Environ. Sci. Technol. 2007, 41, 7403-7408
Secondary Organic Aerosol Formation from the Photooxidation of p- and o-Xylene C H E N S O N G , †,‡,§ K W A N G S A M N A , † B E T H A N Y W A R R E N , †,‡ Q U E N T I N M A L L O Y , †,‡ A N D D A V I D R . C O C K E R , I I I * ,†,‡ Bourns College of EngineeringsCenter for Environmental Research and Technology (CE-CERT), University of California, Riverside, California 92521, and Department of Chemical and Environmental Engineering, Bourns College of Engineering, University of California, Riverside, California 92521
The formation of secondary organic aerosol (SOA) from the photooxidation of xylene isomers (m-, p-, and o-xylenes) has been extensively investigated. The dependence of SOA aerosol formation on the structure of xylene isomers in the presence of NO was confirmed. Generally, SOA formation of p-xylene was less than that of m- and o-xylenes. This discrepancy varies significantly with initial NOx levels. In a NOx-free environment, the difference of aerosol formation between o- and p-xylenes becomes insignificant. Several chemical pathways for the SOA dependence on structure and NOx are explored, with the experimental findings indicating that organic peroxides may be a major key to explaining SOA formation from aromatic hydrocarbons.
Introduction Aromatic hydrocarbons are one of the leading classes of atmospheric reactive volatile organic compounds (VOCs) in the atmosphere (1, 2). They are generally associated with anthropogenic activities such as combustion emissions and industrial solvents. Aromatic hydrocarbons can have direct negative impacts of public health (many are carcinogenic or mutagenic) and indirect negative impacts through their propensity to form important secondary pollutants such as ozone (O3) and secondary organic aerosol (SOA). A study by Derwent et al. (3) has estimated that aromatic hydrocarbons may contribute upward of 30% to O3 formation in Europe. It is believed that SOA impacts global climate change, regional visibility, and human health as a key contributor to fine particulate matter (4-6). Xylenes (m-, p-, and o-xylene) are the most important ambient aromatic hydrocarbons next to toluene and are highly reactive with the hydroxyl radical (OH) (2). The hydroxyl radical rate constant of m-xylene is the highest among all three isomers and about 2 times higher than those of p- and o-xylene. The atmospheric oxidation process of xylenes initiated by OH leads to the production of a wide range of compounds such as phenols, aromatic aldehydes, and unsaturated carbonyl compounds (2), mostly from ringretaining or -fragmenting chemical pathways. These species * Corresponding author phone: (909)781-5695; fax: (909)781-5790. † CE-CERT. ‡ Bourns College of Engineering. § Now affiliated with Atmospheric Science and Global Change Division, Pacific Northwest National Laboratory; Phone: (509) 3724985. 10.1021/es0621041 CCC: $37.00 Published on Web 09/29/2007
2007 American Chemical Society
are expected to be highly reactive and may be oxidized further to product molecules with more functional groups. As a result, semi- and nonvolatile species are expected to be formed and partitioned from the gas phase to the aerosol phase. m-Xylene has been extensively studied in smog chambers and significant SOA formation has been reported (7-9). Only a few studies have focused on p- and o-xylenes, with large variations in SOA formation reported (10, 11). For example, the SOA yield of p-xylene was reported to be less than half of that for the other two xylenes by Izumi and Fukuyama (11). The data of Odum et al. (10) showed comparable SOA yields between m- and p-xylenes under similar conditions. As we pointed out in an earlier study (7), differences in initial NOx levels cause large variations in the SOA formation potential of aromatic hydrocarbons, which is not considered to be a factor in most previous studies (8, 9). In a recent modeling study, Johnson et al. (12) suggested that there are lower unsaturated aldehyde yields from p-xylene as compared to m-xylene, which could be a key factor leading to varied SOA yields among aromatic hydrocarbons. In this study, we present a series of experiments investigating the SOA formation of p- and o-xylenes in the presence and absence of NOx and compare these results with experimental SOA data from m-xylene previously published and acquired from the same chamber facility (7). Our goal is to improve our understanding of the effect of structural features on SOA formation.
Experimental Procedures All experiments were carried out in the UCR Chamber Facility, which has been described in detail elsewhere (13). In brief, the chamber consists of two 90 m3 Teflon reactors that are located inside a temperature controlled and pure air purging enclosure. Eighty blacklights (115W Sylvania 350BL) provided an NO2 photolysis constant of 0.18 min-1 (13). A positive differential pressure was always maintained between the reactors and the enclosure to minimize infiltration of contaminants from the surrounding environment. Temperature and relative humidity (RH) were kept at 27 °C and less than 1.0 and HCo/ NOx < 0.7) and obtained a considerable difference in the SOA production between the two sets of experiments. SOA yield of o- and p-xylene was measured to increase with increasing HCo/NOx (Figure 1). It is noted that three p-xylene photooxidation experiments performed at low HCo/ NOx had nearly indistinguishable SOA yields as the relatively high NOx levels greatly suppressed SOA formation even after large amounts of hydrocarbon were consumed and there was 12 h of irradiation (Figure 1). The dependence of the SOA yield on HCo/NOx from o- and p-xylene is consistent with the observations for m-xylene in our previous study (7). Table S1 contains the Ri and Kom,i values for all empirical fits in Figure 1. SOA Yield Comparison of Xylene Isomers. One of the major purposes of this study is to provide a comprehensive data set for the comparison of SOA formation among xylene isomers. Therefore, we present in Figure 2 SOA yields of oand p-xylene from this study combined with the yield curves for m-xylene from our previous study (7). To aid in comparison between current and past results, only those m-xylene experiments with HCo/NOx > 2.0 were chosen to generate the best fit yield curves that were shown in Figure 2A,B; in Figure 2C, the yield curve of m-xylene was generated by fitting experiments with HCo/NOx ranging from 0.1 to 1.2. We conclude that among the xylene isomers, p-xylene has the lowest SOA yield for any HCo/NOx ratio examined (Figure 2). For the ranges of high (Figure 2A) and medium (Figure 2B) HCo/NOx of this study, the yield curves of o-xylene are very close to the m-xylene yield curve for high HCo/NOx. On the other hand, SOA yields of p-xylene in both HCo/NOx
FIGURE 1. SOA yields from o- and p-xylene photooxidation as a function of aerosol mass concentration. Also shown are yield curves generated (solid: o-xylene and dashed: p-xylene) from the two-product model (eq 1). regimes are obviously lower than the o-xylene and m-xylene yield curves. It seems that the difference of SOA yields between p-xylene and other two xylene isomers in the low HCo/NOx regime is greater than those in the high and medium HCo/NOx regimes. Izumi and Fukuyama (11) have reported that SOA yields of m- and o-xylenes are about 2.4 to 2.7 times the SOA yield of p-xylene with HCo/NOx in the range of 2 to 5, while the Odum et al. (10) results showed similar SOA yields among m-, p-, and o-xylenes under similar conditions (all experiments with HCo/NOx less than 1). Thus, the difference of SOA yields between p-xylene and o-xylene and m-xylene from this study is not as large as reported by Izumi and Fukuyama (11) but much larger than the Odum et al. (10) data. Comparison of SOA Formation between p- and o-Xylene. Figure 3 provides aerosol formation as a function of reacted hydrocarbon for experiments carried out at different HCo/ NOx ranges as well as those performed in the absence of NOx. Each panel represents aerosol formation profiles for two experiments with similar initial hydrocarbon and NOx mixing ratios. SOA production from p-xylene in the low HCo/ NOx regime is generally less than half of that of o-xylene (Figure 3A) for the same amount of reacted hydrocarbon. The differences of SOA formation between the two xylene isomers become much smaller for the medium and high HCo/NOx regimes (Figure 3B,C, respectively). Additionally, the SOA profiles become quite similar for p- and o-xylene in the absence of NOx (Figure 3D). The main atmospheric oxidation route for aromatic hydrocarbons is the reaction with OH, which preferentially adds to the aromatic ring (20), resulting in the formation of a variety of oxidation products through ring-retaining and -opening chemical pathways (2, 21, 22). Major oxidation products include glyoxal, methylglyoxal, unsaturated dicarbonyl, and furanone coproducts from the ring-opening pathways and aromatic aldehydes, aromatic acids, and phenols from the ring-retaining pathways (2, 22). However, it is noted that less than 50% of the mass of total oxidation products has been identified. Therefore, the detailed chemical mechanisms leading to the formation of these oxidation products remain poorly understood. Subsequently, the routes for SOA formation from the oxidation processes of aromatics are unclear. For instance, 3-methyl-2,5-furandione has been identified as a major SOA compound from toluene and xylenes (23), but only a very small amount of 3-methyl-2,5furandione should be in the aerosol phase predicted from its vapor pressure (24).
Recently, aerosol phase heterogeneous reactions have drawn increasing attention and are considered as a possible pathway to produce significant amounts of SOA (25-27). Johnson et al. (12, 28) simulated SOA formation from the photooxidation of benzene, toluene, xylene, and trimethylbenzene using an explicit chemical mechanism (29). Their results suggest that an aerosol phase heterogeneous reaction of organic aldehydes and organic hydroperoxide species could lead to low vapor pressure peroxyhemiacetal compounds and therefore be important contributors to the SOA mass. Johnson et al. (12) also determined that p-xylene has a considerably lower SOA yield than that of m-xylene, consistent with the observation of the present study. This para effect on the SOA formation was attributed to the relatively low yield of unsaturated aldehydes from p-xylene and subsequently lower yield of peroxyhemiacetal products. The ring-opening process of p-xylene favors the formation of unsaturated diketones (e.g., 3-hexene-2,5-dione) over unsaturated aldehydes (e.g., 4-oxo-2-pentenal and 2-methylbutendial), which are expected to be the major dicarbonyls from o- and m-xylenes (22) due to different positions that the methyl group attaches to the benzene ring. p-Xylene has the lowest O3 reactivity among xylene isomers (30). The para effect on O3 reactivity could also be explained by the high yield of unsaturated diketones and low yield of unsaturated aldehydes from p-xylene as compared to other xylene isomers because unsaturated diketones are much less photoreactive than unsaturated aldehydes (31, 32). The correlation between both Johnson et al. and Carter (12, 30) studies seems to indicate that unsaturated aldehydes play a critical role in the SOA formation from photooxidation of xylene isomers. Moreover, in the same studies by Johnson et al. (12, 28), the dependence of SOA formation on HCo/NOx was also found to be dictated by the aerosol phase heterogeneous reaction through the formation of organic hydroperoxide species. As the initial NOx concentration was increased, with the initial concentration of aromatic hydrocarbon fixed, the efficiency of producing organic hydroperoxide species decreases, which in turn reduces the efficiency with which peroxyhemiacetal compounds were formed. This theory is consistent with the changes in SOA yields under different HCo/NOx of each individual xylene isomers, as shown in Figure 1 of this study and our previous study (7). Up to this point, the theory presented by Johnson et al. (12, 28) seems to explain most of our experimental observations. However, as discussed previously, the structural dependence of xylene isomers on SOA formation is also VOL. 41, NO. 21, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Comparison SOA yields from p- and o-xylene photooxidation in (A) high HCo/NOx; (B) medium HCo/NOx; and (C) low HCo/NOx regimes. Also shown are yield curves generated from a two-product model (eq 1). Dashed line is yield curve from photooxidation of m-xylene/NOx (7), which represents experiments with HCo/NOx > 2.0. dependent on HCo/NOx. If peroxyhemiacetal compounds do significantly contribute to SOA formation, one would expect that, when HCo/NOx increases, the yield of unsaturated aldehydes from p-xylene should also increase to elevate the formation of peroxyhemiacetal compounds as compared to those from o- and m-xylenes. On the basis of the current experimental data, the branching ratios of pathways leading to the formation of unsaturated diketones and unsaturated dialdehydes from xylene isomers are mainly determined by the position to which methyl groups attach; there is currently no evidence indicating that these branching ratios are sensitive to different levels of NOx. Figure 4 compares the simulated production of photoreactive R-dicarbonyls from photooxidation of p- and o7406
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FIGURE 3. Aerosol formation profiles as a function of reacted hydrocarbon concentration for experiments in the different HCo/ NOx regimes. (A) Low HCo/NOx; (B) medium HCo/NOx; (C) high HCo/ NOx; and (D) absence of NOx. xylene under different NO conditions using SAPRC-99 (14). Initial p- and o-xylene concentrations were set to 100 ppb, and the initial NO concentration was set to 0, 25, and 50 ppb, respectively. For the simulation without NO, 1 ppm initial H2O2 was added. R-Dicarbonyls in our simulation include glyoxal, methglyoxal, unsaturated dialdehydes, and other highly photoreactive ring-opening products that should represent well the aldehyde species that could react with organic hydroperoxides. It is noted that for most of the simulations, the R-dicarbonyls decay after reaching their maximum concentration. The decay occurs at the very end of simulation when xylene consumption becomes very slow and SOA formation is stable; thus, the decay should have a negligible effect on the SOA formation. Overall, the R-dicarbonyl forming efficiency (amount of R-dicarbonyls produced per unit amout of parent hydrocarbon reacted) of o-xylene is considerably higher than that of p-xylene for all scenarios. The difference of the R-dicarbonyl forming ef-
FIGURE 4. SAPRC-99 simulation of r-dicarbonyls.
FIGURE 5. SAPRC-99 simulation of organic peroxides. ficiency between p- and o-xylene is slightly smaller in the absence of NOx, while this difference is not significant for the other three scenarios. Figure 4 clearly supports our previous assumption that the discrepancy of unsaturated dialdehyde formation between p- and o-xylene is not likely to vary with the initial NOx concentrations. Therefore, the contribution of peroxyhemiacetal compounds to SOA formation may not be as important as proposed by Johnson et al. (12) as other reaction pathways are needed to correlate the structural effects of xylene isomers on SOA formation as well as the combined dependences of SOA formation and structural effect on NOx levels. The yields of organic peroxides from p-xylene are expected to be less than those from other xylene isomers since the key step of O3 formation from the photooxidation of organic hydrocarbons is converting NO to NO2 through the reaction with organic peroxyl radicals formed from OH initiated reaction or photolysis (20). With decreasing NO levels, the negative effect of NO on organic peroxides should be decreased, which in turn reduces the difference between the yield of organic peroxides from p-xylene and those from other xylene isomers. Thus, organic peroxides seem to be a good candidate to explain the experimental observation of the
present study. Figure 5 displays a comparison of organic peroxides from the photooxidation of p- and o-xylene under different NO conditions using SAPRC-99 (14). The same simulation conditions as for the R-dicarbonyls are used, with the initial p- and o-xylene concentrations set to 100 ppb and the initial NO concentration set to 0, 25, and 50 ppb, respectively. The 1 ppm initial H2O2 was added to the simulation without the presence of NO. It is interesting to see that the organic peroxide formation from p- and o-xylenes shows a similar trend to those observed in Figure 2A: organic peroxide formation from p-xylene is lower than that from o-xylene in the presence of NO, while organic peroxide formation from p- and o-xylene is almost identical in the absence of NO. Figure 5 also shows the dependence of organic peroxide formation on initial NO levels. Considering the fact that, for aromatic hydrocarbons, SAPRC-99 uses a highly simplified and parametrized mechanism based on current knowledge, our simulation of R-dicarbonyls and organic peroxides may not be entirely accurate. However, the model does indicate that organic peroxides can be possibly related to SOA formation trends experimentally obtained as stated previously. VOL. 41, NO. 21, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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While there is no direct experimental evidence showing the existence of organic peroxides as SOA species from the photooxidation of aromatic hydrocarbons, organic peroxides have been identified as an important SOA species from the ozonolysis of biogenic hydrocarbons possibly in the form of peroxyhemiacetal compounds (33). In another paper (34), SOA formation from the photooxidation of m-xylene in the absence of NOx was found to be elevated when a considerable amount of CO was added, further supporting the fact that organic peroxides are a contributor to SOA mass. Combined with the result from the present study, it becomes increasingly likely that organic peroxides may be a major contributor to SOA formation from aromatic hydrocarbons. Therefore, it is imperative for further research to identify the role organic peroxides play in the SOA formation from the photooxidation of aromatic hydrocarbons and identify the final fate of the organic peroxides (further oxidation or combination with other molecules) within the atmosphere.
Acknowledgments The authors acknowledge the National Science Foundation for financial support under Grants ATM-0234111 and ATM0449778. The authors also thank Kurt Bumiller, Irina Malkina, and Sally Pederson for help with experiment setup and measurement and William P. L. Carter for helpful discussions.
Supporting Information Available Yield curve parameters, Ri and Kom,i, for Figure 1. This material is available free of charge via the Internet at http:// pubs.acs.org.
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Received for review September 3, 2006. Revised manuscript received May 24, 2007. Accepted July 11, 2007. ES0621041
