Photochemical Aging of Beijing Urban PM2. 5: HONO Production

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Environmental Processes

Photochemical Aging of Beijing Urban PM2.5: HONO Production Fengxia Bao, Meng Li, Yue Zhang, Chuncheng Chen, and Jincai Zhao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00538 • Publication Date (Web): 01 May 2018 Downloaded from http://pubs.acs.org on May 2, 2018

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Environmental Science & Technology

Photochemical Aging of Beijing Urban PM2.5: HONO Production

Fengxia Bao a,b, Meng Li a,b , Yue Zhang a,b, Chuncheng Chen a,b,*, Jincai Zhao a,b a

Key Laboratory of Photochemistry, CAS Research/Education Center for Excellence in

Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China b

University of Chinese Academy of Sciences, Beijing, 100049, P. R. China

Corresponding Authors:

Prof. Chuncheng Chen Email: [email protected]

ACS Paragon Plus Environment

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1

ABSTRACT

2

Photochemical aging represents an important transformation process of aerosol particles in the

3

atmosphere, which greatly influences the physicochemical properties and the environmental

4

impact of aerosols. In this work, we find that Beijing urban PM2.5 aerosol particles release

5

substantial HONO, a significant precursor of ·OH radicals, into the gas phase during the

6

photochemical aging process. The generation of HONO exhibits a high correlation with the

7

amount of nitrate in PM2.5. The formation rate of HONO becomes gradually decreased with the

8

irradiation time, but can be restored by introducing the acidic proton, indicative of the essential

9

role of the acidic proton in the HONO production. Other environmental factors such as relative

10

humidity, light intensity, and reaction temperature also possess important influences on HONO

11

production. The normalized photolysis rate constant for HONO (JHNO3→HONO) is in the range of

12

1.22 × 10-5 s-1 ~ 4.84 × 10-4 s-1, which is 1 ~ 3 orders of magnitudes higher than the reported

13

photolysis rate constant of gaseous HNO3. The present study implies that the photochemical

14

aging of Beijing PM2.5 is an important atmospheric HONO production source.

15


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16


INTRODUCTION

17

Atmospheric aerosol particles, which significantly affect the global climate, air quality and

18

atmospheric chemistry, undergo complex aging processes in the atmosphere1-3. The aging of the

19

particles will change the physical and chemical properties of the particles, but also influences the

20

composition of the surrounding air by uptaking or releasing volatile species4-8. Therefore, the

21

aging process of the atmospheric particles will present great influence on their environmental

22

fate and effect. Among many particle aging processes, photochemistry plays a crucial role in the

23

atmosphere1, since solar radiation provides the energy to initiate many reactions that are difficult

24

to occur under the tropospheric and stratospheric conditions. In the past few decades, the

25

scientific community has put large effort to study aerosol evolution in the atmosphere, but the

26

level of understanding the aging processes, especially the photochemical aging process, of

27

aerosol particles is still low.

28

It is known that the oxidation of nitrogen oxides such as NO2 leads to the formation of nitric

29

acid or nitrate salt9, 10, which is consequently adsorbed on aerosol particles or deposits on other

30

environmental surfaces. It is conventionally considered that such adsorption and deposition

31

process of nitrate would remove the nitrogen oxides from the atmosphere. However, recent

32

studies found that the photolysis of HNO3/NO3- deposited on environmental surfaces can lead to

33

a rapid release of nitrogen oxides into the gas phase (renoxification)11-14. For example, the

34

photolysis of nitric acid/nitrates deposited on the natural surfaces of plant leaves15,

35

particulates in the marine boundary layer14 is reported to generate HONO, NO2, and/or NO. The

36

irradiation of the urban grime from Toronto, Canada12, ground surfaces from Houston, Texas17

37

and urban aerosols from New York15, 18 also leads to the HONO production. In recent years, the

38

PM2.5 (fine particulates with aerodynamic diameters less than 2.5 µm) pollution has been a

16

and


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39

serious environmental problem in cities of northern China19. Together with the high intensity of

40

solar irradiation, the NOx production during the photoaging of PM2.5 may have important

41

influence on the nitrogen cycling in Beijing.

42

Nitrate anion (NO3-), ubiquitous in ambient particles, is reported to be particularly high near

43

urban centers8, as NO3- is primarily formed from the anthropogenic release of NOx1. According

44

to the field observations, NO3-, which is the dominant species in the water-soluble fraction of

45

PM2.5 (constituting up to 25% of the total mass)20, in the atmosphere of Beijing (＞10 µg·m-3) is

46

significantly higher than that in the cites of New York (~ 2 µg·m-3), Hong Kong (~ 1µg·m-3),

47

Shanghai (~ 7 µg·m-3), and Seoul (~ 7 µg·m-3)21.

48

The large amounts of nitrate provide plenty of nitrogen sources for the photochemical

49

transformation of nitrogen oxides. In addition, the suspended airborne PM2.5 can also be fully

50

exposed to the incoming sunlight, and much easier to undergo photochemical reactions than

51

other environmental surfaces. For now, the important renoxification during the photochemical

52

aging of Beijing PM2.5 is not investigated and the influence of the environmental factors in this

53

process is not clear.

54

Gaseous nitrous acid (HONO) is a very important gaseous species to stimulate a series of

55

further photochemical oxidation reactions of natural or industrial organic compounds by

56

producing hydroxyl radicals (·OH)22-24. Recent environmental HONO observations found that

57

HONO concentration in Beijing is high relative to other cities, as summarized in Table S125-27.

58

However, a certain contribution of the high HONO concentration was not yet ascribed to specific

59

sources. For example, field studies at urban sites of Beijing from 2014 to 2016 have shown that

60

the unknown daytime HONO source rate is in the range of 1.26 ~ 3.82 ppbv·h-1 25, 26. In the

61

present work, to shed light on the contribution of the photolysis of nitrate species in PM2.5


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62

particles to the HONO source, we examined the generation of gaseous nitrous acid (HONO)

63

during the photochemical aging process of Beijing urban PM2.5 samples. The production rates for

64

HONO and NO2 were measured on different PM2.5 samples and the photolysis rate constants

65

were estimated. In addition, the effects of various environmental conditions including acidity,

66

relative humidity, light intensity and wavelength on the formation of HONO were also

67

investigated. The contribution of photoaging of PM2.5 to the overall HONO sources was

68

estimated by using the photolysis rate constants measured in the present study, and compared

69

with the reported unknown source rate of HONO in Beijing. Our work provides direct evidence

70

that the photochemical aging of Beijing Urban PM2.5 is an important HONO source.

71

EXPERIMENTAL SECTION

72

Sampling of PM2.5 particles. The ambient fine particulate matter (PM2.5) (aerodynamic particle

73

size < 2.5 µm) was collected on quartz microfiber filters (Whatman, 203 mm × 254 mm) by a

74

600/AFPM1001K High Volume Sampler (see supporting information for details). Samples were

75

collected on the roof of the ten-storey building of the Institute of Chemistry, CAS, Beijing

76

(116°19’21.58’’E, 39°59’22.68’’N) that is a typical heavily polluted urban area. The sampled

77

filters were labeled by the sampling date and stored at - 20 °C in the freezer. Fractions with given

78

surface area from one randomized-chosen filter sample were used to perform the photochemical

79

experiments or other analysis.

80

Photochemical reaction of PM2.5. A custom-made cylindrical quartz vessel was used as the

81

photochemical flow reactor. A Xenon lamp (CEAULIGHT, 300 W) was used to simulate

82

sunlight. To adjust the J value to ambient sunlight condition (solar elevation angle θ = 0o), the

83

radiant output of the solar simulator was quantified by actinometry calibration28 (Supplementary

84

method section and Figure S2). During the photoreaction process, the reactor was kept at a


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85

constant temperature by a HX-205 water circulating bath (Beijing YKKY Technology Co., Ltd).

86

RH was adjusted in the air flow through a water bubbler, and monitored with an on-line RH

87

sensor (Vaisala, HMT130). Since the formation of HONO and NO2 was not changed much in the

88

range of 15% ~ 75% relative humidity, an RH of 60% was typically used. Synthesized air

89

composed of ultra-high-purity nitrogen and ultra-high-purity oxygen mixed at a ratio of 26:7 was

90

used as the carrier gas.

91

On-line gas measurements. Gaseous products HONO and NOx released during the experiment

92

were flushed out of the reactor by the carrier gas and were detected on-line by a MODEL T200

93

NOx analyzer (Teledyne API). This analyzer can directly measure the concentration of NO, and

94

the detection of HONO and NO2 was achieved by converting them into NO with a molybdenum

95

catalyst. For HONO measurement, a Na2CO3 denuder was employed to trap HONO selectively,

96

and HONO concentrations were indirectly measured by the signal difference without and with

97

the carbonate denuder29, 30 (see supporting information for details). The validity of this method

98

for HONO measurement was verified by measuring concentration of NO2- ions with ion

99

chromatography (ICS-900, DIONEX), after HONO was absorbed by water (Figure S3), and by

100

long-path absorption photometer (LOPAP) HONO analyzer31(Figure S4).

101

Calculations of HNO3 photolysis rate constant. The average production rates (mol·s-1) of

102

HONO and NO2 during a period of sample irradiation (PHONO and PNO2) were calculated by the

103

following equations:

104

PHONO=

105

PNO2=

ଵ଴షవ ×ி೒

௧మ ݀‫ݐ‬ ‫ܥ ׬‬ ௏೘ ×଺଴×(௧మ ି௧భ ) ௧భ ுைேை

ଵ଴షవ ×ி೒

௧మ ‫ݐ݀ ܥ ׬‬ ௏೘ ×଺଴×(௧మ ି௧భ ) ௧భ ேைଶ

(1)

(2)


6

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106

where Fg is the flow rate of the carrier gas (L·min-1); Vm (24.5 L·mol-1) is the molar volume of

107

gas at 25 oC and 1 atmosphere of pressure; t1 and t2 (min) are the starting and ending time of the

108

irradiation, respectively; CHONO and CNO2 (ppbv) are the on-line measured concentrations of

109

HONO and NO2.

110

The normalized rate constant of HNO3 photolysis leading to HONO production (JHNO3→HONO)

111

and the normalized rate constant of HNO3 photolysis leading to NO2 production (JHNO3 → NO2)

112

were calculated by the following equations15:

113 114

‫ܬ‬ୌ୒୓య →ୌ୓୒୓ =

௉ౄోొో ேొోష య

௉ొోమ

‫ܬ‬ୌ୒୓య →୒୓మ = ே

ొోష య

×

×

ଷ×ଵ଴షళ ௃౛౮౦

ଷ×ଵ଴షళ ௃౛౮౦

(3)

(4)

115

Jexp (s−1) was the photolysis rate constant of nitrate in the actinometer solution exposed to the

116

experimental light source, which was determined by the method of nitrate actinometry28( detailed

117

in the supporting information). The calculated photolysis rate constants JHNO3→ HONO (s−1) and

118

JHNO3 → NO2 (s−1) have been normalized to tropical noontime conditions on the ground (solar

119

elevation angle θ = 0°) where a photolysis rate constant is ∼3 × 10−7 s−1 for aqueous nitrate and

120

∼7 × 10−7 s−1 for gaseous HNO328. NNO3- (mol) is the amount of NO3- in the PM2.5 sample

121

determined in the extraction solution. In principle, the photolysis rate constants of NO3- should

122

be calculated on the NNO3- that is reachable to the irradiation. However, unlike the homogeneous

123

systems, the inhomogeneity of the PM2.5 filter samples makes it difficult to quantify the amount

124

of the light-reachable NO3-. Accordingly, the NNO3- was estimated on the basis of the total NO3-

125

amount in the PM2.5 sample in this study which is convenient to reflect the NO3- content of the

126

tested sample14, 15, 16. To detect the total NNO3-, the fraction of the sample with given surface area


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127

was rinsed by deionized water and then sonicated for 15 min. The amount of nitrate in the PM2.5

128

was measured by an ion chromatograph. DNO3- was calculated through dividing NNO3- with the

129

geometric area of the sampled filter.

130 131

RESULTS AND DISCUSSION

132

HONO production during the photochemical aging of Beijing urban PM2.5. Figure 1a shows

133

a typical profile for the change in concentrations of HONO and NO2 in the reaction system

134

during the photoaging of Beijing urban PM2.5 under the irradiation of simulated solar light.

135

HONO was initially generated very quickly. However, when the photo-irradiation was turned

136

off, the HONO generation stopped immediately and its concentration in the reactor decayed

137

slowly. Moreover, when the sample was heated up to 60 oC in the dark, little HONO was

138

observed, indicating that the formation of HONO was attributed to the photochemical reaction of

139

particles, but not to the simple thermal reaction. Compared with HONO, another common

140

atmospheric nitrogen oxide NO2 was almost undetectable during the irradiation (Figure 1a, blue

141

line). This phenomenon was universally observed on other PM2.5 samples (Figure S5 and Table.

142

1).

143

The average HONO production rates within the first 15 min of irradiation were further

144

investigated on different PM2.5 samples. As shown in Figure 1b, HONO generation rate exhibited

145

a high correlation with DNO3- in the samples (with a R2 of 0.85), suggesting that the observed

146

HONO production during the photochemical aging of Beijing PM2.5 originates from HNO3/NO3-.

147

We also tried to correlate the HONO production of samples with the pollution level of the

148

sampling day. As shown in Figure S6, the HONO production rate obtained in the present study


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149

exhibited significant positive correlation with the official-released AQI (Air Quality Index) and

150

the concentrations of PM2.5 and NO2 of the sampling day of PM2.5, while the correlation is not

151

very significant with the SO2 concentration. It is possible that, at high pollution level, the

152

concentration of nitrate in the PM2.5 should be accordingly high and consequently a high HONO

153

production rate is obtained, in agreement with Figure 1b.

154 155

Figure 1. (a) HONO and NO2 production from the PM2.5 sample (NOV16) during the

156

irradiation; (b)Average HONO production rate within the first 15 min of irradiation as a function

157

of DNO3-(left, red rhombuses); The photolysis rate constant JHNO3→HONO of PM2.5 surfaces as a

158

function of DNO3- (RH = 60%, temperature = 25 oC)(right, blue circles).

159

The HNO3 photolysis rate constants for HONO (JHNO3→HONO) and NO2 (JHNO3→NO2) production

160

on different PM2.5 samples are summarized in Table 1. The observed JHNO3→HONO values were

161

distributed from 1.22 × 10-5 s-1 to 4.84 × 10-4 s-1 with a mean value of 8.24 × 10−5 s-1. The JHNO3→

162

HONO

163

orders of magnitude, and are similar to the reported values (6.0 × 10−6 s-1 ~ 3.7 × 10−4 s−1) on

164

plant leaves, metal sheets, and construction materials15. It is noted that the photolysis rate

values of PM2.5 samples are higher than that of gaseous HNO3 (about 3 × 10-7 s-1) by 1-3


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165

constants listed in Table 1 are the average rates in the irradiation of the first 15 min. As shown in

166

Figure 2, with extended irradiation time, the HNO3 photolysis rate constants decreased gradually.

167

Furthermore, in the atmosphere, the nitrogen cycle exists among HNO3 and the reactive nitrogen

168

species. The more nitrogen species are formed by the photolysis of HNO3, the rapider

169

transformation of the nitrogen species back to the HNO3 occurs. As a result, the amount of

170

particulate nitrate can be largely maintained by the dynamical nitrogen cycling, besides other

171

external nitrate sources.

172

Table 1. HNO3 photolysis rate constants for HONO (JHNO3 → HONO) and NO2 (JHNO3 → NO2)

173

production and the production ratio of HONO/NO2 on different PM2.5 samplesa. Date

JHNO3→HONO -5

JHNO3→NO2

Yield ratio HONO/NO2

APR24

s-1 2.13±0.22

JUL05

7.31±0.33

1.21±0.13

6.07±0.40

JUL06

12.58±0.03

2.39±0.04

5.26±0.1

AUG31

14.58±0.16

5.05±0.83

2.97±0.47

SEP02

28.22±3.30

5.32±0.78

5.31±0.16

SEP05

34.62±19.54

14.92±9.63

2.40±0.24

SEP06

8.15±1.17

3.59±1.50

2.56±1.40

OCT13

1.30±0.11

0.23±0.00

5.59±0.51

OCT18

1.50±0.32

0.59±0.25

2.65±0.58

OCT25

6.55±0.92

3.17±1.26

2.32±0.88

NOV03

1.79±0.08

0.26±0.02

6.90±0.73

NOV09

1.91±0.30

0.36±0.01

5.30±0.96

NOV16

2.54±0.57

0.67±0.31

4.90±3.67

DEC12

3.26±0.47

1.45±0.03

2.24±0.27

10

s-1 0.46±0.02 10

-5

4.59±0.31


10

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174


DEC20

1.70±0.02

0.52±0.03

3.25±0.12

Median value

8.24

3.43

3.95

Mean value

3.59

1.3

3.55

a

The average HONO and NO2 production within the first 15 min of irradiation were used,

175

and the error bars were obtained by repeating each experiment 2-4 times with different fractions

176

from the same sample.

177

Figure 1b shows the change of the JHNO3→HONO as a function of DNO3- in PM2.5 samples. The

178

JHNO3→HONO value decreased sharply with the increasing DNO3-. Such a decrease may be caused by

179

the screening effect of the PM2.5 particles to the light. As shown in Figure S7, there are plenty of

180

light-absorbing species within PM2.5 particles. These light-absorbing species would hinder the

181

light absorption of nitrate in the inside and underside of the particle. Moreover, it is very often

182

that more than single layer of PM2.5 particles were collected on the filter samples. In the multi-

183

layer situation, the particles in the lower layers of the filter sample would absorb less light than

184

those on the first layer due to the screening effect. Usually, higher DNO3- goes with more PM2.5

185

particles, and the screening effect becomes stronger. Therefore, the JHNO3→HONO decreases with

186

DNO3-. To verify this assumption, we correlated the content (%) of elemental carbon (one of the

187

most important light-absorbers in PM2.5) in different PM2.5 samples (Table S3) to the

188

corresponding photolysis rate constants JHNO3→HONO in Table 1. An apparent negative correlation

189

was observed (Figure S8), indicating that light-absorbing species within PM2.5 particles would

190

really exert light screening effect in the photochemical HONO production. In our experiments,

191

the observed DNO3- of PM2.5 filter samples varied from 1.63 × 10-4 to 3.02 × 10-2 mol∙m-2, which

192

is 1 ~ 3 magnitude higher than the DNO3- on natural surfaces16, 32. The similar JHNO3→HONO values

193

imply that the production of HONO during the photoaging of PM2.5 is more efficient than that on


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194

natural surfaces. Moreover, in the real atmosphere, the screening effect between the suspended

195

PM2.5 particles should be minimized because of the less density of the particles. Thus, the real

196

JHNO3→HONO value in the atmosphere should be higher than that of our laboratory observations. It

197

is also possible that the decrease of the JHNO3→HONO with DNO3- can be partially caused by the

198

confining effect of the solid matrix of particle, which makes the photoproducts of HNO3 inside

199

the particle difficult to escape from the particle bulk or the lower layers of the sample.

200

Influence of acidic protons and H2O. Another notable phenomenon for the photochemical

201

aging of PM2.5 is that the formation rate of HONO becomes gradually decreased with the

202

irradiation time. After 10 h of irradiation, HONO was hardly produced anymore (Figure 2). At

203

this point, 95% of the original NO3- was still left in the sample (Figure S9). Interestingly, when

204

HCl flow was introduced into the reaction system, the HONO production was restored. As a

205

control experiment, the photoaged PM2.5 sample without light irradiation showed no HONO

206

production in HCl flux (Figure S10). These results indicate that the acidic proton plays an

207

important role in the photochemical HONO production. One of a possible role of the proton is to

208

assist the escape of HONO by protonating the photochemically-generated nitrite, which implies

209

that the photolysis of nitrate still proceeds in the absence of protons. If this is the case, other NOx

210

such as NO2 should be formed or nitrite should be accumulated in the particles after the acidic

211

proton was depleted. However, as shown in Figure 2, when HONO production decreased with

212

the irradiation time, the formation of NO2 was scarce. In addition, after the cease of

213

photochemical HONO production, introducing gaseous HCl did not enhance the release of

214

HONO in the dark, but restored HONO production under irradiation, which excludes the

215

possibility of the accumulation of nitrite in the particles by photolysis of nitrate. More

216

reasonably, the photolysis reaction is ceased in the lack of the protons. HNO3, rather than NO3-,


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217

is the main photoactive species for the HONO formation. Besides, even in the HCl flux, the

218

photochemical reaction underwent a rapid decrease and stopped after further 10 h of irradiation

219

(Figure 2). This decrease should not result from the depletion of HNO3, since 66% of the original

220

NO3- was still left after 10 h of irradiation in the presence of HCl (See Figure S9). It is reported

221

that HNO3 can be reduced by VOC (electron donors) emitted by the aerosol surface, leading to

222

HONO formation33. Therefore, it is possible that the decrease in the HONO production is caused

223

by consumption of reactive electron donors (eq. 5). We also note that under environmental

224

conditions, plenty of sulfuric acid, formed by the oxidation of SO2 in the atmosphere, can be

225

uptaken by PM2.5. The HCl-promotive effect implies that the sulfuric acid ubiquitous in the

226

atmosphere would similarly enhance the formation of HONO. However, the experimental

227

validation for this assumption in our laboratory is not feasible because the low-volatile H2SO4 is

228

difficult to be introduced into the reaction system in a gaseous form.

229

HNO3 + 2H+ + 2e- + hv → HONO + H2O

(5)

230

HNO3 + H2O + hv → HONO + 2·OH

(6)

231

HNO3+ hv → NO2 + ·OH

(7)

232

NO2 + H2O+ hv → HONO + ·OH

(8)

233


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234

Figure 2. The decay of HONO and NO2 production from Beijing urban PM2.5 (NOV28) with the

235

irradiation time, and its restoration by introducing HCl flow (bubbled from 1 M HCl solution

236

with the flow rate of 500 mL/min).

237

The influence of H2O on the HONO production was evaluated at different relative humidity

238

(RH) during the photochemical aging of Beijing PM2.5 (Figure 3 and S11). At low RH (< 5%),

239

the formation of HONO was quite slow, and became rapid at intermediate RH (15%~75%).

240

However, at RH > 90%, the photolysis rate decreased. Similar effects have been reported on the

241

photolysis of nitrate adsorbed on aluminum oxide by Grassian and coworkers34, who found that

242

the photolysis was enhanced at low relative humidity (RH < 20%), but depressed at high relative

243

humidity (RH > 45%). There are several possible explanations for the effect of water vapor on

244

HONO production: (i) as shown in eq. 6, H2O molecules can directly participate in the HNO3

245

photolysis reaction probably in the form of H2O/HNO3 clusters35; (ii) as shown in eq.7 and eq.8,

246

H2O molecules may promote the secondary reaction of NO2, produced by the photolysis of

247

HNO3, to HONO, since strong humidity dependence for the heterogeneous transformation of

248

NO2 to HONO has been reported36; (iii) it is also possible that RH can affect phase state of the

249

particle and hence their physical and chemical properties. For example, the higher RH will lead

250

to the decreased viscosity of the matrix, which can accelerate the molecular motion and thus

251

promote the rate of photochemical reactions37. To further evaluate the role of the water in the

252

HONO production, the photochemical reaction was performed at RH = 0%. Under this condition,

253

both HONO and NO2 production were almost below the detection limit (Figure 3a, b). The lack

254

of NO2 production excludes the second explanation. Namely, the HONO production in the PM2.5

255

particles should be generated from the H2O/HNO3 interaction during the photochemical aging

256

rather than from the secondary reaction of photo-generated NO2 with H2O vapor.


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257 258

Figure 3. Cumulative HONO production (red columns) or NO2 production (blue columns) after

259

50 min irradiation of Beijing urban PM2.5 as a function of the relative humidity (Temperature =

260

25 oC).

261

The unique properties of PM2.5 surfaces. In order to evaluate the effect of different

262

heterogeneous surfaces on HONO production, the HONO and NO2 production were detected

263

during photolysis of HNO3 loaded on different substrate surfaces (Figure 4). The HNO3 loaded

264

on quartz microfiber filters and pyrex glass was nearly inert and no HONO or NO2 was detected

265

in our reaction system, in agreement with the reported low photolysis rate of HNO3 (JHNO3→HONO

266

= 2.4 × 10−7 s-1) on quartz glass surfaces38. On the surface of TiO2 and Al2O3, significant NO2

267

production was observed under irradiation, while the production of HONO was still negligible.

268

The HONO/NO2 production ratios for Al2O3 and TiO2 were much less than 1 (Table S2). By

269

contrast, PM2.5 surface showed high activity for the heterogeneous HONO production (more than

270

an order of magnitude over TiO2 and Al2O3), but quite low yield for NO2. The HONO/NO2

271

production ratio for PM2.5 (JUL07) was much larger than 1 (Table S2), which is very distinctive

272

from other heterogeneous interfacial reactions on traditional metal oxide nano-/micro-particles.

273


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274 275

Figure 4. Cumulative (a) HONO (NHONO) and (b) NO2 production (NNO2) divided by the amount

276

of NO3- (NNO3-) absorbed on different supports as the function of irradiation time.

277

The unique photochemical properties of urban PM2.5 to the selective HONO generation may

278

originate from its complicated composition (component analysis of Beijing urban PM2.5 samples

279

is detailed in Table S3). Beijing urban PM2.5 contains various organic acid and other organic

280

species21, 39, which can act as both the proton and electron donors. In addition, the inorganic

281

acids (H2SO3, H2SO4 and HNO3, mostly from the chemical transformation of SOx and NOx) in

282

Beijing urban PM2.5 samples also play very important roles in the photochemical aging process

283

by both providing acidic protons and active reaction environments.

284

Influences of Environmental factors on the HONO generation. In the atmosphere, the

285

heterogeneous chemical reactions on atmospheric PM surfaces are significantly affected by

286

environmental factors (i.e. temperature, light wavelength and intensity). The influences of the

287

typical environmental factors on the HONO formation were further investigated. As shown in

288

Figure S12a, no HONO generation was observed under dark conditions. With the increase of

289

light intensity, the HONO production rate was gradually increased. Furthermore, the wavelength

290

of incident light also affected directly the HONO production (Figure S12b). When the PM2.5


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291

sample was illuminated by using visible light with wavelength λ > 400 nm, the average HONO

292

production rate within 0.5 h irradiation was 2.13 × 10-12 mol·s-1, decreasing 63.1% compared to

293

that of the full spectrum illuminated sample with the value of 5.78 × 10-12 mol·s-1 under identical

294

light intensity. These results imply that HONO generation reaction on urban PM2.5 samples is

295

mainly initiated by UV light ( λ < 400 nm).

296

Temperature is another environmental factor that possesses important influence on the HONO

297

production during the photochemical aging of PM2.5. As shown in Figure 5, under identical light

298

intensity and RH (60%), the HONO production rate increased with the temperature increasing

299

from 5 ~ 40 oC.

300 301

Figure 5. Cumulative HONO production from the PM2.5 sample (NOV16) as a function of the

302

irradiation time (t) at different temperature.

303

Environmental implication. Nitrous acid (HONO) is a very important gaseous species to

304

stimulate a series of further photochemical oxidation reactions of natural or industrial organic

305

compounds by producing hydroxyl radicals (·OH)22, 23, which has considerable impacts on the

306

local atmospheric oxidation capacity. The present study reveals that the photochemical aging

307

process of PM2.5 can be accompanied by the HONO release. It is reported that the nitrate in


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308

Beijing is significantly relative higher than that in other cities21. Together with the heavy PM2.5

309

pollution in Beijing, the photochemical formation of nitrogen oxides should play a more

310

important role in the nitrogen cycle in Beijing. The contribution of photoaging of PM2.5 to the

311

overall HONO sources under environmental conditions can be estimated by using the photolysis

312

rate constants measured in the present study and the concentration of nitrate (CNO3-) in the

313

atmospheric particles. Since the difference in response bandwidth of the nitrate actinometer is

314

not significant between latitude of 0 and 40° N28, the photolysis rate constants in Table 1 was

315

directly used for the estimation in Beijing (39°59’22.68’’N). According to the field

316

measurements at urban sites of Beijing in 2015 and 2016, CNO3- in the atmosphere of Beijing is

317

around 6.64 µg·m-3 (2.62 ppbv) 26. Using this CNO3- value and our observed JHNO3→HONO (1.22 ×

318

10-5 s-1 ~ 4.84 × 10-4 s-1), the noontime HONO source rate from heterogeneous photolysis of

319

HNO3 is estimated to be in the range of 0.12 ~ 4.57 ppbv·h-1, with the mean value of 0.78

320

ppbv·h-1, which is comparable to the reported daytime unknown HONO source rate in Beijing

321

(1.85 ppbv·h-1 in the severe haze period and 1.26 ppbv·h-1 in the clean period25, or 1.30 ~ 3.82

322

ppbv·h-1 26). These comparisons imply that the photochemical aging of Beijing urban PM2.5 is a

323

significant HONO production source. The present study suggests that the inclusion of the photo-

324

induced HONO production on PM2.5 could explain the presence of unexpected high daytime

325

HONO concentrations in both urban and rural areas, and improve the accuracy of the model

326

simulations based on field measurements in London, Huston and Michigan17, 24, 40-42.

327

Our results also show that the HONO production from PM2.5 is affected by specific

328

environmental conditions. For example, the acid can greatly accelerate the HONO release. The

329

atmosphere of southern China often has high air acidity based on model simulation studies43-45,

330

and should greatly enhance the HONO production of PM2.5. Moreover, it is demonstrated that the


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331

high light intensity during daytime promotes the HONO production of PM2.5, indicating the

332

PM2.5 as a HONO source mainly contributes to the daytime HONO production. The large

333

amount of HONO production in the atmosphere can cause a series of further photochemical

334

oxidation reactions of natural or industrial organic compounds by producing hydroxyl radicals

335

(·OH), which further induce the formation of oxidative products such as ozone, peroxy acetyl

336

nitrate (PAN), and a large amount of secondary environmental pollutants. All these oxidative

337

products can react with various organic compounds and stimulate the production of secondary

338

organic aerosols (SOA), causing severe air pollution problems.

339

ACKNOWLEDGMENT

340

This work was supported by NSFC (Nos. 21590811, 21525729, 21521062, 21777168), the

341

“Strategic Priority Research Program” (No. XDA09030200), the “Key Research Program of

342

Frontier Sciences” (No. QYZDY-SSW-SLH028) of the Chinese Academy of Sciences, and the

343

“CAS Interdisciplinary Innovation Team Program”. The authors are grateful to Prof. Pingqing Fu

344

from Institute of Atmospheric Physics (CAS) and Prof. Maofa Ge from Institute of Chemistry

345

(CAS) for their helpful discussion and their supports in PM2.5 samples.

346

ASSOCIATED CONTENT

347

Supporting Information.

348

Supplementary methods; Tables of HONO measurements in Beijing and other cities, and

349

components of PM2.5 samples; Figures showing calibration results and other supplementary

350

materials. The Supporting Information is available free of charge on the ACS Publications

351

website http://pubs.acs.org.


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For Table Of Contents Only


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Photochemical Aging of Beijing Urban PM2. 5: HONO Production

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