Light absorption, fluorescence properties and sources of brown carbon aerosols in the Southeast Tibetan Plateau

Journal Pre-proof Light absorption, fluorescence properties and sources of brown carbon aerosols in the Southeast Tibetan Plateau Guangming Wu, Xin Wan, Kirpa Ram, Peilin Li, Bin Liu, Yongguang Yin, Pingqing Fu, Mark Loewen, Shaopeng Gao, Shichang Kang, Kimitaka Kawamura, Yongjie Wang, Zhiyuan Cong PII:

S0269-7491(19)35431-4

DOI:

https://doi.org/10.1016/j.envpol.2019.113616

Reference:

ENPO 113616

To appear in:

Environmental Pollution

Received Date: 23 September 2019 Revised Date:

10 November 2019

Accepted Date: 11 November 2019

Please cite this article as: Wu, G., Wan, X., Ram, K., Li, P., Liu, B., Yin, Y., Fu, P., Loewen, M., Gao, S., Kang, S., Kawamura, K., Wang, Y., Cong, Z., Light absorption, fluorescence properties and sources of brown carbon aerosols in the Southeast Tibetan Plateau, Environmental Pollution (2019), doi: https:// doi.org/10.1016/j.envpol.2019.113616. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Graphical Abstract

1

Light absorption, fluorescence properties and sources of

2

brown carbon aerosols in the Southeast Tibetan Plateau

3

Guangming Wu1,9, Xin Wan1,9, Kirpa Ram2, Peilin Li1,9, Bin Liu3, Yongguang Yin4, Pingqing

4

Fu5, Mark Loewen1, Shaopeng Gao1, Shichang Kang6,8, Kimitaka Kawamura7, Yongjie

5

Wang1, Zhiyuan Cong1,8*

：

1

Key Laboratory of Tibetan Environment Changes and Land Surface Processes,

7

Institute of Tibetan Plateau Research, Chinese Academy of Sciences (CAS),

8

Beijing 100101, China

9

2

10 11

Institute of Environment and Sustainable Development, Banaras Hindu University, Varanasi 221005, India

3

Chongqing Jinfo Mountain Field Scientific Observation and Research Station for

12

Kaster Ecosystem, School of Geographical Sciences, Southwest University,

13

Chongqing 400715, China

14

4

15 1：

Center for Eco-Environmental Sciences, CAS, Beijing 100085, China 5

17 18

6

State Key Laboratory of Cryospheric Sciences, Northwest Institute of Eco-Environment and Resources, CAS, Lanzhou 730000, China

7

21 22

Institute of Surface-Earth System Science, Tianjin University, Tianjin 300072, China

19 20

State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research

Chubu Institute for Advanced Studies, Chubu University, Kasugai 487-8501, Japan

8

23

Center for Excellence in Tibetan Plateau Earth Sciences, CAS, Beijing 100101, China

24

9

25

Corresponding Author

2： 27 28

University of Chinese Academy of Sciences, Beijing 100049, China

E-mail: [email protected] Address: Building 3, Courtyard 16, Lincui Road, Chaoyang District, Beijing, China. Tel.: +86 010-8424 9408 ＊

29

Abstract

30

Brown carbon (BrC) has been proposed as an important driving factor in climate

31

change due to its light absorption properties. However, our understanding of BrC’s

32

chemical and optical properties are inadequate, particularly at remote regions. This

33

study conducts a comprehensive investigation of BrC aerosols in summer (Aug. 2013)

34

and winter (Jan. 2014) at Southeast Tibetan Plateau, which is ecologically fragile and

35

sensitive to global warming. The concentrations of methanol-soluble BrC (MeS-BrC)

3：

are approximately twice of water-soluble BrC (WS-BrC), demonstrating the

37

environmental importance of water-insoluble BrC are previously underestimated with

38

only WS-BrC considered. The mass absorption efficiency of WS-BrC (0.27- 0.86 m2

39

g-1) is lower than those in heavily polluted South Asia, indicating a distinct contrast

40

between the two sides of Himalayas. Fluorescence reveals that the absorption of BrC

41

is mainly attributed to humic-like and protein-like substances, which broaden the

42

current knowledge of BrC’s chromophores. Combining organic tracer, satellite

43

MODIS data and air-mass backward trajectory analysis, this study finds BrC is

44

mainly derived from bioaerosols and secondary formation in summer, while

45

long-range transport of biomass burning emissions in winter. Our study provides new

4：

insights into the optical and chemical properties of BrC, which may have implications

47

for environmental effect and sources of organic aerosols.

48

Capsule

49

from bioaerosols in summer, while biomass burning emissions in winter.

Optical properties and organic tracers demonstrate that BrC are derived

50 51

1. Introduction

The Tibetan Plateau (TP), with an average altitude of more than 4000 m a.s.l.

52

(above sea level) and area about 2.5

53

geographical area of the Third Pole (Yao et al., 2012). It plays a crucial role in

54

regional and global climate, atmospheric and hydrological circulation as well as

55

ecosystem stability (Qiu, 2008; Yao et al., 2012). With sparse inhabitants and

5：

relatively less emission sources, this region represents relatively cleaner environment.

57

However, recent researches have revealed that the TP has been experiencing more

58

rapid and dramatic warming than other regions (Liu and Chen, 2000; Qin et al., 2009).

59

Along with driving factors like greenhouse gas (e.g., CO2), atmospheric aerosol also

：0

has been considered as an important contributor for such elevation-dependent

：1

warming (Pepin et al., 2015). Atmospheric aerosols could directly heat the ambient

：2

atmosphere by absorbing the solar irradiation and reduce snow albedo due to

：3

deposition on glaciers and thus, warm the surface (Kaspari et al., 2015; Qian et al.,

：4

2015).

106 km2, is recognized as an important

：5

Traditionally, black carbon is considered as the main light absorber in

：：

atmospheric aerosols over the spectrum ranging from ultraviolet to infrared (Bond and

：7

Bergstrom, 2006; Yuan et al., 2019). Nevertheless, recent studies have revealed that a

：8

portion of organic carbon (OC) with brownish to yellowish color also shows light

：9

absorption properties. These OC exhibit strong wavelength dependent (λ-2 - λ-9) and

70

are defined as brown carbon (BrC) (Andreae and Gelencsér, 2006; Laskin et al., 2015).

71

Global modeling studies have demonstrated that BrC, at 550 nm, could contributes to

72

nearly 20% of total absorption caused by carbonaceous aerosols (Chung et al., 2012).

73

Like other aerosols, BrC also can be transported into glacier by wet and/or dry

74

depositions and then accelerating glacier melting (Wu et al., 2016).

75

Carbonaceous aerosols are characterized by high OC to EC (elemental carbon)

7：

ratios over the TP, such as in the central (23.4) (Wan et al., 2015), southern edge (1.91

77

– 43.8) (Cong et al., 2015) and southeast TP (17.67 ± 11.05) (Zhao et al., 2013),

78

indicating that OC may has considerable influence on air quality and climate change.

79

Recent study has pointed out that humic-like substance (HULIS) is main chemical

80

component of BrC over the central TP and can account for approximately 40 - 60% of

81

water-soluble OC (WSOC) and 12 - 31% of OC mass concentrations (Wu et al., 2018),

82

further underlining the importance of atmospheric BrC over the TP. Although there

83

are some progresses concerning BrC aerosols over the TP (Li et al., 2016; Zhang et al.,

84

2017), most of them were focused on water-soluble BrC (WS-BrC). The

85

water-insoluble BrC, which has light absorption capability about 3 times that of

8：

WS-BrC (Chen and Bond, 2010; Liu et al., 2013; Zhang et al., 2013), is less studied

87

yet. Moreover, the source and chemical component of BrC are crucial for

88

understanding the transport and optical property of BrC in the atmosphere, which are

89

also poorly characterized over the TP (Wu et al., 2018; Zhang et al., 2017). These

90

inadequate studies constitute a major uncertainty in the assessment of environmental

91

effect of atmospheric aerosols.

92

In this study, the southeast TP was chosen as the representative site of alpine

93

forest environment over the TP to study the BrC aerosols. Our current study focuses

94

on the temporal variations of light absorption properties of both WS-BrC and

95

methanol-soluble BrC (MeS-BrC). Moreover, fluorescence technique, combined with

9：

PARAFAC (parallel factor) model, is used to investigate the chemical components in

97

BrC. The possible sources of BrC are also discussed based on various organic tracers

98

such as β caryophyllinic acid, levoglucosan, and arabitol. The knowledge on light

99

absorption properties, chemical characteristics and sources of BrC is expected to be

100

helpful to understand the environmental effects of aerosol over the high-altitude

101

regions.

102

2. Sampling site and data analysis

103

2.1 Research site and aerosol sampling

104

The samples were collected at South-East Tibetan plateau Station for integrated

105

observation and research of alpine environment (SETS, 94°44′E, 29°46′N, 3326 m

10：

a.s.l., Figure S1), located in the southeast TP. Attributed to low population density,

107

local emissions from human activities around SETS is minimal (Liu et al., 2017; Zhao

108

et al., 2013). In summer, humid air masses originating from the Indian Ocean can

109

reach this site through the Yarlung Tsangpo Grand Canyon, which deliver abundant

110

precipitation (800-1000 mm). The mean values for air temperature, relative humidity,

111

wind speed and radiative forcing are 13.4 ± 0.8

112

461.1 ± 115.9 W m-2 (only daytime included), respectively, according to an automatic

113

weather station. The warm and wet environment conditions feed an extensive alpine

114

forest in the southeast TP. The main vegetation species are Alpine pine, spruce and fir,

115

which account for 72.9% of total live biomass of natural vegetation over the TP (Luo

11：

et al., 2002). In winter season, this region is mainly under the influence of south

117

branch westerly winds with wind speed and air temperature of 1.9 ± 0.6 m s-1 and -0.2

118

± 4.9

119

diverse of geographical environments are formed over the southeast TP, including

120

large area of glacier, which is sensitive to the radiative forcing deduced by

121

light-absorbing aerosols (e.g., BrC) (Wang et al., 2015). Therefore, SETS is a unique

122

site to investigate the complex interactions between atmosphere, biosphere and

123

hydrosphere.

, 74.5 ± 3.9%, 0.9 ± 0.1 m s-1 and

, respectively. By the active interaction between monsoon and westerlies,

124

In August (15th - 30th) of 2013 and January (1st - 17th) of 2014, total suspended

125

particles (TSP) were collected with a high-volume air sampler (Qingdao Laoshan Co.,

12：

China). All the samples were collected on 20.3 × 25.4 cm2 quartz filters (Whatman,

127

UK), prebaked at 450

128

standard condition (i.e., 0.75 m3 min-1) automatically. For each daytime, the sampling

129

started around 8:00 am (Beijing time) and ended at 7:30 pm, which was followed by

130

the collection of nighttime samples from 8:00 pm to 7:30 am (next day). Each

131

sampling duration time was approximately 11.5 h. The field blank samples were

132

collected about 5 minutes without air flowing through the filter. After the collection,

133

all samples were sealed in pre-combusted glass bottles and kept frozen until analysis.

134

A total of sixty samples were collected with daytime and nighttime samples each

135

amounting to half.

13：

2.2 Analysis of water-/methanol- soluble BrC

for 6 h. The flow rate during the sampling was converted to

137

For the measurement of water-soluble BrC, an aliquot filter about 20 cm2 was

138

extracted with 20 ml pure water (Millipore, USA) under ultrasonication in a brown

139

glass bottle. Then the extracts were filtered by 0.45 µm PTFE filter (Chromafil

140

Xtra-45/25, Macherey-Nagel, Duren, Germany) and divided into two portions. One

141

portion was analyzed for its concentration (represented by water-soluble organic

142

carbon, WSOC) by a total carbon analyzer (TOC-L, Shimadzu, Japan) with detection

143

limit of 4 µg L-1 and standard deviation less than 3%. The other portion was prepared

144

for the light absorption measurement. The parallel experiment, by cutting samples at

145

different regions on filter, shows the aerosol particles are equally deposited on filter

14：

with deviation less than 1.3%.

147

For the methanol extraction, a punch of sample filter (2.54 cm2) was soaked in 5

148

ml HPLC grade methanol (Merck KGaA, Darmstadt, Germany) for two hours without

149

sonication following the method developed by Cheng et al. (2016). The methanol

150

extracts were filtered and ready for optical measurement. The residual filters were

151

further dried for another 2 hours in a 40

152

get prepared for methanol-soluble OC (MeSOC) quantifying (see section 2.4). It

153

should be noted that certain WSOC is also included in the MeSOC, because WSOC is

154

extractable by methanol.

oven to let the methanol evaporated and

155

The light absorption properties of water and methanol extracts were measured by

15：

a UV-Vis Spectrophotometer (UV-3600, Shimadzu, Japan) over the wavelength

157

ranging from 200 to 800 nm in 1 nm stepwise. During the measurement, the solution

158

was kept in a quartz cuvette with optical path-length of 1 cm (l). The light absorption

159

capability of BrC is estimated by mass absorption efficiency (MAE), which was

1：0

derived by normalizing the absorption coefficient (Abs) against its corresponding

1：1

concentration (C, µgC m-3). Absorption Ångström exponent (AAE) refers to the

1：2

wavelength dependence of absorption capability and was derived by fitting the Abs

1：3

values between wavelengths 300 to 400 nm (Hecobian et al., 2010; Wu et al., 2018):

1：4


  = 
−
 × 



 × 

 !"#$

1：5

MAE   = % &'% "#(

1：：

AAE =

)*   $ ⁄  +

)*  + / $

× ln 10 (1) (2) (3)

1：7

Here Aλ refers to the light absorbance at wavelength (λ). The Abs is reported in Mm-1

1：8

where 1 Mm-1 = 10-6 m-1. The Vs and Va is the volume of extraction solvent and the

1：9

air passed through the filter during sampling, respectively. Duplicate measurements

170

show the standard deviation for absorbance is no more than 0.1%.

171

The fluorescence properties (i.e., excitation-emission matrix, EEM) of WS-BrC

172

and MeS-BrC were obtained by Horiba Fluoromax-4 fluorometer in Sc/Rc

173

(signal/reference after correction) model with the excitation wavelength ranging from

174

240 to 455 nm in 5 nm intervals, and the emission wavelength ranging from 290 to

175

550 nm in 2 nm intervals (Fu et al., 2015; Wu et al., 2019). Considering the high

17：

volatility of methanol, the quartz cuvette was sealed with a cap during the

177

measurement to minimize the possible influence from methanol evaporation. After the

178

measurement, a series of calibrations were conducted to ensure the accuracy of the

179

data, which include instrumental bias calibration, inner filter correction, Raman

180

normalization and blank subtraction (Supporting Information) (Murphy et al., 2010).

181

Then the above derived original data (Fori) was further normalized by the volume of

182

extraction solvent (Vs, ml) and the air passed through the filter during sampling (Va,

183

m3) as well as the organic carbon concentrations (C, µg m-3) to account for the

184

different chromophore abundances (Wu et al., 2019):

185

./ = .012 × 



 ×  × %

(4)

18：

The finally derived fluorescence data (Ff) were reported in the unit of RU﹒m2 gC-1

187

(RU: Raman Unit). Furthermore, the fluorescent components in BrC were identified

188

by PARAFAC model with non-negativity constraints, which was validated by

189

split-half and squared error analysis (Murphy et al., 2013). The relative standard

190

deviation for this analysis was below 2%.

191

2.3 Extraction and measurement of HULIS and organic tracers

192

The HULIS is important component of water-soluble BrC and was isolated by

193

solid-phase extraction (SPE) protocol as reported in Wu et al. (2018). Briefly, HULIS

194

was efficiently absorbed on Oasis HLB cartridge (30 µm, 60 mg/cartridge, Waters,

195

USA) at pH = 2, while other organic and inorganic (e.g., ion species) compounds

19：

were passed through the column into effluent. After the HLB cartridge was dried in

197

pure nitrogen, methanol was introduced to elute the HULIS into prebaked glass bottle.

198

Then the eluent was evaporated in pure nitrogen at room temperature to remove

199

methanol. Finally, HULIS was redissolved into ultrapure water for subsequent

200

analysis of concentration and light absorption, following the same methods for WSOC

201

analysis. The extraction efficiency of HULIS was better than 85% (Wu et al., 2018).

202

The

organic

tracers,

including

β caryophyllinic

acid,

cis-pinic

acid,

203

levoglucosan, arabitol and glucose, were extracted with dichloromethane/methanol

204

(v/v = 2:1) solutions under ultrasonication. Then they were measured by a gas

205

chromatography-mass spectrometry (GS-MS) following the protocol reported in Wan

20：

et al. (2017). The reproducibility, estimated based on the repeat experiments, was less

207

than 15 %.

208

2.4 Determination of OC and EC

209

A portion of each sample (0.5 cm2) was analyzed for OC and EC concentrations

210

by a carbon analyzer (DRI model 2001) based on the IMPROVE-A thermal/optical

211

reflectance protocol (Chow et al., 2007). Briefly, OC concentration was measured in

212

pure helium condition as the temperature increased in stepwise to 580

213

EC was analyzed after being oxidized in He/O2 (v/v=98 /2) environment at 580, 740

214

and 840

215

of OC and EC. The detection limit for OC, EC and TC were 0.05 µgC cm-2. The

21：

residual filters after methanol extraction were also measured by the same carbon

217

analyzer described above to get the methanol extracted OC concentrations, which

218

were derived by subtracting the TC content before and after the methanol extraction

219

(Cheng et al., 2016; Li et al., 2019).

220

2.5 Radiative effects of BrC relative to black carbon

. While the

, respectively (Cong et al., 2015). The TC content was defined as the sum

221

The direct solar absorption of BrC relative to black carbon (fBrC/BC) was

222

estimated by the method documented in Kirillova et al. (2014), with the specific input

223

parameters listed in Table 1 and S1. Briefly, the light absorption by species X (i.e.,

224

BrC or BC) is following the Lambert-Beer law:

225 22：

34 3 34

:4 <<= ; ∙%; ∙?<@A

5, 7 = 1 − 8 !9:4 ,;  :

(5)

Where Cx represents mass concentrations of BrC or BC in the atmosphere and hABL

227

corresponds to the height of atmospheric boundary layer (1000 m). The MAE

228

MAE value of BrC or BC calculated at wavelength 365 nm or 632 nm, respectively

229

(The calculation protocol for MAEBC is presented in Supporting Information). The I0

230

is the clear sky Air Mass 1 Global Horizontal solar irradiance (Levinson et al., 2010).

231

Then the solar irradiance absorbed by BrC was normalized against that of BC:

232

BC1%/C% =

E #E D 34  ∙ 4E  ,C1% F 4

E #E D 34  ∙ 4  ,C% F

0,x

is

(6)

E4

233

3. Results and discussions

234

3.1 Abundances and variations of carbonaceous compounds

235

The average abundances of various carbonaceous compounds at SETS are

23：

summarized in Figure 1 and Table S1. The concentration of OC is 3.73 ± 0.89 µgC

237

m-3 and 3.79 ± 1.93 µgC m-3 in summer and winter, respectively and exhibits no

238

seasonal variation. In contrast, EC shows relatively higher abundance in winter (0.96

239

± 0.63 µgC m-3) by nearly 3 times than in summer (0.36 ± 0.23 µgC m-3). Compared

240

to previous study in central TP (Wan et al., 2015), OC concentration at SETS is

241

comparable, while the EC is almost 2-3 times higher. The OC and EC abundances at

242

SETS are also higher than those reported in the southern TP, e.g., Mt. Everest (Cong

243

et al., 2015), NCO-P (Decesari et al., 2010) (Table S2). However, they are lower than

244

those from Manora Peak (Ram et al., 2010) and Kharagpur of India (Srinivas and

245

Sarin, 2014) (Table S2), both of which are most likely impacted by local

24：

anthropogenic emissions. Therefore, although SETS is susceptible to air pollutants

247

than other regions over the TP, its air quality is still relatively pristine compared to

248

South Asia. This finding is consistent with a recent study by Liu et al. (2017) based on

249

integrated usage of satellite and ground-based remote sensing. For carbonaceous

250

aerosols at SETS, OC is a predominant component with OC/EC ratio of 14.1 ± 8.2

251

and 4.57 ± 2.18 in summer and winter, respectively (Figure 1 and Table S1), pointing

252

out the importance of organic aerosols over the TP.

253

The concentrations of WSOC are 1.89 ± 0.57 µgC m-3 and 1.44 ± 0.42 µgC m-3

254

in summer daytime and nighttime, and 1.97 ± 0.68 µgC m-3 and 1.42 ± 0.49 µgC m-3

255

in winter daytime and nighttime, respectively (Figure 1 and Table S1). Similar to OC,

25：

no seasonal variations are found for WSOC abundances, which is expected as WSOC

257

being an important component of OC. However, WSOC/OC ratios (0.45 ± 0.12 in

258

summer and 0.51 ± 0.18 in winter) at SETS are lower than those from other

259

background sites from the northeast TP (0.79, Qilian Shan, 4180ma.s.l.) (Xu et al.,

2：0

2015) and south slope of the TP (0.46 – 0.95, NCO-P, 5079 m a.s.l.) (Decesari et al.,

2：1

2010) as well as Summit, Greenland (0.81 ± 0.2, 3200 m a.s.l.) (Hagler et al., 2007)

2：2

and Alps (54.8% - 75.9%) (Jaffrezo et al., 2005), implying a large fraction of OC is

2：3

water-insoluble at SETS.

2：4

Methanol has a better extraction efficiency for water-insoluble organic aerosols

2：5

than commonly used other organic solvents (e.g., hexane and acetone) (Chen and

2：：

Bond, 2010), and has been frequently used to investigate the chemical characteristics

2：7

and optical properties of water-insoluble BrC in different environments, such as

2：8

high-altitude Tibetan Plateau (Kirillova et al., 2016; Zhu et al., 2018) and urban

2：9

environments (Sarkar et al., 2019; Yan et al., 2017). In this study, the abundance of

270

methanol-soluble organic carbon (i.e., MeSOC) is 3.03 ± 0.93 µgC m-3 and 3.61 ±

271

2.07 µgC m-3 in summer and winter, respectively. The ratio of MeSOC/WSOC is 1.74

272

± 0.42 and 2.02 ± 0.72 in summer and winter, respectively (Table S1), further

273

demonstrating the importance of water-insoluble organic aerosols. In addition,

274

MeSOC/OC ratio is 0.77 ± 0.19 in summer and 0.91 ± 0.20 in winter, suggesting that

275

most of OC can be extracted by methanol. Thus, MeSOC may provides more

27：

comprehensive description of BrC’s optical and chemical characterizations compared

277

to WSOC.

278

3.2 Light absorption properties of BrC

279

3.2.1 Mass absorption efficiency (MAE)

280

The optical parameter of MAE provides a quantitative description for the light

281

absorption capability of BrC and is a key parameter used in model to evaluate the

282

climate effect of BrC (Laskin et al., 2015). The statistical summaries of MAE values

283

of BrC at SETS are listed in Table 1. The average MAE value of WS-BrC (i.e.,

284

MAEWS-BrC) in winter (0.86 ± 0.17 m2 g-1) is approximately 3 times of that in summer

285

(0.27 ± 0.10 m2 g-1), demonstrating that WS-BrC has stronger light absorption

28：

capability in winter than in summer. A previous study at the central TP has highlighted

287

that the optical properties of BrC are strongly related with their sources, namely,

288

biomass burning emitted BrC has stronger light absorption capability than secondary

289

BrC formed in the atmosphere (Wu et al., 2018). Therefore, the different MAEWS-BrC

290

values between summer and winter essentially imply a seasonal variation of BrC’s

291

sources. The MAEWS-BrC values at SETS are comparable to those from the central TP

292

(Nam Co, winter: 0.64 ± 0.40 m2 g-1, summer: 0.25 ± 0.12 m2 g-1) (Wu et al., 2018)

293

and Himalayas (NCO-P, 0.42 – 0.61 m2 g-1) (Kirillova et al., 2016) as well as South

294

Asian outflow (MCOH, 0.5 ± 0.2 m2 g-1) (Bosch et al., 2014), which are relatively far

295

away from densely populated regions. However, they are much lower than those from

29：

more polluted sites, such as, Northwest China (Xi’an, 5.38 m2 g-1 at 340 nm)(Shen et

297

al., 2017), Delhi of India (1.1 - 2.7 m2 g-1) (Kirillova et al., 2014) and the central

298

Indo-Gangetic Plain (Kanpur, 1.16 ± 0.60 m2 g-1) (Satish et al., 2017), which

299

highlights that anthropogenic emissions may have considerable impacts on the light

300

absorption properties of BrC aerosols.

301

The MAE value for methanol-soluble BrC (i.e., MAEMeS-BrC) is 0.34 ± 0.12 m2

302

g-1 in summer and 0.59 ± 0.44 m2 g-1 in winter. The light absorption properties of

303

methanol-extracted BrC show a similar seasonal variation with those of

304

water-extracted BrC, i.e., the MAE values in winter are approximately 2 to 3 times of

305

those in summer (Table 1). Therefore, these observations suggest that both MeS-BrC

30：

and water-soluble BrC show stronger solar absorption capabilities in winter than

307

summer. Comparing the MAE values between methanol- and water-soluble BrC, the

308

MAEMeS-BrC values are higher than those of MAEWS-BrC in summer. In contrast, the

309

situation is reverse in winter, i.e., the MAEMeS-BrC values are lower than those of

310

water-soluble BrC. This demonstrates that more water-insoluble BrC with stronger

311

light absorption capabilities is extracted by methanol in summer, while the

312

water-insoluble BrC in winter is characterized with lower light absorption capability.

313

The discrepancy in the optical properties of MeS-BrC between different seasons

314

should be carefully considered in the estimation of BrC’s climate effect in future

315

studies.

31：

3.2.2 Absorption Ångström exponent (AAE)

317

The AAE reflects the wavelength dependence of BrC’s absorption capability, i.e.,

318

higher AAE value demonstrates that light absorption capability increases more

319

sharply from longer to shorter wavelengths. The AAEWS-BrC, calculated between 300

320

and 400 nm, is relatively higher in summer (7.22 ± 1.45) than winter (5.92 ± 0.43)

321

without obvious diurnal variations (Table 1). According to previous studies, the

322

AAEWS-BrC value lies between 7.37 - 9.28 in the central TP (Nam Co) (Wu et al.,

323

2018), 3.1 - 9.3 in South Asia (New Delhi) (Kirillova et al., 2014), 4.1 - 6.4 in North

324

American (Liu et al., 2014), and 2.23 - 9.48 in central Europe (Utry et al., 2013). All

325

the aforementioned AAEWS-BrC values are 2 to 9- fold higher than that of BC (i.e.,

32：

AAE of BC ~1) (Bond and Bergstrom, 2006). This confirms that it is a common

327

characteristic of BrC to exhibits strong wavelength dependence in light absorption

328

capabilities. BrC aerosols tend to absorb more solar irradiation over the ultraviolet

329

wavelength, which may have considerable impact on various photochemical reactions

330

in the atmosphere (Jo et al., 2016; Laskin et al., 2015; Yan et al., 2018). For example,

331

Jo et al. (2016) found BrC could decrease the annual mean NO2 photolysis rate by 8%

332

and the O3 concentration by 6% in surface air over Asia, where BrC aerosols are

333

abundant.

334

Moreover, the averaged AAE value for methanol-extracted BrC (AAEMeS-BrC) is 5.53

335

± 1.04 in summer and 6.24 ± 1.24 in winter (Table 1). Compared to water-soluble BrC,

33：

AAEMeS-BrC is slightly lower in summer but comparable in winter. This indicates

337

methanol extracts more organic matters, which have stronger absorption capabilities

338

at longer wavelength, from the summer aerosols. Therefore, the BrC aerosols from

339

summer season may play more important role in solar absorption at SETS, because

340

most solar energy occurs at longer wavelength than ultraviolet wavelength (Levinson

341

et al., 2010).

342

3.2.3 Radiative effects

343

Over the solar spectrum ranging from 300 to 700 nm, the fBrC/BC value of

344

WS-BrC is minor, both in summer (4.2 ± 1.9%) and winter (3.4 ± 1.1%) (Table S3).

345

These values are comparable to that at the high Himalayas (4 ± 1%, NCO-P, Nepal)

34：

(Kirillova et al., 2016) but lower than that from South Asia such as 8.78 ± 3.74% at

347

Godavari (Wu et al., 2019) and 40% in the Indo-Gangetic Plain (Srinivas et al., 2016).

348

However, due to the strong wavelength dependence, the fBrC/BC value of WS-BrC

349

increases to 17.6 ± 7.9% in summer and 12.3 ± 4.2% in winter, over the wavelength

350

range of 300 - 400 nm (Table S3). For MeS-BrC, its fBrC/BC values are approximately 1

351

to 2.5 folds larger than WS-BrC, which are attributed to more abundant or stronger

352

light absorption capability of MeS-BrC as discussed in section 3.1 and 3.2.1. Similar

353

to WS-BrC, the absorption contribution of MeS-BrC also increases significantly to

354

32.5 ± 18.4% in summer and 14.7 ± 6.9% in winter over the ultraviolet wavelengths

355

(i.e., 300 - 400 nm).

35：

3.3 Fluorescence properties of BrC

357

Fluorescence technique is based on the absorption and then re-emission of light

358

by fluorophores. Generally, each fluorophore has its specific excitation/emission peak

359

in the excitation-emission matrix, which can act as fingerprint to identify the chemical

3：0

components in atmosphere (Coble, 2007). Recent studies have found that BrC shares

3：1

similar molecular conformation with some fluorophores (Laskin et al., 2015), and thus

3：2

fluorescence technique can be used as a powerful tool to quickly identify the chemical

3：3

components in BrC aerosols (Chen et al., 2016b; Wu et al., 2019; Yan and Kim, 2017).

3：4

In this study, three fluorescence components were identified in WS-BrC from summer

3：5

samples while four from winter samples with explained variance (core consistency) of

3：：

99.4% (81.5%) and 99.6% (47.9%), respectively (Figure 2). The first two components

3：7

(i.e., C1 and C2) belong to HULIS. The component C1 is most likely characterized by

3：8

terrestrially-derived humic acid (Coble, 2007) with similar peak locations ((Ex/Em

3：9

(Excitation/Emission) = ~250/384 - 412 nm) for both summer and winter samples. C2

370

has obvious blue shift in emission wavelength in summer (Ex/Em = ~250/450 – 470

371

nm) relative to winter (Ex/Em = ~250/490 – 510 nm). The HULIS (i.e., C1 and C2)

372

contributes to 67.7 ± 8.2% (summer) and 47.5 ± 11.4% (winter) of total fluorescence

373

intensities (Figure 3), which are lower than that documented at the south slope of

374

Himalayas (i.e., 80.2 ± 4.1%, Godavari, Nepal) (Wu et al., 2019). Meanwhile, the

375

mass concentration of HULIS is 0.83 ± 0.36 µgC m-3 and 0.82 ± 0.33 µgC m-3 in

37：

summer and winter seasons, respectively (Figure 1 and Table S1), which accounts for

377

approximately half of the corresponding WSOC concentrations.

378

Besides HULIS, two other fluorescent components (i.e., C3 and C4) are also

379

identified in SETS aerosols (Figure 2), which refer to protein-like substance (PRLIS)

380

(Coble, 2007). These PRLISs contribute to 32.3 ± 7.4% and 52.5 ± 8.3% of the total

381

fluorescence intensities in summer and winter, respectively, demonstrating that PRLIS

382

is also important component of BrC. Given the fact that HULIS has comparable light

383

absorption capability (i.e., MAEHULIS) to WS-BrC, while its concentration is only half

384

of WSOC (Table 1 and Figure 1), indicates that PRLIS and other unknown

385

compounds may be responsible for near half the absorption of WS-BrC (Figure 3).

38：

As discussed in section 3.1, the abundances of methanol-soluble organic carbon

387

are almost twice of WSOC, which underline the importance of water-insoluble

388

organic aerosols. Therefore, the fluorescence properties of methanol-soluble BrC were

389

also analyzed in this study. By the PARAFAC analysis, three kinds of fluorescent

390

components are identified in MeS-BrC in both summer and winter aerosols with

391

explained variance (core consistency) of 98.6% (91.0%) and 99.3% (86.1%),

392

respectively. They include one HULIS (i.e., C2) and two PRLISs (i.e., C3, C5)

393

(Figure 4) with their corresponding peak locations in EEM presented in Table S4.

394

Comparing the fluorescent components identified in WS-BrC and MeS-BrC,

395

humic-like substance (i.e., C1) in WS-BrC is not found in MeS-BrC. Meanwhile,

39：

more protein-like substances are identified in MeS-BrC with the contributions to total

397

fluorescence intensities of 76.4 ± 23.8% in summer and 84.2 ± 5.9% in winter, which

398

are approximately 2 and 1.5 times larger than those in WS-BrC. As such, the

399

protein-like substances appear to be more important than HULIS in the

400

methanol-soluble BrC. Recent studies have found that methanol extracts contain

401

abundant amines, proteinaceous materials and organic nitrates (Chen et al., 2016a;

402

Duarte et al., 2004), which are important components of BrC (Huang et al., 2018;

403

Song et al., 2018; Wang et al., 2017; Xie et al., 2019). Therefore, the increased

404

protein-like substances may be responsible for the stronger light absorption capability

405

of methanol-extracted BrC than water-soluble BrC in summer (Table 1), which are

40：

possibly derived from vegetation emissions, such as bioaerosols. More detailed

407

discussions are presented in the following section.

408

3.4 Source identification of BrC

409

The light absorption coefficient (Abs) at wavelength 365 nm is directly related

410

with the light absorption capability and abundance of BrC in extraction, which is

411

widely used as proxy of BrC (Chen et al., 2018; Du et al., 2014; Hecobian et al.,

412

2010). Meanwhile, several organic compounds are derived from specific emission

413

sources, and can be used as tracers to investigate the formation of atmospheric

414

aerosols. For example, cis-pinic and β-caryophyllinic acid are the well-recognized

415

biogenic volatile organic carbon (BVOC) tracers (Stone et al., 2012). Cis-pinic acid is

41：

the oxidation product of α-pinene by ozone (Christoffersen et al., 1998). The

417

β-caryophyllinic acid is a tracer for β-caryophyllene (sesquiterpene released from

418

plant foliage) (Jaoui et al., 2007). Glucose and arabitol are mainly related with

419

primary emissions from plant debris and fungal spores, respectively (Wan et al., 2019).

420

Levoglucosan is a unique indicator for biomass burning emission (Bhattarai et al.,

421

2019; Simoneit et al., 1999).

422

In summer season, both OC (r2= 0.05, P=0.14, Figure 5a) and WSOC (r2= 0.0,

423

P=0.30, Figure 5b) exhibit no liner relationships with EC, indicating less influence

424

from combustion emissions. However, the Abs value of WS-BrC at 365 nm

425

(AbsWS-BrC) demonstrates good linear relationships with the above-mentioned BVOC

42：

tracers, i.e., β-caryophyllinic acid (r2=0.58, P<0.01, Figure 6a) and cis-pinic acid

427

(r2=0.63, P<0.01, Figure 6b), which indicates that secondary formations contribute

428

considerably to WS-BrC aerosols at SETS. Furthermore, the AbsWS-BrC is relatively

429

well correlated with β-caryophyllinic acid (r2=0.74, P<0.01, Figure S2a) and cis-pinic

430

acid (r2=0.74, P<0.01, Figure S2c) in summer daytime, but no linear relationships in

431

nighttime (Figure S2b, d). This finding further supports that WS-BrC is formed from

432

secondary reactions of BVOC, which are more efficient in daytime with stronger solar

433

irradiation and higher temperature (Hansen and Seufert, 2003). These precursors for

434

BrC’s secondary formations may be released from local and surrounding forest. Wang

435

et al. (2007) have found that the southeast TP is one of the strongest BVOC (e.g.,

43：

monoterpene and isoprene) emission regions in China with the emission intensity

437

more than 1 mgC m-2 hr-1 and 2.5 mgC m-2 hr-1, respectively.

438

The Abs value of MeS-BrC at 365 nm (AbsMeS-BrC) shows no liner relationships

439

with BVOC tracers (i.e., β-caryophyllinic and cis-pinic acid, Figure S3a, b) in

440

summer but is well correlated with the primary organic tracers for plant debris and

441

fungal spores (i.e., glucose, r2= 0.48, P<0.01, Figure 7a and arabitol, r2=0.41, P<0.01,

442

Figure 7b). These findings indicate that methanol tends to extract more bioaerosols,

443

which is consistent with the fluorescence result, that is, more protein-like substances

444

are identified in methanol extract (section 3.3). Bioaerosols (e.g., fungal spores, plant

445

debris and amino acids) have been frequently documented as important fluorescent

44：

components in atmospheric aerosols (Poehlker et al., 2012; Yue et al., 2019) and could

447

be released from forest at SETS.

448

During the winter season, the concentrations of OC, WSOC and MeSOC are well

449

correlated with EC (Figure 5d, e, f), which highlight the importance of combustion

450

emissions for BrC aerosols at SETS. Moreover, the AbsWS-BrC (r2=0.73, P<0.01,

451

Figure 8c) and AbsMeS-BrC (r2=0.53, P<0.01, Figure 8d) exhibit strong linear

452

relationships with levoglucosan, demonstrating obvious influence of biomass burning

453

emissions. The air mass at SETS is dominated by westerly in winter with 57% coming

454

from northeast India and 43% from Nepal (Figure S4). According to the observations

455

of MODIS satellite (Figure S4), dense active fire spots are identified in the

45：

above-mentioned regions, which demonstrate wide-spread biomass burning activities.

457

According to previous studies, large amounts of air pollutants significantly buildup in

458

South Asia during winter season (Lelieveld et al., 2001). These air pollutants could be

459

transported to SETS within a few days through the Yarlung Tsangpo River valley

4：0

under the influence of prevailing westerly winds (Cao et al., 2010). The aerosol

4：1

vertical profile achieved by Cloud-Aerosol Lidar with Orthogonal Polarization

4：2

(CALIOP, Figure S5) during the sampling period also proved that the polluted smoke

4：3

from South Asia could be transported to SETS.

4：4

4. Summary and implications

4：5

In this study, both water- and methanol- soluble BrC have been investigated in

4：：

the southeast TP. The results show that the averaged abundances of MeS-BrC are 3.03

4：7

± 0.93 µgC m-3 and 3.61 ± 2.07 µgC m-3 in summer and winter, respectively, which

4：8

are nearly twice of those of WS-BrC. This suggests that the climate effect of BrC may

4：9

be underestimated if only WS-BrC was taken into consideration. Previously,

470

MeS-BrC is recognized to exhibit stronger light absorption capability than WS-BrC

471

due to the fact that more organic matters are extractable by methanol. However, this

472

study finds the optical properties of BrC are more complex over TP, i.e., the averaged

473

MAE values of MeS-BrC in summer are higher than those of WS-BrC, while the case

474

in winter is inverse. The seasonal variations of BrC’s optical properties should be

475

considered when estimating the radiative forcing of atmospheric aerosols in future. By

47：

organic tracer (i.e., glucose and arabitol) and fluorescence analysis, this study finds

477

the stronger light absorption of MeS-BrC in summer is mainly attributed to bioaerosol

478

emissions from vegetation. The solar absorption of MeS-BrC is comparable to that of

479

black carbon over wavelength 300-400 nm (32.5±18.4%) in summer, further

480

highlighting that bioaerosols are important light-absorbing compounds in atmosphere.

481

In winter, BrC aerosols are mainly derived from long-range transport of biomass

482

burning emissions from South Asia, as supported by levoglucosan analysis and

483

satellite data. In general, this study deepens our understanding about the seasonal

484

variations of BrC’s optical properties and sources at the forest representative site over

485

TP, which may benefit the estimation of environmental effect of atmospheric aerosols

48：

at high-altitude regions.

487

Acknowledgments

488

This study was supported by the Strategic Priority Research Program of the

489

Chinese Academy of Sciences, Pan-Third Pole Environment Study for a Green Silk

490

Road (Pan-TPE, XDA20040501), and the National Natural Science Foundation of

491

China (41807389 and 41625014). We thank the SETS station for logistical and

492

sampling support. Xin Wan appreciates the financial support from China Postdoctoral

493

Science Foundation (2018M630210). Guangming Wu thanks Xiaolong Zhang from

494

ITPCAS and Shengjie Hou from LAPCCAS for the support of sample pretreatment.

495

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Table 1. The mass absorption efficiency at 365 nm and Absorption Ångström exponent (AAE, calculated between 300 - 400 nm) of BrC and HULIS at SETS. All the data are reported as mean ± standard deviation (S.D.) Summer

Winter

Daytime

Nighttime

Total

Daytime

Nighttime

Total

(n=15)

(n=15)

(n=30)

(n=15)

(n=15)

(n=30)

MAEWS-BrC

0.28 ± 0.11

0.27 ± 0.08

0.27 ± 0.10

0.93 ± 0.14

0.80 ± 0.17

0.86 ± 0.17

MAEMeS-BrC

0.36 ± 0.11

0.32 ± 0.12

0.34 ± 0.12

0.59 ± 0.43

0.59 ± 0.45

0.59 ± 0.44

MAEHULIS

0.30 ± 0.17

0.34 ± 0.17

0.32 ± 0.17

0.95 ± 0.32

0.88 ± 0.41

0.91 ± 0.37

AAEWS-BrC

7.54 ± 1.41

6.90 ± 1.38

7.22 ± 1.45

6.03 ± 0.40

5.81 ± 0.44

5.92 ± 0.43

AAEMeS-BrC

5.92 ± 0.71

5.10 ± 1.16

5.53 ± 1.04

6.08 ± 0.89

6.38 ± 1.48

6.24 ± 1.24

AAEHULIS

7.15 ± 1.76

7.41 ± 2.26

7.28 ± 2.03

6.62 ± 0.86

5.96 ± 1.10

6.29 ± 1.04

Figure 1. The concentrations (column graph) of EC, OC, WSOC and HULIS as well as ratios (pie graph) of HULIS/OC, MeSOC/OC and WSOC/OC at SETS.

Figure 2. The water-soluble fluorescent components (i.e., C1, C2, C3 and C4) identified by PARAFAC analysis in summer (left column) and winter (right column). The components C1 and C2 belong to humic-like substances, C3 and C4 are protein-like substances.

Figure 3. The light absorption and fluorescence intensity contributions of HULIS and PRLIS + other organic matters to WS-BrC at SETS (a), and the specific data distributions of fluorescence (b) and light absorption (c) contributions by HULIS to WS-BrC.

Figure 4. The methanol-soluble fluorescent components (i.e., C2, C3 and C5) identified by PARAFAC analysis in summer (left column) and winter (right column). The naming of methanol-soluble component is following the order as reported for water-soluble components. C2 is humic-like substance, C3 and C5 belong to protein-like substances.

Figure 5. The relationships between EC and OC (a, d), WSOC (b, e) and MeSOC (c, f) in summer (left column) and winter (right column), respectively, at SETS.

Figure 6. The correlations of water-soluble BrC’s absorption coefficient (AbsWS-BrC) with β-caryophyllinic acid (a, c), cis-pinic acid (b, d) in summer (left panel) and winter (right panel) at SETS.

Figure 7. The relationships between AbsMeS-BrC and primary bioaerosol tracers, i.e., glucose (a) and arabitol (b) at SETS.

Figure 8. Correlations of biomass burning tracer (i.e., levoglucosan) with the absorption coefficient of water-soluble (AbsWS-BrC, a, c) and methanol-soluble (AbsMeS-BrC, b, d) BrC in summer (left column) and winter (right column) at SETS.

Highlights


Methanol-soluble BrC are more abundant than water-soluble BrC.




Protein- and humic-like substances are the main chromophores in BrC.




BrC are mainly derived from bioaerosols and secondary formations in summer.




In winter, biomass burning emissions are the dominant sources of BrC.

Declaration of interests


The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: