Direct contribution of ammonia to CCN-size αlpha-pinene secondary organic aerosol formation

Ammonia (NH3), a gasous compound ubiquitiously present in the atmosphere, is involved in the formation of secondary organic aerosol (SOA), but the exact mechanisum is still not well known. This study presents the results 10 of SOA experiments from the photooxidation of α-pinene in the presence of NH3 in the reaction chamber. SOA was formed in nucleation experiment and in seeded experiment with ammonium sulfate particles as seeds. The chemical composition and time-series of compounds in the gasand particlephase were characterized by an on-line highresolution time-of-flight proton transfer reaction mass spectrometer (HR-ToF-PTRMS) and a high-resolution timeof-flight aerosol mass spectrometer (HR-ToF-AMS), respectively. Our results show that for the aerosol particles in 15 cloud condensation nuclei (CCN) size, the mass concentration of ammonium (NH4) was still rising even after the mass concentration of organic component started to decrease due to aerosol wall deposition and evaporation, implying the continuous new formation of particle phase ammonium in the process. Stoichiometric neutralization analysis of aerosol indicates that organic acids have a central role in the formation of particle phase ammonium. Our measurements show a good correlation between the gas phase organic monoand di-carboxylic acids formed in the 20 photooxidation of α-pinene and the ammonium in the particle phase, thus highlighting the contribution of gas-phase organic acids to the ammonium formation in the CCN-size SOA particles. The work shows that the gas-phase organic acids contribute to the SOA formation by forming ammonium salts through acid-base reaction. The changes in aerosol mass, particle size and chemical composition resulting from the NH3-SOA interaction can potentially alter the aerosol direct and indirect forcing and therefore alter its impact on climate change. 25


Introduction
The largest uncertainty in forward projection of global warming is related to our limited knowledge of negative solar radiative forcing associated with aerosols (IPCC, 2013). Formation of secondary organic aerosols (SOA) is one of the main processes that affects the composition and properties of atmospheric aerosols. Formation of SOA occurs through two distinct mechanisms: by increasing the mass of the existing aerosol and through the formation of new 30 particles. Estimate on the SOA formation shows its significance as a source of atmospheric organic aerosol: about 60% of the organic aerosol mass is SOA on the global scale and regionally even more (Hallquist et al., 2009;Jimenez et al., 2009;Kanakidou et al., 2005). Hence, SOA plays an important role in the direct scattering of solar radiation, cloud formation and precipitation, and visibility reduction, and may also have a direct impact on human health. Ammonia (NH3) is ubiquitously present in the atmosphere as a dominant volatile base. The majority of its sources 35 is accounted by emissions from agriculture, livestock, soil and traffic (Huang et al., 2012;Grönroos et al., 2009;Battye et al., 2003). NH3 governs the neutralization of atmospheric aerosol by reacting with the inorganic acids such as sulfuric acid and nitric acid, leading to transformation of a substantial amount of ammonium sulfate (and https://doi.org/10.5194/acp-2020-457 Preprint. Discussion started: 14 May 2020 c Author(s) 2020. CC BY 4.0 License. derivatives) and ammonium nitrate (Seinfeld and Pandis, 2016). These inorganic salts play a vital role in contributing to the fine particle matter (PM2.5) and altering the chemical and physical properties of aerosol particles in the 40 atmosphere. The mechanism between NH3 and the inorganic acids leading to the secondary inorganic aerosol has been well recognized.
A number of studies have shown that NH3 is one of the key species for the new particle formation through ternary and binary nucleation with water and sulfuric acid (e.g. Lehtipalo et al., 2018;Jokinen et al., 2018;Bianchi et al., 2016;Kirkby et al., 2011;Kurten et al., 2007;Kulmala et al., 2000). The nucleated particles are a significant source 45 of atmospheric SOA particles and subsequent growth to a larger size (>50nm) allows them to serve as cloud condensation nuclei (CCN). However, the role of NH3 for the CCN-size SOA particles are still rarely studied. A study, conducted more than a decade ago, demonstrated that NH3 increased the number and volume concentrations of CCNsized SOA particles from α-pinene-ozone system by 15% and 8%, respectively, (Na et al., 2007). Similar results have also been observed in the photooxidation and ozonolysis of a-pinene SOA experiments that SOA mass yield increased 50 by 13% as a response to NH3 addition (Babar et al., 2017). Besides, the addition of NH3 could also promote the SOA formation from photooxidation of vehicle exhaust , anthropogenic VOCs (Wang et al., 2018;Huang et al., 2018) and acrolein (Li et al., 2019). The promotion mechanism of NH3 to SOA formation can be both through base-acid reaction (Schlag et al., 2017;Babar et al., 2017;Na et al., 2007) and by the NH3 uptake to the carbonyl group (Zhu et al., 2018;. As a consequence, the changes in particle size and chemical 55 composition could alter the CCN ability and hygroscopicity of SOA particles (Dinar et al., 2008). Moreover, the reaction of NH3 with SOA decreases the volatility of SOA particle (Paciga et al., 2014), and also results in production of light-absorbing brown carbon compounds that modify the optical properties of the aerosols (Huang et al., 2018;Updyke et al., 2012;Bones et al., 2010). Additionally, the updake of NH3 by SOA can deplete ambient NH3 concentrations, causing indirect reductions in the amount of inorganic ammonium salts in particulate matter (Horne 60 et al., 2018). Therefore, the interaction of NH3 and SOA could alter both direct and indirect aerosol radiative forcing and potentially alter its impact on climate change.
This work presents the results of SOA formation from photooxidation of α-pinene in the presence of NH3 in the nucleation and seeded experiments. The chemical composition of gas-phase and particle-phase compounds were characterized with a high-resolution time-of-flight proton transfer reaction mass spectrometer (HR-ToF-PTRMS) and 65 a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS), respectively. Our experiments show the formation of ammonium salt (NH4 + ) in the CCN-size SOA particles, and our gas phase measurements indicate organic acids are responsible for their formation. The results will have potential applications in studying SOA formation mechanism and the impact of SOA on climate forcing. Finland. The experimental system and experimental procedure have been described in details in Leskinen et al. (2015) and Kari et al. (2019a), respectively. The chamber consists of a 29 m 3 Teflon TM FEP film bag. Ultraviolent (UV) lights with a spectrum centered at 340 nm around the chamber enable photochemical reactions. Two sets of 75 experiments were designed (Table 1): (1) SOA nucleation experiments in the absence of seed aerosols; (2) SOA formation experiments in the presence of ammonium sulfate seeds. Prior to each experiment, the chamber was https://doi.org/10.5194/acp-2020-457 Preprint. Discussion started: 14 May 2020 c Author(s) 2020. CC BY 4.0 License. continuously flushed overnight with laboratory clean air produced by a zero air generator (Model 737-15, Aadco Instruments Inc., USA) and conditioned in a humidifier (Model FC125-240-5MP-02, Perma Pure LLC., USA), aiming at 50 % relative humidity at the typical temperature of 20 °C during the course of each experiment.

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Independent on the type of experiments (nucleation and seeded), the reactants were added into the chamber in the sequence described below. The (NH4)2SO4 (ammonium sulfate, 99 %, Sigma Aldrich) seed was injected to the chamber first. The seed was generated using an atomizer (Topas ATM226, Germany). In two experiments, nitrogen oxide (NO) was added to chamber, and ozone (O3) was introduced into the chamber to convert NO to nitrogen dioxide (NO2) to reach the atmospherically relevant NO2-to-NO ratio of ~ 3 (Kari et al., 2019a). After O3 and NOx were fed, 85 3 µl 9-fold butanol (butanol-d9, 98 %, Sigma Aldrich) was injected to the chamber, from whose consumption the hydroxyl radical (OH) exposure was estimated in each experiment (Kari et al., 2019a). Next, 1 µl and 18 µl of αpinene (≥ 99%, Sigma Aldrich) were added into the chamber for the seeded and nucleation SOA experiments, respectively, corresponding to concentrations of ⁓5 ppb and ⁓100 ppb of α-pinene in the chamber. Last, 5 ml of hydrogen peroxide (H2O2, 30 wt. % in H2O, Sigma Aldrich) was conducted into the chamber by mixing it with 90 purified air flowing at 10 lpm (liter per minute), as a precursor for OH radicals to be generated under UV radiation.
After all the compounds were introduced into the chamber, the chamber was closed, and the compounds were allowed to stabilize for 15 min. Then the UV lights were switched on to initiate photochemistry.

Analytical methods and instrumentation
The size-resolved chemical composition and mass concentration of aerosol particles were measured directly with 95 an on-line high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS, abbreviated as AMS) (DeCarlo et al., 2006). A detailed description of AMS operational procedure is provided in previous publications (Canagaratna et al., 2007;Jayne et al., 2000). In brief, AMS was operated in V -mode in EI -mode. Calibration of ionization efficiency (IE) followed the standard protocol using dried and size-selected ammonium nitrate particles. The data were analyzed using standard AMS data analysis toolkits (Squirrel V1.62D and Pika V1.22D) in Igor Pro Software 100 (version 6.37, WaveMetrics Inc.). For determining mass concentrations, the default relative ionization efficiency (RIE) values were 1.4, 1.1 and 1.2, for organics, nitrate and sulfate, respectively. The RIE for ammonium was 2.95, as determined in the IE calibration. After a comparison to the volume concentration derived from a scanning mobility particle sizer (SMPS TSI 3081 DMA + 3775 CPC) measurement (Fig. S1), a collection factor of 100 % was applied to determine the aerosol mass concentration in the reported results in this work. The positive matrix factorization 105 (PMF) analysis was performed on the high-resolution mass spectra by using the PMF Evaluation Tool V2.08 (Paatero and Tapper, 1994;Ulbrich et al., 2009). The standard error matrices were processed following the principles of applying minimum error estimate, downweighting weak variables, removing bad variables, and downweighing m/z 44 related fragments (Ulbrich et al., 2009). The PMF was evaluated with 1 to 6 factor. Rotation (Fpeak) varied from -1 to 1 at a step of 0.1.
For supporting information we measured the particle concentration and size distribution in a diameter range of 7-800 nm with an SMPS (TSI 3081 DMA + 3775 CPC) and the concentrations of NO, nitrogen oxides (NOx), ozone O3, and sulfur dioxide (SO2) , as well as the relative humidity and temperature inside the chamber. In this study, we lacked the measurement of NH3 concentration in the chamber but estimated it from our AMS measurement results, 125 by assuming that the particulate ammonium salt (NH4 + ) was converted from the gas-phase NH3 (Fig. 1). The maximum NH4 + concentration was in the range of 1.17⁓1.51 µg m -3 , which corresponds to a minimum NH3 concentration level of 1.6 ⁓ 2.1 ppbV in our chamber.

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Aftert the UV lamps were switched on, the photooxidation reaction produced oxidized gas-phase compounds and SOA particles in both nucleation and seeded experiments. The time series of mass concentrations of formed SOA (in green) and ammonium (in orange) along with sulfate and nitrate components measured by AMS are presented in Fig. 1. In the nucleation experiments (the left panels in Fig. 1), we observed rapid increase in the SOA mass concentrations after the photooxidation reaction had started. . After reaching the maximum concentration of 390 -476 µg m -3 , the 135 SOA concentration declined by 27.9 ± 9.2 % at the end of experiment because of particle deposition on the chamber wall and/or aerosol evaporation. The O:C ratio (oxygen to carbon ratio) of the SOA particles were slightly increased from the initial 0.39 ± 0.015 to the final 0.44 ± 0.01 because of aerosol aging. In a distinct contrast, the mass concentrations of ammonium component were still rising at the stage of decreasing SOA masses. Together taking into account the fact that aerosol wall deposition loss was present resulting in decreasing organic mass (and decreasing 140 sulphate mass in the seeded experiments), our results suggest new production of ammonium salts. The newly formed ammonium can be partly attributed to the co-generated nitrate and sulfate as the photooxidized products of NO and SO2 in the chamber. However, the amount of the two inorganic species can't fully interpret the ammonium and we will elaborate this in more detail in Sec. 2. Similar phenomena were also observed in the seeded experiments ( Fig. 1 and Fig. 2). We need to point out that the lower cutoff diameter of SOA particle is about 35nm for AMS measurement (Zhang et al., 2004), and the majority of the SOA mass is dominated by particles in the Aitken-and accumulation modes. Thus the reported results are with SOA in CCN size in this study.

Reaction of ammoina with organic acids
To investigate the monotically increasing profile of the ammonium salt in our chamber experiment, we studied 150 the stoichiometric neutralization of formed SOA particles. The approach proposed by Zhang et al (2007) was adopted, in which the ammonium mass concentrations measured (NH4 + ,mea) in the particles were compared to the stoichiometric ammonium concentrations required (NH4 + ,pre ) to fully neutralize the measured concentrations of SO4 2-, NO3and Cl -: https://doi.org/10.5194/acp-2020-457 Preprint. Discussion started: 14 May 2020 c Author(s) 2020. CC BY 4.0 License.
where NH4 + , SO4 2-, NO3and Clrepresent the mass concentrations (in µg m -3 ) of the species and the denominators correspond to their molecular weights. The factor 18 is the molecular weight of NH4 + .
A comparison between the predicted and measured ammonium masses is displayed in Figs. 2 and S2. In both sets of experiments, the measured ammonium mass concentration was systematically greater than the predicted value.
The trend doesn't show a dependence on the presence of NOx in the chamber. On average, NH4 + ,mea is 400 ± 156 % 160 and 21 ± 11 % greater than NH4 + ,pre at the end of nucleation and seeded experiments, respectively. The large discrepancy between the measured and predicted ammonium concentrations suggested that the current amount of sulfate, nitrate and chloride is insufficient to neutralize the ammonium formed in the particle phase, which indicates that organic component must have played a role in this process. Considering the nature of NH3 as a base compound, the candidate species of organic compounds are attributed to organic acids.

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Since a vast variety of molecular compositions of organic acids may be present in the photooxidation products of a-pinene, it isn't possible to define the amount of individual organic acid required to neutralize the NH3. Therefore, we use the CO2 + ion measured by AMS to represent carboxylic functional group of organic acids. The CO2 + is not only considered as a reliable marker of oxygenated organic aerosol (e.g. Zhang et al., 2005), but is also tightly associated with the formation of organic mono-and di-acids shown in laboratory and field measurements (Yatavelli 170 et al., 2015;Takegawa et al., 2007;Alfarra et al., 2004).
Taking into account the contribution of organic acids to ammonium salt, we reformulate Eq. (1) to: where CO2 + _NH4 is the mass concentration of carboxylic function group (-CO2) representing organic acids which were required to neutralize the ammonium. The denominator 44 is the molecular weight of carboxylic functional group.

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To interpret NH4 + ,mea, NH4 + ,pre should equal to NH4 + ,mea: 180 The time series of estimated CO2 + _NH4 over each experiments is shown in Fig. 3. On average, the CO2 + _NH4 concentration required to explain the observed ammonium concentrations was 48.6 times lower for seeded experiments than for the nucleation experiments. Fig. 3 indicates that organic acids participated in reacting with NH3 much earlier in the low-NOx test (red and green mark) than in the high-NOx experiment (blue mark). Based on the time series in Fig. 3, in the high-NOx test, the time at which the organic acids started to play a role in ammonium 185 formation was delayed by 31 and 100 minutes for the nucleated and seeded experiments, respectively. This observation can be associated to the formation of nitric acid (HNO3) from photooxidation of NOx compounds in the high-NOx conditions. The reaction of HNO3 and NH3 takes precedence over the reaction between organic acids and NH3. In general, the required CO2 + _NH4 accounted for the 27.0 ± 3.1 % of total CO2 + mass in the nucleation experiments and 18.7 ± 6.0 % in the seeded SOA experiments.

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To further verify our conclusion that organic acids are the drivers of the ammonium formation, we explored the size distribution of organic acids (represented by CO2 + ion), ammonium and nitrate at the end of nucleation experiments (Fig. 4). The mode diameters of the three species, determined by performing log-normal fitting on the size distributions, are listed in Table S1. The mode diameter of CO2 + ion is about 5-13 nm greater than that of ammonium and nitrate in the three individual experiments. The slight difference in two species mode diameters might 195 https://doi.org/10.5194/acp-2020-457 Preprint. Discussion started: 14 May 2020 c Author(s) 2020. CC BY 4.0 License. be associated with the lower evaporation rate of organic CO2 + than ammonium ions on the AMS vaporizer. Anyhow, the similarity in the mode diameters and size distributions of three chemical species suggests that they are internally mixed in the physical phase and are originated from the similar formation sources.

Connecting gas compounds by PTRMS to ammonium ion by AMS
To investigate the organic acid species which may potentially contribute to the ammonium formation in our 200 experiments, we first examined the organic monocarboxylic acids in the gas phase formed in the photooxidaiton of α-pinene. Fig. 5(a1) shows the correlation between the concentration of butyric acid (C4H8O2) (or its isomer) measured by PTRMS and the particle-phase ammonium salts measured by AMS in the nucleated SOA experiments.
The excellent linear correlation (coefficient R 2 ≈ 1) of these two species implies that the formation of butyric acid is associated with the ammonium formation. In addition, we also identified a molecular ion C4H8O2 + in the AMS mass 205 spectrum, which also shows an excellent correlation to the formed ammonium (Fig. 5c1) and thus indicates its simultaneous formation with ammonium ion. The simultaneous observation of the same ion molecules in both gas and particle phases gives us confidence to speculate that this ion is derived from gas-phase butyric acid. The observation suggests a reaction between butyric acid and NH3 which enables the production of ammonium butyrate salts (NH4C4H7O2). The formation of the salts favors the condensation of butyric acid on the particle phase 210 contributing to the observed ammonium ions. Although we can't overlook the fact that ammonium butyrate is severely fragmented inside AMS, it is believed that the detected C4H8O2 + ion signal is a residual of parental molecule.
In the same way, we also explored pentanoic acid (C5H10O2) (or its isomer) ( Fig. 3b1 and 3d1) and obtained similar results as with butyric acid.
A similar comparison has also been made in the seeded SOA experiments (Fig. 5a2-d2). Because of the presence 215 of ammonium sulfate seeds, the ammonium attributed to the organic acids reaction (defined as NH4,orgacid) is estimated to be the difference between NH4 + ,pre and NH4 + ,mea. The butyric and pentanoic acids measured by PTRMS were then compared to the calculated NH4,orgacid in the particle phase. We have observed a good correlation of butyric acid to the organic acid-driven ammonium (NH4,orgacid) (r 2 = 0.68-0.73), and a moderate correlation of pentanonic acid to NH4,orgacid (r 2 = 0.22-0.63). Compared with the nucleation experiments, the correlation relationship in the seeded SOA 220 experiments is worse, mainly because the signals in both PTRMS and AMS measurements are weak in the seeded SOA experiments. Similar to the observation in the nucleated SOA experiments, the ion C4H8O2 + in AMS mass spectrum was also identified to correlate to NH4,orgacid. The ion has the same molecular formula as butyric acid. The concentration of C5H10O2 + of the same molecular formula as pentanoic acid was in a concentration level of 10 -3 ug m -3 (Fig. 5d2). Such signal is at the same level as the background noise in AMS measurement, making it challenging to 225 correlate with NH4,orgacid. In general, the results in the seeded SOA experiments are consistent with those in the nucleated SOA experiments, confirming the role of butyric and pentanoic acids in the formation of ammonium salt.
We also extend our study to other four types of gas phase organic acids measured by the PTR-MS. Fig. 4 shows the relationship of the formic acid, acetic acid, propionic acid and pinonic acid measured in the gas phase with the ammonium ion measured by AMS. Surprisingly, the gas phase formic acid, acetic acid and propionic acid were in in 230 good agreement with particle phase ammonium concentration with excellent linear correlation coefficients r 2 ≥ 0.89 in the nucleated SOA experiments and r 2 ≥ 0.69 in the seeded SOA experiments. The good correlation of gas phase organic acids with the particle phase ammonium salt suggests that theses acids played a role in the formation of ammonium. Formic acid and acetic acid are the most abundant organic monoacids in the atmosphere (Chebbi and Carlier, 1996), whose one significant source is from photooxidation of a-pinene and other alkenes and terpenes 235 https://doi.org/10.5194/acp-2020-457 Preprint. Discussion started: 14 May 2020 c Author(s) 2020. CC BY 4.0 License. (Friedman and Farmer, 2018). Propionic and butyric acids are also other important organic acids in this process (Nah et al., 2018a;Nah et al., 2018b;Chebbi and Carlier, 1996). These low molecular weight monoacids have a vapor pressure of 6.8 ~ 8.1 µg m -3 (log10(C * )) (Friedman and Farmer, 2018), and are generally considered to be too volatile to be distributed substantially to the particle phase. However, their presence in the aerosol particles is ubiquitous in various areas over the world, although the levels of these monoacids are one or two order of magnitude lower than 240 those in the gas phase (Nah et al., 2018a;Fisseha et al., 2004;Chebbi and Carlier, 1996). A study conducted in a forest-agriculture area in Atlanta showed that the acetic acid and formic acid are the second and third richest watersoluble organic acids in the particle phase and their molar fractions to the total individual acid concentrations in the particulate phase were 5.8 ± 5.0% and 3.6 ± 3.6%, respectively (Nah et al., 2018a). In addition, the presence of NH3 as a strong base facilitates the shift of the equilibrium of these monocarboxylic acids and NH3 to the particle phase 245 (Barsanti et al., 2009). Under a base environment, a higher molar fraction of formic acid and acetic acid has also been observed in the particle phase (Nah et al., 2018a). These organic acids could exist in the particle phase in chemical forms of ammonium formate, ammonium acetate and ammonium propionate salts (Barsanti et al., 2009;Smith et al., 2008;Becker and Davidson, 1963), and thus contribute to the observed ammonium ions.
In an obvious contrast to the observation of C1-C5 monoacids above, the pinonic acids didn't show a linear  However, the lack of positive correlation between the pinonic acid and ammoniums can be caused by several reasons in this study. Firstly, the vapor pressure of pinonic acid is about 4 ~ 5 orders of magnitude lower than those of C1-C5 monoacids , so pinonic acid can condense on the particle phase independent on NH3.
Meanwhile, the concentration of pinonic acid accounts for less than 1% of the total mono-acids based on the PTR-MS results. Hence, the contribution of pinonic acid to the formed ammonium was estimated to be less than 1%, 260 considering that the acidity strength of pinonic acid is similar to other C1-C5 monoacids (acid dissociation constant (pKa) of pinonic acid is 4.8 (Howell and Fisher, 1958), close to the pKa values of formic acid (3.75), acetic acid (4.75), propionic acid (4.86), butyric acid (4.83) and pentanoic acid (4.84) (Lide, 2007)). Therefore, after α-pinene was completely consumed and pinonic acid formation had ceased in the gas phase (pink region, Fig. 6), the rapid condensation of pinonic acid on the particle phase or chamber wall causes a non-linear correlation of pinonic acid to 265 ammonium. Secondly, the gas-phase pinonic acids could be further reacted away by OH radicals, which also contributes to the non-linear observation. Field experiment has shown the relatively low atmospheric PM concentrations of pinonic acid measured in summer because of the consumption of pinonic acid by OH radicals (Szmigielski et al., 2007).
Analogously, the scattering plots for the gas-phase organic dicarboxylic acids and particle-phase ammonium salts 270 are shown in Fig. 7. We chose malonic acid and succinic acid as representative organic diacids, two of the most abundant dicarboxylic acids measured in the atmospheric aerosols (Chebbi and Carlier, 1996). The nice correlations of the gas-phase malonic and succinic acids to the particle-phase ammonium in this study suggest that diacids contribute to the formation of ammonium in both nucleated and seeded SOA experiments.
Our results qualitatively demonstrate that in the photooxidation of α-pinene, the presence of NH3 drives the gas-275 phase mono-and dicarboxylic acids to the particle phase and promotes the SOA mass concentration in the CCN size.
Previous studied have shown that the presence of NH3 can significantly enhance SOA formation from the apinene/ozone/photooxidation system because of the interaction of NH3 with gas-phase organic acids (Na et al., 2007;Babar et al., 2017), which is consistent with our results. Carboxylic acid is one of the key species in determining SOA physico-chemical property. Our results may prompt us to reconsider the pathway of gas phase organic acids involving 280 in SOA formation in the atmosphere, whether they directly participate partitioning between the gas and particle phases, or they undergo secondary conversion via reaction with NH3 before they condensate on the particle phase in the atmosphere. After SOA are formed, carbonyl group of chemical compounds in SOA particles can also uptake NH3 heterogeneously to form nitrogen-containing compounds (Zhu et al., 2018;Updyke et al., 2012;Dinar et al., 2007) and organic ammonium salts (Schlag et al., 2017). However, in this study the N:C ratios measured 285 by AMS remained nearly constant at 0.002 for E0322 and E0326 and 0.004 for E0327 suggesting that the carbonyl-NH3 heterogeneous reaction could be negligible. The interaction of NH3 and SOA affects the cloud condensation nuclei (CCN) and hygroscopic growth of SOA particles and may have a potential impact on climate change (Dinar et al., 2007). The oxidation level of MO-OOA is represented by an O:C ratio of 0.47 in this study, which is close to the value of 0.48 for MO-OOA determined in New York City in Summer (Sun et al., 2012). Its time series shows a good correlation to the measured gas-phase monocarboxylic and di-acids such as butyric and succinic acids. The second 295 factor LO-OOA is featured by an O:C ratio of 0.38. Its time series is related to the gas-phase oxidant products such as pinonaldehyde. The mass spectra profiles and time series of the two factors are shown in Fig. S3. In this study, the formation of ammonium salts is consistent with MO-OOA factor and also organic mono-and di-acids (Fig. 8). A higher oxidized organic factor is usually associated with the formation of organic mono-and di-acids. These results further suggest that the ammonium has a close relation to the organic acids. Our observation is in agreement with the 300 study by Schlag et al. (2017) where they showed by field data that NH4 is associated with a more oxygenated organic aerosol factor.

Conclusions
The SOA experiments were carried out from photooxidation reaction of α-pinene in the presence of NH3 in a 29m 3 indoor simulation chamber. Experiments were designed for SOA formation in the presence of ammonium 305 sulfate seeds and at the absence of seed aerosols. The chemical composition and time-series of compounds in the gasand particle-phase were characterized by an on-line high-resolution time-of-flight proton transfer reaction mass spectrometer (PTRMS) and a high-resolution time-of-flight aerosol mass spectrometer (AMS), respectively.
After the precursor α-pinene was consumed in the chamber, the mass concentration of organic aerosol in CCNsize was decreased because of aerosol wall deposition or evaporation, the ammonium concentration was still rising,

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suggesting the continuous formation of ammonium. AMS results showed that organic acids were required to neutralize the observed ammonium salt. The CO2 + ion was selected to represent organic acids. The amount of CO2 + required for neutralizing ammonium accounted for the 27.0 ± 3.1 % of total CO2 + mass in the nucleated SOA experiments, and 18.7 ± 6.0 % in the seeded SOA experiments. The good correlation of organic monocarboxylic acids (such as formic acid, acetic acid, propionic acid, butyric acid and pentanoic acid (or their isomers)) in the gas 315 https://doi.org/10.5194/acp-2020-457 Preprint. Discussion started: 14 May 2020 c Author(s) 2020. CC BY 4.0 License. phase to the ammonium salts further qualitatively confirms an affective role of organic acids for the ammonium formation. The same conclusion is also applied to the organic dicarboxylic acids such as malonic and succinic acids.
In addition, the formed ammonium salts correlated well to the more-oxidized oxygenated organic aerosol (MO-OOA), which is consistent with the conclusion that organic acids contributed to the observed particulate ammonium.
Our work firmly shows the direct contribution of NH3 to the CCN-size SOA formation through the organic acids-320 base reaction. The increase in SOA mass and the change of chemical composition due to NH3-SOA interaction could change the hygroscopocity, CCN ability and optical property of aerosol particles, which may alter the aerosol impact on climate change and need to be studied in the future.

Code/Data availability.
The data included in this paper can be obtained by contacting the authors.
LH, EK, AL and AV designed and conducted the experiments. LH and EK performed the data analysis with contributions by DW and AV. LH and AV wrote the paper with contributions from all co-authors.

Competing interests.
The authors declare that they have no conflict of interest.

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(2) In the seeded SOA experiment, because of the presence of ammonium sulfate seeds, the maximum mass concentration of newly formed ammonium salt was estimated from the difference between NH4 + ,pre and NH4 + ,mea, refer to the text for details.      and seeded SOA experiments (right four panels). Top panels: Linear correlation between butyric (C4H8O2) and pentanoic (C5H10O2) monoacids measured by PTRMS measured and the ammonium by AMS; Bottom panels: Linear 775 correlation of two fragmental ions C4H8O2 + and C5H10O2 + to the ammonium measured by AMS. Note that the two fragmental ions have identical ion molecular formula to those of butyric and pentanoic acids. In the seeded SOA experiments, the AMS NH4 + is the difference between NH4 + ,pre and NH4 + ,mea, refer to the text for details.