Formation kinetics and mechanism of ozone and secondary organic aerosols from photochemical oxidation of different aromatic hydrocarbons: dependence of NOx and organic substituent

. Aromatic hydrocarbons (AHs) contribute significantly to ozone and secondary organic aerosol (SOA) formation in atmosphere, but formation mechanisms are still unclear. Herein, photochemical oxidation of nine AHs was investigated in 15 chamber. Only small amount of ozone was produced from direct photochemical oxidation of AHs, while fewer AH substituent number resulted in higher concentrated ozone. Addition of NOx increased ozone and SOA production. Synergetic effect of accelerated NO2 conversion and NO reaction with AHs boosted ozone and volatile intermediate formation. Promoting AH concentration in VOC/NOx ratio further increased formation rates and concentrations of both ozone and SOA. Additionally, ozone formation was enhanced with increasing AH’s substituent number but negligibly 20 affected by their substituent position. Differently, SOA yield decreased with increased substituent number of AHs, but increased with ortho methyl group substituted AHs. Model fitting and intermediate consistently confirmed that increasing substituent number on phenyl ring inhibited generating dicarbonyl intermediates, which however were preferentially produced from oxidation of ortho methyl group substituted AHs, resulting in different changing trend of SOA yield. The restrained oligomerization by increased substituent number was another main cause for decreased SOA yield. These results 25 are helpful to understand photochemical transformation of AHs to secondary pollutants in real atmosphere. to study their photochemical oxidation behavior in an indoor smog chamber system, to compare the formation activity in O 3 and SOA. The influences of NO x , AH concentration and AH substituent on the formation kinetics of O 3 and SOA were studied in detail. All volatile intermediates were qualitatively and quantitatively online analyzed to propose their potential contribution to the formation of O 3 and SOA. The relationship between AH structure, intermediate, and production of O 3 and SOA were established to reveal the transformation mechanism of AHs to O 3 and SOA. The results of this work will further elucidate the photochemical behavior of AHs in the atmosphere and provide reliable experimental data for modeling and prediction in the substituted AHs exhibited higher SOA yield. The preferential formation of variation of dicarbonyl intermediates and restrained oligomerization reaction were responsible for above differences. These results showed more clear understanding of the effect of NO x and organic molecule structures on photochemical oxidation of AHs to form O 3 and SOA, which could provide solidly experimental basis for studying the transformation of AHs to secondary pollutants in the real atmospheric environment.

3776, TSI Inc., USA). The velocities of sheath gas and aerosol flows were set at 3.0 and 0.3 L min -1 , respectively. Under this 95 setting, the particle size range was observed from 13.8 to 723.4 nm. The yield calculation formula of SOA was given in SI.

Formation kinetics and mechanism of O3 and SOA without NOx
The directly photochemical oxidation of nine AHs was first conducted to evaluate the formation potential of O3 and SOA.
Furthermore, to figure out the formation mechanism of O3 from direct AH oxidation, the correspondingly volatile 110 intermediates were also monitored. Toluene was taken as an example to illustrate the concentration variation of volatile intermediates. As Fig. 1b shows, with decrease of toluene's concentration from 996 to 944.5 ppb, the concentrations of nine intermediates increased at different degrees. The concentrations of m/z = 45 (m45, acetaldehyde), m/z = 47 (m47, formic acid) and m/z = 61 (m61, acetic acid or glycolaldehyde) increased faster than others, and peaked at 5 − 9 ppb within 450 min, indicating easily oxidation of toluene to small molecular carbonyl products. The peak concentration of m/z = 99 (m99, 3-115 methyl-2(5H)-furanone or 4-keto-2-pentenal) reached 4.2 ppb, while the productions of m/z = 31 (m31, formaldehyde), m/z = 59 (m59, glyoxal), m/z = 73 (m73, methylglyoxal) were at the same level (ca. 2.4 ppb). The concentrations of some intermediates including m/z = 85 (m85, butanedione), m/z = 87 (m87, butenedione) and m/z = 111 (m111, hexene diketone) were lower than 0.8 ppb within 450 min's reaction duration. Similar variation trends of volatile intermediates were observed from other eight AHs (Fig. S1). 120 It was worth mentioning that most of above intermediates were well-known precursors of O3 and SOA (Li et al., 2016;Ji et al., 2017;Nishino et al., 2010). However, the formation of SOA was not observed in this study. Two reasons might be involved. Since this study was carried out at low RH (< 5%) and without seed particles, no SOA precursor oligomers existed. Furthermore, the concentrations of produced intermediates were too low to trigger the initial nucleation reaction and then generate SOA under low RH condition. Therefore, SOA formation could not be observed in the NOx-free 125 photochemical oxidation of these nine AHs. In general, tropospheric O3 was mainly from NO2 photolysis and the existence https://doi.org/10.5194/acp-2021-29 Preprint. Discussion started: 25 January 2021 c Author(s) 2021. CC BY 4.0 License.
of AHs could enhance O3 formation. However, when the absent of NOx in this study, the low concentrated O3 was observed from AH photochemical oxidation. The possible contributors of these O3 might be intermediates such as carbonyl compounds. In all, our results indicated that direct photochemical transformation of AHs to O3 actually occurred and should be taken into consideration in the atmospheric environment. 130

Formation kinetics and mechanism of O3 in the presence of NOx
To further explore the role of NOx in O3 formation during photochemical oxidation of AHs, about 160 ± 10 ppb of NO2 was added into the reactor. Under this condition, the generated O3 was found significantly increasing and the O3 peak concentrations ranged from 230 to 440 ppb within 100 to 250 min (Fig. 2). All these data were about 200 − 400 ppb higher than those obtained in the absence of NOx (Fig. 1a), indicating quick enhancement of NOx to O3 formation. In this work, the 135 added NO2 was firstly photolyzed under 360 nm's light irradiation to from NO and O( 3 P) (Eq. 1). Then, the latter was oxidized to form O3 (Eq. 2), which further reacted with NO to form NO2 (Eq. 3). Meanwhile, AHs were photochemically oxidized to form RO2 and HO2, both of which then reacted with NO to form NO2 (Eqs. 4 and 5). Clearly, the presence of Furthermore, the effect of AH content on the O3 formation in the presence of NOx (e.g., VOC/NOx ratio) was investigated. Here, the concentration of NOx was maintained constantly and that of AH was gradually increased. For toluene ( Fig. 3a), O3 peak concentration of 250 ppb was achieved after reaction for 420 min under VOC/NOx ratio of 2.47. When increasing VOC/NOx ratio to 6.29, the time needed achieving higher peak O3 concentration of 280 ppb was shortened to be 150 min. All these data confirmed that O3 formation rate and concentration were both accelerated with increased AH 150 concentration. Similar results of shorter reaction time leading to higher O3 concentration were observed for the rest AHs (Fig.   S2). Increasing AH concentration would result in the enhanced formation of RO2 and HO2, both of which reacted with NO to save the O3 consumption. Meanwhile, the photolysis of NO2 to form NO and then O3 was also accelerated. Both of these reasons were responsible for the fast-enhanced formation of O3 with the increased AH concentration in VOC/NOx ratio.
To study the effect of AH's substituent on O3 formation, the O3 peak concentrations of nine AHs obtained at the same 155 VOC/NOx ratio were compared. As Fig. 3b shows, the O3 peak concentrations of nine AHs followed the order of TMB ppb). Clearly, the O3 peak value was positively correlated with the number of AH's substituent, suggesting AHs with more substituent possessed higher O3 production potential at the same VOC/NOx ratio. In previous studies, O3 concentrations from AH oxidation with the presence of NOx were reported as follows: 210 − 320 ppb for benzene, 160 − 300 ppb for toluene, 550 160 − 700 ppb for ethylbenzene, 400 ppb for m-xylene, 300 − 600 ppb for o-xylene, 300 − 350 ppb for p-xylene, 340 − 470 ppb for 123-TMB, 500 ppb for 124-TMB and 400 − 700 ppb for 135-TMB (Wang et al., 2016;Luo et al., 2019;Li et al., 2018;Xu et al., 2015;Jia and Xu, 2013;Lu et al., 2009;Carter, 1997). However, these studies only focused on one or several AHs, and the relationship between AH substituent and O3 formation was still not understood. Our results of O3 concentration were comparable to those in the previous studies. Furthermore, the results obtained in this study clearly confirmed that increasing 165 substituent number of AH correspondingly increased O3 concentration. It was also noticed that the O3 peak concentrations of xylene or TMB isomers were in the same range, suggesting negligible effect of substituent position of AHs to their O3 formation.

Accelerated formation of SOA in the presence of NOx
Besides O3, the effect of AH concentration on formation kinetics of SOA with the presence of NOx was also investigated. As 170 Fig. 4 shows, from photochemical oxidation of toluene, the peak number concentration of SOA increased from 2.0 × 10 4 to 5.5 × 10 4 particle cm -3 with increase of VOC/NOx ratio from 2.37 to 5.58. The time achieving above concentration was shortened from 250 to 120 min, while the median particle size range also increased from 300 − 400 to 400 − 500 nm. Similar results of shorter time leading to higher concentration and larger particle size for SOA were observed for other eight AHs with the increasing VOC/NOx ratio (Figs. S3-S10). 175 Previous studies reported the enhanced SOA yield by increased NOx concentration (Zhao et al., 2018;Hurley et al., 2001;Song et al., 2007;Sarrafzadeh et al., 2016). This was because that NOx mainly influenced the distribution of oxidation products by affecting the RO2 reaction equilibrium, where RO2 easily converted to low-volatile ROOH or ROOR and thus resulted in the nucleation of new particles (Sarrafzadeh et al., 2016). However, in this study, we kept the NOx concentration unchanged, and modified the initial concentration of AHs. The increased AHs could lead to promoted RO2 formation, 180 resulting in more low-volatile products formation. The accumulation of low-volatile products promoted the nucleation of particulate matter and finally increased the yield of SOA.
The particle number and mass concentrations of SOA generated from nine AHs were further compared to evaluate the effect of AH's substituent on the SOA formation. With the increase of substituent number, the number concentration of SOA decreases (e.g., from 6.9 × 10 3 particle m -3 for 135-TMB to 7.8 × 10 4 particle m -3 for toluene) (Fig. 5a). With the progress of 185 the reaction, the mass concentration of SOA increased, and the increase of substituent number shortened the time achieving the peak mass concentration (Fig. 5b). These results revealed that the increase of substituent number of AHs increased SOA's mass concentration but decreased its particle number. AHs with different substituent position also showed different SOA formation characteristics. For xylene, the peak mass concentration of o-xylene (88.6 μg m -3 ) was higher than that of mxylene and p-xylene, while the peak mass concentration of 123-TMB (82.0 μg m -3 ) was significantly higher than those of 124-TMB (31.8 μg m -3 ) and 135-TMB (27.6 μg m -3 ) (Fig. 5b). These phenomena indicated that xylene and TMB with ortho methyl substituent facilitated the SOA formation. Further, the ortho methyl group of isomers (e.g. o-xylene, 123-TMB) could more thoroughly be oxidized, producing more particles (Sato et al., 2010).
It has also been reported that seed particles (e.g., NaCl) and highly relative humidity (up to 90%) can significantly increase the yield of SOA (Wang et al., 2016;Luo et al., 2019;Jia and Xu, 2018). However, in this study, the maximum SOA 195 yield of 25% (Fig. S11) was produced with increasing AH concentration, which was consistent with that from previous researches (Sato et al., 2012;Li et al., 2016;Song et al., 2007;Odum et al., 1997). Further considering the oxidation conditions of low RH (less than 5%) and seedless particles in this study, our results indicated that AH concentration should also be paid much attention to SOA formation although the addition of NOx, seed particles and high RH are all very

important. 200
To further investigate the effect of AH's substituent on SOA yield, a two-product semi-empirical model was employed.
As Fig. S12 shows, the model well fitted the SOA yield of nine AHs and the correspondingly fitting parameters are listed in Table 1. Similarly, high-volatile components were assumed from the photochemical oxidation of AHs and same Kom,2 of 0.005 m 3 μg -1 was assigned. As seen from the table, benzene (0.242 m 3 μg -1 ), toluene (0.162 m 3 μg -1 ) and ethylbenzene (0.422 m 3 μg -1 ) showed higher α2 than that of xylenes (e.g., 0.086 m 3 μg -1 for m-xylene) and TMBs (e.g., 0.082 m 3 μg -1 for 205 123-TMB), indicating the production of more high-volatile products from AHs with less number of substituent. Meanwhile, benzene (0.022 m 3 μg -1 ), toluene (0.027 m 3 μg -1 ) and ethylbenzene (0.023 m 3 μg -1 ) displayed lower Kom,1 in comparison with xylenes (e.g., 0.074 m 3 μg -1 for p-xylene) and TMBs (e.g., 0.085 m 3 μg -1 for 135-TMB), and the corresponding α1 decreased with increasing substituent number. All these results demonstrated that the increase of substituent number on phenyl ring inhibited the generation of low-volatile products, thus reducing the generation of SOA particles, finally leading to the 210 decrease of SOA yield. The result also indicated that the oxidation degree became lower and lower for AHs with increased substituent number, since the oxidation of methyl carbon was more difficult than that of carbon of phenyl ring. Li et al.
reported similar phenomenon at early time, (Li et al., 2016) and is consistent with our results. However, they did not further investigate the relationship of isomer AH with the SOA yield.
In the present study, SOA yield of o-xylene was found higher than that of m-xylene and p-xylene, consistent with the 215 SOA number and mass results. The fitting results showed that the Kom,1 of o-xylene (0.024 m 3 μg -1 ) was much lower than that of m-xylene (0.057 m 3 μg -1 ) and p-xylene (0.074 m 3 μg -1 ), indicating the production of more low-volatile products from oxylene. Similarly, 123-TMB showed the highest SOA yield and lowest Kom,1 among three TMBs. These results further confirmed that AHs with ortho methyl substituent favored the yield of SOA. This might be because that these AHs were more susceptible to be oxidized, formed ring-opening products and finally produced more RO2 than other isomers. Some 220 previous studies have also obtained consistent results (Zhou et al., 2011;Song et al., 2007). In addition, ethylbenzene was also isomeric to xylene, and its SOA yield was higher than that of xylenes. Recent studies have found that the SOA yield during the oxidation of alkanes and alkenes by • OH increased with the carbon chain length (Lim and Ziemann, 2009;Tkacik et al., 2012). Obviously, the length of carbon chain also affected the oxidation degree of AHs. Then, the longer ethyl group https://doi.org/10.5194/acp-2021-29 Preprint. Discussion started: 25 January 2021 c Author(s) 2021. CC BY 4.0 License. led to a higher degree of photochemical oxidation for ethylbenzene than xylenes, promoting the formation of more SOA 225 precursors and finally higher SOA yield.

Enhanced formation mechanism of SOA with NOx
In order to further reveal the enhanced formation mechanism of SOA from the oxidation of AHs with the presence of NOx, the corresponding volatile intermediates were all identified, and quantified comparably. As Fig. 6a shows, with gradual decrease of toluene concentration, the concentrations of small molecule carbonyl products, such as m31 (formaldehyde), 230 m45 (acetaldehyde), m47 (formic acid) and m61 (acetic acid or glycolaldehyde), quickly increased to 16.0, 45.3, 31.0 and 17.0 ppb within 200 min. Acetaldehyde showed the highest concentration, which was by far higher than that obtained without NOx (9 ppb). Followed one were m85 (butanedione), m87 (butenedione) and m111 (hexene diketone) with the peak concentration below 2.8 ppb. The increase of concentration of m59 (glyoxal), m73 (methylglyoxal) and m99 (3-methyl-2(5H)-furanone or 4-keto-2-pentenal) began to slow down after 120 min, and this trend was consistent with the trend of SOA 235 formation (Fig. 5b). Similar variation trend of volatile intermediates for other AHs was also measured (Fig. S13). All these results demonstrated that there was a specific window period, and the intermediates in the gaseous phase were transformed into the particulate phase. The significant increase of SOA occurred after breaking through the window period.
Further comparison of volatile intermediates during photochemical oxidation of AHs with different VOC/NOx ratio was also carried out. For toluene oxidation ( Fig. 6b and 6c), the concentrations of all intermediates increased with the increase of 240 VOC/NOx ratio. The carbonyl intermediates such as m59 (glyoxal) and m73 (methylglyoxal) were believed to be playing important role in the photochemical oxidation of AHs to form SOA (Li et al., 2016;Ji et al., 2017;Nishino et al., 2010). Bloss et al. also found glyoxal and methylglyoxal produced from toluene photochemical oxidation as the main precursor of SOA (Bloss et al., 2005b). In previous studies, the maximum yield of glyoxal and methylglyoxal were obtained as 20% and 17% during toluene photochemical oxidation (Baltaretu et al., 2009;Volkamer et al., 2001;Nishino et al., 2010), which was lower 245 than that of the present study (24% in Fig. 6c). Meanwhile, the yields of glyoxal and methylglyoxal during photochemical oxidation of toluene, xylenes and TMBs increased with increasing AH concentration. Therefore, the increase of AH content in reaction system promoted the photochemical oxidation of AHs to produce more volatile carbonyl intermediates, finally leading to higher SOA yield in this study. Moreover, the yield of m59 (glyoxal) and m73 (methylglyoxal) from benzene photochemical oxidation was found the lowest among all AHs (Figs. 6b, 6c and S14-S22), indicating that the presence of 250 branch chain on phenyl ring inhibited the production of unsaturated carbonyl compounds. This was because that increasing methyl group number of AHs weakened their oxidation reactivity, resulting in the inhibition of ring-opening reaction.
Further, the formation of glyoxal and methylglyoxal from RO2 were also subsequently suppressed. Furthermore, the yields of m85 (butanedione) from AHs containing ortho methyl group (e.g., o-xylene, 123-TMB and 124-TMB) were found higher than that of their isomers, due to that ortho methyl groups of phenyl ring preferred to ring opening and then generation of 255 ketone products (Li et al., 2016). Based on the above results, the possible photochemical oxidation mechanism from AH to SOA were proposed. Toluene was selected as an example. As Fig. 7 shows, phenyl ring of toluene firstly reacted with • OH to produce cresol (Ziemann, 2011;Atkinson, 2007), and then further oxidized to form an intermediate which could occur ring-opening reaction or react with HO2 radicals to form bicyclic peroxide compounds. The latter has been suggested as important SOA precursor from AH 260 photochemical oxidation (Song et al., 2005;Wyche et al., 2009;Nakao et al., 2011;Johnson et al., 2005). The ring-opening intermediates were consisted of saturated and unsaturated dicarbonyl compounds (Jang and Kamens, 2001;Birdsall and Elrod, 2011). However, the possibility of these dicarbonyl intermediates directly partitioning into the particulate phase was very small (Jang and Kamens, 2001), but they could oligomerize to form low-volatility compounds (Forstner et al., 1997;Jang and Kamens, 2001). The oligomerization was an important pathway for SOA formation from AH photochemical 265 oxidation (Sato et al., 2012;Li et al., 2016;Hu et al., 2007). In our study, the detection of m85 (butanedione) and m99 (3methyl-2(5H)-furanone or 4-keto-2-pentenal) proved the formation of ring-opening products. These unsaturated 1,4dicarbonyls were observed to form small cyclic furanone compounds (Jang and Kamens, 2001;Bloss et al., 2005b).
Therefore, the ring-opening products with saturated or unsaturated dicarbonyl groups finally transformed into SOA through oligomerization process. 270 As mentioned above, with the increase of the substituent number on AHs, the yield of SOA decreased. The enhanced ring-opening products and restrained oligomerization reactions by the increased methyl group number were supposed to be the main cause. The methyl group is found to stabilize the ring-opening radicals (Ziemann, 2011). When phenyl ring contained methyl group, the oxidation pathway was prone to ring-opening. The concentrations of m87 and m111 increased with the methyl group number increasing (Fig. S23), meaning that these two intermediates were dominant in the ring-275 opening products. However, they could not oligomerize to further partition into SOA formation (Fig. 7). Both non-cyclic dicarbonyls and cyclic compounds formed by unsaturated dicarbonyls were deemed to have small probability to oligomerize (Li et al., 2016;Kalberer et al., 2004). In previous study, no butanedione and hexene diketone were detected in particulate phase SOA under 300 ± 1 K and dry conditions (RH < 0.1%) in the absence of inorganic seed aerosol (Li et al., 2016).
However, butanedione was measured in gas phase of this study, indicating that it was not the precursor of SOA for the 280 oligomerization reaction. The presence of methyl groups would inhibit the oligomerization to prevent the formation of ring compounds by unsaturated dicarbonyl groups and finally decrease SOA formation.

Conclusions
In this study, no SOA formation was observed from direct photochemical oxidation of AHs, while a small amount of O3 was produced without NOx addition. The presence of NOx significantly increased the productions of O3 and SOA, due to 285 synergetic effect of accelerated NO2 conversion and AH reaction with NO as well as enhanced formation of volatile intermediates. Further increased formation of both O3 and SOA were observed by promoted AH concentration. In addition, increase of AH's substituent number could enhance O3 formation, but decrease SOA yield. The ortho methyl group