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Reactive uptake studies of NO3 and N2O5 on alkenoic acid, alkanoate, and polyalcohol substrates to probe.. Gross, Simone; Iannone, Richard; Xiao, Song; Bertram, Allan K. 2009-08-31

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 This paper is published as part of a PCCP Themed Issue on: Physical Chemistry of Aerosols  Guest Editors: Ruth Signorell and Allan Bertram (University of British Columbia)   Editorial Physical Chemistry of Aerosols Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b916865fPerspective Reactions at surfaces in the atmosphere: integration of experiments and theory as necessary (but not necessarily sufficient) for predicting the physical chemistry of aerosols Barbara J. Finlayson-Pitts,  Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b906540gPapers Water uptake of clay and desert dust aerosol particles at sub- and supersaturated water vapor conditions Hanna Herich, Torsten Tritscher, Aldona Wiacek, Martin Gysel, Ernest Weingartner, Ulrike Lohmann, Urs Baltensperger and Daniel J. Cziczo,  Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b901585j Secondary organic aerosol formation from multiphase oxidation of limonene by ozone: mechanistic constraints via two-dimensional heteronuclear NMR spectroscopy Christina S. Maksymiuk, Chakicherla Gayahtri, Roberto R. Gil and Neil M. Donahue,  Phys. Chem. Chem. Phys., 2009,  DOI: 10.1039/b820005j DRIFTS studies on the photodegradation of tannic acid as a model for HULIS in atmospheric aerosols Scott Cowen and Hind A. Al-Abadleh,  Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b905236d Infrared spectroscopy of ozone and hydrogen chloride aerosols Chris Medcraft, Evan G. Robertson, Chris D. Thompson, Sigurd Bauerecker and Don McNaughton,  Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b905424n IR spectroscopy of physical and chemical transformations in cold hydrogen chloride and ammonia aerosols Evan G. Robertson, Chris Medcraft, Ljiljana Puskar, Rudolf Tuckermann, Chris D. Thompson, Sigurd Bauerecker and Don McNaughton,  Phys. Chem. Chem. Phys., 2009,  DOI: 10.1039/b905425c Formation of naproxen–polylactic acid nanoparticles from supercritical solutions and their characterization in the aerosol phase Moritz Gadermann, Simran Kular, Ali H. Al-Marzouqi and Ruth Signorell,  Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b901744e Measurements and simulations of the near-surface composition of evaporating ethanol–water droplets Christopher J. Homer, Xingmao Jiang, Timothy L. Ward, C. Jeffrey Brinker and Jonathan P. Reid,  Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904070f Effects of dicarboxylic acid coating on the optical properties of soot Huaxin Xue, Alexei F. Khalizov, Lin Wang, Jun Zheng and Renyi Zhang,  Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904129j Spectroscopic evidence for cyclical aggregation and coalescence of molecular aerosol particles J. P. Devlin, C. A. Yinnon and V. Buch,  Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b905018n Photoenhanced ozone loss on solid pyrene films Sarah A. Styler, Marcello Brigante, Barbara D Anna, Christian George and D. J. Donaldson,  Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904180j Quantifying the reactive uptake of OH by organic aerosols in a continuous flow stirred tank reactor Dung L. Che, Jared D. Smith, Stephen R. Leone, Musahid Ahmed and Kevin R. Wilson,  Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904418c Laboratory study of the interaction of HO2 radicals with the NaCl, NaBr, MgCl2·6H2O and sea salt surfaces Ekaterina Loukhovitskaya, Yuri Bedjanian, Igor Morozov and Georges Le Bras,  Phys. Chem. Chem. Phys., 2009,  DOI: 10.1039/b906300e Kinetics of the heterogeneous reaction of nitric acid with mineral dust particles: an aerosol flowtube study A. Vlasenko, T. Huthwelker, H. W. Gäggeler and M. Ammann,  Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904290n Timescale for hygroscopic conversion of calcite mineral particles through heterogeneous reaction with nitric acid Ryan C. Sullivan, Meagan J. K. Moore, Markus D. Petters, Sonia M. Kreidenweis, Greg C. Roberts and Kimberly A. Prather,  Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904217b Mid-infrared complex refractive indices for oleic acid and optical properties of model oleic acid/water aerosols Shannon M. McGinty, Marta K. Kapala and Richard F. Niedziela,  Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b905371a A study of oleic acid and 2,4-DHB acid aerosols using an IR-VUV-ITMS: insights into the strengths and weaknesses of the technique Sarah J. Hanna, Pedro Campuzano-Jost, Emily A. Simpson, Itamar Burak, Michael W. Blades, John W. Hepburn and Allan K. Bertram,  Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904748d Deliquescence behaviour and crystallisation of ternary ammonium sulfate/dicarboxylic acid/water aerosols L. Treuel, S. Pederzani and R. Zellner,  Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b905007hDownloaded by The University of British Columbia Library on 18 April 2011Published on 06 August 2009 on http://pubs.rsc.org | doi:10.1039/B904741GView OnlineLaboratory chamber studies on the formation of organosulfates from reactive uptake of monoterpene oxides Yoshiteru Iinuma, Olaf Böge, Ariane Kahnt and Hartmut Herrmann,  Phys. Chem. Chem. Phys., 2009,  DOI: 10.1039/b904025k Measurement of fragmentation and functionalization pathways in the heterogeneous oxidation of oxidized organic aerosol Jesse H. Kroll, Jared D. Smith, Dung L. Che, Sean H. Kessler, Douglas R. Worsnop and Kevin R. Wilson,  Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b905289e Using optical landscapes to control, direct and isolate aerosol particles Jon B. Wills, Jason R. Butler, John Palmer and Jonathan P. Reid,  Phys. Chem. Chem. Phys., 2009,  DOI: 10.1039/b908270k Reactivity of oleic acid in organic particles: changes in oxidant uptake and reaction stoichiometry with particle oxidation Amy M. Sage, Emily A. Weitkamp, Allen L. Robinson and Neil M. Donahue,  Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904285g Surface tension of mixed inorganic and dicarboxylic acid aqueous solutions at 298.15 K and their importance for cloud activation predictions Alastair Murray Booth, David Owen Topping, Gordon McFiggans and Carl John Percival,  Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b906849j Kinetics of the heterogeneous conversion of 1,4-hydroxycarbonyls to cyclic hemiacetals and dihydrofurans on organic aerosol particles Yong Bin Lim and Paul J. Ziemann,  Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904333k Time-resolved molecular characterization of limonene/ozone aerosol using high-resolution electrospray ionization mass spectrometry Adam P. Bateman, Sergey A. Nizkorodov, Julia Laskin and Alexander Laskin,  Phys. Chem. Chem. Phys., 2009,  DOI: 10.1039/b905288g Cloud condensation nuclei and ice nucleation activity of hydrophobic and hydrophilic soot particles Kirsten A. Koehler, Paul J. DeMott, Sonia M. Kreidenweis, Olga B. Popovicheva, Markus D. Petters, Christian M. Carrico, Elena D. Kireeva, Tatiana D. Khokhlova and Natalia K. Shonija,  Phys. Chem. Chem. Phys., 2009,  DOI: 10.1039/b905334b Effective broadband refractive index retrieval by a white light optical particle counter J. Michel Flores, Miri Trainic, Stephan Borrmann and Yinon Rudich,  Phys. Chem. Chem. Phys., 2009,  DOI: 10.1039/b905292e Influence of gas-to-particle partitioning on the hygroscopic and droplet activation behaviour of -pinene secondary organic aerosol Zsófia Jurányi, Martin Gysel, Jonathan Duplissy, Ernest Weingartner, Torsten Tritscher, Josef Dommen, Silvia Henning, Markus Ziese, Alexej Kiselev, Frank Stratmann, Ingrid George and Urs Baltensperger,  Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904162a Reactive uptake studies of NO3 and N2O5 on alkenoic acid, alkanoate, and polyalcohol substrates to probe nighttime aerosol chemistry Simone Gross, Richard Iannone, Song Xiao and Allan K. Bertram,  Phys. Chem. Chem. Phys., 2009,  DOI: 10.1039/b904741g Organic nitrate formation in the radical-initiated oxidation of model aerosol particles in the presence of NOx Lindsay H. Renbaum and Geoffrey D. Smith,  Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b909239k Dynamics and mass accommodation of HCl molecules on sulfuric acid–water surfaces P. Behr, U. Scharfenort, K. Ataya and R. Zellner,  Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904629a Structural stability of electrosprayed proteins: temperature and hydration effects Erik G. Marklund, Daniel S. D. Larsson, David van der Spoel, Alexandra Patriksson and Carl Caleman,  Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b903846a Tandem ion mobility-mass spectrometry (IMS-MS) study of ion evaporation from ionic liquid-acetonitrile nanodrops Christopher J. Hogan Jr and Juan Fernández de la Mora,  Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b904022f Homogeneous ice freezing temperatures and ice nucleation rates of aqueous ammonium sulfate and aqueous levoglucosan particles for relevant atmospheric conditions Daniel Alexander Knopf and Miguel David Lopez,  Phys. Chem. Chem. Phys., 2009, DOI: 10.1039/b903750k    Downloaded by The University of British Columbia Library on 18 April 2011Published on 06 August 2009 on http://pubs.rsc.org | doi:10.1039/B904741GView OnlineReactive uptake studies of NO3and N2O5on alkenoic acid, alkanoate,and polyalcohol substrates to probe nighttime aerosol chemistrySimone Gross, Richard Iannone, Song Xiao and Allan K. Bertram*Received 9th March 2009, Accepted 17th July 2009First published as an Advance Article on the web 6th August 2009DOI: 10.1039/b904741gHeterogeneous reactions between NO3and N2O5and diethyl sebacate (DES), glycerol,oleic acid (OA), linoleic acid (LA), and conjugated linoleic acid (CLA) were studied tounderstand better nighttime aerosol chemistry. The reactive uptake coefficient of NO3on theliquid alkenoic acids (OA, LA, and CLA) was found to be 40.07, which is higher thanprevious results for unsaturated organics, including alkenoic acids. This reaction could potentiallybe an important loss process of particle-phase unsaturated organic compounds in the atmosphereand in laboratory secondary organic aerosol studies. The reactive uptake coefficient of N2O5onliquid glycerol was also found to be relatively large with a value of (3.2–8.5) C2 10C04, suggestingthat N2O5heterogeneous reactions with alcohols may also be atmospherically relevant. For allmeasurements with OA, CLA, and DES, the reactive uptake coefficients decreased significantlyupon freezing. One possible explanation is that the liquid reaction is due to both a surfacereaction and a bulk reaction and that the freezing process significantly decreases the importanceof any bulk reactions. NO3reactive uptake coefficients for liquid-phase compounds decreased inmagnitude in the order: alkenoic acids 4 DES 4 glycerol. This is different compared to previousgas-phase studies and the difference may be due to the large viscosity of glycerol compared to theother organic compounds studied. N2O5reactive uptake coefficients for liquid-phase compoundsdecreased in magnitude in the order: glycerol 4 LA 4 DES D OA D CLA.1. IntroductionLiquid and solid aerosol particles are abundant in thetroposphere, with concentrations in the range of 102–107aerosol particles in 1 cm3of ambient air. Field measurementshave shown a broad variety of aerosol types, both organicand inorganic. The organic fraction comprises typically10–90% of the total aerosol mass.1This organic materialcan be in the form of pure organic particles, or alternativelythe organic material can be mixed with inorganic material.In the latter case, the organic material can form organiccoatings on surfaces of aqueous particles,2,3or organiccoatings adsorbed on surfaces of solid particles, such asmineral dust.4The composition of this particle-phase organic material isvery diverse, with hundreds to thousands of different organicsidentified.5–9Some of the classes of components in the organicaerosol fraction are alkanes, alkanoic acids, alkenoic acids,dicarboxylic acids, alcohols, and polycyclic aromatichydrocarbons (PAH).10–12Sources of these organics are bothbiological and anthropogenic and range from terrestrialvegetation, airborne microorganisms, cigarettes, automobilesand diesel trucks, to meat cooking operations. Additionally,many particle-phase organics in the atmosphere are secondaryin nature, formed from the oxidation and condensation ofgas-phase precursors.Organic particles or coatings, while in the atmosphere,experience reactions with gas-phase species that may lead tothe modification of the particle or coating composition. Thesereactions, which are often referred to as heterogeneous reac-tions, are of importance for several reasons. First, they maylead to toxic or carcinogenic compounds.13Second, they maychange the hygroscopic and optical properties of organicparticles, and therefore influence the ability of these particlesto act as cloud condensation nuclei, to act as ice nuclei, and toscatter and absorb solar radiation.13–15Third, these reactionscan be a major loss pathway of organic compounds in theatmosphere.13–16Fourth, under certain conditions, these reac-tions can be an important sink for gas-phase species.17Fifth, ithas been suggested that these heterogeneous reactions can leadto rapid volatilization of organic particulate matter18–20andare a major source of volatile organic compounds (VOC) inthe atmosphere.3,21Sixth, heterogeneous reactions may alsohave implications for source apportionment. Specific organicspecies often serve as molecular markers for probing sourcesof organic particles. If heterogeneous reactions change theconcentrations of the selected molecular markers they can leadto errors when calculating source strengths.22In the following, we study heterogeneous reactions betweenNO3and N2O5and five types of organic substrates to betterunderstand atmospheric aerosol chemistry. The focus of thesestudies is to determine the reactive uptake coefficients (g)ofthese gases on the different organic substrates. The reactiveuptake coefficient, a parameter often used to describe aheterogeneous process, is defined as the fraction of collisionswith a surface that leads to reactive loss.Department of Chemistry, University of British Columbia, Vancouver,British Columbia, Canada V6T 1Z1. E-mail: Bertram@chem.ubc.ca7792 | Phys. Chem. Chem. Phys., 2009, 11, 7792–7803 This journal is C13c the Owner Societies 2009PAPER www.rsc.org/pccp | Physical Chemistry Chemical PhysicsDownloaded by The University of British Columbia Library on 18 April 2011Published on 06 August 2009 on http://pubs.rsc.org | doi:10.1039/B904741GView OnlineNO3is an important nighttime oxidant in the troposphere.Concentrations of this radical range from o10 ppt to430 ppt.7,23–26There have only been a few studies that haveexplored the reactive uptake coefficient of NO3on organicsubstrates. In a pioneering study, Moise et al.17studied thereactive uptake coefficient on the following liquid and solidorganics: n-hexadecane, n-octanoic acid, 1-octadecene,1-hexadecene, heptylmethyl nonane, 1-octanol, 7-tetradecene,conjugated linoleic acid, and nonconjugated linoleic acid.Knopf et al.27studied the reactive uptake coefficient of NO3on an alkane monolayer. Gross and Bertram28studied thereactive uptake coefficient of NO3on an alkene monolayer.Gross and Bertram29also studied the reactive uptakecoefficient on solid PAH films. McNeill et al.30studied theNO3uptake coefficient on aqueous aerosols coated with amonolayer of sodium oleate. These combined studies suggestthat NO3reactions with organics may be important in theatmosphere under certain conditions, but more work in thisarea is still needed to fully understand the significance of thesereactions.N2O5is also an important gas-phase species during thenight. It is formed from the reaction between NO2and NO3and canreachconcentrations of upto approximately10ppb.31There have only been a few studies of the reactiveuptake coefficient of N2O5on organics. Gross and Bertram29explored the uptake coefficient of N2O5on PAH surfaces andThornton et al.32studied the uptake coefficient of N2O5onsolid malonic acid and azelaic acid aerosols. In addition, Laiand Finlayson-Pitts33investigated the products formed fromthe heterogeneous reaction between N2O5and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (an unsaturated organiccompound), but uptake coefficients were not reported.We have investigated the reactive uptake of NO3and N2O5on liquid and solid organic substrates. The reactive uptakecoefficients are used to assess the lifetime of these condensed-phase organics in the atmosphere. The potential importance ofthese reactions in laboratory studies of secondary organicaerosol (SOA) formation is also discussed.The specific substances used for this study were diethylsebacate (DES, C14H26O4), an alkanoate; glycerol (C3H8O3),a polyalcohol; and oleic acid (OA, C18H34O2), also known ascis-9-octadecenoic acid; linoleic acid (LA, C18H32O2); andconjugated linoleic acid (CLA, C18H32O2), which is a mixtureof the cis-9, trans-11 and trans-10, cis-12 isomers of linoleicacid. OA, LA and CLA are all alkenoic acids. Fig. 1 provideschemical structures for all organic compounds used in thisstudy. The five organics used in this study represent differentfunctional groups commonly found in atmospheric samplesand therefore serve as models for certain organics in theatmosphere. Two of the compounds (OA and glycerol)have been observed in atmospheric field studies, and diestersof sebacic acid (decanedioic acid) have recently been usedas proxies for saturated organic compounds found in theatmosphere.20,34,35LA and CLA were studied to make directcomparisons between our NO3uptake data with those ofMoise et al.17The compounds OA, CLA, and DES have freezing pointswithin the studied temperature range of 263–303 K. Thisallowed us to investigate differences in reactivity between solidand liquid films. By comparing the results for the liquid andsolid compounds, we examined whether the reaction for theliquid is due to mainly a surface reaction or both a surface anda bulk reaction.17For NO3we find the reactive uptake on the alkenoic acid isfaster than expected based on previous results for otherunsaturated organics. This finding may have implications forthe atmosphere and studies of SOA formation. For N2O5wefind that the reaction with the polyalcohol is also efficientand could potentially play an important role in atmosphericaerosol chemistry.2. Experimental2.1 Experimental setup and procedureExperiments were performed in a rotating-wall flow cellcoupled to a chemical ionization mass spectrometer (CIMS)described in more detail elsewhere.36The rotating glass tube(1.77 cm inner diameter (I.D.), B12 cm length) fit snuglyinside the flow tube. An outer tube that surrounds the flow cellallowed for temperature control to within C61 K. All exposedglass and metal surfaces of the flow tube (other than thereactive surfaceof interest)were coatedbyaninert halocarbonwax or grease to avoid losses of NO3or N2O5on thesesurfaces. Approximately 0.5–1 mL of the liquid organic wasdistributed onto the inner wall of the rotating glass cylinder.A rotation rate of approximately 10 rotations per min wasused in the liquid experiments. This produced a uniform filmapproximately 0.5 mm thick. For conducting experiments onsolid surfaces, the rotating liquid films were rapidly cooled bypassing a refrigerated coolant through the outer jacket of theflow cell. The freezing temperatures (i.e. freezing points) ofeach film were determined with the rotating-wall flow celldiscussed above. The temperature of the flow cell wasdecreased at a rate of approximately 0.5 K minC01, and thefreezing temperatures were determined visually.Total pressures in the flow cell were 2–5 Torr and velocitiesin the flow cell ranged from 20–100 cm sC01for the N2O5experiments, and from 380–810 cm sC01for the NO3experiments. The main carrier gas used in the experimentswas He. O2was also added to the carrier gas to better mimicatmospheric concentrations. O2percentages in the flow cellranged from 10–14% (NO3experiments) to 38–54% (N2O5experiments) of the total flow. In some experiments involvingLA, only He was used as a carrier gas, but no difference inmeasured uptake coefficient was observed compared to thoseexperiments using a mixture of He and O2as a carrier. Theflow was laminar in all experiments, based on the Reynoldsnumber (Re o 5). NO3and N2O5were added through amovable injector that allowed for varying of the reactivedistance and thus the reaction time. The injector positionwas periodically moved during an experiment to expose anincreasing surface area of the organic coating to NO3or N2O5.N2O5was generated by reacting NO2with an excess amountof O3in a flow system as described by Schott and Davidson37and Cosman et al.38N2O5was stored as solid white crystals at197 K. NO3radicals were obtained by thermal conversion ofgaseous N2O5to NO3and NO2at 430 K in a TeflonscoatedThis journal is C13c the Owner Societies 2009 Phys.Chem.Chem.Phys.,2009, 11, 7792–7803 | 7793Downloaded by The University of British Columbia Library on 18 April 2011Published on 06 August 2009 on http://pubs.rsc.org | doi:10.1039/B904741GView Onlineglass oven before entering the movable injector. Due to theshort residence time of the gases in the flow tube reactor(typically 20–100 ms), the recombination of NO3and NO2toN2O5was negligible. N2O5and NO3were both detected asNO3C0in the mass spectrometer after chemical ionization byIC0.IC0was generated by passing a trace amount of CH3IinN2through a210Po source (NRD, model Po-2031).N2O5concentrations were calculated from the NO3C0signaland the known rate constants of the reaction of N2O5with IC0.NO3concentrations were estimated by assuming that all N2O5is converted to NO3and NO2and approximately 20% of theNO3thermally dissociates in the Teflonscoated glass oven basedon well-known gas-phase reaction rates and modeling studiesusing the Acuchem chemical kinetics simulation program.39Conversion of N2O5to NO3and NO2in the oven was verifiedat the beginning of every NO3uptake experiment by addingan excess quantity of NO to the exit of the flow tube toquantitatively convert NO3to NO2. This conversion by NO alsoserved as a convenient way to quantify the background signal inthe NO3experiments. The background signal was typically lessthan 5% of the total signal. NO3concentrations ranged from(3.5–16) C2 1010molecules cmC03and N2O5concentrations rangedfrom (1–20) C2 1011molecules cmC03. The uncertainty in theseconcentrations, based on the uncertainty of the rate constant forthe gas-phase N2O5+IC0reaction, is 40%.40Observed first-orderloss rate coefficients, kobs, were calculated from the depletion ofthe oxidant signal with increasing reaction time. Three typicalplots of the natural logarithm of the NO3signal vs. time areshown in Fig. 2 for glycerol, DES, and OA. The slopes of theFig. 1 Molecular structures for the organic compounds used in this study. Conjugated linoleic acid (CLA) is a mixture of two conjugated isomersof octadecadienoic acids.Fig. 2 Plot of the natural logarithm of the CIMS signals vs. reactiontime for NO3during typical experiments on liquid surfaces of glycerol(293 K), and OA and DES (both at 288 K).7794 | Phys. Chem. Chem. Phys., 2009, 11, 7792–7803 This journal is C13c the Owner Societies 2009Downloaded by The University of British Columbia Library on 18 April 2011Published on 06 August 2009 on http://pubs.rsc.org | doi:10.1039/B904741GView Onlinelinearfitswereusedtodeterminekobs,whichwasinturncorrectedfor concentration gradients that form close to the flow-tubewall by using the procedure developed by Brown.41Uptakecoefficients, g, were calculated from the corrected kobsusing astandard procedure.36Diffusion coefficients used in thesecalculations were taken from Rudich et al.42and Knopf et al.43for NO3and N2O5, respectively.40As NO2is always present in our NO3experiments (due toour method of producing NO3radicals), we also measured thereactive uptake coefficients of NO2for OA, DES, and glycerolat 298 K in the presence of O2. Experimental conditions weresimilar to the conditions discussed above. For all experimentsthe g value was at or below our detection limit (gr1 C2 10C06).2.2 ChemicalsDiethyl sebacate (98%), linoleic acid (Z99%), andconjugated linoleic acid (o1% unconjugated LA impurities)were obtained from Sigma-Aldrich; glycerol (99.9%) and oleicacid (Z99.0%) were purchased from Fisher-Scientific andFluka, respectively. NO2(99.5%) was procured fromMatheson. N2(99.999%), O2(99.993%), and He (99.999%)were purchased from Praxair. O2was passed through anultraviolet light source to generate O3.37,383. Results and discussionTable 1 and Fig. 3 provide mean values of uptake coefficientsfor both NO3and N2O5experiments at different flow celltemperatures. Typically, 5–10 experiments were performed foreach heterogeneous reaction at the specified temperature. Atthe end of every uptake experiment, when the injector wasmoved to a position where the coated organic layer was nolonger exposed to the oxidant flow, no release of NO3or N2O5was observed. This indicates that the uptake of NO3and N2O5for all organic compounds studied was irreversible.The freezing temperatures for DES, OA, and CLA were(273.6 C6 1) K, (284.2 C6 1) K, and (270.3 C6 1) K, respectively.In contrast, for the studied temperature range of (263–303) K,neither the glycerol nor the LA films froze. Both remained asTable 1 Mean values of measured uptake coefficients for reactions ofNO3and N2O5with solid and liquid-phase organic compoundsSurface T/K PhaseUptake coefficient (g)ag(NO3) g(N2O5) C2 10C04OA 302 Liquid 0.21 (+0.79/C00.11) —298 Liquid — 0.54 C6 0.06295 Liquid 0.18 (+0.82/C00.11) —288 Liquid 0.16 (+0.84/C00.06) 0.62 C6 0.06285 Liquid 0.17 (+0.83/C00.09) —283 Solid 0.053 C6 0.011 —277 Solid 0.051 C6 0.014 —268 Solid 0.076 C6 0.026 0.09 C6 0.04DES 298 Liquid (4.1 C6 0.3) C2 10C030.51 C6 0.07288 Liquid (3.6 C6 0.1) C2 10C030.86 C6 0.06278 Liquid (4.1 C6 0.5) C2 10C031.30 C6 0.05272 Solid (3.6 C6 0.5) C2 10C04—268 Solid 0.03 C6 0.29263 Solid (2.5 C6 0.2) C2 10C04—Glycerol 303 Liquid — 8.14 C6 0.35293 Liquid (1.4 C6 0.3) C2 10C036.45 C6 0.58286 Liquid (9.2 C6 0.4) C2 10C045.11 C6 0.61268 Liquid (8.3 C6 0.5) C2 10C043.98 C6 0.74CLA 298 Liquid 0.37 (+0.63/C00.24) 0.46 C6 0.06288 Liquid 0.33 (+0.67/C00.21) 0.49 C6 0.01278 Liquid 0.62 (+0.38/C00.47) 0.39 C6 0.02263 Solid 0.08 C6 0.03 0.14 C6 0.13LA 298 Liquid 0.29 (+0.71/C00.15) 1.68 C6 0.19288 Liquid 0.33 (+0.67/C00.20) 1.67 C6 0.30278 Liquid 0.41 (+0.59/C00.27) 1.65 C6 0.31263 Liquid 0.13 (+0.67/C00.05) 1.36 C6 0.14aUncertainty is calculated using a 20% uncertainty in the diffusioncoefficient unless otherwise indicated.Fig. 3 Measured reactive uptake coefficients as a function oftemperature for NO3(top) and N2O5(bottom) reactions on the fiveorganic compounds studied.This journal is C13c the Owner Societies 2009 Phys.Chem.Chem.Phys.,2009, 11, 7792–7803 | 7795Downloaded by The University of British Columbia Library on 18 April 2011Published on 06 August 2009 on http://pubs.rsc.org | doi:10.1039/B904741GView Onlinesupercooled liquids, even though the lowest temperaturesstudied were below the melting points of glycerol and LA.Melting points for OA, DES, glycerol, and LA are 287 K,278 K, 291 K, and 268 K, respectively; the melting point forCLA was not provided by the supplier.443.1 Reactive uptake of NO3on the alkenoic acids OA, LA,and CLAThe uptake coefficients for all three alkenoic acids in the liquidphase were 40.07 (see Table 1). The flow tube technique usedin this study is typically capable of accessing 10C06o g o 1,however, for our flow rates and pressures, g values greaterthan 0.07–0.1 (depending on the flow conditions) are greatlyinfluenced by gas-phase diffusion to the reactive surface.In this case, a small uncertainty in the observed first-orderrate constant (kobs), or the diffusion coefficient, results in alarge uncertainty in the reactive uptake coefficients.Average uptake coefficients for the frozen alkenoic acidsOA and CLA were approximately 65% and 80% lower,respectively, than the corresponding liquid-phase data. Thissuggests that the net liquid-phase reaction may be a combinationof both a surface reaction and a bulk reaction, since thefreezing process is expected to greatly decrease the importanceof any bulk reactions in our experiments. Alternatively,the reactive uptake for both the liquid- and solid-phaseexperiments might only be due to surface reactions, wherethe liquid-phase surface is much more favourable for uptakeand reactivity.45In these experiments, we cannot discernbetween the two different possibilities. We assume the formeruntil further information is available.For all the liquids studied, the NO3loss was constantwith time, which was expected since the surface was continuouslybeing replenished by the rotation of the liquid film.Furthermore, for all frozen surfaces studied, NO3loss wasalso constant with time, and there was no indication of‘‘chemical aging’’ of the surface due to a decrease in reactivesites. Moise et al.17also reported a constant loss of NO3onfrozen organic liquids. This may be due to evaporation ofsurface products that continuously creates a ‘‘fresh’’ unreactedsurface. Alternatively, there could be some mobility on thefrozen surface and reactants may diffuse into the bulk andreact with subsurface molecules.17The reactive uptake for the liquid (and most likely also forthe solid) is due to the addition of the NO3radical to a CQCbond, based on recent product studies for NO3reactions withliquid OA particles46,47and a study of NO3reactivity on aterminal alkene monolayer.28Results from these productstudies have indicated that the likely mechanism involves theaddition of the NO3radical to the CQC bond to form aC–ONO2functional group at one C atom and an alkyl radicalat the other C atom. O2can be added to the radical to form aperoxy radical (ROOC15), which can then undergo a series ofradical reactions to form products such as carbonyls, alcohols,nitrates, or peroxynitrates.46,47In Table 2, g values for alkenoic acids are compared withg values determined in other studies using unsaturatedorganic substrates. Moise et al.17used a coated-wall flow tubereactor to study both liquid and frozen unsaturated organics,including LA and CLA. Our data for LA and CLA arelarger than the results from Moise et al., even if one considersthe uncertainty in the measurements. Another differencebetween our results and the results from Moise et al.17foralkenoic acids are the changes observed upon freezing theliquid. As mentioned above, we observed a decrease in thereactive uptake coefficient when freezing the liquid. Incontrast, Moise et al. reported that the change in valuesof g upon freezing of liquid alkenoic acids was within theexperimental uncertainty, although Moise et al. reported thatthe uptake coefficients of NO3by n-hexadecane andn-octanoic acid decreased by a factor of 5 upon freezing. Thisbehavior was explained by reactions occurring in the bulk ofthe organic liquid as well as on the surface for n-hexadecaneand n-octanoic acid. It is also interesting to note that unlikethe results for unsaturated organics, our results for saturatedorganics are in good agreement with the results fromMoise et al.17(see section 3.3).Also shown in Table 2 is preliminary data by Ziemannet al.48These authors used an environmental chamber todetermine the reactive uptake coefficient of NO3on liquidOA aerosols at room temperature, and they obtained resultsconsistent with our findings.Both our experiments and the experiments by Moise et al.were performed with a rotating-wall flow reactor, experimentalconditions such as pressure and temperatures were similar,and both experimental apparatuses were validated by studyingknown gas-phase reactions. Moise et al.17studied the gas-phase reaction of 1-butene with NO3and obtained rateconstants consistent with literature values. We measured thesecond order rate constant for the gas-phase reaction ofNO3with NO at room temperature using our flow cell. Weobtained (2.96 C6 0.15) C2 10C011cm3moleculeC01sC01whichagrees well with established literature values in the range of(2.4–3.0) C2 10C011cm3moleculeC01sC01.7,49,50One experimental parameter that differs between ourstudiesand those by Moise et al.17is the NO3concentration range.Moise etal. used(0.5–5)C21012moleculescmC03,arangethat isslightly above that of the (3.5–16) C2 1010molecules cmC03usedin our investigation. It is, however, unlikely that this canentirely explainthediscrepancies betweendatasets.Our recentresults suggest a potential uncertainty in the reactive uptakecoefficients for NO3–unsaturated organic reactions. Morestudies are needed to resolve the apparent discrepanciesbetween the two data sets.McNeill et al. studied the uptake of NO3on aqueousparticles coated with a monolayer of sodium oleate andobtained a value of o10C03.30This study is more similar tostudies on frozen films where only the surface or top fewlayers were accessible. For frozen films, the g values obtainedin this study are at least 40 times greater than those obtainedby McNeill et al. These differences in reactive uptakecoefficients could be rationalized by differences in surface-filmstructure. A monolayer only has one layer available forreaction whereas in frozen films several top layers arepotentially accessible. Frozen films are also expected to havesurface defects such as steps and kinks that may enhancereactivity.7796 | Phys. Chem. Chem. Phys., 2009, 11, 7792–7803 This journal is C13c the Owner Societies 2009Downloaded by The University of British Columbia Library on 18 April 2011Published on 06 August 2009 on http://pubs.rsc.org | doi:10.1039/B904741GView Online3.2 Reactive uptake of NO3on glycerolThe films of glycerol did not freeze for the temperature rangestudied but rather became a supercooled liquid at the lowesttemperatures studied (well below the literature melting pointof glycerol). The uptake coefficients measured for liquidglycerol were in the range of (7.8–17) C2 10C04(which includesmeasurement uncertainty).There has only been one previous study of the reactiveuptake coefficient of NO3on a liquid-phase saturated alcohol.Moise et al.17studied 1-octadecanol and obtained a value of7.1 C2 10C03for the liquid. The mechanism for the NO3reactionwith the alcohol, based on gas-phase reactions, likely involveshydrogen abstraction.17,51,52However, product studies areneeded to confirm the reaction mechanism.3.3 Reactive uptake of NO3on DESThe uptake coefficients measured for liquid DES were(3.5–4.6) C2 10C03(this range includes measurement uncertainties).Freezing of DES resulted in a decrease of g of about one orderof magnitude. This suggests that the reactive uptake of NO3on the liquid is due to both a surface and a bulk process.There are no previous studies of NO3reactive uptake on liquidor solid alkanoates available for comparison. Gas-phasemeasurements suggest the reaction rates for saturated esters aresimilar to those for saturated hydrocarbons.53Thus, the presenceof the ester functional group does not significantly affect thereactivity of the alkyl chain. Table 2 provides a comparison ofour results for the alkanoate with previous measurements of theuptake of NO3with solid and liquid alkanes by Moise et al.17Inthis case, our results for the alkanoate DES are in reasonableagreement with the measurements by Moise et al. Consideringgas-phase reactions between NO3and alkanoates, the mechanismis probably due to abstraction of an H atom, similar to theglycerol + NO3mechanism but product studies are needed tolend credence to this hypothesis.3.4 Overall trend in NO3reactivityFor the solid surfaces studied, the trend in the reactive uptakecoefficient is alkenoic acid 4 alkanoate. This trend is consistentwith the trend observed for gas-phase reactivity, assuming thealkenoic acid reactivity is due to the CQC bond (a reasonableassumption based on recent product studies).46,47Forexample, the gas-phase reaction rate coefficients for propeneand methyl propionate are 9.5 C2 10C015cm3moleculeC01sC01,and o3.3 C2 10C017cm3moleculeC01sC01, respectively.53,54The trend in reactive uptake coefficients for the liquids isalkenoic acid 4 alkanoate 4 polyalcohol. This is differentfrom the gas-phase reactivity trend, which is alkenoic acid 4alcohol4alkanoate(again assumingthe alkenic acidreactivityis due to the CQC bond). For example, the gas-phase reactionrate coefficient for propanol is 1.5 C2 10C015cm3moleculeC01sC01,which is between the rate coefficient for propene and methylpropionate.54(Although, keep in mind that the data on gas-phase reactivity for alkanoates is very limited.) For the liquid,the reactive uptake coefficient of glycerol is lower than expectedbased on gas-phase reactivity. For a possible explanation,we used the resistor model of gas–liquid interactions55andassumed that for all liquids the reaction occurs mainly in thebulk (since g values for experiments with frozen DES, OA, andCLA were significantly lower than for the corresponding liquidexperiments). If the reaction occurs in the bulk, the reactiveuptake coefficient canbe explained with the following equation:1g¼1aþcavg4HRTffiffiffiffiffiffiffiffiffiffiffiffiDkrxnp ð1Þwhereaistheprobabilitythatamoleculethatstrikesthesurfaceenters into the bulk, cavgis the mean molecular velocity of NO3,H is the Henry’s law solubility constant, R is the gas constant,Tisthetemperature,DisthediffusioncoefficientforNO3intheorganic liquid, and krxnis the first-order rate constant forreaction in the liquid.Eqn (1) shows that the reactive uptake coefficient is afunction of the bulk-phase rate coefficient (krxn), the Henry’slaw solubility, and the diffusion coefficient of NO3in theliquid. It is interesting to the note that the viscosity of liquidglycerol is more than an order of magnitude higher than thatof liquid OA or liquid DES (viscosity data for LA and CLAare not available, to our knowledge). Viscosities at 293–298 KTable 2 Comparison of uptake coefficient data for the reaction of NO3on liquid and solid surfaces of saturated and unsaturated organicsSurfaceLiquid surface Solid surfaceRef.T/K gliquid T/K gsolidSaturatedDES 278–298 (3.5–4.5) C2 10C03263–272 (2.3–4.1) C2 10C04This studyn-Hexadecane 293 (2.6 C6 0.8) C2 10C03283–289 (3.8 C6 1.0) C2 10C04Moise et al.17Heptamethyl nonane 253 (2.1 C6 0.8) C2 10C03234 (2.6 C6 0.9) C2 10C03Moise et al.17UnsaturatedOA 285–302 Z0.07 268–283 (3.8–10) C2 10C02This study298 0.13 C6 0.02 — — Ziemann et al.aLA 288 Z0.13 — — This study288 (1.5 C6 0.2) C2 10C02248–263 (1.1 C6 1.3) C2 10C02Moise et al.17CLA 278–298 Z0.12 263 0.08 C6 0.03 This study273 (7.9 C6 1.2) C2 10C03253–263 (7.8 C6 1.4) C2 10C03Moise et al.171-Hexadecene 277 (2.3 C6 0.9) C2 10C03254–274 (1.8 C6 0.3) C2 10C03Moise et al.171-Octadecene 293 (1.6 C6 0.3) C2 10C03283 (1.4 C6 0.1) C2 10C03Moise et al.177-Tetradecene 246 (5.8 C6 2.0) C2 10C03283 (5.2 C6 2.0) C2 10C03Moise et al.17aUnpublished data.This journal is C13c the Owner Societies 2009 Phys.Chem.Chem.Phys.,2009, 11, 7792–7803 | 7797Downloaded by The University of British Columbia Library on 18 April 2011Published on 06 August 2009 on http://pubs.rsc.org | doi:10.1039/B904741GView Onlinefor glycerol, OA,and DESare 1500mPas,5643.8 mPa s,57and5.88 mPa s,58respectively. The diffusion coefficient of aspecies in a liquid is related to the viscosity through theStokes–Einstein equation:D ¼kT6pZrð2Þwhere D is the diffusion coefficient, k is the Boltzmannconstant, T is the temperature, Z is the viscosity of the liquid,and r is the radius of the diffusing species. Eqn (2) and theliquid viscosity data suggest that the diffusion coefficient ofNO3is more than an order of magnitude smaller in glycerolthan OA and DES. This information [together with eqn (1)]leads us to hypothesize that the NO3–glycerol reactive uptakecoefficient is lower than the NO3–DES reactive uptakecoefficient because of the small diffusion coefficient of NO3in liquid glycerol [ultimately resulting in a smaller H(Dkrxn)0.5value for eqn (1)]. Measurements of reactive uptake coefficientson different alcohols with different viscosities may provide anadequate data set for resolving this discrepancy.3.5 Reactive uptake of N2O5on glycerolFor all organic compounds studied, the N2O5reactive uptakecoefficient was the largest on glycerol. Uptake coefficientsranged from (3.2–8.5) C2 10C04(range incorporates uncertaintiesin the measurements). Our studies are the first measurementsof thereactiveuptakecoefficients ofN2O5oncondensed-phasealcohols. However, reactions between N2O5and saturatedalcohols have been observed in the gas phase.51,52In addition,the condensed-phase reaction between N2O5and saturatedalcohols is known to produce organonitrates.59The mechanismfor the N2O5reaction with an alcohol has been suggested tooccur via a six-membered ring, leading to an organic nitrateand HNO3.51The suggested mechanism is shown in Fig. 4.3.6 Reactive uptake of N2O5on the alkenoic acids OA, LA,and CLAForthealkenoicacids,liquid-phasereactionswithN2O5yieldedreactive uptake coefficients in the range of (3.7–19.7) C2 10C05.Solid-phase reactions of OA and CLA with N2O5gaveg o 3 C2 10C05.The decrease in g below the freezing pointsuggests that the reaction for the liquid was due to a combinationof surface and bulk reactions. For all N2O5studies on bothliquid and solid films, the loss of N2O5was constant over time,similar to the NO3experiments.There have been no previous studies of the reactive uptakecoefficients of N2O5on a liquid or solid alkenoic acid.However, Lai and Finlayson-Pitts studied the heterogeneousreaction between N2O5and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (an unsaturated organic compound) on theinside surface of a glass reaction cell, but uptake coefficientswere not reported.33Major products identified in this previousstudy included molecules with nitro and nitrate functionalgroups. The initial step inthereactionmechanism was thoughttobedirectreactionofN2O5withthedoublebondleadingtoanitronitrate which can undergo subsequent reactions. This wasconsistent with an early study by Stevens and Emmons.603.7 Reactive uptake of N2O5on DESReactive uptakecoefficients determined forliquidDES wereinthe range of (4.3–14)C210C05and wereo6C210C06for the solid.As g dropped significantly upon freezing, this again suggeststhat reaction on the liquid occurs both at the surface and inthe bulk.As shown in Fig. 3, a significant negative temperaturedependence was observed for liquid DES. To explain thistrend we again use the resistor model and eqn (1), whichassume the reaction for the liquid occurs mainly in the bulk.Diffusion coefficients and rate constants typically show apositive temperature dependence. Increasing either D or krxnwould lead to higher results for g and cannot explain thenegative temperature dependence for DES with N2O5. Themean molecular velocity, cavg, also has a positive temperaturedependence and therefore leads to a negative temperaturedependence for g. However, increasing the temperature from278 K to 298 K increases cavgby only 3.5%, while the valueof g for the DES + N2O5reaction decreased by a factor of2.6 over this temperature range. Therefore the observedtemperature dependence cannot be due to changes in themolecular velocity. H, Henry’s law solubility constant, alsohas a negative temperature dependence12and we thereforeassume that the observed increase in g at lower temperatures isdue to the higher solubility of N2O5in the liquid, leading tomore N2O5in the liquid bulk and thus a higher probability ofreactive uptake.There have been no previous studies of N2O5on liquid orsolid alkanoates for comparison. Also, we are not aware ofany gas-phase reactions between N2O5and alkanoates. Theonly other N2O5heterogeneous study for pure saturatedorganics that we are aware of is a study by Thornton et al.32These authors measured the uptake of N2O5on solidmalonic acid and azelaic acid aerosols and determined uptakecoefficients to beo10C03and (5 C6 3) C2 10C04, respectively. Thevalues they obtained for azelaic acid are slightly higher thanour result for DES, but the difference may also be due to thepresence of water in the experiments by Thornton et al.32Therelative humidity (RH) used in the azelaic acid experiments byThornton et al. was 85% humidity, whereas we had a drycarrier gas.Others have also studied the reactive uptake coefficientof N2O5on aqueous organic particles, but we have notincluded this in the discussion, since the reactivity in theseexperiments was most likely due to the hydrolysis reaction ofFig. 4 Reaction mechanism of N2O5with a saturated alcohol basedon the gas-phase mechanism of methanol in reaction with N2O5assuggested by Langer and Ljungstro¨ m.517798 | Phys. Chem. Chem. Phys., 2009, 11, 7792–7803 This journal is C13c the Owner Societies 2009Downloaded by The University of British Columbia Library on 18 April 2011Published on 06 August 2009 on http://pubs.rsc.org | doi:10.1039/B904741GView OnlineN2O5and not a reaction between N2O5and the organiccompound.30,32,38,61–633.8 Overall trend in N2O5reactivityThe trend in the reactive uptake coefficients for the threefrozen organic compounds in reaction with N2O5isCLA 4 OA 4 DES. N2O5uptake coefficients for the liquidorganic compounds studied at 298 K are in the order: glycerol4 LA 4 DES E OA E CLA. There is very little data onN2O5–organic gas-phase reactions for comparison. However,a recent study by Pfrang et al.64provides insight into therelative reactivity of an OH functional group vs. a doublebond. These authors studied the reaction between N2O5andpentenols and the identified products were mainly unsaturatednitrates. These gas-phase results suggest that the OH functionalgroup is more reactive than the double bond toward N2O5,which is consistent with our liquid-phase uptake results.4. Atmospheric implications4.1 Oxidation lifetimes for single component liquid organicparticlesTo determine if the reactions we investigated in this studyare important in the atmosphere, we calculated the oxidationtime scale from our experimental data and compared thistime scale with the atmospheric residence time of theaerosol particles. This analysis is similar to the analysisrecently used by Robinson et al.22to determine the effectof OH and O3reactions on the molecular composition oforganic aerosols in the regional context. Following Robinsonet al., we calculated the oxidation lifetime using the followingequation:tliquid¼NtotF¼4NArr3Mgcavg½oxC138ð3Þwhere tliquidis the oxidation lifetime of a single-componentliquid particle, Ntotis the total number of organic molecules inthe particle, F is the flux of oxidant into the particle, r is theparticle radius, r is the particle density (assumed to be thedensity of the pure liquid), NAis Avogadro’s number, M isthe molecular weight of the organic molecule, g is the reactiveuptake coefficient, [ox] is the concentration of the gas-phaseoxidant, and cavgis the average velocity of the oxidant in thegas phase.22This time scale was derived by setting the totalnumber of oxidation events equal to the initial number ofmolecules in the particles.22This simple calculation gives anestimate for the time needed for all the molecules in theparticles to be oxidized.We carried out these calculations for OA, DES, andglycerol using NO3,N2O5, and OH as the oxidants (OHcalculations were performed for sake of comparison). Oxidantmixing ratios used in the calculations were 25 ppt for NO3,1 ppb for N2O5and 0.06 ppt for OH (all 24 h averages).NO3and N2O5concentrations used in these experimentsroughly correspond to moderate pollution levels.65Itshould be noted, however, that tropospheric NO3and N2O5mixing ratios are highly variable. For example, recentmeasurements by Penkett et al.66over western Europesuggest an average NO3mixing ratio of 350 ppt, and aircraftmeasurements by Brown et al. indicate NO3mixing ratios ofup to several hundred ppt in the boundary layer overnorth eastern USA.26OH concentrations used in thesecalculations correspond roughly to summer conditions in anurban environment.67,68The results of these calculations are shown in Fig. 5 as afunction of particle size. For reference, the size of organicparticles in the atmosphere can range from a few nanometresto approximately 10 mm, depending on the source andenvironment. In an urban environment the mass mediandiameter for particles in the accumulation mode is typicallybetween 0.2–0.6 mm.7,12,69Global 3D models of210Pb andcarbonaceous aerosols suggest aerosol residence times of5–15 days,70,71while radioactive tracer measurements suggestan atmospheric residence time of the order of 10 days for someregions influenced by pollution.72In Fig. 5 we have indicatedFig. 5 Atmospheric lifetimes, t, of liquid droplets as a function ofparticle diameter during oxidations by NO3,N2O5, or OH at 298 K. Itis assumed that the whole particle consists of OA (a), DES (b) orglycerol (c). For NO3and N2O5, reactive uptake coefficientsdetermined in this study were used for liquids at approximately 298 K.For the reaction of OA with NO3, an uptake coefficient of 0.5 waschosen. For OH, a reactive uptake coefficient of 0.5 was assumed forall substrates.84This journal is C13c the Owner Societies 2009 Phys.Chem.Chem.Phys.,2009, 11, 7792–7803 | 7799Downloaded by The University of British Columbia Library on 18 April 2011Published on 06 August 2009 on http://pubs.rsc.org | doi:10.1039/B904741GView Onlinetimes of 5 and 15 days with horizontal lines to allow for easiercomparison.Fig. 5a shows that the NO3oxidation lifetime for OA-likeparticles is short compared to aerosol particle residencetimes. Even for N2O5, the oxidation lifetime is comparableto the residence time of atmospheric aerosols. The predictedshort lifetimes of OA shown in Fig. 5 are in contrast tofield measurements together with source fluxes thatsuggest lifetimes of OA are of the order of days.22,73One possible explanation for this discrepancy is a differencein the phase (i.e. liquid vs. solid vs. semi-solid) between thesubstrates investigated in our work and the OA-containingparticles in the atmosphere.36,74–80One of the main sourcesof OA in the atmosphere is meat cooking operations.The phase behaviour for these multicomponent particles isnot well understood, but based on thermodynamic arguments,these particles are likely to be solid–liquid mixtures.36In solid–liquid mixtures, the lifetime of OA can be muchlonger due to trapping of the OA in or by the solidstructure. This has been shown previously for O3reactionswith OA-containing particles.36,74–80Other possibleexplanations for the differences between the laboratory studiesand the field measurements are differences in diffusion andsolubility of NO3in pure OA, compared to OA-containingparticles in the atmosphere.74,75,81Note that although meatcooking aerosols are likely to be solid–liquid mixtures, amajority of organic particles in the atmosphere are likely tobe liquid, since for most aerosols the concentration of anyspecific organic is small.82Fig. 5b shows the oxidation lifetime of DES particles. ForNO3and N2O5heterogeneous reactions, the oxidation lifetimeis comparable (or shorter) than the residence time of aerosolparticles in the atmosphere. Also, the oxidation lifetimeassociated with NO3is comparable to the oxidation lifetimeassociated with OH.Fig. 5c shows the oxidation lifetime of glycerol particles.The oxidation lifetime associated with N2O5heterogeneousreactions is short compared to the residence time of aerosolparticles. Glycerol has three OH groups which probablyenhance its reactivity compared to alcohols which containonly one OH group. Studies of the reactive uptake coefficientof N2O5on other alcohols (i.e. diols or compounds with asingle OH group) would be interesting.The results in Fig. 5 were calculated for three specificorganic compounds. In addition, the results correspondto single-component organic aerosols. Atmospheric aerosolsare complex mixtures containing multiple components.Nevertheless, the present results are important first outcomesfor understanding the importance of these heterogeneousreactions and provide a ‘‘back-of-the-envelope estimate’’ forthe lifetime of condensed phase organics in the atmosphere.In the future, measurements and calculations involvingmore complex aerosols (which are better models foratmospheric aerosols) will be required. Also, the apparentdiscrepancies between our work and the work by Moiseet al.17need to be resolved. However, until more data isavailable, the results shown in Fig. 5 may be useful forestimating the importance of NO3and N2O5on the lifetimeof organic species in atmospheric aerosols.4.2 Importance of these heterogeneous reactions to secondaryorganic aerosol (SOA) studiesOne area where these heterogeneous reactions may be importantis in SOA experiments with NO3radicals. In these studies,NO3heterogeneous reactions may occur during secondaryorganic aerosol formation and aging. To illustrate this pointwe refer to a recent SOA study by Ng et al.83These authorsstudied the SOA formation from reactions of isoprene withnitrate radicals using NO3concentrations of approximately140 ppt. These authors proposed that heterogeneous reactionsmay have been important in their studies. Here we use our newreactive uptake results to support this suggestion.We assume here that in general SOA particles are liquidand the results from our studies are applicable to particlesgenerated in SOA studies. This, of course, needs to be verifiedinfuturestudies,butthisanalysisisstillinsightfulforestimatingthe importance of NO3heterogeneous chemistry in SOAlaboratory experiments.To estimate if NO3heterogeneous reactions may beimportant in the SOA studies by Ng et al.83we calculate theoxidation lifetime of unsaturated organic molecules and saturatedorganic molecules in the condensed-phase using eqn (3), with aparticle diameter of 200 nm and [NO3] = 140 ppt. For thecalculations of the oxidation lifetime of the unsaturated andsaturated organics we used the reactive uptake coefficients forOA and DES determined in our studies, respectively. Usingeqn (3) resulted in oxidation lifetimes of 7.6 min and 18.2 hfor unsaturated and saturated organics, respectively. Theexperiment length used by Ng et al. was approximately1–2 h, so heterogeneous reactions between NO3and saturatedorganics likely did not play an important role (at least notbased on our DES reactive uptake coefficients). However, forthe unsaturated compounds, the oxidation lifetime is muchshorter than the experiment length, suggesting heterogeneousreactions may have been important. Unsaturated organicswere suggested products of the SOA chemistry, accordingto Ng et al. These authors have also suggested that theunsaturated organics may have partitioned into the condensedphase and reacted through heterogeneous chemistry. Ourresults and calculations above provide support for the lattersuggestion.In SOA studies with NO3,N2O5is often used as the sourceand N2O5concentrations are higher than NO3. As a result,N2O5heterogeneous reactions are potentially important insome of these studies as well. Considering our reactiveuptake coefficient results for alkenoic acids and DES, N2O5concentrations would need to be roughly 500 ppb in order forN2O5heterogeneous reactions with unsaturated and saturatedorganics to be important in SOA studies lasting approximately1 h (which is roughly the time scale of some chamberexperiments). However, if the results for glycerol are applicableto SOA studies, N2O5heterogeneous chemistry will beimportant at even lower N2O5concentrations.Again, these calculations correspond to single-componentorganic aerosols. SOA aerosols are also complex mixturescontaining multiple components. In the future, measurementsand calculations involving more complex aerosols (which arebetter models for SOA) will be required.7800 | Phys. Chem. Chem. Phys., 2009, 11, 7792–7803 This journal is C13c the Owner Societies 2009Downloaded by The University of British Columbia Library on 18 April 2011Published on 06 August 2009 on http://pubs.rsc.org | doi:10.1039/B904741GView Online4.3 Oxidation time scale for surface organicsTocalculate theoxidationtimescaleforasolidorganicsurfaceunder atmospheric conditions, the following equation16,29was used:tsolid¼4Ntotgcavg½oxC138ð4Þwhere tsolidis the oxidation lifetime of a solid surface (i.e. thetime needed for 63% of the surface molecules to be oxidized)and Ntotis the number of organic molecules on the surface.For these calculations we used g values determined fromheterogeneous reactions of solid organic surfaces and an Ntotvalue of 1 C2 1014molecules cmC02. Calculations were performedfor both OA and DES. The same oxidant concentrations usedin section 4.1 were used in eqn (4).The results for these calculations are shown in Table 3. Thetrends are similar to the trends observed for the liquid, buttheoxidationtimescales areshortersince, inthiscase, onlythesurface is oxidized. Recently, Moise and Rudich16carried outa similar analysis for NO3and OH to determine the processingtime for an organic coated aerosol. Their time scales weresimilar to ours except for NO3with a solid unsaturatedorganic, since our measured reactive uptake coefficients for asolid unsaturated organic are larger than the values used byMoise and Rudich.Eqn (4) was derived by assuming that the reactionprobability is proportional to the fraction of unreacted surfacesites. This is a simple expression for estimating the lifetime ofsurface organics and has been used in previous studies tocalculate the surface lifetime of organic molecules. Recentlyit has been shown that many atmospheric cases fit aLangmuir–Hinshelwood mechanism.84,85At this point, ananalysis involving a Langmuir–Hinshelwood mechanism(which should be more accurate) is not possible since theparameters necessary for describing these heterogeneousreactions with the Langmuir–Hinshelwood mechanism arenot known. Nevertheless, our calculations are important firstoutcomes for assessing the importance of these heterogeneousreactions.4.4 Fates of NO3and N2O5Above, we discussed the fate of particle-phase organics. Here,we discuss the fate of NO3and N2O5based on the modelingstudy by Moise et al.17These authors used a box model tostudy the effect of NO3–organic heterogeneous reactions ongas-phase [NO3t] (defined as [NO3]+[N2O5]). UsinggNO3=3C2 10C03, they observed a decrease in [NO3t]of B10% for certain atmospheric conditions. Increasing gNO3to 1 C2 10C02resulted in a 26% decrease of [NO3t] for certainconditions.AreactiveuptakecoefficientforNO3of1C210C02ismore consistent with our experimental results for unsaturatedorganic compounds. However, since the uptake of NO3forunsaturated organics is large, these molecules should berapidly oxidized in the atmosphere (unless matrix effects playa role). In this case, the large uptake coefficients will only beimportant close to emission sources and only if there is a largesource of aerosol unsaturated organics. Thomas et al. reacheda similar conclusion when modeling O3heterogeneousreactions with particle-phase unsaturated organics.865. ConclusionsLaboratory studies of the reactive uptake coefficient (g) forheterogeneous reactions between NO3and N2O5and fivedifferent organic compounds were conducted to better under-stand the atmospheric importance of these reactions. Thespecific substances used for this study contain functionalgroups commonly found in atmospheric samples of aerosolparticles and may thus aid in the interpretations of certainorganic compounds in the atmosphere. This study addsimportant g values to a relatively small set of data and high-lights a potential uncertainty in NO3–alkenoic acid hetero-geneous reactions.The reactive uptake coefficients were used to show that NO3heterogeneous reactions with liquid-phase unsaturated andsaturated organics may be important in the atmosphere andthat N2O5heterogeneous reactions with alcohols could also beimportant. Experimental data also suggests that NO3andN2O5heterogeneous reactions could be important in somesecondary organic aerosol (SOA) studies.Measurements and calculations were carried out forsingle-component organic aerosols and substrates. Theseresults are important first outcomes for understanding theimportance of these heterogeneous reactions and provide a‘‘back-of-the-envelope estimate’’ for the lifetime of condensedphase organics in the atmosphere. However, atmosphericaerosols are complex mixtures. In the future, measurementsand calculations involving more complex aerosols will berequired. Until further studies have been conducted, theresults contained herein will be useful for estimates of at leastan upper limit on the influence of NO3and N2O5on lifetimesof organic species in atmospheric aerosols.AcknowledgementsThe authors would like to acknowledge financial support fromthe Natural Science and Engineering Research Council ofCanada (NSERC) and the Canada Research Chair Program.We also thank P. J. Ziemann and P. Campuzano-Jost forseveral helpful discussions regarding material related to themanuscript.References1 M. Kanakidou, J. H. Seinfeld, S. N. Pandis, I. Barnes,F. J. Dentener, M. C. Facchini, R. Van Dingenen, B. 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