO self-reaction.of iodine between the aerosol and gas phases would tend to As shown in Table 2 (top), for the case of 0.5 pptv Ix thefavor most iodine being in the gas phase. However, given predicted 03 destruction D(O3) due to iodine never exceedsthe impact of a 14-day aerosol sink, it would seem prudent that estimated from HOx alone by more than 8%. This leadsto more fully explore the issue of heterogeneous loss of to a relatively small 0 to 12-km column D(O3) effect of onlyiodine.2%. Based on what we believe to be a more typical source3.2 Ozone Destructionscenario, involving 1.5 pptv Ix, Table 2 (middle) indicatesthat for some altitude/latitude ranges, as much as a 24%As discussed earlier in the text, the two most significant increase in D(O3) might occur. In all cases the general trendiodine-related 03 sinks involve the reaction sequences (R2), in D(O3) is seen as involving increasing iodine effects with

DAVIS ET AL.: POTENTIAL IMPACT OF IODINE ON TROPOSPHERIC OZONE LEVELS 2143Table 2. Diurnally Averaged, Column IntegratedOzone Destruction from IodineLatitude Altitude Without Ix, With .5 pptv Ix a, %Effectmolec/cm2/s molec/cm2/s0-18 0-1 4.84E+ 10 4.91E+ 10 1.410-18 1-2 4.57E+ 10 4.65E+ 10 1.740-18 2-4 8.86E+ 10 9.01E+ 10 1.740-18 4-6 5.57E+ 10 5.72E+ 10 2.620-18 6-8 3.70E+ 10 3.82E+ 10 3.270-18 8-10 0.85E+10 0.91E+10 7.81(28.4E+ 10) b (29.0+ 10) b (2.24) b18-42 0-1 5.10E+10 5.17E+10 1.3118-42 1-2 7.69E+10 7.77E+10 1.0418-42 2-4 9.01E+10 9.15E+10 1.5718-42 4-6 5.98E+10 6.12E+10 2.2718-42 6-8 2..36E+10 2.47E+10 4.4618-42 8-10 1.33E+10 1.40E+10 5.6418-42 10-12 0.39E+10 0.41E+10 6.81(31.9E+10) b(32.5E+10) b (1.98) bLatitude Altitude Without I•, With 1.5 pptv I• a, % Effectmolec/cm2/s molec/cm2/s0-18 0-1 4.84E+ 10 5.05E+ 10 4.240-18 1-2 4.57E+10 4.81E+10 5.220-18 2-4 8.86E+ 10 9.32E+ 10 5.200-18 4-6 5.57E+ 10 6.01 E+ 10 7.840-18 6-8 3.70E+ 10 4.06E+ 10 9.800-18 8-10 0.85E+10 1.05E+10 23.81(28.4E+10) b(30.3E+10) b (6.71) b18-42 0-1 5.10E+10 5.30E+10 3.9418-42 1-2 7.69E+10 7.93E+10 3.1118-42 2-4 9.01E+10 9.43E+10 4.7118-42 4-6 5.98E+10 6.39E+10 6.8218-42 6-8 2.36E+10 2.68E+10 13.5118-42 8-10 1.33E+10 1.156E+10 17.4418-42 10-12 0.39E+10 0.47E+10 21.30(31.9E+ 10) b(33.8E+ 10) b (5.98) bLatitude Altitude Without Ix, With 7 pptv I• a, %Effectmolec/cm2/s molec/cm2/s0-18 0-1 4.84E+ 10 5.80E+ 10 200-18 1-2 4.57E+10 5.67E+10 240-18 2-4 8.86E+ 10 11.0E+ 10 240-18 4-6 5.57E+ 10 7.60E+ 10 360-18 6-8 3.70E+ 10 5.40E+ 10 460-18 8-10 0.85E+10 1.89E+10 123(28.4E+10) b(37.3E+10) b(32) b18-42 0-1 5.10E+10 6.04E+10 1818-42 1-2 7.69E+ 10 8.80E+ 10 1418-42 2-4 9.01E+10 11.0E+10 2218-42 4-6 5.98E+10 7.89E+10 3218-42 6-8 2.36E+10 3.93E+10 6618-42 8-10 1.33E+10 2.60E+10 9518-42 10-12 0.39E+10 0.85E+10 122(31.9E+10) b(41.1E+10) b(29) bRead 4.84E+10 as 4.84xl 010o• I• levels at all altitudes have been scaled to the "reference" curvegiven in Figure 3.bColumn integrated numbersincreasing altitude. However, it must be recognized that thistrend is strongly influenced by the rapidly decreasingcontribution to D(O3) from HOx chemistry alone. As aresult, although the percent change in D(O3) is largest forhigh-altitudes, the total column effect on D(O3) is actuallymore influenced by middle free tropospheric altitudes.A similar trend in D(O3) is seen for I• levels of 7 pptv,e.g., Table 2 (bottom). In this case, the upper altitude valuesof D(O3) are shifted by factors of 2 or more due to theinclusion of iodine chemistry. The resulting enhancement intotal column 03 destruction is approximately 30%. If thisevaluation of the PEM-West A tropical regime is furtherextended to the tropopause (based on generic high-altitudetropical trace gas compositions), we find that total 03 columndestruction is increased by only an additional 2% over thatcited above for 0-10 kin. This result once again makes thepoint that although very large effects can be found fromiodine at high-altitudes, these altitudes tend to contribute lessto the total column D(O3) effect.Taking for the moment the most extreme case for theimpact of iodine on the troposphere (e.g., I• levels of 7 pptv),the total-column D(O3) results from Table 2 (Sx10 lømolecules/cm2/s) indicate that this photochemical destructionflux would be nearly 1.5 times larger than the 03 depositionflux to the ocean [e.g., Kawa and Pearson, 1989; Lenschowet al, 1982]. Equally noteworthy is the fact that, asdiscussed by Davis et al. [this issue], even before addingiodine, the PEM-West A tropical regime has been evaluatedas having an imbalance between 03 photochemicalproduction and destruction of between 3 and 12x10 lømolecules/cm2/s. (The lower deficit number assumessignificant high-altitude (e.g., 10-17 km) production of 03due the presence of high levels of lightning-generated NOx.)If, however, this region is at steady state for 03 as suggestedby Davis et al., [this issue, 1995] then an additional 8x10 lømolecules/cm2/s 03 loss due to iodine raises still furtherquestions concerning our level of understanding of thetropical ozone budget. Most likely, it means that during thetime period of PEM-West A, I• levels were much closer tothose labeled earlier as "typical," i.e., 1.5 pptv. Quiteclearly, the optimum location for seeing a maximum iodineeffect was not that defined by the PEM-West A mission asevidenced by the low primary productivity levels estimatedfrom the low observed levels of CH3I.3.3. Modulation of HO2/OH and NO2/NO RatiosBased on (R8), (R14), and (R7), one would predict thatwith increasing levels of Ix, shifts should occur in both theratios HO2/OH and the NO2/NO. But, as seen in Tables 3(top) and 3 (middle) only minor changes are seen in theseratios when I x levels are limited to 0.5 and 1.5 pptv. Bycomparison, for I• levels of 7 pptv the HO2/OH ratio isobserved to shift by as much as-18 to -24%, depending onlatitude and altitude. The trend in this ratio is one whichshows increasing negative values with increasing altitude.The average for all altitudes and latitudes is estimated hereto be -15%. This leads to an average increase in the level ofOH by 9%; for example, the OH effect typically defines 60%of the total change in the HO2/OH ratio. Given this shift inOH, if I• levels were to persist at the 7 pptv level for anextended period of time, one would also estimate acorrespondingly 9% shorter lifetime for OH-controlled longlivedtrace gases such as CH4 and CH3CC13.Concerning changes in the ratio NO2/NO, from Table 3(bottom) it can be seen that 7 pptv of I• shifts the value ofthis ratio by a maximum of 11%. Furthermore, this shift isonly observed for the highest altitudes examined. Theaverage shift over all altitudes and latitudes is approximately5%. Thus on average, the magnitude of the iodine shift in

2144 DAVIS ET AL ß POTENTIAL IMPACT OF IODINE ON TROPOSPHERIC OZONE LEVELSTable 3. Tropospheric Impact of Iodine on Ratios HO2/OH and NO2/NOHO2/OH, HO2/OH, HO2/OH, NO2/NO, NO2/NO , NO2/NO ,Latitude Altitude Without I x With 0.5 pptv I x, % Effect • Without I x With 0.5 pptv Ix' % Effect b0-18 0-1 109 108 -0.7 1.30 1.31 0.20-18 1-2 105 104 -0.9 1.20 1.21 0.20-18 2-4 78 77 -1.1 1.21 1.21 0.20-18 4-6 81 80 -1.4 0.87 0.88 0.20-18 6-8 76 75 -1.5 0.68 0.68 0.20-18 8-10 66 64 -2.2 0.32 0.32 0.518-42 0-1 91 90 -0.7 1.55 1.55 0.218-42 1-2 82 82 -0.7 2.00 2.00 0.118-42 2-4 85 84 -0.9 1.46 1.46 0.11842 4-6 71 70 -1.2 1.14 1.14 0.21842 6-8 64 63 -1.5 0.70 0.70 0.31842 8-10 41 40 -1.5 0.47 0.47 0.518-42 10-12 32 32 -0.9 0.19 0.19 0.8HO2/OH, HO2/OH, HO2/OH, NO2/NO, NO2/NO, NO2/NO,Latitude Altitude Without Ix With 1.5 pptv Ix a % Effect • Without Ix With 1.5 pptv Ix a % Effect b0-18 0-1 109 106 -2.2 1.30 1.31 0.70-18 1-2 105 103 -2.7 1.20 1.21 0.60-18 2-4 78 75 -3.2 1.21 1.22 0.70-18 4-6 81 77 4.0 0.87 0.88 0.60-18 6-8 76 73 4.3 0.68 0.69 0.60-18 8-10 66 61 -6.4 0.32 0.32 1.518-42 0-1 91 89 -2.1 1.55 1.56 0.71842 1-2 82 81 -2.1 2.00 2.01 0.41842 24 85 83 -2.6 1.46 1.46 0.418-42 4-6 71 68 -3.5 1.14 1.15 0.51842 6-8 64 62 4.5 0.70 0.71 0.91842 8-10 41 39 4.3 0.47 0.48 1.61842 10-12 32 31 -2.7 0.19 0.19 2.3HO2/OH, HO2/OH, HO2/OH, NO2/NO, NO2/NO, NO2/NO,Latitude Altitude Without I• With 7 pptv Ix, % Effect • Without I x With 7 pptv Ix, % Effect b0-18 0-1 109 98 -9.4 1.30 1.34 3.30-18 1-2 105 93 -11.4 1.20 1.24 2.90-18 24 78 67 -13.3 1.21 1.25 3.10-18 4-6 81 67 -16.5 0.87 0.90 3.20-18 6-8 76 63 -17.3 0.68 0.70 3.20-18 8-10 66 50 -24.2 0.32 0.34 7.518-42 0-1 91 83 -9.1 1.55 1.59 3.11842 1-2 82 75 -9.2 2.00 2.04 2.11842 2-4 85 76 -11.1 1.46 1.49 2.11842 4-6 71 60 -14.6 1.14 1.17 2.518-42 6-8 64 53 -18.0 0.70 0.73 4.31842 8-10 41 34 -17.2 0.47 0.50 7.418-42 10-12 32 29 -11.6 0.19 0.21 10.9For reference purposes the OH levels for 0ø-18øN latitude and 30 ø zenith angle are 3.6E+6 for 0-1 km and3.8E+6 for 6-8 km. For all altitudes and I x levels, increasesOH contribute roughly 60% to the total changein the HO2/OH ratio.' I x levels at all altitudes have been scaled to the "reference".• Based on steady state modeling conditions.this ratio does not appear to provide a satisfactory case of the marine BL but did so for a wide range of medianexplanation for the total enhancement in NO 2 levels observed values of CH3I. Since only the total source of iodine isduring PEM-West A although it might have been one of the important, not the specific form, we have here been able tocontributing factors (for further details, see Crawford et al. backouthe level of CH3I for BL conditions that corresponds[this issue]).to our high-altitude 7 pptv Ix scenario. For this specific case,BL levels of CH3I would be in the range of 10 to 13 pptv.Based on this input, we find that the C&D standard model4. Comparison With Previous Studiespredicts an approximate 25% enhancement in D(O3) over thatThe two most relevant tropospheric iodine modeling estimated from HOx related chemistry alone. As shown instudies that have preceded this work have been those reported Table 2 (bottom), this is basically the same enhancementby Chameides and Davis [1980] and ChaOqeM and Crutzen predicted from this work (i.e., 20%) for BL conditions in the[1990]. Chameides and Davis (C&D) only examined the tropics. Although this agreement appears to be quite good,

DAVIS ET AL.: POTENTIAL IMPACT OF IODINE ON TROPOSPHE•C OZONE LEVELS 2145it turns out that it is actually quite fortuitous. For example,on closer inspection one finds that C&D estimated IO levelsthe findings of Davis et al. [this issue] who report that forthe time period of PEM-West A this region displayedare nearly 1 order of magnitude higher than those estimated significant net-photochemical column destruction even in thein this work. As a result, both the IO self-reaction and thereaction with HO2 are found to contribute significantly toD(O3). The reason for C&D's high IO levels now appears tobe largely due to a major difference in the assignment of kvalues to the three critical processes (R6), (RS), and (R9).All three processes represent loss reactions for IO; and allthree have now been assigned new k values based on morereliable kinetic and photochemical data. The new values arenearly 1 order of magnitude higher than the C&D values.Cha•eld and Crutzen [1990] (C&C) also explored thequestion of tropospheric destruction of ozone by iodineabsence of iodine chemistry. This most likely suggests thatduring PEM-West A, Ix never reached the 7 pptv level. Thisconclusion would seem to be in agreement with theobservations that BL levels of CH3I were also low indicatinga marine region having low primary productivity.The influence of iodine on the radical ratio HO2/OH wasfound to be most significant for the upper free troposphereand for I,, levels of 7 pptv. For this I,, level the averageeffect for all altitudes was approximately 15%. The effect ofiodine on the NO2/NO ratio was also found to maximize athigh-altitude but never exceeded 11% even for the highest Ixalthough the main thrust of the study of these investigators scenario. If the maximum estimated shift in the HO2/OHwas the possible impact of iodine on the oxidation of marineDMS. These authors concluded that iodine was probably aninsignificant sink for ozone; however, this conclusion, wasbased on BL CH3I levels of only 3 pptv. In addition, C&Cbased their assessment on a k value for (RS) which hasrecently been reevaluated based on new kinetic data as being1 « orders of magnitude higher [DeMore et al., 1994]. TheJ value for (R16) was also taken to be nearly 3 orders ofmagnitude lower than that estimated here. All 3 factors havebeen found to significantly minimize the tropospheric effectsof iodine. This point was demonstrated here by carrying outsimulations using PEM-West A median values for allphotochemical species but invoking the C&C CH3I, (RS), and(R16) values. The D(O3) results in this case were within afactor of 1.5 of C&C, thus agreeing with the C&C evaluationin showing no significant effect on ozone from iodinechemistry.As noted above, the availability of new kinetic data wasthe basis for the k value assigned to (RS) in this study.However, in the case of (R16) no spectroscopic data areavailable from which to evaluate this J value [Sander, 1986;denkin, 1993]. C&C assumed values ranging from 3x10 '4 to2xl 0 '5 s '•, but this is 2 orders of magnitude lower than the Jratio were to be realized, this would result in a 9% increasein atmospheric levels of OH and thus lead to an approximate9% decrease in the lifetimes of OH-controlled trace gasessuch as CH 4 and CH3CC13.Of interest also is the fact that median levels of CH3I inthe 8-10 km altitude range (e.g., for the western tropicalPacific) reached 0.15 pptv, with individual observationsreaching 0.3 to 0.7 pptv. Although these altitudes are stillsignificantly removed from the tropical tropopause, if themixing ratios observed for CH3I represent only a fraction ofthe total iodine being convectively pumped into the uppertroposphere, it is still possible that Solomon et al. 's [1995]requirement of a few tenths of one pptv of iodine in thelower stratosphere might be reached on some occasions. Inthis context, because of the high sensitivity for detection ofiodine species like CH3I, together with its short lifetime of 2-4 days, CH3I would appear to be an excellent tracer for thetransport of marine BL air into the upper troposphere andlower stratosphere.Major uncertainties in the proposed iodine chemistryinclude the total marine iodine source strength as well as thereliability of several k and J values. In particular, it will beimportant in future field studies to include not onlyvalue calculated from the recently reported absorption measurements of CH3I but also those for other iodocarbons.spectrum for C1202 [DeMore et al., 1994]. We have takenwhat we believe is a rather conservative value for (R16),9xJ(C1202) which is approximately 3 orders of magnitudeIn addition, both the critical IO reaction involving HO2 (RS)and the self-reaction (R9) should be further investigated, asshould the photochemical cross section for the species I202.greater than C&C. This estimate is still 1 order of Finally, the efficiency for removal of iodine species due tomagnitude lower than the J value for (R6), involving thephotolysis of IO.high-altitude aerosol must be resolved. Quite significantogaining more direct insight into all aspects of the proposediodine chemistry would be direct observations of the radical5. Summary and Conclusionsspecies I and IO.The CH3I observations from PEM-West A, in conjunction Acknowledgments. This work was supported in part by fundswith updated J and k values for several tropospheric iodine from the National Aeronautics and Space Administration underreactions, suggesthat under certain conditions the addition grants NCC-1-148 and NAG-1438 and by the AtmosphericChemistry Project of the Climate and Global Change Program of theof iodine chemistry to current models leads to predictedNational Oceanic and Atmospheric Administration. The author D.enhancements in the rate of destruction of ozone. It alsoD. Davis would also like to thank the project office at NASApredicts shifts in the critical free radical ratio HO2/OH. Langley Research Center as well as the flight crews at NASA AmesAlthough the percent effect on D(O3) was found to increase Research Center for their strong support of the PEM-West A fieldsteadily with increasing altitude, the mid-troposphere was program. Finally, he would like to acknowledge many helpfulfound to be the major contributor to the 0 to 12 km columndiscussionswith William Chameides and Hanwant Singh.integrated value of D(O3). The total column effect on D(O3)was estimated to be only 6% for Ix levels of 1.5 pptv; Referenceshowever, this increased to 30% at tropical latitudes for theAnlauf, K. G., R. E. Mickle, and N. B. A. Trivett, Measurement ofhigh Ix scenario involving 7 pptv. A potential enhancement ozone during Polar Sunrise Experiment 1992, J. Geophys. Res.,in D(O3) of 30% represents a major impact on the 03 budget 99, 24,345-25,353, 1994.at tropical latitudes. In fact, it is difficult to reconcile with Atkinson, R., D. L. Baulch, R. A. Cox, R. F. Hampson Jr., J. A.

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