Direct formation of HONO through aqueous-phase photolysis of organic nitrates

. Organic nitrates (RONO 2 ) are secondary compounds whose fate is closely related to the transport and removal of NO x in the atmosphere. Despite their ubiquitous presence in submicron aerosols, the photo-chemistry of RONO 2 has only been investigated in the gas phase, leaving their reactivity in condensed phases poorly explored. This work aims to address this gap by investigating, for the ﬁrst time, the reaction products and the mechanisms of aqueous-phase photolysis of four RONO 2 (i.e., isopropyl nitrate, isobutyl nitrate, α -nitrooxy acetone, and 1-nitrooxy-2-propanol). The results show that the reactivity of RONO 2 in the aqueous phase differs signiﬁcantly from that in the gas phase. In contrast to the gas phase, where RONO 2 release NO x upon photolysis, the aqueous-phase photolysis of RONO 2 leads primarily to the direct formation of nitrous acid

Furthermore, NOx removal from the atmosphere can occur via either RONO2 deposition to the Earth's surface or by 30 transformation into a less reactive chemical compound, such as nitric acid (Hu et al., 2011;Nguyen et al., 2015). Therefore, their atmospheric reactivity and fate must be considered to accurately predict pollution transport on a regional scale. This is especially important for world regions, such as Europe and North America, where the relative importance of RONO2 in NOx transport and removal is increasing due to the overall transformation of NOx into RONO2 (Romer Present et al., 2020).
RONO2 are not only present in the gas phase, as some of them have low volatility and can partition into condensed phases. 35 As a result, RONO2 account for a significant fraction of submicron organic aerosol, ranging from 5% to 77% (Kiendler-Scharr et al., 2016;Ng et al., 2017). RONO2 reactivity in condensed phases may differ from that in the gas phase and may affect their role as NOx reservoirs. For instance, it is well-established that the hydrolysis of tertiary and allylic RONO2 serves as a fast and permanent sink of NOx in the atmosphere, as the nitrate group is transformed into nitric acid (Darer et al., 2011;Hu et al., 2011;Rindelaub et al., 2015). However, only a small fraction of RONO2 (between 9 % and 34 % for α-and β-40 pinene related RONO2) undergoes hydrolysis (Takeuchi and Ng, 2018;Wang et al., 2021). Other aqueous-phase reactions are thus to be considered: photolysis and •OH oxidation. Our previous studies have emphasized the significance of aqueousphase reactivity for atmospherically relevant RONO2, such as isoprene and terpene nitrates, with intermediate to high water solubilities (González-Sánchez et al., 2021. At typical cloud/fog conditions (liquid water content, LWC, of 0.35 g m -3 ), the aqueous-phase photoreactivity can act as a major sink (> 50 %) for water-soluble RONO2 (KH > 10 5 M atm -1 ), while 45 at very low LWC (3 •10 -5 g m -3 ), it can serve as a major sink for very highly water-soluble RONO2 (KH > 10 9 M atm -1 ).
Nevertheless, the fate of the nitrate group during these processes is still unknown, and it is uncertain whether this reactivity acts as a NOx sink or as an additional transport mechanism.
This work intends to address these questions for the aqueous-phase photolysis of RONO2. To do so, the fate of four RONO2 (i.e., isopropyl nitrate, isobutyl nitrate, α-nitrooxyacetone, and 1-nitrooxy-2-propanol) was experimentally investigated. The 50 aqueous-phase photolysis primary and secondary reaction products were identified and quantified, and the fate of the nitrate group was elucidated with support from theoretical calculations. The atmospheric implications of these findings are discussed.

Experimental setup 55
The aqueous-phase photolysis experiments were conducted using the experimental setup previously described in detail by (see lamp spectra in González-Sánchez et al., 2023). A constant distance of 18.4 cm between the lamp and the water surface was carefully maintained in all experiments.

Photolysis experiments
Prior to each photolysis experiment, the photoreactor was filled with mili-Q water and the RONO2 was added. The solution 65 was stirred for 30 min in the dark to ensure complete dilution of the RONO2. Meanwhile, the lamp was turned on for 10 min to stabilize the light beam. The first aliquot was sampled when the reactor was placed under the light beam, marking the reaction time zero. Photolysis reactions were performed for 4 to 7 h at 298.0 ± 0.2 K. The specific experimental conditions of all photolysis experiments are appended in Table 1.
During the reaction, aliquots were regularly sampled for offline analyses. The pH of the reaction mixture was measured 70 using a 9110DJWP pH Probe (Thermo Scientific). UHPLC-UV analyses were performed to monitor the RONO2 decay or to identify and quantify carbonyl compounds after DNPH derivatization. HPIC-CD analyses were conducted to quantify HNO2, HNO3 and organic acids. At the end of the reaction, the remaining volume was used to perform liquid-liquid extraction and GC-MS analyses to identify the formed oxidized RONO2. In experiment 1, the headspace of the reactor was monitored with a NOx analyzer to investigate the possible formation of these compounds. 75

a) Measurements of RONO2.
A binary eluent of H2O and CH3CN was used for all analyses, with a flow rate of 400 μL min -1 . Two gradients were used depending on the polarity of the compounds. For isopropyl nitrate and isobutyl nitrate, the gradient started at H2O/CH3CN 80/20 (v/v) and was gradually adjusted to 50/50 (v/v) over 3 min, held at this proportion for 1 min, and then set back to 90 80/20 (v/v) within 10 s until the end of the run at 5 min. For more polar compounds, i.e., α-nitrooxyacetone and 1-nitrooxy-2-propanol, a similar gradient was employed but the initial and final proportions were adjusted to H2O/CH3CN 90/10 (v/v) to optimize their retention times (rt). All RONO2 were detected at their maximum absorbance wavelength at 200 nm .

b) Measurements of carbonyl compounds.
To derivatize the carbonyl compounds, 500 µL of the aqueous sample was mixed with 450 µL of 0.005 M DNPH and 50 µL of 0.1 M HCl. The mixture was allowed to react for 24 hours to achieve high yields. A specific method was developed for separating and quantifying the formed hydrazones. The gradient, with a flow rate of 400 μL min -1 , started from H2O/CH3CN 80/20 (v/v) for 1 min, then was gradually adjusted to 30/70 (v/v) over 6 min, held at this proportion for 1 min, and then set 105 back to 80/20 (v/v) within 10 s until the end of the run, at 9 min.

HPIC-CD
The formation of HNO2 and HNO3 and organic acids such as formic acid and acetic acid was quantified using a DIONEX ICS-3000 High-Performance Ionic Chromatography (HPIC) with a DIONEX IonPac™ AG11-HC precolumn (4 x 50 mm) and a DIONEX IonPac™ AS11-HC column (4 x 250 mm) coupled to a CD25 conductivity detector.
A binary eluent gradient method composed of H2O and NaOH 0.1 mol L -1 aqueous solution was optimized to separate the 115 formed organic acids at relatively short retention times. At a flow rate of 1 mL min -1 , the gradient started at H2O/NaOH 0.1 mol L -1 96/4 (v/v) for 10 min, then gradually to 50/50 (v/v) during 12 min, then went back within a minute to 96/4 (v/v), and was held at this proportion until the end of the analyses at 25 min. The injection volume was 200 µL, and a constant flow of H2SO4 0.05 M continuously passed through the suppressor at a flow rate of 3 mL min -1 .

GC-MS
A Clarus ® 680 Gas Chromatograph (GC, Perkin Elmer) equipped with an Elite-5MS Capillary Column (Perkin Elmer) with 30 m length, 0.25 mm diameter, and 0.25 μm of film thickness coupled to an AxION ® iQT TM Quadrupole/Time of Flight-125 Mass Spectrometer (MS, Perkin Elmer) was used to qualitatively detect and identify oxidized RONO2 formed during the aqueous-phase photolysis experiments. RONO2 were extracted and preconcentred from the remaining solution after the end of each photolysis experiment. The remaining solutions were stored at ∼4 °C for up to 48 h before the analyses. 100 mL of the remaining solution were extracted using 3 x 20 mL of dichloromethane in a separatory funnel. UHPLC-UV analyses of the aqueous phase before and after the extraction confirmed that all RONO2 efficiently partitioned to 130 dichloromethane. The extracts were washed with 20 mL of mili-Q water and were concentrated in a TurboVap II system (Biotage). The concentration workstation used a nitrogen flow at 11 psi and a water bath at 30 °C to evaporate dichloromethane until a 500 µL sample was obtained.
One μL of the concentrated extract was then injected into the GC-MS. The carrier gas was helium at a flow rate of 1 mL min -1 . A split of 20:1 was used due to the high concentration of the compounds. The injector temperature was set to increase 135 from 60 °C to 200 °C within 1 min to prevent RONO2 thermolysis. The following program was set in the oven: 30 °C for 10 min; increase until 300 °C at a 15 °C min -1 rate; and hold for 10 min at 300 °C before the end of the analyses.
The analytes were detected with a Time-of-Flight Mass Spectrometer using electron impact ionization with an electron energy of 70 eV and an ion source temperature of 250 °C. The ion source was turned on 5 -7 minutes after the analysis started, to avoid the saturation of the source due to the solvent signal. The detector performed full scan measurements from 140 m/z = 30 to 300 amu. The mass-to-charge ratio of the ion NO2 + (m/z = 46), specific to RONO2, was extracted to detect these https://doi.org/10.5194/egusphere-2023-1169 Preprint. Discussion started: 9 June 2023 c Author(s) 2023. CC BY 4.0 License.
compounds. Seven known RONO2 were analyzed by GC-MS to investigate their retention times and fragmentation patterns (Section S1).

NOx analyzer in the reactor's headspace
A CLD 88p Ecophysics NOx analyzer was used to determine if ·NO and ·NO2 were formed and partitioned into the gas-145 phase headspace of the solution during the photolysis of isopropyl nitrate. Indeed, both ·NO and ·NO2 are highly volatile compounds (KH = 1.8 ·10 -3 M atm -1 and KH = 2.0 ·10 -2 M atm -1 , respectively, Sander, 2015) i.e. from 30 to 10 7 times more volatile than the investigated RONO2. Therefore, if any ·NO or ·NO2 were formed during the aqueous-phase photolysis, they would have partitioned to the reactor's headspace.
As the NOx analyzer monitored the headspace of the reactor, a specific experimental setup consisting of a hermetic one-liter 150 three-neck round-bottom flask was used (Fig. S2). It was irradiated by the lamplight beam on its side. Note that since the reactor's headspace was also illuminated, photolysis of isopropyl nitrate could occur in the reactor's headspace. However, although isopropyl nitrate is highly volatile, most of the compound remained in the aqueous phase in the time scale of the experiments (only 3 % of isopropyl nitrate partitioned into the reactor's headspace after 7 h, González-Sánchez et al., 2023).
The NOx Analyzer LOD is 0.1 ppbv for both ·NO and ·NO2. Considering the gas phase dilutions performed downward the 155 reactor, ·NOx could be detected, if formed, at concentrations higher than ∼2 ppbv using this set-up. Although the CLD 88p Ecophysics NOx analyzer uses a photolytic converter, interferences with the RONO2 were observed. A slight proportion (less than 0.6 %) of gas-phase isopropyl nitrate was detected as ·NO2. Further details are given in Section S2.
In addition, a control experiment was performed to test the efficiency of the gas-phase ·NO2 photolysis and conversion to ·NO under our experimental conditions by bubbling gas-phase ·NO2 into the reactor's aqueous phase and photolyzing it with 160 the lamplight. The experimental setup is depicted in Fig. S3.

Molar yield determinations
The molar yields of the primary reaction products were determined by plotting their concentrations against ∆[ 2 ], that represents the consumption of the parent organic nitrate (Eq. 1).
(1) 165 Since the reaction products were susceptible to undergo photolysis over time, the yields were calculated for the initial aliquots, sampled during the first 1-2 h of reaction. The evaporation rate of some RONO2 could be non-negligible compared to photolysis , ∆[ 2 ] was thus systematically corrected from evaporation using Eq. (2-3).

Theoretical calculations
Theoretical simulations of the photolysis reaction of isopropyl nitrate photolysis were performed in a model of aqueous solution and in the gas phase. For building the model in the gas phase, snapshots from a 10 ps QM MD dynamics were performed using a thermostat at 300 K. For building the model in aqueous solution, a water box of (22.5 Angstrom) 3 was built and equilibrated using Amber99 TIP3P water parameters. Isopropyl nitrate was then soaked in the boxed, and re-180 equilibrated in the following protocol (see Section S3): 1) An NVT MM MD at fixed isopropyl geometry during 125 ps; 2) An NPT MM MD at fixed isopropyl during 1 ns; and 3) An NPT B3LYP/6-31G*//Amber99 QM/MM PBC MD relaxing the full system for 12 ps (Bonfrate et al., 2023). The snapshots were taken from the latter, discarding the first 2 ps. For each snapshot, a water droplet of 10 Angstrom was extracted, including a spherical wall potential to avoid evaporation of water during the excited-state dynamics. In each snapshot (gas phase and aqueous solution), non-adiabatic excited state molecular 185 dynamics were operated using Tully's fewest switch surface hopping algorithm (Huix-Rotllant et al., 2023). The trajectories were started from the second excited state (S2). Excited states were computed using mixed-reference time-dependent densityfunctional theory, which can describe the multi-configuration character of wavefunctions during photolysis at the cost of a density-functional theory calculation (Huix-Rotllant et al., 2023;Lee et al., 2018).

Results 190
The results of aqueous phase photolysis of organic nitrates are presented stepwise. Since NOx are the known major primary products formed in the gas-phase photolysis of RONO2, this process is first examined in Section 3.1 which describes the attempt to measure any formation and partitioning of NOx to the headspace of the reactor. Sections 3.2, 3.3, and 3.4 present the identified reaction products in the aqueous phase including HNO2, HNO3, carbonyls, organic acids, and oxidized RONO2, and their associated yields. All results are reported in Table S1. Finally, Section 4 provides a detailed discussion of 195 the mechanisms involved focusing on the fate of the nitrate group.

Absence of NOx in the reactor's headspace
Experiment 1 investigated isopropyl nitrate (1 mM) photolysis by analyzing the reactor's gas-phase headspace with a NOx analyzer (Fig. 1a). Prior to turning on the lamp, •NO2 signal increased up to ∼ 150 ppb, corresponding to a fraction of gasphase isopropyl nitrate that was photolyzed inside the NOx analyzer photolytic converter (see Section S2 for further details).
Once the lamp was turned on (shown in shaded blue in Fig. 1a), the aqueous-phase photolysis of isopropyl nitrate started, but no ·NO signal was detected, while the ·NO2 signal peaked at 800 ppb within ∼10 min of photolysis. However, this signal did not correspond to ·NO2, as demonstrated by the control experiment where ∼ 800 ppb of ·NO2(g) were bubbled through the same volume of ultrapure water. When the lamp was turned on (shown in shaded blue in Fig. 1b), ·NO2(g) was effectively photolyzed, forming ·NO(g). In this experiment, barely any ·NO2(g) partitioned to the aqueous phase (confirmed by the 205 absence of aqueous-phase HNO2 or HNO3, measured by HPIC), and thus the photolysis of ·NO2(g) exclusively occurred in the reactor's headspace. From this control experiment, it was concluded that if the measured ·NO2 signal represented actual •NO2(g) directly formed in Experiment 1, it would be photolyzed to produce measurable amounts of ·NO(g).
Since no ·NO(g) was observed when the lamp was turned on in Experiment 1 (Fig. 1a), one can conclude that no substantial amounts of ·NO2(g) were present in the system. The signal detected as ·NO2(g) likely corresponded to another volatile N-210 containing compound that was detected by the NOx analyzer as ·NO2 signal (as isopropyl nitrate does).  Table 1); and b) photolysis of ·NO2(g) bubbled in water.

Formation of HNO2 and HNO3 215
HNO2 and HNO3 were formed during RONO2 aqueous-phase photolysis. Both compounds were detected as NO2 -, and NO3using HPIC-CD but their formation as acids was inferred by the observed fast decrease of pH (Fig. S4) and was confirmed by theoretical calculations (see Section 3.5). quantified during the aqueous-phase photolysis of α-nitrooxyacetone due to its fast hydrolysis in the HPIC system that used high pH eluents, where the molecule decomposes into lactate and NO2 - .

225
The figure shows that HNO2 was efficiently formed as a primary product during all RONO2 aqueous-phase photolysis reactions. HNO2 formation slowed down over time due to its fast oxidation to HNO3 whose time profiles present exponential growth due to its secondary formation. Since this conversion is fast, HNO3 formation of the first aliquots has been included in the HNO2 primary yields, assuming that all HNO3 was formed via HNO2 oxidation. The detailed chemistry of HNO2/HNO3 that validates this approach is discussed in Section 3.5. The HNO2 yields ranged from 40 to 59 % for isopropyl 230 nitrate (Exp 2 and 4), 59 to 62 % for 1-nitrooxy-2-propanol (Exp. 11 and 12), was of 31 ± 7 % for isobutyl nitrate (Exp. 8) and was higher than 28 % for α-nitrooxyacetone (Exp. 10).

Formation of carbonyl compounds and organic acids
The formation of primary and secondary carbonyl compounds and organic acids was observed during the aqueous-phase photolysis of RONO2 (Fig. 3). 235 For isopropyl nitrate (Fig. 3a), the main primary reaction product was acetone with yields ranging from 32 to 88 % (Exp. 2-240 4) and acetaldehyde was formed primarily with lower yields (5 %). Additionally, hydroxyacetone, formic acid and acetic acid were formed as secondary products. These compounds were likely formed via acetone photooxidation (Poulain et al., 2010). For isobutyl nitrate (Fig. 3b), formaldehyde and acetone were the main non-nitrogen-containing photolysis products (primary yields of 37 -39 % and 20 -32 %, respectively, Exp. 8-9). Additionally, isobutyraldehyde was detected as a minor product (with a primary yield of 4 -5 %). For α-nitrooxyacetone (Fig. 3c), formaldehyde and formic acid appeared as 245 primary products while hydroxyacetone and acetaldehyde were likely secondary products. The formation yields were found to be 96 ± 5 % and 79 ± 3 % for formic acid and formaldehyde, respectively. Other reaction products such as acetic acid and methylglyoxal were identified but not quantified due to interferences in the analyzers, caused by hydrolysis of αnitrooxyacetone or oligomerization of methylglyoxal (see Section S4). For 1-nitrooxy-2-propanol (Fig. 3d), formaldehyde and acetaldehyde were identified as the main primary reaction products with yields of 63 -71 % and 50 -70 %, 250 respectively. Furthermore, lactaldehyde was detected as a primary product with a minor yield of 8 -14 %. Formic acid and acetic acid were observed as secondary products, likely formed via the photooxidation of formaldehyde and acetaldehyde.

Secondary formation of oxidized RONO2
GC-MS analyses at the end of each reaction were performed to seek for nitrogen-containing organic products. For isopropyl nitrate, Fig. 4a compares the gas chromatograms obtained for the sample analyzed after 7 h of photolysis with one obtained 255 during a control experiment of isopropyl nitrate in the dark. In both chromatograms, m/z = 46 (which corresponds to NO2 + fragment) was extracted to display chromatographic peaks related to RONO2 compounds. The figure shows the formation of at least 5 oxidized RONO2 molecules (IP1, IP2, IP3, IP4, and IP5), with IP3 presenting an intensity of one magnitude higher than the others.
The observed compounds were less volatile than isopropyl nitrate (which rt = 6 min, not shown in Fig. 4a) given their higher 260 retention times and thus were probably oxidized species. The mass spectra of IP1 to IP5 confirm that all compounds were RONO2 with similar chemical structures as isopropyl nitrate (included in Fig. 4b bottom right for comparison). Apart from the NO2 + fragment, other fragments observed for isopropyl nitrate were detected. Fragments such as C3H7 + (m/z = 43), and C2H4ONO2 + (m/z = 90) were observed in IP2, IP3, and IP5 (and also IP1 for m/z = 43). Note that m/z = 43 can also correspond to an oxygenated fragment (C2H3O + ) but the resolution of 1 amu did not allow for separation from C3H7 + 265 fragments. Additionally, a specific fragment of a RONO2 bearing its nitrate group on a primary carbon atom (CH2ONO2 + at m/z = 76) was observed for IP1, IP3, IP4, and IP5. Since IP3 and IP5 combine this fragment with a fragment specific for the secondary nitrate group (C2H4ONO2 + at m/z = 90), these compounds might be dinitrates. This is the case for the most intense chromatographic peak (IP3). IP3 was thus assigned to the 1,2-propyl dinitrate molecule due to its mass spectra. Additionally, IP2 was assigned to 2-nitrooxy-1-propanol due to the C3H7O + and C2H5ONO2 + fragments (m/z = 59 and m/z = 90, 270 respectively). These identifications are consistent with the proposed mechanism (see Section 4.2). However, the absence of standards prevented from precise identification and quantification of these compounds.  Hints of the formation of an oxidized RONO2 were also observed in the non-derivatized UHPLC-UV analyses. An unidentified peak was detected at a retention time close to isopropyl nitrate (2.7 vs. 2.4 min). The peak presented similar UV absorption spectra to the RONO2 standards ( Figure S5) and was thus assigned to be IP3 (1,2-propyl dinitrate) due to its 280 major concentrations). The compound was a secondary product since its occurrence started after 2 hours of reaction. A rough estimation of its concentration was performed using average calibration curve parameters obtained for isopropyl nitrate, isobutyl nitrate, α-nitrooxyacetone, and 1-nitrooxy-2-propanol. Assuming that IP3 was a dinitrate, it represented 9 % of the consumed nitrogen at the end of the reaction.
The secondary formation of oxidized RONO2 was also confirmed for isobutyl nitrate and 1-nitrooxy-2-propanol. For 285 isobutyl nitrate, two unidentified peaks assigned to oxidized RONO2 (IB1 and IB2) were observed by UHPLC-UV. Both compounds present UV-Vis absorption spectra identical to isobutyl nitrate at lower retention times (1.6 min for IB1 and 3.1 min for IB2 vs 3.4 min for isobutyl nitrate) related to a higher polarity of the molecules. Their time profiles show that both compounds were formed through secondary reactions (Fig. S6). GC-MS analyses (performed after preconcentration of the sample) allowed for the detection of up to 9 oxidized RONO2. For 1-nitrooxy-2-propanol, four oxidized RONO2, including 290 α-nitrooxyacetone, were observed by GC-MS. The chromatograms and mass spectra as well as comments on the identification of the formed molecules are presented in SI (Section S5).
For α-nitrooxyacetone, no oxidized RONO2 were found neither in UHPLC-UV analyses nor in GC-MS analyses.

N budget during aqueous-phase RONO2 photolysis 295
Gas-phase photolysis of RONO2 is known to induce homolytic rupture of the RO-NO2 bond releasing ·NO2 to the atmosphere with yields close to 100 % (Talukdar et al., 1997;Carbajo and Orr-Ewing, 2010). This reactivity turns RONO2 into NOx reservoirs and shifts pollution transportation from the local to the regional scale. Our results show that, in the aqueous phase, a primarily formation of HNO2 (with yields ranging from 31 to 62 %) is followed by a secondary formation of HNO3. Therefore, one of the main questions about the aqueous-phase photolysis of RONO2 is if they can (or not) 300 regenerate NOx that would partition to the gas phase.
To address this question, we explored the viability of two different chemical pathways that lead to NO2 -/HNO2 and NO3 -/HNO3 in the aqueous phase. The first explored pathway was the direct formation of ·NO2,(aq) followed by its known aqueous reactivity (i.e., hydrolysis and reactivity towards other radicals). This pathway was rejected since ·NO and ·NO2 should be observable in the system under this scenario (see details in Section S6). The second explored pathway was the direct 305 formation of HNO2 in the aqueous phase. This pathway was confirmed by theoretical calculations for isopropyl nitrate aqueous-phase photolysis. Herein, the discussion focuses on this pathway and the secondary chemistry of the photolysis products in our system. Finally, a conclusion is given with proposed mechanisms of aqueous phase photolysis reactions of https://doi.org/10.5194/egusphere-2023-1169 Preprint. Discussion started: 9 June 2023 c Author(s) 2023. CC BY 4.0 License.

Direct formation of HNO2 in the aqueous phase
Theoretical calculations were performed to evaluate if the direct formation of HNO2 is possible in the aqueous phase. The results showed that the formation of HNO2 is thermodynamically favorable. Figure 5 represents the relative energy diagram of isopropyl nitrate aqueous-phase photolysis, showing that it is indeed a possible reaction. Upon photon absorption, isopropyl nitrate is in the first excited state (1) and relaxes rapidly to the minimum of this state (at 74.72 kcal mol -1 ). From 315 the excited state, it undergoes an internal conversion to the ground state through a degenerated point between the excited and the ground state, a conical intersection (2). The -ONO2 presents a pyramidal structure (instead of triangular) in the conical intersection. The process is very fast since the energy barrier is 5.47 kcal mol -1 while there is an excess of ∼ 36 kcal mol -1 of nuclei kinetic energy. From there, the molecule can come back to the ground state. However, there is enough energy to cross the transition state (4) and undergo dissociation into acetone and HNO2. 320 To further investigate the reaction intermediates, excited state non-adiabatic have been performed. In Fig. 6, a reactive trajectory in the excited state is depicted. Initially, the R-ONO2 is in a trigonal planar conformation. Once the photon is absorbed, the group displays a pyramidal conformation that allows a non-radiative conversion from the excited to the ground state via a conical intersection. This leads directly to the dissociation of ·NO2, which diffuses towards water. Interaction of ·NO2 with water favors the 180-degree twist, in which nitrogen is pointing towards water molecules, favoring thus a 325 conformation in which a proton transfer is favored, occurring in less than 1 ps. Despite this happening to be the main reaction channel, other reactions are possible in which direct formation of acetaldehyde or dissociation of HNO2 in ·OH and ·NO are observed. This is due to the excess of vibrational energy of the photoproducts encapsulated in a water cavity of a diameter around 7 Å, which prevents their diffusion. Still, in longer timescales the photoproducts will either react with water or dissipate the energy to the solvent.  Using the same type of dynamics, it was calculated that the initial •NO2 radical formation mechanisms also occurs during the 335 gas-phase photolysis of isopropyl nitrate. Likely, the energy dissipation through collisions with other molecules does not take place fast enough in the gas phase due to the absence of a cavity that keeps the fragments together. Therefore, in the gas-phase, the great amount of energy (∼110 kcal mol -1 ) held by the RONO2 upon photon absorption provokes the dissociation of the O-N bond and a subsequent diffusion of the resulting fragments as in aqueous solution. The main difference lies in the fact that the diffusion separates the resulting fragments at large distance, without the possibility of 340 proton transfer. This explains the observed direct formation of ·NO2,(g) (Talukdar et al., 1997). In contrast, in the cavity, collisions with the solvent are frequent, and thus, the photolysis likely follows the pathway with the minimum energy barrier, that leads to the formation of HNO2 and acetone.

345
Timescales are just indicative.
These calculations agree with our observations where the photolysis of isopropyl nitrate provided the direct concomitant formation of HNO2 and acetone as shown in Fig. 7. Furthermore, minor trajectories where the carbon backbone structure of isopropyl nitrate breaks leading to the formation of acetaldehyde and other species have been also experimentally observed since acetaldehyde was determined to be a primary product with low yields (∼4 %). 350

Secondary chemistry of HNO2 in the aqueous phase
Once formed in the solution, HNO2 was highly reactive as shown by its time profiles (in Fig. 7). It may disproportionate to 355 yield ·NO and ·NO2 (R1).
However, this reaction is quite slow under our experimental conditions (rate constant of 28.6 M -1 s -1 , Vione et al., 2004).
Nevertheless, considering the lamp actinic flux, the photolysis/photooxidation of HNO2 was likely its major sink. The photolysis of HNO2 and NO2is known to form ·NO and a ·OH radical (R2-4) (Mack and Bolton, 1999;Fischer and 360 Warneck, 1996;Kim et al., 2014). HNO2 can also decompose due to the additional energy of the RONO2 photolysis.
HO2· radicals were likely formed by the photooxidation of organic compounds. Since ·OH radicals were formed through HNO2/NO2photolysis they could attack the organic molecules present in the photoreactor (i.e., the RONO2, as no scavenger was used). Upon oxygen addition, the ·OH attack yielded peroxy radicals. The formation of peroxy radicals was confirmed by the dissolved oxygen time profiles: during each photolysis experiment, dissolved [O2] underwent slight decay due to the reaction of alkyl radicals (R·) and oxygen (Fig. S7). 380 Peroxy radicals can readily react with ·NO to form peroxynitrites (ROONO) that can isomerize to RONO2 (R11).
In our experiments, the formation of oxidized RONO2 during isopropyl nitrate and isobutyl nitrate photolysis was confirmed by GC-MS, and UHPLC-UV analyses ( Fig. 4 and Section S4). The possibility to form oxidized RONO2 via the 390 aforementioned reactions is consistent with the substantial number of compounds displaying the NO2 + fragment found by GC-MS analyses (up to 6 compounds for isopropyl nitrate photolysis and up to 8 for isobutyl nitrate). Nevertheless, ROONO2, if formed, were not detected due to their thermolysis during the analysis.
During isopropyl nitrate photolysis, the main formed oxidized RONO2 (IP3 in Fig. 4) was suspected to be a dinitrate (1,2propyldinitrate) since its mass spectra conjugate mass fragments that correspond to both primary (m/z = 76, CH2ONO2 + ) and 395 secondary nitrate groups (m/z = 90, CH(ONO2)CH3 + ). The formation of this compound through secondary photochemistry of HNO2/NO2agrees well with the observed secondary time profile of this product. An equivalent compound was observed during isobutyl nitrate photolysis (IB6 in Fig. S4.1).

Isopropyl nitrate aqueous-phase photolysis proposed mechanism 400
Conjugating all the reactions mentioned in the discussion, Fig. 8 proposes a complete mechanism of isopropyl nitrate aqueous-phase photolysis. Isopropyl nitrate photolyzes into acetone and nitrous acid. Nitrous acid undergoes equilibrium in the aqueous phase with nitrite. Both HNO2 and NO2can undergo photolysis yielding ·NO and ·OH radicals (R5 to 7). ·OH radicals can react with isopropyl nitrate yielding an alkyl radical that upon oxygen addition forms a peroxy radical. The peroxy radical can decompose into products (i.e., acetone, formic acid, acetic acid, hydroxy acetone, and acetaldehyde which also could be issued from acetone photooxidation), or react with ·NO or ·NO2, to form a dinitrate or a peroxynitrate. The dinitrate likely 410 corresponds to the compound detected by GC-MS (IP3 in Fig. 4) and is formed secondarily in agreement with the proposed mechanism. Additionally, IP3 was estimated to account for 18 % of the reactive N at the end of the reaction, in agreement with the 20 % of isopropyl nitrate estimated to undergo ·OH oxidation. Furthermore, HNO3 is formed through secondary reactions such as ·NO2 hydrolysis (R1) or peroxynitrite isomerization (R11).

Isobutyl nitrate and 1-nitrooxy-2-propanol aqueous-phase photolysis proposed mechanism 415
The primary formation of HNO2 was also observed during the photolysis of isobutyl nitrate and 1-nitrooxy-2-propanol in the aqueous phase (Fig. 2). The determined yields were 31 % and 59-62 %, for isobutyl nitrate (Fig. 9a), and 1-nitrooxy-2propanol ( Fig. 9b), respectively. Although no DFT calculations were performed specifically for these molecules, they likely undergo a similar photolysis process to the one detailed for isopropyl nitrate, where an adjacent hydrogen atom is captured by the -NO2 leaving moiety (as shown in Fig. 6). 420 Nevertheless, the formation of carbonyl products concomitant to HNO2 was different from those expected from the main isopropyl nitrate mechanism. The corresponding carbonyl compounds were only observed in minor proportions: yields of 5 % isobutyraldehyde and 8-10 % lactaldehyde were obtained respectively for isobutyl nitrate and 1-nitrooxy-2-propanol. The major carbonyl products were formed after the breakdown of the organic chain, probably due to the excess energy the molecules have after light absorption. This pathway has been observed during the isopropyl nitrate calculations although as a 425 minor pathway, leading to the formation of acetaldehyde. Figure S8 clearly shows that the carbonyl products formed concomitantly to HNO2 were acetone and formaldehyde (yields of 20-32 % and 37-39 %, respectively) during isobutyl nitrate photolysis, and formaldehyde and acetaldehyde (yields of 63-71 % and 50-70%, respectively) during 1-nitrooxy-2propanol photolysis.
The proposed pathways for their photolysis are given in Fig. 9. Further studies should be conducted to understand the 430 breakdown of the organic chain.

α-Nitrooxyacetone aqueous-phase photolysis proposed mechanism
During α-nitrooxyacetone photolysis, NO2could not be measured due to its base-catalyzed hydrolysis in the HPIC system (at pH = 12, Brun et al., 2023), but NO3was quantified and showed a secondary formation. Primary formation of HNO2 was 440 thus expected with a minimum yield of 28 %. Due to the formation of various carbonyl compounds, the photolysis mechanism was likely following various pathways (Fig. 10).

Conclusions and atmospheric implications
This work has investigated the fate of the nitrate group during the aqueous-phase photolysis of four RONO2 species: isopropyl nitrate, isobutyl nitrate, 1-nitrooxy-2-propanol, and α-nitrooxyacetone. Our findings suggest a completely different reactivity from the gas phase one. While RONO2 releases NOx back to the atmosphere upon photolysis in the gas phase, HNO2 is directly formed in the aqueous phase. 450 HNO2 was detected as a primary compound along with other primary products such as carbonyl compounds or organic acids.
The direct formation of HNO2 by aqueous-phase photolysis was confirmed by DFT theoretical calculations and was supported by the absence of direct •NO2 formation. Therefore, aqueous-phase photolysis of RONO2 represents both a sink of NOx and a source of atmospheric HNO2 (or HONO). The latter is an important precursor of •OH and •NO radicals. During our experiments, these secondarily formed 455 radicals were shown to be trapped in the aqueous phase, producing HNO3 and functionalized RONO2. In the atmosphere, this reactivity can potentially contribute to the sink of NOx, a source of •OH radicals in condensed phases, and an additional source of SOAaq. Aqueous-phase photolysis has been reported to be negligible in the RONO2 sinks in the atmosphere due to the hindering effect of the "solvent cage" . Nevertheless, the mechanisms of this reactivity might be relevant for more significant reactions such as the aqueous-phase •OH oxidation of RONO2, or potentially their 460 heterogeneous photolysis. Therefore, further work should be done to better assess the role of RONO2 in NOx sink and transport, in the formation of atmospheric HONO and SOA.