Environ. Sci. Technol. 2006, 40, 6934-6937
Seasonal Variation of 2-Methyltetrols in Ambient Air Samples XIAOYAN XIA AND PHILIP K. HOPKE* Center for Air Resources Engineering and Science, Clarkson University, Potsdam, New York 13699-5708
PM2.5 samples were collected from June to December 2005 in Potsdam, New York and analyzed for polar organic compounds by GC/MS. The major compounds that were identified in the samples included 2-methyltetrols (2methylthreitol and 2-methylerythritol), levoglucosan, cispinonic acid, and mannitol. 2-Methyltetrols were quantified during the analysis. A seasonal variation for these two diastereoisomers was observed, with the highest concentrations occurring during the summer and the lowest concentrations occurring during the winter. OC/EC analyses of these samples were also performed. The variation of the carbon contribution of 2-methyltetrols to OC was found to follow the same pattern as the concentration variation of 2-methyltetrols. During summer, the period of high photochemical activity, the maximum carbon contribution of 2-methyltetrols to OC was 2.8%. The observation of high 2-methyltetrol concentrations during the summer indicates isoprene is a significant summertime source of secondary organic aerosol in this rural area in the northeastern United States.
Introduction Fine particulate matter (with aerodynamic diameters less than 2.5 µm) affects visibility, human health, and climate (1, 2). Organic material may contribute between 20 and 50% to the total fine aerosol mass at continental mid-latitudes (3). These organic compounds can be classified into primary organic aerosol (POA) if they are emitted directly in particulate form or secondary organic aerosol (SOA) if they are formed from nucleation reactions of gas-phase oxidation products of volatile organic compounds (VOCs) or uptake of these oxidation products to pre-existing aerosol. To control ambient SOA concentrations, the major precursors must be identified and the SOA formation mechanisms must be established. VOCs from both biogenic sources and anthropogenic sources can serve as SOA precursors. Formation of SOA from O3, •OH, and •NO3 initiated reactions of monoterpenes has been widely studied in laboratory systems (4, 5). Some of the SOA products have been detected in ambient particle samples (6). Isoprene has generally not been considered to be a SOA precursor (7). However, Claeys et al. (8) first identified two diastereoisomeric 2-methyltetrols in samples collected in the Amazonian rain forest. They proposed isoprene as the precursor for formation of 2-methyltetrols in the atmosphere because of the presence of the C5 isoprene skeleton in these compounds and the high emission rates of isoprene. They * Corresponding author
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also suggested that the contribution to SOA formation by the photochemical oxidation of isoprene was not negligible. Since 2-methyltetrols were first identified by Claeys and associates, they have been detected in ambient air samples collected in different regions such as Hungary (9), Finland (10), and the Eastern United States (11). Claeys et al. (8) initially proposed that 2-methyltetrols could form from selfand cross-reactions of peroxy radicals from •OH initiated reactions of isoprene. Later, they proposed a multiphase acidcatalyzed oxidation pathway for isoprene based on their experimental synthesis of 2-methyltetrols (12). Edney et al. (11) found that 2-methyltetrols and 2-methylglyceric acid are possible indicator compounds for isoprene SOA from laboratory irradiated isoprene/NOx/SO2/air mixtures. In their laboratory studies, they found that the presence of SO2 significantly enhanced SOA formation from isoprene photooxidation. This result supported the hypothesis that acidcatalyzed reactions may play an important role during the SOA formation. Recent studies (13, 14) have measured the yields of isoprene photochemical oxidation products over a range of experimental conditions such as isoprene and NOx concentrations, etc.; and formation of secondary organic particle phase compounds from isoprene gas-phase oxidation products was performed by using chamber and field studies. Kroll et al. (13) investigated the mechanism of SOA formation by isoprene photooxidation. They used hydrogen peroxide as radical precursor and found yields were affected by NOx concentrations. Bo¨ge et al. (14) reacted isoprene and its gasphase oxidation products with hydrogen peroxide in the presence of acidic particles and observed the formation of 2-methyltetrols. It was also found that 2-methyltetrols were enriched in the fine size fraction in their study. In the present study, the ambient concentrations of 2-methyltetrols in the PM2.5 were measured between June and December 2005 in Potsdam, NY, a location just north of the Adirondack State Park, six million acres of mixed coniferous/deciduous forest in northern New York State. The measurement program also included monitoring for levoglucosan, cis-pinonic acid, and mannitol, as well as OC and EC.
Experimental Section Sample Collection. A high-volume Tisch PM2.5 sampler (Tisch Environmental model TE-1000 PUF with PM2.5 inlet) was used to collect ambient PM2.5 samples on the roof of the Science Center at Clarkson University. The samples were collected for 24 h every other day between June 2 and December 7, 2005 except for times when the instrument required maintenance. The sampler operated at a flow rate of 220 L per min. A total of 69 samples were collected during this study period: 11 samples in June, 5 samples in July, 13 samples in August, 14 samples in September, 9 samples in October, 12 samples in November, and 5 samples in December. One field blank was collected for each month. Pallflex Tissuequartz filters (90 mm diameter) were baked for 12 h at 550 °C prior to their use in the sampler to minimize organic blank concentrations. After collection, the samples were stored in a freezer at -10 °C until analysis. Analysis for Organic and Elemental Carbon. The samples were analyzed for organic carbon and elemental carbon using the NIOSH 5040 protocol in a Sunset thermal/optical carbon aerosol analyzer (15). A punch (1 cm2) from each filter was used for the OC/EC analysis. The detailed analytical procedure is given by Na et al. (16). 10.1021/es060988i CCC: $33.50
2006 American Chemical Society Published on Web 10/20/2006
FIGURE 2. Daily concentration of 2-methyltetrols.
FIGURE 1. Total ion chromatograph of ambient air sample trimethylsilylated derivatives: peak 1, 2-methylthreitol; peak 2, 2-methylerythritol; peak 3, cis-pinonic acid; peak 4, levoglucosan; peak 5, mannitol. Sample Preparation for GC/MS Analysis. One-fourth of each filter was extracted using a Dionex accelerated solvent extractor ASE300 with a mixture of methylene chloride and methanol in the ratio of 4:1 (17). Before extraction, D-threitol was spiked onto the filters as an internal standard. The extraction heating time in the ASE300 was 5 min. The static time, static solvent extraction time, was 5 min. The solvent flush volume, defined as the amount of solvent to flush through the cell expressed as a percentage of the cell volume, was 60% with a purge time of 100 s. The extraction temperature was 100 °C and the extraction pressure was 1500 psi. After extraction, the solution was concentrated to 1 mL by blowing high purity nitrogen across the samples using a Turbovap LV evaporator. The extract was divided into two portions. One part was stored in the refrigerator while the other portion was dried under a high-purity nitrogen flow and derivatized with bis (trimethylsilyl) trifluoroacetamide (BSTFA) plus 1% trimethylchlorosilane (TMCS) as a catalyst (9, 17). Using the derivatization method of Edney et al. (11), 500 µL of BSTFA + 1% TMCS in pyridine (4:1 in volume) was added to the reaction vial to dissolve the residue. The mixture was heated at 70 °C for 1 h, and 1 µL of the derivatized sample was analyzed by GC/MS. 2-Methyltetrols Analysis by GC/MS. The derivatized samples were analyzed for 2-methyltetrols using a Thermo TRACE GC coupled with a PolarisQ ion trap mass spectrometer. A 30 m Rtx-5MS capillary column with 0.25 mm internal diameter and 0.25 µm film thickness was used. The stationary phase was 5% diphenyl and 95% dimethyl polysiloxane. The temperature program in the GC/MS analysis began with a constant 60 °C for 2 min. It was increased to a final temperature of 270 °C with a ramp of 8 °C/ min, and then held constant for 20 min. Mass spectra were recorded in the mass range of m/z 50-550 using the electron ionization (EI) mode. The ion source temperature was 200 °C and the transfer line temperature was 300 °C. The mass spectra of 2-methyltetrols were compared with reported mass spectral data (11, 12, 18). The selected ions m/z 219 and 277 were used to determine the concentrations of the 2-methyltetrols.
Results and Discussion Figure 1 shows a typical total ion chromatograph (TIC) of trimethylsilylated derivatives of the ambient air sample. The
FIGURE 3. Monthly average concentration of 2-methyltetrols. upper figure (A) is the TIC of the sample collected on July 25, while the lower panel (B) shows the TIC of a sample collected on November 20, 2005. The major identified polar compounds are 2-methylthreitol, 2-methylerythritol, cispinonic acid, levoglucosan, and mannitol. Among these identified polar compounds, 2-methylthreitol and 2-methylerythritol were quantified in order to provide information on SOA formation and to evaluate the 2-methyltetrols carbon contribution to the OC concentration over time. Figure 2 shows the daily concentrations of 2-methylthreitol, 2-methylerythritol, and total 2-methyltetrols in the Potsdam samples. The concentrations of 2-methylthreitol and 2-methylerythritol were higher during July and August than in the other months. The maximum 2-methylthreitol concentration was 54 ng/m3 during July and August while the maximum concentration of 2-methylerythritol reached 77 ng/m3 during this period. After October 11, the date of the first extensive frost in the area, the concentration of 2-methyltetrols dropped sharply and remained extremely low. The concentrations of 2-methylthreitol and 2-methylerythritol on December 7 were 0.02 and 0.07 ng/m3, respectively. Figure 3 shows the monthly average concentrations of 2-methylthreitol, 2-methylerythritol, and their total concentrations. This trend is similar to that of isoprene emissions (19). Zhang et al. (19) found that isoprene emissions from deciduous trees vary with different seasons. The isoprene emission rates were much higher in summer than in spring and fall due to the change of light, temperature, and leaf age. The light intensity and temperature could also affect the isoprene reactions and result in the higher concentrations of 2-methyltetrols in summer. Figure 4 shows the relationship between 2-methylthreitol and 2-methylerythritol concentrations. The average ratio of the concentration of 2-methylthreitol to VOL. 40, NO. 22, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Scatter plot of 2-methylthreitol versus 2-methylerythritol.
FIGURE 6. Total concentrations of 2-methyltetrols versus the solar radiation intensity measured at Huntington Research Forest.
FIGURE 5. Total concentrations of 2-methyltetrols versus average daily temperature.
FIGURE 7. Monthly average 2-methyltetrol carbon contributions to OC.
2-methylerythritol was 0.58 ( 0.04. The squared correlation coefficient between these concentrations is 0.94, which is similar to the value of 0.95 obtained by Ion et al. (9). The high correlation suggests the same formation pathway for these two diastereoisomers. The total concentration of 2-methyltetrols reported by Claeys et al. (8) in Amazonian rain forest particle samples was 64.7 ng/m3 during the day and 49.2 ng/m3 during the night. The total concentration of 2-methyltetrols observed in boreal forest aerosol samples collected at Hyytia¨la¨, Finland (10) was 26.3 ng/m3 in summer and 0.46 ng/m3 in fall. The concentrations of 2-methylthreitol and 2-methylerythritol observed at K-puszta, Hungary during summer 2003 by Ion et al. (9) ranged from 1 to 34 and 1 to 85 ng/m3, respectively. The concentrations of 2-methyltetrols were significantly higher during the summer months than later in the year. The concentrations of cis-pinonic acid, a compound detected in R-pinene SOA formation, and mannitol, a proposed marker for fungal spores, followed the same pattern. In contrast, the relative concentration of levoglucosan, a marker for biomass burning, shows the opposite trend with higher concentrations during colder months. This pattern suggests that wood burning was more important during the winter than the summer. Figure 5 shows the total concentrations of 2-methyltetrols as a function of the average daily temperature, with all the low temperature values showing negligible 2-methyltetrol concentrations occurring after October 11. The total concentrations of the 2-methyltetrols were correlated to the base of the natural logarithm raised to the ratio of the sample temperature to the freezing point of water in degrees Kelvin.
It is clear that the concentrations of 2-methyltetrols closely follow the temperature prior to October 11, 2005. At that time, the trees moved quickly to a dormant state and essentially ceased isoprene production. The squared correlation coefficient between total tetrol concentration and this temperature function was 0.68. Photochemical production of 2-methyltetrols may also be affected by other meteorological variables such as solar radiation intensity. Figure 6 shows the trends of 2-methyltetrols concentration and average daily solar radiation intensity, which was measured at Huntington Research Forest, approximately 75 km south of Potsdam. The average solar radiation intensity during July and August was 79.03 W/m2, decreasing to 0.82 W/m2 during November and December. Unlike the relationship with temperature, the concentration of 2-methyltetrols shows no correlation with solar radiation. Other factors that could affect the concentrations of 2-methyltetrols are plant species and leaf surface area. During the summer, isoprene can be emitted from deciduous and coniferous trees with intense solar radiation; the average leaf area index around Potsdam area is about 2.64. During winter time, coniferous trees are the main emission sources of isoprene; the average leaf area index value is about 0.82. The average leaf area can affect the emission of isoprene, and this variation may further affect the concentrations of the 2-methyltetrols. OC/EC analysis was also performed for each sample in this study. The monthly average contribution of the 2-methyltetrols to the measured OC is shown in Figure 7. The carbon contributions of 2-methylthreitol and 2-methylerythritol in
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the summer are on average 0.7% and 1.2%, respectively. These ratios gradually decrease in the subsequent months. During the colder months, November and December, the average carbon contributions of 2-methylthreitol and 2-methylerythritol became 0.012% and 0.024%, respectively. The OC concentration remained relatively constant compared to the concentration of 2-methyltetrols. From the above results, it can be seen that the carbon contribution of isoprene oxidation products to the observed OC cannot be ignored during the high photochemical productivity season. Isoprene should be considered as a major producer of SOA. As one of the high-production, biogenic VOCs, it plays an important role in the formation of SOA in this rural area. According to the Claeys et al. (8) estimate, the global contribution of isoprene oxidation to SOA formation would be 2 Tg if the annual global emissions of isoprene is 500 Tg and the annual yield of 2-methyltetrols is 0.2%. Since isoprene accounts for about half of all natural VOC emissions, it is necessary to further determine its contribution to the total SOA formation. It is necessary to investigate the SOA yields from other biogenic VOCs such as sesquiterpenes to compare their contributions to total global SOA formation.
Acknowledgments We thank Dr. Tadeusz E. Kleindienst (National Exposure Research Laboratory) for the informative discussion and providing the internal standard for 2-methyltetrols analysis. We also thank Dr. Thomas M. Holsen and his student HyunDeok Choi (Clarkson University) for providing us access to the solar radiation meteorology data website. We thank Ping Li and Shankar Kiran Sondekoppa Gopalakrishna for performing the organic and elemental carbon analyses. This work was supported in part by the New York State Energy Research and Development Authority under contract 6083.
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Received for review April 25, 2006. Revised manuscript received July 5, 2006. Accepted September 18, 2006. ES060988I
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