Evidence for the Existence of Organosulfates from β-Pinene

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Environ. Sci. Technol. 2007, 41, 6678-6683

Evidence for the Existence of Organosulfates from β-Pinene Ozonolysis in Ambient Secondary Organic Aerosol ¨LLER,† Y O S H I T E R U I I N U M A , † C O N N Y MU ¨GE,† T O R S T E N B E R N D T , † O L A F BO MAGDA CLAEYS,‡ AND H A R T M U T H E R R M A N N * ,† Leibniz-Institut fu ¨ r Tropospha¨renforschung (IfT), Permoserstrasse 15, D-04318 Leipzig, Germany, and Department of Pharmaceutical Sciences, University of Antwerp (Campus Drie Eiken), Universiteitsplein 1, BE-2610 Antwerp, Belgium

The formation of organosulfates from the gas-phase ozonolysis of β-pinene in the presence of neutral or acidic sulfate particles was investigated in a series of indoor aerosol chamber experiments. The organosulfates were analyzed using high-performance liquid chromatography (LC) coupled to electrospray ionization-time-of-flight mass spectrometry (MS) in parallel to ion trap MS. Organosulfates were only found in secondary organic aerosol from β-pinene ozonolysis in the presence of acidic sulfate seed particles. One of the detected organosulfates also occurred in ambient aerosol samples that were collected at a forest site in northeastern Bavaria, Germany. β-Pinene oxide, an oxidation product in β-pinene/O3 and β-pinene/ NO3 reactions, is identified as a possible precursor for the β-pinene-derived organosulfate. Furthermore, several nitroxy-organosulfates originating from monoterpenes were found in the ambient samples. These nitroxy-organosulfates were only detected in the nighttime samples, suggesting a role for nighttime chemistry in their formation. Their LC/ MS chromatographic peak intensities suggest that they represent an important fraction of the organic mass in ambient aerosols, especially at night.

Introduction The emissions of biogenic volatile organic compounds (BVOCs) have been estimated to be a significant contributor to the total VOC budget in the atmosphere (1). Monoterpenes are among the most important class of BVOCs as they react rapidly with atmospheric oxidants such as O3, OH, and NO3 to form low-volatile organic compounds. These compounds can form secondary organic aerosol (SOA) by either nucleation and/or condensation. Recent chamber studies on BVOC oxidation have shown that particle-phase reactions can also play a significant role in SOA formation as monoterpene oxidation products appear to oligomerize readily in the particle phase under both acidic and nonacidic conditions (2-10). Although dimers and oligomers are abundant and ubiquitous in laboratory-formed * Corresponding author phone: +49-341-235-2446; fax: +49-341235-2325; e-mail: [email protected]. † Leibniz-Institut fu ¨ r Tropospha¨renforschung. ‡ University of Antwerp. 6678

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monoterpene oxidation SOA, the atmospheric importance of such dimers and oligomers is still unclear as no atmospheric evidence of such compounds has been found so far. Recent studies have reported organosulfates from oxidation of aromatic compounds, isoprene, and monoterpenes in the presence of acidic sulfate seed particles (11-14). The proposed structures of the organosulfates indicate that the terminal sulfate group can react further leading to organosulfate dimers containing a sulfate bridge with MW values of >400. Additionally, the atmospheric relevance of organosulfate formation from R-pinene and isoprene SOA has been supported by their detection in ambient aerosol collected in the southeastern U.S. (14, 15). Although this suggests that organosulfates from monoterpene and isoprene SOA are relevant to ambient aerosol, their concentrations and ubiquity in ambient atmospheric aerosols are not yet clear. In this study, we present the formation of organosulfates in monoterpene oxidation products by a series of aerosol chamber studies of β-pinene ozonolysis under acidic and nonacidic conditions. The importance of monoterpenederived organosulfates is supported by field evidence from samples collected in a Norway spruce forest in northeastern Bavaria, Germany, during the BEWA campaign (Regional biogenic emissions of reactive volatile organic compounds from forests: Process studies, modeling and validation experiments (16)). The detection and structural elucidation of organosulfates is carried out using one- or two-dimensional highperformance liquid chromatography (1D/2D-HPLC) simultaneously coupled to electrospray ionization-time-of-flight mass spectrometry (ESI-TOFMS) and electrospray ionization-ion trap mass spectrometry (ESI-ITMSn) for accurate mass measurements and fragmentation experiments, respectively.

Experimental Procedures Chamber Experiments. Table 1 summarizes the experimental conditions of R- and β-pinene ozonolysis experiments. A 9.1 m3 indoor chamber with a surface to volume ratio (S/V) of 3 m-1 was used for the β-pinene ozonolysis experiments in the presence of sulfuric acid seed particles (strongly acidic). A newly built indoor aerosol chamber with a volume of 17.7 m3 (S/V ) 2.1 m-1) was used for the β-pinene ozonolysis experiments in the presence of sodium sulfate (neutral) and ammonium sulfate/sulfuric acid (acidic) seed particles and for R-pinene ozonolysis experiments with ammonium sulfate/ sulfuric acid seed particles. The same instrumental setup and experimental procedure were followed for both chamber experiments. No OH scavenger was used in this study. A detailed chamber operational procedure can be found in the Supporting Information. BEWA Field Samples. A detailed description of the BEWA campaign can be found elsewhere (16). The samples analyzed in this study were taken at the forest ecosystem research site of the Bayreuth Institute for Terrestrial Ecosystem Research (BITO ¨ K), which is located in the Fichtelgebirge mountain range in northeastern Bavaria, Germany, and is dominated by Norway spruce. The sampling was carried out at two heights (12 and 24 m) of a 30 m tower located in the middle of the forest (776 m above sea level, 50°08′32′′ N and 11°52′03′′ E). Aerosol samples were collected on prebaked quartz fiber filters (MK 360, Munktell, Falun, Sweden) using a DHA-80 high-volume sampler with an automatic filter changer and a PM2.5 preseparator (Digitel Elektronik AG, Hegnau, Switzerland). The samplers were programmed to collect two sets of samples (daytime, 10 a.m. to 6 p.m., and nighttime, 10.1021/es070938t CCC: $37.00

 2007 American Chemical Society Published on Web 09/05/2007

TABLE 1. Initial Experimental Conditions Used in r-Pinene and β-Pinene Ozonolysis Experiments seed particle

hydrocarbon

date

concn of atomizing solns (M)

β-pinene β-pinene β-pinene β-pinene β-pinene β-pinene R-pinene R-pinene

01/06/06 02/06/06 12/10/06 13/10/06 16/10/06 17/10/06 13/07/06 18/09/06

0.1 0.1 0.06 0.06 0.03/0.05 0.03/0.05 0.03/0.05 0.03/0.05

initial mixing ratio

seed type

hydrocarbon (ppbV)

ozone (ppbV)

R.H. (% )

T (°C)

H2SO4 H2SO4 Na2SO4 Na2SO4 (NH4)2SO4/H2SO4 (NH4)2SO4/H2SO4 (NH4)2SO4/H2SO4 (NH4)2SO4/H2SO4

300 300 300 300 300 300 100 100

98 101 98 99 96 98 72 60

52 53 36 44 37 36 36 36

22 23 24 22 24 23 24 23

10 p.m. to 6 a.m.) over three consecutive days (total 24 h) at 500 L min-1 to obtain enough aerosol mass for organic speciation. The filter samples were kept in a freezer at -22 °C until analysis. Two sampling periods (19-21 July 2002 and 28-30 July 2002) were chosen for the analysis of organosulfates as these days were selected as “golden days” based on other available data sets and as sections of these samples were already analyzed in detail for OC/EC, inorganic ions, and monoterpene oxidation tracers (17). Analytical Methods. Sample Preparation. One-half of the Teflon filter from the chamber experiments was cut into small pieces using ceramic scissors and extracted in 1 mL of methanol under ultrasonication for 10 min. The resulting extract was filtered through a 0.2 µm Teflon syringe filter (Acrodisc Pall, NY) to remove insoluble materials and was blown to dryness under a gentle stream of N2 at 10 °C. The residue was reconstituted in 250 µL of a acetonitrile/water solution (80/20, v/v) for HPLC/ESI-TOFMS analysis. Approximately 5% of the quartz fiber filters from the BEWA campaign were used for extraction. The BEWA samples were extracted and reconstituted in the same fashion as the Teflon filters for HPLC/ESI-TOFMS analysis. The extract solutions were stored in a freezer (-22 °C) and repeatedly analyzed over months. No evidence for hydrolysis of organosulfates was found. 1D/2D-HPLC/ESI-TOFMS and ESI-ITMSn. 1D/2D-HPLC/ ESI-TOFMS (one- or two-dimensional high-performance liquid chromatography simultaneously coupled to electrospray ionization-time-of-flight mass spectrometry) and ESIITMSn (electrospray ionization-ion trap tandem mass spectrometry) were used to characterize organosulfates from both the chamber and BEWA samples. A schematic diagram of the setup and detailed separation conditions are given in Supporting Information (Figure 1S). Accurate mass measurements were performed using a TOF mass spectrometer (micrOTOF, Bruker Daltonics, Bremen, Germany) at a mass resolution of 10000 fwhm. Tandem mass spectrometry experiments were carried out using a Bruker Esquire 3000 plus ion trap mass spectrometer (Bruker Daltonics). Both mass spectrometers were operated in the negative ionization mode; hence, all compounds reported here are detected as their deprotonated molecules [M - H]-. The 1D separation was used to obtain the MW distribution of the analytes, tentative quantification of organosulfates, and cleanup for 2D separation. While the 1D separation was not aimed at baseline separation of all peaks, it was still useful for accurate mass determination of target compounds. Selected 1D fractions were further analyzed using a 2D column. This strategy ensures that dimers, oligomers, and organosulfates detected in 1D chromatograms are not caused by artifact formation in the ion source. A monomeric organosulfate detected in β-pinene ozonolysis and BEWA samples was

filter

init. seed particle volume concn [µm3cm-3]

particle volume concn at 2.5 h [µm3 cm-3]

Teflon Teflon Teflon Teflon Teflon Teflon Teflon Teflon

25 35 15 17 19 19 23 32

110 125 123 139 128 127 121 169

tentatively quantified using a standard of (1S)-(+)-camphor10-sulfonic acid (99% purity, Sigma-Aldrich). We have chosen this compound as a surrogate because it has a bicyclic skeleton with a terminal sulfonate group that is comparable to a β-pinene-derived organosulfate. However, no quantification was made for nitroxy-organosulfates from the BEWA samples. The regression coefficient of a calibration curve was better than 0.999 (quadratic fitting, six points, 3.6144 mg L-1). Mass calibration in TOFMS was performed using a sodium formate solution. The concentrations for organosulfates given in this study are only estimations based on the use of a surrogate compound which shows similar chromatographic and mass spectrometric behavior as the target analyte. Additionally, ion suppression in the ion source might have caused underestimation of organosulfates, especially for the BEWA samples. Therefore, their concentrations might contain large uncertainties. In addition to the 1D C8 separation, 1D C18 separation was also carried out for selected samples. Better separation obtained from the 1D C18 column was essential for tandem mass spectrometry experiments for compounds with isobaric isomers. The abbreviations 1DC8 HPLC, 1DC18 HPLC, and 2DC18 HPLC will be used throughout this work to differentiate between the two different columns employed for the 1D and 2D separation.

Results and Discussion Detection of Organosulfates from Chamber-Produced β-Pinene Ozonolysis SOA. Figure 2S (Supporting Information) shows base peak chromatograms (BPCs) and average mass spectra (3.0-5.0 min and 5.0-9.0 min) from the analyses of β-pinene ozonolysis SOA for three different seed particle acidities using the 1DC8 HPLC/ESI-TOFMS setup. No difference was found between the mass spectra from the three different acidities in the range where monomers eluted (3.05.0 min). Although high-MW compounds were always present under all three conditions, some signals above m/z 200 were only found under acidic and strongly acidic conditions. Additionally, intense signals indicative of higher MW compounds (m/z > 400) were found under both acidic conditions (i.e., m/z 435). The m/z 357 compound was the most abundant dimer under all three conditions; TOF MS and MS2 ion trap spectra are given in Figure 3S (Supporting Information). It is noted that an m/z 357 compound has also been observed in R-pinene ozonolysis SOA by Docherty et al. (4) and has been attributed to a peroxycarboxylic acid dimer in which the two monomeric units are connected via a peroxy bridge. Among the compounds detected under both acidic and strongly acidic conditions, an m/z 249 compound resulted in the second most intense peak after the m/z 357 compound. Accurate mass and the MS2 ion trap data for the m/z 249 ion are illustrated in Figure 4S (Supporting InforVOL. 41, NO. 19, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Proposed reaction pathways for the formation of m/z 249 and m/z 263 compounds in β-pinene ozonolysis SOA. Three organosulfate precursors from a σ-complex pathway are shown in a dashed box. Two possible m/z 249 organosulfate [M - H]- structures are shown in a solid box. mation). The m/z 249 MS2 product ion spectrum shows an abundant m/z 97 ion (HSO4-), indicating the presence of a sulfate group. The possibility of an ion source artifact was ruled out since this compound eluted much later than the sulfate peak (1.4-2.3 min; m/z 97 and 195) and coeluting compounds that could produce ion source adducts were not found. Moreover, the m/z 249 compound was detected under both acidic (HPLC) and basic (CE) separation conditions. The presence of a sulfate group was further supported by the isotopic pattern obtained with high-resolution MS showing characteristic features of an organic compound containing a sulfur atom, i.e., the difference between the second and third isotopic peaks is less than 0.001 Da and their intensity ratio is approximately 2 (Figure 4S, middle). The [M - H]ion formula, C10H17O5S-, was inferred from the m/z 249 accurate mass data, and its theoretical mass and isotopic distribution agreed very well with measured data (approximately 1.1-1.2 ppm error). On the basis of these results, the m/z 249 compound is identified as an organosulfate. While the above results show evidence of a C10 organosulfate in β-pinene ozonolysis SOA, a commonly accepted ozonolysis mechanism does not explain the formation of C10 organosulfate precursors for exocyclic β-pinene. After cycloaddition of ozone to the exocyclic double bond in β-pinene, a primary ozonide is believed to decompose into a pair of formaldehyde (C1) and C9 alkyl peroxide which then leads to a number of C9 oxidation products such as nopinone and pinic acid (Figure 1, pathway A). However, the presence of a C10 organosulfate compound indicates that there is an 6680

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alternative reaction pathway which produces C10 precursor compounds. Indeed, C10 β-pinene oxidation products in both the gas and particle phase are reported in the literature (18). The formation of such C10 precursors can be explained by a σ-complex in which ozone forms an oxygen carbon σ-bond at one of the carbon atoms of the carbon-carbon double bond (Figure 1, pathway B). The σ-complex decomposes further to form oxirane, aldehyde, and alcohol during the ozonolysis of sterically unhindered alkenes (19). In order to obtain evidence for such C10 precursors, the gas-phase product study of β-pinene ozonolysis was conducted in a flow tube (see Supporting Information for the experimental conditions). A GC/MS chromatogram from the flow tube experiment shows a peak corresponding to oxirane (β-pinene oxide) at 12.67 min revealing a mass spectrum identical to that of the reference β-pinene oxide (Figure 5S, Supporting Information). The reaction of hydroxyl radicals with β-pinene can also lead to a C10 precursor (Figure 1, pathway C), though no C10-diol was found in this study or reported in the literature. Obtained yields of β-pinene oxide were approximately 0.05 regardless of reacted β-pinene (Figure 6S, Supporting Information). This corresponds to the formation of approximately 25 µg m-3 β-pinene oxide based on the amount of β-pinene consumed by ozone. Roughly 2% conversion of β-pinene oxide equals to a tentatively estimated concentration of the m/z 249 organosulfate in this study (∼0.6 µg m-3, Table 1S, Supporting Information). Reaction of pinanediol, formed by hydrolysis of β-pinene oxide, with sulfuric acid is expected to sulfate the terminal hydroxyl group

which is sterically less hindered. This is also consistent with the formation of the m/z 97 (HSO4-) product ion instead of m/z 96 (SO4- radical); the latter product ion is expected in the case of sulfation at the C(2) position for which a stable tertiary neutral radical can be eliminated during fragmentation in the ion trap. The formation of an m/z 249 organosulfate is also possible from myrtanal. However, this pathway is less likely than the β-pinene oxide pathway because the yield of myrtanal is much lower than that of β-pinene oxide. The much smaller peak of an m/z 263 compound (Figure 7S, Supporting Information) compared to that of the m/z 249 compound is possibly due to much lower yields of myrtenol. It is noted that β-pinene oxide was also detected as a product from the β-pinene/NO3 reaction (20). Interestingly, an organosulfate [M - H]- signal at m/z 249 detected by Surratt et al. (14) from the photooxidation of R-pinene in the presence of SO2 was not observed in R-pinene ozonolysis SOA in the presence of acidic seed particles (Figure 8S, Supporting Information). A possible explanation for the different behaviors of R- and β-pinene is that ozonolysis of R-pinene is more likely to follow a ring-opening primary ozonide pathway than the ring-retaining oxirane formation pathway owing to the strained structure of R-pinene primary ozonide. However, the reaction of R-pinene and NO3 is known to produce the corresponding oxirane (R-pinene-2,3-oxide) which can further react with acidic sulfate to form organosulfates. The reported yields of the R-pinene oxide for the R-pinene/NO3 system are 0.021 (21), 0.03 (22), and 0.035 (23). Even if these values are somewhat lower than the β-pinene oxide yield determined in this study, R-pinene can be an important source of organosulfates as the estimated global emission of R-pinene is higher than that of β-pinene (24). Indeed, evidence of ring-retaining organosulfates in R-pinene SOA under high-NOx and SO2 conditions has been reported for chamber-produced SOA as well as for ambient samples (14). An m/z 435 compound, which was only detected under acidic and strongly acidic conditions, was further analyzed using 1DC8 HPLC/TOFMS and ITMSn. Figure 9S (Supporting Information) shows the m/z 435 extracted ion chromatogram (EIC), TOFMS data, ion trap MS2 and MS3 product ion spectra for the m/z 435 compound found under acidic conditions, and a possible structure for the [M - H]- ion. Note that the proposed [M - H]- ion structure contains pinic and norpinonic acid parts, which is reasonable since both monomers are known β-pinene oxidation products. The TOF accurate mass data allow us to infer an [M - H]- ion formula of C19H31O9S- (Figure 9S, second top). The organosulfate structure is further confirmed by the m/z 97 (HSO4-) product ion formed in the MS3 m/z 435 f m/z 265 experiment (Figure 9S, fourth top). The m/z 435 MS2 product ion spectrum shows m/z 265; the neutral loss of 170 u is consistent with a norpinonic acid part, while lack of an m/z 97 ion in the m/z 435 MS2 spectrum suggests that the sulfate group is not terminal and points to an internal sulfate bridge. In this respect, it is noted that large organosulfates were also detected in limonene ozonolysis SOA in the presence of acidic sulfate seed particles (11). No quantification attempt was made for these higher MW organosulfates owing to lack of suitable standard or surrogate compounds. Nonetheless, their intensities imply that they could contribute significantly to monoterpene oxidation SOA in the presence of acidic seed particles. Atmospheric Evidence. Organosulfate from β-Pinene Ozonolysis (m/z 249) in BEWA Samples. Figure 2 shows the comparison of 1DC8 total ion chromatograms (TICs), 2DC18 EICs, TOF mass spectra, and m/z 249 MS2 ion trap data for (A) β-pinene ozonolysis SOA under the acidic condition and (B) the BEWA13 daytime sample (28-30 July 2002, 24 m). On the basis of the extremely good agreement between the retention times, TOF MS data, and the MS2 ion trap data, the

m/z 249 compound found in the BEWA sample was attributed to the β-pinene-derived organosulfate. This compound was also detected in other BEWA samples (Figure 10S, Supporting Information). Much higher intensities of the m/z 249 peaks were observed in samples collected during a period with higher temperatures (BEWA12-15, 28-30 July 2002), possibly corresponding to a higher monoterpene emission. Additionally, the 28-30 July 2002 samples were characterized by easterly winds from eastern central Europe with high photochemical activity, whereas the 19-21 July 2002 samples were characterized by westerly winds with relatively low photochemical activity (25). These meteorological conditions might have also influenced the formation and concentration of the organosulfate precursor compounds. The particle phase concentrations of the m/z 249 compound are estimated to be in the range of 1-9 ng m-3, which are similar to pinic acid concentrations determined in our earlier study (17). No diel cycle or height dependency was found for the m/z 249 compound. The above results prove that organosulfates can be formed not only under chamber conditions (high VOC and seed particle concentrations) but also under ambient conditions. Other Organosulfates in BEWA Samples. Organosulfates with m/z 294, 342, and 373. Previously reported organosulfates originating from the photooxidation of monoterpene with [M - H]- ions at m/z 294 (14, 15) and m/z 342 (15) were also found in the BEWA samples. Furthermore, an m/z 373 organosulfate was also detected in the BEWA samples. Figure 3 shows TOFMS m/z 294 EICs. Figure 11S shows m/z 342 and 373 EICs, respectively. The [M - H]- ion formulas derived from the TOF accurate mass data were C10H16NO7S-, C10H16NO10S-, and C10H16N2O11S-, for m/z 294, 342, and 373, respectively. These ion formulas confirm that the [M - H]ions contain a monoterpene (C10H16) part as well as nitrogen and sulfur atoms. All three nitroxy-organosulfates were detected only in the nighttime samples, suggesting that their formation involves nighttime chemistry. There was no evidence of photolysis as traces of corresponding organosulfates without nitroxy groups were not found in the daytime samples. Furthermore, the m/z 342 organosulfate was only detected in the samples taken at 12 m height. The chromatographic peak intensities observed in LC/ESI-MS suggest that the m/z 294 compounds are important contributors to the organic fraction of PM at night, since they reached about 40 times larger peak areas than the β-pinene-derived m/z 249 compound or pinic acid detected at m/z 185. No clear relationship was found between the gas-phase NOx (25), particle-phase nitrate, sulfate (17), and nitroxy-organosulfate concentrations for the BEWA samples. The exact formation mechanisms for these nitroxy-organosulfates are not clear at present though nitroxy terpeneol and oxiranes formed in the reaction of monoterpenes and NO3 most likely serve as precursors. Further study is also needed to explain the diel cycle of these compounds. Two organosulfates with higher [M - H]- ion intensities (m/z 294 and m/z 373) were further analyzed using ion trap MS. Figure 12S (Supporting Information) compares the TOFMS and ion trap MSn data for the five m/z 294 compounds detected in the BEWA14 sample. All the m/z 294 ions are isobaric (C10H16NO7S-) though their fragmentation patterns show distinct differences. A neutral loss of 63 u (HNO3) resulting in an m/z 231 product ion is only observed for peaks 3, 4, and 5, while a neutral loss of 47 u (HNO2) resulting in m/z 247 is characteristic of peaks 1, 2, and 4. The HSO4ion (m/z 97) is detected for peak 2, while the SO4- radical ion (m/z 96), indicative of a sulfate group at a branched position, is observed for peaks 1, 3, 4, and 5. This suggests that the m/z 294 compounds can originate not only from R-pinene as proposed by Surratt et al. (14) but also from other monoterpenes such as, for example, limonene. However, the VOL. 41, NO. 19, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. 1DC8 HPLC/ESI-TOFMS TIC, 2DC18 HPLC/ESI-TOFMS m/z 249 EIC, and TOF mass spectra and ITMS2 spectra for the m/z 249 compound from (A) β-pinene ozonolysis SOA under the acidic condition and (B) BEWA13 daytime sample between 28 and 30 July 2002. A gray area in the 1DC8 TIC chromatogram shows the 1D fraction that was applied to the 2D column.

FIGURE 3. 1DC8 HPLC/ESI-TOFMS m/z 294 (top, C10H16NO7S-) EICs obtained for BEWA samples. corresponding precursor monoterpene for each peak could not be deduced based on the fragmentation patterns solely. Therefore, only possible nitroxy-organosulfate structures are shown in Figure 12S. Figure 13S (Supporting Information) shows 1DC8 and 2DC18 m/z 373 EICs, TOF MS and ion trap MSn spectra, and plausible structures for the m/z 373 ion detected in the BEWA14 sample. The [M - H]- ion formula (C10H16N2O11S-) indicates two nitrate groups; the presence of a nitrate group is evident from the m/z 373 MS2 product ion spectrum which shows a neutral loss of 63 u (HNO3), while the presence of a second nitrate group is supported by the m/z 373 f m/z 310 MS3 product ion spectrum which reveals an additional neutral loss of 63 u. Both the MS2 and the MS3 spectra show a HSO4ion, suggesting that the sulfate group is not at a branched 6682

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position. The presence of a sulfate and two nitrate groups in this compound points to a precursor monoterpene that likely has two double bonds (e.g., limonene), as a monoterpene containing one double bond cannot accommodate all three functional groups at the same double bond position. Further work is required to fully elucidate the chemical structure of this compound. Atmospheric Implications. In this study, we provide evidence of a β-pinene-derived organosulfate in both the β-pinene ozonolysis SOA under acidic conditions and ambient aerosols sampled at a forest site in northeastern Bavaria, Germany. The concentration of the β-pinene-derived organosulfate (m/z 249) was estimated to be as high as that of known monoterpene oxidation tracer compounds such as pinic acid. Furthermore, nitroxy-organosulfates originating

from monoterpene SOA were detected in the ambient samples. Their very high abundances in the nighttime samples suggest an important role of nighttime chemistry in their formation. The MWs of the nitroxy-organosulfates can be as high as those of dimers previously detected in monoterpene oxidation experiments under acidic conditions (∼400). These compounds are amphiphilic (i.e., both hydrophilic and hydrophobic) and may play an important role in aerosol microphysics. The results obtained for the chamber SOA and the BEWA field samples suggest that organosulfates from the oxidation of monoterpenes could possibly explain a part of the missing organic mass in ambient aerosols, especially at night. More studies are needed in order to quantify the particle-phase concentrations of both organosulfates and nitroxy-organosulfates and to elucidate their chemical structures and gain insights into their formation pathways.

Acknowledgments This study was supported by Deutsche Forschungsgemeinschaft (DFG) under grant number HE-3086/4-1 and by the Bundesministerium fu ¨ r Bildung und Forschung (BMBF) through the BEWA project as part of the Atmospheric Research Program 2000 under grant number 07ATF25. Research at the University of Antwerp was supported by the Belgian Federal Science Policy Office and the Research Foundation-Flanders (FWO).

Supporting Information Available Details on the chamber operation, flow tube experiments, 2D HPLC setup, HPLC separation conditions, β-pinene oxide yields, CE/ESI-ITMS results, and 1D/2D-HPLC/ESI-TOFMS and ESI-ITMSn results for chamber and ambient samples including chromatograms and mass spectra. This material is available free of charge via the Internet at http:// pubs.acs.org.

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Received for review April 20, 2007. Revised manuscript received July 23, 2007. Accepted July 25, 2007. ES070938T

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