Environ. Sci. Technol. 2007, 41, 2357-2363
Controlled OH Radical Production via Ozone-Alkene Reactions for Use in Aerosol Aging Studies ANDREW T. LAMBE, JIEYUAN ZHANG, AMY M. SAGE, AND NEIL M. DONAHUE* Center for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213
We present a novel method for continuous, stable OH radical production for use in smog chamber studies, especially those focused on organic aerosol aging. Our source produces OH radicals from the reaction of 2,3-dimethyl-2butene and ozone and is unique as a method that requires neither NOx nor UV photolysis of a radical precursor. Typical radical concentrations are in the range of (4-8) × 106 molec cm-3 and are easily sustainable over experimental time scales of several hours. We discuss design considerations, radical production capability under different operating conditions, and the core source chemistry. As a proof of concept we present preliminary results from oxidation of n-hexacosane aerosol observed with an Aerodyne Aerosol Mass Spectrometer. The extent of hexacosane oxidation is sufficient to significantly change the organic aerosol mass spectrum by virtue of fast heterogeneous uptake of OH radicals at the particle surface, with a calculated uptake coefficient γ ) 1.04 ( 0.21.
1. Introduction Hydroxyl radicals (OH) are the dominant oxidizing agent in the troposphere, but their role in oxidizing condensed-phase organics in atmospheric particles is uncertain. Based purely on the expected collision rate of OH with atmospheric particles, the lifetime of condensed-phase organics could be as short as a few days (1-3). However, the implications of such organic aerosol processing are poorly understood. Aging changes particle hygroscopicity, which may affect aerosol climate forcing behavior (4-7). It may complicate the use of condensed-phase organic markers used to track primary aerosol sources (1). Aged particles may also have different toxicity and health effects than their unaged precursors. These uncertain but potentially significant effects necessitate the research of aging with OH in a laboratory setting. Until now, heterogeneous chemistry involving aging with OH has been done almost exclusively in flow-tube reactors (3, 8-11). In these experiments, high radical concentrations (108-1010 molec cm-3) and short exposure times (e30 min) are used to oxidize films. Because average tropospheric OH concentrations are on the order of 106 molec cm-3 (12), a potential problem with this approach is error incurred in extrapolating short-term, high-intensity oxidation to longterm, low-intensity oxidation more representative of the atmosphere. This is a special challenge for very reactive compounds with uptake coefficients (γ) near unity, where experimental conditions often result in losses that are ratelimited by diffusion, either in the gas phase near the surface or in the condensed phase itself. To that end, smog-chamber * Corresponding author e-mail:
[email protected]. 10.1021/es061878e CCC: $37.00 Published on Web 02/21/2007
2007 American Chemical Society
studies with experimental time scales of several hours are a promising alternativesthey may facilitate a much more direct comparison of atmopsheric oxidation and laboratory experiments. There are established OH sources for smog-chamber studies, typically involving photolysis of a radical precursor (methyl nitrite, hydrogen peroxide, etc.). While perfectly suitable for some applications, these methods have some shortcomings for heterogeneous oxidation experimentssthey typically require some combination of hard UV, high NOx, or extensive radical cycling. To address these shortcomings, we have developed a novel OH source employing alkene ozonolysis that yields a high radical flux under low-NOx conditions without the use of UV light. The ozonolysis mechanism described by Criegee (13) has been studied for decades, including extensive experimental and theoretical work appropriate to the gas phase. The essential features are that ozone first adds across the double bond of an alkene to form a primary ozonide, which almost immediately decomposes into an aldehyde or ketone and a coproduct known as the Criegee Intermediate (CI). The chemistry of the CI is complex; however, unimolecular decomposition to form OH radicals is well documented (1420). We exploit this in the design of our OH source.
2. Design Considerations 2.1. Design Constraints. This source is designed to study heterogeneous aging by OH in various model aerosol systems, with experiment time scales that approach the oxidation time scales over which these processes occur (1-3). Changes in particle mass and/or number concentrations, chemical composition, and oxidant uptake rate are all of interest. Similar experiments have been performed in our laboratory using ozone (21) and elsewhere using NO3 radical (22) as the oxidizing agent. OH yields from ozonolysis depend strongly on the alkene; 2,3-dimethyl-2-butene (tetramethylethylene, TME) is the alkene of choice in our application because TME ozonolysis results in an OH yield near unity (14, 15, 17, 19, 20). Furthermore, acetone is the coproduct to the CI, and smaller oxygenated species including formaldehyde, carbon monoxide, and carbon dioxide from the organic radical fragment are produced along with OH radicals. These are all high vapor pressure compounds that will not participate directly in aerosol chemistry; the only potential sources of low vaporpressure compounds are the OH + TME reaction and decomposition of the CI followed by reaction with HO2 and RO2 radicals. While ozonolysis of other alkenes can also result in high OH yields, many of these species produce low vapor pressure products that partition appreciably into the condensed phase to form secondary organic aerosol (SOA). For our application, we wish to restrict the condensed-phase chemistry to the model species of interest, making this an undesirable side effect. However, this could be exploited as a feature for study of heterogeneous aging in the presence of condensation. The generalized reaction scheme under consideration is
O3 + TME f OH + products;
TME kO 3
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OH + target f products;
ktarget OH
where ‘target’ is the species or system whose oxidation by OH radical is our primary focus. There are three obvious issues with this source design. First, OH production will require high ozone concentrations, which could influence the heterogeneous chemistry. Second, the organic radical chemistry following OH production should be understood. Finally, OH reacts with TME at the collisional rate and, without careful design, that reaction will immediately consume most of the freshly generated OH. We shall discuss the first two issues below, but the third issue has the potential to fatally compromise this design. With relatively rapid source chemistry and experimental time scales of hours, we need to maintain steady-state TME and ozone levels by controlling fluxes (Θ) into the reaction chamber. We expect ozone and TME to react with unit stoichiometry (16). This therefore requires a reactant flux balance, with ΘTME ) ΘO3. At steady state
∂CO3 ∂t
TME CTMECO3 ) 0 ) ΘO3 - kO 3
∂CTME TME ) ΘTME - kO CTMECO3 ) 0 3 ∂t
9
3. Experimental Section We measure OH indirectly by monitoring the decay of two gas-phase tracer species that react with OH but not O3. This is equivalent to the competitive oxidation technique employed to indirectly measure OH yields from ozonolysis (15, 17). Here we use relative kinetics to verify that the decay of both tracer species is due to reaction with OH. When the log concentration of one tracer species is plotted versus the other, the slope should be equal to the ratio of their rate constants with OH. However, because of dilution, we also need a third, nonreactive, tracer. We used cyclopropane (99%, Aldrich) or acetonitrile (HPLC grade, Fisher). For acetonitrile, kOH ) 2.2 × 10-14 cm3 molec-1 s-1 (23), resulting in an oxidation lifetime much longer than experimental time scales. The dilutioncorrected relative rate equation is given by
(1) ∂ln(C1(t)/Cn(t)) (2)
However, we want to minimize the rate of TME + OH, so CTME needs to be small. This constitutes an inhomogeneous solution to eq 1, with a high initial ozone concentration skewing the steady-state conditions toward high ozone and low TME levels. This in turn ensures that ktarget OH Ctarget > kTME OH CTME. Therefore, a limitation of this source is that species that are reactive toward both OH and ozone cannot easily be studied. 2.2. Design Elements. 2.2.1. Capillary Flow System. TME (99+%, Aldrich) is delivered to a reaction chamber via capillary injection. The TME is held in a reservoir with a constant head pressure maintained in a 1-L stainless-steel cylinder immediately above the reservoir. At the base of the TME reservoir is a length of 50-µm i.d. glass capillary tubing, through which the TME is forced by the pressure gradient. By adjusting the head pressure from 10 to 25 psi on a 3 m length of capillary tubing, we can maintain a steady TME flow of between approximately 0.03 and 0.12 mL h-1, which is far lower and much more stable than flows achievable through traditional means. Changing the capillary length affects the range of possible flow rates. The downstream end of the capillary is held in a tee connecting to a carrier flow line, through which clean carrier gas flows at 4 L min-1. At these TME flows, the TME forms a steady-state droplet at the end of the capillary, with evaporation balancing the flow from the capillary into the droplet. The flow can be further reduced using a flow splitter. In some of our experiments, we used a rotameter to control the reactant stream and a needle valve to control the waste stream by balancing flows. There are a few of advantages to this design. First, a capillary in tandem with a flow splitter permits a customizable design and a wide range of flows. Second, the volumetric TME flow can be measured directly. We thus have accurate control of the flow, which can be altered by changing the pressure drop, capillary length, and flow ratios at the splitter. More detailed discussion of the capillary flow system as well as a schematic diagram can be found in the Supporting Information. 2.2.2. Ozone Generation. Ozone is generated by passing a corona discharge through O2 or by vacuum-UV photolysis of an O2 stream using an unshielded mercury pen-ray lamp (UVP*, 11SC-1 with power supply PS-1). The lamp is housed in a 1/2′′ ultratorr tee, and it is held in place with a pair of 2358
silicone o-rings. O2 is passed into the connector adjacent to the lamp; ozone feeds out of the third connection into the reaction chamber. The ozone and TME streams remain separate until inside the chamber, which is necessary because of the short lifetime of OH radicals.
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∂ln(C2(t)/Cn(t))
)
k1 - kn k1 = k2 - kn k2
(3)
To calculate COH, we perform a separate pseudo-first-order analysis on each tracer species, assuming that COH is roughly constant over the analysis interval. Performing this analysis on both tracer species serves as additional verification that we are in fact producing OH radicals at the desired level. From their reaction rate expressions, the slope of this line is log-linear and equal to the loss rate of the tracer species. Corrected for dilution, this equation is given by
∂ln(Ci(t)/Cn(t)) ) kOHCOH ∂t
(4)
and dividing both sides of this equation by kOH yields COH. Data reported in this paper were taken from a number of relative kinetics experiments conducted in 2- and 10-m3 Teflon chambers (Welch Fluorocarbon), at T ) 22-25 °C and p ) 1 atm air. Experiments typically lasted 3-4 h. Prior to measurement, the bag was flushed for several hours with clean air supply (using a HEPA filter to eliminate particles), and background samples were taken to ensure bag cleanliness. Tracer species were then introduced to the chamber using a septum injector. The initial ozone concentration was established using a corona-discharge ozone generator (Azco HTU500AC). Production of OH radicals was initiated by simultaneous capillary injection of TME and flow of ozone into the chamber at volumetric fluxes that were matched according to prior calibrations. We monitored TME and tracer concentrations using gas chromatography with flame ionization detection (GC-FID, Perkin-Elmer AutoSystem XL; J&W Scientific DB-624 capillary column, 30 m × 0.530 mm coupled to a preconcentrator (Entech 7100A)) and/or proton-transfer reaction mass spectrometry (PTR-MS, Ionicon Analytik). The inlet and drift tube temperatures in our PTR-MS are maintained at 80 °C to minimize condensation, and the drift voltage is set at 500 V to minimize fragmentation at the higher Tdrift while maintaining relatively low water cluster concentrations. Ozone levels were monitored using a Dasibi ozone monitor (1008-PC). Only proton-transfer reactions with a reaction enthalpy greater than 165.2 kJ mol-1 are thermodynamically favorable in the PTR-MS (24), so we selected our tracers accordingly. We used combinations of alcohols, alkynes, and furan.
better signal-to-noise achieved from using higher ‘target’ concentrations is offset by the increased difficulty in sustaining higher oxidant levels. For example, where we used a small amount of one tracer, we achieved OH radical levels about 50% greater than experiments performed under similar operating conditions with higher concentrations of two tracers, but this comes at the expense of having a less certain estimate of COH. The relative importance of better performance versus better characterization is situationally dependent, and so it is difficult to devise a generally optimal strategy. The specific chemistry of different tracer species can affect the OH source performance. When both ethanol and butanol were used as tracers, COH was 2-3 times higher than with other tracers under similar operating conditions. With our current understanding of the source chemistry, we cannot quantitatively explain these results; it is likely due to complicated HOx chemistry. FIGURE 1. OH radical concentrations achieved from operating at different ozone levels with different tracer combinations during gas-phase relative rate experiments at ΘTME ∼ 109 molec cm-3 s-1. Error bars show 1 σ precision. Data from experiments 10 and 11 as listed in Table 1 in the Supporting Information are omitted from the figure to isolate known mixing issues in those experiments. The observed trend is toward higher OH radical concentrations at higher ozone concentrations, as indicated by the limiting lines. However, this trend is less pronounced at lower ΘTME and is influenced by specific tracer levels and chemistry in addition to ozone levels. For example, COH is much higher when two alcohol tracers are used. In aging experiments, we obtained particle number and size information with a TSI 3080 Scanning Mobility Particle Size (SMPS) operated with sheath and aerosol flows of 2.5 and 1.0 lpm. We monitored condensed-phase chemsitry with an Aerodyne electron-impact quadrupole Aerosol Mass Spectrometer (AMS). The temperature of the AMS vaporizer was maintained at T ) 600 °C.
4. Results and Discussion We tested our OH source over a range of conditions with a number of tracer species, in order to assess the optimal operating parameters. The initial hypothesis used to constrain the operating phase space was that increasing ozone levels would result in higher OH concentrations. The production rate of OH radicals, POH, is unchanged by increasing CO3 for a given Θ because POH ) yOH × kCTMECO3 ) (ΘTME ) ΘO3), where yOH is the OH yield. However, increasing CO3 lowers CTME, which effectively lowers the loss rate of OH radicals, LOH, and thus potentially increases steady-state OH levels because CTME × CO3 remains constant. The general trend observed in Figure 1 supports this hypothesis, although this trend is relatively modest, and direct comparison from one experiment to the next is complicated by use of different tracer concentrations in different experiments. The obvious complication is HOx (OH + HO2) cycling, and loss is not directly constrained. Using ΘTME ∼ 109 molec cm-3 s-1, our OH source can consistently produce radicals in the range of (4-8) × 106 molec cm-3 at high ozone concentrations and in some cases greater than 107 molec cm-3. At these fluxes, increasing ozone levels much beyond 2-3 ppm has a negligible effect on COH, as seen in Figure 1. These observations receive further scrutiny in the Supporting Information, where we develop a diagnostic model of the source chemistry. In these experiments, tracer concentrations are high enough that the OH oxidation lifetimes with respect to these tracers are competitive with other radical sinks inherent in the core OH source chemistry. This shows that our oxidation efficiency is high. However, it also shows that judicious choice of the level of the ‘target’ species is necessary. Potentially
Chamber volume can have a substantial effect on our radical production capability. This is because chamber mixing time, which is typically a few minutes, is similar to, if not greater than, the TME oxidation lifetime with respect to ozone under typical operating conditions. For example, at 1 ppm ozone, the TME lifetime is about 35 s. Thus, all other factors being equal, a longer mixing time will hinder our ability to uniformly and efficiently oxidize the contents of our reaction chamber. As shown by Table 1 in the Supporting Information, comparison of measurements taken in our 2- and 10-m3 reaction chambers highlight the effect of increased mixing time on the performance of our OH source. In experiments where a single TME injection site was used with our 10-m3 reaction chamber (experiments 10 and 11), OH levels were only 50-60% of those at similar ozone levels in our small chamber. When we used a second injection site at the opposite end of the chamber (experiments 12-14), this enabled us to reach OH levels that were comparable with those obtained in our small chamber under otherwise similar operating conditions. Because time scales for aging experiments are on the order of hours, whereas mixing time scales within the chamber are on the order of minutes, OH source mixing is not a confounding factor in our calculated OH concentrations; the only appreciable effect is slightly lower performance. We anticipate that taking further steps to decrease chamber mixing time, such as additional injection points, will improve the performance of our OH source. Applications. We shall now identify regimes where our design is feasible for aging model aerosol systems in smog chamber studies. Our objective is to find experimental conditions where we can generate good signal-to-noise for OH-induced particle oxidation while at the same time staying as close as possible to ambient conditions. Good signal-tonoise requires that the total oxidized organic mass be large enough to measure and that a reasonable fraction of the molecules in each particle be oxidized. As a first-order metric we shall use the oxidation time scale of individual particles, defined as the total number of molecules in a particle, nOA, divided by the oxidant flux across the surface of the particle, ΦOH in molec s-1.
τOA )
nOA ΦOH
(5)
With a polydisperse aerosol suspension of particles, it is more useful to consider the volume averages of these values -COA is the aerosol molecule concentration in molecules cm-3, and ΘOH is the flux across all surface area in a volume of gas. The flux of OH molecules in a volume is VOL. 41, NO. 7, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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ΘOH )
∂COH 1 ) γcjAsCOH ∂t 4
(6)
where γ is the uptake coefficient of the oxidant by the aerosol, jc is the mean molecular speed of OH molecules in the gas phase, and As is the aerosol surface area per unit volume. The mean speed is
jc )
x
8RT πMOH
(7)
The surface area per unit volume is
As )
6COA Fp D hp
(8)
where COA is the mass concentration of particles (the conventional measure), while the number concentration of molecules with molecular weight MOA in that volume is
COA )
NACOA MOA
(9)
The oxidation lifetime is thus
τOA )
COA 2 NAFpD hp ) ΘOH 3 MOAγcjCOH
(10)
Provided that the production rate of OH is sufficient to maintain COH in the presence of a large particle concentration (POH . ΘOH), we find an intuitive set of constraints. We need a high oxidant concentration and relatively small particles to minimize the oxidation time scale. We expect aerosol surfaces to have OH uptake coefficients of γ ) 0.1-1 (9). Figure 2 encapsulates the desired operating regime for our smog chamber aging experiments and indicates that oxidation lifetimes approaching experimental time scales can be achieved with COH ∼ 107 molec cm-3 and sufficiently small particles. OH Uptake on Hexacosane. As a test of this source, we introduced particles generated by vaporizing n-hexacosane (C26H54) into our 10 m3 chamber. The initial suspended particle concentration was COA ) 1000 µg m-3, with a surfacearea weighted mean particle diameter of 340 nm. We calculated an average COH ) (6.3 ( 0.5) × 106 molec cm-3 during this experiment. In Figure 3a we show the initial hexacosane aerosol mass spectrum, with individual fragment peaks plotted as a fraction of the total organic signal. The fragmentation pattern is a classic signature of an n-alkane, with a ‘picket fence’ of (CH3(CH2)n) alkyl peaks separated by 14 amu (the mass of CH2). Figure 3b shows the AMS spectrum after nearly 5 h exposure to OH, while Figure 3c shows the residual AMS spectrum derived from subtracting a scaled initial spectrum from the final spectrum. The initial spectrum was scaled by the ratio of the m/z ) 71 fragments in the two spectra, which ensured a non-negativity constraint in the residual. Figure 3a-c reveals a significant change in composition caused by exposure of the hexacosane aerosol to our OH source. The most obvious feature is a sharp change in the ratio of the m/z ) 57 signal to the m/z ) 43 signal, clearly visible in the linear insets for Figure 3a,b as a roughly 20% decrease in the relative abundance of m/z ) 57 over the course of the experiment. The effect is unambiguously tied to the OH sourcesa blank experiment with 4.5 ppm of ozone but no TME did reveal small changes in the mass spectrum over time, but they were at most 20% of those observed here. The difference spectrum in Figure 3c is a first estimate of the reaction product spectrum, as the maximum plausible 2360
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FIGURE 2. τOA as a function of OH concentration and mean particle diameter. Gp ) 1 g cm-3, γ ) 1, c¯ ) 609 m s-1, MOA ) 300 g mol-1. Color bar indicates the corresponding aerosol oxidation lifetime, τOA, in hours. hexacosane contribution to the final spectrum has been removed. There are three interesting features: First, there is a significant shift toward smaller fragments, suggesting the addition of functional groups along the carbon chain that results in different fragmentation patterns and/or some reduction in carbon number in the reaction products. Second, the spectrum is considerably more filled in, which is consistent with more complicated fragmentation of an array of reaction products. Finally, there are significant signs of oxidation, with large increases at peaks 1 amu heavier than the alkyl radical ‘picket fence’. One of these peaks is at m/z ) 44, which is commonly used in the AMS community as a sign of oxidation (25, 26). However, the effect is more pronounced on a fractional basis at very high m/z, >240 amu, where a succession of peaks one mass higher than the dark green peaks are significantly larger than their alkyl counterparts. These features could be consistent with aldehyde (CH3(CH2)nCHO) formation. To further confirm that the changes in the mass spectrum are in fact caused by heterogeneous uptake of OH, in Figure 4 we show a semilog plot of the key masses seen in Figure 3, normalized by the total organic signal. The kink shortly after 2:00 p.m. was caused by an increase in TME flow, which in turn increased the OH concentration by 80% (as measured by tracer decay). By plotting the log ratio of the mass fraction markers of key alkyl fragments normalized by their initial mass fractions, we can calculate the first-order loss rate and take the inverse to obtain the oxidation lifetime. We can then use eq 10 to calculate γ. Performing this analysis separately for each of the markers shown in Figure 4, in both phases I and II, we calculate an average γOH ) 1.04 ( 0.21 for hexacosane assuming FP ) 0.8 g cm-3. This value is consistent with what we would expect for efficient uptake of OH radicals to the particle surface. Approximately 20% of the hexacosane was oxidized during the experiment, and yet the decays appear to remain pseudofirst-order. The OH radicals should have a very small reactodiffusive length, and so the reactions should occur at or near the particle surface. The continuing first-order decay suggests that hexacosane is not being selectively depleted at the surface. This is one advantage of long time scale experiments such as this onesthere is ample time for diffusion in the particle to equilibrate any radial concentration gradients. However, it also suggests that hexacosane is not dramatically less surface active than its reaction products. We have ruled out significant heterogeneous oxidation by ozone, but a few other systematic effects may bias this result in either direction. (1) Significant unaccounted-for evaporation (EOA) resulting from scission along the hexacosane carbon backbone, as has been proposed elsewhere for model alkane surfaces (3), would bias γ low. (2) Condensation of aerosol mass (COA) (presumably SOA
FIGURE 3. AMS spectra for aging of hexacosane (m ) 366 amu) by OH. Inset figures show linear spectra up to m/z ) 125 amu. In all figures, ‘picket fence’ features due to alkyl chain fragments (CH3(CH2)n) are shown in dark green, starting with n ) 2 (propyl, m/z)43), while m/z ) 44 (CO2+ or CH3 CHO+, due to oxidation) is shown in pink. (a) Initial AMS mass spectra for n-hexacosane. (b) Final mass spectra after aging by OH. (c) Residual mass spectra obtained from subtracting the spectrum in (a) from (b), scaled by the ratio of the m/z ) 71 fragment (pentyl) in the final spectrum from the initial spectrum to maintain a positive signal at all masses.
formation from the TME oxidation) would bias γ high. (3) Some of the reaction products almost certainly produce fragments at the three selected masses, and this source term (i.e., P57,P71,P85) has not been accounted for; this would bias γ low. (4) Secondary removal of hexacosane could lead to an overall stoichiometry greater than 1; this would bias γOH high. As an example, γOH is proportional to first-order loss rate of the m/z ) 71 fragment, k71, which is
appear as growth or shrinkage of the particles, evident as a shift in the size distribution. During this experiment the particles grew by at most 30%. Furthermore, it is unlikely that OH oxidation of hexacosane will trigger secondary reactions in the condensed phase. This would require alkyl radicals to react with hexacosane rather than the abundant oxygen in our chamber; we assume that peroxy radical formation dominates. Overall, we estimate a lower-bound estimate for OH uptake of γOH,low > 0.7.
∂ln(f71/f71(0)) ) (P - L)71 - f71(C - E)OA (11) ∂t
To the extent that γ < 1 is an appropriate constraint, secondary production of fragments at m/z 57, 71, and 85 is minor. Our best estimate is thus 0.7 eγOH e1 for pure hexacosane. This is a significant improvement over calculated
k71 )
Any difference in condensation and evaporation would
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FIGURE 4. Relative evolution of key organic mass spectral markers during aging with OH radical. Total COA drops by about a factor of 3, but features are plotted as a fraction of the total organic signal in a given spectrum. During phases I and II, COH ) (4.4 ( 0.3) × 106 and (7.0 ( 0.1) × 106 molec cm-3, respectively. Log ratio of m/z ) 57, 71, and 85 mass fractions, markers for hydrocarbon-like organic aerosol (25, 26), are normalized by their initial mass fractions to cancel wall loss effects (a constant offset is applied to m/z ) 57 and 85 traces to remove overlap). The slope of the fractional decay, k, is the inverse of the oxidation lifetime, from which γOH can be calculated using eq 10. Labels correspond specifically to m/z ) 71. uptake coefficients for highly reactive organics from coated wall flow tube studies, which can typically only constrain γOH g 0.2 because of diffusion limitations (3, 8). The results are thus consistent with progressive oxidation of hexacosane, leading to condensed-phase products that produce both oxygen-contaning fragments such as aldehydes and also small alkyl ions. To the extent that these results can be extended to all atmospheric particles with abundant organic content at the particle surface, they suggest that heterogeneous oxidation of organic particles by OH radicals may indeed be highly efficient, causing substantial loss of supposedly stable molecular markers and also transforming aerosol properties such as hygroscopicity and the organic volatility distribution.
5. Future Work This paper presents preliminary results employing a new OH production method for the study of organic aerosol aging. We anticipate using the method described here in a quantitative assessment of condensed-phase oxidation kinetics in the future. In particular, studies of aerosols dominated by saturated hydrocarbons and thus susceptible to OH (and not ozone) attackssuch as paraffins, steranes, motor oil, and diesel exhaustsare of interest in order to better understand oxidation during atmospheric transport. All radical sources have drawbacks. The disadvantages of this source are that a large ozone concentration is required and also that the TME chemistry must be kept in mind. For example, the RO2 arising from both TME Criegee Intermediate decomposition and from any OH + TME reactions certainly engage in gas-phase chemistry. It is possible that some multifunctional reaction productsssuch as products of RO2 - RO2 and HO2 - RO2 reactions following OH addition to TMEs might have low enough vapor pressures to influence either SOA experiments or heterogeneous oxidation experiments, depending on the organic aerosol levels in the experiments and the resulting phase partitioning (27). In future work, we intend to experiment with different alkenes to verify that changes in condensed-phase composition are independent of source chemistry. The advantages of this source are that it produces a very high flux of OH radicals at low NOx, that it can be used in the dark without any complex precursor synthesis, and that 2362
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the OH levels can be maintained indefinitely. These factors make it a useful tool for well-controlled, longer-duration smog-chamber studies.
Acknowledgments This work is supported by grant R832162 from the EPA STAR program through the National Center for Environmental Research. The PTR-MS and AMS used in this work were obtained by grant ATM-0420842 through the National Science Foundation.
Supporting Information Available Capillary flow system, modeling source chemistry and operating conditions, gas-phase OH tracer experimental conditions, and operating conditions and tracer concentrations (Table 1). This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review August 4, 2006. Revised manuscript received November 14, 2006. Accepted January 19, 2007. ES061878E
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