Role of NH3 in the Heterogeneous Formation of Secondary Inorganic

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A: Environmental, Combustion, and Atmospheric Chemistry; Aerosol Processes, Geochemistry, and Astrochemistry 3

Role of NH in the Heterogeneous Formation of Secondary Inorganic Aerosols on Mineral Oxides Weiwei Yang, Qingxin Ma, Yongchun Liu, Jinzhu Ma, Biwu Chu, Ling Wang, and Hong He J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b05130 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 11, 2018

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The Journal of Physical Chemistry

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Role of NH3 in the heterogeneous formation of secondary

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inorganic aerosols on mineral oxides

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Weiwei Yanga, b, d, Qingxin Maa, b, c*, Yongchun Liua, b, c, Jinzhu Maa, b, c, Biwu Chua, b, c,

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Ling Wanga, b, Hong Hea, b, c

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a

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Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences,

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Beijing 100085, China

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b

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Beijing 100049, China

State Key Joint Laboratory of Environment Simulation and Pollution Control,

College of Resources and Environment, University of Chinese Academy of Sciences,

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c

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Environment, Chinese Academy of Sciences, Xiamen 361021, China

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d

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Institute of Environmental and Applied Chemistry, College of Chemistry, Central

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China Normal University, Wuhan 430079, China.

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*Correspondence to: Qingxin Ma ([email protected])

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*Tel.: 86-10-62849337; fax: 86-10-62849337

Center for Excellence in Urban Atmospheric Environment, Institute of Urban

Key Laboratory of Pesticide and Chemical Biology of Ministry of Education,

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ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT

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In this work, a relationship between the role of NH3 and the properties of mineral

20

oxides (α-Fe2O3, α-Al2O3, CaO and MgO) in the evolution of NO3-, SO42- and NH4+

21

has been established. It was found that the promotion effect of NH3 was more

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favorable for the formation of NO3- (or SO42-) and NH4+ on acidic α-Fe2O3 and

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α-Al2O3 due to acid-base interactions between NO2 with NH3 or between SO2 and

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NH3, while this effect was weaker on basic CaO and MgO possibly due to their basic

25

nature. The acid-base interaction (NO2/SO2 with NH3) overpowered the redox

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reaction (SO2 with NO2) on Fe2O3 owing to its unique redox chemistry. However, the

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opposite was found on basic CaO and MgO for the formation of SO42- and NO3-.

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Under equivalent concentration conditions, the two synergistic effects did not further

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strengthen on Fe2O3, CaO and MgO due to a competition effect. However, in

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NH3-rich situations, a synchronous increase of SO42-, NO3- and NH4+ occurred on

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Fe2O3. On acidic Al2O3, the favorable adsorption of NH3 on the surface as well as the

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existence of NO2 with an oxidizing capability synergistically promoted the formation

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of SO42-, NO3- and NH4+.


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1. INTRODUCTION

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Mineral dust with an emission rate of 1000-3000 Tg per year, is the second most

36

important type of aerosol particles.1-3 Originating from windblown soil, mineral dusts

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have similar chemical components to crustal rock, in which aluminum, iron, calcium

38

and magnesium oxides are the abundant dominant oxides.4, 5 The rich defects, oxygen

39

species, hydroxyls and metal atoms on those oxides provide many reactive sites for

40

the adsorption and transformation of atmospheric trace gases, participating in the

41

formation of new particles and aging existing aerosols.6-9 Therefore, the reactions of

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trace gases on mineral dusts, i.e., heterogeneous reactions, are important processes in

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determining the composition of the gaseous troposphere.10-12

44

Nitrogen dioxide (NO2) is one of the most important gaseous pollutants in the

45

atmosphere. The high chemical reactivity of NO2 makes it an important processor for

46

the formation of acid rain and the depletion of ozone.13, 14 Nitrate coating onto mineral

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components in aerosols has always been observed in field measurements, suggesting a

48

non-negligible role of heterogeneous atmospheric chemistry in the contribution of

49

nitrate.15,

50

reaction of NO2 on mineral oxides indeed results in the production of nitrate species,

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which can become an NO2 sink.17, 18 Moreover, the existence of a mixture of gases

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causes the heterogeneous formation of significant secondary species on the mineral

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dusts.19, 20 For example, many studies have focused on the coexisting reactions of SO2

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and NO2 on mineral dusts, in which a synergistic effect existing in those two gases

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could remarkably promote the buildup of SO42- and NO3-, especially during heavy

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haze periods.21-23

16

Laboratory and model studies have confirmed that the heterogeneous

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Since pre-industrial times, NH3 as an important basic gas, has been injected into the

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atmosphere at an increasing rate of two to five-fold that of pre-industrial times, with

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further increases expected over the next 100 years due to the need in agricultural

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growth.24 Field observations have established a good correlation between NH3 and

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SO42-, NO3- and NH4+ in PM2.5.25 The NH3 is considered to be involved in the ternary

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nucleation process in the NH3-SO2-H2O system.26-29 Notably, environmental chamber

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modeling found that the existing surface of the aerosol played a positive role in the

64

degradation of precursor gases (i.e., SO2, NO2 and NH3) and that the degradation rates

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of these precursor gases increased with an increase in the initial ratio of (NH3)/(NO2 +

66

SO2).30 Our recent lab study found that the existence of NH3 led to the synergistic



67

formation of SO42- and NH4+ on typical mineral dusts.31 Obviously, the NH3 plays an

68

important role in the heterogeneous atmospheric chemistry. While its role in the

69

formation of nitrate or its role relative to NO2 in the formation of sulfate on mineral

70

dusts in the multi-gas coexisting system is still paid little attention in lab study.

71

Actually, the heterogeneous reactivity depends greatly on the properties of mineral

72

oxides, such as the acid-base nature, or the redox properties.31-34 For the

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heterogeneous reaction of NO2, basic MgO and CaO were more active compared to

74

acidic SiO2.35 A similar phenomenon also occurred with the heterogeneous reaction of

75

SO2 in which, among the same category group as Al2O3, the basic Al2O3 was the most

76

active.32 Furthermore, as an ubiquitous oxide in soil as well as in rust coatings and

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bricks, Fe2O3, with its unique Fe2+/Fe3+ redox chemistry, favors the formation of SO42-

78

and the heterogeneous conversion of NO2.32,

79

focused on the relationship of the oxide nature (including acid-base and redox

80

properties) with its heterogeneous reactivity in multi-gas coexisting system.

33, 36-38

However, few studies have

81

In the present study, α-Fe2O3, α-Al2O3 and CaO and MgO particles were chosen as

82

model mineral oxides to investigate the effect of NH3 on the heterogeneous reaction

83

of NO2 as well as on the complex coexisting system of SO2, NO2 and NH3. The

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surface products were detected in detail and quantitatively via in situ diffuse

85

reflectance infrared Fourier transform spectroscopy (DRIFTS) combined with ion

86

chromatography (IC) techniques. Possible mechanisms and atmospheric implications

87

were further proposed.

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2. MATERIALS AND METHODS

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2.1. Materials. The α-Fe2O3 was prepared using a precipitation method, as we

90

previously reported, while α-Al2O3 was obtained after the calcination of AlOOH

91

(SASOL) at 1200°C.31 CaO and MgO were purchased from Sinopharm Chemical

92

Reagent Beijing Co., Ltd. The Brunauer-Emmett-Teller (BET) surface areas of

93

α-Fe2O3, α-Al2O3, CaO and MgO were 24.5, 5.5, 57.8 and 29.1 m2 g-1, respectively, as

94

measured by a Quantachrome Quadrasorb SI-MP. X-ray diffraction (XRD) patterns of

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those samples were confirmed on a computerized PANalytical X'Pert Pro

96

diffractometer equipped with a Cu Kα radiation source (Figure S1). SO2 standard gas

97

(25 ppm in N2, Beijing Huayuan Gases Inc.), NO2 standard gas (25 ppm in N2, Beijing

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Huayuan Gases Inc.), NH3 standard gas (36 ppm in N2, Beijing Huayuan Gases Inc.),

99

and high purity N2 and O2 (99.999%, Beijing AP BEIFEN Gases Inc) were used as the ACS Paragon Plus Environment

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100 101

gas mixture. 2.2. In situ DRIFTS experiments. The in situ DRIFTS experiments were

102

performed on an FTIR spectrometer (Nicolet iS50, ThermoFisher Scientific Co., USA)

103

equipped with a high-sensitivity MCT/A detector cooled by liquid nitrogen. Before

104

each experiment, the sample was pretreated at 573 K for 120 min in a stream of

105

synthetic air (80 % N2 and 20 % O2) of 100 mL min-1 to remove adsorbed species.

106

The sample was then cooled down at 303 K until the baseline became stable and

107

subsequently exposed to 1 ppmv NO2, 1 ppmv SO2, or/and 1 ppmv NH3 for 8-9 h. All

108

the spectra were recorded at a resolution of 4 cm-1 for 30 scans in the spectral range of

109

4000 to 600 cm-1.

110

2.3. IC measurements. IC was used to measure the surface products formed on the

111

samples. The reacted particles were preserved with 1% formaldehyde, which was

112

diluted with ultrapure water (after boiling for 20 min to remove dissolved oxygen) to

113

10 mL (specific resistance ≥ 18.2 MΩ cm) and then sonicated for 30 min at 298 K.

114

The leaching solution was obtained through a 0.22 µm PTFE membrane filter and

115

then was analyzed using Wayee IC-6200 ion chromatography equipped with TSKgel

116

Super IC-CR cationic or SI-524E anionic analytical column. An eluent of 3.5 mM

117

Na2CO3 was used at a flow rate of 0.8 mL·min−1. The nitrate and sulfate ions

118

separated by anionic column appeared at 5.344 min and 7.532 min successively; while

119

the ammonium ions separated by cationic column was detected at 7.843 min (Figure

120

S2).

121

3. RESULTS

122

3.1. The role of NH3 in the NO2-NH3-mineral dust system. To investigate the

123

effect of NH3 on the heterogeneous reactions of NO2 on mineral dust, in situ DRIFTS

124

experiments were performed in a gas flow mixture of 1 ppmv NO2 and 1 ppmv NH3

125

balanced with synthetic air at room temperature (303 K). The DRIFTS spectra are

126

shown in Figure 1. The absorption bands and their detailed assignments are given in

127

Table 1. Figure 1a shows the DRIFTS spectra of an individual reaction of NO2 over

128

α-Fe2O3 as a function of time. Several bands ranging over 1600-1000 cm-1 appeared

129

and grew in intensity as time increased. The bands between 1600 and 1200 cm-1 are

130

assigned to the degenerate ν3 mode of nitrate species coordinated onto the surface.39

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Generally, ν3 can split into two bands, one at a higher wavenumber (ν3, high) and one at

132

a lower wavenumber (ν3, low), varying due to the different bonding configurations of



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133

monodentate, bidentate and bridging nitrates.40 In the present study, the bands at 1563

134

(ν3, high) and 1259 cm-1 (ν3, low) were assigned to bidentate nitrates, and the band at

135

1288 cm-1 (ν3, low) was assigned to monodentate nitrate with no observation of its

136

higher wavenumber band due to low coverage.19, 41-43 The weak adsorption band at

137

1006 cm-1 (ν1) was assigned to nitrate species, possibly in the bidentate state.43 With

138

exposure time increasing, solvated nitrate species were observed at 1417, 1379, 1347

139

cm-1.19,

140

monodentate nitrito products were also detected, as indicated by the presence of the

141

bands at 1155 cm-1 (ν3) and 1092 cm-1 (ν1), respectively.38,

142

consumption of hydroxyl groups (OH) at the bands of 3689 and 3667 cm-1 indicated

143

that the formation of nitrate/nitrito species were due to the interaction of NO2 with

144

those OH sites, accompanied by the production of water molecules with the stretching

145

adsorption mode and bending adsorption mode at the 3567 and 1632 cm-1 bands.18

44, 45

In addition, a small amount of bridging nitro-nitrito and bridging 42

The obvious

146 147

Table 1. Vibrational assignments of nitrate and nitro-nitrito products formed during

148

the reaction of NO2 with mineral dusts. Surface species Monodentate NO3Bidentate NO3

-

Bridging dentate NO3-

Ion-coordinated /solvated NO3nitro-nitrito, NO2N2O4

α-Fe2O3 ν3, low ν3, high ν1 ν3, low

1283-1286 19, 38, 41 1520 1006-1017 39, 41, 46 1259-126519, 38, 40, 41

ν3, high ν1 ν3, low ν3, high ν1 ν2 ν

156319, 38, 40, 41 100643 19, 27 1232 19, 27 1614

ν

1417-1424, 1379, 13418, 19, 41, 44, 45 1215, 1157, 109219, 38,

ν

17348, 19

α-Al2O3

CaO

MgO 152243, 44

1265

39, 49

159039, 49

42

149


106141, 47, 48 126644

126444

1596-158539, 41

1632-162339, 41 97241 80741, 47 1333-1328, 84841 120040, 41

1654-163849

1364, 131441 122019 17328, 19

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150 151 152

Figure 1. (a) In situ DRIFTS spectra of individual reaction of 1 ppmv NO2 and (b)

153

simultaneous reaction of 1 ppmv NO2 and 1 ppmv NH3 on α-Fe2O3 as a function of

154

time.

155 156

When NH3 was added into the gas flow with NO2, as shown in Figure 1b, in

157

addition to the formation of the nitrate (ca. 1424, 1379, 1347, 1286, 1265, 1017 cm-1)

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and nitrito species (1157, 1092 cm-1) mentioned above, new NH3 absorption bands

159

appeared. The bands at 1607 and 1198 cm-1 were assigned to the δas and δs modes of

160

NH3 coordinated onto Lewis acid sites, accompanied with the νas mode at 3365 cm-1

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and νs mode at 3258 cm-1 split with the overtone of the asymmetric NH3 deformation

162

at 3160 cm-1.50-52 Meanwhile, the deformation vibration mode of NH4+ bound to

163

Brønsted acid sites was also present at the band of 1447 cm-1.51 In contrast to the case

164

without NH3 (Figure 1a), the introduction of NH3 resulted in a significant increase in

165

the coverage of monodentate nitrate by the growth of the band at 1286 cm-1 (ν3, low)

166

and the presence of its ν3,

167

adsorption bands of the solvated nitrate species were enhanced, spreading from 1424

168

to 1347 cm-1 with a new resolved band at 1399 cm-1, which was possibly due to the

169

increased surface hygroscopicity promoted by the addition of NH3.41 Additional bands

170

at 1232 and 1215 cm-1 were due to the newly formed bridging nitrate and bidentate

171

nitrito species.19, 38 These results indicated that the presence of NH3 promoted the

high

mode at the 1520 cm-1 band.39 Furthermore, the



172

conversion of NO2 to nitrate and nitrito species on the surface of α-Fe2O3. Notably,

173

the surface OH still acted as active sites, although the adsorption bands of the

174

produced water molecules overlapped partially with those of NH3 at 3546 and 1622

175

cm-1.

176

177 178 179

Figure 2. Comparison of the final DRIFTS spectra for (a) α-Fe2O3, (b) α-Al2O3, (c)

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CaO and (d) MgO after the reaction with 1 ppmv NO2 or/and 1ppmv NH3 for 8.5 h in

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a gas flow of 100 mL·min-1 at 303 K.

182 183

To clearly illustrate the interactions between NO2 and NH3 on dust particles, the

184

final DRIFTS spectra for the individual and simultaneous reactions are compared in

185

Figure 2. On α-Fe2O3 (Figure 2a), NH3 adsorbed predominantly on the Lewis acid

186

sites (3365, 3268, 1604, and 1185 cm-1) after the individual reaction for 8.5 h. When

187

NO2 and NH3 coexisted in the gas flow for 8.5 h, the NH3 absorption band at 1185

188

cm-1 grew in intensity and blueshifted to 1198 cm-1 and the bands at 3365, 3258 and

189

3160 cm-1 simultaneously increased, indicating the accumulation of NH3 adsorbed

190

species.53 The strengthened adsorption band at 1447 cm-1 indicated that an increasing

191

amount of NH3 preferentially converted into NH4+ with the coexistence of NO2. With

192

respect to the NO2, the addition of NH3 not only promoted the accumulation of nitrate,

193

as mentioned above but also led to the increase of nitro-nitrito species at the bands of

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1215 and 1157 cm-1.19, 38

195

The α-Al2O3, CaO and MgO as typical acidic and basic oxides were used further to


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196

investigate the influence of NH3 on the heterogeneous reaction of NO2 (Figure 2b-d).

197

Their detailed reactions with NO2 in the absence and presence of NH3 versus time are

198

provided in Figure S3. In the individual reaction (Figure 2b), NO2 interacted weakly

199

with α-Al2O3, with the formation of a relatively small amount of bidentate nitrate at

200

the bands of 1590 (ν3, high) and 1265 cm-1 (ν3, low). 29, 43 The absorption configurations of

201

nitrate on α-Al2O3 showed some similarity with that on α-Fe2O3, although the latter

202

exhibited a much higher activity towards the adsorption of NO2. By comparison, the

203

adsorption of NO2 was favorable on basic CaO and MgO with bridging nitrates (at the

204

bands of 1623, 972, 807 cm-1 on CaO, and 1654 cm-1 on MgO) prevailing on the

205

surfaces.40, 41, 47, 48 In addition to solvated nitrate (1333 and 848 cm-1 on CaO, and

206

1364 cm-1 on MgO), some bidentate nitrate (at bands of 1596, 1266, 1061 cm-1 on

207

CaO) and monodentate nitrate (at band of 1522 cm-1 on MgO) were also found over

208

those two types of basic oxides. In the case of NH3, almost no adsorption was

209

observed on CaO and MgO and only weak adsorption of NH3 (3356, 3285, 1617 and

210

1230 cm-1) on Lewis acid sites was present on α-Al2O3.

211

After exposure to both NO2 and NH3 for 8.5 h, no positive influence of NH3 on the

212

formation of nitrate was observed for α-Al2O3, CaO and MgO. For α-Al2O3, it was

213

possibly due to its extremely low surface area while for CaO and MgO, the surface

214

basicity may not be in favor of the adsorption of NH3 and further the interaction of

215

NH3 with NO2. Noting the enhanced disturbance in the OH region ranging from 3731

216

to 3605 cm-1 in the simultaneous reaction on CaO, an intense interaction of OH with

217

the reactants occurred due to the coexistence of NO2 and NH3.

218

To better clarify the effect of NH3 on the heterogeneous reactions of NO2 on

219

mineral dusts, the integrated areas obtained under different conditions versus the

220

reaction time are exhibited in Figure 3. As seen from Figure 3a, a significant

221

synergistic effect between NO2 and NH3 was observed on the surface of α-Fe2O3.

222

Because of the poorly observed bands for the adsorbed NH3 on α-Al2O3, only the

223

integrated area for the NO3- bands is given here (Figure 3b). Almost no obvious

224

promotion effect of NH3 was detected during its interaction with NO2. Similar results

225

were also found over CaO and MgO (Figure 3c-d). These proved that the promotion

226

effect of NH3 for the heterogeneous reaction of NO2 was more obvious on acidic

227

oxides than on basic oxides.



228 229 230

Figure 3. Comparison of the integrated areas obtained from DRIFTS spectra over (a)

231

α-Fe2O3 (NO3-, 1328-1219 cm-1; NH4+, 1230-1100 cm-1), (b) α-Al2O3 (NO3-,

232

1290-1189 cm-1), (c) CaO (NO3-, 1124-866 cm-1) and (d) MgO (NO3-, 1720-1200 cm-1)

233

under individual and simultaneous conditions. The error bar represents the standard

234

deviation from three repeated experiments.

235

3.2. The role of NH3 in the NO2-SO2-NH3-mineral dust coexisting system. The

236

abovementioned results suggested a promotion effect of NH3 for the formation of

237

nitrate species on mineral dusts. Our recent research has also indicated that the

238

presence of NH3 could promote the heterogeneous formation of sulfate/sulfite species.

239

The acid-base interaction was proposed to be responsible for the synergistic effect

240

between SO2 and NH3 on the surface of mineral dusts.31 As reported previously, the

241

coexistence of NO2 with SO2 promotes the formation of sulfate significantly through a

242

redox process, in which the dimer intermediate, N2O4, of the adsorbed NO2 oxidized

243

the adsorbed SO2 to form a sulfate species, while N2O4 was reduced to form a nitrite

244

species,8, 54 as shown below,

245

MN2 O4,ads +MSO3 →MNONO2 +MSO4 (1)

246

MNONO2 +MO→2MNO2 (2)

247

Given the complex evolution relationship among those gases, it is necessary to

248

elucidate the effect of NH3 in the mixed SO2-NO2-NH3 atmospheres. Therefore, SO2

249

was further added into the reaction system, and the heterogeneous formation of SO42-,

250

NH4+ and NO3- versus time is exhibited in detail in Figure S4 and S5. The final


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251

DRIFTS curves for the mineral oxides after exposure to different atmospheres for 8.5

252

h are thereafter compared in Figure 4.

253

254 255 256

Figure 4. Comparison of the final DRIFTS spectra for (a) α-Fe2O3, (b) α-Al2O3, (c)

257

CaO and (d) MgO after the reaction with different gas components for 8.5 h.

258 259

Compared to the individual reaction of SO2, the formation of sulfate was clearly

260

promoted in the presence of NH3 on acidic Fe2O3, which was indicated by the growth

261

of the bands at 1236, 1151 and 1020 cm-1 (Figure 4a, green line).37 In contrast, this

262

promotion effect was weakly observed on Al2O3, CaO and MgO, especially for the

263

latter two basic oxides (Figure 4b-d, green lines). Only sulfite species were detected

264

on Al2O3 and CaO, and the corresponding bands were present at 932 and 940 cm-1,

265

respectively. The adsorption of SO2 on MgO with the coexistence of NH3 was similar

266

to that without NH3 and that the weak formation of sulfate and sulfite appeared

267

separately at the bands of 1213 and 1028 cm-1.8 In the SO2 and NH3 reaction group,

268

the adsorption of NH4+ bound to the Brønsted acid sites was predominant on Fe2O3,

269

which was represented with the bands of 1683 and 1429 cm-1 attributed to δs(NH4+)

270

and δas(NH4+), which were connected to ν(NH4+) in the high frequency region, at 3049

271

and 2858 cm-1. In addition, a single adsorption band for ν(NH3) on Lewis acid site

272

was still observed at 3216 cm-1.31 For Al2O3, the formation of NH4+ on Brønsted acid

273

sites at the band of 1461 cm-1 (δas(NH4+)) and the adsorption of NH3 on Lewis acid

274

sites at the bands of 3360, 3281, 3202 cm-1 (ν(NH3)) and 1230 cm-1 (δs (NH3)) were



275

both present.31 However, the bands assigned to adsorbed NH3 were far weaker than

276

the broad adsorption of H2O at the band of 3538 cm-1. The adsorption of NH3 on CaO

277

and MgO was barely observed possibly due to their basic nature.

278

The SO2 reaction in the presence of NO2 was different (Figure 4, blue lines). In

279

contrast to the experiments with SO2 and SO2+NH3, additional sulfate bands at 1340

280

and 1100 cm-1 appeared on the surface of Fe2O3 and at 1308 cm-1 on Al2O3.55, 56 More

281

obviously, the intensities of the bands at 1188, 1150 and 1072 cm-1, which

282

corresponded to the newly formed sulfate species, were very strong on CaO,

283

accompanied by the significant reduction of sulfite species at the band of 940 cm-1.48

284

For MgO, a large amount of sulfate accumulated on the surface, which was

285

characteristic of the remarkably increasing bands at 1213 and 1161 cm-1. In addition,

286

the weak adsorption of sulfite species at the band of 1028 cm-1 vanished at the same

287

time.57

288

With regards to the NO2-adsorbed species, the bands at 1328 cm-1 for CaO and

289

1314 cm-1 for MgO were due to solvated nitrate and ion-coordinated nitrate,

290

respectively.41 Some nitrate bands, such as 1267 cm-1 on Fe2O3, 1260 cm-1 on Al2O3

291

and 982 cm-1 on CaO, partially overlapped with those assigned to sulfate species,

292

while others mostly appeared at the bands ranging from 1683 to 1536 cm-1. The weak

293

adsorption of N2O4 was also observed at approximately 1732 cm-1 on Fe2O3 and

294

MgO.8, 19 These results confirmed that NO2 promoted the oxidation of SO2 to sulfate,

295

which largely proceeded through the abovementioned redox process (eqn. 1). While

296

the promotion effect by NH3 can be driven by the acid-base interaction since no

297

transformation from sulfite to sulfate occurred in this situation. Essentially, those

298

results align with our previous findings.8, 31

299

When SO2, NH3 and NO2 coexisted in the gas flow (Figure 4, red lines), the NH3

300

adsorption was obviously enhanced in the high wavenumber region from 3400 to

301

2800 cm-1, particularly for the basic oxides of CaO and MgO. Meanwhile, the growth

302

of the sulfate bands (1188 and 1072 cm-1) on CaO, and the increase of both the sulfate

303

(1314, 1213 and 1161 cm-1) and nitrate (1638 and 1536 cm-1) bands on MgO were

304

also observed. By comparison, the synchronous growth of SO42-, NO3- and NH4+ was

305

not evident on Fe2O3 and Al2O3. A slight increase in the NH4+ at band of 1429 cm-1

306

and the formation of a new sulfate band at 1298 cm-1 occurred on the former, while

307

only a weak enhancement of the NH4+ band at 1461 cm-1 was shown on the latter.

308

Based on the DRIFTS results, the acid-base interaction between NO2/SO2 with NH3 ACS Paragon Plus Environment

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309

and the redox process between NO2 and SO2 can both occur with the reactions of the

310

three gases. However, of the two mechanisms, the one that is prioritized might be

311

determined by the properties of mineral oxides.

312

3.3. Quantitative analysis of surface products. To elucidate the role of NH3 in the

313

heterogeneous reactions of SO2, NO2 and NH3 on mineral oxides, quantitative

314

analysis of the surface products resulting from those reacted particles in DRIFTS

315

experiments were conducted by IC measurements, as shown in Figure 5. Detailed data

316

that is displayed in Figure 5 has also been summarized in Table S1 and S2. In Figure

317

5a, compared to the individual reactions (SO2/NO2/NH3 reaction), two kinds of gases

318

(SO2 and NH3, NO2 and NH3, SO2 and NO2) coexisting in the system resulted in the

319

synergistic formation of SO42-, NO3- and NH4+ on the mineral oxides. With the

320

coexistence of NH3 (SO2+NH3, NO2+NH3), its promotion function for the formation

321

of SO42- and NO3- was more obvious on acidic Fe2O3 and Al2O3 (especially on Fe2O3)

322

than on basic CaO and MgO. In contrast, the NO2 promotion effect played a more

323

predominant role in the formation of SO42- and NO3- on the basic oxides of CaO and

324

MgO in the SO2+NO2 reaction group.

325 326

Figure 5. The IC results showing the quantity of surface products formed on the

327

reacted particles, which were collected from the DRIFTS cell after reactions under

328

different conditions for 8.5 h. All the concentrations of reactants in (a) were 1 ppmv,

329

and the amounts of SO42- and NO3- formed on MgO follows the right y-axis, while

330

others following the left y-axis. The concentration of NH3 varied from 0.1 ppmv to 1

331

ppmv in (b), while NO2 and SO2 remained at a constant concentration of 0.1 ppmv in

332

this case.



333

However, when the three kinds of gases were introduced into the system

334

simultaneously, the amount of SO42-, NO3- and NH4+ formed did not further increase,

335

and instead, decreased in comparison with that for the coexistence of SO2 and NH3, or

336

NO2 and NH3 on Fe2O3 and that for the coexistence of SO2 and NO2 on CaO and

337

MgO, but not for Al2O3. These results indicate that the acid-base interaction (between

338

SO2 and NH3, or NO2 and NH3) and the redox process (between SO2 and NO2) may

339

exhibit a competition effect towards the formation of SO42-, NO3- and NH4+.

340

Taking the evolution of SO42- on Fe2O3 as an example, both NH3 and NO2 would

341

compete for the SO2 molecules to induce the transformation of SO2 to SO42-. It

342

seemed that NH3 has a more promotional effect than NO2 on the formation of SO42-

343

on this type of acidic oxide. However, with equal inlet concentrations of the reactants,

344

NH3 would be limited in the interaction with SO2 due to the competition effect of NO2,

345

in that a portion of NH3 can also interact with NO2. Therefore, the amount of SO42-

346

formed under this circumstance would be lower than the amounts formed for the

347

coexisting gases (SO2 and NH3 or SO2 and NO2). To prove this hypothesis, NH3 was

348

increased to up to ten times the concentration of SO2 and NO2, as shown in Figure 5b.

349

All the amounts of SO42-, NO3- and NH4+ increased significantly compared to those in

350

the low concentration situation. These results confirmed that both SO2 and NO2 were

351

favorable for the reaction with NH3 on acidic Fe2O3, further investigation is needed to

352

determine which is more attractive to NH3 or whether there exists an interaction

353

between SO2 and NO2.

354

For other types of basic oxides of CaO and MgO, the interaction between SO2 and

355

NO2 is dominant but can be weakened by the presence of NH3, which may interact

356

with both SO2 and NO2 at the same time. The case of the weakened promotion effect

357

in the three kinds of gas coexisting system did not occur on Al2O3. The results implied

358

that the different roles of NH3 (via an acid-base interaction with SO2 or NO2) and NO2

359

(via a redox process with SO2) are closely related to the properties of the mineral

360

oxides, which will be discussed in detail in a later part.

361

4. DISCUSSION

362

4.1. In the NO2-NH3-mineral dust reaction system. DRIFTS and the IC

363

investigation suggested that the synergistic effect between NO2 and NH3, and that

364

between SO2 and NH3, or SO2 and NO2 varies on mineral oxides with different

365

acidic/basic properties. In the NO2 and NH3 reaction group, the synergistic formation


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366

of NO3- and NH4+ was clearly detected on acidic Fe2O3 and Al2O3, especially on

367

Fe2O3, by both DRIFTS and the IC measurements. The DRIFTS spectra (Figures 1,

368

2b) show that the OH groups were the main active sites for the adsorption of NO2 on

369

Fe2O3 and Al2O3, which caused the predominant formation of bidentate nitrate on

370

these two types of oxides. This type of nitrate configuration was due to the adsorption

371

of NO2 on the hydroxyls, followed by a disproportionation reaction of two

372

adsorbed-NO2 molecules through the Langmuir-Hinshelwood (LH) mechanism, with

373

α-Fe2O3 as an example,18, 44 -

-

2FeOOH···NO2,ads →FeO+ NO3 +FeO+ NO2 + H2 O

374

LH (3)

375

In this reaction, the production of nitrate was accompanied by the formation of nitrite

376

species, which were confirmed by the presence of bands at ca. 1215, 1157 and 1092

377

cm-1 on α-Fe2O3. Then, the nitrite species were transformed into nitrate rapidly by

378

excess gaseous NO2 through the Eley-Rideal (ER) mechanism, as expressed in eqn. 4,

FeO+ NO-2 +NO2, g →FeO+ NO-3 +NO g

379

ER (4)

380

No nitrite and adsorbed NO species were observed on α-Al2O3 possibly due to their

381

extreme low concentrations or poor signals. Since the CaO and MgO surfaces were

382

not as hydroxylated as the Fe2O3 and Al2O3 surfaces (Figure 2c, d), no consumption of

383

OH groups was observed when NO2 reacted individually, and the main product was a

384

bridging nitrate. In this case, NO2 may coordinate directly onto the exposed metal

385

atoms (Lewis acid sites), interacting with adjacent adsorbed NO2 (possibly in the form

386

of N2O4) to form nitrite and gaseous NO.42 The reaction equations were listed as

387

follows,

388

O-M+NO2, g →O-M-NO2, ads

389

2O-M-NO2, ads →2M-N2 O4 →2M-NO3 +NO g

(5) (6)

390

In addition, the presence of water on the oxide surface would involve the formation of

391

solvated nitrate (e.g., ca. 1680 cm-1) and solvated nitrite species, as shown in eqns.

392

7-8:18, 41

393

(MO)-NO3, ads +H2 O→MOOH···HNO3

(7)

394

(MO)-NO2, ads +H2 O→MOOH···HNO2

(8)

395

When NH3 was introduced simultaneously with the reaction of NO2, it

396

preferentially adsorbed on Lewis acid sites (Figure 1b). The preferential adsorption of

397

NH3 (as seen in Figure 1b) quickly increases the surface basicity, promoting the



398

adsorption of NO2. Due to their different adsorption sites, NO2 mainly adsorbed on

399

the OH sites while NH3 coordinated on the exposed metal atoms, and a synergistic

400

effect can be found on Fe2O3.58 In addition, a small amount of NH4+ (1447 cm-1)

401

present on the surface of Fe2O3 suggests that the nitric or nitrous acid formed through

402

eqns. 7-8 participated in the neutralization of adsorbed-NH3:

403

HNO3 +NH3 →NH4 NO3

(9)

404

HNO2 +NH3 →NH4 NO2

(10)

405

Since surface hydroxyls served as the main adsorption sites for water molecules,

406

highly hydroxylated Fe2O3 was favored for the formation of nitric or nitrous acid

407

through reactions 7-8 followed by the formation of NH4+ through reactions 9-10.59-61

408

In contrast, no obvious promotion effect of NH3 occurred with the formation of

409

NO3- for basic CaO and MgO. As postulated above, NO2 mainly coordinated onto

410

exposed metal atoms, meaning that it competed with NH3 for Lewis acid sites. A

411

previous study found that NO3- had a closer connection to the metal atoms in basic

412

oxide, such as CaO and MgO, than in acidic oxides, such as α-Al2O3, TiO2, and

413

γ-Fe2O3.41 The strong bond between NO3- and Ca (or Mg) in the present study may

414

further limit the adsorption of NH3 on the same Ca (or Mg) atoms. However, nitrate

415

connecting to metal atoms showed a strong electron affinity, which would increase the

416

Lewis acid strength of the metal ion and may promote the adsorption of NH3 to

417

certain extent. Furthermore, a small amount of other types of nitrate species including

418

bidentate, monodentate and solvated nitrate species present on CaO and MgO

419

suggests that a small fraction of NO2 did not competed with NH3 for the Lewis acid

420

sites, possibly through the reaction in eqns. 3-4. Therefore, the coexistence with NH3

421

did not reduce the amount of nitrate but promoted its formation slightly, as found from

422

the IC results. The obvious consumption of OH ranging from 3731 to 3605 cm-1

423

(Figure 2c) in the simultaneous reaction confirms the enhanced interactions of NO2

424

and NH3 with the surface, which may account for the additional formation of nitrate

425

and ammonia species on basic oxides.

426

4.2. In the NO2-SO2-NH3-mineral dust reaction system. Since our previous

427

studies have explained the mechanisms regarding the synergistic effect between SO2

428

and NH3 and between SO2 and NO2 in detail, which have been simply mentioned

429

earlier as the acid-base interaction mechanism and redox mechanism, the complex

430

relationship between the NH3 (or NO2) role and the properties of the mineral dusts

431

would be revealed on the basis of these given mechanisms.8, 31 The IC results showed ACS Paragon Plus Environment

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432

that the synergistic effect between SO2 and NH3, or NO2 and NH3 prevailed over the

433

synergistic effect between SO2 and NO2 on Fe2O3, especially for SO2 for the

434

formation of SO42-, while the reverse was found over CaO and MgO for the formation

435

of SO42- and NO3-.

436

It has been demonstrated that SO2 mainly interacted with basic O2- sites at the steps

437

and kinks in CaO and MgO to form sulfite species, and only a small amount of sulfate

438

formation can be determined by the four-coordinated oxide anions on the steps and

439

corners of the MgO particles under dry conditions, even in the presence of O2.57 In the

440

individual reaction of SO2 in this work, the amount of sulfate obtained by the IC

441

measurements on CaO and MgO (and Al2O3) can be partially derived from the

442

aqueous oxidation during the measurement process because the dissolved oxygen

443

cannot be excluded completely and the particles might not be preserved very well

444

with formaldehyde. After this correction, the transformation of SO2 and NO2 to SO42-

445

and NO3- still require an oxidation process, in which CaO and MgO had no oxidizing

446

ability but only NO2 can act as an oxidant (Figure 3-4, eqns.1-6). Moreover, given the

447

basic nature, CaO and MgO were more favorable for the adsorption of NO2 than NH3.

448

Therefore, the synergistic effect between SO2 and NO2 has a more promotional effect

449

than that between SO2 (or NO2) and NH3 for the formation of SO42-.

450

The case of Fe2O3 is somewhat different as Fe2O3 itself can transform SO2 into

451

sulfate species, driven by the Fe(III)/Fe(II) redox process by a series of free-radical

452

propagation, termination, and product-formation reactions, or by molecular oxygen

453

activation on oxygen vacancy sites.37,

454

adsorption of NH3 was fast, even with the coexistence of NO2 (Figure 1b), possibly

455

due to the acidic nature of Fe2O3. The quickly enhanced surface basicity would

456

promote the adsorption of SO2 rapidly, which then transformed into SO42- from the

457

Fe(III)/Fe(II) redox process. Furthermore, due to the acidic nature, the adsorption of

458

NO2 as a dimer of N2O4 may be limited, which is a key intermediate promoter for the

459

oxidation of SO2.8 Therefore, the promotional effect of NH3 seemed more obvious

460

than that of NO2 in the formation of SO42- on acidic Fe2O3. For another acidic oxide of

461

Al2O3, the promotion effect of NH3 was basically equal to that of NO2 for the

462

formation of SO42-. However, neither Al2O3 nor NH3 has ability to oxidize SO2 to

463

SO42- in the way Fe2O3 or NO2 does. The remarkably increased amount of SO42- in the

464

presence of NH3 was possibly due to the dramatically enhanced adsorption of water,

465

as indicated by the enhanced 3538 cm-1 band in Figure 4b. which was favorable for

62

The DRIFTS spectra showed that the



466

the conversion of surface-coordinated sulfite to sulfate species.57

467

It is noted that the formation of nitrate species was promoted significantly by SO2

468

on all mineral oxides. One possible reason can be the re-oxidation of the produced

469

nitrite in eqn. 2.63, 64 Especially with the aid of O2, the NO resulting from the eqns. 4

470

and 6 would combine with nitrite species to form nitrate species, as shown below,54

471

2MNO2 +NOg +O2 →MNO3 (11)

472

IC results (Figure 5a) showed that the addition of a third gas did not further

473

increase the amounts of SO42-, NO3- and NH4+ on Fe2O3, CaO and MgO compared to

474

the amounts in the presence of two gases, which was possibly due to the complex

475

competition effect among those synergistic effects (SO2 and NH3, NO2 and NH3, and

476

SO2 and NO2) with equivalent gas concentrations. On Al2O3, however, the amounts of

477

SO42- and NH4+ were higher in the SO2, NO2 and NH3 reaction group compared to the

478

two-gas reaction groups, indicating that both the acid-base interaction and redox

479

reaction mechanism exhibit a synergistic influence on the formation of SO42- and

480

NH4+. The acidic nature of Al2O3 induced the adsorption of NH3, promoting the

481

adsorption of SO2, which was oxidized into SO42- by NO2 (in the form of N2O4) at the

482

same time. In turn, the formed SO42- increased the Brønsted acid sites on the surface

483

and then promoted the formation of NH4+.31

484

5. CONCLUSION AND ATMOSPHERIC IMPLICATIONS

485

The promotional effect of NH3 on the heterogeneous formation of NO3-, SO42- and

486

NH4+ has been elucidated in detail, which relied greatly on the acid-base and redox

487

properties of mineral oxides. For acid oxides such as α-Fe2O3 and α-Al2O3, the NH3

488

exerts a significant promotion effect on the formation of nitrate species. Moreover, it

489

acted as a counterpart role and was even more important relative to the NO2 in

490

promoting the formation of SO42-. Particularly for α-Fe2O3, its redox chemistry

491

significantly facilitated the promotional effect of NH3. Evidence from the remarkably

492

formed SO42-, NO3- and NH4+ under NH3-rich conditions hinted us that an effective

493

control of the emission of NH3 might be a potential strategy to reduce secondary

494

inorganic aerosols in NH3-rich areas.65 However, the prevailing effect of NO2 over

495

NH3 for the formation of SO42- and NO3- on basic CaO and MgO is indicative of a

496

more effective way to reduce the formation of secondary species in areas containing

497

an abundance of basic aerosols.21, 66 Given the varied minerology of dusts with source

498

location, a differential elaboration of the evolution mechanism of secondary species is


Page 18 of 26

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499

necessary.

500

In the system involving the reaction with the coexistence of SO2, NO2 and NH3

501

sharing equal inlet concentrations, however, the synergistic effect initiated separately

502

by NO2 or NH3 will not multiply to obtain further enhanced formation of SO42-, NO3-

503

and NH4+ on the oxides with redox or basic properties. Nevertheless, the IC results of

504

α-Al2O3 illustrate that acidic oxides with non-oxidizing ability are favorable towards

505

the synergistic adsorption of SO2 (or NO2) and NH3 (driven by NH3) and the

506

simultaneously transformation of SO2 (or NO2) and NH3 into SO42- (or NO3-) and

507

NH4+ (oxidized by NO2). Thus, an enhanced synergistic effect is expected on this type

508

of acid oxide in the system of coexisting SO2, NO2 and NH3.



Page 20 of 26

509

ASSOCIATED CONTENT

510

Supporting Information is available free of charge

511

http://pubs.acs.org.

512

XRD patterns of mineral oxides. The chromatogram of NO3-, SO42- and NH4+. In situ DRIFTS

513

spectra of mineral oxides during exposure to different reaction atmospheres versus reaction time.

514

The tables summarizing the amounts of surface products formed on mineral oxides under different

515

conditions.

516

ACKNOWLEDGMENTS

517

This research was financially supported by the National Key R&D Program of China

518

(2016YFC0202700) and the National Natural Science Foundation of China

519

(91744205), and the Project funded by China Postdoctoral Science Foundation (no.

520

2017M622485).

521

AUTHOR INFORMATION

522

Corresponding Author

523

*E-mail:

524

86-10-62849337

525

Notes

526

The authors declare no competing financial interest.

Qingxin

Ma

([email protected]);

Tel.:

527


via the Internet at

86-10-62849337;

fax:

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Role of NH3 in the Heterogeneous Formation of Secondary Inorganic

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