Environ. Sci. Technol. 2010, 44, 8453–8459
Methyl-Nitrocatechols: Atmospheric Tracer Compounds for Biomass Burning Secondary Organic Aerosols YOSHITERU IINUMA, OLAF BÖGE, RICARDA GRÄFE, AND HARTMUT HERRMANN* Leibniz-Institut fu ¨ r Tropospha¨renforschung (IfT), Permoserstr. 15, D-04318 Leipzig, Germany
Received August 26, 2010. Revised manuscript received October 8, 2010. Accepted October 8, 2010.
Detailed chemical analysis of wintertime PM10 collected at a rural village site in Germany showed the presence of a series of compounds that correlated very well with levoglucosan, a known biomass burning tracer compound. Nitrated aromatic compounds with molecular formula C7H7NO4 (Mw 169) correlated particularly well with levoglucosan, indicating that they originated from biomass burning as well. These compounds were identified as a series of methyl-nitrocatechol isomers (4methyl-5-nitrocatechol, 3-methyl-5-nitrocatechol, and 3-methyl6-nitrocatechol) based on the comparison of their chromatographic and mass spectrometric behaviors to those from reference compounds. Aerosol chamber experiments suggest that m-cresol, which is emitted from biomass burning at significant levels, is a precursor for the detected methyl-nitrocatechols. The total concentrations of these compounds in the wintertime PM10 were as high as 29 ng m-3, indicating the secondary organic aerosol (SOA) originating from the oxidation of biomass burning VOCs contributed non-negligible amounts to the regional organic aerosol loading.
Introduction Natural and anthropogenic biomass burning emits large amounts of gases and particles into the atmosphere (1), impacting the environment (2) and affecting public health (3) significantly. Several hundreds of compounds are detected both in the gaseous and particulate samples collected from laboratory biomass combustion (e.g., refs 4-7). These compounds largely originate from the pyrolysis of biopolymers such as lignin, cellulose, and hemicellulose. Some of these compounds are specific to biomass burning and widely used as tracer compounds for the presence of biomass burning smoke in the ambient atmosphere. For example, levoglucosan (1,6-anhydro-β-glucopyranose), which is derived from the pyrolysis of cellulose and hemicellulose, is commonly used as an indicator for the presence of primary aerosols originating from biomass burning due to its relatively high emission factors from biomass combustion (e.g., refs 4, 8-10). Substituted phenols including alkylphenols and methoxyphenols are another class of biomass burning tracer compounds that originate from the pyrolysis of lignin (11). Unlike levoglucosan, this class of compounds can be found in both the gas-phase and the particle-phase owing to their wide range of volatilities (4). * Corresponding author phone: +49-341-235-2446; fax: +49-341235-2325; e-mail:
[email protected]. 10.1021/es102938a
2010 American Chemical Society
Published on Web 10/21/2010
Biomass burning is typically considered as a source of primary organic aerosol (POA) (e.g., refs 1, 12,) and only a few studies are available concerning secondary organic aerosol (SOA) formation from the oxidation of volatile or semivolatile organic compounds emitted from biomass burning (13-16). The emission factors of volatile and semivolatile organic compounds are comparable to those of particulate organic carbon (4) and the oxidation of semivolatiles such as substituted phenols is likely to form SOA, contributing significantly to organic aerosol loadings in the atmosphere. In the present work, we report the presence of novel biomass burning secondary organic aerosol (BBSOA) tracer compounds that correlate very well with levoglucosan in PM10 field samples collected in a rural village of Seiffen, Germany during winter 2007-2008. On the basis of the results from laboratory experiments, aerosol chamber experiments, and the comparison to liquid chromatographic and mass spectrometric features of authentic standard compounds, these compounds are identified as 3-methyl-5-nitrocatechol, 4-methyl-5-nitrocatechol, and 3-methyl-6-nitrocatechol originating primarily from the photooxidation of m-cresol that is emitted from biomass burning at considerable levels (4). SOA yields are determined for the reaction of m-cresol with OH radicals in the presence of NOx.
Experimental Section Field Samples. The ambient PM10 samples were collected between October 22, 2007 and March 30, 2008 at the rural village of Seiffen, Saxony, Germany (50°38′50” N, 13°27′08” E, 647 m above mean sea level). The sampling site was located in a residential area and was significantly influenced by the emissions from domestic wood combustion. The PM10 samples were collected every four days on a prebaked quartz fiber filter using a Digitel DHA-80 high volume sampler (Digitel, Elektronik AG, Hegnau, Switzerland) for 24 h from midnight to midnight (720 m3 total sampling volume). The mixing ratios of NOx (NO and NO2) were delivered from the colocated trace gas measurement station operated by Saxon State Office of the Environment, Agriculture and Geology. Aerosol Chamber Samples. Details of the chamber facilities and the operational procedures have been described elsewhere (17). Briefly, a 19 m3 indoor aerosol chamber was used for the reaction of m-cresol with OH radicals in the presence of NOx and (NH4)2SO4/H2SO4 (0.034/0.057 mM) seed particles. OH radicals were produced through the photolysis of methylnitrite in the presence of excess NO. The NO and NO2 mixing ratios were continuously monitored using a NOx analyzer (Model 42S, Thermo Environmental Instruments, Franklin, MA, U.S.A.) and the mixing ratio of the m-cresol was measured with a proton-transfer-reaction mass spectrometer (PTR-MS, IONICON, Innsbruck, Austria). The number and volume particle size distributions (3-900 nm) were measured by a differential mobility particle sizer (DMPS). The SOA sample was collected on a glass fiber filter coated with fluorocarbon (47 mm diameter, PALLFLEX T60A20, PALL, NY, USA) after a reaction time of 75 min. The sampling time was 1 h with a total sampling volume of 1.8 m3. Synthesis of Reference Compounds. Mixtures of methylnitrocatechols are made from 3-methylcatechol or 4-methylcatechol and used for the identification of precursor substituted phenols. In addition, higher purity 3-methyl-6nitrocatechol was synthesized for the quantification. The detailed procedures for the synthesis of standard compounds are given in the Supporting Information, SI. VOL. 44, NO. 22, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
8453
Sample Preparation and Analytical Method. Field Samples. A portion of the PM10 quartz fiber filter (9.42 cm2) was spiked with an internal standard (trans-cinnamic d7 acid, 98 atom % D, Aldrich, St. Louis, MO, USA) and extracted in 1 mL of LC-MS grade methanol for 30 min under ultrasonication. Nonsoluble materials in the extract were removed by a syringe filter (0.2 µm, PTFE Acrodisc, Pall, NY) and the extract was dried under a gentle stream of nitrogen at 10 °C. The dry residue was redissolved in 200 µL of a methanol/ water solution (50/50, v/v) for the analysis of methylnitrocatechols. Another portion of the PM10 filter (6.28 cm2) was used for levoglucosan analysis. Detailed extraction procedures are described elsewhere (18). Chamber Samples. A half of the PTFE coated glass fiber filter from the chamber experiment (8.68 cm2) was extracted and reconstituted in the same manner as the field samples for the methyl-nitrocatechols analysis. The extracts from both the field and the chamber filters were analyzed within 24 h in most cases. HPLC/(-)ESI-TOFMS and UPLC/(-)ESI-QTOFMS. The concentrations of methyl-nitrocatechols and methoxyphenols in the field sample extracts were determined by an Agilent 1100 series HPLC coupled with a Bruker micrOTOF mass spectrometer equipped with an electrospray ionization source (HPLC/(-)ESI-TOFMS). An Agilent Zorbax SB-C18 column (150 × 3 mm, 5 µm, 80 Å) was used for the separation. The eluent composition used for the separation was (A) 0.1% acetic acid in water and (B) methanol. The following gradient elution program was employed: the concentration of eluent B was increased from 10% to 90% in 20 min, held constant for 15 min, and then decreased to 10% to re-equilibrate for 10 min. The flow rate was 0.5 mL min-1. The operational parameters for micrOTOF was as follows: ion polarity, negative; ESI nebulizer (N2), 1.5 bar; dry gas (N2), 10 L min-1; capillary voltage, 4.5 kV; end plate offset, -0.5 kV; ion source temperature, 200 °C; and scan rage, m/z 50-1200. The aerosol chamber sample was analyzed using a Waters Acquity ultra performance liquid chromatography coupled to a Synapt HDMS electrospray ionization ion mobility spectrometer and quadrupole time-of-flight mass spectrometer (UPLC/(-)ESIQTOFMS). Details of the operating parameters for the UPLC/ (-)ESI-QTOFMS is described elsewhere (19). Selected field sample extracts were also analyzed by the UPLC/(-)ESIQTOFMS to compare the chromatographic behaviors and the fragmentation patterns of target compounds with the chamber sample and standard compounds. For fragmentation experiments, the transfer collision energy of 20 V was applied. The quantification of 3-methyl-6-nitrocatechol and 4-methyl-5-nitrocatechol (Santa Cruz Biotechnology, Inc., CA, USA) for both field and chamber samples was performed by running a series of standard solutions (0.05-6.25 mg L-1, 8 points, quadratic fitting, R2 > 0.998) containing 1.6 mg L-1 trans-cinnamic d7 acid as an internal standard. As the synthesized 3-methyl-6-nitrocatechol contained a minor amount of 3-methyl-5-nitrocatechol, the ratio of integrated peak areas was used to obtain a fraction of 3-methyl-6nitrocatechol that was present in the standard solution. 3-Methyl-5-nitrocatechol was quantified from a calibration curve obtained for 3-methyl-6-nitrocatechol. The same procedure was used to quantify nine methoxyphenols (vanillin, vanillic acid, 3-hydroxy-4-methoxybenzoic acid, conyferyl aldehyde, homovanillic acid, 3,5-dimethoxy-4hydroxyacetophenone, syringic acid, sinapic acid and an isobaric isomer of vanillin). A calibration curve obtained for vanillin was used to quantify the vanillin isomer. HPAEC-PAD. The concentrations of levoglucosan, a tracer compound for biomass burning, in the field samples were determined by a high-performance anion-exchange chromatography with pulsed amperometric detection method 8454
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 22, 2010
(HPAEC-PAD, ICS-3000, Dionex, Sunnyvale, CA, USA). Details about the method are provided in previous work (18).
Results and Discussion Identification and Quantification of Methyl-Nitrocatechols in Ambient Samples. Figure 1a shows a typical extracted ion chromatogram (EIC) and mass spectra of m/z 168 compounds from the analysis of an ambient PM10 sample collected at Seiffen. Two major and one minor m/z 168 peaks are detected in the ambient samples. High-resolution (-)ESI-TOFMS data reveals that they are isobaric isomers with the chemical formula of C7H6NO4- and m/z 168.0297. On the basis of the C to H ratio and a number of nitrogen and oxygen atoms, these compounds are postulated to be a nitro-aromatic compound with either one methyl group and two hydroxyl groups (methyl-nitrocatechols) or one methoxy group and one hydroxyl group (nitroguaiacols). Their chromatographic behaviors and fragmentation patters were compared to commercially available nitroguaiacols (4-nitroguaiacols and 2-methoxy-5-nitrophenol, Sigma-Aldrich) and laboratory produced methyl-nitrocatechols from 3-methylcatechol and 4-methylcatechol. The retention times of both 4-nitroguaiacol and 2-methoxy-5-nitrophenol are very close to the retention times of the first and second overlapping m/z 168 peaks detected around 6 min in the field sample (Figure 1S, SI). However, the MS2 product ion spectra of both the nitroguaiacols show abundant m/z 153 (C6H3NO4•-), m/z 123 (C6H3O3-), and m/z 95 (C5H3O2-) ions that are not present in the MS2 product ion spectra of m/z 168 compounds detected in the field samples (Figure 1S, SI), indicating that these compounds are structurally different. The retention times of the laboratory produced methyl-nitrocatechols are the same as the m/z 168 compounds in the field samples and the MS2 product ion spectra also show the same ions at m/z 138 (C7H6O3•-), m/z 137 (C7H5O3-), and m/z 109 (C6H5O2-) with similar intensity ratios (Figure 1S, SI). On the basis of the chromatographic behaviors and the fragmentation patterns of m/z 168 compounds, we conclude that the m/z 168 compounds are methyl-nitrocatechols rather than nitroguaiacols with 3-methylcatechol and 4-methylcatechol skeletons. In order to elucidate the exact structure of these methylnitrocatechols, their chromatographic retention times and fragmentation patterns were further compared to the reference compounds of 4-methyl-5-nitrocatechol, 3-methyl-5nitrocatechol, and 3-methyl-6-nitrocatechol. Figure 1c,d illustrates EIC chromatograms and accurate mass data for the reference compounds of 4-methyl-5-nitrocatechol, 3-methyl-5-nitrocatechol, and 3-methyl-6-nitrocatechol and Figure 2S, SI shows the fragmentation data for these reference compounds and the ambient sample. The very good agreement of the retention times and the fragmentation patterns between the ambient sample and the methyl-nitrocatechol reference compounds confirms that the m/z 168 compounds in the ambient samples are 4-methyl-5-nitrocatechol, 3-methyl-5-nitrocatechol, and 3-methyl-6-nitrocatechol. The concentrations of three identified methyl-nitrocatechols in the ambient samples are determined using reference compounds. Figure 2 shows the time series for levoglucosan, a known biomass burning tracer compound, and the methylnitrocatechols. The concentrations of the methyl-nitrocatechols follow very closely those of levoglucosan with a strong correlation (R2 ) 0.749), indicating that the methyl-nitrocatechols likely originate from the same source as levoglucosan, i.e., biomass burning. It is noted that only very weak correlation was found between the methyl-nitrocatechols and PM10 nitrate (R2 ) 0.4). This does not exclude a possibility of nitration reactions in the particle phase. The concentrations of PM10 nitrate were 2 to 3 orders of magnitude higher than those of the methyl-nitrocatechols, hence the availability of the methyl-nitrocatechol precursors were likely a limiting
FIGURE 1. UPLC/(-)ESI-TOFMS extracted ion chromatograms and accurate mass data of m/z 168 compounds for (a) an ambient sample collected on December 17, 2007 at Seiffen, Germany, (b) a chamber sample collected from an m-cresol/NOx photooxidation experiment, (c) 3-methyl-5-nitrocatechol and 3-methyl-6-nitrocatechol reference compounds, and (d) 4-methyl-5-nitrocatechol reference compound.
FIGURE 2. Time series for the concentrations of PM10 bound methyl-nitrocatechols and levoglucosan, and the mixing ratios of NO and NO2. A correlation between methyl-nitrocatechols and levoglucosan are shown in the insert. factor for their formation. Table 1 summarizes the concentration ranges and average concentrations for the detected methyl-nitrocatechols. The total concentration of the methylnitrocatechols was as high as 29 ng m-3 with an average concentration of about 5 ng m-3 during the sampling period. These values were higher than the sum of nine biomass burning tracer methoxyphenols determined in the same PM10 samples (average 4.4 ng m-3, max. 17 ng m-3, Table 1S, SI,), indicating the importance of the methyl-nitrocatechols in biomass burning smoke. The Formation of SOA and Methyl-Nitrocatechols from the Photooxidation of m-Cresol. In our previous work, we falsely assigned the m/z 168 compound as 2-nitroguaiacol,
TABLE 1. Concentrations of 4-Methyl-5-Nitrocatechol, 3Methyl-5-Nitrocatechol, and 3-Methyl-6-Nitrocatechol Determined from Field Samples Collected between October 2, 2007 and March 30, 2008 in Seiffen (n = 45)
4-methyl-5-nitrocatechol 3-methyl-5-nitrocatechol 3-methyl-6-nitrocatechol total
range (ng m-3)
average (ng m-3)
0.0-11 0.0-2.0 0.0-16 0.02-29
2.0 0.37 2.9 5.2
VOL. 44, NO. 22, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
8455
8456
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 22, 2010
9.9% 821 4.9% 410 0.7% 60 4.2% 352 m-Cresol/CH3ONO/UV
52.8
38.6
95
5
14.7
8.3 22.6 ( 0.5 43 ( 2
4.9%
wt% in SOA ng m-3 wt% in SOA ng m-3 wt% in SOA ng m-3 wt% in SOA ng m-3 10-6 cm-3 m-3 10-6 cm-3 m-3 °C ppb ppb
ppb
ppb
%
(G ) 1)
total 3-methyl-6nitrocatechol 3-methyl-5nitrocatechol 4-methyl-5nitrocatechol SOA yield ∆SOA volume initial seed volume mean T mean RH initial CH3ONO initial NO ∆HC initial HC
TABLE 2. Initial Conditions for Chamber Experiments and Concentrations of 4-Methyl-5-Nitrocatechol, 3-Methyl-5-Nitrocatechol and 3-Methyl-6-Nitrocatechol Determined from Chamber Experiment
owing to the lack of a suitable reference compound and high resolution MS2 data (10). We postulated that the nitrophenols in biomass burning smoke likely form in the combustion flame rather than the subsequent atmospheric processes. The ratios of levoglucosan to the methyl-nitrocatechols found in the ambient PM10 samples from the present study are often lower than those from the direct emission study (i.e., higher methyl-nitrocatechol concentrations), indicating that there are additional processes that contribute to the methylnitrocatechol formation or levoglucosan elimination. Here we present one plausible formation pathway of the methylnitrocatechols from m-cresol photooxidation in the presence of NOx. It has been reported that o-, m-, and p-cresols are emitted at significant levels from the combustion of wood (4). The sum of their emission factors from pine wood combustion was shown to be about one-third of levoglucosan, and cresols were almost exclusively found in the gas-phase, contributing significantly to VOC emissions from biomass burning (4). The atmospheric oxidation of cresols in the gas-phase have been studied in the past for both their reaction kinetics (20-28) and the gas-phase products (20, 21, 23, 27). The reactions of cresols with OH radicals in the presence of NOx lead to the formation of various isomers of methyl-benzoquinones,methylcatecholsandmethyl-nitrophenols(20,21,23). Among three cresol isomers, only m-cresol oxidation yields both 3-methylcatechol and 4-methylcatechol that are used for the preparation of the identified methyl-nitrocatechols. The oxidation of o-cresol or p-cresol leads to either 3-methylcatechol or 4-methylcatechol (23). On the basis of the high emission factor of m-cresol from biomass burning (4) and the high yields of both the methylcatechols (23), an m-cresol/NOx photooxidation experiment was conducted in an indoor aerosol chamber to obtain evidence for the presence of 4-methyl-5-nitrocatechol, 3-methyl-5-nitrocatechol and 3-methyl-6-nitrocatechol in m-cresol SOA. The initial experimental conditions and time series of chamber experiments are given in Table 2 and Figure 3S (SI). The results from the analysis of the chamber sample show three m/z 168 compounds corresponding to three isomers of methyl-nitrocatechols in the ambient samples and laboratory prepared standard compounds (Figure 1). Their fragmentation patterns agree very well with those of the ambient samples and the standard compounds, confirming the presence of 4-methyl-5-nitrocatechol, 3-methyl-5-nitrocatechol and 3-methyl-6-nitrocatechol in m-cresol/NOx photooxdation SOA (Figure 2S of the SI)). Table 2 summarizes the results from the analysis of the m-cresol/NOx photooxdation SOA. The SOA yield at the end of the photooxidation was about 5% and the sum of three methyl-nitrocatechols accounted for approximately 10% of the formed SOA mass with 4-5% each for 4-methyl-5-nitrocatechol and 3-methyl6-nitrocatechol. Figure 3 shows the time-dependent aerosol growth curve that illustrates the effect of multigeneration oxidations on SOA formation (29-33). A hook-like shape of the growth curve is characteristic of the hydrocarbon oxidation in which the later generation oxidation products and/or the particle phase reactions play an important role in the SOA formation. In the case of the m-cresol oxidation, it is likely that the first generation semivolatile oxidation products such as methylcatechols are further oxidized to form low-volatile methyl-nitrocatechols both in the gas- and particle-phases, contributing to the SOA formation (Figure 4). Indeed, first generation oxidation products such as methylcatechols and methylbenzoquinone were detected at only trace levels in the m-cresol/NOx photooxidation SOA (data not shown), consistent with the feature of the growth curve obtained in the present study. With regard to other oxidation products, methyl-nitrophenols are unlikely to react further with OH radicals in the m-cresol/NOx photooxidation
FIGURE 3. Time-dependent growth curve obtained for m-cresol/ NOx photooxidation. The curve is fitted to a two-component SOA growth model described by Kroll and Seinfeld (33) in which one component is treated significantly less volatile than the other. ri is the mass based stoichiometric coefficient for product i and Ki* (m3 µg-1) is the gas-particle partitioning equilibrium constant for product i. system forming the methyl-nitrocatechols because the rate constants for the reactions of nitrophenols with OH radicals are much smaller than that of phenol with OH (20). It is noted that the reaction of toluene with OH in the presence of NOx is known to produce cresols with a yield of about 20% (34-36). Toluene is emitted from various sources such as gasoline-powered vehicles (37) and biomass burning (4), and it is one of the most abundant anthropogenic VOCs in polluted urban areas (38). Sato et al. (39) reported the formation of unidentified m/z 168 SOA compounds from the oxidation of toluene in the presence of NOx though the contribution of the m/z 168 compounds to the toluene SOA is smaller than that of nitrocresols (
