"Science, Faculty of"@en . "Chemistry, Department of"@en . "DSpace"@en . "Xiao, Song, Bertram, Allan K. 2011. Reactive uptake kinetics of NO3 on multicomponent and multiphase organic mixtures containing unsaturated and saturated organics. Physical Chemistry Chemical Physics 13(14) 6628-663"@en . "Bertram, Allen K."@en . "Xiao, Song"@en . "Bertram, Allan K."@en . "2012-02-03T00:00:00"@en . "2011-02-03"@en . "We investigated the reactive uptake of NO3 (an important night-time oxidant in the atmosphere) on binary mixtures containing an unsaturated organic (methyl oleate) and saturated molecules (diethyl sebacate, dioctyl sebacate, and squalane) which we call matrix molecules. These studies were carried out to better understand the reactivity of unsaturated organics in multicomponent and multiphase atmospheric particles. For liquid binary mixtures the reactivity of methyl oleate depended on the matrix molecule. Assuming a bulk reaction, varied by a factor of 2.7, and assuming a surface reaction HSmatrixKSmatrixkSoleate varied by a factor of 3.6, where and HSmatrixKSmatrixkSoleate are constants extracted from the data using the resistor model. For solid\u00E2\u0080\u0093liquid mixtures, the reactive uptake coefficient depended on exposure time: the uptake decreased by a factor of 10 after exposure to NO3 for approximately 90 min. By assuming either a bulk or surface reaction, the atmospheric lifetime of methyl oleate in different matrices was estimated for moderately polluted atmospheric conditions. For all liquid mixtures, the lifetime was in the order of a few minutes (with an upper limit of 35 min). These lifetimes can be used as lower limits to the lifetimes in semi-solid mixtures. Our studies emphasize that the lifetime of unsaturated organics (similar to methyl oleate) is likely short if the particle matrix is in a liquid state."@en . "https://circle.library.ubc.ca/rest/handle/2429/33767?expand=metadata"@en . "6628 Phys. Chem. Chem. Phys., 2011, 13, 6628\u00E2\u0080\u00936636 This journal is c the Owner Societies 2011 Cite this: Phys. Chem. Chem. Phys., 2011, 13, 6628\u00E2\u0080\u00936636 Reactive uptake kinetics of NO3 on multicomponent and multiphase organic mixtures containing unsaturated and saturated organics S. Xiao and A. K. Bertram* Received 27th November 2010, Accepted 3rd February 2011 DOI: 10.1039/c0cp02682d We investigated the reactive uptake of NO3 (an important night-time oxidant in the atmosphere) on binary mixtures containing an unsaturated organic (methyl oleate) and saturated molecules (diethyl sebacate, dioctyl sebacate, and squalane) which we call matrix molecules. These studies were carried out to better understand the reactivity of unsaturated organics in multicomponent and multiphase atmospheric particles. For liquid binary mixtures the reactivity of methyl oleate depended on the matrix molecule. Assuming a bulk reaction, Hmatrix ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Dmatrixkoleate p varied by a factor of 2.7, and assuming a surface reaction HSmatrixK S matrixk S oleate varied by a factor of 3.6, where Hmatrix ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Dmatrixkoleate p and HSmatrixK S matrixk S oleate are constants extracted from the data using the resistor model. For solid\u00E2\u0080\u0093liquid mixtures, the reactive uptake coefficient depended on exposure time: the uptake decreased by a factor of 10 after exposure to NO3 for approximately 90 min. By assuming either a bulk or surface reaction, the atmospheric lifetime of methyl oleate in different matrices was estimated for moderately polluted atmospheric conditions. For all liquid mixtures, the lifetime was in the order of a few minutes (with an upper limit of 35 min). These lifetimes can be used as lower limits to the lifetimes in semi-solid mixtures. Our studies emphasize that the lifetime of unsaturated organics (similar to methyl oleate) is likely short if the particle matrix is in a liquid state. 1. Introduction Organic material contributes about 20\u00E2\u0080\u009390% to the total fine aerosol mass in the troposphere.1,2 This organic material can be in the form of pure organic particles or alternatively the organic can be internally mixed with inorganic material.3,4 Adding to the complexity, the organic material can consist of thousands of different organic compounds with a range of functional groups.5,6 Organic and mixed organic\u00E2\u0080\u0093inorganic particles can also be solids, liquids, liquid\u00E2\u0080\u0093liquid mixtures, liquid\u00E2\u0080\u0093solid mixtures or glasses.7\u00E2\u0080\u009316 While in the atmosphere these organic and mixed organic\u00E2\u0080\u0093inorganic particles can undergo reactions with gas- phase species such as OH,17\u00E2\u0080\u009320 O3, 21\u00E2\u0080\u009325 NO3 26\u00E2\u0080\u009333 and Cl.34\u00E2\u0080\u009336 These heterogeneous reactions can be important for several reasons.18,37\u00E2\u0080\u009339 As an example, heterogeneous reactions have implications for source apportionment. Specific organic species often serve as molecular markers for probing sources of organic particles. If heterogeneous reactions change the concen- trations of the selected molecular markers they can lead to errors when calculating source strengths.40 Despite the potential importance of organic heterogeneous chemistry in the atmosphere and the fact that organic particles in the atmosphere are complex, there have been relatively few heterogeneous chemistry studies using multicomponent or multiphase organic mixtures. Recent studies using multi- component and multiphase organics have mainly involved O3, OH and Cl chemistry. See for example ref. 9, 21, 24, 29, 34, 35, 41\u00E2\u0080\u009359. Recently we studied the reactive uptake coefficient of NO3 on single-component organics and concluded that the NO3-alkene reaction could potentially be an important loss process of particle-phase unsaturated organic compounds in the atmosphere and in laboratory secondary organic aerosol studies.27 However, these conclusions were based on measurements with single-component substrates. The NO3 kinetics may be different in multicomponent and multiphase mixtures based on past studies using multicomponent and multiphase mixtures with O3, OH and Cl. See for example ref. 9, 18, 23, 24, 29, 35, 41\u00E2\u0080\u009344, 46, 47, 52, 54, 57, 58 and 60. In the following we investigate NO3 reactive uptake on multicomponent and multiphase mixtures containing an unsaturated organic. For the unsaturated organic we used methyl oleate (see Fig. 1). Based on previous work we expect that the reaction between NO3 and the carbon\u00E2\u0080\u0093carbon double bond is efficient and the ester functional group does not play a significant role in the chemistry or kinetics.27 Other molecules used in this study were diethyl sebacate (DES), dioctyl sebacate (DOS) and squalane. These molecules cover a range of viscosities, Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada PCCP Dynamic Article Links www.rsc.org/pccp PAPER D ow nl oa de d by T he U ni ve rs ity o f B rit ish C ol um bi a Li br ar y on 1 8 A pr il 20 11 Pu bl ish ed o n 02 M ar ch 2 01 1 on h ttp :// pu bs .rs c. or g | do i:1 0.1 039 /C0 CP 026 82D View Online This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 6628\u00E2\u0080\u00936636 6629 molecular weights and functional groups (See Fig. 1 and Table 1). Also these molecules have low vapor pressures (which is a prerequisite for the flow tube studies). Here we refer to these saturated organics as matrix molecules. At a temperature of 278 K we studied the following binary mixtures: methyl oleate\u00E2\u0080\u0093DES, methyl oleate\u00E2\u0080\u0093DOS and methyl oleate\u00E2\u0080\u0093squalane. At this temperature the mixtures were all liquid. This allowed us to probe multicomponent liquid mixtures and assess the effect of the matrix molecules on the NO3 uptake kinetics. At 268 K we studied binary mixtures of methyl oleate\u00E2\u0080\u0093DES. At this temperature the binary system was a solid\u00E2\u0080\u0093liquid mixture. This allowed us to probe the effect of particle phase on the NO3 chemistry. In all experiments the concentration of methyl oleate in the binary mixtures were always kept less than 4 wt% methyl oleate. At these concen- trations physical properties of the binary mixtures, such as solubility and molecular diffusion, will be controlled mainly by the matrix molecules. In this paper we present the reactive uptake coefficient measurements for these multicomponent and multiphase mixtures. The reactive uptake coefficient (g) is defined as the fraction of collisions with a surface that leads to reactive loss. The results are analyzed using the resistor model, and the results from this analysis are then used to assess the effect of the matrix molecules on the NO3 uptake kinetics and the lifetime of unsaturated organics in the atmosphere. 2. Experimental 2.1 Experimental setup Experiments were conducted in a cylindrical, rotating-wall flow tube reactor coupled to a chemical ionization mass spectrometer (CIMS). The setup and procedure are similar to several recent studies.9 A rotating Pyrex tube (B12 cm length, 1.77 cm inner diameter) fitted snugly inside the flow tube reactor. The inside wall of the glass tube provided a surface for a thin coating of the studied organic material. NO3 entered the flow tube through a movable injector. By varying the distance between the injector tip and the exit of the flow tube, loss of NO3 can be determined as a function of reaction distance and thus reaction time. NO3 radicals were obtained by thermal conversion of gaseous N2O5 to NO3 and NO2 at 430 K in a Teflon coated glass oven before entering the movable injector. N2O5 was generated by reacting NO2 with an excess amount of O3 in a flow system as described by Schott and Davidson61 and Cosman et al.62 N2O5 was trapped and stored as solid white crystals at 197 K. After thermal conversion of N2O5 to NO3 and NO2, the recombination of NO3 and NO2 was negligible due to the short residence time of the gases in the flow tube reactor (typically 20\u00E2\u0080\u0093100 ms), NO3 was detected as NO3 \u0002 in the mass spectrometer after chemical ionization by I\u0002 which was generated by passing a trace amount of CH3I in N2 through a 210Po source (model Po-2031, NRD). Total pressures in the flow cell during experiments were typically 2.6\u00E2\u0080\u00933.2 Torr whereas flow velocities ranged from 380\u00E2\u0080\u0093600 cm s\u00021. The carrier gas through the cell was a mixture of O2 (B10\u00E2\u0080\u009315%) in He. NO3 concentrations for all experiments were estimated as (3.5\u00E2\u0080\u009316) \u0003 1010 molecules cm\u00023 by assuming that all N2O5 is converted to NO3 and NO2 and approximately 20% of the NO3 thermally dissociates in the Teflon coated glass oven based on well-known gas-phase reaction rates and modeling studies using the Acuchem chemical kinetics simulation program.63 Quantitative conversion of N2O5 to NO3 and NO2 in the oven was confirmed by adding high levels of NO to the exit of the flow tube. This conversion reaction with NO also served as a convenient way to quantify the background signal in the NO3 experiments. NO was added in excess which completely titrated NO3 to NO2. Any remaining signal at mass 62 after titration by NO was assigned to the background. The background signal was typically less than 10% of the total signal. The uncertainty of the NO3 concen- tration, based on the uncertainty of the rate constant for the gas-phase N2O5 + I \u0002 reaction, is 40%.64 Fig. 1 Molecular structures of the organic compounds used in this study. Table 1 Properties of the organic compounds used in this study Compound Molecular Formula Molecular weight (g mol\u00021) Viscosity at 293\u00E2\u0080\u0093298 K (mPa s) Diffusion Coefficienta at 293 K (cm2 s\u00021) Methyl oleate C19H36O2 296.49 N/A N/A DES C14H26O4 258.36 5.88 78 1.8 \u0003 10\u00026 DOS C26H50O4 426.67 25 b 4.3 \u0003 10\u00027 Squalane C30H62 422.81 36.0 79 3.0 \u0003 10\u00027 a The diffusion coefficient is calculated by the Stokes\u00E2\u0080\u0093Einstein equation27 and by assuming the radius of the diffusing species (NO3) was the same as O3 as done in a recent paper. 80 b Taken from www.kicgroup.com/dos.htm. D ow nl oa de d by T he U ni ve rs ity o f B rit ish C ol um bi a Li br ar y on 1 8 A pr il 20 11 Pu bl ish ed o n 02 M ar ch 2 01 1 on h ttp :// pu bs .rs c. or g | do i:1 0.1 039 /C0 CP 026 82D View Online 6630 Phys. Chem. Chem. Phys., 2011, 13, 6628\u00E2\u0080\u00936636 This journal is c the Owner Societies 2011 Observed first-order loss rate coefficients, kobs, were calculated from the depletion of the oxidant signal with increasing reaction time. Typical plots of the natural logarithm of the NO3 signal as a function of time are shown in Fig. 2. The slopes of the linear fits were used to determine kobs, which was in turn corrected for concentration gradients that formed close to the flow-tube wall by using the procedure described by Brown.65 Uptake coefficients (g) were calculated from the corrected rate constants, kcorr, using a standard procedure. 9 Diffusion coefficients of NO3 used in these calculations were taken from Rudich et al.66 The two main sources of uncertainty for the uptake coefficient measurements were the gas phase NO3 diffusion coefficient and the measurement of kobs. We calculated the error from gas phase diffusion by assuming a 20% uncertainty for the NO3 diffusion coefficient. 67 The uncertainty for the gas phase diffusion coefficient of NO3 in Helium is about 8%, and for NO3 in O2 is about 20%. In our study, the carrier gas is a mixture of He and O2. To be conservative we used the larger uncertainty (20%) as the uncertainty of NO3 in the He\u00E2\u0080\u0093O2 mixture. For the uncertainty of kobs, we used the standard deviation (1s) of the measurements. Reported errors include both the uncertainty from the diffusion coefficient and uncertainty from measuring kobs. The vapor pressure of pure methyl oleate is 4 \u0003 10\u00025 Torr at 25 1C, and in the mixtures it should be decreased by more than an order of magnitude assuming Raoult\u00E2\u0080\u0099s Law behaviour. At these low vapor pressures, the loss due to gas-phase reactions between methyl oleate and NO3 should be less than 0.1% of the observed loss of NO3. For reactive uptake studies on liquids, approximately 0.5 to 0.8 ml of the liquid was added to the inner wall of a rotating glass cylinder. A rotation rate of B10 rotations min\u00021 was used for all experiments to ensure an even coating of the liquid on the inside of the glass tube. For reactive uptake studies on solid\u00E2\u0080\u0093liquid mixtures, a smooth solid\u00E2\u0080\u0093liquid film was prepared following the procedure outlined by Knopf et al.9 First a liquid mixture of methyl oleate in DES (1.4 wt% methyl oleate) was prepared. This liquid mixture was added to a glass tube at room temperature and rotated. Next, the glass tube was rapidly immersed into liquid N2. Subsequently, the tube was taken out of the liquid N2 and located inside the flow tube reactor at (268 \u0004 1) K. There was no apparent change in the phase after the reactions. 2.2 Measurements of the temperature-composition phase diagram for mixtures of methyl oleate in DES The temperature-composition phase diagram for methyl oleate\u00E2\u0080\u0093DES mixtures is not known. We determined this phase diagram by means of differential scanning calorimetry (DSC). The phase diagram was necessary to determine properties of the solid phase (e.g. pure solid DES or a solid solution containingmethyl oleate andDES) that formed in the experiments mentioned above as well as determine mass partitioning between solid and liquid phases. Determination of the phase diagram consisted of the following steps: 40 mL of liquid mixture (methyl oleate and DES) were added to a sample pan. The temperature of the sample was decreased to \u000250 1C, and then increased to 30 1C at a rate of 5 1C min\u00021. The phase diagram was constructed from the melting peaks in the thermogram.68 2.3 Chemicals Diethyl sebacate (98%) and squalane (99%) were obtained from Sigma-Aldrich; Methyl oleate (Z 99%) and dioctyl sebacate (Z 97%) were purchased from Fluka; NO2 was purchased from Matheson. N2 (99.999%), O2 (99.993%), and He (99.999%) were purchased from Praxair. O3 was produced by photolysis of O2. 3. Results 3.1 Reactive uptake coefficients of NO3 on single component organics The resistor model is used to analyze the reactive uptake data for binary mixtures. Ideally, for this analysis the NO3 reactive uptake coefficients on the pure matrix molecules are available. Table 2 provides the uptake coefficients for NO3 on the pure matrix molecules, as well as the uptake on pure methyl oleate for comparison. The uptake result for methyl oleate is in good agreement with measurements of other unsaturated organics (oleic acid, linoleic acid and conjugated linoleic acid).27 Also, the uptake coefficient of NO3 on methyl oleate is about 2\u00E2\u0080\u00933 orders of magnitude higher than those of NO3 on saturated organics (DES, DOS and squalane). This trend is roughly consistent with the trend observed in the gas phase.69 The uptake Fig. 2 Plot of the natural logarithm of the NO3 signal vs. reaction time from several experiments. The substrates used in these studies were liquid DES and two liquid binary mixtures of methyl oleate and DES (0.57 and 1.72 wt% methyl oleate). Table 2 Measured uptake coefficients of NO3 on single-component organic compounds Compound T (K) Phase g DES 278 Liquid (4.4 \u0004 0.4) \u0003 10\u00023 DES 272 Solid (3.6 \u0004 0.5) \u0003 10\u00024a DOS 278 Liquid (3.9 \u0004 0.3) \u0003 10\u00023 Squalane 278 Liquid (5.2 \u0004 0.4) \u0003 10\u00023 Methyl oleate 278 Liquid (1.4 +8.6/\u00020.5) \u0003 10\u00021 a This uptake coefficient was obtained from our previous work.27 D ow nl oa de d by T he U ni ve rs ity o f B rit ish C ol um bi a Li br ar y on 1 8 A pr il 20 11 Pu bl ish ed o n 02 M ar ch 2 01 1 on h ttp :// pu bs .rs c. or g | do i:1 0.1 039 /C0 CP 026 82D View Online This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 6628\u00E2\u0080\u00936636 6631 coefficient of NO3 with solid DES is about 90% lower than the corresponding liquid-phase data. 3.2 Reactive uptake coefficients of NO3 on binary liquid mixtures containing methyl oleate Fig. 3 shows the measured uptake coefficients of NO3 on different binary mixtures as a function of the methyl oleate concentration. For all matrices studied the addition of small amounts of methyl oleate (less than 4 wt%) significantly increases the reactive uptake coefficient. Also, the magnitude of increase depends of the type of matrix. For example at approximately 2.3 wt% methyl oleate the reactive uptake coefficient in DES increased by a factor of 20 compared to the pure case, but in squalane the reactive uptake coefficient only increased by a factor of 4. To check whether the uptake is reversible or irreversible, at the end of every experiment we moved the injector to a position where the coated organic mixture was no longer exposed to the NO3 flow. The absence of any release of NO3 indicated that the uptake was irreversible. 3.3 Analysis of the reactive uptake coefficient data using the resistor model To analyze the liquid uptake results presented in Fig. 3, we used the resistor model for gas-substrate interactions.70 If the reaction occurs in the bulk and the reactive uptake coefficient is not limited by the mass accommodation coefficient (i.e., a c g, where a is the mass accommodation coefficient) then the following equation applies for our binary liquid mixtures (see Appendix). g2mixture \u0002 g2matrix \u00C2\u00BC \u00C3\u00B04HmatrixRT\u00C3\u009E2Dmatrixkoleate c2NO3 Moleate \u00C3\u00B01\u00C3\u009E where gmixture is the reactive uptake coefficient of NO3 in the two component mixture, gmatrix is the reactive uptake coefficient of NO3 with the pure matrix molecules, Hmatrix is the Henry\u00E2\u0080\u0099s law solubility constant of NO3 in the matrix, R is the gas constant, T is the temperature, Dmatrix is the diffusion coefficient for NO3 in the matrix, koleate is the bulk second- order rate constant for the NO3 reaction with methyl oleate, cNO3 is the mean molecular velocity of NO3, andMoleate is the molarity of the methyl oleate in each matrix. According to eqn (1), a plot of (g2mixture \u0002 g2matrix) vs. Moleate is expected to yield a straight line. In contrast to eqn (1), if the reaction occurs on the surface and assuming the reactive uptake coefficient is not limited by the adsorption coefficient, the following equation applies for our binary liquid mixtures (see Appendix). gmixture \u0002 gmatrix \u00C2\u00BC 4HSmatrixRTK S matrixk S oleate cNO3 Moleate \u00C3\u00B02\u00C3\u009E where gmixture is the reactive uptake coefficient of NO3 with the binary mixture, gmatrix is the reactive uptake coefficient of NO3 with the corresponding pure matrix, H S matrix is the surface Henry\u00E2\u0080\u0099s law equilibrium analogous to a Henry\u00E2\u0080\u0099s law equilibrium for bulk condensed phase, KSmatrix is an equilibrium constant linking the surface concentration to the bulk concentration of the organic liquid, kSoleate is the second-order rate constant for the NO3 reaction with methyl oleate at the surface, and Moleate is the molarity of methyl oleate in each matrix. If the reaction occurs at the surface and the assumptions outlined above are valid, then a plot of (gmixture \u0002 gmatrix) vs. Moleate is expected to yield a straight line. In Fig. 4 panels a\u00E2\u0080\u0093c, we have plotted (g2mixture \u0002 g2matrix) vs. Moleate and panels d\u00E2\u0080\u0093f, we have plotted (gmixture \u0002 gmatrix) vs. Moleate. Fig. 4 shows that the data can be fit reasonably well by assuming either a bulk reaction or a surface reaction. To evaluate the goodness-of-fit for the two different models (bulk and surface), we calculated w2 values. Smaller w2 values represents a better fit to the data. The results from these calculations are included in Fig. 4. Based on the w2 values, kinetics for DOS and squalane mixtures is explained well by both the bulk and surface model. For DES, the kinetic data fit better to the surface model than the bulk model, although even the bulk model does a reasonable job of describing the trend in the reactive uptake data. Table 3 shows values of Hmatrix ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Dmatrixkoleate p and HSmatrixK S matrixk S oleate determined from the slopes of the lines shown in Fig. 4. If the reaction occurs in the bulk, then the Hmatrix ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Dmatrixkoleate p values vary by a factor of 2.7. If the reaction occurs on the surface, then the HSmatrixK S matrixk S oleate values vary by a factor of 3.6. This shows that the matrix has an effect on the kinetics as expected. It is also interesting to compare the trends observed for the different matrices. For example, the trend in Hmatrix ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Dmatrixkoleate p is DES > DOS > squalane. This trend is the same as the trend in the diffusion coefficients (Dmatrix) of the matrices (see Table 1). Fig. 3 Measured uptake coefficients of NO3 on binary liquid mix- tures containing methyl oleate. Some of the error bars for methyl oleate in DES exceed maximum y-values shown in this figure. Typically, when the g value is greater than 0.05 such as the last three data points for the methyl oleate\u00E2\u0080\u0093DES mixtures, the gas-phase diffusion of NO3 to the reactive surface greatly influences the measured g values. In this case, a small uncertainty in the diffusion coefficient will result in a large uncertainty in the measured g value. All experiments were carried out at (278 \u0004 1) K. D ow nl oa de d by T he U ni ve rs ity o f B rit ish C ol um bi a Li br ar y on 1 8 A pr il 20 11 Pu bl ish ed o n 02 M ar ch 2 01 1 on h ttp :// pu bs .rs c. or g | do i:1 0.1 039 /C0 CP 026 82D View Online 6632 Phys. Chem. Chem. Phys., 2011, 13, 6628\u00E2\u0080\u00936636 This journal is c the Owner Societies 2011 3.4 Temperature-composition phase diagram for methyl oleate\u00E2\u0080\u0093DES mixtures Shown in Fig. 5 are results from the differential calorimetry measurements. According to the phase diagram, methyl oleate and DES are miscible in the liquid state but immiscible in the solid state. The eutectic temperature for the binary system was determined to be (250.7 \u0004 0.5) K. 3.5 Reactive uptake coefficient measurements of NO3 on solid\u00E2\u0080\u0093liquid mixtures containing methyl oleate and DES The solid triangle in Fig. 5 shows the temperature and composition at which we studied the reactive uptake coefficient of partially solid mixtures of methyl oleate and DES. According to the phase diagram, the mixture consists of solid DES in equilibrium with a binary liquid mixture of approximately 45 wt% methyl oleate in DES. Contrary to measurements with liquids, the measured reactive uptake coefficients for these mixtures decrease with time. This is illustrated in Fig. 6. After approximately 90 min the reactive uptake coefficient on the solid\u00E2\u0080\u0093liquid mixture decreased by a factor of 10. In contrast the reactive uptake coefficient of NO3 on a liquid methyl oleate\u00E2\u0080\u0093DES mixture with the same weight percent methyl oleate did not decrease with time as expected. Fig. 6 illustrates that the phase of the mixture can significantly influence the kinetics, consistent with previous measurements using O3, Cl and OH as the oxidants. See for example ref. 9, 18, 29, 35, 41\u00E2\u0080\u009344, 47, 52, 54 and 60. In our studies the liquid\u00E2\u0080\u0093solid mixture is likely to have a surface that is partially solid DES and partially a liquid mixture of methyl oleate and DES. As the carbon\u00E2\u0080\u0093carbon double bonds in the exposed liquid regions are oxidized, the uptake is expected to decrease, consistent with observations. During the 90 min exposure approximately 4 \u0003 1016 molecules of NO3 were lost to the surface. Assuming that one molecule of NO3 reacts with one molecule of methyl oleate and that one monolayer of methyl oleate corresponds to roughly 6 \u0003 1014 molecules cm\u00022, then during the 90 min exposure approximately 10 monolayers of methyl oleate is oxidized. This is consistent with only the top few monolayers of the material being available for reaction when the material is in the semi-solid state. 4. Atmospheric implications 4.1 Lifetime of unsaturated organics in liquid organic particles Next we use the kinetic parameters for the liquids, to estimate the lifetime of condensed-phase unsaturated organics in the Fig. 4 Plot (g2mixture \u0002 g2matrix) (panel a, b, c) and (gmixture \u0002 gmatrix) (panel d, e, f) as a function ofMoleate. Panel a and d correspond to the reaction of NO3 with a methyl oleate\u00E2\u0080\u0093DES mixture, panel b and e correspond to the reaction of NO3 with methyl oleate\u00E2\u0080\u0093DOS mixtures, panel c and f correspond to the reaction of NO3 with methyl oleate\u00E2\u0080\u0093squalane mixtures. Table 3 Hmatrix ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Dmatrixkoleate p and HSmatrixK S matrixk S oleate values determined from the slopes of the lines in Fig. 4. The reported uncertainties are based on the standard deviation (1s) of the slopes in Fig. 4 Matrix molecule Hmatrix ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Dmatrixkoleate p \u00C3\u00B0cmM1=2 atm\u00021 s\u00021\u00C3\u009E HSmatrixKSmatrixkSoleate (L cm\u00022 atm\u00021 s\u00021) DES 69.4 \u0004 5.8 281.3 \u0004 11.8 DOS 35.4 \u0004 2.4 120.9 \u0004 4.2 Squalane 26.1 \u0004 1.5 78.0 \u0004 2.7 D ow nl oa de d by T he U ni ve rs ity o f B rit ish C ol um bi a Li br ar y on 1 8 A pr il 20 11 Pu bl ish ed o n 02 M ar ch 2 01 1 on h ttp :// pu bs .rs c. or g | do i:1 0.1 039 /C0 CP 026 82D View Online This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 6628\u00E2\u0080\u00936636 6633 atmosphere. If the reaction occurs in the bulk then the following equation can be used together with parameters shown in Table 3 to estimate the atmospheric lifetime.71\u00E2\u0080\u009373 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi \u00C2\u00BDUnsaturatedOrganic\u0005t q \u00C2\u00BC ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi \u00C2\u00BDUnsaturatedOrganic\u00050 q \u0002 3PNO3Hmatrix ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Dmatrixkoleate p 2rparticle t \u00C3\u00B03\u00C3\u009E where PNO3 is the NO3 partial pressure in the atmosphere, rparticle is the radius of the particle in the atmosphere, [UnsaturatedOrganic]0 is the initial concentration of the unsaturated organic in the particle and [UnsaturatedOrganic]t is the concentration of an unsaturated organic after reaction time t. If the reaction occurs at the surface then the following equation together with parameters in Table 3 can be used to estimate the lifetime of an unsaturated organic in the atmosphere.71 ln \u00C2\u00BDUnsaturatedOrganic\u0005t \u00C2\u00BDUnsaturatedOrganic\u00050 \u00C2\u00BC \u0002 3PNO3H S matrixK S matrixk S oleate rparticle t \u00C3\u00B04\u00C3\u009E Shown in Table 4 are the calculated lifetimes of unsaturated organics using eqn (3) and (4) and the parameters listed in Table 3, and assuming a radius of 100 nm, and a NO3 concentration of 25 ppt NO3 (24 h average). The NO3 concentration corresponds to roughly moderately polluted levels.74 Several conclusions can be drawn from Table 4. First, comparing the calculations assuming bulk with the calculations assuming surface, the lifetimes only differ by a factor of 1.5. Second, the lifetimes differ by only factor of 3 when comparing different liquid matrices. Third, regardless of the liquid matrix, or the assumption of surface vs. bulk, the lifetimes are short (all less than 35 min) for liquids. Hence we can conclude that the lifetime of unsaturated organics (similar to methyl oleate) are likely short in the atmosphere if the particle matrix is in a liquid state and NO3 concentrations are approximately 25 ppt. These lifetimes are comparable to the lifetimes reported for O3 with oleic acid9,40 (a molecule similar to methyl oleate) but are considerably shorter than the lifetimes for OH with oleic acid.40 Significant amounts of oleic acid, an unsaturated compound similar to methyl oleate, have been observed in the atmosphere. Fig. 6 Measured uptake coefficient (g) of NO3 on mixtures of methyl oleate with DES (composition = 1.37 wt% methyl oleate). Experiments were carried out by continuously exposing the surface to NO3, and periodically measuring the NO3 uptake coefficients. The open symbols represent experiments carried out at 278 K and correspond to a liquid, whereas the solid symbols represent experiments carried out at 268 K and correspond to a solid\u00E2\u0080\u0093liquid mixture. Table 4 Estimated atmospheric lifetimes of unsaturated organics, tunsaturated, using parameters determined from studies with methyl oleate in different matrices (DES, DOS and squalane) System used for determining kinetic parameters tunsaturated (min) a Assuming bulk reaction Assuming surface reaction Liquid mixture of methyl oleate in DES 13.0 8.0 Liquid mixture of methyl oleate in DOS 25.7 18.4 Liquid mixture of methyl oleate in squalane 34.8 28.5 a When calculating the atmospheric lifetime it was assumed that the mole fraction of the unsaturated organic in the particle was 0.1 and the particle diameter was 200 nm. Fig. 5 Temperature-composition phase diagram for the methyl oleate\u00E2\u0080\u0093DES system. S and L indicate solid and liquid phases, respectively. MO represents methyl oleate. The symbols \u00E2\u0080\u0099 and . represent the melting temperatures of DES and methyl oleate, respectively, in the binary mixtures. The symbol K represents the measured eutectic temperature of the mixture. Each point represents the average of two runs. The symbol m represents the conditions at which the uptake kinetics was investigated. The line and curves were added to guide the eye. D ow nl oa de d by T he U ni ve rs ity o f B rit ish C ol um bi a Li br ar y on 1 8 A pr il 20 11 Pu bl ish ed o n 02 M ar ch 2 01 1 on h ttp :// pu bs .rs c. or g | do i:1 0.1 039 /C0 CP 026 82D View Online 6634 Phys. Chem. Chem. Phys., 2011, 13, 6628\u00E2\u0080\u00936636 This journal is c the Owner Societies 2011 This suggests that particles containing oleic acid in the atmo- sphere are most likely not liquids, rather solids, semi- solids or glasses, based on our NO3 kinetics. A similar conclusion has been made by others based on measured reaction rates between O3 and liquid oleic acid in the laboratory. 4.2 Lifetime of unsaturated organics in semi-solid organic matrices The lifetime of unsaturated organics in semi-solid organic matrices is difficult to estimate from our measurements. Until further measurements are available, the results for the liquid mixtures can be used as a lower limit to the lifetime of unsaturated organics in semi-solid organic matrices based on Fig. 6. Appendix 1. Derivation of eqn (1) According to the resistor model, if the reaction occurs in the bulk, and if NO3 can react with both methyl oleate and the matrix molecules, and if the reactive uptake coefficient is not limited by the mass accommodation coefficient (i.e., a c g, where a is the mass accommodation coefficient) then the following equation applies for our binary liquid mixtures:70,75,76 gmixture \u00C2\u00BC 4HmixtureRT ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Dmixture\u00C3\u00B0kmatrixMmatrix \u00C3\u00BE koleateMoleate\u00C3\u009E p cNO3 \u00C3\u00B0A1\u00C3\u009E where Hmixture corresponds to the Henry\u00E2\u0080\u0099s law solubility constant of NO3 in the mixture, Dmixture corresponds to the diffusion coefficient for NO3 in the mixture, kmatrix is the second- order rate constant for the NO3 reaction with matrix molecules, and Mmatrix is the molarity of the matrix molecules in the mixture. In this study, the amount of the reactant (methyl oleate) is always small (wt% o 4%) in the mixture. As a result the Henry\u00E2\u0080\u0099s law solubility constant and the diffusion coefficient of NO3 in the mixture is approximately the same as the Henry\u00E2\u0080\u0099s law solubility constant and the diffusion coefficient of NO3 in pure matrix molecules (i.e. Hmixture E Hmatrix and Dmixture E Dmatrix where Hmatrix is the Henry\u00E2\u0080\u0099s law solubility constant of NO3 in the pure liquid of matrix molecules, and Dmatrix is the diffusion coefficient of NO3 in the pure liquid of matrix molecules). Substituting these approximations into eqn (A1) results in the following equation. g2mixture \u00C2\u00BC \u00C3\u00B04HmatrixRT\u00C3\u009E2Dmatrix c2NO3 kmatrixMmatrix \u00C3\u00BE \u00C3\u00B04HmatrixRT\u00C3\u009E 2Dmatrix c2NO3 koleateMoleate \u00C3\u00B0A2\u00C3\u009E For our study, g2mixture varies at least by a factor of 5, but Mmatrix only varies by 3%. Hence we assume that the first term in eqn (A2) is constant and equal to g2 for a pure liquid of matrix molecules. We refer to this as g2matrix which can be calculated from the g value in Table 1. After making this assumption and substitution we have the following: g2mixture \u0002 g2matrix \u00C2\u00BC \u00C3\u00B04HmatrixRT\u00C3\u009E2Dmatrixkoleate c2NO3 Moleate \u00C3\u00B0A3\u00C3\u009E Eqn (A3) is equivalent to eqn (1) above. A similar equation to eqn (A3) was used to describe the uptake coefficient of NO3 on an aqueous solution that had two parallel bulk reactions: a reaction with water and a reaction with ions.75,76 2. Derivation of eqn (2) According to the resistor model, if NO3 can react with both methyl oleate and the matrix molecules at the surface and the reactive uptake coefficient is not limited by the adsorption coefficient (i.e., S c g, where S is the adsorption coefficient) then the following equation applies for our binary liquid mixtures.70,71,77 gmixture \u00C2\u00BC 4RTHSmixtureK S mixturek S matrixMmatrix cNO3 \u00C3\u00BE 4RTH S mixtureK S mixturek S oleateMoleate cNO3 \u00C3\u00B0A4\u00C3\u009E Employing approximations similar to the ones used to derive eqn (A2) above, we derive eqn (A5) below. gmixture \u00C2\u00BC 4RTHSmatrixK S matrixk S matrixMmatrix cNO3 \u00C3\u00BE 4RTH S matrixK S matrixk S oleateMoleate cNO3 \u00C3\u00B0A5\u00C3\u009E Employing approximations similar to the ones used to derive eqn (A3) above, we derive eqn (A6): gmixture \u0002 gmatrix \u00C2\u00BC 4RTHSmatrixK S matrixk S oleate cNO3 Moleate \u00C3\u00B0A6\u00C3\u009E Eqn (A6) is equivalent to eqn (2) in the main text. 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"Reviewed"@en . "Vancouver : University of British Columbia Library"@en . "Royal Society of Chemistry"@en . "10.1039/C0CP02682D"@en . "Attribution-NonCommercial-NoDerivatives 4.0 International"@en . "http://creativecommons.org/licenses/by-nc-nd/4.0/"@en . "Faculty"@en . "Reactive uptake kinetics of NO3 on multicomponent and multiphase organic mixtures containing unsaturated and saturated organics."@en . "Text"@en . "http://hdl.handle.net/2429/33767"@en .