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Letter

Aerosol Liquid Water Driven by Anthropogenic inorganic salts: Implying its key role in the haze formation over North China Plain Zhijun Wu, Yu WANG, Tianyi TAN, Yishu ZHU, Mengren Li, Dongjie SHANG, Haichao Wang, Keding Lu, Song Guo, Limin Zeng, and Yuanhang Zhang Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.8b00021 • Publication Date (Web): 12 Feb 2018 Downloaded from http://pubs.acs.org on February 13, 2018

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Environmental Science & Technology Letters

ACS Paragon Plus Environment


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Aerosol Liquid Water Driven by Anthropogenic Inorganic Salts:

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Implying Its Key Role in the Haze Formation over North China Plain

3 4 5 6 7 8 9 10

Zhijun Wu*, Yu Wang#, Tianyi Tan, Yishu Zhu, Mengren Li, Dongjie Shang, Haichao Wang, Keding Lu, Song Guo, Limin Zeng, Yuanhang Zhang State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China # Now at Centre for Atmospheric Sciences, School of Earth and Environmental Sciences, University of Manchester, Manchester M13 9PL, UK. 2

11

*

12

Abstract

13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

This study reveals the aerosol liquid water (ALWC) in PM2.5 ranged from 2% up to 74% associating with the secondary inorganic fraction rose from 24% to 55% and ambient relative humidity (RH) increased from 15% to 83% in the atmosphere over Beijing. Unexpectedly, the secondary inorganic fraction in PM2.5 increased with an increase in the ambient RH, which is a meteorological parameter independent of anthropogenic activities, indicating the presence of a feedback mechanism driven by Henry’s law and thermodynamic equilibrium. During haze episodes, simultaneously elevated RH levels and anthropogenic secondary inorganic mass concentrations resulted in an abundant ALWC. The condensed water could act as an efficient medium for multiphase reactions, thereby facilitating the transformation of reactive gaseous pollutants into particles and accelerating the formation of heavy haze. The ALWC was well correlated with the mass concentrations of both nitrate and sulfate, indicating both nitrate and sulfate salts play key roles in determining the ALWC. Coincident with a significant reduction in SO2 emissions throughout China, nitrates will become a dominant anthropogenic inorganic salt driving the ALWC. Thus, the abundance of the ALWC and its effects on the aerosol chemistry and climate should be reconsidered.

Corresponding author: Zhijun Wu ([email protected])


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33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

(Photo by: Zhijun Wu)



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1 Introduction

57

Condensed water is an important component of atmospheric particles and

58

significantly contributes to ambient aerosol mass1-3, optical properties4-5, and

59

gas-particle transformations through complex physical and chemical mechanisms6,

60

thereby playing profound roles on air quality degradation7. Water in the particulate

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phase has been proposed as a plasticizer since its presence results in changes to the

62

particle phase state, which subsequently has a substantial effect on molecular

63

diffusion on both particle and bulk surfaces8-11. Condensed water provides the

64

medium

65

Aqueous-phase reactions, including N2O5 hydrolysis, can irreversibly drive gas

66

uptake14-15 and enhance the formation of secondary inorganic aerosols (SIAs)16, which

67

are used to refer to sulfate, nitrate, and ammonium in this study. Recent field surveys,

68

modeling results, and experimental studies also highlighted the role of aerosol liquid

69

water in the formation of global secondary organic aerosols (SOAs)12,

70

Condensed water facilitates the partitioning of water-soluble, polar, organic precursors

71

into condensed phases25 and subsequent aqueous-phase reactions into more

72

oxygenated organic components6. The aerosol liquid water content (ALWC) is

73

determined according to the ambient relative humidity (RH), aerosol mass

74

concentration, and chemical composition4. Sulfate and nitrate salts represent the most

75

important components that drive the particle hygroscopicity and ALWC in areas with

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intense anthropogenic activities26-29. Since the ALWC attributed to anthropogenic

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SIAs may significantly influence the air quality5, 30 and regulate biogenic aqueous

78

SOAs formation17,

79

provide an explanation of atmospheric physicochemical processes.

for

the

multiphase

chemistry

throughout

the

atmosphere6,

12-13

.

17-24

.

31-33

, understanding the ALWC and its determinants is vital to

80

Heavy haze events, which occur frequently throughout the atmosphere over the

81

North China Plain (NCP), threaten the health of millions of people. Moreover, the

82

outflow of Asian air pollution could have an impact on global weather patterns34.

83

Secondary aerosols was considered a main source of the particulate pollution during

84

such haze events35-36. Recently, the reactive nitrogen chemistry within aerosol liquid


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water was found to be a source of particulate sulfates during haze events over China16,

86

37

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role on the additional formation of oxidized SOAs38. A recent study showed that the

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uptake coefficient of N2O5 could be high and that it was related to a high ALWC in

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Beijing, thereby increasing the formation of nitrates39. These findings highlight the

90

importance of the ALWC in secondary aerosol formation, and thus, in the generation

91

of heavy haze events. An estimation based on measurements of the chemical

92

composition over Beijing showed that the average ALWC was the highest compared

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with other locations around the world3. However, a thorough understanding of the

94

association of the ALWC with the ambient RH, chemical composition, and particulate

95

matter pollution levels in the atmosphere over Beijing is still lacking.

. Observations from Beijing revealed that aqueous-phase processes play a dominant

96

The ALWC was calculated in the present study on basis of long term data sets

97

provided by aerosol mass spectrometer (AMS), hygroscopicity-tandem differential

98

mobility analyzer (H-TDMA), and measurements from filter samples. Furthermore,

99

the effects of particle chemical compositions and of the RH and air pollution levels on

100

the ALWC were evaluated. We discovered that simultaneously elevated levels of the

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RH, inorganic fraction, and PM2.5 mass concentration resulted in an abundant ALWC

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associated with anthropogenic SIAs during haze episodes throughout the atmosphere

103

over Beijing. This abundant ALWC may play a profound role in visibility degradation,

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aerosol mass loading, and secondary aerosol formation.

105

2 Materials and Methods

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The measurements were conducted at the Peking University Atmosphere

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Environment Monitoring Station (PKUERS), which is located in the northwestern

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urban area of Beijing. At the sampling site, various meteorological parameters,

109

including the wind speed, wind direction, ambient RH, and temperature, were

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measured by a weather station. The non-refractory particle chemical compositions

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were measured using an Aerodyne High-Resolution Time-of-Flight Aerosol Mass

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Spectrometer (HR-ToF-AMS). Data consisting of daily filter-based water-soluble ions



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in PM2.5 were obtained from filter samples in addition to laboratory ion

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chromatography analysis. An H-TDMA and a scanning mobility particle sizer (SMPS)

115

and aerodynamic particle sizer (APS) system were respectively employed to measure

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the size-resolved hygroscopic growth factors (HGFs) and particle number size

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distribution (PNSD) from 3 nm to 10 µm. The dataset utilized for the calculation is

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listed in Table S1. A detailed description of the instrumentation and data validation

119

can be found in the supporting information (SI).

120

First, the ALWC was calculated using the ISORROPIA-II thermodynamic

121

model40 with AMS particle mass concentrations of three inorganic species (NH4+,

122

SO42−, and NO3−). The results were then compared with the ALWC derived from the

123

HGF-PNSD. The measured ambient RH and temperature were set as the input

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parameters. ISORROPIA-II was operated while assuming that the particles are

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“metastable”, meaning that a solid formation was prevented in the model. The ALWC

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calculated during the reverse mode, in which the known quantities were the

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temperature, the RH, and the precursor concentrations in the aerosol phase40, was

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ultimately taken (see the SI). The ALWC contributed by the organic fraction was

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calculated using a method in the literature41 (see the SI).

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Second, the ALWC was calculated using the ISORROPIA-II thermodynamic

131

model with four filter-based inorganic species (NH4+, Cl-, SO42−, and NO3−) and four

132

cations (K+, Na+, Ca2+, and Mg2+). The organic fraction was not considered in the

133

calculation of the ALWC, and therefore, the ALWC may have been underestimated.

134

Finally, the ALWC was calculated from the particle hygroscopicity

135

measurements. In this method, the volume of wet particles at the ambient RH was first

136

calculated by combining the size-resolved HGFs measured via the H-TDMA with the

137

PNSDs. Then, the ALWC was calculated by subtracting the volume of dry aerosol

138

particles from that of wet particles. A brief introduction is given in the SI. A detailed

139

description of this method can be found in Bian et al.42.

140

A comparison (Fig. S1) was conducted between the ALWC calculated using

141

ISORROPIA-II both with and without water contributed by organic compounds and

142

the ALWC calculated using the HGFs and PNSDs (the data were collected in June ACS Paragon Plus Environment

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2014). Considering the uncertainties in both the ISORROPIA-II model and the

144

HGF-PNSD method (see the explanation in the SI), we concluded that the ALWC

145

calculated using ISORROPIA-II agreed well with that calculated from the

146

size-resolved HGFs and PNSDs. The abovementioned comparisons mutually validate

147

the HGF-PNSD method and the ISORROPIA-II model, and thus, their results can be

148

used to calculate the ALWC within polluted atmospheric environments such as that

149

over Beijing.

150

The following analysis employed the ALWC results calculated using

151

ISORROPIA-II with three-year filter-based ion concentrations in addition to the

152

ALWC observations derived from nine-month size-resolved HGFs and PNSDs. The

153

former (shown in Fig. 1 and 2) were used to gain insight into the relationship between

154

the ALWC and the particle chemical composition, ambient RH, and PM2.5 mass

155

concentrations. The latter (displayed in Fig. 3) provided a picture of the seasonal and

156

diurnal variations in the ALWC.

157

3 Results and Discussion

158

Fig. 1 displays the time series of the PM2.5 chemical composition and the

159

meteorological parameters during two typical heavy haze episodes (Episode I and II).

160

The NOx and SO2 concentrations are displayed in Fig. S2. On 25 November 2015, a

161

clean air mass was carried over the study area by strong northerlies, resulting in a

162

relatively low PM2.5 mass concentration of 24.8 µg/m3. Over the following 6 days, the

163

wind speed decreased to 1 m/s, indicating the occurrence of stagnant conditions. Such

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stagnant weather conditions are typically associated with air masses that move slowly

165

from the south and southwest of Beijing and spend a greater amount of time over

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industrialized regions, thereby resulting in higher mass concentrations of particulate

167

matter43-44. The observed PM2.5 mass concentrations increased up to 480.4 µg/m3 in a

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step-wise fashion. The NOx and SO2 concentrations increased to 172.5 ppbv and 19.3

169

ppbv, respectively (refer to Fig. S2), indicating high gas precursor concentrations. The

170

heavy haze episode ended with the appearance of a strong northerly. The second haze



171

episode began on 3 December 2015 with very similar variations in the PM2.5 mass

172

concentration, chemical composition, and meteorological parameters.

173 174

Figure 1. Time series of the PM2.5 concentrations and chemical composition (left

175

y-axis) and of the ambient relative humidity (right y-axis). The wind speed is

176

represented by the size of the gray circles.

177

The measured PM2.5 chemical composition (Fig. 1) revealed that the SIA mass

178

concentration increased with an increase in the PM2.5 mass concentration and

179

accounted for approximately 61% and 55% of the PM2.5 on 1 and 9 December,

180

respectively, which represent the heaviest haze days. Markedly, the ambient RH

181

continued to rise during the heavy haze episodes. For example, the RH increased from

182

23% on 3 December 2015 to 68% on 9 December 2015 during Episode II. The ALWC

183

was 211.5 and 103.6 µg/m3 on 1 and 9 December, respectively, and accounted for 44%

184

and 48% of the PM2.5. These observations demonstrated that the RH, the inorganic

185

fraction of PM2.5, and the PM2.5 mass concentration increased simultaneously during

186

the heavy haze episodes and potentially brought about the elevated ALWC within the

187

PM2.5.

188

To confirm that the results observed during the individual air pollution episodes

189

are generally applicable to the atmosphere over Beijing, a three-year filter-based

190

dataset was employed to investigate the relationships among the ALWC, the inorganic

191

fraction of PM2.5, the PM2.5 mass concentration, and the ambient RH. The SIA mass


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fraction, PM2.5 mass concentration, and aerosol liquid water mass concentration during

193

the entire sampling period were grouped and averaged corresponding to an RH bin

194

width of 10%. The SIA fraction (circles) plotted as a function of the RH is displayed in

195

Fig. 2(a). The circles are colored according to the ALWC, and the sizes of the circles

196

are scaled to the PM2.5 mass concentration. The data displayed in Fig. 2(a) were

197

provided in the Table S2. As was the case for the individual pollution episodes (Fig. 1),

198

the long-term measurements revealed that both the dry PM2.5 mass concentration and

199

the SIA fraction increased with an elevated ambient RH. However, the PM2.5 mass

200

concentration decreased (as indicated by the circle sizes) as the RH exceeded 80%

201

because wet deposition, which typically occurred at higher RH conditions (>80%). On

202

average, as ambient RH increased from 15% to 83%, the SIA mass fraction in PM2.5

203

increased from 24% to 55%, and the ALWC increased from 2% to 74% (refer to

204

Table S2).

205

As displayed in Fig. 1(a), the SIA mass fraction was positively correlated with

206

the ambient RH. This is unexpected, because inorganic compounds are related to

207

anthropogenic activities and should not change with variations in the ambient RH

208

without considering global climate change caused by human activities. Therefore, a

209

driving force must exist for gaseous pollutants, such as SO2 and NOx, for the

210

transformation of particulate SIAs. We propose a scenario as follows. If a highly

211

concentrated aqueous solution of, e.g., 30% RH, absorbs more inorganic matter due to

212

N2O5 hydrolysis or SO2 uptake and oxidation reactions, the solution should be forced

213

to absorb more water to maintain an equilibrium with water vapor1, i.e., in this

214

example, to maintain an aqueous solution water activity of 0.3. A subsequent increase

215

in the RH would result in larger particles with greater water content and inorganic

216

fraction compared with a particle that does not take up inorganic matter. Any gas

217

would partition into an aqueous solution following Henry’s law. If a gas such as N2O5

218

is removed from the air and converted into an inorganic species which would remain

219

in a condensed phase, the species will be depleted from the gas and condensed phases.

220

However, Henry’s law is applicable, suggesting that the abovementioned depletion



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from the gas phase via uptake should balance the depletion in the condensed phase via

222

the aqueous phase chemistry. Alternatively, if this gas is constantly emitted or

223

supplied from the Anthropocene, it will constantly partition into the condensed phase

224

in preparation for a chemical reaction to occur, thus triggering the feedback

225

mechanism. This means that Henry’s law become a driving force to reach equilibrium

226

concentration, enhanced water uptake due to enhanced nitrate mass, but the

227

thermodynamic equilibrium with water vapor will ensure that the aqueous solution

228

water activity remains equal to the RH. To maintain thermodynamic equilibrium

229

while adjusting for reactive uptake, the particles must increase their water and

230

inorganic contents, thereby increasing the inorganic mass fraction. We should note

231

that the SOA might participate such feedback process: The enhanced aerosol liquid

232

water facilitate the aqueous phase SOA formation20-22, thereby, increased the SOA

233

mass. Considering the lower hygroscopicity of organic aerosols compared to SIA45-47,

234

the enhanced ALWC associated with aqueous SOA may be insignificant. While, the

235

SOA may influence the feedback process in other ways. For example, the gas uptake

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(e.g. of N2O5) might be reduced when particles are covered by organic films48-49, thus,

237

weaken the feedback loop. 1.0

1.0

(a) 3

20 10

= 99

]

0.6

0.6

0.4

0.4

0.2

0.2

0.0 0

20

SIA fraction > 0.6 0.5-0.6 0.4-0.5 0.3-0.4 0.2-0.3 < 0.2 All data points

0.8

ALWC

30

]

= 40

3

SIA Mass/PM2.5 Mass

40

PM2.5 [µg/m Water [µg/m

50

0.8

(b)

40 60 RH [%]

80

0.0 0

20

40 60 RH [%]

80

100

238 239

Figure 2. Panel (a): Inorganic fraction in PM2.5 (circles) as a function of the relative humidity. The

240

circles are colored according to the aerosol liquid water concentration, and the sizes of the circles are

241

scaled to the PM2.5 mass concentration. Panel (b): Aerosol liquid water content (ALWC) of condensed


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water with respect to the dry PM2.5 mass concentration. The ratios are grouped according to the

243

inorganic fraction in PM2.5.

244

Fig. 2 (b) displays the ALWC as a function of the ambient RH. The data points

245

are grouped according to the inorganic fraction in PM2.5. The data in different groups

246

were fitted by log(y)=a*x+b, as shown by the colored curves in Fig. 2 (b). The ALWC

247

generally increased with an increase in the ambient RH, and the ALWC associated

248

with higher inorganic fractions was much larger than that associated with lower

249

inorganic fractions at a constant RH. In other words, an increase in the ALWC for a

250

given RH increment was greater for particles with larger SIA mass fractions (refer to

251

Fig. 2 (b)). As discussed above, the polluted days were typically associated with a

252

higher inorganic fraction in PM2.5. In addition, our observations showed that the

253

contribution of nitrates to the SIA mass concentration increased significantly with an

254

increase in the PM2.5 mass concentration (Fig. S3). Nitrates accounted for 25% of the

255

SIAs for PM2.5 concentrations lower than 50 µg/m3 and 45% for PM2.5 concentrations

256

greater than 150 µg/m3. This means that nitrates become a dominant inorganic

257

component during heavy haze events. It is well known that ammonium nitrate has a

258

lower deliquescence RH (DRH=61.8%) than ammonium sulfate (DRH=79.9%)1.

259

Thus, at even lower RH levels, the dominant nitrate particles containing other soluble

260

components may absorb water and increase the ALWC28.

261

Fig. 3 shows the detailed monthly and diurnal variations in the ALWC of PM2.5.

262

During the observation period from May 2014 to January 2015, the highest ALWC

263

was observed in July in association with the highest ambient RH. The ALWC was

264

abundant during November 2014 and January 2015, during which haze episodes were

265

frequent. As expected, the diurnal patterns showed a higher ALWC during the

266

nighttime due to the higher ambient RH after sundown. The ALWC abundance at

267

night may influence the nighttime chemistry, including N2O5 hydrolysis15 and the

268

nitrate radical (NO3) oxidation of biogenic volatile organic compounds50. The

269

nighttime formation of nitrates may drive greater inorganic fractions and accordingly

270

greater ALWCs.


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20

100

16

80

12

60

8

40

3

24

ALWC (µ µg/m )

Hour (h)


20

4

0

0 May

271 272 273

July

Sep Month

Jan

Nov 2014

2015

Figure 3: Monthly and daily average trends of the aerosol liquid water content (ALWC) from May 2014 to January 2015.

274

The long-term observations in our study showed that the ALWC was well

275

correlated with the nitrate and sulfate mass concentrations (see Fig. S4), indicating

276

that both nitrate and sulfate salts play key roles in determining the ALWC. This

277

suggests that a reduction in both the sulfate and the nitrate fraction could efficiently

278

reduce the ALWC over the NCP. Differently, a substantial fraction of the particulate

279

ALWC over the Eastern U.S. could be attributed to anthropogenic sulfate

280

components19. The ALWC over the Italian Po Valley is driven by locally formed

281

anthropogenic nitrates17. Since 2007, the emission of anthropogenic SO2 throughout

282

China has declined by 75%51. This is not true for the emission of nitrogen oxides,

283

however. As a consequence, the nitrate concentration is expected to become more

284

important than that of sulfates. This implies that emission controls on nitrogen oxides

285

will constitute an efficient method for reducing the particle mass loading and ALWC

286

over the NCP in the future.

287

The elevated ALWC on haze days trends toward an increasing particle mass

288

concentration, thereby increasing the light scattering properties of submicron particles

289

and leading a degradation of the visibility4, 52. Most importantly, the enhanced ALWC

290

increases several trace gases (such as N2O5 and HO2) uptake coefficients by diluting

291

the absolute concentration of solute, regulating the acidity or acting as a reactant in

292

the bulk14, 53-54. Furthermore, the increased aerosol surface area due to hygroscopic


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aerosol growth leads to the increasing of heterogeneous reaction rate53. Previous

294

studies reported that a high ALWC could speed up the uptake coefficient of N2O515, 55,

295

thereby enhancing the formation of particulate nitrates. In addition, the ALWC could

296

serve as a reactor for the transformation of SO2 to sulfate in the aqueous phase16, 37, 56.

297

Furthermore, the ALWC could facilitate the formation of SOAs through the aqueous

298

phase chemistry and photochemistry12, 18, 22, 31. Thus, an increase in the ALWC during

299

pollution episodes over Beijing would significantly increase the reaction rates and

300

probabilities of adsorbed species in ambient aerosols and therefore increase the uptake

301

coefficients of many trace gas compounds. An elevation in the ALWC due to increased

302

ambient RH levels, water-soluble inorganic fractions, and aerosol mass loading may

303

lead to a positive feedback loop wherein aqueous particles readily uptake pollutants

304

that could react to form additional anthropogenic inorganic components; these

305

components could then uptake additional water, further accelerating the aerosol mass

306

accumulation over time periods of pollution57.

307

The NCP is China’s largest steel production and cement manufacturing region;

308

these industries are strongly dependent upon coal. Sulfur- and nitrogen-containing

309

gaseous precursors such as SO2 and NOx that are emitted from the combustion of

310

fossil fuels are transferred into hygroscopic particle constituents (i.e., sulfate and

311

nitrate salts) via atmospheric chemistry processes. As a result, particulate matter in the

312

atmosphere over the NCP carries an abundance of SIA components58, leading to high

313

SIA-derived ALWCs at elevated RH levels during haze episodes due to the triggered

314

positive feedback loop. Such a mechanism may also be an effective pathway for

315

producing the ALWCs in other environments, especially regions in Southeast Asia that

316

exhibit an increased fossil fuel consumption59-60. The SIAs associated with condensed

317

water particles may provide the relevant media through which anthropogenic SIAs may

318

modulate the formation and properties of biogenic SOAs. Previous field observations

319

have already provided evidence for the influence of the SIA-derived ALWC on the

320

formation of SOAs17, 19, 22. An increase in the water uptake ability due to SIAs may

321

affect climate change at a regional scale by altering cloud microphysics and optical

322

properties. We must note that changes in the particulate matter constituents that drive ACS Paragon Plus Environment


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the ALWC and its effects on the aerosol chemistry coincident with significant

324

reductions in global SO2 emissions, especially in China, should be reconsidered.

325 326

Notes

327

The authors declare no competing financial interest.

328

Acknowledgement

329

This work is supported by the following projects: National Natural Science

330

Foundation of China (41475127, 41571130021) and National Key R&D Program of

331

China (2016YFC0202800: Task 1). The authors would like to greatly thank Min Hu,

332

Yuxuan Bian, Ying Chen, and Hongyu Guo for useful discussion and thank three

333

anonymous reviewers for their hints and suggestions.

334

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