Impact of firework on nitrooxy-organosulfates in urban aerosols
during Chinese New Year Eve

Abstract. Little is known about the formation processes of nitrooxy-organosulfates (nitrooxy-OSs) by nighttime chemistry. Here we characterize nitrooxy-OSs at a molecular level in firework-related aerosols in urban Beijing during Chinese New Year. High-molecular-weight nitrooxy-OSs with relatively low H / C and O / C ratios and high unsaturation, which are potentially aromatic-like nitrooxy-OSs, considerably increased during the New Year’s Eve. We find that large quantities of carboxylic-rich alicyclic molecules possibly formed by nighttime reactions. The sufficient abundance of aliphatic-like and aromatic-like nitrooxy-OSs demonstrates that both anthropogenic and biogenic volatile organic compounds are essential precursors of urban secondary organic aerosols (SOA). Besides, more than 98 % of nitrooxy-OSs were extremely low-volatile organic compounds that could easily partition into and consist in the particle phase, and affected the volatility, hygroscopicity, and even toxicity of urban aerosols. Our study provides new insights into the formation of nitrooxy-organosulfates from anthropogenic emissions through nighttime chemistry in the urban atmosphere.


. Moreover, nitrooxy-OSs can alter the surface hygroscopicity of aerosol particles because of their water-soluble and fat-soluble properties, promoting the production of cloud condensation nuclei (Schindelka et al., 2013) and also increasing the light absorption of organic aerosols (Nguyen et al., 2012).
The formation of nitrooxy-OSs due to nighttime chemistry is less understood so far. Surratt et al. (2008) suggested that nitrooxy-OSs can be formed from the combination of organonitrates and sulfates under acidification, while organonitrates are preferably produced by nighttime NO3 radical oxidation than daytime photooxidation (Rollins et al., 2012;Kiendler-Scharr et al., 2016;Huang et al., 2019). Iinuma et al. (2007) reported that some monoterpene nitrooxy-OSs (e.g., 15 C10H17NO7S, C10H18N2O7S, and C5H10N2O11S) were only detected in nighttime aerosols, indicating the importance of NO3 radicals in the nighttime chemistry. Fireworks are frequently conducted as traditional activities to celebrate popular festivals in particular with the New Year's Eve, emitting large quantities of pollutants into the atmosphere (Vecchi et al., 2008;Huang et al., 2012). It is found that lots of semi-volatile to volatile organic compounds, such as n-alkanes and polycyclic aromatic hydrocarbons (PAHs), released during firework-related events (Sarkar et al., 2010;Feng et al., 2012), can be essential 20 precursors of anthropogenic nitrooxy-OSs in aerosols (Tao et al., 2014;Riva et al., 2015). Even though, the knowledge of chemical and physical behaviors of nitrooxy-OSs in firework-related urban aerosols is very sparse, particularly for highmolecular-weight (HMW) compounds because of their molecular complexity.
To fill this research gap, the molecular characterization of HMW nitrooxy-OSs in firework-related aerosols during nighttime is reported in this study based on the measurements from Fourier transform ion cyclotron resonance mass spectrometry  ICR MS) with ultrahigh resolution and mass accuracy. FT-ICR MS has been proven to be a powerful tool to reveal the complicated organic matter in environmental samples at a molecular level (Dzepina et al., 2015;Qi et al., 2020;Qi et al., 2021). Our study presents elemental compositions, classifies the organic mixtures into different categories to identify potential origins of nitrooxy-OSs and to investigate their possible chemical structures and precursors. The volatility of different nitrooxy-OSs is predicted and discussed as well. and Normal N (Normal Day nighttime) (Xie et al., 2020b). Detailed sample information can be found in Table S1. The total suspended particle (TSP) samples were collected on prebaked quartz filters (Pallflex) using a high-volume aerosol sampler, and then were refrigerated at −20°C until analyzation. Field blank filters were collected following the same procedure.
Besides, two-day air mass backward trajectories show that the air was primarily originated from the clean northwest region 10 during the sampling period.

FT-ICR MS analysis
The method to extract water-soluble organic carbon (WSOC) fractions from each aerosol samples were taken from our previous studies (Xie et al., 2020a;Xie et al., 2020b). After extracted with ultrapure Milli-Q water, WSOC fractions were eluted from a solid-phase extraction cartridge (Oasis HLB, Waters, USA) using methanol, and were analyzed using a 15.0 GmbH,Bremen,Germany) with the negative ESI ionization mode. The detection mass ranges were 180−1000 Da. The mass spectra were internally calibrated via the Data Analysis software. The mass accuracy was within 1ppm and the peaks of signal to noise ratio higher than 6 were assigned for further analysis.
The formulae containing C, H, N, O, and S atoms, namely CHONS compounds, in WSOC fractions of urban aerosols were assigned from the ESI FT-ICR MS. The number and the total intensity of CHONS compounds attributed to 14-29% and 10-20 28% of the total assigned compounds, respectively ( Figure S1). As the predominant fraction occupying more than 85%, CHONS compounds with O/S ³ 7 were tentatively deemed to be nitrooxy-OSs in present work, supporting the assignment of a -OSO3H group and a -ONO2 group in molecules (Kuang et al., 2016;Wang et al., 2016). However, other sulfur-containing compounds (e.g. sulfonates) might also be introduced and impact the analyzation due to lack of the structure information of ions from tandem MS experiments (El Haddad et al., 2013;Riva et al., 2015). 25 Table 1 shows that slightly higher compounds in number frequency was found in normal nighttime samples (1094 in Normal N) than in normal daytime (885 in Normal D), which was in agreement with previous studies (Pinxteren et al., 2009;O'Brien et al., 2014;Wang et al., 2016). However, 690 nitrooxy-OSs were observed in NYE D before the firework periods, while it considerably increased up to 2050 in NYE N during the firework periods, indicating a pronounced increment at night during the firework event. This can be explained by significant precursors emitted from fireworks (Kong et al., 2015) that produce nitrooxy-OSs via the nighttime NO3 radical chemistry (Riva et al., 2015). Meanwhile, the heavy emissions of nitrogen oxide during the fireworks event could elevate the production rate of NO3 radicals (Ljungström and Hallquist, 1996;Kiendler-Scharr et al., 2016), and previous study showed a good correlation between NO3 and the total concentration of nitrooxy-OSs 5 at the night (Nguyen et al., 2014).

General molecular characterization of nitrooxy-OSs
Different from the detected compounds in NYE D, here were 1411 nitrooxy-OSs that were only detected in NYE N, which contributed 69% of total number of nitrooxy-OSs in the sample ( Figure 1). Moreover, their relative intensities also accounted for nearly half of that in NYE N. These results indicate that extensive burning of firecrackers offered many specific precursors for the formation of new nitrooxy-OSs. 10 Nitrooxy-OSs were identified as N1O7S1-N1O13S1 and N2O7S1-N2O14S1 species (Figures 2 and S2). Here, N1O7S1 compounds refer to formulae containing one nitrogen, seven oxygen, and one sulfur elements, and so for the other species.
Their number concentrations decrease with the increase of oxygen content in molecules. N1OnS1 species are the predominant nitrooxy-OSs, the number and the intensity of N1OnS1 species occupied 65-82% and 62-80% of the total detected nitrooxy-OSs, respectively. During firework periods, up to 1300 species of N1OnS1 were detected in NYE N, twice as many as other 15 samples. The effect of pyrotechnics on nitrooxy-OSs become more substantial with the increased oxygen atom number ( Figure 2). Similarly, the intensity of N1OnS1 species doubled in NYE N compared to other samples. N2OnS1 species may have other nitrogenous functional groups (e.g., amino and nitro groups) in addition to nitrate functional group. During nonfirework periods, there were an average of 300 species of N2OnS1, but the number increased to 724 in NYE N. Moreover, the contribution of fireworks to N2OnS1 species was higher than that of N1OnS1 compounds, possibly because some released 20 amino acids and their derivatives react to form nitrooxy-OSs with two nitrogen-containing functional groups.  respectively. Compared with 16 these compounds in NYE D, there were up to 83 compounds in NYE N. These results possibly indicate that pyrotechnic emissions have substantial impacts on the formation of aromatic-like nitrooxy-OSs.

Van Krevelen diagram division
The Van Krevelen (VK) diagram is widely applied to depict the evolution of organic mixtures and to identify possible origins of organic aerosols by differentiate major known categories of the natural and anthropogenic organic matter (Noziere 5 et al., 2015;Bianco et al., 2018). Here, we applied the VK to investigate nitrooxy-OSs in firework-related aerosols. The particularly seven classification areas are shown in Figures 3 and S4, and their stoichiometric ranges are displayed in Table   S2. The two most populated regions correspond to carboxylic-rich alicyclic molecules (CRAMs-like)/lignin-like (49−66% and 40−49% of the total number and intensity) and aliphatic/peptides-like (21−33% and 24−38% of the total number and intensity) classes, which followed by carbohydrates-like (6−12% and 11−19% of total number and intensity) and tannins-like 10 (4−7% of total number and intensity) classes. It is found that more than 98% of nitrooxy-OSs belong to these four categories.
Previous studies reported that the majority of WSOC fractions were lignin-, lipids-, and aliphatic/peptides-like classes in aerosols and cloud water (Wozniak et al., 2008;Zhao et al., 2013;Bianco et al., 2018). However, nitrooxy-OSs in the present work had relatively high O/C ratios, potentially because they were consistent with covalently bound HSO4 − (Romero and Oehme, 2005) in HMW sulfur-containing compounds (Wozniak et al., 2008). Besides, nitrooxy-OSs also had high H/C 15 ratios, indicating that sulfation, nitration, or functionalization processes led to mostly saturated compounds.
Although the number of all seven types of nitrooxy-OSs increased obviously in NYE N compared with other samples, the impacts of fireworks on nitrooxy-OSs are different in categories. As for the most abundant CRAMs-like nitrooxy-OSs, they were more abundant during the nighttime (620 in Normal N) than daytime (375 in Normal D). However, NYE N contained about 1354 CRAMs-like compounds, which were three times more than NYE D. The relative contribution of the number of 20 these compounds was 60% in NYE N, substantially higher than that in NYE D (45%). The total intensity of such compounds in NYE N was about eight times higher than those in NYE D. These observations demonstrate that nighttime oxidation is important in the formation of CRAMs-like nitrooxy-OSs, especially in the presence of abundant firework-related precursors.
CRAMs contain the structures of carboxylated alicyclic and large and fused nonaromatic rings with a high ratio of substituted carboxyl groups (Bianco et al., 2018). However, some CRAMs-like nitrooxy-OSs located in the aromatic area 25 (AI > 0.5). Thus, there may exist some aromatic-like compounds with some degree of alkylation that have been mistaken for nonaromatic class (Kourtchev et al., 2016;Tong et al., 2016). Also, it was worth noting that nitrooxy-OSs of aromatic region could be lignin-like compounds, which contains aromatic rings in their chemical structures. The results are consistent with the high unsaturation of compounds in the region as described above. contributions of both the number and the total intensity of aliphatic/peptides-like nitrooxy-OSs were more abundant during daytime (~40%) than nighttime (~30%). However, there are 427 species of aliphatic/peptides-like nitrooxy-OSs in NYE N, which is twice as many as other samples. The intensity of each compound was also higher in NYE N with bigger symbol sizes (Figure 3b), and the total intensity was about three times higher than other samples. These results demonstrate the importance of anthropogenic precursors for the formation of aliphatic/peptides-like nitrooxy-OSs, though they could be more 5 susceptible to photochemical reactions than nighttime chemistry from biogenic precursors.
Contrary to CRAMs-like, the carbohydrates-like nitrooxy-OSs with high intensity have high H/C and O/C ratios and saturation, indicating highly oxidized and alkylated. Compared with other reported sulfur-free compounds (Wozniak et al., 2008;Bianco et al., 2018), the abundance of carbohydrates-like nitrooxy-OSs increased significantly. The increase is reasonable because carbohydrates and their derivatives are polyhydroxy aldehydes, polyhydroxy ketones and organic 10 compounds, which can be hydrolyzed to form polyhydroxy aldehydes or polyhydroxy ketones, tending to generate OSs and nitrooxy-OSs. Moreover, N2OnS1 species were more abundant than N1OnS1 species (Figure S5), indicating a trend toward easily functionalization. Although the number of carbohydrates-like compounds in NYE N was similar to other samples, the intensity of them increased, potentially indicating an increase of the concentration of them. As for tannins-like classes, which are also highly oxygenated organic compounds (Bianco et al., 2018), their content in NYE N was also more abundant than 15 other samples.
Considering the lipids-like, unsaturated hydrocarbons and aromatic structures classes, only less than 1.5% compounds are in these regions. Lipids-like organics, containing monoglycerides, diglycerides, fats, fat-soluble vitamins, and sterols, primarily originate from biogenic materials and phospholipids (Gurganus et al., 2015;Bianco et al., 2018), but most nitrooxy-OSs are of secondary origin. Unsaturated hydrocarbons compounds are mostly composed of carbon and hydrogen atoms, while 20 nitrooxy-OSs have lots of heteroatoms. As for aromatic compounds, they are mainly produced by combustions as the indicators of anthropogenic origin. The limited number of nitrooxy-OSs detected in this region may be due to the tendency of some alkylated compounds to fall into other categories (e.g., CRAMs-like). OSs of DBE (4-10) and C number (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) in the nighttime was higher than that during the daytime, the number of nitrooxy-OSs with DBE and C number in the range of 4-18 and 10-20 was even higher in the NYE N. These highly unsaturated nitrooxy-OSs are aromatics, which may be originated from firework-related aromatic VOCs or PAHs (Riva et al., 2015). Nitrooxy-OSs have an extensive range of unsaturation with DBE values ranging from 0 to 23 (Figure 4). Previous studies have reported that nitrooxy-OSs (e.g. C10H17NO7S (1), C9H15NO8S (2)) can be formed from biogenic VOCs (e.g. a-pinene and limonene) (Iinuma et al., 2007a;Surratt et al., 2008;Cai et al., 2020). C10H17NO7S and C9H15NO8S (Figure 4) have the same degree of unsaturation as their precursors and show the strongest intensity among all nitrooxy-OSs, which demonstrate that a-pinene and limonene are the primary precursors of biogenic nitrooxy-OSs. A continuous series of corresponding 5 family series was also detected in firework-related aerosols, namely CnH2n-3NO7S (n=9-22) and CnH2n-3NO8S (n=9-24) ( Figure 5).

Subgroups and potential precursors
The nitrooxy-OSs were divided into three main categories to illustrate the molecular difference. Group A comprises aliphatic-like nitrooxy-OSs (DBE ≤ 2), which is featured by long alkyl carbon chains with high saturation; Group B contains aromatic-like nitrooxy-OSs (Xc > 2.5) with high unsaturation; Group C represents the rest compounds similar to biogenic 10 nitrooxy-OSs with a moderate extent of saturation. As for the aliphatic-like nitrooxy-OSs, such as C18H35NO9S (3) and C12H25NO7S (4) (Figure 4), they have saturated and long carbon chains, which may source from precursors such as longchain alkenes, alkanes, and fatty acids by photooxidation. Both had relatively high intensities and consecutive family series i.e. CnH2n-1NO9S (n=9-22) and CnH2n+1NO7S (n=6-24), respectively ( Figure 5). These precursors may also produce nitrooxy-

OSs under high NOx conditions or break double bonds to form intermediate products through photooxidation, and then form 15
nitrooxy-OSs with sulfates. Some possible formation mechanisms are proposed in Figure 4(g). It is noted that, except for compounds (1) and (2), the intensity of each aliphatic-like nitrooxy-OSs are higher than others in daytime samples, highlighting the importance of photooxidation to their generation.
The number of the aromatic-like nitrooxy-OSs were the most abundant among all measured nitrooxy-OSs. Figures S6(d) and S8(h) showed that there were slight differences between NYE D and NYE N for aliphatic and biogenic nitrooxy-OSs. 20 Nonetheless, compared with the daytime, the number of aromatic-like nitrooxy-OSs considerably enhanced in NYE N in particular with the HMW ones. Riva et al. (2015) using side by side comparison experiments proved that the generation of OSs and sulfonates from PAHs was enhanced with the acidified sulfate seed aerosols existed. Their results implied that aromatic-like nitrooxy-OSs might efficiently be generated through PAHs and sulfate ions released from fireworks at night without the participation of photochemistry. For instance, the aromatic-like nitrooxy-OSs with multiple benzenes can be 25 generated from carboxyl compounds, which is a oxidation products of pyrene (Juhasz and Naidu, 2000). Some possible structures, such as C18H15NO11S (5), C16H13NO9S (6), and C11H15NO8S (7), and proposed formation mechanisms are displayed in Figures 4, respectively. Their corresponding family series (i.e. CnH2n-21NO11S (n=17-28), , and CnH2n-7NO8S (n=8-29)) have more carbon atoms than the biogenic and aliphatic nitrooxy-OSs ( Figure 5), possibly because they are formed via the polymerization process or derived from aromatic compounds with high carbon content. 30 Besides, it is noted that the intensity of each aromatic-like nitrooxy-OSs was lower than the other two groups and decreased with the increase of unsaturation (Figures 4 and Table S3). These may because the water solubility of aromatic-like nitrooxy-OSs decreases with the increasing unsaturation. However, these nitrooxy-OSs are possibly present in the non-WSOC fractions, which requires further investigations.

Volatility characteristics and molecular corridors
Molecular corridors that constrained by two boundary lines of sugar alcohols CnH2n+2On with O/C = 1 and linear n-alkanes CnH2n+2 with O/C = 0 are used for a better understanding of the chemical and physical properties in SOA evolution (Shiraiwa et al., 2014;Li et al., 2016;Shiraiwa et al., 2017). From Figure 6(a-b), more than 98% of nitrooxy-OSs detected in present study were located in the region of extremely low-volatile organic compounds (ELVOC) with saturation mass concentration 5 et al., 2011;Murphy et al., 2014) and a molar mass higher than 250 g mol −1 . Moreover, the volatility varies among N1OnS1 and N2OnS1 species because of the differences in their molecular composition and structures.
Compared with N1OnS1, many of the N2OnS1 species have lower volatility and higher O/C ratio, nearing the sugar alcohols CnH2n+2On line. Lots of these compounds with higher intensity were found in NYE N than other samples, which suggests that highly oxidized compounds are produced in large quantities via nighttime chemistry after firework emissions. Moreover, the 10 volatility of nitrooxy-OSs potentially decreased with the increase of molecular weight and unsaturation. Compared with compounds in NYE D, numbers of nitrooxy-OSs with low volatility were only detected in NYE N, possibly because of the increase of HMW aromatic-like nitrooxy-OSs affected by firework emissions. Besides, it was worth noting that the volatility of nitrooxy-OSs was lower than that of OSs (Xie et al., 2020a), possibly because they are highly functionalized compounds.

The molecular corridor includes three primary parts, consisting of low, intermediate, and high O/C ratio corridors (LOC, 15
IOC, and HOC) (Shiraiwa et al., 2014). The plentiful gas-phase oxidation products of alkanes fall into LOC, which near the alkanes line. Conversely, the aqueous-phase reaction and autoxidation products were in HOC, closing to the sugar alcohols line. The IOC corridor is the area connecting LOC and HOC. Nitrooxy-OSs observed in this work are dominantly found in the IOC molecular corridors. N1OnS1 are closer to the LOC corridors, while N2OnS1 are closer to the HOC corridors. These results suggest that most of firework-related nitrooxy-OSs were possibly gas-and/or particle-phase autoxidation or 20 dimerization products. For instance, nitrooxy-OS can be formed through hydroxynitrate gas-phase products reactively up taking onto acidified sulfate seed aerosols through the esterification of the hydroxyl group with sulfuric acid . Jay and Stieglitz (1989) also found hydroxynitrates produced by the oxidation of a-pinene induced by NO3 at night. Also, several newly firework-related nitrooxy-OSs with higher molecular weight were possibly generated from dimerization and oligomerization in the particle phase. These results demonstrate that the dimerization and functionalization of nitrooxy-25 OSs can be substantially enhanced in the particle-phase with rising pollutant concentrations and varying reaction scenarios.
Although nitrooxy-OSs have been frequently reported within aerosols O'Brien et al., 2014;Cai et al., 2020), deposited sediment (Zhang et al., 2016), and atmospheric water such as cloud water (Zhao et al., 2013), rain (Altieri et al., 2009), or fog (Mazzoleni et al., 2010), they were different from those in the present work because of the unique chemical reactions during the fireworks (Figure 6c). Organics in atmospheric water with aqueous phase reactions are highly oxidized 30 and close to the sugar alcohol line. Although a fraction of cloud-water nitrooxy-OSs overlap with those in aerosols, the firework-related nitrooxy-OSs in our work showed a higher molar mass and lower volatility than urban aerosols reported previously O'Brien et al., 2014), especially at NYE night, potentially because of increased dimerization and oligomerization reactions.

Summary and Prospective
The present study provides unique information about the important contributions of anthropogenic precursors, as well as biogenic precursors, to the formation of nitrooxy-OSs in ambient aerosols during the firework events. Instead, numbers of 5 nitrooxy-OSs were potentially derived from alkene, fatty acids, and aromatics, and their derivatives compared to biogenicrelated nitrooxy-OSs. The surfactant properties of ambient aerosol particles may be influenced after coupling with hydrophilic functional groups of nitrooxy and sulfate, which affect the formation of cloud condensation nuclei. Furthermore, influenced by the fireworks emission, a lot of organonitrates in the gas-phase can partition into the particles phase by forming nitrooxy-OSs with low volatility as ELVOC, thus participating in the organic nitrogen cycle. Besides, they also 10 affect the NOx cycle in the atmosphere. Our results highlight that fireworks emission considerably contributes to the formation of nitrooxy-OSs and will have an important influence on atmospheric physical and chemical processes.
Nevertheless, nighttime chemistry of NO3 radicals were substantially involved in the generation of nitrooxy-OSs, particularly for aromatic-like compounds. Such complex mechanisms warrant further investigations.

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Data availability. The dataset for this paper is available upon request from the corresponding author (fupingqing@tju.edu.cn).
Competing interests. The authors declare that they have no conflict of interest.