Degradation Kinetics and Secondary Organic Aerosol Formation 1 from Eugenol by Hydroxyl Radicals 2

Abstract. Methoxyphenols are an important organic component of wood-burning emissions and considered to be potential precursors of secondary organic aerosols (SOA). In this work, the rate constant and SOA formation potential for the OH-initiated reaction of 4-allyl-2-methoxyphenol (eugenol) were investigated for the first time in an oxidation flow reactor (OFR). The rate constant was (8.01 ± 0.40) × 10−11 cm3 molecule−1 s−1 as determined by the relative rate method. The SOA yield first increased and then decreased as a function of OH exposure, and was also dependent on eugenol concentration. The maximum SOA yields (0.11–0.31) obtained at different eugenol concentrations could be expressed well by an one-product model. The carbon oxidation state (OSC) increased linearly and significantly as OH exposure rose, indicating that a high oxidation degree was achieved for SOA. In addition, the presence of SO2 (0–198 ppbv) and NO2 (0–109 ppbv) was conducive to increasing SOA yield, for which the maximum enhancement values were 38.57 % and 19.17 %, respectively. The N / C ratio (0.032–0.043) indicated that NO2 participated in the OH-initiated reaction, subsequently forming organic nitrates. The results could be helpful for further understanding the SOA formation potential from the atmospheric oxidation of methoxyphenols and the atmospheric aging process of smoke plumes from biomass-burning emissions.



Introduction
Wood combustion is a major contributor to atmospheric fine particulate matter (PM) (Bruns et al., 2016), which could contribute approximately 10−50% of the total organic fraction of atmospheric aerosols (Schauer and Cass, 2000).In some regions with cold climates, wood smoke-associated aerosols are estimated to account for more than 70% of PM2.5 in winter (Jeong et al., 2008;Ward et al., 2006).Recently, significant potential of secondary organic aerosol (SOA) formation from wood smoke emissions has been reported (Bruns et al., 2016;Gilardoni et al., 2016;Tiitta et al., 2016;Ciarelli et al., 2017;Ding et al., 2017).In addition, the organic compounds derived from wood combustion and their oxidation products may contribute significantly to global warming due to their light-absorbing properties (Chen and Bond, 2010).It has been reported that wood smoke particles are predominant in the inhalable size range (Bari et al., 2010) and that their extracts are mutagenic (Kleindienst et al., 1986).Exposure to wood smoke can result in adverse health effects associated with acute respiratory infections, tuberculosis, lung cancer, cataracts, etc. (Bolling et al., 2009).Therefore, wood combustion has multifaceted impacts on climate, air quality, and human health.Methoxyphenols produced by lignin pyrolysis are potential tracers for wood smoke, and their emission rates are in the range of 900−4200 mg kg -1 fuel (Schauer et al., 2001;Simpson et al., 2005;Nolte et al., 2001).The highest level of methoxyphenols in the atmosphere always appears during a wood smoke-dominated period, with observed values up to several mg m -3 (Schauer and Cass, 2000;Schauer et al., 2001;Simpson et al., 2005).Methoxyphenols are semi-volatile aromatic compounds with low molecular weight, and many of them are found to mainly exist in the gas phase at typical ambient temperature (Simpson et al., 2005;Schauer et al., 2001).Thus, methoxyphenols can be chemically transformed through gas-phase reactions with atmospheric oxidants (Coeur-Tourneur et al., 2010a;Lauraguais et al., 2012Lauraguais et al., , 2014aLauraguais et al., , 2014bLauraguais et al., , 2015Lauraguais et al., , 2016;;Yang et al., 2016;Zhang et al., 2016;El Zein et al., 2015).The corresponding rate constants control their effectiveness as stable tracers for wood combustion and atmospheric lifetimes.In recent years, the rate constants for the gas-phase reactions of some methoxyphenols with hydroxyl (OH) radicals (Coeur-Tourneur et al., 2010a;Lauraguais et al., 2012Lauraguais et al., , 2014bLauraguais et al., , 2015)), nitrate (NO3) radicals (Lauraguais et al., 2016;Yang et al., 2016;Zhang et al., 2016), chlorine atoms (Cl) (Lauraguais et al., 2014a) and ozone (O3) (El Zein et al., 2015) have been determined.Some studies have indicated significant SOA formation from 2,6-dimethoxyphenol (syringol) and 2-methoxyphenol (guaiacol) with respect to their reactions with OH radicals (Sun et al., 2010;Lauraguais et al., 2012Lauraguais et al., , 2014b;;Ahmad et al., 2017;Yee et al., 2013;Ofner et al., 2011).Although biomassburning emissions have been indicated to have great SOA formation potential via atmospheric oxidation (Bruns et al., 2016;Gilardoni et al., 2016;Li et al., 2017;Ciarelli et al., 2017;Ding et al., 2017), SOA formation and growth from methoxyphenols are still poorly understood.Besides, the observed SOA levels in the atmosphere cannot be well explained by the present knowledge on SOA formation, which reflects the fact that a large number of precursors are not taken into account in the SOA-formation reactions included in the atmospheric models (Lauraguais et al., 2012).
Its average emission concentration and factor in beech burning are 0.032 μg m -3 and 1.52 μg g -1 PM, respectively, which are both higher than those (0.016 μg m -3 and 0.762 μg g -1 PM) of guaiacol (Bari et al., 2009).It has even be detected in human urine after exposure to wood smoke (Dills et al., 2006).Eugenol has been observed to mainly distribute in the gas phase in wood smoke emissions (Schauer et al., 2001), and its gas/particle-partition coefficient is lower than 0.01 (Zhang et al., 2016), thus indicating the importance of its gas-phase reactions in the atmosphere.For this reason, the aim of this work was to determine the rate constant and explore the SOA formation potential for eugenol in the gas-phase reaction with OH radicals using an Oxidation Flow Reactor (OFR).In addition, the effects of SO2 and NO2 on SOA formation were investigated.
To our knowledge, this work represents the first determination of the rate constant and SOA yield for the gas-phase reaction of eugenol with OH radicals.

Experimental section
The detailed schematic description of the experimental system used in this work is shown in Figs.S1 and S2.The gas-phase reactions were conducted in the OFR, whose detailed description has been presented elsewhere (Liu et al., 2014).Before entering into the OFR, gas-phase species were mixed thoroughly in the mixing tube.The reaction time in the OFR was 26.7 s, calculated according to the illuminated volume (0.89 L) and the total flow rate (2 L min -1 ).OH radicals were generated by photolysis of O3 in the presence of water vapor using a 254 nm UV lamp (Jelight Co., Inc.), and their formation reactions have been described elsewhere (Zhang et al., 2017).The concentration of OH radicals was governed by O3 concentration and relative humidity (RH).O3 concentration was controlled by changing the unshaded length of a 185 nm UV lamp (Jelight Co., Inc.).O3 was produced by passing zero air through an O3 generator (Model 610-220, Jelight Co., Inc.), and its concentration was in the range of 0.94−9.11ppmv in this work measured with an O3 analyzer (Model 205, 2B Technology Inc.).RH and temperature in the OFR were (44.0 ± 2.0)% and (301 ± 1) K, respectively, measured at the outlet of the OFR.The steady-state concentrations of OH radicals were determined using SO2 as the reference compound in separate calibration experiments.
It is a widely-used method for calculating OH exposure in the OFR, but could not well describe the potential OH suppression caused by the added external OH reactivity (Zhang et al., 2017;Lambe et al., 2015;Simonen et al., 2017;Li et al., 2015;Peng et al., 2015Peng et al., , 2016)).The decay of SO2 from its reaction with OH radicals (9 × 10 -13 cm 3 molecule -1 s -1 ) (Davis et al., 1979) was measured by a SO2 analyzer (Model 43i, Thermo Fisher Scientific Inc.).The concentration of OH radicals ([OH]) in this work ranged from approximate 4.5 × 10 9 to 4.7 × 10 10 molecules cm -3 , and the corresponding OH exposures were in the range of 1.21−12.55× 10 11 molecules cm -3 s or approximate 0.93 to 9.68 d of equivalent atmospheric exposure.Assuming that particles are spherical and non-porous, the average effective particle density could be calculated to be 1.5 g cm -3 using the equation ρ=dva/dm (DeCarlo et al., 2004), where dva is the mean vaccum aerodynamic diameter measured by HR-ToF-AMS and dm is the mean volume-weighted mobility diameter measured by SMPS.The mass concentration of particles measured by HR-ToF-AMS was corrected by SMPS data in this work using the same method as Gordon et al. (2014).Eugenol and reference compounds were measured by a proton-transfer reaction time-of-flight mass spectrometer (PTR-QiToF-MS) (Ionicon Analytik GmbH).More experimental details were described in the supplementary information.

Rate constant
In order to investigate the possible photolysis of eugenol and reference compounds at 254 nm UV light in the OFR, the comparative experiments were conducted with UV lamp turned on and turned off.The normalized mass spectra of eugenol and reference compounds in the dark and light were shown in Fig. S3.The results showed that no Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-749Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 10 September 2018 c Author(s) 2018.CC BY 4.0 License.significant decay (<5%) by photolysis was observed and could be neglected.According to the results reported by Peng et al. (2016), the photolysis of phenol and 1,3,5trimethylbenzene could be ignored when the ratio of exposure to 254 nm and OH is lower than 1 × 10 6 cm s -1 , of which value in this work also met this condition.In addition, the initial concentration of eugenol was determined with UV lamp turned on.Therefore, the effect of photolysis could be neglected in this work.
The rate constant for the gas-phase reaction of eugenol with OH radicals was determined by the relative rate method, which can be expressed as the following equation (Coeur-Tourneur et al., 2010a;Yang et al., 2016;Zhang et al., 2016): where CE0 and CEt are the initial and real-time concentrations of eugenol, respectively.
kE is the rate constant of the eugenol reaction with OH radicals.CR0 and CRt are the initial and real-time concentrations of reference compound, respectively.kR is the rate constant of the reference compound with OH radicals, of which values for m-xylene and 1,3,5-trimethylbenzene are 2.20 × 10 -11 and 5.67 × 10 -11 cm 3 molecule -1 s -1 , respectively (Kramp and Paulson, 1998;Coeur-Tourneur et al., 2010a).
Data obtained from the reactions were plotted in the form of Eq. ( 1) and were well fitted by linear regression (R 2 > 0.97, Fig. 1).A summary of the slopes and the rate constants are listed in Table 1.The errors in kE/kR are the standard deviations generated from the linear regression analysis and do not include the uncertainty in the rate constants of the reference compounds.The rate constants are (7.54 ± 0.28) × 10 -11 and the rate constant was calculated to be 6.50 × 10 -11 cm 3 molecule -1 s -1 (Table 1), which is lower than that obtained in this work.Inaccurate performance of the AOP WIN model has been observed for other multifunctional organics due to the inaccurate representation of the eletronic effects of different functional groups on reactivity (Coeur-Tourneur et al., 2010a;Lauraguais et al., 2012).This suggests that it is necessary to determine the rate constants of multifunctional organics through lab experiments.
The rate constant determined in this work can be used to calculate the atmospheric lifetime of eugenol with respect to its reaction with OH radicals.Assuming a typical [OH] for a 24 h average value to be 1.5 × 10 6 molecules cm -3 (Mao et al., 2009), the corresponding lifetime of eugenol was calculated to be 2.31 h with the average rate constant of 8.01 × 10 -11 cm 3 molecule -1 s -1 .This short lifetime indicates that eugenol is too reactive to be used as a tracer for wood smoke emissions, and also implies the possible fast conversion of eugenol from gas-phase to secondary aerosol during the transportation process.
The rate constant obtained in this work is about 2 orders of magnitude faster than that for eugenol with NO3 radicals (1.6 × 10 -13 cm 3 molecule -1 s -1 ) (Zhang et al., 2016),  Lauraguais et al., 2012).The reactivity of eugenol toward OH radicals is slightly higher than those of guaiacol and 2,6-dimethylphenol, while slightly slower than that of syringol.The presence of two methoxyl groups (−OCH3) in syringol activates the electrophilic addition of OH radicals to the benzene ring by donating electron density through the resonance effect (Lauraguais et al., 2016).The activation effect of the methoxyl group is much larger than those of alkyl groups (McMurry, 2004).In a recent study, the reported energy barrier of NO3 electrophilic addition to eugenol was about 2fold than that of 4-ethylguaiacol, indicating that the activation effect of the allyl group (−CH2CH=CH2) is lower than that of the ethyl group (−CH2CH3) (Zhang et al., 2016).
These results are in accordance with the activation effects of the substituants toward the electrophilic addition of OH radicals (McMurry, 2004).

Effects of eugenol concentration and OH exposure on SOA formation
In this work, a series of experiments were conducted in the OFR with different eugenol concentrations.The SOA yield was determined as the ratio of the SOA mass concentration (M0, μg m -3 ) to the reacted eugenol concentration (△[eugenol], μg m -3 ) (Kang et al., 2007).The experimental conditions and maximum SOA yields are listed in Table 2. Fig. S4 shows the plots of the SOA yield versus OH exposure at different eugenol concentrations.Higher concentrations resulted in higher amounts of condensable products and subsequently increased SOA yield (Lauraguais et al., 2012).SOA mass also directly influences the gas/particle partitioning, because SOA can serve as the adsorption medium for oxidation products, and higher SOA mass generally results in higher SOA yield (Lauraguais et al., 2012(Lauraguais et al., , 2014b).In the OFR, in all cases the SOA yield first increased and then decreased as a function of OH exposure (Fig. S4).This trend is the most common phenomenon observed in PAM reactor studies (Lambe et al., 2015;Ortega et al., 2016;Simonen et al., 2017).In this work, according to the SO2 decay in the presence of eugenol and the OFR exposure estimator (v2.3) developed by Jimenez's group based on the estimation equations reported in the previous work (Li et al., 2015;Peng et al., 2015Peng et al., , 2016)), the maximum reduction of OH exposure by eugenol in the OFR was approximately 30%.Although OH suppression by eugenol was not well determined in the OFR for the positive influence of SO2 on SOA formation, OH radicals were expected to be the main oxidant due to the fast reaction rate constant of eugenol toward OH radicals obtained in this work.The decrease of SOA yield at high OH exposure is possibly contributed from the C−C bond scission of gas-phase species by further oxidation or heterogeneous reactions involving OH radicals, which would generate a large amount of fragmented molecules that could not condense on aerosol particles (Lambe et al., 2015;Ortega et al., 2016;Simonen et al., 2017).
SOA yield can be described using a widely-used semi-empirical model on the basis of the absorptive gas-particle partitioning of semi-volatile products, in which the overall SOA yield (Y) is given by (Odum et al., 1996): where αi is the mass-based stoichiometric coefficient for the reaction producing the semi-volatile product i, Kom,i is the gas-particle partitioning equilibrium constant, and M0 is the total aerosol mass concentration.
The SOA yield data in Table 2 can be plotted in the form of Eq. ( 2) to obtain the yield curve for eugenol (Fig. 2).The simulation of experimental data indicated that an one-product model could accurately reproduce the data (R 2 = 0.98), while the use of two or more products in the model did not significantly improve the fitting quality.Odum et al. (1996) reported that the SOA yield data from the oxidation of aromatic compounds could be fitted well using a two-product model.However, an one-product model was also efficient for describing the SOA yields from the oxidation of aromatics including methoxyphenols (Lauraguais et al., 2012(Lauraguais et al., , 2014b;;Coeur-Tourneur et al., 2010b).The success of simulation with an one-product model in this work is likely to indicate that the products in SOA have similar values of αi and Kom,i, i.e., that the obtained αi (0.36 ± 0.02) and Kom,i (0.013 ± 0.002 m 3 ug -1 ) represent the average values.
In this work, considering that the composition of SOA was not determined, the volatility basis set (VBS) approach was not applied to simulate SOA yields.theoretical partition model (Lauraguais et al., 2012(Lauraguais et al., , 2014b)).In other words, SOA yield was approximately an upper limit for eugenol oxidation in the OFR.In view of the residence time in this work, it seems be in contradiction with the recommendation of longer residence time made by Ahlberg et al. (2017), who found that the condensation of low-volatility species on SOA in the OFR was often kinetically limited at low mass concentrations.In our recent experiments (not published), the SOA yields for guaiacol oxidation by OH radicals obtained under the similar experimental conditions as this work, could be comparable to those obtained in the chamber studies (Fig. S6).This suggests that the effect of kinetic limitations on SOA condensation for the OH-initiated oxidation of methoxyphenols in this system might be not important.
Elemental ratios (H/C and O/C) could provide insights into SOA composition and chemical processes along with aging (Bruns et al., 2015).As shown in Fig. 3, O/C ratio of SOA increases and H/C ratio decreases with increasing OH exposure, because oxygen-containing functional groups are formed in the oxidation products.In addition, the organic mass fractions of m/z 44 ( 2 CO  ) and m/z 43 (mostly 2 3 C H O  ), named f44 and f43, respectively, could also provide information about the nature of SOA formation.In this work, the OSC values obtained at low (272 μg m -3 ) and high (1328 μg m -3 ) concentrations of eugenol were compared.As shown in Fig. 3, OSC values for low concentration (0.035−1.78) were much larger than those for high concentration (0.0036−1.09), and increased linearly (R 2 > 0.96) with OH exposure of (1.21−12.55)× 10 11 molecules cm -3 s.The results are well supported by the evolution of SOA mass spectra obtained by HR-ToF-AMS at the same eugenol concentrations (Fig. S8).
Similar trends have been observed in the smog chamber and PAM reactor (Simonen et al., 2017;Ortega et al., 2016).The OSC value in this work extends as high as 1.78, which is in good agreement with that observed for ambient LV-OA, up to 1.9 (Kroll et al., 2011) reducing hydrogen in the molecules in the particle-phase to increase OSC values despite the overall loss of SOA mass (Ortega et al., 2016).

Effect of SO2 on SOA formation
As shown in Fig. 4, the presence of SO2 favored SOA formation, and the sulfate concentration increased linearly (R 2 = 0.99) as a function of OH exposure.The maximum SOA yield enhancement of 38.57% was obtained at OH exposure of 5.41 × 10 11 molecules cm -3 s, and then decreased with the increase of OH exposure due to the fragmented molecules formed through the oxidation of gas-phase species by high OH exposure (Lambe et al., 2015;Ortega et al., 2016;Simonen et al., 2017).The SOA yield and sulfate concentration both increased linearly (R 2 > 0.97) as SO2 concentration increased from 0 to 198 ppbv at OH exposure of 1.21 × 10 11 molecules cm -3 s (Fig. S9).
Compared to the initial SOA yield (0.049) obtained in the absence of SO2, the SOA yield (0.066) obtained in the presence of 198 ppbv SO2 was enhanced by 34.69%.In previous studies, Kleindienst et al. (2006) reported that the SOA yield from α-pinene photooxidation increased by 40% in the presence of 252 ppbv SO2; Liu et al. (2016b) recently found that the SOA yield from 5 h photochemical aging of gasoline vehicle exhaust was enhanced by 60−200% in the presence of ~150 ppbv SO2.
As shown in Figs. 4 and S7, the increase of sulfate concentration was favorable for SOA formation.In this system, it is difficult to completely remove trace NH3, thus the 3 NO  −H2O systems (http://www.aim.env.uea.ac.uk/aim/model2 /model2 a.php; Liu et al., 2016b).The detailed description of the calculation method has been represented elsewhere (Liu et al., 2016b).The elevated concentration of sulfate in the particle phase with the increases of SO2 concentration and OH exposure is an important reason for the enhanced SOA yields (Kleindienst et al., 2006;Liu et al., 2016b).Cao and Jang ( 2007) indicated that SOA yields from the oxidation of toluene and 1,3,5trimethylbenzene increased by 14−36% in the presence of acid seeds, with [H + ] of 240−860 nmol m -3 compared to those obtained in the presence of nonacid seeds.Similar results concerning the effect of particle acidity on SOA yields were reported in other studies (Kleindienst et al., 2006;Liu et al., 2016b;Jaoui et al., 2008;Xu et al., 2016).
However, Ng et al. (2007b) found that particle acidity had a negligible effect on SOA yields from photooxidation of aromatics, possibly due to the low RH (~5%) used in their work.The water content of aerosol plays an essential role in acidity effects (Cao and Jang, 2007).Under acidic conditions, the gas-phase oxidation products of eugenol would be partitioned more quickly into the particle-phase and further oxidized into low volatility products, or produce oligomeric organics by acid-catalyzed heterogeneous reactions, subsequently enhancing SOA yields (Cao and Jang, 2007;Jaoui et al., 2008;Liu et al., 2016b;Xu et al., 2016).In addition, the formed sulfate not only serves as the substrate for product condensation and likely participates in new particle formation (NPF) (Jaoui et al., 2008;Wang et al., 2016), but also enhances the surface areas of particles to facilitate heterogeneous reactions on aerosols (Xu et al., 2016).These roles of sulfate are also favorable for increasing SOA yields.Recently, Friedman et al. (2016) have indicated that SO2 could participate in the oxidation reactions of α-and β-pinene and perturbs their oxidation in the OFR, but this possible effect could be ignored in this work due to the relatively high RH and the negligible S/C ratio observed by HR-ToF-AMS (Friedman et al., 2016).

Effect of NO2 on SOA formation
It is well known that high NOx concentration almost always plays a negative role in NPF and SOA formation because the reaction of NO with RO2 radicals results in the formation of more volatile products compared to the reaction of HO2 with RO2 radicals (Sarrafzadeh et al., 2016).Previous studies reported that nitro-substituted products were the main products for SOA formed from OH-initiated reactions of phenol precursors including methoxyphenols, in the presence of NOx (Finewax et al., 2018;Ahmad et al., 2017;Lauraguais et al., 2012Lauraguais et al., , 2014b)).Thus, the effect of NO2 on SOA formation from eugenol oxidation by OH radicals was investigated.As shown in Fig. 5, the nitrate concentration measured by HR-ToF-AMS increased as a function of OH exposure in the presence of 40 ppbv NO2, but it was much lower than the sulfate concentration (Fig. 4) even though the OH rate constant for NO2 was faster than that for SO2 (Davis et al., 1979;Atkinson et al., 1976).The possible explanation is that the formed HNO3 mainly exists in the gas phase, and the relatively high temperature (301 ± 1 K) is not favorable for gaseous HNO3 distribution in the particle phase (Wang et al., 2016).It has been indicated that the temperature range for the greatest loss of nitrate is 293−298 K (Keck and Wittmaack, 2005).As illustrated in Fig. 5, the SOA yield enhancement and N/C ratio both increased firstly and then decreased with rising OH exposure.The increase of NO2 concentration (40−109 ppbv) is beneficial to SOA yields (0.053−0.062),N/C ratio (0.032−0.041), and nitrate formation (4.29−6.30μg m -3 ) (Fig. S10).Compared to the presence of 41 ppbv SO2, the maximum SOA yield enhancement (19.17%) in the presence of 40 ppbv NO2 is lower.For most aromatic precursors, the addition of ppbv levels of NO2 should have a negligible effect on SOA formation, because the rate constants of OH-aromatic adducts with O2 and NO2 are on the order of approximate 10 - 16 and 10 -11 cm 3 molecule -1 s -1 , respectively (Atkinson and Arey, 2003).But, for phenol precursors only about 0.5 ppbv NO2 is enough to compete with O2 for the reaction with OH-aromatic adducts (Finewax et al., 2018).Therefore, the enhancement effect of NO2 on SOA formation might be relevant to the special case of phenols or methoxyphenols but not other aromatic precursors.
It is noteworthy that the N/C ratio is in the range of 0.032−0.043,suggesting that NO2 participated in the OH reaction of eugenol, through the addition to the OH-eugenol adduct (Peng and Jimenez, 2017).Recently, Hunter et al. (2014) found that NO2 participated in the OH reactions of yclic alkanes, and the N/C ratios were in the range of 0.031−0.064,higher than those obtained in this work.The nitro-substituted products are reported to be the main reaction products of the OH reactions of guaiacol and syringol in the presence of NO2 (Lauraguais et al., 2014b;Ahmad et al., 2017).The Ncontaining products might be also formed through the reactions involving with NO3 radicals, which are possibly generated by the reaction between NO2 and O3 in this system (Atkinson, 1991).But, the specific contribution of NO3 radicals could not be quantified in this work.The relative low volatility of these products could reasonably contribute to SOA formation (Duporté et al., 2016;Liu et al., 2016a).In addition, higher NO2/NO ratio favors the formation of nitro-substituted products, which are potentially involved in NPF and SOA growth (Pereira et al., 2015).Ng et al. (2007a) also indicated that NOx could be beneficial to SOA formation for sesquiterpenes, due to the formation of low volatility organic nitrates and the isomerization of large alkoxy radicals, resulting in less volatile products.The decrease in the N/C ratio at high OH exposure suggested that more volatile products were generated through the oxidation of particle-phase species by OH radicasls.
The NO  / 2 NO  ratios measured by HR-ToF-MS are widely used to identify inorganic and organic nitrates.The NO  / 2 NO  ratios for inorganic nitrates have been reported to be in the range of 1.08−2.81(Farmer et al., 2010;Sato et al., 2010).The ratio ranged from 2.06 to 2.54 in this work as determined by HR-ToF-AMS using ammonium nitrate as the calibration sample.However, the NO  / 2 NO  ratios during oxidation of eugenol in the presence of 40 ppbv NO2 were 3.98−6.09.They were higher than those for inorganic nitrates and consistent with those for organic nitrates (3.82−5.84)from the photooxidation of aromatics (Sato et al., 2010).The abundance of organic nitrates could be estimated from the N/C ratios determined in this work.
Assuming that the oxidation products in the SOA retain 10 carbon atoms, the yields of organic nitrates are in the range of 32−43%, which are comparable to those reported in earlier studies (Liu et al., 2015;Hunter et al., 2014).Liu et al. (2015) reported that the nitrogen-containing organic mass contributed 31.5 ± 4.4% to the total SOA derived from m-xylene oxidation by OH radicals.Hunter et al. (2014) estimated the organic nitrate yields of SOA to be 31−64%, formed in the OH-initiated reactions of acyclic, monocyclic, and polycyclic alkanes.This range obtained in this work should be the upper limit due to the possibility of C−C bond scission of gas-and particle-phase organics oxidized by high OH exposure.Besides, the maximum yield of nitrates for a single reaction step is expected to be approximately 30% (Ziemann and Atkinson, 2012), this suggests that multiple reaction steps are needed.

Atmospheric implications
Biomass burning not only serves as a major contributor of atmospheric POA, but also has great SOA formation potential through atmospheric oxidation (Bruns et al., 2016;Gilardoni et al., 2016;Li et al., 2017;Ciarelli et al., 2017;Ding et al., 2017).Recent studies have indicated that SOA formed from biomass burning plays an important role in haze pollution in China (Li et al., 2017;Ding et al., 2017).Residential combustion (mainly wood burning) could contribute approximately 60−70% to SOA formation in winter at the European scale (Ciarelli et al., 2017).In addition, methoxyphenols are the major component of OA from biomass burning (Bruns et al., 2016;Schauer and Cass, 2000).Based on our results and those of previous studies (Sun et al., 2010;Lauraguais et al., 2012Lauraguais et al., , 2014b;;Ahmad et al., 2017;Yee et al., 2013;Ofner et al., 2011), it should pay more attenion on the SOA formation from the OH oxidation of biomass burning emissions and its subsequent effect on haze evolution, especially in China with nationwide biomass burning and high daytime average [OH] in the ambient atmosphere ((5.2−7.5)× 10 6 molecules cm -3 ) (Yang et al., 2017).Meanwhile, the potential contributions of SO2 and NO2 to SOA formation should also be taken into account, because the concentrations of NOx and SO2 could be up to close 200 ppbv in the severely polluted atmosphere in China (Li et al., 2017).Although eugenol concentrations in this work are higher than those in the ambient atmosphere, the results obtained in this work could provide new information for SOA formation from the atmospheric oxidation of methoxyphenols, and might be useful for SOA modeling, especially for air quality simulation modeling of the specific regions experiencing serious pollution caused by fine particulate matter.
N-containing products formed from the oxidation of methoxyphenols could contribute to water-soluble organics in SOA (Lauraguais et al., 2014b;Yang et al., 2016;Zhang et al., 2016), which have been widely detected in atmospheric humic-like substances (HULIS) (Wang et al., 2017).Due to their surface-active and UV-lightabsorbing properties, HULIS could influence the formation of cloud condensation nuclei (CCN), solar radiation balance, and photochemical processes in the atmosphere  (Wang et al., 2017).The high reactivity of methoxyphenols toward atmospheric radicals suggests that SOA was formed from their oxidation processes with relatively high oxidation level, subsequently leading to SOA with strong optical absorption and hygroscopic properties (Lambe et al., 2013;Massoli et al., 2010).Therefore, SOA formed from the reactions of methoxyphenols with atmospheric oxidants might have important effects on air quality and climate.In addition, the experimental results from this study would help to further the understanding of the atmospheric aging process of smoke plumes from biomass-burning emissions.

Conclusions
For the first time, the rate constant and SOA foramtion for the gas-phase reaction of eugenol with OH radicals were investigated in an OFR.The second-order rate constant of eugenol with OH radicals was (8.01 ± 0.40) × 10 -11 cm 3 molecule -1 s -1 , measured by the relative rate method, and the corresponding atmospheric lifetime was 2.31 h.In  e Corresponding OH exposure of maximum SOA yields.
f Corresponding atmospheric aging time of maximum SOA yields, calculated using a typical [OH] in the atmosphere in this work (1.5 × 10 6 molecules cm -3 ) (Mao et al., 2009).
Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-749Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 10 September 2018 c Author(s) 2018.CC BY 4.0 License.An Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) was applied to perform online measurement of the chemical composition of particles and the non-refractory submicron aerosol mass (DeCarlo et al., 2006).The size distribution and concentration of particles were monitored by a scanning mobility particle sizer (SMPS), consisting of a differential mobility analyzer (DMA) (Model 3082, TSI Inc.) and a condensation particle counter (CPC) (Model 3776, TSI Inc.).
Fig.S5shows a plot of the SOA mass concentration (M0) versus the reacted eugenol concentration(△[eugenol]). Its slope was 0.37 as obtained using linear least-squares fitting, which is very close to the αi value (0.36).This suggests that the low-volatility products formed in the reaction almost completely disperse on the particle phase according to the Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-749Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 10 September 2018 c Author(s) 2018.CC BY 4.0 License.
addition, the significant SOA formation of eugenol oxidation by OH radicals was observed.The maximum SOA yields (0.11−0.31) obtained at different eugenol concentrations could be expressed well by an one-product model.SOA yield was dependent on OH exposure and eugenol concentration, which firstly increased and then decreased as a function of OH exposure due to the possible C−C bond scission of gasphase species by further oxidation or heterogeneous reactions involving OH radicals.The OSC and O/C ratio both increased significantly as a function of OH exposure, suggesting that SOA became more oxidized.The presence of SO2 and NO2 was helpful Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-749Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 10 September 2018 c Author(s) 2018.CC BY 4.0 License. to increase SOA yield, and the maximum enhanced yields were 38.57% and 19.17%, respectively.The observed N/C ratio of SOA was in the range of 0.032−0.043,indicating that NO2 participated in the OH-initiated reaction of eugenol, consequently producing organic nitrates.The experimental results might be helpful to further understand the atmospheric chemical behavior of eugenol and its SOA formation potential from OH oxidation in the atmosphere.

Figure 1 .
Figure 1.Relative rate plots for gas-phase reaction of OH radicals with eugenol.

Figure 2 .
Figure 2. Maximum SOA yield as a function of SOA mass concentration (M0) formed

Figure 3 .
Figure 3. OSC, H/C, and O/C vs. the OH exposure for SOA formed at two eugenol

Figure 4 .Figure 5 .
Figure 4. Evolution of the enhanced SOA yield and sulfate formation as a function of
b c Calculated using US EPA AOP WIN model (US EPA, 2012).

Table 2 .
Experimental conditions and results.