Ion Mobility Distributions during the Initial Stages of New Particle

Nov 9, 2010 - In this study ion mobility spectrometer (IMS) is used to monitor and analyze gas phase changes during the formation of secondary organic...
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Environ. Sci. Technol. 2010, 44, 8917–8923

Ion Mobility Distributions during the Initial Stages of New Particle Formation by the Ozonolysis of r-Pinene A N N A - K A I S A V I I T A N E N , * ,† ERKKA SAUKKO,† ANNELE VIRTANEN,† ¨ , ‡ J A M E S N . S M I T H , §,|,⊥ PASI YLI-PIRILA JORMA JOUTSENSAARI,§ AND ¨ KELA ¨† JYRKI M. MA Aerosol Physics Laboratory, Department of Physics, Tampere University of Technology, P.O. Box 692, FI-33101 Tampere Finland, Department of Environmental Science, University of Eastern Finland, P.O. Box 1627, FI-70211, Kuopio, Finland, Department of Physics and Mathematics, University of Eastern Finland, P.O. Box 1627, FI-70211, Kuopio, Finland, Finnish Meteorological Institute, Kuopio Unit, P.O. Box 1627, FI-70211 Kuopio, Finland, and Atmospheric Chemistry Division, National Center for Atmospheric Research, P.O. Box 3000, Boulder, Colorado 80307, United States

Received May 10, 2010. Revised manuscript received September 9, 2010. Accepted October 26, 2010.

An ion mobility spectrometer (IMS) was used to study gas phase compounds during nucleation and growth of secondary organic aerosols (SOA). In this study SOA particles were generated by oxidizing R-pinene with O3 and OH in a 6 m3 reaction chamber. Positive ion peaks with reduced mobilities of 1.59 cm2(Vs)-1 and 1.05 cm2(Vs)-1 were observed 7 min after R-pinene and ozone were added to the chamber. The detected compounds can be associated with low volatility oxidation products of R-pinene, which have been suggested to participate in new particle formation. This is the first time that IMS has been applied to laboratory studies of SOA formation. IMS was found suitable for continuous online monitoring of the SOA formation process, allowing for highly sensitive detection of gas phase species that are thought to initiate new particle formation.

Introduction Secondary organic aerosols (SOA), consisting of a large variety of organic compounds with low vapor pressures, are formed by reactions of biogenic and anthropogenic volatile organic compounds (VOCs) with atmospheric oxidants such as hydroxyl (OH) radicals, nitrate (NO3) radicals, and ozone (O3) (1). Recent estimates of the importance of SOA formation show that it may be as significant as primary organic aerosol emissions: about 60-70% of the organic aerosol mass is SOA on the global scale, while regionally SOA may be even more * Corresponding author phone: +358 3 3115 2133; fax: +358 3 3115 2600; e-mail: [email protected]. † Aerosol Physics Laboratory, Tampere University of Technology. ‡ Department of Environmental Science, University of Eastern Finland. § Department of Physics and Mathematics, University of Eastern Finland. | Finnish Meteorological Institute. ⊥ National Center for Atmospheric Research. 10.1021/es101572u

 2010 American Chemical Society

Published on Web 11/09/2010

important (2, 3). Hence, formation of SOA in the atmosphere has a significant impact on the climate and human health. Monoterpenes and isoprene are globally important biogenic species whose oxidation products are low volatility compounds that play a key role in SOA formation (4, 5). Monoterpenes (e.g., R-pinene, β-pinene, ∆3-carene, and limonene) account for about 11% of all global biogenic VOC emissions to the atmosphere (6). For example, VOC emissions from Scots pine (one of the most common species in the boreal forests) consist of up to 90% monoterpenes (7). Globally the most abundant monoterpene is R-pinene, comprising about 25% of the total terpene emissions (3), and thus it is widely used as a model compound in biogenic SOA formation studies. Both nucleation and growth of fresh particles are assumed to occur by a phase transition of ambient condensable vapors; the importance of the gas phase is clear. Nucleation of single component vapor occurs when its saturation ratio in gas phase reaches a threshold. However, mixtures of distinct vapors can nucleate in much lower concentrations than what is needed for single component systems (8). In addition, the growth of the newly formed particles due to vapor mixtures may occur well below saturation conditions of any single component involved. Quite often condensing sulfuric acid (9), amine compounds (10), and various volatile organic compounds (e.g. ref 11) are observed to drive the particle growth. Recent measurements by Laaksonen et al. (5) indicated that during new particle formation events in the Finnish boreal forest particle growth is dominated by VOC oxidation products. Studies of gas phase compounds before and during the new particle formation and growth may offer new information related to compounds involved. Laboratory studies of the gas phase may also help to identify the essential compounds responsible for atmospheric new particle formation. For such laboratory studies, high-volume, Teflon film reaction chambers are commonly employed. In the gas phase, VOCs can be studied and measured using both off-line and online methods. These methods were reviewed in ref 12, where e.g. PTR-MS was highlighted as a sensitive real-time measurement technique. The main disadvantages of PTR-MS technique are considered to be the inability to distinguish structural isomers as well as the complexity of recorded spectra. Another common measurement technique, gas chromatography - mass spectrometry (GC-MS), overcomes some of the disadvantages of PTR-MS by providing the ability to distinguish structural isomers; its primary disadvantages include long integration times and relatively lower sensitivity compared to PTR-MS. Both techniques have difficulties sampling species with very low volatility such as those species believed responsible for SOA formation. Thus, there is a need for complementary measurement techniques. Here, we report the use of an ion mobility spectrometer (IMS) to study the gas phase composition of reaction products that are aerosol precursors formed from the oxidation of R-pinene. Ion mobility spectrometry is based on electrical mobility of ions. In this method, the gas phase sample is artificially ionized and gated into a drift tube, where ions will drift to a collector plate under influence of an electric field. Detailed information about ion mobility spectrometry can be found from, e.g. ref 13. Even though the IMS is a simple measurement technique, there are some benefits compared to more sophisticated instruments. From the point of view of atmospheric sciences, one of the main benefits of the IMS technique is the real time measurement response, which provides continuous online observations of changes in the VOL. 44, NO. 23, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Measurement setup during the experiments. gas phase. It is known that different structural isomers, e.g. some monoterpenes, exist in the atmosphere (e.g. ref 14). Since the IMS technique is based on electrical mobility and not on mass analysis, chemical specificity is possible for such species (e.g. ref 15). Since analyte compounds are ionized via primary ionization of carrier gas molecules, fragmentation is limited, and therefore the ion distribution can be relatively simple. While as a mobility analyzer the IMS has high sensitivity and specificity, it is incapable of exact mass analysis. These aspects favor the use of IMS as a complementary technique for laboratory as well as ambient atmospheric observations. In fact, ion mobility spectrometry has already been used for nucleation studies of sulfur containing compounds (16) as well as for studies of the formation of multimers (17). Here the IMS is used to study SOA formation in parallel with aerosol measurement techniques. In our experiments, SOA formation was caused by oxidizing R-pinene with O3 in a reaction chamber. The aims of the study are to test whether the IMS can be used to observe nucleation and to gain new insights into the species responsible for nucleation and the subsequent growth of the stable clusters into particles.

Experimental Section The measurement setup is presented in Figure 1. The reaction chamber used in this study was a 6 m3 chamber made from Teflon FEP film, which was supported by a steel frame. The chamber is described in ref 18. The chamber was situated indoors, and the temperature and lighting of the system were well controlled. Prior to each experiment, the chamber was flushed overnight with laboratory compressed clean air. The air was processed by active charcoal, PurafilTM select, and absolute HEPA filter to remove non-methane hydrocarbons, NOx, and particles. The relative humidity of the compressed air was 2-4%. OH from the ozonolysis of R-pinene was not suppressed by the addition of an “OH scrubber,” thus the species reported here are produced from the combination of ozone and OH chemistry. The main test compound, R-pinene, was purchased from Fluka, and its purity was >97% (GC). The exact proton affinity (PA) of R-pinene is not available, but it has been estimated to be g209 kcal/mol (g875 kJ/mol) (19). This is relatively high compared to PA of water, 691 kJ/mol (20). Based on this, R-pinene is detectable with IMS. Prior the chamber experiments, the chamber air was first humidified by bubbling it through distilled water and introduced into the chamber to adjust the chamber RH to 30% before every experiment. The temperature and RH were measured continuously by a RH and temperature transmitter (model HMP50; Vaisala, Inc.). Next, the R-pinene was injected into the reaction chamber from a heated gas bottle. For VOC off-line analysis, the VOCs were sampled onto Tenax TA absorbent for 15-30 min with a flow rate of 0.23 L/min. The collected VOCs were analyzed with GC-MS. The analysis methods used are described in detail elsewhere (21, 22). After the R-pinene injection, ozone enriched air (700 ppb, 40 L/min) generated by a UV lamp generator was introduced into the chamber for typically 8-10 min. The ozone enriched air was mixed with the chamber air until the desired concentration in the chamber was reached. The ozone 8918

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concentration was monitored at the inlet and inside the chamber by two O3 analyzers (model 1008-RS; Dasibi Environmental Corp.). Finally, the chamber was closed so that the adjusted conditions (RH, T) remained stable throughout each experiment. The IMS was used to investigate the gas phase chemistry throughout the experiments. In addition, the IMS also measured the background ion distribution of the pressurized air before every experiment. In this way it was possible to distinguish changes as a result of the gas phase chemistry of the chamber from background contamination of the pressurized air, chamber walls, and humidification system. The IMS used in this study was a commercial time-of-flight instrument (model RAID 1; Bruker Daltonics GmbH). The instrument has been slightly modified by removing the membrane to allow for direct gas sampling. This modification resulted in some loss of sensitivity due to interaction of water vapor with the sampled gases. Gases were sampled at 0.4 L min-1 into the ion source, which consisted of a 63Ni source of an activity of 15 mCi. This configuration created an approximate ion-molecule reaction time of 0.2 ms in the ion source. The ions were gated into the drift region with a pulse width of 0.3 ms, where they were then exposed to a constant drift field of 254 Vcm-1. The effective drift tube length of RAID 1 has been determined using standard reference compound 2,6-di-tert-butylpyridine resulting in 5.764 cm (23). The maximum drift time of the device after the gate is 30 ms, corresponding a mobility of 0.8 cm2(Vs)-1. The instrument operates at atmospheric pressure and was operated at room temperature without internal heating, resulting in a drift chamber temperature of around 30 °C. A Faraday cup detector and a sensitive electrometer recorded the current of ions that traverse the length of the drift cell. The RAID 1 data system recorded the time-of-flight of ions and their current in every 20 s. The sample rate was 58.3 µs, and for one individual spectrum an accumulation of four was used. The measured time-of-flight was converted to the mobility and reduced to normal conditions according to K0 )

d 273K P Et T 101.3kPA

(1)

where K0 is reduced mobility, d is the effective drift tube length, E is the electrical field, t is the time-of-flight, T is the temperature, and P is the pressure (13). The data were converted to current versus electric mobility and normalized with respect to the operating temperature and pressure. For each mobility value the quantity of dQ/dK was calculated by which the charge (dQ, in Coulombs) on the graph would correspond with the actual ion concentration. The exact mobility was determined with a peak detection routine when it was feasible and manually when required. As the algorithm was only able to distinguish the peaks correctly only with high peak currents, the peak area data were obtained with a simple summation of the data from a constant, suitable width on the mobility axis around the determined peak mobility. In Figure 2 the ion distribution of R-pinene is presented. The concentration was not controlled but increased until the reaction ions were lost and detectable responses for both monomer and dimer were achieved. The concentration can be considered to be significantly higher than the one used in the chamber experiments and these data are presented here as a demonstration of typical spectra obtained with this instrument. As Figure 2 shows, three ion mobility peaks were measured for R-pinene and identified based on literature (15, 24) as the charged monomer, an additional product ion, and the dimer, with reduced mobilities 1.69, 1.59, and 1.20 cm2(Vs)-1, respectively. The particle size distributions were measured by a scanning mobility particle sizer (SMPS). The SMPS consisted

FIGURE 2. The positive ion distribution from the calibration measurement of r-pinene at high concentration (solid line). The background spectrum shows reaction ion peak (RIP) and some impurity (dashed line). When the r-pinene was introduced to the system, three different peaks were observed (a-c) and identified as (a) monomer, (b) additional product ion, and (c) dimer of r-pinene.

TABLE 1. Summary of the Experiments Presented in This Study experiment number

purpose

r-pinene concentrationa

O3 concentrationa

RH

Figures

Exp. 1 Exp. 2 Exp. 3

reaction chamber experiment, the effect of ozone reaction chamber experiment, oxidation of R-pinene reaction chamber experiment, oxidation of R-pinene

45 ppb 31 ppb

38 ppb 43 ppb 86 ppb

30% 30% 30%

3 4 and 5 not shown

a

The peak concentration after the injection.

of Condensation Particle Counter (model 3022A; TSI, Inc.) and Differential Mobility Analyzer (model 3081; TSI, Inc.) with a measurement range from 10-700 nm. The total particle volume concentration was calculated from the SMPS number concentration data by fitting a log-normal particle size distribution and calculating the volume concentration from the fitted distribution. This was done in order to improve accuracy by eliminating noise caused from single particle events during the scan in large particle sizes, which have a large effect on total particle volume. The clocks of every device involved were synchronized before each experiment.

Results Table 1 summarizes the different experiments performed in this study. In Exp. 1 the reaction chamber was introduced with 38 ppb of ozone (no R-pinene). The purpose of this experiment was to study the background effect of ozone into the ion mobility distribution to be able to compare it to the R-pinene oxidation experiments (Exps. 2 and 3). The averaged ion mobility distributions for both positive and negative polarities during Exp. 1 are shown in Figure 3a-d. For the positive ions, the background distribution (Figure 3a) shows a main peak with mobility of 2.1 cm2(Vs)-1 formed from the positive reactant ions (RIP), consisting mainly of H+(H2O)n and H3O+(H2O)n (13, 25). The peak at reduced mobility of around 2.3 cm2(Vs)-1 most likely indicates NH4+(H2O)n (25). In addition minor impurities are present, with the most intense one being at a reduced mobility of ∼1.4 cm2(Vs)-1. The negative ion background distribution (Figure 3c) consists mainly of negative reactant ions (peak around 2.2 cm2(Vs)-1), most likely O2- (H2O)n (13) with some impurity peaks. After

the ozone addition no significant changes were observed in the positive ion distribution (Figure 3b) compared to background distribution (Figure 3a). For the negative polarity, clear peaks were detected at reduced mobilities 2.3 cm2(Vs)-1 and 1.8 cm2(Vs)-1 approximately 13 min after the ozone addition was started. Also some broadening around the peak 1.3 cm2(Vs)-1 was observed (Figure 3d). Thus, these peaks in the negative ion distribution are considered to be caused by ozone chemistry. During Exp. 1, new particle formation was not observed, nor were there any low mobility ion peaks that may be indicators of nucleation. The results of Exp. 2 are shown in Figures 4a-f and 5a-c. For these experiments, 45 ppb of R-pinene was introduced into the reaction chamber (at -20 min, shown by the vertical line in Figure 5). Ozone addition started 20 min after that (time zero, the second vertical line in Figure 5). A peak O3 concentration of 43 ppb was reached in 10 min. In the positive ion distribution during Exp. 2, R-pinene addition did not change the background ion mobility distribution (Figure 4a-b). This indicates that either the concentration was too low for the device to be able to measure the same peaks as earlier in R-pinene calibration measurement or the concentration was too low to allow dimer formation. After the ozone addition clear changes in the positive ion mobility distribution were observed (Figure 4c and 5b). Two new peaks in the positive ion distribution with reduced mobilities of 1.59 cm2(Vs)-1 and 1.05 cm2(Vs)-1 can be separated from the background ion mobility distribution. Thus, the detected positive ion mobility peaks are considered to be formed as a result of R-pinene oxidation. VOL. 44, NO. 23, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. The averaged spectra of positive (a-b) and negative (c-d) ion distributions during Exp. 1 (only O3 at 38 ppb). In the positive ion distributions the main background peaks are the reaction ion peak (RIP, 2.1 cm2(Vs)-1), ammonium (NH4+(H20)n, 2.3 cm2(Vs)-1), and some impurities with the most intense peak around 1.4 cm2(Vs)-1. In the positive polarity, compared to the background ion distribution (a), no significant changes were observed after ozone addition (b). In the negative ion distributions the background spectrum (c) shows the reaction ion peak (RIN, 2.2 cm2(Vs)-1) and some impurities. After the ozone addition peaks with reduced mobilities 2.3 cm2(Vs)-1 (peak1-) and 1.8 cm2(Vs)-1 (peak2-) were measured as well as some broadening of the peak around 1.3 cm2(Vs)-1 (peak3-) (d). The shading guides the eye to follow the significant mobilities discussed in this study. The spectra were averaged so that the background spectrum is the average of 118 individual spectra; the ozone spectrum is the average of 96 individual spectra. Figure 5a-b plots the time evolution of the negative and positive ion distributions. In Figure 5c the time behavior of the peaks 1.59 cm2(Vs)-1 and 1.05 cm2(Vs)-1 are compared to the particle volume concentration. As it can be seen from Figure 5 the peaks 1.59 cm2(Vs)-1 and 1.05 cm2(Vs)-1 appeared approximately 7 min after the ozone addition began (time zero in Figure 5). During Exp. 2 the formation of new particles (mobility diameter >10 nm) was observed approximately 20 min after ozone addition (Figure 5c). The particle size and the total volume kept growing throughout the measurement period of Exp. 2. In the negative ion spectra (Figure 4d) the RIN forms the main background peak with a reduced mobility of ∼2.2 cm2(Vs)-1. Also note that the R-pinene addition alone does not change the ion distribution significantly (Figure 4e). After ozone addition we observe two new peaks with reduced ion mobilities 2.3 cm2(Vs)-1 and 1.8 cm2(Vs)-1 as well as some broadening of the peak 1.3 cm2(Vs)-1 (Figure 4f), which were also measured in Exp. 1 (pure O3 experiment, ref Figure 3d). Based on the correspondence of these peaks, we conclude that these species are caused by ozone reactions with trace impurities and are not associated with R-pinene oxidation products or to new particle formation. In Exp. 3, an R-pinene concentration of 31 ppb and ozone concentration of 86 ppb was used. We have not plotted these results but will summarize these observations as follows. In the positive ion distributions the peaks 1.59 cm2(Vs)-1 and 1.05 cm2(Vs)-1 were again detected after the ozone addition and confirm the observations of Exp. 2. During this experiment the background distribution before the R-pinene injection was not recorded, which prevents us from isolating the effect of ozone addition alone. The negative ion mobility 8920

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FIGURE 4. The averaged spectra of positive (a-c) and negative (d-f) ion distributions during the Exp. 2 (45 ppb r-pinene, 43 ppb O3). In the positive ion distribution the main background peak with reduced mobility 2.1 cm2(Vs)-1 is the reaction ion peak (RIP). There is some ammonium present showing a peak around 2.4 cm2(Vs)-1 as well as other impurities with the most intense peak around 1.4 cm2(Vs)-1. The r-pinene addition does not dramatically change the positive ion distribution (b). After ozone addition the peaks with reduced mobilities 1.59 cm2(Vs)-1 (peak1+) and 1.05 cm2(Vs)-1 (peak2+) are detectable in the positive ion distributions (c). In the negative polarity the chamber background shows again the reaction ion peak (RIN, 2.2 cm2(Vs)-1) and some impurities (d). r-Pinene addition does not have significant influence on the negative ion distribution (e) while after the ozone addition a similar distribution was measured as in Exp. 1 (see Figure 3d, peaks 1-3) (f). The shading guides the eye to follow the significant mobilities discussed in this study. The spectra were averaged depending on measurement length, so that the background spectrum consists of 13 individual spectra, the r-pinene addition of 30 individual spectra, and the ozone addition of 60 individual spectra from the end of each measurement period. distributions for Exp. 3 were similar to those measured during Exp. 1 and 2. During Exp. 3 new particle formation was observed.

Discussion Secondary organic aerosol formation and growth were studied in a reaction chamber by oxidizing R-pinene with O3. During the oxidation experiment, the gas phase components were measured by ion mobility spectrometry simultaneously with particle size distribution measurements using SMPS. From the positive ion distribution two R-pinene oxidation related ion mobility peaks, with reduced mobilities of 1.59 cm2(Vs)-1 and 1.05 cm2(Vs)-1, were observed. These changes in gas phase components were observed 7 min after mixing of ozone and R-pinene but 13 min before observation of new particles (diameter >10 nm), indicating that IMS can be used to study gas-phase reactions and compounds at the initial stage of new particle formation. Our background measurements show that R-pinene (Figure 2) and ozone (Figure 3) by themselves do not produce these ions; the measured ion mobility peaks can only be associated with the oxidation of R-pinene. Since the R-pinene concentration in the chamber experiments was kept relatively low, formation of R-pinene dimers is not expected, nor does the measured peak with reduced mobility 1.05 cm2(Vs)-1 cor-

of (355 ( 71) amu indicating to some large monomer or most likely to a dimer formed by some compound alone or by different compounds (heterodimer). The mobility diameter of an ion can also be estimated from the reduced mobility according to (30) K0 ) 2.2458 × 10-22·Dp-1.9956

FIGURE 5. a-c. (a) Negative ion distribution; (b) positive ion distribution, and (c) particle volume concentration (particle >10 nm in diameter) plotted with selected ion mobility peak areas during the Experiment 2 (45 ppb r-pinene and 43 ppb ozone). The vertical lines at -20 min and 0 min mark the r-pinene and ozone injections, respectively. respond to the mobility of the R-pinene dimer. In addition, these peaks do not correspond with those reported in the literature for R-pinene (15, 24, 26). Borsdorf et al. (15, 24) postulated that R-pinene forms a monomer and a dimer in the positive polarity with reduced mobilities around 1.7 cm2(Vs)-1 and 1.2 cm2(Vs)-1, respectively. In those studies also the peak with reduced mobility 1.59-1.60 cm2(Vs)-1 was measured. These results agree with the R-pinene calibration measurement shown in Figure 2. The ion mobility peak 1.06 cm2(Vs)-1 has been previously measured and reported only with IMS based on UV-ionization in humid conditions (RH 100%), but it was not analyzed further (26). In the negative ion distributions, R-pinene oxidation related changes were not observed. The changes seen in the negative ion distribution, two ion mobility peaks 1.8 cm2(Vs)-1 and 2.3 cm2(Vs)-1 as well as the broadening around the peak 1.3 cm2(Vs)-1, were connected only to the ozone addition and are thus not believed to derive from R-pinene oxidation. Without exact mass spectrometer measurements or compound specific calibration the exact identification of the measured peaks cannot be made. While the mass-to-charge (m/z) ratio cannot be measured directly by IMS, nevertheless it is possible to estimate m/z according to ion mobility. Several different conversion formulas from mobility to m/z have been proposed (e.g. refs 13, 27, and 28). One commonly used conversion between mass m and reduced mobility K0 for mass area 35.5-2122 amu is the Kilpatrick-fit (27) K0 ) exp[-0.0347(ln(m))2 - 0.0376ln(m) - 1.4662] (2) As a rule of thumb, bigger molecules have lower mobility. Ion structure also has an influence in inferring m/z from mobility; however, this is not considered in eq 2. Based on Kilpatrick’s relation and 20% accuracy recommended in ref 29 for the m/z to mobility conversion, the m/z corresponding to the peak 1.59 cm2(Vs)-1 is (129 ( 26) amu, which indicates an oxidized compound with a structure similar to that of R-pinene. The peak at 1.05 cm2(Vs)-1 corresponds to an ion

(3)

where Dp is the diameter in meters, and K0 is the reduced mobility in m2(Vs)-1. Using eq 3, the corresponding diameters for the peaks 1.59 cm2(Vs)-1 and 1.05 cm2 V s-1 are roughly 1.1 and 1.4 nm, respectively. The diameter of the assumed oxidation product dimer, 1.4 nm, is in the range of generally assumed smallest stable particle size in the atmosphere (31). Several R-pinene ozonolysis products have been measured and identified (e.g. refs 3, 14, and 32). Thus, there are many candidates for the peaks measured here. To further complicate matters, the measured mobility peak may contain several different compounds with equal or near-equal mobilities. Many R-pinene oxidation products are reported to have a tendency to form dimers, and different oxidation products can also attach to each other forming heterodimers (33, 34). The peak at 1.05 cm2(Vs)-1 is likely associated with one of these high molecular weight, low volatility species. The mechanism for the formation of this low mobility species was previously thought to be particle-phase reactions (35); we have observed that these compounds also exist in the gas phase. The formation of these oxidation product peaks has a similar time evolution to the volume of newly formed particles. The volume concentration of particles increased during the experiment, indicating that the oxidation products were moved to the particle phase continuously. Thus, the compounds forming the ion mobility peaks are coincident with, and might have influence on, new particle formation and growth during this experiment. More studies are planned to find out the structure and implication of these products, especially of the low mobility assumed dimer, to new particle formation. We have presented observations of the appearance of oxidation products of R-pinene and their connection with particle formation and growth. Here, the R-pinene was used as a model compound for monoterpenes. In the boreal forest the total monoterpene concentration has been reported to be roughly in the range of 50-400 ppt, and the R-pinene concentration was in the range of 48-109 ppt depending on season (36). Here, the R-pinene concentration was 45 ppb, which is higher than that observed in the boreal forest. In chamber studies the R-pinene concentration varies typically from a few ppb to a few hundred ppb (32, 37-40), which is consistent with the concentrations used in this study. The atmospheric ozone concentration at ground level is in the range of 10-100 ppb (14), which corresponds the concentration used in this study. Based on the current study it seems plausible that the IMS used here may not have the required sensitivity for direct atmospheric sampling, where R-pinene concentration may be lower by as much as an order of magnitude. However, it may be sufficient for measuring oxidation products, as discussed below. For example experimental conditions may have an influence on the sensitivity of the IMS and will be the focus of future studies. For example, the formation of pinonaldehyde, one of the R-pinene oxidation products, is influenced by the humidity so that humid conditions favor the pinonaldehyde yield (41). In addition, other gas phase compounds could compete for ionization. Thus, in some cases we might be able to measure oxidation products with initial R-pinene concentrations other than those reported here. In addition, the sample losses through the inlet are likely and difficult to completely eliminate owing to the very low vapor pressures expected for some of these compounds. VOL. 44, NO. 23, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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These issues should be addressed in order to obtain quantitative information of the concentration of the observed species; however, none of these are suspected to invalidate the measured mobilities that we report. Our observations show that the ion mobility spectrometer is a useful instrument for monitoring and analyzing the gas phase in parallel with aerosol measurement devices. The IMS may provide additional time-resolved information of the compounds involved in new particle formation, in situ, and in real time. Future planned experiments include coupling our IMS instrument to a mass spectrometer, which will provide more specific information about compounds we observed.

(11)

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Acknowledgments A.-K.V. acknowledges the Ministry of Education and the Academy of Finland (project No. 118780) and Finnish Foundation for Technology Promotion for funding. J.S. acknowledges the financial support of the Saastamoinen Foundation. J.J and P.Y.-P. acknowledge the support by the Academy of Finland (decision no. 110763, 131019). The National Center for Atmospheric Research is sponsored by the U.S. National Science Foundation.

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