to-HONO Conversion on Soil Surfa - ACS Publications

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Article

Evidence for Quinone Redox Chemistry Mediating Daytime and Nighttime NO-to-HONO Conversion on Soil Surfaces 2

Nicole K. Scharko, Erin T. Martin, Yaroslav B. Losovyj, Dennis G Peters, and Jonathan D. Raff Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01363 • Publication Date (Web): 25 Jul 2017 Downloaded from http://pubs.acs.org on July 27, 2017

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Environmental Science & Technology

1 2 3 4

Evidence for Quinone Redox Chemistry Mediating Daytime and Nighttime NO2-to-HONO Conversion on Soil Surfaces

5

Jonathan D. Raff a,b,*

Nicole K. Scharko,a Erin T. Martin,b Yaroslav Losovyj,b Dennis G. Peters,b and

6 7 8

a

School of Public and Environmental Affairs, Indiana University, Bloomington, IN 47405-2204 b

Department of Chemistry, Indiana University, Bloomington, IN 47405-7102

9 10 11

ABSTRACT. Humic acid (HA) is thought to promote NO2 conversion to nitrous acid (HONO)

12

on soil surfaces during the day. However, it has proven difficult to identify the reactive sites in

13

natural HA substrates. The mechanism of NO2 reduction on soil surrogates comprised of HA

14

and clay minerals was studied by use of a coated-wall flow reactor coupled to cavity-enhanced

15

spectroscopy.

16

correlated to the abundance of C‒O moieties in HA determined from X-ray photoelectron spectra

17

of the C1s region. Twice as much HONO was formed when NO2 reacted with HA that was

18

photoreduced by irradiation with UV-visible light compared to the dark reaction; photochemical

19

reactivity was correlated to the abundance of C=O moieties rather than C‒O groups. Bulk

20

electrolysis was used to generate HA in a defined reduction state. Electrochemically reduced

21

HA enhanced NO2-to-HONO conversion by a factor of two relative to non-reduced HA. Our

22

findings suggest that hydroquinones and benzoquinones, which are interchangeable via redox

23

equilibria, contribute to both thermal and photochemical HONO formation. This conclusion is

24

supported by experiments that studied NO2 reactivity on mineral surfaces coated with the model

25

quinone, juglone. Results provide further evidence that redox-active sites on soil surfaces drives

26

ground-level NO2-to-nitrite conversion in the atmospheric boundary-layer throughout the day,

27 28

while amphoteric mineral surfaces promote the release of nitrite formed as gaseous HONO.

Conversion of NO2 to HONO in the dark was found to be significant and

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TOC/Abstract Art

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31 32

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INTRODUCTION

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Nitrous acid (HONO) is an important precursor to hydrogen oxide radicals (HOx ≡ OH + HO2)

37

that contribute to ground-level ozone and aerosol production.1-4 Concentrations of HONO in the

38

lower atmosphere fluctuate from highs of hundreds of parts per trillion (ppt) at night to minima

39

of tens of ppt during the daytime.5-11 Although daytime concentrations seem low, the short

40

photolysis lifetime of HONO (~10 min) requires a strong source flux.12 In addition, vertical

41

gradients of HONO are observed, which provide strong evidence for ground surfaces as the

42

dominant source of HONO to the atmospheric boundary-layer.8-10,13,14 Unfortunately, the exact

43

mechanism(s) responsible for the formation of HONO remain obscure. Accumulation of HONO

44

in the nocturnal boundary-layer had previously been attributed to hydrolysis of NO2 on ground

45

surfaces according to reaction (1):15-17 surfaces 2NO 2 + H 2O → 2H + + NO 2− + NO 3−

(1)

46

However, it is possible that this reaction is limited to situations involving high NO2

47

concentrations18 or when NO2 is a photoproduct generated in a solvated environment.19

48

Additional sources may also contribute to nocturnal emissions of HONO from soil, e.g.,

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ammonia-oxidizing bacteria and archaea,20,21 reactions of NO2 with phenolic compounds,22-25 or

50

iron-bearing minerals in soil.26 With respect to daytime HONO sources, the following are a few

51

of the mechanisms that have been proposed: (1) photolysis of nitrophenols;25,27 (2) displacement

52

of NO2‾ by strong acids produced during photooxidation events;28 (3) surface photochemistry

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involving NO3‾;19,29-31 and (4) reduction of NO2 on irradiated soot or humic acid (HA).32-38

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Reduction of NO2 by irradiated soil organic matter is likely an important daytime source due to

55

the widespread distribution of soil in the terrestrial environment.39 Ammann and coworkers were



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the first to demonstrate NO2-to-HONO conversion on HA surfaces irradiated with UV-visible

57

light.32,33 They suggested that the reactive sites responsible for NO2 photoreduction might be

58

phenolic in nature, although it was not possible from their data to identify the reducing

59

intermediates in natural HA substrates.33 The mechanism was instead inferred from model

60

systems that showed efficient uptake of NO2 by aqueous solutions of phenols.23,24 Furthermore,

61

studies show that UV-visible irradiation of mixtures containing photosensitizers (e.g.,

62

benzophenone or methylene blue) and polyphenols generate phenolic radicals40 that can reduce

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NO2.25,32 Such model systems are relevant to soil systems as soil organic matter is, in part,

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derived from polyphenolic lignin and tannin precursors.

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Redox properties of soil organic matter have been attributed to the presence of quinone

66

moieties.41-45 Quinones are a class of aromatic compounds that readily transition between three

67

oxidation states (benzoquinones, semiquinones, and hydroquinones) depending on redox

68

conditions (Scheme 1).46,47 Fully oxidized benzoquinones are electron-acceptors that readily

69

convert to semiquinone radicals, which are thought to give rise to the persistent electron

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paramagnetic signature observed in organic matter isolates.48-51 Semiquinone radicals may be

71

oxidized to reform the benzoquinones, or semiquinones may undergo coupled proton–electron

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transfer to generate hydroquinones, which are the fully reduced endmember. Additionally, inter-

73

conversion between benzoquinones and hydroquinones is possible via comproportionation

74

reactions

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Furthermore, benzoquinones are photoactive; when excited with light and in the presence of

76

hydrogen donors, they will generate semiquinone radicals.53 It is therefore reasonable to

77

hypothesize that soil organic matter can reduce NO2 via the semiquinone or hydroquinone

(i.e.,

one-electron

transfers

between

benzoquinones


and

hydroquinones).52

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species generated via dark or photochemistry (Scheme 1), potentially enabling it to facilitate

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NO2-to-HONO conversion at nighttime as well as during the day.

Scheme 1. Quinone Redox Chemistry and Potential Intermediates Responsible for NO2-to-HONO Conversion by Organic Matter. Color shadings indicate oxidation state.

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To our knowledge, a role of quinone redox chemistry in promoting both daytime and nighttime

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HONO formation on soil surfaces has not been explored in natural HA systems, although

82

quinone redox chemistry is thought to have an important impact on nutrient cycling and the fate

83

of pollutants in soil and aquatic systems.52,54 The objective of this work is to characterize both

84

thermal and photochemical pathways that convert NO2 to HONO on soil organic matter. The

85

approach was to expose coatings of humic acid isolates or a model quinone to NO2 in a flow tube



86

and to monitor changes in NO2 and HONO in the dark and during irradiation with UV-visible

87

light. Surrogate soil systems were constructed that reflect the important role that mineral–organic

88

matter interactions play in controlling release of HONO from soil surfaces. The reactivity was

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compared to the physical-chemical properties of the humic acid isolates to identify the chemical

90

moieties contributing to NO2-to-HONO conversion. Additionally, substrates containing

91

electrochemically reduced humic acid isolates were generated to investigate the role of

92

quinone/hydroquinone redox chemistry on the conversion process.

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EXPERIMENTAL SECTION

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Chemicals. Suwannee River Humic Acid Standard II (SRHA II, Cat # 2S101H), Elliott Soil

95

Humic Acid Standard IV (ESHA, Cat # 4S102H), Pahokee Peat Humic Acid (PPHA, Cat #

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1S103H) and Leonardite Humic Acid Standard (LHA, Cat # 1S104H) were purchased from the

97

International Humic Substance Society (IHSS). The terrestrial HAs include ESHA, PPHA, and

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LHA, whereas the single aquatic HA studied is SRHA. An industrial sourced HA from Sigma-

99

Aldrich was used for comparison (called “Sigma” in this work). Kaolinite (Fluka, natural),

100

aluminum sulfate hydrate (98%), sodium phosphate monobasic monohydrate (ACS reagent,

101

≥98%), sodium phosphate dibasic heptahydrate (ACS reagent, 98−102%), diquat (Pestanal®,

102

analytical standard), and juglone (97%) were obtained from Sigma-Aldrich. Sodium hydroxide

103

(2 N, Baker analyzed® reagent) was from J. T. Baker, and isopropyl alcohol (UltimAR®) was

104

from Macron chemicals.

105

Coated-Wall Flow Tube Experiments. Experiments investigating uptake of NO2 on

106

substrate-coated Pyrex tubes were carried out in a jacketed (23 ± 0.02 °C) Pyrex horizontal flow

107

tube (2 cm i.d. × 100 cm) equipped with a movable injector.55 The interior walls of the flow tube


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were coated with a perfluorinated polymer to minimize wall effects, and Teflon tubing and

109

fittings were used throughout the apparatus. The substrate is coated on the inner surface of a 1.7

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cm i.d. × 5.08 cm section of Pyrex tubing that is then inserted into the flow tube. The carrier gas

111

was zero air at 295 K (or nitrogen gas for the electrochemical experiments), and the total flow of

112

gas exiting the flow tube was 2100 cm3 min–1; 1000 cm3 min–1 of this flow was directed to the

113

cavity-enhanced absorption spectrometer (CEAS) for simultaneous detection of NO2 and HONO.

114

The CEAS system was described previously.19 The integration time was 20 s and the number of

115

scans averaged was 30. All experiments were carried out at ambient temperature and pressure.

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The detection limit for NO2 and HONO is 0.7 ppb and 0.2 ppb, respectively. For all experiments,

117

the relative humidity (RH) of the carrier gas was adjusted by flowing zero air through a glass frit

118

submerged in ultrapure water, followed by dilution with zero air in a 3-L reservoir equipped with

119

a RH gauge and temperature probe (Vaisala, HMT130).

120

In a typical uptake experiment, a coated tube is inserted into the flow tube and a mixture of NO2

121

in humid air (30% RH) is directed through the movable injector into the CEAS bypassing the

122

substrate coating; this is done to quantitate the initial NO2 concentration. After 20 min, the

123

injector was pulled back 36 cm behind the substrate to expose it to NO2; exposures were 130 min

124

long for the HA and 30 min when juglone was the substrate. Due to the 10 min averaging times

125

required by the CEAS, we are unable to determine the initial uptake coefficients for the reaction

126

of NO2 with HA. The reported uptake coefficients (see SI for derivation) are derived from

127

averages of data collected over the ~130 min long exposure times following the initial uptake

128

event. Hence, they approximate a steady-state uptake coefficient that is more applicable to

129

longer-term exposures of soil surfaces to atmospheric NO2. During photolysis experiments, the

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substrate was simultaneously exposed to the NO2 mixture and the filtered output (λ > 280 nm) of 7 ACS Paragon Plus Environment


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a 200 W Hg arc lamp (Hamamatsu). The lamp was positioned adjacent to the flow tube such that

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the entire coated surface was irradiated.

133

actinometry, see SI) reaching the center of the flow tube was 5.4 × 1016 and 2.8 × 1016 photons

134

cm–2 s–1 in the absence and presence of the coated sample, respectively.

135

Substrates and Coating Procedure. In the environment, soil organic matter occurs in

136

close association with clay minerals. With this in mind, we prepared model soil substrates

137

consisting of 96% kaolinite, 2% HA and 2% aluminum sulfate [Al2(SO4)3] (by weight). The

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following rationale was used for constructing this model soil system:

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phyllosilicate clay mineral commonly found in a wide range of soils; (b) the amount of HA (2%)

140

and the pH level of 5 were chosen based on values found in locally collected soil samples

141

(Crider-Urban land complex), which had a measured organic matter content of 2.5% and a pH of

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5.09;21 and (c) the added Al2(SO4)3 hydrolyzes to form an Al (hydr)oxide coating on the kaolinite

143

surface.56 The resulting Al (hydr)oxide coating is amphoteric and carries a net positive surface

144

charge at near neutral pH due to the presence of surface Al-OH2+ groups. The net positive

145

charge facilitates electrostatic binding of negatively charged HA isolates to the surface; this

146

produces a highly uniform coating of HA on kaolinite particles. In addition, the Al-OH2+ groups

147

are capable of protonating NO2‾ at a bulk pH that is 3‒4 units above the pKa of HONO.57 Lastly,

148

the Al (hydr)oxide coating is redox inactive, ensuring that results are not impacted by metal

149

redox chemistry.

150

In all cases, aqueous slurries consisting of the above-mentioned mixture were pH adjusted to 5

151

with 2 N NaOH prior to coating the flow reactor. The pH was measured with a benchtop pH

152

meter equipped with pH/ATC probe (Thermos Scientific Orion Star A211 and Ross Ultra

153

pH/ATC triode) at a substrate to water ratio of 1:2 (w/w). Each coating was prepared by

The photon flux (determined by means of NO2


(a) Kaolinite is a

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dripping a slurry of substrate in nanopure water (Millipore, 18.2 MΩ cm) onto the inner surface

155

of a glass tube and spinning the tubing to ensure that the substrate coats the entire inner surface.

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Excess substrate was removed, and the coated tube was immediately placed in a drying chamber

157

and allowed to dry overnight in air (or nitrogen gas). The resulting coating was uniform to the

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eye with a mass of 0.6 g and a thickness of 70 µm (Figure S2). The amount of substrate coating

159

the flow reactor walls was chosen based on two sets of experiments (not shown) that aimed to

160

investigate NO2-to-HONO conversion on substrates consisting of local soil or 2%

161

juglone/kaolinite (w/w). In both cases, a mass loading of 0.6 g (22 mg cm‒2) was well within the

162

region where additional soil mass had no influence on the uptake kinetics.

163

experiments, the substrate-coated tubes were placed in a RH-stabilized air equilibration chamber

164

for 30–45 min prior to being transferred to the flow tube. This is done to ensure that all

165

substrates contain similar amounts of moisture prior to and during the experiments.

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Electrochemical Experiments.

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aqueous solutions with 0.10 M sodium phosphate (pH 7) as the supporting electrolyte. Oxygen

168

was removed from the system by means of constant argon flow through the electrochemical cell,

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and the cell was darkened to avoid photolysis of humic material. In all cases, the reference

170

electrode was a homemade saturated calomel electrode (SCE), and data were processed with the

171

aid of OriginPro 2015 software. For cyclic voltammetry, an EG&G PAR 2273 instrument was

172

used with PowerSuite software. Cells and procedures used have been described in previous

173

publications.58,59

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Working electrodes were glassy carbon disks press-fitted into Teflon rods to yield an area of

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0.071 cm2, and the auxiliary electrode was a coiled platinum wire. Working electrodes were

176

polished with 0.05-µm alumina (Buehler) and sonicated in solvent between scans. Controlled-

Prior to all

All electrochemical measurements were performed in



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potential (bulk) electrolyses were performed with the aid of a PARC model 173 potentiostat. A

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locally written LabView program was used for data collection. A two-compartment (divided)

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cell was used as previously described in the literature.58,60 The anode compartment consisted of

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a carbon rod auxiliary anode in water containing sodium phosphate buffer, and was separated

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from the cathode compartment by a sintered-glass disk backed by an agar–solvent–electrolyte

182

plug. For bulk electrolyses performed in this cell, the cathode compartment contained 20 mL,

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and carbon working electrodes were reticulated vitreous carbon discs with an approximate area

184

of 200 cm2 (RVC 2X1-100S, Energy Research and Generation, Inc.).

185

Once the HA was reduced or processed (meaning that it went through the same process as the

186

reduced HA without applied potential, termed here as “non-reduced”), the solution was dried

187

with nitrogen and the recovered HA was mixed with kaolinite and Al2(SO4)3 and pH adjusted to

188

5 as described above. The coating was prepared as described above. All subsequent drying, RH-

189

stabilization, and experiments were carried out with nitrogen gas to prevent the oxidation of HA.

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The average time between retrieving the HA from the electrolysis cell to exposing the coating in

191

the flow reactor was 4 days. Flow reactor experiments involving exposure of electrochemically

192

reduced HA to NO2 were carried out in N2 to minimize the oxidation of reduced HA in air.

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X-ray

194

measurements were performed on a PHI VersaProbe II instrument equipped with a

195

monochromatic Al Kα source. Humic acid samples were used as purchased and pressed onto

196

double-sided tape, which was placed on a sample plate. Experiments were run at the base

197

pressure of 4−8 × 10−10 Torr. All XPS spectra were obtained at room temperature without

198

heating samples due to fear of decomposing the samples. Under these conditions, we believe

199

that at least a couple of monolayers of water are still present on the sample surface. Thus, we

Photoelectron

Spectroscopy

(XPS).

X-ray photoelectron


spectroscopy

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assume that substrate modifications due to dehydration reactions are negligible.

201

experiments, the X-ray power of 65 W at 15 kV was used with beam size of 260 µm at take-off

202

angles of 45°. Utilizing the Fermi edge of the valence band for metallic silver for XPS (Hell

203

line), the instrument resolution was determined to be 0.3 and 0.15 eV, respectively. Minima of

204

10−60 scans were collected for the spectra, using 0.05−0.1 eV step and 23 eV pass energy.

205

Spectra were recorded using SmartSoft-XPS v2.0 (PHI) and processed using MultiPack v9.0

206

(PHI). The instrument was calibrated to give a binding energy of 284.8 for the C1s line of

207

adventitious (aliphatic) carbon presented on the non-sputtered samples.

208

contain elemental composition from the survey scan and the percent carbon from the high-

209

resolution carbon region. The carbon region was deconvoluted using the binding energies,

210

assignments and full width at half maximum (FWHM) from Monteil-Rivera et al;61 See Tables

211

S1 and S2. A similar procedure was used by Gerin et al.62 and by Fimmen et al.63 to specifically

212

provide evidence of quinone and phenol content in dissolved organic matter. All XPS spectra

213

were obtained using pure humic acid isolates to achieve highest possible signal-to-noise ratio for

214

subsequent peak deconvolution. No significant differences in the C1s region were observed

215

between samples comprised of pure HA isolates and those where HA was coated onto kaolinite

216

other than the presence of interfering K2p3/2 lines in the latter (Figure S3).

217

RESULTS AND DISCUSSION

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NO2-to-HONO conversion on Leonardite Humic Acid (LHA). To investigate the

219

conversion of NO2 to HONO on humic acid surfaces under dark conditions, NO2 was exposed to

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a kaolinite/Al (hydr)oxide substrate containing 2% LHA, and the amounts of NO2 and HONO

221

present were monitored for 140 min. Figure 1A shows the amounts of NO2 and HONO before

222

(between 0 and 10 min) and during (10−140 min) exposure to ~60 ppb of NO2 in air. After 11 ACS Paragon Plus Environment

In all

Tables S1 and S2


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exposure of the substrate for 130 min to NO2, the average amount by which the initial NO2

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concentration decreased following exposure to the surface (i.e., ∆NO2) was 6.4 ppb; the average

225

increase in HONO concentration in the flow tube was 5.6 ppb. The resulting HONO/∆NO2 ratio

226

is 0.88, and is higher than expected if one considers only the surface hydrolysis reaction of NO2

227

(Equation 1), which would result in a ratio of 0.5.17,64 That the ratio is slightly less than unity

228

can be due to the following: (i) reactions of NO2 that do not lead to HONO;25,65 (ii) small

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amounts of HONO that remain adsorbed to the substrate (e.g., as NO2¯); or (iii) secondary

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reactions of HONO before it is emitted.55,66 Over an extended period of time, the amount of NO2

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taken up by the surface and the amount of HONO generated decreased slightly. This implies that

232

surface reactive sites are consumed during the course of the reaction. In the absence of LHA, no

233

significant uptake of NO2 was observed on the Al (hydr)oxide-coated kaolinite substrate, and the

234

amount of HONO formed, if any, was below the limit of detection (0.2 ppb, which corresponds

235

to an uptake coefficient of 1 × 10–6); see Figure S4. This is consistent with the study by Liu et al.

236

who used a more sensitive chemiluminescence analyzer to measure uptake coefficients between

237

5 × 10‒8 and 3 × 10‒8 for the adsorption of NO2 on kaolinite.67 This is three orders of magnitude

238

lower than the uptake coefficients we measured for the reaction of NO2 on humic acid coated

239

kaolinite substrates (Figure S4).

240

experiments was due to the LHA present in the substrate and not a direct reaction of NO2 with

241

the kaolinite. Further control experiments performed by replacing LHA with 2% citric acid

242

showed no reactivity between NO2 and the substrate in the dark (Figure S6). This confirms that

243

carboxylate functional groups are not responsible for the reactivity of the humic acid sample.

244

The role of LHA in reducing NO2 is therefore consistent with the presence of reduced species

245

(e.g., semiquinones) capable of donating electrons.68

This shows that the reduction of NO2 to HONO in our


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We compared the reactivity of LHA coatings toward NO2 in the dark to those irradiated with

247

UV-visible light. Figure 1B shows that, upon exposure to light, the average uptake of NO2 was

248

13.4 ppb and the amount of HONO generated was 10.7 ppb. When compared to results from the

249

dark experiment (Figure 1A), light exposure enhanced the uptake of NO2 and HONO emitted by

250

a factor of ~2. When exposed to light, LHA is photo-reduced and as a result has a greater

251

capacity to reduce NO2 relative to what is observed in the dark. Whereas the reactivity gradually

252

decreases for the substrates in the dark, the amount of NO2 taken up by the surface and the

253

amount of HONO generated during photochemical experiments remained relatively constant

254

over the course of the experiment. In addition, both photochemical and thermal pathways have

255

similar HONO yields with HONO/∆NO2 ratios of 0.88. The comparison between thermal and

256

photochemical reactivity of NO2 with LHA (Figure 1A & B) suggests that light activates reactive

257

sites that are inactive in the dark, and can regenerate them photochemically after they are

258

consumed.

259

The amount of HONO formed from humic acid surfaces in the dark is surprising considering that

260

previous studies suggest this process to be negligible when compared to HONO produced

261

photochemically in the same system.33,34,37,38 Comparison of our results to those of previous

262

studies is complicated by differences in experimental conditions used. These include: (i)

263

differences in lamp intensities and emission spectra; (ii) the phase that the humic acid was in

264

(e.g., dissolved, aerosolized, or present as a coating); and (iii) the composition and corresponding

265

point of zero charge of the substrate. Of these differences, point of zero charge of the substrate is

266

likely the most critical. Previous studies used two-component substrates comprised of humic

267

acid and water (present as the solvent or adsorbed to a surface), where speciation of N(III) and

268

partitioning between dissolved and gas phase was governed by aqueous equilibria alone. For 13 ACS Paragon Plus Environment


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those systems, the bulk aqueous pH of the system needed to be below the pKa of nitrite (~3) for

270

significant HONO outgassing to occur; HONO release was suppressed at higher pH values.34 In

271

our system, the additional contribution of surface acidity of the Al (hydr)oxide/kaolinite mineral

272

surface must be considered. As discussed above, conversion of adsorbed nitrite to HONO is

273

favored on amphoteric surfaces with high points of zero charge, where highly acidic M-OH2+ (M

274

= metal e.g., Al or Fe) groups may be abundant.

275

(hydr)oxide/kaolinite surfaces are capable of protonating nitrite at a bulk pH that is 3‒4 units

276

above the pKa of HONO.57 Thus, while the organic matter coatings are responsible for reducing

277

NO2 to NO2‾, it is the amphoteric mineral surface that facilitates protonation of NO2‾ and release

278

of gaseous HONO over a wide pH range.

279

Reactivity of Electrochemically Reduced Humic Acid.

280

photochemical reduction of NO2 to HONO by organic matter involves a semiquinone

281

intermediate that is formed following photo-excitation of a benzoquinone, as shown in Scheme

282

1. Previous studies have attributed the free-radical signature in electron paramagnetic resonance

283

studies of organic matter to the presence of stable semiquinone moeties. 48 If stable semiquinone

284

groups are generated when LHA is irradiated with UV-visible light, we should be able to form

285

them electrochemically. If formed, we expect NO2-to-HONO conversion on reduced HA to be

286

enhanced relative to the non-reduced HA—similar to the type of enhancement that was observed

287

for the photochemical vs. dark experiments shown in Figure 1A and B. LHA was chosen for this

288

particular study due to its high electron-accepting capacity and presumably high abundance of

289

reducible quinone groups.41

290

Samples of electrochemically reduced LHA were generated in bulk electrolysis cells via the

291

mediated electrochemical reduction technique.69,70 The reduced substrates were subsequently

Our previous study showed that Al


We hypothesize that the

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isolated and used as a substrate for flow reactor studies. The advantage of electrochemically

293

reducing HA at an electrode is that we avoid the use of strong chemical reductants70 that can by

294

themselves reduce NO2. The disadvantage of the system is that LHA is a bulky material that

295

limits the rate of electron transfer and makes observation of direct reduction by cyclic

296

voltammetry difficult. For this reason, diquat dibromide monohydrate (diquat) was used as a

297

mediator for electron transfer. Figure 1C shows cyclic voltammograms recorded for diquat alone

298

and in the presence of 1 or 2 g L–1 LHA at a glassy carbon electrode. Low concentrations lead to

299

limited peak currents.

300

mediator, and the marked increase in cathodic current at –0.64 V vs. SCE upon addition of LHA.

301

For the purpose of preparing reduced LHA, controlled-potential electrolyses were performed at –

302

0.84 V vs. SCE with LHA concentrations of 2 g L-1 and 30 µM diquat. An average of 1.3 mmol

303

e– were transferred per gram of LHA, which agrees with previously reported values.46

304

Figure 1D displays the average uptake of NO2 and HONO emitted when ~60 ppb of NO2 was

305

exposed to flow reactor coatings comprised of either non-reduced or electrochemically reduced

306

LHA, potassium phosphate, and diquat at pH 5. In both the dark and when irradiated with UV-

307

visible light, the electrochemically reduced LHA was found to be most reactive, producing twice

308

as much HONO as the non-reduced substrate. Interestingly, regardless of the reduction method

309

employed (photochemical or electrochemical), the reduced LHA substrates have the capacity to

310

enhance the uptake of NO2 and HONO formation by a factor of 2 relative to their non-reduced

311

counterparts. This similarity in enhancement factors supports the idea that semiquinones and

312

phenols generated from (electrochemical or photochemical) reduction of benzoquinone moieties

313

are responsible for NO2-to-HONO conversion in the photochemical system.

However, notable is the reversible behavior visible for the diquat



Page 16 of 38

314

It is worth noting that the reactivity of the electrochemically modified substrates (Figure 1D)

315

appears to be lower than untreated substrates (Figure 1A & B). We suspect this is due to the

316

presence of diquat in the latter experiments. Diquat is a herbicide known to bond strongly to soil

317

particles.71 It is possible that diquat molecules form strong electrostatic interactions with the

318

carboxylate and phenolate groups within the HA,72 thus limiting the access of NO2 to HA

319

reactive sites. An alternative explanation is that diquat lowers reactivity by scavenging electrons

320

before they can be transferred to NO2.

321

Structure and Reactivity of HA in the Dark. It is well known that HAs from different

322

origins have different reducing capacities that depend on their structure and the functional groups

323

present.41,73 To understand better how HA structure impacts the conversion of NO2 to HONO, a

324

series of NO2 uptake experiments was carried out on various types of HA in the absence of light.

325

Figure 2A shows the direct relationship between the average amounts of HONO generated and

326

the corresponding amount of NO2 reacted on the substrates. The trend for the amount of HONO

327

formed and NO2 reacted is Sigma < LHA < ESHA < PPHA < SRHA. Linear regression of the

328

data in Figure 2A yields a slope that corresponds to a [HONO]/∆NO2 ratio of 0.84 ± 0.02 (R2 =

329

0.99).

330

The differences in reactivity among the various substrates are not entirely surprising as each HA

331

is expected to be comprised of different moieties that influence their structure and reducing

332

capacities.68 To understand why certain HAs form more HONO than others, the dark reactivity

333

was correlated to chemical properties of the various HAs. Figure 2B shows the average amount

334

of HONO (from Figure 2A) plotted versus the electron-donating capacity (EDC) for each HA (at

335

0.61 V, pH 7), as measured by Aeschbacher et al.41 These authors showed that the EDC for HA

336

derived from aquatic environments is greater than what it is for terrestrial HAs.41 Our data are 16 ACS Paragon Plus Environment

Page 17 of 38


337

generally consistent with this observation. As shown in Figure 2B, the aquatic HA (SRHA) has

338

the highest reactivity and EDC, followed by the terresitrial HAs; the industrial-sourced HA

339

(Sigma) has the lowest reactivity and EDC. This is consistent with previous findings that

340

organic matter from natural waters and soil has a higher phenol content than does HA from

341

Sigma-Aldrich.41 It should be noted that the EDC is a measure of the total moles of electrons

342

donated by reduced HA to an electrode, whereas the dark reactivity we measure here reflects the

343

electrons transferred to NO2 and subsequently released as HONO(g). Thus, lack of a tight

344

correlation between reactivity and EDC may indicate that some of the electrons are transferred to

345

components in the coating other than NO2 (i.e., trace metals, other organic moieties, or

346

dioxygen). However, our attempts at correlating reactivity to trace metal content (i.e., trace

347

metal abundances provided by IHSS or determined by XPS) did not yield any significant

348

correlations.

349

We used X-ray photoelectron spectroscopy (XPS) to analyze directly the C/O moieties of the

350

humic acid isolates,63 with the goal of relating surface composition to the observed reactivity.

351

High-resolution C1s spectra of all HA isolates were fit to quantitate the distribution of carbon

352

corresponding to known functional groups. Many studies have been devoted to interpreting and

353

quantitating the C1s spectra of materials in environmental and biological systems.61-63,74

354

Although it is possible that multiple functional groups can occur at the same binding energy, we

355

use the approach of Gerin et al. where the functional groups considered are as follows: (a) C‒C

356

and C‒H groups at 284.9 eV; (b) C‒O and C‒N groups at 286.4 eV; (c) C=O and O‒C‒O groups

357

at 287.5 eV; and (d) O=C‒O, O=C‒N, and CO32‒ at 288.7 eV (Tables S1 & S2).62 Of particular

358

interest are the peaks at 286.4 and 287.5 eV, which include contributions from

359

phenol/hydroquinone and benzoquinone functional groups, respectively. For all experiments, no



Page 18 of 38

360

statistically significant correlations were observed between the fitted areas determined for the

361

peak centered at 288.7 eV and elemental N or amino acid content (as reported by the

362

International Humic Substance Society)75 of the HA isolates studied here. In addition, we

363

excluded from our analysis peaks at 284.9 eV, which could be influenced by carbon

364

contamination from the room air. Thus, in the discussion below, we limit our analysis to the

365

peaks at 286.4 and 287.5 eV.

366

Peak areas derived from high-resolution C1s spectra were plotted along with the amount of

367

HONO formed during the reaction of NO2 with the substrate to infer relationships between

368

structure and reactivity (Figure 2C & D). The strongest linear correlation was found between the

369

[HONO]dark and the peak area corresponding to C‒O (Figure 2C). There was no significant

370

correlation between [HONO]dark and the C=O peak area (Figure 2D), suggesting that C=O

371

functionalities may not be participating in the observed reactivity. The statistically significant

372

positive correlation to C‒O content and the lack of any relationship to C=O content provides

373

evidence in favor of the hypothesis that the reducing ability of HA is driven by semiquinones or

374

hydroquinones, which are capable of donating electrons to NO2.

375

HA Structure and Photochemical Reactivity. Figure 3A shows the relationship between

376

the amounts of HONO generated photochemically ([HONO]total) and amounts of NO2 reacted

377

when NO2 is flowed over various humic acid substrates that are irradiated with UV-visible light

378

(λ < 280 nm). Although the amount of HONO produced is different for each humic acid, the

379

overall conversion efficiency is similar regardless of HA origin. A linear regression of the data

380

in Figure 2A yields a slope that corresponds to a [HONO]/∆NO2 of 0.88 ± 0.02 (R2 = 0.99)—

381

similar to what was observed for the reaction of NO2 with HA in the dark.


Page 19 of 38


382

Data shown in Figure 3A reflect the total reactivity of the substrate due to thermal and

383

photochemical processes. To display the amount of HONO formed and NO2 reacted due to

384

photochemistry alone, [HONO]photo, we subtracted the data in Figure 2A from those displayed in

385

Figure 3A (i.e., [HONO]photo = [HONO]total – [HONO]dark). The amount of NO2 taken up on the

386

surface due to the photochemistry alone was calculated in the same way and is referred to as

387

∆[NO2]photo. As shown in Figure 3B, there are differences in the photochemical reactivity of the

388

various humic acids when [HONO]photo is plotted vs. ∆[NO2]photo. In particular, the most reactive

389

substrates are the non-peat terrestrial humic acids EHSA and LHA, whereas SRHA and PPHA

390

display the lowest photochemical reactivity. To help explain these trends, we used UV-visible

391

absorption spectroscopy and the C1s XPS spectra to provide information regarding the

392

chromophores responsible for the photochemistry.

393

Figure 3C displays the aqueous-phase UV-visible absorbance spectra for the HA series, along

394

with the normalized light intensity of the excitation source used (filtered output from a Hg lamp).

395

For the most part, the amount of HONO formed tracks the amount of light absorbed by the HAs

396

between 300‒380 nm, which is the range encompassing the two most intense emission lines from

397

the Hg arc lamp. Overlap between each HA absorbance spectrum and the lamp emission

398

spectrum was calculated by summing the product of HA absorbance and normalized lamp

399

intensity over the entire spectrum (280‒500 nm). Results (Figure 3D) show a statistically

400

significant positive correlation between the amount of HONO produced photochemically and the

401

spectral overlap between the chromophore and lamp. Highest reactivity belongs to non-peat

402

terrestrial HA (LHA and ESHA) with the highest spectral overlap; the aquatic humic acid

403

SRHA, with its lower spectral overlap, was least active photochemically. We attribute this to a

404

higher abundance of benzoquinone moieties in non-peat terrestrial HA vs. the aquatic organic



Page 20 of 38

405

matter.76 LHA originates from lignite coal, whereas ESHA is derived from agricultural soil

406

composed of highly degraded material; both are highly oxidized and have high electron-

407

accepting capacity indicative of the presence of benzoquinones.41 Indeed, the higher absorbance

408

of LHA and ESHA in the tail extending into the low-energy region of the visible spectrum is

409

thought to be due in part to the presence of benzoquinone moieties.41 For example, the molecule

410

juglone, which can be thought of as a surrogate for quinones in natural organic matter, possesses

411

a strong absorbance band between 350‒500 nm (Figure 3C). Aquatic humic acid SRHA is not as

412

highly oxidized (i.e., aged) and has a higher phenol content and relatively lower abundance of

413

benzoquinone moieties. Although a role of non-aromatic ketones in this process cannot be

414

completely ruled out, we believe they do not play a role in this process since such molecules

415

require wavelengths between 230‒320 nm for excitation.

416

Further support for the importance of benzoquinone functional groups in promoting

417

photochemical NO2-to-HONO conversion comes from linear correlations between [HONO]photo

418

and the peak areas of the C‒O and C=O bands derived from the XPS spectra (Figure 3E & F). In

419

stark contrast to the NO2 dark reaction, the photochemical reactivity is not correlated to the

420

presence of C‒O groups which include phenolic moieties (Figure 3E). Instead, a statistically

421

significant correlation was found between [HONO]photo and C=O groups (Figure 3F). This is

422

further evidence that benzoquinones are the chromophores responsible for photochemical

423

reduction of NO2 to HONO on surfaces consisting of HA.

424

Taken together, the data are consistent with a mechanism by which reduced species such as

425

semiquinones and hydroquinones are generated when benzoquinone moieties present in HA are

426

excited by UV-visible light (Scheme 1). Following their reaction with NO2, the phenols are re-

427

oxidized to the benzoquinone state,65 which can be cycled back to a phenol upon absorption of a 20 ACS Paragon Plus Environment

Page 21 of 38


428

photon, or in the presence of a chemical reductant. This mechanism accounts for differences in

429

reactivity seen in Figures 1A and 1B. The efficiency of the NO2 + HA reaction in the dark

430

decreases over time as semiquinone and hydroquinone reactive sites are oxidized to

431

benzoquinones (Figure 1A). In contrast, the photochemical reactivity observed in Figure 1B is

432

relatively constant over the course of the experiment (>2 h) since photochemistry provides a

433

mechanism to regenerate semiquinones and hydroquinone reactive sites.

434

Quinone/Hydroquinone Chemistry in a Model System. To confirm that benzoquinone

435

moieties in HA can facilitate NO2-to-HONO conversion, we repeated the experiments shown in

436

Figure 1A and B by replacing the 2% HA coating on the kaolinite/Al (hydro)oxide substrate with

437

one containing juglone (5-hydroxy-1,4-napthalenedione).

438

compound that contains neighboring phenol and para-quinone moieties.77 As discussed above

439

and shown in Figure 3C, juglone absorbs UV-visible light between 300 and 500 nm. Thus, we

440

expect juglone to react with NO2 via both the thermal and photochemical pathways shown in

441

Scheme 1.

442

Figure 4A shows changes in the HONO and NO2 concentrations following exposure of juglone-

443

coated substrate to 60 ppb of NO2 in the dark. Following exposure, the initial NO2 concentration

444

dropped by 6.1 ppb and the amount of HONO increased by ~3.7 ppb relative to background

445

levels.

446

kaolinite/Al(hydro)oxide alone (Figure S5), we attribute the observed HONO formation in the

447

dark to reaction of NO2 with the phenol group present in juglone.23,24

448

experiment, but this time flowed NO2 over a juglone-containing substrate that was irradiated

449

with UV-visible light (λ > 280 nm). As shown in Figure 4B, the average drop in [NO2] is 8.1 ppb

450

while the HONO concentration increased by 6.1 ppb. Exposure to UV-visible light enhances

Since we did

not

observed

significant

Juglone is a naturally occurring

NO2-to-HONO conversion


when

We repeated this


451

HONO formation by a factor of ~2, which is comparable to the enhancement factors observed

452

for the humic acids studied here. Quinones such as juglone will generate an excited triplet state

453

when they absorb a photon (Scheme 1).53 This reactive species is capable of reacting with

454

hydrogen donors (e.g., from solvent or a neighboring organic molecule) to generate

455

semiquinones and, upon further reduction, a hydroquinone.53 Thus, we propose that NO2 is

456

reduced to HONO by a semiquinone or hydroquinone species generated photochemically from

457

juglone.65

458

It is interesting that the reactivity of the juglone-coated kaolinite is comparable to most of the

459

humic acid substrates studied, even though humic acid substrates contain only a fraction of the

460

quinone/hydroquinone moieties compared to the juglone substrate. Although one would expect

461

reactivity to scale with surface concentration, we believe the higher density of electron rich

462

chromophores may actually enhance secondary losses of HONO (via electrophilic aromatic

463

substitution), which would decrease the yield of HONO and explain the lower NO2-to-HONO

464

conversion efficiencies of juglone compared to the humic acid substrate (Table S6-S8). It is also

465

possible that light screening effects, energy transfer, or reactions between excited state organic

466

species (e.g., irreversible phenoxide radical couplings)50 lowered the apparent reactivity of

467

juglone at these relatively high surface densities. Future work will explore the chemistry of these

468

model systems in more detail.

469

Environmental Implications.

470

semiquinones) are important reactive sites for NO2 reduction in the dark, while benzoquinone

471

moieties are key to the photochemical reactivity of NO2 on HA-containing surfaces. This is

472

consistent with the prevailing theory that the electron-donating and accepting capacities of humic

473

acid are due to phenols and benzoquinones, respectively, which are end members of the same

The present results suggest that hydroquinones (and


Page 22 of 38

Page 23 of 38


474

reversible redox couple.42,43,45-47,73

475

studies utilizing model phenols22,24,25 that showed significant NO2-to-HONO conversion in the

476

dark, and studies showing negligible HONO formation when NO2 was exposed to surfaces

477

coated with natural HA isolates.33,34 Another novel insight provided by this work is that the

478

efficiency of NO2-to-HONO conversion on soil surfaces will depend on soil surface pH. Soil

479

organic matter exists in strong association with soil minerals78-80 whose surface acidity

480

(determined by the density of M-OH2+ groups) can have a major impact on the speciation of

481

adsorbed acids such as HONO. Thus, soil surfaces not only provide redox-active sites that

482

promote NO2 reduction (via reduced functional groups in organic matter), but also provide the

483

surface acidity required to volatilize nitrite as gaseous HONO.

484

The potential environmental significance of quinone-centered heterogeneous chemistry on both

485

the thermal (dark) and photochemical HONO sources is supported by a comparison of reaction

486

efficiency derived from our laboratory results and published field campaigns. We quantitate the

487

reaction efficiency of the heterogeneous reaction in terms of a (net) reactive uptake coefficient

488

(γNO2), which reflects the fraction of NO2 collisions with a surface that result in formation of a

489

HONO molecule. From our coated-wall flow reactor experiments, we find that γNO2 ranges from

490

6 × 10−6 to 4.6 × 10−5 for the thermal reaction (Figure S4A); when the HA surfaces are irradiated

491

with UV-visible light, γNO2 varies between 1.8 × 10−5 and 8 × 10−5 (Figure S4B). Our values of

492

γNO2 are higher than those derived in previous laboratory studies of HA reactivity,33,34 likely due

493

to our use of amphoteric mineral substrates capable of protonating nitrite. This suggests that

494

NO2-to-HONO conversion is more efficient on substrates containing humic acid that is

495

associated with minerals.

This mechanism helps to clarify discrepancies between



496

Values of γNO2 calculated from the coated-wall flow-reactor experiments reported here are

497

consistent with those derived from field measurements.14,81,82 For example, the range of

498

nocturnal γNO2 values calculated from measured HONO gradients in Colorado was 0.2‒1.6 ×

499

10−5; a value of 8 × 10−6 provided the best fit to the data.81 For daytime HONO production, a

500

modeling study by Wong et al. found that a γNO2 value of 6 × 10−5 was needed to account for the

501

daytime levels of HONO observed;14 this is in the range of uptake coefficients calculated here

502

(Figure S4B). Calculated γNO2 values derived from our HA-clay substrates appear consistent with

503

values required to explain nighttime and daytime HONO formation on real boundary-layer

504

surfaces.

505

The relative importance of the HA mechanism in promoting daytime NO2-to-HONO conversion

506

on boundary-layer surfaces is a matter of recent debate. A recent field campaign observed that

507

daytime HONO levels were relatively constant even when NO2 levels varied, which revealed that

508

daytime HONO does not depend on NO2(g).11 In contrast, nighttime HONO concentrations were

509

correlated to ambient NO2, supporting the idea that redox chemistry on ground surfaces may be

510

more important during the night than in the daytime. This makes sense since NO2 concentrations

511

are higher at night due to the absence of the major daytime sinks for NO2 (photolysis and

512

reaction with OH). Thus, the higher concentrations of NO2 at night may make up for the less

513

efficient NO2-to-HONO conversion rate in the dark relative to photochemical production rates.

514

The most important role for daytime photochemistry may be to regenerate semiquinone or

515

hydroquinone moieties within soil organic matter after they have been depleted during the

516

previous night. This is similar to the finding that actinic light can help regenerate reactive sites

517

on soot surfaces that were previously deactivated via dark reactions.36 Therefore, daytime


Page 24 of 38

Page 25 of 38


518

photochemistry centered in soil organic matter may not only be capable of reducing NO2 during

519

the daytime, but also activating soil surfaces for nighttime HONO production.

520

ASSOCIATED CONTENT

521

Supporting Information.

522

The Supporting Information is available free of charge on the ACS Publications website at DOI:

523

10.1021/acs.est.#######. Details of UV-visible spectroscopy measurements; NO2 actinometry

524

measurements; XPS spectral analysis and results; tables containing results for thermal,

525

electrochemical, and photochemical experiments; description of the calculations used for the

526

determination of uptake coefficients and results; and results from control experiments.

527

AUTHOR INFORMATION

528

Corresponding Author

529

*E-mail: [email protected]; Phone: +1 (812) 855-6525. Address: 702 N. Walnut Grove

530

Ave., Rm 308, Bloomington, IN 47405-2204.

531

Notes

532

The authors declare no competing financial interest.

533

ACKNOWLEDGMENT

534

We gratefully acknowledge support from a National Science Foundation CAREER Award

535

(AGS-1352375), the U.S. Department of Energy’s Office of Science via their Biological and

536

Environmental Research (BER) program (DE‐SC0014443), and Indiana University. N.K.S. was

537

supported by a U.S. Environmental Protection Agency’s Science to Achieve Results (STAR)

538

Graduate Fellowship program.



539


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540

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FIGURES

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Figure 1. (A & B) Concentration of NO2 and HONO over time before and after NO2 (1 atm at 30% RH) is exposed to kaolinite/Al2(SO4)3 substrates containing 2% LHA at pH 5 under the following conditions: (A) in the dark and (B) when exposed to UV-visible light (λ > 280 nm). The error bars represent uncertainty of the least-squares fit of the reference spectrum to the experimental data. (C) Cyclic voltammograms recorded in solutions containing redox mediator diquat and leonardite humic acid (LHA) at pH 7 (0.1 M potassium phosphate) and at a scan rate of 0.10 V s−1. (D) Average ∆NO2 and the amount of HONO generated over the period of 130 min when 60 ppb of NO2 is exposed to non-reduced diquat-LHA and reduced diquat-LHA substrates in the dark and when irradiated with UV-visible light. Error bars represent 95% confidence interval, and the letters represent whether there is a significant difference using an ANOVA oneway at a level of 0.05.

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Figure 2. Average amount of HONO generated over a period of 130 min when 60 ppb of NO2 is exposed to various HA substrates [kaolinite/Al2(SO4)3] adjusted to pH 5, at 30% RH in the dark as a function of: (A) The average difference between initial [NO2] (first 10 min before reaction) and the [NO2] determined every 10 min for 130 min when the HA substrate is exposed to NO2, (B) electron-donating capacity (at 0.61 V, pH 7),41 XPS C1s spectral peak areas corresponding to abundance of (C) C−O and (D) C=O and O−C−O functionalities in HA. All error bars represent 95% confidence interval and red lines represent linear fits and gray areas represent 95% confidence bands.

797


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799 800 801 802 803 804 805 806 807 808 809 810

Figure 3. (A) Average amount of HONO generated and NO2 reacted when 60 ppb of NO2 (in 1 atm air at 30% RH) is exposed to various HA substrates [kaolinite/Al2(SO4)3] adjusted to pH 5 and irradiated with UV-visible light (λ > 280 nm). (B) Amount of HONO formed and NO2 taken up due to photochemistry alone for the HA-containing substrates indicated. (C) Aqueous phase UV-visible spectra of HA isolates (15 mg L−1 in 0.1 M potassium phosphate buffer, pH 7), juglone (15 mg L−1 in isopropyl alcohol) and the normalized light-intensity of the Hg arc lamp with 280 nm cutoff filter. Average of the difference between HONO generated in the light and dark as a function of: (D) The sum of UV absorbance for the HAs multiplied by the normalized light intensity, XPS C1s spectral peak areas corresponding to abundance of (E) C−O and (F) C=O functionalities in HA. All error bars represent 95% confidence interval and red lines represent linear fits and gray areas represent 95% confidence bands. 37 ACS Paragon Plus Environment


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Figure 4. Concentration of NO2 and HONO over time before and during exposure of NO2 to substrates consisting of 2% juglone in kaolinite/Al2(SO4)3 under the following conditions: (A) in the dark and (B) when irradiated with UV-visible light. Dashed lines represent when the substrates were exposed to NO2. Error bars represent uncertainty of the least-squares fit of the reference spectrum to the experimental data.

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to-HONO Conversion on Soil Surfa - ACS Publications

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