A study of gas-aerosol equilibrium and aerosol ph in the remote marine boundary layer during the First Aerosol Characterization Experiment (ACE 1)

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 5, NO. D3, PAGES 7,3-7,34, JULY 6, 2 A study f gas-aersl equilibrium and aersl H in the remte marine bundary layer during the First Aersl Characterizatin Exeriment (ACE ) A.M. Fridlind and M. Z. Jacbsn Deartment f Civil and Envirnmental Engineering, Stanfrd University, Stanfrd, Califrnia Abstract. A thermdynamic equilibrium mdel was alied t study the interactins f gas-hase NH3, HNO3, and HC with size-reslved aersls and estimate aersl H in the remte marine bundary layer during the First Aersl Characterizatin Exeriment (ACE ). Analysis f mdel results and field measurements indicates that accumulatin-mde aersls were rbably in equilibrium with NH3, HNO3, and HC simultaneusly. The largest carse-mde aersls did nt aear t be in equilibrium with HNO3, but may have been in equilibrium with NH 3 and HC. The estimated H f accumulatin-mde aersls was -2, a functin rimarily f the amunt f sulfate relative t sea salt resent in that mde. By cntrast, the estimated equilibrium H f carsemde aersls was 2-5, a functin rimarily f relative humidity and gas-hase HC. Prir t exsure t HC, the estimated H f fresh sea sray aersls was 7-9, a functin rimarily f relative humidity. Sensitivity tests shwed that the drying f aersls during the samling rcess may have vlatilized u t 3% f NH4 +.. Intrductin Natural and anthrgenic aersls are believed t lay an imrtant rle in climate thrugh their direct effect f scattering and absrbing radiatin and their indirect effect f serving as clud cndensatin nuclei. Measurements and mdeling studies have fund that marine aersl cmnents may cmrise a significant cmnent f bth the direct effect [Quinn and Cffman, 999; M. Z. Jacbsn, Glbal direct radiative frcing due t multicmnent anthrgenic and natural aersls, submitted t Jurnal f Gehysical Research, 999] and the indirect effect [Jnes and Sling, 997] f natural aersls n climate. Hwever, bth effects are strngly deendent un the size, number cncentratin, and chemical and tical rerties f the aersls [e.g., Hrvath, 993]. In rder t better quantify the characteristics f natural marine aersls, the First Aersl Characterizatin Exeriment (ACE ) was designed t measure the hysical, chemical, and tical rerties f aersls, as well as gas cncentratins, in the remte marine bundary layer [Bates et al., 998a]. Field data indicate that remte marine aersls, thse nt significantly affected by either anthrgenic r natural cntinental surces, are cmsed rimarily f sea salts (Na +, CI-, SO4 2-, Mg 2+, ca 2+, K +, CO32-, and Br-, in rder f ceanic mlar abundance), nitrate (NO3-), ammnium (NH4+), nn-sea-salt sulfate (SO42-), methanesulfnate (CH3SO3-), assciated hydrgen ins (H+), and ther rganics [e.g., Fitzgerald, 99; O'Dwd et al., 997]. The size distributin is ften cmsed f fur mdes, including an ultrafine mde, a nucleatin r Aitken mde, an accumulatin mde, and a sea salt r carse mde [e.g., Bates et al., 998b; O'Dwd et al., 997]. During ACE, in terms f dry gemetric diameter, these mdes were bserved at 5-2 nm, Cyright 2 by the American Gehysical Unin. Paer number 2JD //2JD nm, 8-3 nm, and >3 nm, resectively, in the number size distributin [Bates et al., 998b]. Aside frm aersl mass advected frm cntinents r entrained frm the free trshere, the mass is rduced exclusively by sea sray emissins and gast-article cnversin. Sea sray aersls are emitte directly by cean surface disturbances at bth accumulatin-mde and carse-mde sizes, and may be enriched un emissin with ceanic surface active rgani cmunds [e.g., Barger and Garrett, 97; Middlebrk et al., 998} All ther significant cmnents f sluble mass (NH4 +, NO3-, nn-sea-salt SO4 2-, CHTSO3-, and erhas additinal rganic secies) are secndary, resulting frm gas-t-article cnversin rcesses. Such rcesses include nucleatin, cndensatin, disslutin, and aqueus-hase xidatin f disslved intermediates. Whereas ambient NH 3 is accumulated as NH4 + rimarily by the acidic sulfate aersls, ambient HNO 3 is accumulated as NO 3- rimarily by the less acidic sea sray aersls. When acids are added t aersls cntaining sea salt, C- may be dislaced as HC, which may in turn be scavenged by fresh sea sray, enhancing C- in thse articles [e.g., Ericksn et al., 999]. A hst f ther hysical rcesses influences the bundary layer cncentratin and chemical characteristics f the marine aersl ulatin, including cagulatin, dry desitin, sedimentatin, clud frmatin, reciitatin scavenging, hrizntal advectin, and vertical exchange with the free trshere. Within this envirnment the nature f the artitining f chemical secies between the gas and aersl hases deends un the characteristics f the aersl ulatin, the characteristics f the secies, and the degree f gas-aersl equilibrium btained. Many gases may interact reversibly with the aersl hase, including NH 3, HNO3, HC, CH3SO3H, HCOOH, CH3COOH, SO 2, and 3. Amng these, nly NH 3, HNO3, and HC have been rven t interact with a reservir in the aersl hase that has been bserved t cmrise a significant fractin f ttal aersl mass in remte marine regins [e.g., Parung et al., 986; Quinn et al., 992], thereby exerting significant reversible influence n aersl hysical, chemical, and tical rerties. 7,3

2 7,326 FRIDLIND AND JACOBSON: GAS-AEROSOL EQUILIBRIUM AND AEROSOL H Limited evidence frm remte marine air masses indicates that HCOOH and CH3COOH may als fall int this categry [e.g., Matsumt et al., 998], but neither was measured during ACE, and sensitivity tests indicated that they wuld nt be exected t cntribute significant aersl mass at equilibrium under ACE cnditins. Current field data indicate that remte marine aer- sls that are nt fully neutralized are likely t be in equilibrium with ambient NH 3 [Quinn et al., 992], a result surted by mdeling studies f ACE aersls [Quinn et al., 998] and rttyical aersls under mre lluted cnditins [Meng and Seinfeld, 996]. By cntrast, field data gathered under lluted cnditins indicate that smaller sea sray aersls are likely t be in equilibrium with ambient HNO3 and HCI whereas larger aersls may nt be [Keene and Savie, 998, 999]. This bservatin has als been surted under sme cnditins by a mdeling study [Ericksn et al., 999]. Slutin H is an imrtant characteristic f aersls that may be clsely assciated with gas-aersl equilibrium. The H f remte marine aersls may be critical t aqueus-hase chemistry in the marine bundary layer [e.g., Chameides and Stelsn, 992; Clegg and Tumi, 997; Keene et al., 998; Sievering et al., 992], but few effrts have been made t determine aersl H since it is extremely difficult t reliably estimate frm measurements [Keene and Savie, 998]. Previus estimates have been based un indirect bservatin f marine aersls r mdels f rttyical aersls (Table ). Estimated H values under remte marine cnditins have ranged frm - t 3 in the accumulatin mde and 2- in the carse mde. The gals f this study were t aly a thermdynamic equilibrium mdel t the size-reslved aersl field data gathere during ACE in rder () t estimate whether aersls were equilibrated with gas-hase NH 3, HNO 3, and HC; (2) t test the sensitivity f gas-aersl equilibrium t the aersl samling rcess (aersls were heated and dried during samling) in rder t ascertain whether vlatile material shifted hase due t the samling rcess; and (3) t estimate the H f the aersls. Table. Estimates f Marine Aersl H Based n Indirect Observatins r Mdel Results H Cnditins Methd Surce Accumulatin mde Carse mde 6-6-9* remte bservatin Winkler [986] remte mdel this wrk cntinental mdel Zhu et al. [992] remte mdel Katshevski et al. [ 999] remte mdel Katshevski et al. [999] remte bservatin Winkler [986] remte mdel Ericksn et al. [999] remte mdel Keene et al. [ 998] remte mdel Vgt et al. [996] remte mdel Chameides and Stelsn [992] cntinental bservatin Keene and Savie [999] cntinental mdel Sander and Crutzen [996] remte mdel this wrk cntinental mdel Ericksn et al. [999] cntinental mdel Zhu et al. [992] Marine cnditins were designated as arximately either "remte," meaning free f anthrgenic r natural cntinental influence, r mre "cntinental." *Values btained frm leaching slutins f marine aersls. Table 2. Range and Median f Measurements Vaffable Range Median Aersl mass, ng m '3 Na Mg Ca K NH CI- 5-,9 344 Br ' CH3SO NO 3' NH3, t Pressure, mb Temerature, øc Ambient 4-3 Samle Relative humidity, % Ambient Samle Data Aersl, gas, and meterlgical measurements were made ver the Suthern Ocean (4ø-55øS, 35ø-6øE) between Nvember 8 and December, 995, n the Natinal Oceanic and Atmsheric Administratin (NOAA) R/V Discverer shi. This segment f the ACE exeriment was chsen due t the availability f NH 3 measurements, and all 9 aersl samles that were cllecteduring the segment were included in this study. 2.. Aersl Data Aersls were measured with a seven-stage Berner-tye cascade imactr, as described by Quinn et al. [998]. In chrmatgrahy was used t determine ttal sluble CI-, Br, NO3-, SO42-, CH383- ' Na +, NH4 +, K +, Mg2+, and Ca 2+. A mass clsure study using ther data gathered n the Discverer cncluded that these ins accunted fr all aersl mass within exerimental uncertainty [Quinn and Cffman, 998], indicating that rganics and insluble dust were nt a dminant cmnent. The samles were cllected frm an inlet arximately 8 m abve sea level and dried by heating rir t entering the imactr. Mean aerdynamic diameters at the dry samle relative humidity were 5,.32,.49,.89,.7, 3.3, and 7.4!am, reslving the accumulatin and carse aersl mdes. Samle exsure erids were 6-4 hurs, crresnding t an average samle vlume f 26 m 3. The range and median f the aersl measurements (summed ver the imactr stages) are shwn in Table 2. Of the 9 aersl samles cllected, 4 samles were significantly cntinentally influenced (samles and 36), as defined by 2-hur mean radn cncentratins exceeding mbq m -3, and the remaining 5 samles were mre reresentative f ure maritime air [Whittlestne et al., 998] Gas Data Gas data used were measured CO 2 and NH 3, and estimated HNO 3 and HCI. CO 2 was cllected frm an inlet arximately 3 m abve sea level and an average reresentative value f 36 m was used fr all samles. NH 3 was measured frm an inlet arximately 7 m abve sea level, with a samle exsure erid f 48 t 84 hurs, as described by Quinn et al. [998]. The range and median f the measurements are included in Table 2.

3 FRIDLIND AND JACOBSON: GAS-AEROSOL EQUILIBRIUM AND AEROSOL H 7,327 Since HNO 3 and HC measurements were unavailable, their cncentratins were estimated frm ther data sets. HNO3 was estimated as t, the mean summertime value measured under clean marine cnditins n the cast f New Zealand at 4øS, 75øE [Allen et al., 99T[ Other measurements f HNO 3 in the marine bundary layer in the Suthern Hemishere are cmarable, with a mean f less than 3 t measured at ø-3øs ver the Atlantic Ocean [Paenbrck et at., 992]. HC was estimated as t, a tyicalw value measured in bundary layer air under clean marine cnditins [Graedel and Keene, 995; Kajii et al., 997; Pszenny et al., 993; Singh, 995]. The sensitivity f the results t the estimated HNO 3 and HC! cncentratins is addressed belw in sectins 4.2 and Meterlgical Data Ambient... rneterlical... (l:- tn,qe'd wer ' 3-rain averayne nreqr - sure, temerature, and relative humidity, cllected frm instruments lcated arximately 3 m abve sea level. Temerature and relative humidity were als mnitred in the heated aersl samling line. All meterlgical variables were averaged ver each aersl samling erid t btain reresentative values. The range and median f the reresentative values are included in Ta- ble Gas-Aersl Equilibrium Mdel The fllwing rvides a descritin f the gas-aersl equilibrium mdel that was alied t the data. A cncetual verview is first given, fllwed by a technical descritin, a summary f the initial c{ nditins, and a discussin f the rimary sumtins. 3.. Cncetual ()ver, iew Observed gas and size-reslved aersl secies were assumed t be in thermdynamic equilibrium at bserved temeratures, ressures, and relative humidities. The thermdynamic equilibrium f the gas-aersl system can be cncetualized as the result f the cmetitive demand f the nnvlatile secies in each aersl size bin fr the available vlatile cmunds in the system. The nnvlatile aersl secies cannt migrate amng the size bins, and thus their redicted size distributins always equal their initial distributins. ttwever, the vlatile aersl secies may migrate amng the varius aersl size bins via the gas hase until equilibrium has been btained. Thus the redicted size distributins f the vlatile secies may differ frm the measurements with which they were initialized. Deviatins between the redicted and measured aersl size distributins f the vlatile secies result frm three general surces f errr: () errr in the mdel assumtins (e.g., lack f actual equilibrium between the gas and aersl hases), (2) errr in the mdel calculatins (e.g., errr in the estimated water utake f sea salts), r (3) errr in the estimated r measured cncentratins f gases r aersls (e.g., exerimental uncertainty in measured C- cncentratin). In this study, the thermdynamic equilibrium f the gasaersl system was calculated under the ambient cnditins in the marine bundary layer (high relative humidity and lw temerature) and als under the samling cnditins in the article imactr (lw relative humidity and higher temerature). The equilibrium gas-aersl artitining f the vlatile aersl secies may be esecially sensitive t changes in the relative humidity f the system due t the cncmitant change in the liquid water cntent f the aersls Technical Descritin The EQUISOLV II mdel alies the Zdanvskii-Stkes- Rbinsn equatin t estimate liquid water cntent and Brmley's methd t estimate mean mixed activity cefficients [Jacbsn, 999a, b]. Additinal thermdynamic data added t the mdel fr this study were the øc dissciatin, water activity, and binary activity cefficients alicable t CH3SO3H(a q) slutin [Clegg and Brimblecmbe, 985; Cvingtn et al., 973]. Gashase secies cnsidered were H2, CO2, NH3, HNO3, and HC. Aersl-hase secies cnsidered were H2(aq), H2CO3(aq), H2SO4(aq), Cl-h. SO3H(aq), NH4 +, Na +, Mg 2+, C +, K +, H +, NO3-, HSO4', SO4 2-, Cl-, Br-, Ct SO3-, HCO3-, CO32-, NH4NO3(s), NH4Ci(s), NH4HSO4(s), (N H4)2SO4(s), (N}4)3(SO4)2(s), NtI4HCO3(s), NaNO3(s), NaCl(s). Na- [tso4(s), Na2SO4(s ), NaHCO3(s), Na2CO (s), KNO3(s ), KCI(s), KHSO4(s), K2SO4(s), KHCO3(s), K2CO3(s), Ca(NO3)2(s), CaC2(s), CaSO4(s), CaSO4-2H2(s), CaCO3(s), MgC2(s), Mg(NO3) 2, MgSO4(s), and MgCO3(s). The inuts fr each samle were aersl mass cncentratins, gas cncentratins, and meterlgical variables. Aersl inuts were the bserved sluble mass f Na ', CI-, Ca 2+, Mg 2+, K +, Br-, SO42-, Ct3SO3-, NH4 +, and NO - in each f the seven size bins. Initial charge irabalances in each size bin were atlributed t unmeasured tt + and CO32-. Gas-hase inuts were bserved CO2 and NH 3, and estimated }tno 3 and }IC. Meterlgical inuts were bserved ressure, temerature. and relative humidity (either in the marine bundary layer r in the article imactr). Mdel ututs were the equilibrium cncentratin {)f all gas secies and the cncentratin f each aqueus, inic, and slid cmnent in each size bin f the aersl hase, including equilibrium,xater cntent and }t. The ttal sluble mass f a cmund was then calculated as the sum f its liquid, inic, and slid cncentratins in rder t cmare it with bservatins. Fr examle, the ttal sluble qo42' wuld be the sum f the SO42- mass redicted as t2so4(aq), HSO4-, SO42-, NH4 t$o4(s), and all ther slids cntaining sulfate. Since multile size bins were used, the thermdynamic equilibrium slutin was unique nly if n slids were redicted that cntained tw vlatile secies (i.e., NH4NO3(s). Ntt4Cl(s), r Ntt4HCO3(s) ) [Jacbsn, 999b]. Such slids were never redicted in the ACE I samles, ermitting a unique slutin in all cases. All aersl field data were rerted as functins f the mean aerdynamic diameter at the samling relative humidity, and this value was assumed t be the initial gemetric diameter f the mdel size bin under bth ambient and samling cnditins. Mdel results indicated that gemetric diameter did nt tyically deviate frm the mean aerdynamic diameter by mre than 8% and dry gemetric diameter did nt tyically deviate frm it by mre than 3% Furthermre, mdel results were indeendent f the bin size secificatin since Kelvin effects are small fr articles larger than - [ m diameter [e.g., Jacbsn, 999a]. All field data and mdel redictins are rerted here as a fimctin f the dry aerdynamic diameter Initial Cnditins The bserved gas-aersl systems were first cnsidered at the ressures, temeratures, and relative humidities bserved in the marine bundary layer (sectin 2.3). Gases were initialized as

4 --- 7,328 FRIDLIND AND JACOBSON: GAS-AEROSOL EQUILIBRIUM AND AEROSOL H measured r estimated (sectin 2.2). In the aersl hase each f the 9 aersl samles was first initialized with the bserved secies in each size bin (sectin 2.). Hwever, exerimental uncerideal and sea-salt samles t indeendent equilibratin with NH 3, HNO3, and HC was tested t elucidate the rle f each gas in cntrlling aersl H. tainty in the majr sea-salt ins ften created large charge imbalances and discntinuities in H frm ne size bin t the next. Thus each f the 9 aersl samles was als initialized using a charge-balancing rcedure: CI', Mg 2+, Ca 2+, K +, and sea-salt 3.4. Mdel Assumtins Alying the mdel t the available data invlved a number f SO42- were als scaled t measured Na + using standard sea-salt imrtant assumtins regarding () the simlificatin f the cmsitin [Stumm and Mrgan, 996]; Br- was nt ermitted t exceed the sea-salt Br-:Na + rati, but was nt raised if it was deleted [e.g., Ayers et al., 999]; and all f the secndary secies, including nn-sea-salt SO4 2- (defined as that exceeding the sea-salt rati t Na+), CH3SO3-, NH4 +, and NO3-, were unchanged. Initializing with the charge-balancing rcedure reserved the rimary characteristics f the marine aersls and eliminated the rimary influence f exerimental uncertainty n the results. In additin t mdeling the 9 ACE samles, tw rttyical samles were derived t reresenthe behavir f generic remte marine aersls. First, an idealized samle was derived frm the clean marine samles by using the median values f measured meterlgical inuts, NH 3, Na +, CH3SO3-, NH4 +, NO3-, and nn-sea-salt SO42-, and scaling CI-, Ca 2+, Mg 2+, K +, Br-, and sea-salt SO42- t the median Na + in each size bin (Figure ). Secnd, a ure sea-salt samle was derived by remving hysical gas-aersl system (articles were unifrm and internally mixed), (2) the rerties f the bservatins (gases and aersls were statinary in time, gas-aersl artitining was insensitive t the aersl samling rcess), and (3) the thermdynamic state f the gas-aersl system (articles were r were nt deliquescent, gas-aersl equilibrium was btained). Because these assumtins are imrtant t the interretatin f the mdel results, they are discussed here.in detail rir t resentatin f results in sectin Unifrm internal mixture. Particles n a given imactr stage were assumed t be a well-mixed internal mixture f the measured secies. This was rbably a gd assumtin in the carse mde, but erhas smetimes less accurate in the accumulatin mde. Tw distinct tyes f articles, sea-salt articles and ammnium-sulfate articles, have been fund t dminate the aersl in remte marine regins accrding t analysis by single-article methds [Gras and Ayers, 983; Mclnnes et al., all secndary secies (CH3SO3-, NH4 +, NO3-, and nn-sea-salt 994], hygrscic grwth [Berg et al., 998} and vlatility SO42-) frm the ideal samle. Simultaneus equilibrium with all gases was mdeled fr the ideal and sea-salt samles, as well as fr the 9 ACE samles using bth charge-imbalanced and charge-balanced data. The charge-balance data were cnsidered mre reresentative f actual cnditins; hwever, the results using charge-imbalanced data are als included here in rder t entirely establish the imact f the charge-balancing rcedure. [Kreidenweis et al., 998]. These tw tyes crresnd t the tw dminant surces f remte marine articles discussed abve: sea sray and nucleatin. Whereas essentially all f the carse-mde articles have been fund t be sea salt [Pdsfai et al., 995; Gras and Ayers, 983; Kreidenweis et al., 998], a mixture f searate sea-salt and ammnium-sulfate articles has been bserved in the accumulatin mde [Berg et al., 998; Gras and Ayers, 983; Kreidenweis et al., 998} Hwever, evidence Several sensitivity tests were then erfrmed n the charge- has als indicated that almst all accumulatin-mde aersls balanced, ideal, and sea-salt samles. The sensitivity f the gasaersl artitining t the ambient gas cncentratins was tested larger than 3 m may cntain sea salt [Murhy et al., 998]. In the accumulatin mde, deviatins frm unifrmity may be by initializing each gas t a minimum value f zer and a maximum value f twice its measured r estimated cncentratin. The sensitivity f the gas-aersl artitining f NH 3, HNO 3, and HC t the heated, dry samling cnditins was tested by setting temerature and relative humidity t values reresentative f the samling cnditins (sectin 2.3). Finally, the sensitivity f the imrtant t gas-aersl interactins because an internal mixture f nn-sea-salt sulfate with sea salt may behave very differently frm an external mixture. Secifically, an equimlar internal mixture f aqueus NH4HSO4(s) and NaCl(s) wuld emit essentially all Cr and NH4 + as HC and NH 3 under tyical ACE cnditins, whereas an equimlar external mixture wuld retain bth 3 ' ' ' ' ' '''l ' ' ' ' ' '''-- Na +, 2 [] NH4 + ' Mg 2+ =. C- ) ø ß NO 3-, SO 2-,- - 4 A CH3SO3- -2 D (m) Figure. Size distributin f aersl secies in the ideal samle (Ca 2' and Br- nt shwn).

5 FRIDLIND AND JACOBSON' GAS-AEROSOL EQUILIBRIUM AND AEROSOL H 7, NH 3, r= ß NH 3, r= [] HNO3, t=l -- HNO3, r= a HC, r= ß HC, ct=5-4 I I I I I I I I J I I D (m) Figure 2. Calculated time required fr aersls t btain 95% equilibratin with a given gas as a functin f the accmmdatin cefficient f the gas, r. secies. This behavir has been verified by individual article data in the marine bundary layer with resect t HC emissin [e.g., Mclnnes et al., 994]. During the ACE erid studied here, hygrscicity data gathered n the Discverer indicated that a range f -8% f the 65-m articles were externally mixed sea salt [Berg et al., 998]. Hwever, data were nt gathered at larger article sizes, where the accumulatin-mde mass was cncentrated Statinarity. Fr mdeling urses, the gas and aersl mass cncentratins were assumed t be cnstant during the time that each aersl samle was cllected. Hwever, gas and aersl cncentratins may have varied significantly during aersl samling erids. Other data sets rvide evidence fr diurnal and air mass variatins in HNO 3 and HC [Keene et al., 993; Paenbrck et al., 992]. Particle size distributins measured n the Discverer during samling als indicated that the aersl ulatin was nt cnstant ver the lng imactr samling erids [Bates et al., 998b]. The deviatins f redictins frm measurements due t the inaccuracy f this assumtin were nt quantified due t lack f data describing shrt-term deviatins in gas and inic mass encuntered Humidity-insensitive artitining. The measured gas and vlatile articulate mass cncentratins were assumed t be valid under bth samling and ambient cnditins. Hwever, the samling rcess (heating and drying) may have vlatilized sme aersl secies (such as NH4 + r NO3- ). If vlatilizatin tk lace, the ambient gas cncentratin lus the samled aersl cncentratin f a given vlatile cmund wuld reresent an underestimate f the ttal vlatile mass actually resent in the marine bundary layer. The ssible extent f this underestimate under ACE cnditins is quantified in sectins Slid frmatin. Under ambient cnditins the re- quirement f hmgeneus slid frmatin was cnsidered t discurage slid frmatin abve the crystallizatin relative humidity (CRH). Slids were assumed t frm nly belw their CRH, and n slids were redicted in any samle. Hwever, under the dry samling cnditins n the imactr substrates, hetergeneu slid frmatin n the substrate media was assumed t ermit slids t frm clse t their deliquescence relative humidity (DRH). Slids were then assumed t frm when the relative humidity fell belw their DRH, and slids redicted t frm in sme samles included (NH4)2SO4(s), (NH4)3H(SO4)2(s), NaCl(s), Na2SO4(s), KCI(s), K2SO4(s ), CaSO4-2H2(s ), MgSO4(s), and MgCO3(s). Sensitivity tests indicated that these assumtins generally had negligible influence n the results excet in the three cases where significant CaSO4-2H2(s) influenced the redicted artitining f nitrate at the dry samling relative humidity, as discussed in sectin Gas-aersl equilibrium. Gas-aersl equilibrium was assumed by the mdel fr all gases resent. Whereas the equilibratin f aersls with atmsheric water var and CO2 is generally fast due t the high cncentratins f thse gases, equilibratin with trace gases at lwer cncentratins may take much lnger. Gas-aersl equilibrium may nt always have been btained by the majrity f articles f a given size under ACE cnditins. An uer limit t the indeendent equilibratin time f each aersl size bin f the ideal samle with each ambient gas was estimated with the methd used by Jacb [985] fr clud drs, excet that mdel-calculated gas-aersl artitining was used in lace f the effective Henry's law cefficients in rder t accunt fr the activity cefficients in cncentrated aersl slutins [e.g., Quinn et al., 992]. The equilibratin times (Figure 2) reresent the arximate time required t reach 95% equilibratin f the aqueus hase during a diffusin-limited transfer rcess. Fr examle, if all f the equilibrium NH4 + in a given size bin were instantaneusly dislaced t the gas hase, the equilibratin time wuld be the time required fr the article t reabsrb 95% f the dislaced gas. A range f gas accmmdatin cefficients was used due t uncertainty in the values. The accmmdatin cefficient f HC ver a water surface has been measured as 5, while a tyical range f - was used fr NH 3 and HNO3 due t lack f data [Seinfeld and Pandis, 998, and references therein]. Measurements f HNO3 utake by aqueus NaCl(s) articles suggest that the accmmdatin cefficient is likely t be relatively large [Abbatt and Waschewsky, 998]. Similarly, measurements f NH 3 utake by sulfuric acid slutins suggest that the accmmdatin cefficient may increase frm -- t near unity as slutin acidity increases [Swartz et al., 999]. Overall, the results suggest that accumulatin-mde aersls in the smallest three size bins equilibrated with all gases within - hurs, a time that was rbably significantly shrter than the mean lifetime f the articles in that size range. Whereas all f the larger articles were als likely t be in equilibrium with

6 ß...,... 7,33 FRIDLIND AND JACOBSON: GAS-AEROSOL EQUILIBRIUM AND AEROSOL H.3 #27.3 # Ol.3... #3.2 D (vm).2 #3 D (m), #32 D (m) i,... ' D ( m) # # D (m).3 Ol #35 D (m) D ( m) D (m) lo. #37 ß #38 i D (r) 3 {} i #39 I {} D ( m).3 #4 V i. D (m) #4 D (m).2.2. D (m).:3.., # #43 D (m).2 D ( m), #45 I. D (m) D (m) D (m).-..3 ideal 'E sea salt.2.2,..,.., -- i lo. i. D (m) D (m) D (m) Figure 3. Cmarisn f bserved NH4 + (circles with uncertainty bars) with calculated NH4 + using chargeimbalance data (dashed lines) and charge-balancedata (slid lines).

7 FRIDLIND AND JACOBSON: GAS-AEROSOL EQUILIBRIUM AND AEROSOL H 7,33 NH 3, estimated equilibratin times with HNO 3 and HC were ---3 hurs, times that may have been n the rder f the mean lifetime f the larger articles. Thus, whether the articles larger than m were in equilibrium with HNO 3 and HC may have deended un their lifetimes, which in turn may have varied significantly acrss the carse mde due t differential lss rates by sedimentatin and dry desitin [e.g., Slinn and Slinn, 98] = O v v e e.2- v - v ß a H<3 3<H<4 H>4 4. Results.- øe - Mdel results fr aersl equilibrium with each gas, NH 3, HNO3, and HC, are analyzed in sectins The mdeled H f the aersls is then analyzed in sectin Aersl Equilibratin With NH 3 Overall, the acidic sulfates in the accumulatin mde had the greatest demand fr the available ammnia in the system, whereas the less acidic sea salts in the carse mde had the least demand (Figure 3). The generally gd match between chargebalanced calculatins and bservatins, cmbined with the shrt exected equilibratin times (Figure 2), suggests that the aersls were rbably in equilibrium with NH 3. Using a different mdel and assuming a fully external mixture f sea salts and nn-sea- salt SO4 2-, Quinn et al. [998] als cncluded that aersls aeared t be in equilibrium with NH 3 during ACE Accumulatin-mde NH4 +. In the accumulatin mde, NH4 + was cmmnly underredicted (Figure 3), and the degree f underredictin was clsely crrelated with increasing cncentratins f sea salt relative t sulfate (Figure 4). Physically, HC was released frm the articles cntaining sea salt, increasing H and reducing the demand f the articles lbr NH4 +. Hwever, it is likely that accumulatin-mde articles were actually in equilibrium with bth NH 3 and HC (Figure 2). Why, then, did equilibrium redictins deviate frm bservatins in sme samles? The reasn fr underredictin f accumulatin-mde NH4 + cannt be cnclusively determined here. The three general tyes f errr that may ccur, as discussed in sectin 3., include errr in mdel assumtins, errr in mdel calculatins, and errr in initialized values. With resect mdel assumtins, it is nted.4- "'7' ,, t- 2:.4- Z Na +(bs.)/so42-(bs.) Figure 4. Rati f calculated equilibrium NH4 + t bserved NH4 + in the.32- m size bin as a functin f the rati f b- served Na + t SO42- in that size bin n a mass basis NH 3 (t) Figure 5. Calculated NH4 + in the 3.4- m size bin as a functin f equilibrium NH 3 and H in that size bin. that any degree f deviatin frm the assumed unifrm internal mixture f sea salt with nn-sea-.salt sulfate wuld result in un- derredictin f NH4 + by the mdel. With resect t errr in mdel calculatins, the EQUISOLV II mdel has been cmared with the AIM2 and SCAPE2 mdels fr the behavir f the fundamental Na+-C--NH4+-SO42- system encuntered here [Zhang et al., 998], and the AIM2 mdel has in turn been cmared with cius labratry data [Clegg et al., 998], making significant mdel errr unexected. With resect t errr in measurements, it is nt immediately exected that the errr wuld be significantly skewed t result in underredictin f accumulatinmde NH4 +. In summary, the hysical cause f underestimated submicrn NH4 + was the assumed internal mixture f large quantities f sea salt with the accumulatin-mde SO42- (Figure 4). Hwever, it cannt be crrbrated that sme degree f external mixture r nnunifrmity amng the articles was actually resnsible fr the deviatin between mdel results and bservatins Carse-mde NH4 +. In the carse mde, bservatins indicated that the sea-salt aersls had little demand fr NH4 +, but significant equilibrium NH4 + was redicted in abut ne third f the samles (Figure 3). This was clsely crrelated with tw factrs: () high equilibrium NH 3 gas and (2) lw equilibrium H in the carse mde (Figure 5). High equilibrium NH 3 did nt aear t exlain the discreancy between redictins and bservatins. NH 3 cncentratins >2 t were measured during six samles (samles 3-36), but NH4 + was rarely measured in the carse mde. Furthermre, lack f equilibrium with the gas hase did nt aear t exlain this discreancy between mdel results and bservatins. When the equilibratin times in Figure 2 were adjusted t accunt fr utake f significant NH 3 by the carse mde, the equilibratin time f all excet the largest size bin remained less than hurs and less than the equilibratin times with HNO 3, which had sufficient time t accumulate significantly as NO3-. Lw estimated H in the carse mde aeared mre likely t exlain instances f verredicted NH4 + in the carse mde. As discussed belw (sectin 4.4), mdeled carse-mde H was cntrlled by equilibratin with HC at a given relative humidity. Cnsistent with this finding, tw changes in the initial cnditins raised equilibrium H abve 3.5 and eliminated the equilibrium carse-mde NH4 + in all charge-balanced samles: () reducing relative humidity t 65% and (2) reducing equilibrium HC belw 65 t in all cases, which required remving all initial HC as

8 7,332 FRIDLIND AND JACOBSON: GAS-AEROSOL EQUILIBRIUM AND AEROSOL H I I I bs. (a) (b) (c) I..8 ' '.6 ".4.2. I. 2,,-7 I. I.99.98, I.96 Z.94 I I I.93 est. (a) (b) (c) --- l I i ]l ii est. (a) (b) (c) Figure 6. Cmarisn f the bserved r estimated articulate fractin with that calculated in equilibrium with initial gas cncentratins f (a) zer, (b) bserved r estimated values, and (c) duble the bserved r estimated values. Bxes san % f the data ints abve and belw the median, with whiskers and utliers indicating the range f the remaining data. well as 2-4% f the initial Cr. This leads t the ssible cnclusins that () NH4 + was resent at the ambient relative humidity but was vlatilized during the measurement rcedure r (2) ambient HC was less than-65 t. Furthermre, the mdel results indicate that an equilibrium value f 65 t f HC culd be achieved nly if the ttal chlride (gas lus aersl hase) in the system were sufficiently deleted relative t the ttal sdium. This in turn wuld imly a lifetime f HCI that was shrter than the lifetime f the chlride-deleted aersls, which seems ssible due t the seed f lss f HC t the cean surface and the relatively lng lifetime f the submicrn aersls that are mst deleted. Aside frm these changes in redicted H and NH4 + in the carse mde, mdel results were insensitive t minr ttal chlride deletin. Overall, hwever, these hytheses cannt be cnfirmed since HC was nt measured n the Discverer Sensitivity f equilibriumf(nh4+). When NH 3 was dubled, an average f 5% f the additinal gas was absrbed by the articles (Figure 6). Hwever, this average includes significant additinal absrtin in the carse mde in many samles. Cnsidering nly the ideal case, where carse-mde absrtin was less, abut 3% f the additinal gas was absrbed, 2% in the accumulatin mde and % in the carse mde. Equilibrium f the aersls n the imactr substrates with NH 3 was likely since mst article samling erids were at least twice as lng as the estimated equilibratin times (Figure 2). Overall, mean f(nh4 +) shifted frm.65 under ambient cnditins t.45 under samling cnditins (Figure 7), indicating that -3% f NH4 + may have been vlatilized during the samling rcess. Other mdel runs indicated that ure NH4HSO4(s ) slu- tin exhibited negligible vlatilizatin f NH4 + when subjected t drying, whereas NaCI(s)-NH4HSO4(s ) slutin exhibited-4% vlatilizatin. Thus the extent f redicted vlatilizatin was heavily deendent un the assume degree f internal mixture f the ammnium sulfates with the sea salts in the accumulatin mde, and 3% shuld be cnsidered an uer limit t the ssible vlatilizatin Aersl Equilibratin With HNO 3 In all samles, equilibrium NO 3- was distributed rrtinally t sea salt (Figure 8). The cmmn underredictin f NO 3- in the smallest articles and the cnsistent verredictin f NO 3- in the largest articles, cmbined with the lng exected equilibratin times f large articles (Figure 2), suggesthat the smaller articles were in equilibrium with HNO 3 whereas the largest ar- ticles were nt Accumulatin-mde NO3'. The aersls in the accumulatin mde had negligible demand fr HNO3 relative t the less acidic sea salts in the carse mde. Hwever, a significant fractin f NO 3- was bserved in the secnd size bin in eight samles (samles 27, 3, 33-37, and 45), and was als ften measured in the third size bin. Underredictin f accumulatinmde NO 3- in thse samles was caused in art by lw HNO 3 redicted at equilibrium due t the absrtin f mst initial gas by the carse-mde articles. When the equilibrium f the aersls in nly the three smallest size bins with initial HNO 3 was calculated (in rder t revent absrtin by the largest articles), significant NO 3- was redicted in the third size bin in mst samles but still nt in the secnd size bin ' f (N H 4+) at ambient RH.7I ß ß.6[/... I...,,,, I,,,, f (N 3 ) at ambient RH.97'.96.95,,, I... I f(cf) at ambient RH Figure 7. Calculated articulate fractin f ammnium, nitrate, and chlride at samle versus ambient relative hu- nidity. Nte varying axis scales.

9 ...,......, FRIDLIND AND JACOBSON: GAS-AEROSOL EQUILIBRIUM AND AEROSOL H 7,333..2, 5. #27,-,. '.2, 5 #28,-,.,2,.2, 5 # #3 D (m) f-,.5 O 'fi f #3 ==.2[ :, O. 5 lo D (m) ' -,.5,..-,. 'E.2 a, 5 #32... D (lam).5 '. '.5 8 #33 D (m). t t 5.2 #34 I D (m) lo.2 D (m) 6.4 = O E.2 #36,.5 D (r) D ( m) " -5., #37 5 # Ol D (m),, 5 Ol /, -- 5 Z. E 2 () #39 D (m). z,.. () 5 5 ; t.,-. () "-E () 2 () lo I #4() D (m).-- 2 "*E #4... ' D (in) '"" 5 5 z.= 5 'Z -- ().5 ',-. 'f-,.., E.2 #42 D (m),--,. "'.2 #43 D (m).-,.2 "E O. #44 I D ( m) '-' 5 =.5 *,-. ',.5 z.= 'f '.5 'f-'.(i., E.2 #45 D (m),--,. 'E.2 ideal D (m),-,.2 'E O. sea salt D (m) '-" z.= D (m) ' D (m). O. i,, ß D (m) Figure 8. Observed NO 3- (circles with uncertainty bars) and calculated NO 3- fr charge-imbalanced data (dashed lines) and charge-balancedata (slid lines). Nte unique x axis scale fr samle 33.

10 7,334 FRIDLIND AND JACOBSON: GAS-AEROSOL EQUILIBRIUM AND AEROSOL H E 2 #27 'E 2 #28 'E 2 # , 5 -= 'U 5,U 5 i D (m) lo D (m) D ( m)?'e 2 '--= 5 #3 'E 2 5 =' #3 "E 2 #32, 5 = 'U 5 lo D ( m) D (r) D (m) 'E 2 5 #33?E 2 5 #34 E 2 #35, 5 -=,U 5 'U 5 D ( m) D lo D (m) lo D (r) v "E 2 #36 #37 ' E 2 #38 5 g' l 5 ß '= 'U 5 (). D (m) D (r) D (r) E 2 #39 "E 2 #4 E 2 #4 3 5 lo 'U 5 "E #42 D (m) -- '-' #43 'E 2 ==' 5 = D (m) ' 'E 2 5 #44,. D (r) ' 'U 5,U g' lo, 5 -= #45 lo. i lo D (r) D (r) D (r), ideal 'E 2 =' : sea salt i. D (m) D (m) 'E 2 = lo 'U 5 D (m) Figure 9. Cmarisn f bserved C- (circles with uncertainty bars) with calculated C- fr charge-imbalanced data (dashed lines) and charge-balance data (slid lines).

11 FRIDLIND AND JACOBSON: GAS-AEROSOL EQUILIBRIUM AND AEROSOL H 7,335 Fresh sea sray may have accunted fr NO 3' bserved in the secnd size bin. When CH3SO 3- and nn-sea-salt SO4 2- were further remved frm the three smallest size bins, leaving nly nitrate-enriched sea sray, redicted NO 3- exceeded bservatins in all three size bins. Thus sufficient sea salt and HNO 3 was resent t accunt fr all bserved NO 3- if sme degree f external-mixture (fresh sea sray) were cnsidered in the accmnulatin mde. Furthermre, accumulatin-mde NO 3- was highly crrelated with sea sray rductin: seven f the nine samles with eak CI- exceeding lag m -3(indicative f recent sea sray rductin) als exhibited NO 3- in the secnd size bin, accunting fr all excet ne such samle. It is als ssible that dust surfaces accunted fr the bserved NO 3- in the secnd size bin, but this seems less likely. Ttal marine NO 3- has been reviusly crrelated with cntinental influence [e.g., Prser and Savie, 989]. Of the fur samles cnsidered cntinentally influenced (samles and 36), three did exhibit NO 3- in the secnd size bin, but this did nt accunt fr the remaining majrity f such cases. While the charge- balancing rcess remved the influence f disslved crustal secies, NO 3- was als underredicted in the charge-imbalanced case (Figure 8). A mass clsure study als indicated that insluble dust was nt a significant cmnent f aersl mass [Quinn and Cffman, 998] Carse-mde NO3'. The largest carse-mde articles did nt aear t be at equilibrium with HNO 3. In the tw largest size bins, equilibrium NO 3' always exceeded measured NO3-. Furthermre, the discreancy between the bserved and calculated size distributins f NO 3- was indeendent f the assumed initial cncentratin f HNO3: the equilibrium distributin was always shifted t the larger articles relative t the bserved distributin. Observatins made under mre lluted cnditins measurements (Figure 9). Overall, the size distributin f CIwas determined rimarily by the size distributin f sea-salt catins, which had the greatest demand fr the chlride in the sy s- tem. Sea-salt C- was als dislaced as HCI by nn-sea-salt sulfate Dislacement f CI'- Nn-sea-salt sulfates were cncentrated relative t sea salt in the smallest size bins, resulting in increasing C- deletin with decreasing size (Figure ). This rcess deleted a large tractin f the C- resent in the smallest have indicated that larger sea-salt articles may be subsaturated size bins (35-%), but nly a small fractin f the ttal chlwith HNO 3 relative t micrn-sized articles [e.g., Keene and Savie, 998], and such behavir has als been bserved in castal regins [e.g., Pakkanen, 996]. It has been rsed that this culd be due t kinetic transrt limitatins t equilibrium (e.g., Figure 2) r reactin rate limitatins related t the frmatin and dislacement f HC, but it has nt yet been satisfactrily exride resent in the system (.9-8.5%). The deletin f sea-salt CI~ frm the ideal samle was 3% f the ttal, cnsistent with ttal lsses f -3% bserved in the ACE regin [Ayers' et al., 999]. Hwever, the redicted deletin was deendent un estimated initial HCI and the effect f the charge-balancing rcedure n ttal CI-, as well as the assumed degree f internal mixlained [e.g., Keene et al., 998; ten Brink, 998]. ture in the accumulatin mde. Uncertainties in these factrs Sensitivity f equilibrium f(no3'). Regardless f initial HNO 3, the sea-salt articles were caable f absrbing almst all available gas (Figure 6), cnsistent with the results f ther mdeling studies [e.g., Keene et al., 998]. When HNO 3 was dubled, an average f 97% f the additinal gas was absrbed recluded quantifying ttal lss frm each samle. Nnetheless, the attern f lss frm the s nallest fur size bins was deendent nly un the fact that enrichment by secndary acids was mst significant in thse size bins. Thus results suggest that mst lss wuld ccur even if the largest size bins by the articles. Additinally, equilibrium NO 3- was always dis- tributed rrtinally t sea salt, regardless f the assumed initial cncentratin f HNO 3. In mst samles the smnling rcess slightly increased equilibrium f(no3- ) due t the decreased H f the mre cncentrated aersl slutin remaining after redicted slid frmatin (Figure 7). N slids cntained nitrate, leaving all nitrate in the remaining slutin. In three samles (samles 28, 4, and 4), hwever, the redicted frmatin f CaSO4-2H2(s) at the samling relative humidity in the intermediate size bins decreased the water cntent sufficiently t significantly lwer disslved NO3-. Because f the small number f samles where nitrate artitining was significantly changed, we cnclude that the artitining f nitrate was generally unchanged by the samling rcess Aersl Equilibratin With HCI The equilibrium size distributins f CI' (with estimated HC) were almst always within the exerimental uncertainty f the t [ i [ i i iii I i! [ i i i : i i i i D (m) Figure. Calculated chlride deletin f charge-balanced samles as a functin f aersl size. were nt in equilibrium with the ambient gases. The equilibratin times in Figure 2 suggest that equilibrium with HC culd have been btained 2- times mre quickly than with HNO 3, but the times may still be lng fr the largest articles. Overall, exerimental uncertainty in CI' made it imssible t estimate whether individual size bins had btained equilibrium with HC, in cntrast t the case f nitrate Sensitivity f equilibriumf(cl'). When HCI was dubled, an average f less than 5% f the additinal HC was taken u by the articles (Figure 6). Hwever, the fact that exerimental uncertainty in C- measurements (u t +% f.5-.9 lug m -3) may have exceeded any estimate f the gas hase reservir (-3 t, r lag m -3) made it difficult draw firm cnclusins regarding actual sensitivity t gas cncentratins. Similar t f(no3-),f(c-) was generally unchanged un aersl dehydratin, an average f less than % lwer than under ambient cnditins (Figure 7). Unlike NO3-, hwever, mst C- was redicted in slid frms at the dry samling relative humidity.

12 ,336 FRIDLIND AND JACOBSON: GAS-AEROSOL EQUILIBRIUM AND AEROSOL H #27 #2 /, #29 lo / lo a / /- I ' O (m) D (m) (m) g3 g3 g32 /... / a ' ' D (.m) D (.m) (.m) $33 $34 - a. 5 I D (vm) D (.m) D (vm) $36 / $37 $38 / a. 5. I I D (vm) D (vm) D (vm) $39 $4 $4 l l / / a. 5. lo. lo. lo D (,m) D (,m) D (,m),..., g42 ' ' ' g43 g44 ' a. 5. D (.m) D (m) D (.m)...,...,...,... g45 ideal sea salt a. 5. I I D (,m) D (,m) D (,m) Figure. Calculated H fr charge-imbalanced data (dashed lines) and charge-balancedata (slid lines) Aersl H In the charge-balanced case, similar H. rfiles were redicted in all samles (Figure ). The larger size bins generally exhibited unifrm H values f 2-5, whereas the first and secnd size bins generally exhibited lwer H values f Submicrn versus suermicrn H. The mdeled H in the submicrn size bins was rimarily a functin f the relative acidity f thse size bins (Figure 2). By cntrast, the mdeled H f the suermicrn size bins was rimarily a functin f relative humidity (Figure 3). The strng crrelatin between relative humidity and carse-mde H indicates that relative humid-

13 FRIDLIND AND JACOBSON: GAS-AEROSOL EQUILIBRIUM AND AEROSOL H 7,337 2'- ß -- - ß - ee,,, I,, I,,, I f I, t I i-] g Na*(bs.)/$42-(bs.) Figure 2. Calculated H in the.32-r size bin as a functin f the rati f bserved Na + t SO42- in that size bin n a mass basis. ity cntrlled the influence f HC n the mdeled sea-salt H mre than either enrichment by secndary acids r the ambient cncentratin f HC in the range redicted. The imrtance f HCI t the tt f sea-salt articles has been nted by revius authrs [Keene and Savie, 998, 999], as has the imrtance f relative humidity and liquid water cntent [Winkler, 986; Zhu et al., 992]. Whereas the H f the acidic accumulatin-mde articles was a weak increasing functin f relative humidity, the H f the carse-mde articles was a strng decreasing functin f relative humidity. Other runs with rttyical aersl slutins at tyical ACE aersl and gas cncentratins indicated that the H f NH4HSO4(s) slutins increased with increasing relative humidity, regardless f whether the gas-aersl artitining f NH 3 was cnsidered. Hwever, the redicted H f NaCl(s) and NaCl(s)-NH4HSO4(s) slutins increased with increasing relative humidity when the gas-aersl artitining f HC was neglected, but decreased with increasing relative humidity when the gas-aersl artitining f HC was cnsidered. Using a mdel that included HC artitining, Chameides and Stelsn [992] als redicted decreasing H f bulk marine aersls with increasing relative humidity Imact f each gas n H. Overall, mdeled equilibrium H was determined by the cmsitin f the aersl in each size bin, as well as the interactin with ambient gases. The rle f these tw influences in the ideal and sea-salt samles was searated by calculating the H resulting frm indeendent equilibrium with each gas (Figure 4). When the ideal samle was equilibrated with NH 3 alne, the mst acidic size bins were slightly neutralized as initial NH 3 was K, l d. 3 Ll, :... ' When equilibratcd with u r... 3 at,an,.,, ' " the nitric acid was distributed mre unifrmly relative t sea salt (see Figures 8 and 9), lwering H in the larger size bins. When equilibrated with HC alne, the H f the smallest tw size bins remained lw, while all f the larger size bins reached a unifrm H that was determined by the interactin with relative humidity and ambient HC. The small aersls emitted HC, raising H, whereas the larger aersls absrbed HC, lwering it. Because f the additinal imrtance f relative humidity t sea-salt H, the equilibrium f HC with the sea-salt samle was als calculated as a functin f relative humidity (Figure 5). The H was a strng functin f relative humidity thrughuthe range f HC, but was mre weakly deendent un HC abve arximately 5 t. Since mst measurements indicate that tyical marine HC levels exceed 5 t (see references in sectin 2.2), these results suggesthat relative humidity may be the mst imrtant determinant f equilibrium sea-salt H. Cmaring the equilibrium H values in Figure with thse in Figures 4 and 5 gives insight int the tential H f nnequilibrium marine article ulatins. Freshly emitted sea-salt articles are likely t begin with a H f 7-9. Interactin with HC at a given relative humidity may then lwer H t as little as 2, deending un relative humidity and ambient HC. Micrnsized sea-salt articles rbably reach equilibrium H, while the largest articles may r may nt attain this H in their lifetimes. Only significant SO4 2- wuld lwer H further t -2, which may ccur nly fr submicrn articles Cmarisn with ther estimates f H. In the accumulatin mde the H redicted in this wrk was generally -2, within the range f- t 3 that has been reviusly estimated r bserved in clean marine envirnments (Table ). In the carse mde the equilibrium H redicted here was 2-5, verlaing the range f 3- that has been bserved r estimated in clean marine envirnments. The existing ranges f estimated H are heavily deendent un the mdels that have been alied. In articular, the mdel treatment and redicted cncentratin f HC was fund t be critical t the carse-mde H estimated here. Of the fur mdels that have been alied t estimate carse marine aersl H in clean envirnments, Katshevski et al. [999] did nt cnsider HCI dynamics, btaining H values f 6-, similar t the H redicted here in fresh sea sray r at very lw HC (Figure 5). The remaining three mdels did cnsider the artitining f HC, but als redicted higher H values, cnsistent with thse estimated here at lwer HCI cncentratins. Chameides and Stelsn [992] redicted H values >8, but did nt rert HC; Ericksn et al. [999] redicted H values f 5-8 and <5 t HC; and Vgt et al. [996] redicted H values f 5-6 and -2 t HC at 76% relative humidity. In summary, the equilibrium carsemde H values redicted in this wrk aear t be cnsistent with ther estimates when the rle f HC is cnsidered. 5. Cnclusins The gas-aersl artitining f ammnium, nitrate, and chlride during the ACE I exeriment was analyzed by cmaring ß - ß ß I... I... I... I... I... t, - ß - ß Relative humidity (%) Figure 3. Calculated H in the 3.4-m size bin as a functin f ambient relative humidity.

14 7,338 FRIDLIND AND JACOBSON: GAS-AEROSOL EQUILIBRIUM AND AEROSOL H ideal with NH 3 ideal with HNO 3 lo ideal with HCI 5 = 5 5 Ol D (m) D (m) D ( m) sea salt with Nl l sea salt with HI O 3 3 lo lo sea salt with H I D (m) D ( m) D ( m) v v v Figure 4. Calculated H f ideal and ure sea-salt samles in equilibrium indeendently with NH 3, HNO 3, and HC at initial gas cncentratins f zer (dtted lines), the bserved r estimated values (dashed lines), and duble the bserved r estimated values (slid lines). equilibrium mdel results with measurements. The abve analysis surts the fllwing general cnclusins:. All aersls aeared likely t be in equilibrium with NH 3 based n cmarisn f bservatins with redictins and analysis f gas-aersl equilibratin times. Particles were sensitive t increased NH 3, absrbing at least 2% f additinal gas when initial NH 3 was dubled. Mdel results indicated that u t 3% f NH4 + resent under ambient cnditins may have been vlatilized during the samling rcess. 2. Submicrn and micrn-sized aersls were likely in equilibrium with HNO 3, but the largest articles aeared t be undersaturated. Particles were sensitive t increased HNO 3 and were caable f absrbing essentially all additinal gas when initial HNO 3 was dubled. 3. Smaller articles aeared likely t be in equilibrium with HC, whereas larger articles may r may nt have been, based un calculated gas-aersl equilibratin times. Emissin f HC due t dislacement by secndary acids was rbably nearly cmlete (clse t equilibrium) since secndary acids were cncentrated in the smallest articles and the smallest articles were likely t achieve equilibrium quickly. 4. The estimated equilibrium H f the accumulatin-mde aersls was -2, whereas the estimated equilibrium H f the HC (t) 95% RH % RH... 55% RH Figure 5. Calculated H f the sea-salt samle as a functin f initial HC and relative humidity. carse-mde aersls was 2-5. The H f sea sray may have varied in the range f 7-9 fr fresh articles dwn t 2 fr aged articles at high relative humidity and high HC. Under ACE cnditins the mdeled equilibrium H f the accumulatinmde articles was rimarily a functin f the amunt f sulfates resent relative t sea salt. By cntrast, the mdeled equilibrium H f the carse-mde articles was rimarily a functin f HC cncentratin and relative humidity. While it was ssible t draw cnclusins frm the available data based n the results f sensitivity tests, the mdel remained uncnstrained by the bservatins in several imrtant ways. With resect accumulatin-mde H and NH 3 equilibrium, the mdel was sensitive t assumtins regarding aersl mixing state. Hygrscicity data gathered n the Discverer at 65 m indicated that an external mixture was ften resent at that diameter, but n data were gathered at larger sizes [Berg et al., 998]. Additinal data n aersl mixing state u t at least m in diameter wuld have rvided an imrtant mdel cnstraint. With resect t carse-mde H and HNO 3 and HC equilibrium, data fr bth gas secies wuld clearly have r- vided imrtant cnstraints. Cnstraining chlrine is cmlicated by the fact that carse-mde aersl H may be quite sensitive t HC, but the ttal chlrine in the system is dminated by CI-, which rbably cannt be measured mre recisely withut sacrificing time and size reslutin f the aersl measurements. Acknwledgments. This material is based un wrk surted under a Natinal Science Fundatin Graduate Fellwshi, Natinal Science Fundatin agreements ATM and ATM-9648, and Natinal Aernautics and Sace Administratin agreement NIP--3. We are grateful t the fllwing researchers fr rviding ACE data: Patricia Quinn (article inic mass and NH 3 data), Timthy Bates and James Jhnsn (meterlgical and radn data), and Richard Feely (CO 2 data). References Abbatt, J.P. D., and G. C. G. Waschewsky, Hetergeneus interactins f HOBr, HNO 3, and NO 2 with deliquescent NaC aersls at rm temerature, J. Phys. Chem. A, 2, , 998. Allen, A. G., A. L. Dick, and B. M. Davisn, Surces f atmsheric methanesulhnate, nn-sea-salt sulhate, nitrate, and related secies ver the temerate Suth Pacific, Atms. Envirn., 3, 9-25, 997. Ayers, G. P., R. W. Gillett, J. M. Cainey, and A. L. Dick, Chlride and brmide lss frm sea-salt articles in Suthern Ocean air, J. Atms. Chem., 33,299-39, 999.

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16 7,34 FRIDLIND AND JACOBSON: GAS-AEROSOL EQUILIBRIUM AND AEROSOL H Whittlestne, S., J. L. Gras, and S. T. Siems, Surface air mass rigins during the First Aersl Characterizatins Exeriment (ACE ), J. Gehys. Res., 3, 6,34-6,35, 998. Winkler, P., Relatins between aersl acidity and in balance, in Chemistry f Multihase Atmsheric Systems, edited by W. Jaeschke, , Sringer-Verlag, New Yrk, 986. Zhang, Y., C. Seigneur, J. H. Seinfeld, M. Z. Jacbsn, S. L. Clegg, and F. S. Binkwski, A cmarative review f inrganic aersl thermdynamic equilibrium mdules: Similarities, differences, and their likely causes, rert, Crd. Res. Cunc., Inc., Atlanta, Ga., 998. Zhu, X., J. M. Prser, F. J. Miller, D. L. Savie, and G. W. Brass, The slubility f ferric in in marine mineral aersls at ambient relative humidities, Mar. Chem., 38, 9-7, 992. A.M. Fridlind and M. Z. Jacbsn, Deartment f Civil and Envirnmental Engineering, Stanfrd University, Stanfrd, CA (fridlind@ stanfrd.edu; ce.stanfrd.edu) (Received Nvember, 999; revised March 24, 2; acceted March 28, 2.)