mixed-phase relative rates technique for measuring aerosol reaction

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1 GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L17805, doi: /2006gl026963, 2006 A mixed-phase relative rates technique for measuring aerosol reaction kinetics John D. Hearn 1 and Geoffrey D. Smith 1 Received 19 May 2006; revised 7 July 2006; accepted 31 July 2006; published 7 September [1] A mixed-phase relative rates approach for measuring rates of reaction in aerosols is presented. Using this method the rate of reaction of methyl oleate (MO) particles, normalized to the gas-particle collision rate, was measured to be g MO = 1.12 (±0.36) 10 3 with 2-methyl-2-butene as the gas-phase reference. This value compares favorably with our previously published value of measured using an absolute technique. Reaction of bis(2-ethylhexyl) sebacate (BES) particles with Cl and OH radicals was also studied using acetone and hexanal, respectively, as the gas-phase references. The rates of reaction of BES, normalized to the gas-particle collision rate, were measured to be g BES = 1.8 ( ) and g BES = 2.0 ( ) with Cl and OH, respectively. These fast rates of reaction (g BES > 1) imply that secondary reactions, perhaps involving radical chain mechanisms, could impact the rate at which organic particles are oxidized in the atmosphere. Citation: Hearn, J. D., and G. D. Smith (2006), A mixed-phase relative rates technique for measuring aerosol reaction kinetics, Geophys. Res. Lett., 33, L17805, doi: / 2006GL radical species since they are very reactive. While it is possible to expose particles to an O 3 concentration of 100 ppm in a flow tube, it is only possible to expose them to a sustained OH concentration of 1 ppb since the OH self-reaction is very efficient. Additionally, the radical concentration in the flow tube is transient, so it is difficult to measure it and the reaction time to which the particles are exposed accurately. [4] Here, we present the first implementation of a mixed-phase relative rates approach to the study of gasparticle reaction kinetics. The approach is very similar to the relative rate method which has been used for many years in the study of gas-phase kinetics [Finlayson-Pitts and Pitts, 2000]. By using a gas-phase reference compound with a known rate constant, this technique eliminates the need to determine the radical concentration or the reaction time. While these measurements of aerosol reaction kinetics are the first to be made using a gas-phase reference species, a similar mathematical formalism for relative rate aerosol kinetics was also very recently proposed by Donahue and co-workers [Donahue et al., 2005]. 1. Introduction [2] Recently, there has been a growing interest in understanding how organic aerosols in the lower atmosphere are chemically transformed through reactions with trace gases such as O 3,NO 3, Cl and OH [Finlayson-Pitts and Pitts, 2000]. There has been a limited number of studies on the kinetics of these reactions, and most have been carried out by observing the rate of loss of the gas-phase species. Only a few experiments have been conducted in which the rate of loss of the condensed-phase species is monitored, whether it be in a film or in particles. One exception to this is the reaction of O 3 with oleic acid, an unsaturated fatty acid often used to represent meat-cooking aerosol. Multiple groups have studied this reaction, as well as that between O 3 and methyl oleate, the methyl ester of oleic acid [Moise and Rudich, 2000; Morris et al., 2002; Smith et al., 2002; Katrib et al., 2004; Thornberry and Abbatt, 2004; Hearn et al., 2005; Zahardis et al., 2005]. Here, we use the reaction of O 3 with methyl oleate particles as a test case to validate a mixed-phase relative rates approach to measuring gasparticle kinetics in a flow tube. [3] Reactions between radicals, such as Cl and OH, and organic aerosols are more difficult to carry out in a flow tube system for a variety of reasons. First and foremost, it is difficult to maintain a high concentration of gas-phase 1 Chemistry Department, University of Georgia, Athens, Georgia, USA. Copyright 2006 by the American Geophysical Union /06/2006GL Experimental [5] The mixed-phase relative rates experiments were carried out at atmospheric pressure in a flow tube 1 m long, 2.54 cm inner diameter, as described in our previous works [Hearn et al., 2005; Hearn and Smith, 2005]. The aerosol particles were introduced into the rear of the flow tube along with either gas-phase O 3 (for both the O 3 and OH reactions) or Cl 2 (for the Cl reaction). The gas-phase reference compound (2-methyl-2-butene, acetone or hexanal for reaction with O 3, Cl or OH, respectively) was also added at the rear of the flow tube with typical concentrations of approximately molecules/cm 3. The total flow of gas through the flow tube was approximately 2 SLPM (standard liters per minute) resulting in an average reaction time between the gas-phase reactants and the particles of 15 seconds. The Cl radicals were created by photodissociation of Cl 2 in the flow tube using the third harmonic (355 nm) of a pulsed Nd:YAG laser (Quanta-Ray PRO-250, Spectra-Physics) directed along the length of the flow tube. The OH radicals were produced by photodissociation of O 3 with the fourth harmonic (266 nm) of the Nd:YAG laser to generate O( 1 D) which reacted with water vapor added to the flow tube: O 1 D þ H2 O! OH þ OH ð1þ [6] While the O 3, Cl and OH concentrations certainly were not uniform along the length of the flow tube, the gasphase reference compounds and the particles were well- L of5

2 mixed and subject to the same exposure (i.e. reactant concentration and reaction time). In essence, the gas-phase reference compound was used as a chemical clock with which to measure the integrated exposure of the particles to the gas-phase reactant making it is unnecessary to know the absolute concentrations and reaction times. Such an approach is similar in concept to that used to measure the photochemical age of air masses by monitoring relative concentrations of VOCs in field studies [Kleinman et al., 2003]. [7] The concentration of O 3 in the flow tube was molecules/cm 3 and was varied by adjusting the N 2 flow through the O 3 cold trap. The concentrations of Cl and OH were varied by adjusting the power of the photodissociation laser and were estimated to be molecules/cm 3 and molecules/cm 3, respectively. In this way it was possible to obtain an entire decay trace in as little as five minutes, and multiple experiments could be carried out in a short amount of time. The concentrations of both the gas-phase (reference) and particle species exiting the flow tube were monitored using Aerosol CIMS (chemical ionization mass spectrometry), as described previously [Hearn and Smith, 2004]. Briefly, the gases and particles were sampled through a flow-limiting heated glass capillary at 1.5 standard liters per minute (SLPM). The vapor from the heated particles was then ionized chemically and the ions were filtered and detected with a quadrupole mass spectrometer (ABB Extrel). All species were detected as the protonated ion ([M + H] + ) through proton transfer with H + (H 2 O) 2 generated by flowing 3.5 SLPM of N 2 through a 210 Po static eliminator (NRD, LLC). [8] The particles were generated by either directly nebulizing or nucleating heated vapor of methyl oleate (MO) (Aldrich, 99% purity) or bis (2-ethylhexyl) sebacate (BES) (Aldrich, 97% purity). All aerosol distributions were monodisperse (geometric standard deviation 1.20) as verified with a scanning-mobility particle sizer (TSI model 3936). The MO particles had mass-weighted diameters of 750 nm, 850 nm and 950 nm. The mass-weighted diameters of the BES particles were 142 nm and 190 nm for the OH and Cl experiments, respectively. O 3 was generated using a commercial ozonizer (Pacific Ozone Technologies) and introduced into the flow tube from a cold trap ( 80 C) containing silica gel on which it was stored. Cl 2 (National Welders, UHP grade), 2-methyl-2-butene (MB) (Acros, 95% purity), acetone (J.T. Baker, 99.5% purity), hexanal (Aldrich, 98% purity), and hexanes (J. T. Baker, 99.6% purity) were used without further purification. N 2 (National Welders, 99.99% purity) was used as the carrier gas. Figure 1. Observed loss of 850 nm methyl oleate (MO) particles reacting with O 3. Exposure to O 3 is determined from the observed loss of gas-phase 2-methyl-2-butene. The rate of reaction of MO normalized to the gas-particle collison rate is calculated to be g MO = 9.4 (±1.2) 10 4 from the initial slope of the MO decay trace. A plot of the residuals demonstrates that the quadratic fit (solid line) describes the data better than the exponential fit (dashed line), indicating that the reaction is limited by O 3 diffusion in the particle. 3. Results 3.1. Ozonolysis Reaction [9] The reaction between O 3 and methyl oleate (MO) particles was studied using 2-methyl-2-butene (MB) as the gas-phase reference compound. In addition, hexanes were added in the gas phase as radical scavengers because it is well known that ozonolysis of unsaturated organics in the gas phase can produce OH radicals [Atkinson and Aschmann, 1993; Atkinson, 1997; Paulson et al., 1998]. These OH radicals could complicate the relative rates analysis if they react with MO and MB at different rates. [10] A representative plot of a typical decay trace is shown in Figure 1 in which the MO signal is plotted as a function of exposure to O 3. In other experiments, this exposure has been calculated from measured O 3 concentrations and gas flow rates. Here, the exposure is calculated directly from the measured loss of the gas-phase reference compound, MB, using the integrated rate law for its reaction with O 3 : O 3 Exposure ¼ dmb ½ Š dt Z t 0 ½ ¼ ko ½ 3 Š½MBŠ ð2þ O 3 ŠðÞdt t ¼ 1 k ln ½MBŠ ½MBŠ 0 where k is the second-order rate constant for the reaction of O 3 with MB and is 4.1 (±0.5) cm 3 /molecule/sec. [Witter et al., 2002]. The slope of the fit to the initial data can then be used to calculate the rate of reaction of methyl oleate, g MO, as shown previously [Smith et al., 2002; Katrib et al., 2005]: g MO ¼ g j ¼ d ½MO Š= ½ MO Š 0 4 RT dð½o 3 ŠtÞ c V S A ð3þ ½MOŠ 0 ð4þ Here, R is the gas constant, T is the temperature, c is the mean speed of O 3, and V/S A (= d/6) is the volume-to-surface area ratio of the particles. Since we observe the rate of loss 2of5

3 Figure 2. Relative rates plot for the reaction of Cl radicals with 190 nm bis(2-ethylhexyl) sebacate (BES) particles and gas-phase acetone. The linear fit to the initial portion of the decay trace (solid line) yields a rate of reaction of BES, normalized to the gas-particle collision rate, of g BES = 1.8 ( ). For comparison, dashed and dotted lines represent initial slopes corresponding to g BES = 1.0 and 0.1, respectively. of the particle species (methyl oleate), we draw a distinction between g MO, representing the rate of reaction of the methyl oleate in the particles (normalized to the gas-particle collision rate) and g, the uptake coefficient, which represents the rate of loss of gas-phase O 3 (also normalized to the gas-particle collision rate). These two quantities will differ if other processes, such as secondary reactions, affect the rate of loss of the particle species and not the gas-phase species. To account for this difference, we have introduced an additional term, j, which represents the reaction yield of reaction of the particle species. In the absence of secondary reactions, j = 1 and g MO = g. [11] From 22 separate experiments with three sizes of particles (mass-weighted diameters of 750 nm, 850 nm and 950 nm), we obtain a value of g MO = 1.12 (±0.36) The uncertainty includes two standard deviations of the measured values of g MO as well as the uncertainty reported for the gas-phase reference rate constant. This value is in good agreement with our previously reported value of 1.23 (±0.10) 10 3 which we obtained by monitoring the loss of methyl oleate and measuring the O 3 concentration and reaction time directly [Hearn et al., 2005], indicating that this mixed-phase relative rates technique is viable. [12] The shape of the decay curve provides additional evidence that this technique can be used to measure the loss of MO quantitatively. From our previous work we know that the decay of MO is expected to demonstrate a quadratic shape [Hearn et al., 2005] since there is a competition between diffusion into the particle and reaction [Finlayson- Pitts and Pitts, 2000; Morris et al., 2002]: ½MOŠ ¼ 1 3H pffiffiffiffiffiffiffiffi 2 Dk 2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ½O 3 Št! ð5þ ½MOŠ 0 ½MOŠ 0 d where H is the Henry s Law solubility of O 3 in MO, D is the O 3 diffusion constant in MO, k 2 is the second-order rate constant for reaction between O 3 and MO, d is the particle diameter, and t is the reaction time. The least-squares quadratic fit (solid line) in Figure 1, indeed, does a good job of describing the data. Alternatively, if the reaction were to occur at the surface instead of inside the particle, the decay would be described by an exponential function (dashed line) [Finlayson-Pitts and Pitts, 2000; Morris et al., 2002]. However, it is clear from the plot of the residuals in Figure 1 that this fit does not describe the data (R 2 = 0.94) as well as the quadratic fit does (R 2 = 0.98). Thus, the ability to resolve the shape of this decay curve over a dynamic range of nearly two orders of magnitude serves as further confirmation of the validity of the mixed-phase relative rates technique Cl Radical Reaction [13] The mixed-phase relative rates technique is particularly well-suited to the study of reactions involving radical species since they generally react quickly and their concentrations may not be uniform or constant within the flow tube. We have used this technique to study the reaction of Cl radicals with BES particles (mass-weighted diameter = 190 nm) using acetone as the gas-phase reference compound. By continuously varying the photodissociation laser power, we obtain a decay trace of BES, as shown in Figure 2, where the Cl exposure is calculated using the value of the rate constant for the Cl + acetone reaction provided by Wine and co-workers (k = 2.09 (±0.31) cm 3 /molecule/sec.) (P. H. Wine, unpublished data, 2006). The rate of loss of BES can then be calculated from the initial slope measured from Figure 2 based on the reaction of the first 25% of BES, yielding g BES = 1.8 ( ). This uncertainty includes the 95% confidence interval on the slope of the linear least-squares fit to the initial data and the reported uncertainty on the Cl + acetone rate constant. Also, since gas-phase diffusion of Cl to the particles may restrict the rate of reaction under these experimental conditions [Finlayson-Pitts and Pitts, 2000], the measured rate of BES loss is a lower limit on the rate of BES reaction. We have included this underestimation in the upper limit of the uncertainty assuming a diffusion coefficient of Cl in one atmosphere of N 2 equal to 0.2 cm 2 /sec and a Cl uptake coefficient of g =1[Fuchs and Sutugin, 1971; Pöschl et al., 2005]. We note that this uncertainty represents the upper limit on this effect since gas-phase diffusion will limit the observed loss of BES less if g <1. [14] The large value of g BES is consistent with the only other study in which the uptake of Cl on organic surfaces was measured and yielded g > 0.1 [Moise and Rudich, 2001]. Furthermore, this value is consistent with the rate constants for analogous reactions between Cl and large organic molecules in the gas phase which are known to occur very efficiently (near the collision rate) [Atkinson, 1997]. The fact that g BES > 1 implies that the reaction yield, j, is larger than unity, indicating that there are additional loss processes for BES other than reaction with Cl OH Radical Reaction [15] The reaction of OH radicals with BES particles (mass-weighted diameter = 142 nm) was also studied using the mixed-phase relative rates technique. In these experiments, the OH concentration was varied by adjusting the photodissociation laser power in a step-wise fashion, and 3of5

4 Figure 3. Relative rates plot for the reaction of OH radicals with 142 nm bis(2-ethylhexyl) sebacate (BES) particles and gas-phase hexanal. Circles represent averages of the multiple measurements made by varying the photodissociation laser fluence in a step-wise manner. The linear fit yields an initial rate of reaction of BES, normalized to the gas-particle collision rate, of g BES = 2.0 ( ). the loss of BES was monitored vs. the exposure to OH, as determined from the loss of the gas-phase reference compound, hexanal, and the rate constant for the OH + hexanal reaction (k = 2.86 (± 0.13) cm 3 /molecule/sec.) [D Anna et al., 2001]. The rate of loss of BES can then be calculated from the initial slope measured from Figure 3 based on the reaction of the first 25% of BES, yielding g BES = 2.0 ( ). The uncertainty includes the 95% confidence interval on the slope of the linear least-squares fit to the data as well as the reported uncertainty on the OH + hexanal rate constant (which is only the uncertainty on the rate constant relative to that for OH + 1-butene [D Anna et al., 2001]). Additionally, the upper limit of the uncertainty accounts for the fact that OH diffusion to the particles may limit the observed rate of reaction of BES. Here, we have assumed a value of the diffusion coefficient of OH in one atmosphere of N 2 of 0.2 cm 2 /sec. and an OH uptake coefficient of g = 1. The measured rate of BES reaction, g BES = 2.0, is similar in magnitude to the range of values (g = 0.1 1) from the only other studies to have measured the rate of uptake of OH on organic surfaces [Bertram et al., 2001; Molina et al., 2004]. The efficient reaction of OH with BES is also consistent with many previous studies showing that OH reacts with large gasphase organics at nearly the collision rate [Atkinson, 1997]. 4. Discussion and Conclusions [16] The relative rates approach has been extended to the study of heterogeneous reactions between organic aerosols and O 3, Cl and OH. The ability of the Aerosol CIMS technique to measure the concentrations of gas-phase and particle species concurrently makes these experiments possible. The good agreement between the value of the uptake coefficient measured for ozonolysis of MO with this approach and the value measured previously using an absolute approach indicates that it is suitable for such quantitative measurements. As with any relative rates measurement, the accuracy of the calculated rate of reaction will depend on the accuracy of the reference rate constant. Also, the reference compound must be chosen such that its rate of reaction is comparable to that of the species to be measured. There are additional concerns, unique to this mixed-phase relative rates technique, as well. For example, the output of the particle source must be stable over the timescale of the experiment (minutes), otherwise the signal from the particle species could drift and create an artificially fast or slow observed rate of decay. Furthermore, the aerosol distribution must be monodisperse (geometric standard deviation < 1.2) and constant since the observed rate of reaction is proportional to the surface area-to-volume ratio of the particles. [17] The BES particles were observed to react very efficiently with both the Cl and OH radicals. The large values of the rate of reaction (g BES > 1) as measured from the BES loss indicate that additional reactions may be occurring in the particles after the initial reaction with the radical. For instance, the organic radical created, R, could react with Cl 2 (in the absence of O 2, as in these experiments) to regenerate a Cl radical (and create RCl) in a catalytic chain mechanism. Thus, the reaction of one Cl radical from the gas phase could initiate the removal of multiple BES molecules in the particle resulting in a value of g BES > 1. We have, in fact, observed evidence for such a mechanism from the identification of an organic chloride product, RCl, observed as [BES + Cl] + in the mass spectrum (not shown). Another possibility is that Cl 2 in or on the particles may photodissociate thereby increasing the Cl radical concentration in the particles. Experiments are underway to determine how significant this mechanism might be. [18] A radical chain mechanism may also be responsible for the large observed rate of BES reaction with OH (g BES > 1). After the OH radical abstracts an H atom from BES, it is believed that RO and RO 2 radicals are created, analogous to similar gas-phase reactions [Finlayson-Pitts and Pitts, 2000] in the presence of O 2 and O 3. We note that while no O 2 was added in these experiments, some O 2 was generated through the photodissociation of O 3. These RO and RO 2 radicals can then participate in a variety of decomposition, isomerization or reaction pathways [Eliason et al., 2003; Molina et al., 2004; Docherty and Ziemann, 2006] which can lead to the further functionalization of the BES molecules as well as the re-generation of radicals, thus propagating the radical chain. Alternatively, the chain could be terminated by RO 2 radical recombination in which alcohol and carbonyl (i.e. ketone or aldehyde) products are created [Ingold, 1969]. While the elucidation of such mechanisms is beyond the scope of the present work, recent studies indicate that they may be active in the radicalinitiated oxidation of organic surfaces [Eliason et al., 2004; Molina et al., 2004]. [19] It is also conceivable that some of the observed loss of BES could have resulted from reaction with HO 2 radicals created as a byproduct of OH generation. However, modeling of the gas-phase kinetics indicates that [HO 2 ] [OH], implying that the reactive uptake coefficient of HO 2 on the BES particles would have to be unity to explain the fast observed rate of BES loss. Given the very inefficient reaction of HO 2 with most organic species in the gas phase (e.g. k = cm 3 /molecule/sec. for HO 2 + propane 4of5

5 [Tsang, 1988]), it seems unlikely that it would react with unit probability when encountering a BES particle. [20] In the atmosphere, radical chain mechanisms would lead to the oxidation of organic particles more rapidly than indicated by just the rate of OH uptake since g BES > g. Consequently, the particles may become CCN (cloud condensation nuclei) active more quickly. Furthermore, trace species used as markers for specific aerosol sources, such as levoglucosan in wood smoke particles [Simoneit et al., 1999] and cholesterol in meat-cooking aerosol [Rogge et al., 1991], will have shorter atmospheric lifetimes than predicted based solely on the rate at which OH reacts with those molecules. In summary, the secondary radical reactions could play a significant role in determining the physical and chemical properties of ambient aerosol as well as influencing source apportionment studies. Using the mixed-phase relative rates technique developed here, we will continue to investigate radical-initiated reactions to quantify the contribution of this secondary chemistry to the oxidative processing of organic aerosol as well as to identify the effects of other trace gas species present in polluted environments, such as NO and NO 2. [21] Acknowledgment. This work was supported by the National Science Foundation (ATM ). References Atkinson, R. (1997), Gas-phase tropospheric chemistry of volatile organic compounds: 1. alkanes and alkenes, J. Phys. Chem. Ref. Data, 26, Atkinson, R., and S. M. Aschmann (1993), OH radical production from the gas-phase reactions of O 3 with a series of alkenes under atmospheric conditions, Environ. Sci. Technol., 27, Bertram, A. K., A. V. Ivanov, M. Hunter, L. T. Molina, and M. J. Molina (2001), The reaction probability of OH on organic surfaces of tropospheric interest, J. Phys. Chem. A, 105, D Anna, B., W. Andresen, Z. Gefen, and C. J. 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