Heterogeneous Photochemical Conversion of NO2 to HONO on the

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Heterogeneous Photochemical Conversion of NO2 to HONO on Humic Acid Surface under Simulated Sunlight Chong Han, Wangjin Yang, Qianqian Wu, He Yang, and Xiangxin Xue Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b05101 • Publication Date (Web): 13 Apr 2016 Downloaded from http://pubs.acs.org on April 18, 2016

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Environmental Science & Technology

1

Heterogeneous Photochemical Conversion of NO2 to HONO on Humic Acid

2

Surface under Simulated Sunlight

3

CHONG HAN*, WANGJIN YANG, QIANQIAN WU, HE YANG, XIANGXIN XUE

4

School of Metallurgy, Northeastern University, Shenyang, 110819, China

5 6

ABSTRACT

7

The poor understanding of HONO sources in the daytime highlights the importance

8

of the heterogeneous photochemical reaction of NO2 with aerosol or soil surfaces. The

9

conversion of NO2 to HONO on humic acid (HA) under simulated sunlight was

10

investigated using a flow tube reactor at ambient pressure. The uptake coefficient (γ)

11

of NO2 linearly increased with irradiation intensity and HA mass in the range of 0-2.0

12

µg/cm2, while it decreased with NO2 concentration. The HONO yield was found to be

13

independent of irradiation intensity, HA mass and NO2 concentration. The

14

temperature (278-308 K) had little influence on both γ and HONO yield. Additionally,

15

γ increased continuously with relative humidity (RH, 7%-70%), and a maximum

16

HONO yield was observed at 40% RH. The heterogeneous photochemical reaction of

17

NO2 with HA was explained by the Langmuir-Hinshelwood mechanism.

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1. INTRODUCTION

45

HONO plays an important role in the tropospheric chemistry since its photolysis

46

may account for 30-60% of OH radical production during the daytime.[1, 2] The OH

47

radical can participate in many oxidation cycles and controls the budget of O3 and

48

lifetime of most trace gases.[3-5] Tropospheric HONO sources are still not well

49

understood. Several sources have been discussed in the past such as the direct

50

emission from combustion processes and the homogeneous reaction of OH with NO.

51

However, these sources cannot account for actual HONO concentration in the

52

atmosphere.[6, 7]

53

A number of studies showed that there was a relationship between HONO and

54

NO2,[7-9] suggesting that the heterogeneous reactions of NO2 with aerosol and soil

55

surfaces may be an important HONO source.[2, 10-12] HONO was found to be produced

56

by the disproportionation reaction of NO2 and H2O adsorbed on mineral dust, glass

57

and building,[12-15] and by the redox reaction of NO2 with soot.[16-18] According to the

58

field observation and model computation results, the HONO concentration reached

59

10-1000 ppt regardless of photolysis of HONO in the daytime,[12] indicating that there

60

are significant but unknown daytime sources of HONO. Recently, it has been reported

61

that ultraviolet or visible light can obviously enhance the heterogeneous conversion of

62

NO2 to HONO on TiO2,[19-22] soot,[23] pyrene,[24, 25] polyphenols species,[26, 27], and

63

humic acid.[28, 29]

64

Humic substances originate from the degradation of biological materials and are the

65

most abundant group of organic species on the earth surface.[29] Humic substances are

66

found on the aerosol surfaces because of soil abrasion or biomass burning. Volatile

67

organic compounds and especially aromatic species in biomass burning aerosols may

68

be transformed into humic like substances by atmospheric oxidation processes.[30, 31] 3



69

Humic acids (HA) represent a mixture of macromolecular organics with complex

70

structures and chemical compositions, including various functional groups OH,

71

COOH, C=O, C-O, C-H, etc.[32, 33] The photochemical conversion of NO2 to HONO

72

requires a reduction step and a reductant is needed for that. Coincidently, HA contains

73

aromatic moieties as light absorbers and large amounts of phenolic functionalities as

74

electron donors.[34]

75

It was found that ultraviolet and visible light can significantly increase the uptake

76

of NO2 on HA,[28, 29] which can become a sink of NO2. In addition, HONO yields up

77

to 80 % were obtained.[28, 29] Photo-enhanced conversion of NO2 to HONO on ice and

78

snow containing HA was observed and depended on the HA content.[35, 36] Under

79

ultraviolet light, the HONO formation by the reaction of NO2 with soil containing 2.3

80

wt% organics was also enhanced.[28] These results demonstrate that the heterogeneous

81

photochemical reaction of NO2 with HA may be an important HONO source during

82

the daytime. However, the temperature- and mass-dependence of the photochemical

83

reaction of NO2 with HA is still unknown.

84

In this work, the heterogeneous photochemical conversion of NO2 to HONO on HA

85

under simulated sunlight was investigated using a flow tube reactor coupled to a NOx

86

analyzer. Effects of various environmental conditions including irradiation intensity,

87

HA mass, NO2 concentration, temperature, and relative humidity (RH) on NO2 uptake

88

coefficient and HONO yield were examined in detail. A reaction mechanism for the

89

heterogeneous photochemical reaction of NO2 with HA was proposed.

90 91

2. EXPERIMENTAL SECTIONS

92

Materials. HA sodium salt (Aldrich) was used to prepare the HA coating. HA sodium

93

salt was dissolved in deionized water and was then acidified to pH=4.0 using HCl. 4


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The inner surface of a quartz tube (20 cm length, 1.0 cm inner diameter) was coated

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with 1.0 mL HA aqueous solution (0.05-1.0 mg/mL). The HA coating on the quartz

96

tube was produced by drying HA aqueous solution in a N2 stream at room temperature.

97

The HA coating thickness was adjusted by varying the HA mass deposited on the

98

quartz tube. NO2 standard gas (50 ppmv in N2, Dalian Special Gases Co., LTD) and

99

high purity N2 (99.99%, Shenyang Gases Co., LTD) were used as received.

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Flow Tube Reactor. The heterogeneous uptake experiments of NO2 with HA were

101

performed in a horizontal cylindrical coated-wall flow tube reactor (34 cm length, 1.6

102

cm inner diameter), [37, 38] which was fully made of quartz and shown in Figure S1.

103

The temperature was controlled by circulating water through the outer jacket of the

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flow tube reactor. High purity N2 was used as the carrier gas and the total flow rate in

105

the reactor was 900 mL·min-1, ensuring a laminar regime at ambient pressure. NO2

106

was introduced into the reactor through a movable injector with 0.6 cm diameter and

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its concentration was in the range of 30-160 ppb. During the experiments, relative

108

humidity (RH) was adjusted by varying the ratio of dry N2 to wet N2 and measured by

109

a hygrometer (Center 314). Two xenon lamps (250 W) were used to simulate sunlight,

110

which had a continuous emission in the range of 350-700 nm and a dominant

111

wavelength at 480 nm. They were located above and below the reactor, respectively.

112

Changes of irradiation intensity were achieved by changing the distance of lamps to

113

the reactor. It was confirmed that NO2 photolysis, NO2 uptake, and HONO and NO

114

formation on the quartz tube were negligible during the control experiments.

115

Gases measurements. A chemiluminescence NOx analyzer (THERMO 42i) was used

116

to detect NO2 and NO. The detection of NO was based on the reaction of NO with O3,

117

which produced a characteristic luminescence with an intensity that was proportional

118

to the NO concentration. For the detection of NO2, a molybdenum catalyst was used 5



119

to convert NO2 to NO. A quartz tube (10 cm length, 0.6 cm inner diameter) filled with

120

1.0 g of crystalline Na2CO3 was introduced to trap HONO between the exit of the

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flow tube reactor and the NOx analyzer. It is well-known that HONO can cause

122

interference when using molybdenum converters.[23] When gaseous HONO passed

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this quartz tube, almost all HONO molecules can contact with Na2CO3, achieving the

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high trapping efficiency of HONO. As shown in Figure S2, the trapping efficiency of

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HONO by this quartz tube was higher than 99% at steady state. NO and NO2 together

126

with HONO were detected using a bypass tube while NO and NO2 were measured

127

using this quartz tube. Thus, HONO can be indirectly quantified from the difference

128

between the NO2 signals with and without the Na2CO3 tube. This HONO detection

129

technique has been widely employed to measure the HONO concentration during the

130

heterogeneous reactions of NO2 with various materials.[23-25, 39] A new Na2CO3 tube

131

was installed before each uptake experiment to avoid saturation effects. Little NO2

132

might be trapped by the Na2CO3 tube, which has been corrected for the calculation of

133

the uptake coefficient and the product yield.

134

Uptake Coefficient. The kinetics of the heterogeneous reaction of NO2 with HA can

135

be described by assuming a pseudo first-order reaction. Figure S3 shows the linear

136

relationship between the natural logarithm of NO2 concentration and reaction time.

137

The first-order rate constant (kobs) is proportional to the geometric uptake coefficient

138

(γ), ln ( C0 Ct ) γ c = kobs = t 2rtube

139

140 141 142

(1)

where C0 and Ci are NO2 concentration at time t = 0 and i, respectively; rtube, t and

c

are the flow tube radius, the exposure time, and the average molecular velocity

of NO2, respectively. The kobs was determined by pulling the injector back to the end 6


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of the sample tube. The geometric inner surface area of the whole sample tube was

144

used to calculate γ.

145

A radial concentration gradient in the gas phase may be formed since the loss of

146

NO2 at the HA surface may be too rapid to be recovered by the diffusion of NO2,

147

which may cause diffusion limitations. Therefore, a correction for the gas-phase

148

diffusion limitation was taken into account for γ using the Cooney-Kim-Davis (CKD)

149

method.[40, 41] γ was determined by averaging the signal within the first 1.0 hour.

150 151

3. RESULTS AND DISCUSSION

152

Temporal Changes of NO2, HONO and NO. Figure 1 shows temporal changes of

153

NO2 and NO concentrations when HA was continuously exposed to NO2 in the dark

154

and under irradiation. In the absence of light, there was a slight decrease in the NO2

155

signal accompanied by a small increase in the NO signal. However, upon exposure to

156

simulated sunlight, NO2 constantly decreased, whereas NO almost remained

157

unchanged. When the light was turned back off, NO2 quickly recovered to its initial

158

concentration and NO2 desorption from the HA surface was not observed, confirming

159

that the reactive uptake is the main contributor to the NO2 loss. The presence of

160

significant amounts of HONO was verified by passing the effluent gas flow through

161

the Na2CO3 tube which trapped acidic gases before detection. These results confirm

162

that HA has a very weak reactivity towards NO2 in the dark, whereas sunlight can

163

significantly enhance the conversion of NO2 to HONO on HA. After irradiation for 3

164

h, a steady-state uptake of NO2 appeared, and the reactivity of HA decreased by 44%

165

compared to that at the initial stage.

7



166 167

Figure 1. Temporal changes of NO2, HONO and NO during the heterogeneous

168

reaction of NO2 with HA in the dark and under irradiation. The HONO concentration

169

was obtained from the difference of the detector signals without and with the

170

carbonate tube in the sampling line. Reaction conditions: irradiation intensity of 194.5

171

W/m2, HA mass of 15.9 µg/cm2, NO2 concentration of 30 ppb, temperature of 298 K,

172

and RH of 22%.

173 174

Dependence on Irradiation Intensity. Figure 2 shows γ and HONO yield for the

175

reaction of NO2 on HA at different irradiation intensity. The errors represent the

176

standard deviations based on three independent experiments. γ increased from (1.67 ±

177

0.14) × 10-6 at 68.5 W/m2 to (4.37 ± 0.45) × 10-6 at 194.5 W/m2. The HONO yield,

178

defined as the ratio of the formed HONO to the lost NO2, was independent of the

179

irradiation intensity with an average value of 74.0%.

180

As shown in Figure 2, γ had a linear relationship with the irradiation intensity

181

(slope: (2.24 ± 0.33) × 10-8 m2/W). γ in the dark was extrapolated to be (1.89 ± 3.05)

182

× 10-7 from the intercept of the linear equation, which is 23 times less than the value

183

obtained at the highest intensity (194.5 W/m2). The linear influence of irradiation

184

intensity on NO2 uptake was also observed for the photochemical reactions of NO2 8


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with urban grime,[24] pyrene,[25] HA,[28, 29] ice containing HA,[35] and fluoranthene,[39]

186

under visible or ultraviolet light.

187 188

Figure 2. γ and HONO yield for the reaction of NO2 on HA at different irradiation

189

intensity. Reaction conditions: HA mass of 15.9 µg/cm2, NO2 concentration of 30 ppb,

190

temperature of 298 K, and RH of 22%.

191 192

Dependence on HA Mass. The HA film may be a mixture of solid precipitates, so

193

that it may exhibit a powder-like structure. To test possible pore diffusion of NO2 on

194

the HA surface, the response of NO2 uptake and HONO formation to the HA mass

195

was measured. Figure 3 shows a typical mass dependence of γ. Two regimes were

196

observed: the first one, where γ linearly increased with the HA mass in the range of

197

0-1.6 µg/cm2; the second one, where it was independent on the HA mass in the range

198

of 4.8-15.9 µg/cm2. The linear dependence of γ on the HA mass indicates that whole

199

HA samples (the full thickness) reacts with NO2. The maximum probe mass per unit

200

area of NO2 was calculated to be 2.0 µg/cm2, which was achieved by extending the

201

corresponding straight line to the average value of γ in the range of 4.8-15.9 µg/cm2. It

202

means that only the top layers of HA can react with NO2 at the large HA mass (>2.0

203

µg/cm2). Assuming that the HA density is 1.0 g/cm3, NO2 uptake has already reached 9



204

the saturation at a 20 nm depth. For this thickness, the “Active” or “Fuchs” surface

205

area, which is available for the interaction of diffusing gases with the particle, would

206

be the same order of magnitude with the geometric surface area.[42-44] Thus, the

207

geometric surface can be used to calculate γ. Even for higher surface roughness, the

208

use of the geometric surface is also justified for a photochemical surface reaction, due

209

to the exact offset of higher surface area by lower irradiance for tilted surfaces. It is

210

also concluded that the photochemical reaction of NO2 with HA is not mass limited

211

under atmospheric conditions, where soil layers and organic aerosol surfaces would

212

have much larger thickness. In addition, as shown in Figure 3, there were no changes

213

of the HONO yield in the HA mass range of 0-15.9 µg/cm2.

214 215

Figure 3. γ and HONO yield for the reaction of NO2 on HA at different HA mass.

216

Reaction conditions: irradiation intensity of 194.5 W/m2, NO2 concentration of 30 ppb,

217

temperature of 298 K, and RH of 22%.

218 219

Dependence on Initial NO2 Concentration. Figure 4 shows the changes of γ and

220

HONO yield as a function of NO2 concentration. A negative dependence of γ with

221

increasing NO2 concentration from 30 ppb to 160 ppb was observed. γ decreased from

222

(4.37 ± 0.45) × 10-6 at 30 ppb to (1.57 ± 0.25) × 10-6 at 160 ppb, indicating that the 10


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reaction was less efficient at higher NO2 concentration. HONO yields were

224

independent of NO2 concentration in the range of 30-160 ppb.

225 226

Figure 4. γ and HONO yield for the reaction of NO2 on HA at different NO2

227

concentrations. The fitting line for γ was based on the Langmuir-Hinshelwood

228

mechanism using equation (10). Reaction conditions: irradiation intensity of 194.5

229

W/m2, HA mass of 15.9 µg/cm2, temperature of 298 K, and RH of 22%.

230 231

Dependence on Temperature. The dependence of γ and HONO yield for the reaction

232

of NO2 with HA on temperature was investigated by varying the temperature from

233

278 K to 308 K. Figure 5 shows that γ and HONO yield at different temperatures can

234

be considered as constant within the range of experimental uncertainty, which was

235

studied here for the first time.[28, 29] The lack of temperature dependence indicates that

236

the photochemical reaction of NO2 with HA is not a thermally-controlled process and

237

its excitation energy originates mainly from photons. Within the temperature range of

238

278-308 K, the average values of γ and HONO yield were (3.76 ± 0.64) × 10-6 and

239

(75.37 ± 2.05)%, respectively.

11



240 241

Figure 5. γ and HONO yield for the reaction of NO2 on HA at different temperature.

242

Reaction conditions: irradiation intensity of 194.5 W/m2, HA mass of 15.9 µg/cm2,

243

NO2 concentration of 30 ppb, and RH of 22%.

244 245

Dependence on Relative Humidity. Figure 6 shows the evolutions of γ and HONO

246

yield at different RH levels. The dependence of γ and HONO yield on RH exhibited a

247

different behavior. γ continuously increased from (3.44 ± 1.03) × 10-6 at 7% RH to

248

(6.34 ± 0.26) × 10-6 at 70% RH. The HONO yield had a maximum value of (80.7 ±

249

4.1)% at 40% RH, suggesting that an advantageous RH existed for the HONO

250

formation on HA. There were diverse changes for γ with RH on HA aerosol surfaces,

251

where γ appeared to drop at low (60%) RH while it was almost

252

constant at intermediate RH (20%-60%).[29] This difference may be related to the

253

physical states of HA (film and aerosol). A positive uptake trend with increasing RH

254

was observed for N2O5 and HO2 on HA.[45, 46] Although γ and the HONO yield varied

255

greatly, the NO yield was unchanged with RH.

12


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256 257

Figure 6. γ and HONO yield for the reaction of NO2 on HA at different relative

258

humidity (RH). Reaction conditions: irradiation intensity of 194.5 W/m2, HA mass of

259

15.9 µg/cm2, NO2 concentration of 30 ppb, and temperature of 298 K.

260 261

Reaction Mechanism. On the basis of experimental results and previous studies, a

262

series of possible pathways are proposed in Scheme1. The linear relationship between

263

γ and irradiation intensity determined the photochemical nature of the reaction of NO2

264

with HA. Therefore, the active sites on HA such as aromatic carbonyls (Ar-C=O) are

265

excited by irradiation first, which is proportional to the photon number. At the same

266

time, some excited sites may be deactivated by the direct photolysis or the energy

267

transfer to other molecules.[47] The excited aromatic carbonyls can react with phenolic

268

electron donors (Ar-OH) to form reduced ketyl radicals (Ar-(C·)-OH), which then

269

react with adsorbed NO2 to form nitrite/HONO.[27] Finally, NO2- is in equilibrium

270

with HONO, depending on the acidity of HA. Here, most HONO was released to the

271

gas-phase since the pH of the HA coatings was adjusted to 4. Ar-C=O

272 273

hv (1)

Ar-C=O*

Ar-OH (2)

Ar-(C.)-OH

H2O(g)+NO2(g) (3)

NO2(ads)...Ar-(C.)-OH...H2O(ads)

(4)

Productsox+HONO

Scheme 1. Proposed photochemical reaction mechanism of NO2 with HA

274 13



275

As seen in Figure 4, an inverse dependence of γ on NO2 concentration indicates that

276

the Langmuir-Hinshelwood surface-mediated mechanism may be responsible for the

277

heterogeneous photochemical reaction of NO2 with HA. As described in equations

278

(2)-(3), NO2 is in rapid equilibrium between the gas phase and the surface and the

279

reaction takes place between the adsorbed species.

280

k3  →[ NO2 ]ads [ NO2 ]g + [ S ] ←

(2)

k−3

281

k4 [ NO2 ]ads  → Products

(3)

282

where [NO2]g and [NO2]ads are the gaseous and adsorbed NO2, respectively, and [S] is

283

the surface active site. If the surface reaction is considered as the rate-limiting step,

284

the first-order reaction rate can be written as

285

−

d [ NO2 ]ads = v = k4 [ NO2 ]ads = k4 [ S ]T θ NO2 dt

(4)

286

where [S]T is the total number of surface active sites, θNO2 is the fraction of sites

287

occupied by NO2. If the NO2 adsorption on HA followed a Langmuir isotherm, θNO2 is

288

given by equation (5),[48]

289

θ NO = 2

K NO2 [ NO2 ]g 1 + K NO2 [ NO2 ]g

(5)

290

where KNO2 is the Langmuir adsorption constant of NO2 and [NO2]g is the gas phase

291

concentration of NO2. Substituting equation (5) into equation (4) yields

292

293

294

295

v=

k4 [ S ]T K NO2 [ NO2 ]g 1 + K NO2 [ NO2 ]g

(6).

Thus, the pseudo first-order rate coefficient is obtained,

k1, NO2 =

k4 [ S ]T K NO2 1 + K NO2 [ NO2 ]g

(7).

By inserting equation (7) into equation (8), 14


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296

297

298

299

300

γ cNO

k1, NO2 =

2

(8),

2rtube

the dependence of γ on NO2 concentration can be described by equation (9),

γ=

( 2r

tube

cNO2

) k [S ] K 4

T

NO2

(9).

1 + K NO2 [ NO2 ]g

Equation (9) can be simplified to equation (10),

γ=

a 1 + K NO2 [ NO2 ]g

(

(10),

) k [S ] K

301

where a = 2 rtube

302

4, where the well-fitting result was obtained. According to the fitting results, KNO2 is

303

calculated to be (7.85 ± 1.00) × 10-13 cm3 and the maximum reaction rate (k4[S]T) of

304

NO2 with HA under simulated sunlight is (12.3 ±1.0) s-1.

cNO2

4

T

NO2

. This equation was used to fit the data in Figure

305

The Langmuir-Hinshelwood mechanism seems to appropriately describe the effect

306

of the NO2 concentration on γ. However, the missing temperature dependence of γ is a

307

hint that the Langmuir-Hinshelwood mechanism may be responsible for the reaction

308

of NO2 with HA at a limited temperature range, because KNO2 generally depends on

309

the temperature. An alternative explanation has been provided by Stemmler et al.,[28, 29]

310

who suggested that the decrease in γ with increasing NO2 concentration could be due

311

to the competition between the deactivation of the excited reductive sites in HA and

312

their reactions with adsorbed NO2. Based on the steady-state kinetics, they have

313

established a similar numerical equation to predict the trend of γ as a function of NO2

314

concentration.[28, 29]

315

HONO was the major product of the photochemical uptake of NO2 on HA while the

316

conversion of NO2 to NO was a minor reaction pathway (Figure 1). If the secondary

317

reaction of HONO leading to NO and NO2 was significant, the NO yield should 15



318

greatly increase at higher NO2 concentration that can produce more HONO. In

319

contrast, the NO yield was almost independent of NO2 concentration, confirming a

320

minor contribution from HONO disproportionation. The products may also contain

321

non-volatile surface species containing N since total HONO and NO were always less

322

than the removed NO2. The species containing N such as nitro compounds may

323

release gaseous HONO and NO by photolysis and hydrolysis, as reported in the cases

324

of soot, PAHs and tannic acid. [23, 24, 26, 49, 50]

325

If NO2 uptake is greatly affected by the H2O competitive adsorption, a decrease of γ

326

would be expected with increasing RH. However, the simple disproportionation

327

reaction of NO2 with adsorbed water should be a minor pathway as the corresponding

328

γ (10-7-10-8) is significantly lower than that (10-6) observed in this study.[15, 51] The

329

multilayer uptake of gaseous species on the solid surfaces is commonly described

330

through the Brunauer-Emmett-Teller (BET) model, which predicts that the equivalent

331

layer numbers (LH2O) of water on the surfaces can be described by equation (11),[52]

332

LH2O =

cRH (1 − RH ) 1 + ( c − 1) RH 

(11)

333

where c is a thermodynamic parameter that reflects the strength of interaction of the

334

gas with the substrate and which determines the shape of the BET isotherm. Here, c =

335

50 and 500 was used as a range of values for water adsorption on a polar surface. As

336

shown in Figure S4, the relative enhancement (γRH/γ7%) of γ with increasing RH may

337

be related to the amount of water taken up by HA. As the RH increases, the significant

338

hydration of the hydrophilic functional groups in HA may activate more surface sites

339

to become accessible for NO2 uptake, leading to a continuous increase of γ with RH.

340

H2O was also necessary to generate HONO from the formed nitrite via H+ transfer.[27]

341

As a result, the HONO yield had an obvious increase from 7% to 40% RH (Figure 6).

16


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342

With increasing adsorbed water layers on HA, HONO may become more soluble, and

343

thus the HONO yield distinctly decreased at high RH (>50%). In the presence of

344

condensed phase H2O at high RH, the formation of more nitro compounds may be

345

also a reason of the decrease of the HONO yield.

346

As described in the experimental section, N2 was used as the carrier gas without

347

O2. It is noticed that O2 may compete with NO2 as an electron acceptor to react with

348

the reduced sites. The experimental data showed that the presence of 20% O2 in the

349

carrier gas resulted in a 50% decrease for γ compared to that in the absence of O2, and

350

it did not influence the HONO yield. The reactive radicals from the photosensitized

351

reactions can react with O2 to produce O2-, which may be a reductant responsible for

352

the photochemical uptake of NO2 on organics.[27] In view of the importance of O2- as

353

an electron carrier, Stemmler et al. has confirmed the role of O2- in the photochemical

354

uptake of NO2 on HA through two experiments,[28] where H2O2 as the product of O2-

355

disproportionation (2O2-+2H+→H2O2+O2) was measured and the reactivity of NO that

356

had a similar reactivity of NO2 towards O2- was examined. The absence of significant

357

amounts of H2O2 and reactivity of NO on irradiated HA surface indicated that O2- was

358

probably not involved in the observed conversion of NO2 to HONO.[28]

359 360

4. ATMOSPHERIC IMPLICATIONS

361

It is well-known that sunlight contains ultraviolet, visible and infrared light. A

362

comparison of two different visible light sources (400-700 nm and 500-700 nm range)

363

showed little differences with respect to the conversion of NO2 to HONO on HA,[28]

364

suggesting that the reaction was weakly dependent on the wavelength in the visible

365

region. The HONO from the reaction of NO2 with HA under ultraviolet light (300-420

366

nm) was approximately larger by a factor of 2 than that under visible light,[28] which 17



367

indicated a wavelength dependence towards the ultraviolet range. The absorbance

368

(0.06-0.20) of HA at 300-420 nm was significantly larger than that (0.01-0.06) at

369

420-700 nm,[29] which should contribute to the difference in the HA reactivity under

370

ultraviolet and visible light. Therefore, compared to single ultraviolet or visible light,

371

simulated sunlight with a full spectrum should be more appropriate for evaluating the

372

actual conversion of NO2 to HONO on HA under atmospheric conditions.

373

Under typical atmospheric conditions with irradiation intensity of 400 W/m2 (48o

374

Solar Zenith Angle), NO2 concentration of 30 ppb, temperature of 298 K and RH of

375

30%, γ of NO2 on HA was extrapolated to be 1 × 10-5 from the present experimental

376

data. This value is lower than the one reported by Stemmler et al.,[28, 29] which may be

377

due to the different experimental conditions such as NO2 concentration, HA status

378

(film or aerosol), type and irradiation intensity of light source, and RH. At the same

379

time, it is higher than γ for NO2 on gentisic acid,[26] tannic acid,[26] pyrene,[24, 25] and

380

fluoranthene.[39] According to simulation results, γ must be higher than 10-4 to have an

381

appreciable impact on NOx and O3 concentrations for the interactions of NO2 with

382

aerosols.[53] Small γ determined in the present study implies that the photochemical

383

reaction of NO2 with HA on aerosol surfaces might be unimportant for NOx and O3

384

concentrations. During the uptake process of NO2, the changes in the structures and

385

compositions of HA may modify its environmental effects including light scattering

386

and absorption, hygroscopicity, and residence of HA in the atmosphere.

387

Under atmospheric conditions mentioned above, the HONO yield and amount were

388

extrapolated to be (74 ± 6)% and (1.6 ± 0.2) × 1018 molecules·m-2·h-1, respectively.

389

Our results confirm the conclusions of Stemmler et al., who proposed that the

390

photochemical interaction of NO2 with soil is a main contributor to the HONO source

391

strength in the near ground atmosphere.[28, 29] Assuming 2.0-5.0 mass% of organics 18


Page 18 of 27

Page 19 of 27


392

in soil, the formation rate of HONO from the sunlight-enhanced reaction of NO2 and

393

HA in soil is estimated to be 120-300 ppt·h-1 at the ground surfaces up to 10 meters

394

height.

395

396

ASSOCIATED CONTENT

397

Supporting Information

398

Diagram of the flow tube reactor. Trapping of HONO by a quartz tube filled with

399

Na2CO3. The linearity of ln(C0/Ct) against t. Relative enhancement (γRH/γ7%) of γ and

400

equivalent layer numbers (LH2O) of water at different RH. This material is available

401

free of charge via Internet at http://pubs.acs.org.

402 403

AUTHOR INFORMATION

404

Corresponding Author

405

*E-mail

406

+86-024-83687719

407

Notes

408

The authors declare no competing financial interest.

address:

[email protected];

phone:

+86-024-83684086;

fax:

409 410

ACKNOWLEDGEMENTS

411

This research was financially supported by the National Natural Science Foundation

412

of China (21407020). We thank Samar G. Moussa for her help with the language.

413 414

References

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