The formation of nitro-aromatic compounds under high NO x-1 anthropogenic VOCs dominated atmosphere in summer in Beijing , 2 China 3

Abstract. Nitro-aromatic compounds (NACs), as important contributors to ultraviolet light absorption by brown carbon, have been widely observed in various ambient atmospheres, however, few field studies has been focused on their formation in urban atmospheres. In this work, NACs in Beijing were comprehensively quantified and characterized in summer, along with major components in fine particulate matter and selected volatile organic compounds. Field observations in this high NOx-anthropogenic VOCs dominated urban atmosphere were analyzed to investigate the NAC formation and influence factors. The total concentration of quantified NACs was 6.63 ng/m3, higher than other summertime studies (0.14–6.44 ng/m3). 4-Nitrophenol (4NP, 32.4 %) and 4-nitrocatechol (4NC, 28.5 %) were the most abundant ones among all the quantified NAC species, followed by methyl-nitrocatechol (MNC), methyl-nitrophenol (MNP) and dimethyl-nitrophenol (DMNP). The oxidation of toluene and benzene in the presence of NOx were found to be more dominant sources of NACs than biomass burning emissions. The NO2 level was an important factor influencing the secondary formation of NACs. A transition from low- to high-NOx regimes coincided with a shift from organic- to inorganic-dominated oxidation products. The transition thresholds were NO2∼20 ppb for daytime and NO2∼25 ppb for nighttime conditions. Under low-NOx conditions, NACs increased with NO2, while the NO3− concentrations and (NO3−)/NACs ratios were lower, implying organic-dominated products. Above the NOx regime transition values, NO2 was excess for the oxidation of ambient VOCs. Under this condition, NAC concentrations did not further increase obviously with NO2, while the NO3− concentrations and (NO3−)/NACs ratios showed significant increasing trends, when shifting from organic- to inorganic-dominated products. Obvious nighttime enhancements of 3M4NC and 4M5NC, daytime enhancements of 4NP, 2M4NP and DMNP indicated their different formation pathways. The aqueous-phase oxidation was the major formation pathways of 4M5NC and 3M5NC, and photo-oxidation of toluene and benzene in the presence of NO2 could be more important for the formation of nitrophenol and its derivatives. Thus, the (3M4NC + 4M5NC)/4NP ratios was employed to indicate the relative contribution of aqueous-phase and gas-phase oxidation to NAC formation. The relative contribution of aqueous-phase pathways was observed to increase at elevated ambient RH and remain constant at RH > 30 %. In addition, the concentrations of VOC precursors (e.g. toluene and benzene) and aerosol surface area were also important factors promoting NAC formation, and photolysis was an important loss pathway of NACs.


Various gas-phase and condensed-phase oxidation of anthropogenic VOC precursors are also important contributors to NAC formation, especially in urban atmospheres (Harrison et al., 2005).The main reactions leading to the secondary Results and discussion

Concentration and composition of NACs
The average concentration of quantified NACs was 6.63 ng/m 3 , ranging from 1.27 to 17.70 ng/m 3 in summer in Beijing.
The concentrations of total NACs as well as individual species across this and prior studies were summarized and compared in Figures 2, S1 and Table S1.The total NAC concentration was higher than other studies conducted in summer in mountain, rural or urban environments (Teich et al., 2017;Kitanovski et al., 2012;Kahnt et al., 2013;Zhang et al., 2013;Chow et al., 2016;Wang et al., 2018b), and comparable to the studies in summertime Wangdu, China (Teich et al., 2017;Wang et al., 2018b) (Figure 2).Each NAC species (NP, NC, MNP and MNC), except DMNP, also showed elevated concentrations in Changping, compared with those reported in other summertime studies (Figure S1).Influenced by the outflow from urban Beijing air masses, the site was under typical high-NO x conditions (Wang et al., 2018a), implying abundant potential secondary formation of NACs during the observation period.A recent study suggested that nocturnal biogenic VOCs (BVOCs) oxidation would transfer from low-to high-NO x regimes and nearly all the BVOCs would be oxidized by NO 3 lower than that in summer in Hong Kong (Zhao et al., 2016), which was more favorable for the partitioning of nitrophenol products from gas phase into particle phase.

Temporal variation and sources of NACs
Temporal variations of the total quantified NAC concentrations in this study are shown in Figure 3, along with particulate organics, nitrate, potassium ion, toluene, benzene, wind speed and RH.The correlation analysis between NACs and other chemical components or meteorological conditions is shown in Table S2.During the observation period, four pollution episodes (episodes I, II, III, IV) were identified based on the organic aerosols, marked by gray shading in Figure 3. Elevated NAC concentrations were observed during each pollution episode, coincided with the increasing of toluene, benzene and potassium.Their good inter-correlations (Table S2) suggested the sources of NACs from biomass burning emissions and secondary oxidation of anthropogenic VOCs (benzene, toluene and their oxidation products).It was noticed that NACs showed stronger correlations with toluene (r=0.70) or benzene (r=0.64)than those with potassium (r=0.49),indicating the NO x oxidation of anthropogenic VOCs as more dominant sources of NACs than biomass burning in summer in Beijing.and S4), indicating different formation pathways for NAC species.Good inter-correlations were observed among nitrophenol and its derivatives (2M4NP, 3M4NP, DMNP, r=0.56-0.88),as well as among nitrocatechol and its derivatives (3M6NC, 3M5NC, 4M5NC, r=0.49-0.84).However, the correlations between nitrophenol derivatives and nitrocatechol derivatives (r=0.05-0.45)were lower (Table S2).The correlation analysis indicated that the formation and loss pathways as well as the influence factors were similar within each group (NP and NP derivatives, NC and NC derivatives), while different between them.NC and its derivatives showed stronger correlations with toluene, benzene and K + , compared with NP and its derivatives (Table S2).This was because the factors influencing the particulate NP distribution could be more complicated.Gas-phase oxidation represents an important formation pathway for NP, thus the distributions of particulate NP could largely dependent on gas-particle partitioning or their gas-phase loss pathways (e.g.photolysis).
Obvious nighttime enhancements of 4M5NC and 3M5NC were observed during the whole observation period, especially during the first pollution episode (Figure 4).Strong inter-correlations between 4M5NC and 3M5NC and their similar temporal variations indicated the similar formation pathways.It has been suggested that aqueous-phase oxidation (including photooxidation and nighttime oxidation) is an important formation pathway for atmospheric MNC, especially in polluted high-NO x environments and relatively acidic particles (pH around 3) (Vidovic et al., 2018;Frka et al., 2016).
4M5NC and 3M5NC showed relatively stronger correlations with RH compared with other NAC species (Table S2) implying the importance of water in their formation processes and the aqueous-phase pathway.During the campaign, particles were generally acidic with a pH range of 2.0-3.7 and under high-NO x conditions (Wang et al., 2018c;Wang et al., 2018a), which were suitable environments for the aqueous-phase oxidation formation of MNC.The daytime correlations between 4M5NC or 3M5NC and RH or NO 2 were stronger than the nighttime (Table S3).The aqueous-phase NO x oxidation could be more dependent on ambient RH and NO 2 levels during the daytime, due to the lower RH and NO 2 concentrations than those at night (Figures 3, S2).MNC also showed good correlations with potassium, as MNC could also form via the oxidation of VOC precursors (e.g.cresol, catechol) emitted from biomass burning (Iinuma et al., 2010;Finewax et al., 2018;Olariu et al., 2002).3M6NC showed different temporal variations from 4M5NC or 3M5NC (Figures 4, S3) and their correlations were lower than that between 4M5NC and 3M5NC (Tables S2, S3), which suggested different formation pathway of 3M6NC from those of 4M5NC or 3M5NC.The quantum calculations have predicted the formation of 3M5NC via aqueous-phase electrophilic substitution and nitration by NO 2 + , while the formation of 3M6NC was negligible due to higher activation barriers for nitration of 3-methylcatechol to form 3M6NC (Frka et al., 2016).A dominant presence of 3M5NC in ambient aerosols was also expected according to the theoretical predictions (Frka et al., 2016).The 3M5NC concentration was obviously higher than that of 3M6NC in summer in Beijing, which is consistent with the study in Frka et al. (2016).
Different from the nighttime enhancements of 4M5NC and 3M5NC, 4NP, 2M4NP and DMNP showed obvious daytime enhancements during the whole campaign (Figures 4, S4), indicating the importance of photochemical oxidation for the formation of NP and its derivatives.Previous study also suggested the daytime gas-phase oxidation of aromatics could represent the major source of nitrophenols, while the contribution from nighttime reaction of phenol with NO 3 radicals was relatively lower (Yuan et al., 2016).We did not find obvious correlation between 4NP and NO 2 when considering the whole period (Table S2), while good correlations were observed when treating the daytime and nighttime conditions separately (Table S3).The strong correlations between 4NP and benzene, toluene or NO 2 during daytime and nighttime indicated its formation via oxidation of benzene and toluene in the presence of NO 2 (Table S3).The formation mechanisms of nitrophenol were different during daytime (OH-initiated photooxidation of aromatics in the presence of NO 2 ) and nighttime (NO 3 -initiated oxidation of aromatics) (Harrison et al., 2005;Yuan et al., 2016;Sato et al., 2007;Ji et al., 2017;Olariu et al., 2002), thus the role and influence of NO 2 on NAC formation were different.For DMNP, 2M4NP and 3M4NP, they also showed good correlations with benzene, toluene and NO 2 during daytime, which indicated their potential formation pathways via photooxidation of aromatics in the presence of NO 2 .However, DMNP and 3M4NP didn't show correlations with benzene, toluene or NO 2 at night.Their correlations with RH were higher at night, implying the possible formation related to aqueous-phase pathways, while the potential precursors or mechanisms remain unknown.the NO 3 -/NAC ratios as a function of NO 2 levels (Figure 5).The variation of (NO 3 -)/NACs ratios was employed to illustrate the relative abundance of inorganic nitrate and oxidized organic nitrogen.The variation during daytime and nighttime were separately considered due to the different atmospheric conditions and oxidation mechanisms (Figure 1).
Generally, higher concentrations of NACs and nitrate were observed with elevated NO 2 concentration levels, with nonlinear responses (Figure 5).During the daytime, the NACs increased with NO 2 , and the NO 3 -concentrations and (NO 3 -)/NACs ratios were lower at low-NO x conditions (NO 2 < 20ppb).As NO 2 increased to higher than 20 ppb, the NAC concentration did not obviously increase with NO 2 any more, indicating the transition from NO x -sensitive to NO x -saturated regimes for NAC secondary formation.At the same time, the NO 3 -concentrations and (NO 3 -)/NACs ratios showed significant increasing trends (Figure 5a, b, c).It was likely that the daytime NO 2 was excess for the oxidation of ambient VOCs and the NAC formation at NO 2 > 20 ppb.The excess NO 2 would be oxidized to form inorganic nitrate, indicating the shift of products from organic-to inorganic-dominated conditions.Similarly, the transitions from the low-to high-NO x regimes and oxidation products from organic-to inorganic-dominated were also observed at NO 2 ~25 ppb at night under the ambient environments in summer in Beijing (Figure 5d, e, f).The nighttime NAC formation would become independent of NO 2 concentrations and inorganic nitrate dominated the NO x oxidation products at NO 2 > 25 ppb.The simplified mechanisms and schematic diagram of the competing formation of inorganic nitrates and NACs are shown in Figure S5.The nighttime NO 2 transition value (~25 ppb) was higher than the daytime one (~20 ppb).The higher anthropogenic VOC precursors (Figure S2) and different oxidation mechanisms at night (Figure 1) were the potential reasons for elevated NO 2 transition value.
Taking NACs as an example, the result implied that the transition from low-to high-NO x regimes and the oxidation products shifting from organic-to inorganic-dominated conditions existed in the anthropogenic VOCs-NO x interacted conditions in polluted urban atmospheres.However, the mechanisms as well as transition thresholds were less understood compared with the well-known BVOCs-NO x interaction atmospheres.The NO x transition regimes of more organic nitrogen formation and the oxidation mechanisms during daytime and nighttime deserve comprehensive investigation in anthropogenic VOCs-NO x interacted urban atmospheres.Due to the complex VOC composition in ambient atmosphere and the limited number of VOC species measured in this study, the NO x regime transition value was expressed by NO 2 concentrations under the condition of urban Beijing atmospheres rather than NO 2 /VOC or NO x /VOC ratios.The NO x regime transition values deserve further investigation based on comprehensive lab simulation and field observations, which could be expressed by NO x /VOC ratios and applied to various atmospheric environments.
The formation pathways of different NAC species vary from each other based on the analysis in sections 3.2, thus the role and influence of NO 2 on their formation are different.The variation of NAC compositions as a function of NO 2 levels is shown in Figure 6 to investigate the influence of NO 2 on NAC compositions.The contributions of nitrocatechol and its derivatives among the total NACs increased and those of nitrophenol and its derivatives decreased at elevated NO 2 concentrations.The NAC composition remained relatively constant at NO 2 >20 ppb, which was approximately the transition value from low-to high-NO x regimes.The role of elevated NO 2 in promoting NC (and its derivatives) formation was more obvious than that for NP (and its derivatives).The oxidation of aromatics (e.g.benzene, toluene and VOCs emitted from biomass burning) in the presence of NO 2 represent the major formation pathway of NC and its derivatives based on the analysis in section 3.2.The formation of NC and its derivatives would increase obviously as the increasing of ambient NO 2 concentration levels.The distributions of nitrophenol and its derivatives could also be obviously influenced by the gas-particle partitioning and their loss pathways (e.g.photolysis), thus their increasing trend as a function of NO 2 was not as obvious as those of NC and its derivatives.

Other influence factors of NACs formation
Nitration of aromatic hydrocarbons (e.g.benzene and toluene) represents the major source of NACs in summer in Beijing.NACs generally increased with the increasing of anthropogenic toluene and benzene (Figure 7).During daytime, when toluene was higher than 0.6 ppb and benzene higher than 0.4 ppb, the NACs concentrations would not increase further with VOC concentrations (Figure 7a, b).It was likely that toluene or benzene was in excess and the NAC formation became independent of anthropogenic VOC precursors under this condition.Similarly, the nighttime formation of NACs would be independent of anthropogenic VOCs, when toluene was higher than 1 ppb and benzene higher than 0.6 ppb (Figure 7c, d).
The transition values of toluene or benzene was higher at night than those during the daytime.This could be due to the significantly higher NO 2 levels (Figure S2), with higher capacity to oxidize VOC precursors, and different oxidation mechanisms at night.
Though the total NACs didn't show good correlations with ambient RH, good correlations between 3M4NC, 4M5NC and RH were observed (Table S2, Figure 8).The NAC products (e.g.nitrophenols) dominated by gas-phase formation pathways were less affected by ambient RH.Aqueous-phase oxidation represented the major formation pathway of 3M4NC and 4M5NC during the campaign, based on the analysis in section 3.2 and previous studies (Vidovic et al., 2018;Frka et al., 2016).Elevated ambient RH would favor the water uptake of aerosols and decrease the aerosol viscosity, which favors the exchange of organic precursors or other gas molecules into the particles, mass diffusion of reactants and chemical reactions within the particles (Vaden et al., 2011;Booth et al., 2014;Renbaum-Wolff et al., 2013;Shrestha et al., 2015;Zhang et Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-1256Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 10 January 2019 c Author(s) 2019.CC BY 4.0 License.Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-1256Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 10 January 2019 c Author(s) 2019.CC BY 4.0 License.
To further investigate the secondary formation of NACs, the time series and day-night variations of individual NAC species are shown in Figures 4, S3 and S4.Obvious daytime enhancements of 4NP, 2M4NP and DMNP, nighttime enhancements of 3M4NC and 4M5NC were observed, and other NAC species didn't show obvious day-night variations (Figures 4, S3 , 7 Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-1256Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 10 January 2019 c Author(s) 2019.CC BY 4.0 License.

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Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-1256Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 10 January 2019 c Author(s) 2019.CC BY 4.0 License.Discussion 4.1 The NO 2 control of NACs formation NO x oxidation of anthropogenic VOC precursors represented the dominant sources of NACs in summer in Beijing.To further investigate the impacts of NO 2 on NAC secondary formation, we plot the concentrations of NACs, nitrate (NO 3 -) and

Figure 2
Figure 2 Summary of NAC concentrations cross this and prior studies (see TableS1for the data and references therein).The NAC concentrations in summer correspond to the left axis and other seasons correspond to the right axis.

Figure 3
Figure 3 Time series of (a) wind speed (WS) and relative humidity (RH), (b) benzene and toluene, mass concentrations of (c) K + , (d) organics and nitrate, and (e) NACs.The pollution episodes are marked by gray shading.

Figure 5
Figure 5 Concentrations of NACs, nitrate and NO 3 -/NAC ratios as a function of NO 2 concentration bins during daytime and nighttime.The markers represent the mean values and whiskers represent 25 and 75 percentiles.

Figure 6
Figure 6 Variation of NAC compositions as a function of NO 2 concentration bins.

Figure 7
Figure 7 Concentrations of NACs as a function of toluene and benzene concentration bins during daytime and nighttime.The markers represent the mean values and whiskers represent 25 and 75 percentiles.The r value in each panel represents the correlation coefficient between NACs and toluene or benzene.