# Superoxide and Nitrous Acid Production from Nitrate Photolysis Is Enhanced by Dissolved Aliphatic Organic Matter

Xinke Wang, Evan Dalton, Zachary Payne, Sébastien Perrier, M. Riva, Jonathan Raff, C. George

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## To cite this version:

Xinke Wang, Evan Dalton, Zachary Payne, Sébastien Perrier, M. Riva, et al., Superoxide and Nitrous Acid Production from Nitrate Photolysis Is Enhanced by Dissolved Aliphatic Organic Matter. Environmental Science and Technology Letters, 2021, 8 (1), pp.53-58. 10.1021/acs.estlett.0c00806. hal-03159673

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Submitted on 13 Oct 2021

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1 Superoxide and nitrous acid production from nitrate photolysis is enhanced by
2 dissolved aliphatic organic matter

3 Xinke Wang,†,# Evan Z. Dalton,‡,# Zachary C. Payne,‡ Sebastien Perrier,† Matthieu Riva,†

4 Jonathan D. Raff,‡,§,* and Christian George†,*

5 †Univ Lyon, Université Claude Bernard Lyon 1, CNRS, IRCELYON, F-69626, Villeurbanne,
6 France.

7 ‡Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington,
8 Indiana 47405, United States.

9 §O'Neill School of Public and Environmental Affairs, Indiana University, 1315 East 10th Street,
10 Bloomington, Indiana 47405, United States

11 *To whom correspondence should be addressed. Email: christian.george@ircelyon.univ-lyon1.fr,
12 jdraff@indiana.edu

13 #These authors contributed equally to this work

14
**ABSTRACT:** Nitrate anion ($NO_3^−$) is ubiquitous in the environment and its photochemistry produces nitrous acid (HONO), a major source of tropospheric hydroxyl radical (OH). Enhanced HONO(g) emissions have been observed from $NO_3^−(aq)$ photolysis in field studies, although the underlying reasons for this enhancement are debated. Here, we show that the enhancement is induced by changes in secondary nitrate anion photochemistry due to dissolved aliphatic organic matter (DAOM). Increased yields of superoxide radical ($O_2^−$) and HONO were observed when $NO_3^−$ solutions (pH 6) were photolyzed in the presence of DAOM surrogates of varying solubility. An additional experiment titrated with additional DAOM showed a further increase in $O_2^−(aq)$ and HONO(g) simultaneously with decreased yields of gaseous nitric oxide (NO) and nitrogen dioxide ($NO_2$). To our knowledge, this is the first time that superoxide was directly observed as an intermediate in nitrate photolysis experiments, produced through DOAM oxidation by $OH_{(aq)}$. Herein, we suggest that enhanced HONO(g) emissions from $NO_3^−(aq)$ photolysis result from the reaction of $O_2^−(aq)$ with $NO_{2(aq)}$ and $NO_{(aq)}$ to respectively form peroxynitrate ($OONO_2^−$) and peroxynitrite ($OONO^−$), which are precursors to nitrite ($NO_2^−$). Overall, this points to an important role of $O_2^−$ in aqueous aerosol chemistry, which is currently under-appreciated.

## INTRODUCTION

Nitrate anion ($NO_3^−$) is a ubiquitous, photochemically-active compound in the atmospheric condensed phase, surface water, and on boundary layer surfaces. The photochemistry of nitrate anions has been extensively studied over the last decades due to its importance as a major source of aqueous hydroxyl radical (OH).¹,² Several studies have shown that pH, nitrate concentration and the presence of organic OH scavengers play important roles in the nitrite yield, which lead to the large variations in the measured values of the quantum yield $Φ(NO_2^−)$ from 0.2% to 4%.³⁻⁶ Benedict
et al.⁵ showed that Φ(NO₂⁻) is constant at high pH, but decreases with decreasing pH in more acidic solutions (pH < 5) due to HONO₄(s) loss to the gas phase⁴ or less effective scavenging of OH₄(s) by bicarbonate/carbonate. Therefore, nitrite and NO₂ quantum yields must be assessed in the presence of organic OH scavengers, which prevent conversion of aqueous NO₂⁻ and NO₂ back into NO₃⁻ by OH radicals.⁴,⁷ While the influence of OH scavengers on the primary products of nitrate photolysis is well established, the impact of organic matter on secondary nitrate photoproducts is poorly understood.

Factors that affect NO₂⁻(aq) yields influence daytime HONO(g) concentrations in the lower troposphere. Photolysis of HONO(g) is a major daytime source of OH(g) radicals that accelerates photochemical transformations leading to pollutant degradation and the formation of aerosols and tropospheric ozone.⁸⁻¹⁰ Various HONO(g) sources have now been identified, including soil bacteria and fuel combustion,¹¹,¹² gas phase reaction between OH and NO,¹³,¹⁴ photosensitized conversion of NO₂ on surfaces,¹⁵,¹⁶ and heterogeneous conversion of NO₂ on wet surfaces¹⁰ or soot particles.¹⁷,¹⁸ Recent studies suggest that HONO(g) was the major product of surface adsorbed and particulate NO₃⁻ photolysis,¹⁹⁻²¹ which is surprising since the known aqueous phase photochemistry predicts nitrite formation to be a relatively minor photolysis channel. However, there is still a significant missing daytime source of HONO(g),¹³,²² which limits the accuracy of models used to predict and study air pollution and changes to the global climate system.

Superoxide radical (and its conjugate base HO₂) has recently been hypothesized to play a role in HONO(g) emission enhancement from nitrate photolysis in environmental systems. Scharko et al. postulated the reaction between HO₂/O₂⁻(aq), formed from organic compound oxidation, and NO₂(aq) is a significant pathway, up to 20%, for aqueous NO₂ to NO₂⁻ conversion via the intermediate production and decomposition of aqueous peroxynitrate (OONO₂⁻).⁴ However, to our
61 knowledge no studies have attempted to prove this mechanism by direct measurement of
62 HO₂/O₂⁻(aq) concentrations in combination with nitrogen photoproducts during nitrate photolysis in
63 solutions with dissolved organic matter.

64 We investigate herein the yields of NO₂(g), HONO(g) from nitrate photolysis, and provide
65 the first direct measurements of O₂⁻(aq) from nitrate photochemistry in the presence and absence
66 of carboxylic acids and alcohols used to represent dissolved aliphatic organic matter (DAOM)
67 found in environmental aqueous systems. These results articulate a critical secondary mechanism
68 to explain enhanced HONO(g) emission from nitrate photolysis and suggest a key role of O₂⁻(aq) and
69 DAOM in atmospheric multiphase chemistry.

## 70 EXPERIMENTAL

71 Photochemical experiments were conducted with batch reactors containing phosphate buffered
72 solutions (pH ~6) of nitrate in the presence and absence of aliphatic carboxylic acids or alcohols.

73 Static solutions were irradiated using optically filtered Hg-Xe arc lamps (λ > 295 nm). Zero air
74 was flowed through the headspace to detect HONO(g) and NOₓ(g) using a long path absorption
75 photometer (QUMA, Model LOPAP-03) and chemiluminescence analyzer (CLD 88CYP),
76 respectively. Reported NOₓ concentrations have not been corrected for secondary photolysis,
77 which amounted to 0.7–2% and 2–7% for HONO(g) and NO₂(g), respectively. Potential gaseous
78 reaction interferences are listed in Table S3, although their influence is expected to be negligible.

79 In separate experiments, superoxide was quantitated *in situ* with a flow injection
80 chemiluminescence (FIC) analyzer (FeLume, Waterville Analytical; Figure S1B).²³ These samples
81 were stirred to reproducibility measure aqueous superoxide. The chemiluminescence probe, methyl
82 cypridina luciferin analog (MCLA), was used to selectively trap dissolved O₂⁻/HO₂.²⁴⁻²⁷ Additional
experimental details are provided in the Supplementary Information and described in detail
elsewhere.²⁸

## 85 RESULTS AND DISCUSSION

86 **NO₂ and HONO Production.** Experiments were conducted with 1 mM of surface-active DAOM
87 surrogates that form an organic film at the air-water interface under static conditions. Irradiation of
88 phosphate-buffered NO₃⁻ solutions with or without nonanoic acid (NNA, 1 mM, pH=5.8) or 1-
89 octanol (1 mM, pH=6.0) produced statistically similar concentrations of gaseous NO₂ (Figures 1A
90 and S3A). Integration of these data, after baseline subtraction of average pre-photolysis
91 concentrations, reveals a 9% enhancement in NO₂(g) emission rate in the presence of NNA, whereas
92 1-octanol caused an 8% decrease in NO₂(g) emission rate (Table S1). This supports observations by
93 Reeser et al²⁹ who showed that the NO₂(g) yield from static nitrate photolysis experiments is
94 sensitive to organic matter: 1-octanol lowered NO₂ emission relative to pure nitrate solutions, while
95 octanoic acid increased NO₂(g) emission. Reeser and coworkers suggest this is due to organic
96 surface films affecting NO₂(g) partitioning, although our results indicate that secondary
97 photochemistry also plays a role. Specifically, while 1-octanol and nonanoic acid are both effective
98 OH scavengers, 1-octanol leads to two times more O₂⁻(aq) production than does nonanoic acid
99 (Figure S4). Both HOₓ(aq) species react quickly with NO₂(aq), although in the case of the 1-octanol
100 system, higher O₂⁻(aq) concentrations result in a lower steady-state NO₂(g) concentration.

101 As shown in Figure 1B, photochemical HONO(g) production from nitrate solutions
102 containing surface-active organics is enhanced relative to pure nitrate solutions. Nitrous acid
103 formed in pure NO₃⁻ solutions reaches a steady-state, in contrast to the faster rate of HONO
104 accumulation in the presence of the OH scavengers. For example, between 20–75 min, the HONO(g)
emission rate for solutions containing NNA and 1-octanol is a factor of 2.4 and 3.2 higher, respectively, relative to the pure $NO_3^-$ solution (Table S1). This is consistent with the view that $OH_{(aq)}$ reacts with $HONO_{(aq)}$ to limit its accumulation.$^{3-6}$

To elucidate the role of DAOM solubility on product yields, we carried out similar experiments on homogeneous solutions containing one of several highly water-soluble organics: acetic (pH=5.7), propionic (pH=5.7), and valeric acids (pH=5.7), and ethanol (pH=6.0). Relative to the pure nitrate solution, emission of $NO_{2(g)}$ and $HONO_{(g)}$ were changed by a factor of between 0.58–1.01 for $NO_{2(g)}$ and 1.96–4.10 for $HONO_{(g)}$. Concentration ranges for $NO_{2(g)}$ and $HONO_{(g)}$ were comparable to experiments with surface-active organics, although different temporal trends for $NO_{2(g)}$ were observed (Figures 1C and D). At $t=20$ min, the amount of $NO_{2(g)}$ generated in the presence of soluble organics was only half of what was generated in the presence of surface-active organic molecules; subsequently, $NO_{2(g)}$ concentration increased at a constant rate, reaching concentrations between 4–8 ppb by $t=75$ min. Differences in DAOM mixing, $HO_x$ accumulation rates, and consumption of DAOM over time are likely responsible for observed differences in $NO_{2(g)}$ trends in Figure 1A, C, as discussed further in Section S7. The increasing trend in $HONO_{(g)}$ emission observed over the course of all experiments is due to the accumulation of $NO_2^-$$_{(aq)}$ and the slow diffusion of $HONO_{(g)}$ out of the solution under static conditions.

On average, secondary chemistry throughout the bulk solution leads to ~27% increase ($p < 0.05$) in the $HONO:NO_2$ ratio ($R_{enhance}$, Table S1) for homogeneous DAOM solutions. Compared to DAOM solutions with surface active organics (1-octanol and NNA), solutions of water soluble DAOM species produced on average 19±4% ($p<0.001$) less $NO_{2(g)}$ and 6.0±2.1% ($p<0.5$) more $HONO_{(g)}$. Note that differences in $HONO$ emissions between DAOM types were not statistically significant, which was due to the large standard deviation between the $HONO(g)$ data from
different homogenous DAOM species. Ethanol and propionic acid solutions yield a faster HONO(g) emission and therefore better NO₂-to-HONO conversion rates than surface-active DAOM solutions, whereas acetic and valeric acid solutions emit HONO(g) more slowly (Figure 1D, Table S1).

Regardless of DAOM type, our data agrees with a study by Scharko et al. which exhibited a suppression of NO₂(g) yields from nitrate photolysis in the presence of water-soluble organic molecules due to secondary chemistry converting NO₂(aq) to HONO(aq).⁴,¹⁶ Notably, this is a static multiphase system; other possible factors impacting NO₂(g) and HONO(g) levels are discussed in section S7.

**O₂⁻ Production.** Production of HONO(g) from nitrate photolysis in the presence of DAOM is hypothesized to involve HO₂/O₂⁻(aq) as an intermediate.⁴,⁵ We tested this hypothesis by measuring the concentration profiles for O₂⁻(aq) formed when nitrate solutions were photolyzed with UV-visible radiation (Figure S4). Irradiation of each phosphate buffered solution (pH=6) yielded a burst in O₂⁻(aq) concentration that quickly reached a maximum, followed by slight decline to quasi-steady-state levels. Photolysis of aqueous nitrate alone produces superoxide (~10 nM). This likely originates from production⁵,⁷ and subsequent photolysis of peroxynitrous acid (HOONO) to form NO(aq) and HO₂(aq) (Scheme 1a).³⁰⁻³³ The addition of NNA (pH=6.0) or 1-octanol (pH=6.0) to nitrate solutions enhances O₂⁻(aq) production 3- and 5-fold, respectively, relative to pure nitrate solutions, whereas blanks experiments do not produce significant amounts of superoxide (Figure S5). This O₂⁻(aq) enhancement agrees well with the 2- to 5-fold enhancement (R_enhance) in HONO:NO₂ ratios observed for nitrate solutions containing organic solutes (see Table S1), indicating a potential link between HONO(g) formation and HO₂/O₂⁻(aq) chemistry.

We therefore measured the response of NO<sub>y(g)</sub> species to increases in HO₂/O₂⁻(aq) that occur when ethanol is added to photolyzed solutions of nitrate and NNA. As shown in Figure 2,
photolysis of nitrate and NNA forms O₂⁻(aq), NO₂(g), HONO(g), and NO(g). Production of NO(g) is accounted for by the photolysis of NO₂⁻/HONO [j(HONO)=5.0×10⁻³ s⁻¹] and NO₂ [j(NO₂)=1.6×10⁻² s⁻¹] in the photochemical cell, as described in Section S8. Titration with ethanol simultaneously increases HONO(g) and O₂⁻(aq) while decreasing NO₂(g) and NO(g) concentrations. Furthermore, the total amount of excess HONO(g) accumulated (+45 ppb) is roughly equal to the total loss of NOₓ(g) (-56 ppb) observed. Plotting the total amount of HONO(g) produced vs. total NOₓ(g) consumed (Figure S6) shows that ~80% of the NOₓ(g) formed during the titrations is converted to HONO(g); the missing 20% is likely retained in solution as NO₂⁻(aq). Sharp increases in superoxide occur immediately before significant jumps in the HONO:NOₓ ratio (Figure S7), supporting the hypothesis that reaction of HO₂/O₂⁻(aq) and NOₓ(aq) are responsible for the observed HONO(g) emission enhancement.

**O₂⁻ and HONO Production Mechanism.** It is well known that organic matter scavenges OH(aq) produced from the photolysis of NO₃⁻.⁴,⁵ Lower OH(aq) concentrations impede reactions (1)–(3) and consequently increase the amount of NO₂(aq) and HONO/NO₂⁻(aq) formed.

$$NO_2^- + OH \rightarrow NO_2 + OH \qquad k_5=1 \times 10^{10} \text{ M}^{-1} \text{ s}^{-1} \qquad (1)$$

$$HONO + OH \rightarrow NO_2 + H_2O \qquad k_6=3 \times 10^9 \text{ M}^{-1} \text{ s}^{-1} \qquad (2)$$

$$NO_2 + OH \rightarrow H^+ + NO_3^- \qquad k_7=4.5 \times 10^9 \text{ M}^{-1} \text{ s}^{-1} \qquad (3)$$

The lowest NO₂(g) yield is indeed observed for acetic acid which has the slowest reaction with OH(aq) (Table S2), while NO₂(g) yield is higher for ethanol or valeric acid, which are the most reactive DAOM species (Figure 1C). However, our results show that homogenous organics lead to a lower initial concentration of NO₂(g) compared to pure nitrate solution photolysis (Figure 1A and C, t=20 min), indicating that secondary chemistry involving species such as HO₂/O₂⁻(aq) significantly affect the dissolved concentration of NO₂(aq).
The data support that reactions of NOₓ(aq) with O₂⁻(aq) are significant pathways by which HONOₐ(q) production is enhanced by DAOM. Superoxide reacts rapidly with NO₂(aq) (k=4.5×10⁹ M⁻¹ s⁻¹),¹,⁷,³⁴ to form peroxynitrate (OONO₂⁻), which thermally decomposes to form NO₂⁻(aq) and O₂(aq) (1.4 s⁻¹, Scheme 1c).³⁵,³⁶ Rapid reaction between O₂⁻(aq) and NOₐ(q) (4.3×10⁹ M⁻¹ s⁻¹, Scheme 1d)³⁷ is expected to be an additional pathway to NO₂⁻(aq) formation. The product, peroxynitrite (OONO⁻), may either undergo internal rearrangement to form NO₃⁻ (1.0 s⁻¹),⁷ or disproportionate to form NO₂⁻(aq) and HOONO₂(aq) (3.0×10⁴ M⁻¹ s⁻¹).³⁸ Therefore, the enhanced levels of O₂⁻(aq) would react with NOₓ(aq) and lead to higher HONO(g) emissions from DAOM solutions relative to what is seen for pure nitrate solutions. Notably, the combined production rate of HONO(g) and NO₂(g) [P(NO₂ + HONO), Table S1] is similar for all nitrate solutions, regardless of whether organics are present. This indicates that the HONO(g) enhancement in solutions containing organic compounds is due to a shift in the product distribution from NO₂(g) to HONO(g) rather than a change in the rate of product formation from NO₃⁻ photolysis.

**Influence of RO₂ on measured species.** Differences in NOₓ-to-HONO conversion between different DAOM surrogates in Figure 1 are likely due to secondary reactions impacting O₂⁻(aq) levels. Reactions between OH(aq) with DAOM leads to alkylperoxy radicals (RO₂), which form O₂⁻/HO₂(aq) via mechanisms that depend on RO₂(aq) structure. Alcohols react with OH(aq) to preferentially form α-hydroperoxyl radicals (Scheme 1b), which decompose unimolecularly to HO₂(aq), with rate constants that increase with degree of methyl substitution.³⁹ This unimolecular decomposition could explain the higher HONOₐ(q) concentrations in the presence of ethanol (Figure 1D). Carboxylic acid peroxyl radicals form less HO₂(aq) due to their stability and propensity to dimerize; the dimers eventually decompose to release HO₂/O₂⁻(aq) (Scheme 1b).⁴⁰,⁴¹ However, the system is more complicated since peroxyl radicals may further act as O₂⁻(aq) and NOₓ(aq) sinks that
yield alkyl hydroperoxyl radicals and organic nitrates/nitrites.⁴⁰,⁴²,⁴³ This may explain the trends observed for acetic, propionic, and valeric acids. Carboxylic acids with shorter alkyl chains have slower OH(aq) reaction rates (Table S2), which seems to correspond to lower NO₂(g) emissions since more OH(aq) is available to oxidize NO₂(aq). Differences in HONO(g) production do not follow the same trend, likely due to unknown differences in RO₂(aq) stability and reactivity towards NO₂(aq), NO(aq), and O₂⁻(aq). All considerations point toward the central role of RO₂(aq) decomposition and reactivity toward HO₂/O₂⁻(aq), or NOₓ(aq), which are insufficiently defined and warrant future investigations.

**Environmental Implications.** The presented work illustrates that nitrate photolysis in the actinic range ($\lambda > 290$ nm) produces HO₂/O₂⁻(aq) and HONO(g) in the presence of DAOM, with an enhancement stemming from secondary photochemistry between O₂⁻/HO₂(aq) and photoproduced NOₓ(aq). Using production rate of NO₂ and HONO as proxies for the nitrate photolysis rate, the observed j(NO₃⁻) are not significantly different from one another (Table S4), regardless whether organics are present, indicating the role of DAOM is to increase the HONO/NO₂⁻(aq) yield at the expense of NOₓ(aq). Previous studies focused on the importance of HONO production from aerosol nitrate and nitric acid use enhancement factors (EF) [i.e., the ratio of j(NO₃⁻) in the aerosol phase to j(HNO₃) in the gas phase] to describe release of HONO and NOₓ from aerosol nitrate photolysis.
As shown in Table S4, the EF calculated for pure nitrate and nitrate in the presence of organic molecules indicates an overall slower rate of NO₃⁻(aq) photolysis relative to HNO₃(g). This reflects the factor of 10 lower quantum yield of nitrate photolysis in bulk solution compared to that of HNO₃(g), which is due to rapid recombination of NO₃⁻(aq) products in the solvent cage. Nitrate adsorbed to surfaces will have an incomplete solvent cage, leading to enhanced quantum yields.⁴⁴,⁴⁵ Enhancement factors of 10–50 have been reported for HONO measurements in field
217 campaigns<sup>21,46,47</sup> which are likely due to surface nitrate photolysis occurring in the presence of
218 organics. Importantly, reactions between NO<sub>x</sub> and HO<sub>2</sub>/O<sub>2</sub><sup>-</sup> in the aerosol phase explain how
219 colocation of nitrate with organic matter can translate the rate enhancement factor into the
220 additional HONO production observed in the field. This chemistry is applicable to many aqueous
221 environments (e.g., cloud droplets, aqueous aerosols, surface water) and may represent a significant
222 source of HONO(g) in areas where surface waters or aqueous microlayers contain high
223 concentration of nitrate and DAOM. This chemistry should therefore be considered in models for
224 nitrate photochemistry and air quality.

225 **SUPPORTING INFORMATION.** Additional experimental details, discussion, 4 tables and 8
226 figures are provided. These include: materials; NOₓ and HONO measurements; quantitation of O₂⁻;
227 factors impacting NO₂ and HONO; additional experiment involving titration of ethanol to
228 irradiated NO₃⁻ solution; tabulated analysis of NO₂-to-HONO conversion for experiments;
229 schematics of the experimental setup including lamp spectrum.

230 **Notes**

231 The authors declare no competing financial interest.

232 **ACKNOWLEDGEMENTS**

233 Work at IRCELYON project was supported by the ANR-RGC programme (project ANR-16-
234 CE01-0013, A-PolyU502/16). Work at Indiana University was supported by grants from the U.S.
235 National Science Foundation (NSF) (AGS-1352375 and CHE-1609752). E.Z.D. was supported by
236 a U.S. NSF Graduate Research Fellowship (2019286041). We are grateful to Prof. Whitney King
237 (Colby College) for helpful discussions regarding the FIC analyzer.
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<figure><img src="image_3.png" /></figure>

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<caption>**Figure. 1.** NO₂(g) and HONO(g) concentration profiles during the irradiation of phosphate buffered nitrate solutions (pH 5.8 ± 0.2). The production of (A) NO₂(g) and (B) HONO(g) is in the presence or absence of NNA or 1-octanol, and the production of (C) NO₂(g) and (D) HONO(g) is in the presence of acetic acid, propionic acid, valeric acid, or ethanol, respectively. The yellow region corresponds to the irradiation period (duration 1 hour) starting at time = 16 min.</caption>
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Simultaneous measurement of NO(g), NO₂(g), HONO(g), and O₂⁻(aq) arising when solutions of nitrate and nonanoic acid (10 mM, 1 mM, pH 5) are irradiated with UV-visible light. Green dashed lines indicate points in time when 100 µL aliquots of a solution containing ethanol (1.5 M) and DTPA (50 µM) were added to initiate HO₂/O₂⁻(aq) formation. For the control experiment, aliquots of DTPA (50 µM) alone was added to the control. The yellow region corresponds to the irradiation period.</figcaption></figure>
$$
(a) \quad \mathrm{NO}_3^- \xrightleftharpoons[\mathrm{p}K_a = 6.8]{\mathrm{h}\nu, \mathrm{H}^+} \mathrm{NO}_2 + \mathrm{OH}^- \longrightarrow \mathrm{HOONO} \xrightleftharpoons[\mathrm{p}K_a = 6.8]{\mathrm{h}\nu} \mathrm{NO} + \mathrm{HO}_2
$$

$$
(b) \quad \mathrm{OH} + \underset{\mathrm{RO}_2, \mathrm{NO}}{\overset{\mathrm{R}}{\mathrm{H}}} \xrightarrow{\mathrm{O}_2} \underset{\mathrm{H}}{\mathrm{OO}} \underset{\mathrm{R}'}{\overset{\mathrm{R}}{\mathrm{H}}} \xrightarrow{\substack{\mathrm{R} = \mathrm{OH} \\ (\mathrm{R}' = \mathrm{OH})}} \underset{\mathrm{H}}{\mathrm{O}} \overset{\mathrm{R}}{=} \underset{\mathrm{R}'}{\mathrm{H}} + \mathrm{HO}_2
$$
$$
\begin{array}{r@{\,}c@{\,}c@{\,}c@{\,}c@{\,}c@{\,}c}
 & & & & & & \\
 & \mathrm{O} & & \mathrm{O} & & \mathrm{O} & \\
 & \mathrm{H} & & \mathrm{H} & & \mathrm{H} & \\
 & \mathrm{R} & & \mathrm{R} & & \mathrm{R} & \\
 & \mathrm{O}_2 & & \mathrm{O}_2 & & \mathrm{O}_2 & \\
 & & & \mathrm{R}' & & \mathrm{R}' & \\
 & & & & & & \\
\end{array}
$$

$$
(c) \quad \mathrm{HO}_2 \xrightleftharpoons[\mathrm{p}K_a = 4.8]{\mathrm{H}^+ + \mathrm{O}_2^-} \mathrm{NO}_2 \xrightleftharpoons[\mathrm{p}K_a = 3.2]{\mathrm{OONO}_2^-} \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{O}_2 + \mathrm{NO}_2^- \xrightleftharpoons[\mathrm{p}K_a = 3.2]{+\mathrm{H}^+} \mathrm{HONO}
$$

$$
(d) \quad \mathrm{O}_2^{-} + \mathrm{NO} \rightleftharpoons \mathrm{OONO}^{-} \quad \begin{array}{@{}c@{\quad}c@{\quad}c@{}}
 & \mathrm{HOONO} & \mathrm{NO}_2^{-} + \mathrm{HOONO}_2 & \mathrm{p}K_a = 5.9 \\
 \mathrm{OONO}^{-} & \mathrm{OONO}_2^{-} + \mathrm{H}^{+} & \mathrm{OONO}_2^{-} & \mathrm{p}K_a = 4.8 \\
 \mathrm{NO}_3^{-} & & & \\
\end{array}
$$

**Scheme 1.** Reaction mechanisms for HO₂/O₂⁻(aq) and NO₂⁻(aq) formation.
408 **TOC**
409 **For Table of Contents Use Only**
410 Title: Superoxide and nitrous acid production from nitrate photolysis is enhanced by dissolved
411 aliphatic organic matter
412 Authors: Xinke Wang, Evan Z. Dalton, Zachary C. Payne, Sebastien Perrier, Matthieu Riva,
413 Jonathan D. Raff, and Christian George

<figure><img src="image_5.png" /></figure>

414