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Environmental Pollution 257 (2020) 113616

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Environmental Pollution

journal homepage: www.elsevier .com/locate/envpol

Light absorption, fluorescence properties and sources of brown carbonaerosols in the Southeast Tibetan Plateau*

Guangming Wu a, i, Xin Wan a, i, Kirpa Ram b, Peilin Li a, i, Bin Liu c, Yongguang Yin d,Pingqing Fu e, Mark Loewen a, Shaopeng Gao a, Shichang Kang f, h, Kimitaka Kawamura g,Yongjie Wang a, Zhiyuan Cong a, h, *

a Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences (CAS),Beijing 100101, Chinab Institute of Environment and Sustainable Development, Banaras Hindu University, Varanasi 221005, Indiac Chongqing Jinfo Mountain Field Scientific Observation and Research Station for Kaster Ecosystem, School of Geographical Sciences, Southwest University,Chongqing 400715, Chinad State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, CAS, Beijing 100085, Chinae Institute of Surface-Earth System Science, Tianjin University, Tianjin 300072, Chinaf State Key Laboratory of Cryospheric Sciences, Northwest Institute of Eco-Environment and Resources, CAS, Lanzhou 730000, Chinag Chubu Institute for Advanced Studies, Chubu University, Kasugai 487-8501, Japanh Center for Excellence in Tibetan Plateau Earth Sciences, CAS, Beijing 100101, Chinai University of Chinese Academy of Sciences, Beijing 100049, China

a r t i c l e i n f o

Article history:Received 23 September 2019Received in revised form10 November 2019Accepted 11 November 2019Available online 14 November 2019

* This paper has been recommended for acceptanc* Corresponding author. Building 3, Courtyard 16, Li

Beijing, China.E-mail address: [email protected] (Z. Cong

https://doi.org/10.1016/j.envpol.2019.1136160269-7491/© 2019 Elsevier Ltd. All rights reserved.

a b s t r a c t

Brown carbon (BrC) has been proposed as an important driving factor in climate change due to its lightabsorption properties. However, our understanding of BrC’s chemical and optical properties are inade-quate, particularly at remote regions. This study conducts a comprehensive investigation of BrC aerosolsin summer (Aug. 2013) and winter (Jan. 2014) at Southeast Tibetan Plateau, which is ecologically fragileand sensitive to global warming. The concentrations of methanol-soluble BrC (MeS-BrC) are approxi-mately twice of water-soluble BrC (WS-BrC), demonstrating the environmental importance of water-insoluble BrC are previously underestimated with only WS-BrC considered. The mass absorption effi-ciency of WS-BrC (0.27e0.86m2 g�1) is lower than those in heavily polluted South Asia, indicating adistinct contrast between the two sides of Himalayas. Fluorescence reveals that the absorption of BrC ismainly attributed to humic-like and protein-like substances, which broaden the current knowledge ofBrC’s chromophores. Combining organic tracer, satellite MODIS data and air-mass backward trajectoryanalysis, this study finds BrC is mainly derived from bioaerosols and secondary formation in summer,while long-range transport of biomass burning emissions in winter. Our study provides new insights intothe optical and chemical properties of BrC, which may have implications for environmental effect andsources of organic aerosols.

© 2019 Elsevier Ltd. All rights reserved.

1. Introduction

The Tibetan Plateau (TP), with an average altitude of more than4000m a.s.l. (above sea level) and area about 2.5 ⅹ 106 km2, isrecognized as an important geographical area of the Third Pole (Yao

e by Baoshan Xing.ncui Road, Chaoyang District,

).

et al., 2012). It plays a crucial role in regional and global climate,atmospheric and hydrological circulation as well as ecosystemstability (Qiu, 2008; Yao et al., 2012). With sparse inhabitants andrelatively less emission sources, this region represents relativelycleaner environment. However, recent researches have revealedthat the TP has been experiencing more rapid and dramaticwarming than other regions (Liu and Chen, 2000; Qin et al., 2009).Along with driving factors like greenhouse gas (e.g., CO2), atmo-spheric aerosol also has been considered as an important contrib-utor for such elevation-dependent warming (Pepin et al., 2015).Atmospheric aerosols could directly heat the ambient atmosphere

G. Wu et al. / Environmental Pollution 257 (2020) 1136162

by absorbing the solar irradiation and reduce snow albedo due todeposition on glaciers and thus, warm the surface (Kaspari et al.,2015; Qian et al., 2015).

Traditionally, black carbon is considered as the main lightabsorber in atmospheric aerosols over the spectrum ranging fromultraviolet to infrared (Bond and Bergstrom, 2006; Yuan et al.,2019). Nevertheless, recent studies have revealed that a portionof organic carbon (OC) with brownish to yellowish color also showslight absorption properties. These OC exhibit strong wavelengthdependent (l�2 - l�9) and are defined as brown carbon (BrC)(Andreae and Gelencs�er, 2006; Laskin et al., 2015). Global modelingstudies have demonstrated that BrC, at 550 nm, could contributesto nearly 20% of total absorption caused by carbonaceous aerosols(Chung et al., 2012). Like other aerosols, BrC also can be transportedinto glacier by wet and/or dry depositions and then acceleratingglacier melting (Wu et al., 2016).

Carbonaceous aerosols are characterized by high OC to EC(elemental carbon) ratios over the TP, such as in the central (23.4)(Wan et al., 2015), southern edge (1.91e43.8) (Cong et al., 2015) andsoutheast TP (17.67± 11.05) (Zhao et al., 2013), indicating that OCmay has considerable influence on air quality and climate change.Recent study has pointed out that humic-like substance (HULIS) ismain chemical component of BrC over the central TP and can ac-count for approximately 40e60% of water-soluble OC (WSOC) and12e31% of OC mass concentrations (Wu et al., 2018), furtherunderlining the importance of atmospheric BrC over the TP.Although there are some progresses concerning BrC aerosols overthe TP (Li et al., 2016; Zhang et al., 2017), most of themwere focusedonwater-soluble BrC (WS-BrC). The water-insoluble BrC, which haslight absorption capability about 3 times that of WS-BrC (Chen andBond, 2010; Liu et al., 2013; Zhang et al., 2013), is less studied yet.Moreover, the source and chemical component of BrC are crucial forunderstanding the transport and optical property of BrC in the at-mosphere, which are also poorly characterized over the TP (Wuet al., 2018; Zhang et al., 2017). These inadequate studies consti-tute a major uncertainty in the assessment of environmental effectof atmospheric aerosols.

In this study, the southeast TP was chosen as the representativesite of alpine forest environment over the TP to study the BrCaerosols. Our current study focuses on the temporal variations oflight absorption properties of both WS-BrC and methanol-solubleBrC (MeS-BrC). Moreover, fluorescence technique, combined withPARAFAC (parallel factor) model, is used to investigate the chemicalcomponents in BrC. The possible sources of BrC are also discussedbased on various organic tracers such as b-caryophyllinic acid,levoglucosan, and arabitol. The knowledge on light absorptionproperties, chemical characteristics and sources of BrC is expectedto be helpful to understand the environmental effects of aerosolover the high-altitude regions.

2. Sampling site and data analysis

2.1. Research site and aerosol sampling

The samples were collected at South-East Tibetan plateau Sta-tion for integrated observation and research of alpine environment(SETS, 94�440E, 29�460N, 3326m a.s.l., Figure S1), located in thesoutheast TP. Attributed to low population density, local emissionsfromhuman activities around SETS is minimal (Liu et al., 2017; Zhaoet al., 2013). In summer, humid air masses originating from theIndian Ocean can reach this site through the Yarlung Tsangpo GrandCanyon, which deliver abundant precipitation (800e1000mm).The mean values for air temperature, relative humidity, wind speedand radiative forcing are 13.4± 0.8 �C, 74.5± 3.9%, 0.9± 0.1m s�1

and 461.1± 115.9Wm�2 (only daytime included), respectively,

according to an automatic weather station. The warm and wetenvironment conditions feed an extensive alpine forest in thesoutheast TP. The main vegetation species are Alpine pine, spruceand fir, which account for 72.9% of total live biomass of naturalvegetation over the TP (Luo et al., 2002). In winter season, this re-gion is mainly under the influence of south branch westerly windswith wind speed and air temperature of 1.9± 0.6m s�1 and-0.2± 4.9 �C, respectively. By the active interaction betweenmonsoon and westerlies, diverse of geographical environments areformed over the southeast TP, including large area of glacier, whichis sensitive to the radiative forcing deduced by light-absorbingaerosols (e.g., BrC) (Wang et al., 2015). Therefore, SETS is a uniquesite to investigate the complex interactions between atmosphere,biosphere and hydrosphere.

In August (15th - 30th) of 2013 and January (1st - 17th) of 2014,total suspended particles (TSP) were collected with a high-volumeair sampler (Qingdao Laoshan Co., China). All the samples werecollected on 20.3� 25.4 cm2 quartz filters (Whatman, UK), pre-baked at 450 �C for 6 h. The flow rate during the sampling wasconverted to standard condition (i.e., 0.75m3min�1) automatically.For each daytime, the sampling started around 8:00 a.m. (Beijingtime) and ended at 7:30 p.m., which was followed by the collectionof nighttime samples from 8:00 p.m. to 7:30 a.m. (next day). Eachsampling duration time was approximately 11.5 h. The field blanksamples were collected about 5min without air flowing throughthe filter. After the collection, all samples were sealed in pre-combusted glass bottles and kept frozen until analysis. A total ofsixty samples were collected with daytime and nighttime sampleseach amounting to half.

2.2. Analysis of water-/methanol-soluble BrC

For the measurement of water-soluble BrC, an aliquot filterabout 20 cm2 was extracted with 20ml pure water (Millipore, USA)under ultrasonication in a brown glass bottle. Then the extractswere filtered by 0.45 mm PTFE filter (Chromafil Xtra-45/25,Macherey-Nagel, Duren, Germany) and divided into two portions.One portion was analyzed for its concentration (represented bywater-soluble organic carbon, WSOC) by a total carbon analyzer(TOC-L, Shimadzu, Japan) with detection limit of 4 mg L�1 andstandard deviation less than 3%. The other portionwas prepared forthe light absorption measurement. The parallel experiment, bycutting samples at different regions on filter, shows the aerosolparticles are equally deposited on filter with deviation less than1.3%.

For the methanol extraction, a punch of sample filter (2.54 cm2)was soaked in 5ml HPLC grade methanol (Merck KGaA, Darmstadt,Germany) for 2 h without sonication following the method devel-oped by Cheng et al. (2016). The methanol extracts were filteredand ready for optical measurement. The residual filters werefurther dried for another 2 h in a 40 �C oven to let the methanolevaporated and get prepared for methanol-soluble OC (MeSOC)quantifying (see section 2.4). It should be noted that certain WSOCis also included in the MeSOC, because WSOC is extractable bymethanol.

The light absorption properties of water and methanol extractswere measured by a UVeVis Spectrophotometer (UV-3600, Shi-madzu, Japan) over the wavelength ranging from 200 to 800 nm in1 nm stepwise. During the measurement, the solutionwas kept in aquartz cuvette with optical path-length of 1 cm (l). The light ab-sorption capability of BrC is estimated by mass absorption effi-ciency (MAE), which was derived by normalizing the absorptioncoefficient (Abs) against its corresponding concentration (C, mgCm�3). Absorption Ångstr€om exponent (AAE) refers to the wave-length dependence of absorption capability and was derived by

G. Wu et al. / Environmental Pollution 257 (2020) 113616 3

fitting the Abs values between wavelengths 300e400 nm(Hecobian et al., 2010; Wu et al., 2018):

Abs�Mm�1

�¼ðAl �A700Þ�

Vs

Va � l� ln ð10Þ (1)

MAE�m2 g�1

�¼ Abs

�Mm�1�

C�mgC m�3

� (2)

AAE¼ lnðAbsðl1Þ=Absðl2ÞÞlnðl2=l1Þ

(3)

Here Al refers to the light absorbance at wavelength (l). The Absis reported inMm�1 where 1Mm�1¼10�6m�1. The Vs and Va is thevolume of extraction solvent and the air passed through the filterduring sampling, respectively. Duplicate measurements show thestandard deviation for absorbance is no more than 0.1%.

The fluorescence properties (i.e., excitation-emission matrix,EEM) of WS-BrC andMeS-BrC were obtained by Horiba Fluoromax-4 fluorometer in Sc/Rc (signal/reference after correction) modelwith the excitationwavelength ranging from240 to 455 nm in 5 nmintervals, and the emissionwavelength ranging from 290 to 550 nmin 2 nm intervals (Fu et al., 2015; Wu et al., 2019). Considering thehigh volatility of methanol, the quartz cuvette was sealed with acap during the measurement to minimize the possible influencefrom methanol evaporation. After the measurement, a series ofcalibrations were conducted to ensure the accuracy of the data,which include instrumental bias calibration, inner filter correction,Raman normalization and blank subtraction (Supporting Informa-tion) (Murphy et al., 2010). Then the above derived original data(Fori) was further normalized by the volume of extraction solvent(Vs, ml) and the air passed through the filter during sampling (Va,m3) as well as the organic carbon concentrations (C, mg m�3) toaccount for the different chromophore abundances (Wu et al.,2019):

Ff ¼ Fori � Vs

Va � l � C(4)

The finally derived fluorescence data (Ff) were reported in theunit of RU.m2 gC�1 (RU: Raman Unit). Furthermore, the fluorescentcomponents in BrC were identified by PARAFAC model with non-negativity constraints, which was validated by split-half andsquared error analysis (Murphy et al., 2013). The relative standarddeviation for this analysis was below 2%.

2.3. Extraction and measurement of HULIS and organic tracers

The HULIS is important component of water-soluble BrC andwas isolated by solid-phase extraction (SPE) protocol as reported inWu et al. (2018). Briefly, HULIS was efficiently absorbed on OasisHLB cartridge (30 mm, 60 mg/cartridge, Waters, USA) at pH¼ 2,while other organic and inorganic (e.g., ion species) compoundswere passed through the column into effluent. After the HLB car-tridgewas dried in pure nitrogen,methanol was introduced to elutethe HULIS into prebaked glass bottle. Then the eluent was evapo-rated in pure nitrogen at room temperature to remove methanol.Finally, HULIS was redissolved into ultrapure water for subsequentanalysis of concentration and light absorption, following the samemethods for WSOC analysis. The extraction efficiency of HULIS wasbetter than 85% (Wu et al., 2018).

The organic tracers, including b-caryophyllinic acid, cis-pinicacid, levoglucosan, arabitol and glucose, were extracted withdichloromethane/methanol (v/v¼ 2:1) solutions under ultra-sonication. Then they were measured by a gas chromatography-

mass spectrometry (GS-MS) following the protocol reported inWan et al. (2017). The reproducibility, estimated based on therepeat experiments, was less than 15%.

2.4. Determination of OC and EC

A portion of each sample (0.5 cm2) was analyzed for OC and ECconcentrations by a carbon analyzer (DRI model 2001) based on theIMPROVE-A thermal/optical reflectance protocol (Chow et al.,2007). Briefly, OC concentration was measured in pure heliumcondition as the temperature increased in stepwise to 580 �C.While the EC was analyzed after being oxidized in He/O2 (v/v¼ 98/2) environment at 580, 740 and 840 �C, respectively (Cong et al.,2015). The TC content was defined as the sum of OC and EC. Thedetection limit for OC, EC and TC were 0.05 mgC cm�2. The residualfilters after methanol extraction were also measured by the samecarbon analyzer described above to get the methanol extracted OCconcentrations, which were derived by subtracting the TC contentbefore and after the methanol extraction (Cheng et al., 2016; Liet al., 2019).

2.5. Radiative effects of BrC relative to black carbon

The direct solar absorption of BrC relative to black carbon (fBrC/BC) was estimated by the method documented in Kirillova et al.(2014), with the specific input parameters listed in Table 1 and S1.Briefly, the light absorption by species X (i.e., BrC or BC) is followingthe Lambert-Beer law:

I0 � II0

ðl;XÞ¼1� e��MAEl0 ;X

�l0l

�AAEX

$CX$hABL(5)

where Cx represents mass concentrations of BrC or BC in the at-mosphere and hABL corresponds to the height of atmosphericboundary layer (1000m). The MAEl0,x is MAE value of BrC or BCcalculated at wavelength 365 nm or 632 nm, respectively (Thecalculation protocol for MAEBC is presented in Supporting Infor-mation). The I0 is the clear sky Air Mass 1 Global Horizontal solarirradiance (Levinson et al., 2010). Then the solar irradiance absor-bed by BrC was normalized against that of BC:

fBrC=BC ¼

ðI0ðlÞ$

I0 � II0

ðl;BrCÞdlðI0ðlÞ$

I0 � II0

ðl;BCÞdl(6)

3. Results and discussions

3.1. Abundances and variations of carbonaceous compounds

The average abundances of various carbonaceous compounds atSETS are summarized in Fig. 1 and Table S1. The concentration of OCis 3.73± 0.89 mgC m�3 and 3.79± 1.93 mgC m�3 in summer andwinter, respectively and exhibits no seasonal variation. In contrast,EC shows relatively higher abundance in winter (0.96± 0.63 mgCm�3) by nearly 3 times than in summer (0.36± 0.23 mgC m�3).Compared to previous study in central TP (Wan et al., 2015), OCconcentration at SETS is comparable, while the EC is almost 2e3times higher. The OC and EC abundances at SETS are also higherthan those reported in the southern TP, e.g., Mt. Everest (Cong et al.,2015), NCO-P (Decesari et al., 2010) (Table S2). However, they arelower than those from Manora Peak (Ram et al., 2010) and Khar-agpur of India (Srinivas and Sarin, 2014) (Table S2), both of which

Table 1Themass absorption efficiency at 365 nm and Absorption Ångstr€om exponent (AAE, calculated between 300 and 400 nm) of BrC and HULIS at SETS. All the data are reported asmean± standard deviation (S.D.).

Summer Winter

Daytime (n¼ 15) Nighttime (n¼ 15) Total (n¼ 30) Daytime (n¼ 15) Nighttime (n¼ 15) Total (n¼ 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± 0.17MAEMeS-BrC 0.36± 0.11 0.32± 0.12 0.34± 0.12 0.59± 0.43 0.59± 0.45 0.59± 0.44MAEHULIS 0.30± 0.17 0.34± 0.17 0.32± 0.17 0.95± 0.32 0.88± 0.41 0.91± 0.37AAEWS-BrC 7.54± 1.41 6.90± 1.38 7.22± 1.45 6.03± 0.40 5.81± 0.44 5.92± 0.43AAEMeS-BrC 5.92± 0.71 5.10± 1.16 5.53± 1.04 6.08± 0.89 6.38± 1.48 6.24± 1.24AAEHULIS 7.15± 1.76 7.41± 2.26 7.28± 2.03 6.62± 0.86 5.96± 1.10 6.29± 1.04

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


are most likely impacted by local anthropogenic emissions.Therefore, although SETS is susceptible to air pollutants than otherregions over the TP, its air quality is still relatively pristinecompared to South Asia. This finding is consistent with a recentstudy by Liu et al. (2017) based on integrated usage of satellite andground-based remote sensing. For carbonaceous aerosols at SETS,OC is a predominant component with OC/EC ratio of 14.1± 8.2 and4.57± 2.18 in summer and winter, respectively (Fig. 1 and Table S1),pointing out the importance of organic aerosols over the TP.

The concentrations of WSOC are 1.89± 0.57 mgC m�3 and1.44± 0.42 mgC m�3 in summer daytime and nighttime, and1.97± 0.68 mgC m�3 and 1.42± 0.49 mgC m�3 inwinter daytime andnighttime, respectively (Fig. 1 and Table S1). Similar to OC, noseasonal variations are found for WSOC abundances, which is ex-pected as WSOC being an important component of OC. However,WSOC/OC ratios (0.45± 0.12 in summer and 0.51± 0.18 in winter)at SETS are lower than those from other background sites from thenortheast TP (0.79, Qilian Shan, 4180ma.s.l.) (Xu et al., 2015) andsouth slope of the TP (0.46e0.95, NCO-P, 5079m a.s.l.) (Decesariet al., 2010) as well as Summit, Greenland (0.81± 0.2, 3200ma.s.l.) (Hagler et al., 2007) and Alps (54.8%e75.9%) (Jaffrezo et al.,2005), implying a large fraction of OC is water-insoluble at SETS.

Methanol has a better extraction efficiency for water-insolubleorganic aerosols than commonly used other organic solvents (e.g.,hexane and acetone) (Chen and Bond, 2010), and has beenfrequently used to investigate the chemical characteristics andoptical properties of water-insoluble BrC in different environments,such as high-altitude Tibetan Plateau (Kirillova et al., 2016; Zhuet al., 2018) and urban environments (Sarkar et al., 2019; Yan

et al., 2017). In this study, the abundance of methanol-solubleorganic carbon (i.e., MeSOC) is 3.03± 0.93 mgC m�3 and3.61± 2.07 mgC m�3 in summer and winter, respectively. The ratioof MeSOC/WSOC is 1.74± 0.42 and 2.02± 0.72 in summer andwinter, respectively (Table S1), further demonstrating the impor-tance of water-insoluble organic aerosols. In addition, MeSOC/OCratio is 0.77± 0.19 in summer and 0.91± 0.20 in winter, suggestingthat most of OC can be extracted by methanol. Thus, MeSOC mayprovides more comprehensive description of BrC’s optical andchemical characterizations compared to WSOC.

3.2. Light absorption properties of BrC

3.2.1. Mass absorption efficiency (MAE)The optical parameter of MAE provides a quantitative descrip-

tion for the light absorption capability of BrC and is a key parameterused in model to evaluate the climate effect of BrC (Laskin et al.,2015). The statistical summaries of MAE values of BrC at SETS arelisted in Table 1. The average MAE value of WS-BrC (i.e., MAEWS-BrC)in winter (0.86± 0.17m2 g�1) is approximately 3 times of that insummer (0.27± 0.10m2 g�1), demonstrating that WS-BrC hasstronger light absorption capability in winter than in summer. Aprevious study at the central TP has highlighted that the opticalproperties of BrC are strongly related with their sources, namely,biomass burning emitted BrC has stronger light absorption capa-bility than secondary BrC formed in the atmosphere (Wu et al.,2018). Therefore, the different MAEWS-BrC values between sum-mer and winter essentially imply a seasonal variation of BrC’ssources. The MAEWS-BrC values at SETS are comparable to those


from the central TP (Nam Co, winter: 0.64 ± 0.40m2 g�1, summer:0.25± 0.12m2 g�1) (Wu et al., 2018) and Himalayas (NCO-P,0.42e0.61m2 g�1) (Kirillova et al., 2016) as well as South Asianoutflow (MCOH, 0.5± 0.2m2 g�1) (Bosch et al., 2014), which arerelatively far away from densely populated regions. However, theyare much lower than those from more polluted sites, such as,Northwest China (Xi’an, 5.38m2 g�1 at 340 nm) (Shen et al., 2017),Delhi of India (1.1e2.7m2 g�1) (Kirillova et al., 2014) and the centralIndo-Gangetic Plain (Kanpur,1.16± 0.60m2 g�1) (Satish et al., 2017),which highlights that anthropogenic emissions may have consid-erable impacts on the light absorption properties of BrC aerosols.

The MAE value for methanol-soluble BrC (i.e., MAEMeS-BrC) is0.34± 0.12m2 g�1 in summer and 0.59± 0.44m2 g�1 in winter. Thelight absorption properties of methanol-extracted BrC show asimilar seasonal variation with those of water-extracted BrC, i.e.,the MAE values in winter are approximately 2e3 times of those insummer (Table 1). Therefore, these observations suggest that bothMeS-BrC and water-soluble BrC show stronger solar absorptioncapabilities in winter than summer. Comparing the MAE valuesbetween methanol- and water-soluble BrC, the MAEMeS-BrC valuesare higher than those of MAEWS-BrC in summer. In contrast, thesituation is reverse in winter, i.e., the MAEMeS-BrC values are lowerthan those of water-soluble BrC. This demonstrates that morewater-insoluble BrC with stronger light absorption capabilities isextracted by methanol in summer, while the water-insoluble BrC inwinter is characterized with lower light absorption capability. Thediscrepancy in the optical properties of MeS-BrC between differentseasons should be carefully considered in the estimation of BrC’sclimate effect in future studies.

3.2.2. Absorption Ångstr€om exponent (AAE)The AAE reflects thewavelength dependence of BrC’s absorption

capability, i.e., higher AAE value demonstrates that light absorptioncapability increases more sharply from longer to shorter wave-lengths. The AAEWS-BrC, calculated between 300 and 400 nm, isrelatively higher in summer (7.22± 1.45) than winter (5.92± 0.43)without obvious diurnal variations (Table 1). According to previousstudies, the AAEWS-BrC value lies between 7.37 and 9.28 in thecentral TP (Nam Co) (Wu et al., 2018), 3.1e9.3 in South Asia (NewDelhi) (Kirillova et al., 2014), 4.1e6.4 in North American (Liu et al.,2014), and 2.23e9.48 in central Europe (Utry et al., 2013). All theaforementioned AAEWS-BrC values are 2 to 9- fold higher than thatof BC (i.e., AAE of BC ~1) (Bond and Bergstrom, 2006). This confirmsthat it is a common characteristic of BrC to exhibits strong wave-length dependence in light absorption capabilities. BrC aerosolstend to absorb more solar irradiation over the ultraviolet wave-length, which may have considerable impact on various photo-chemical reactions in the atmosphere (Jo et al., 2016; Laskin et al.,2015; Yan et al., 2018). For example, Jo et al. (2016) found BrCcould decrease the annual mean NO2 photolysis rate by 8% and theO3 concentration by 6% in surface air over Asia, where BrC aerosolsare abundant.

Moreover, the averaged AAE value for methanol-extracted BrC(AAEMeS-BrC) is 5.53± 1.04 in summer and 6.24± 1.24 in winter(Table 1). Compared to water-soluble BrC, AAEMeS-BrC is slightlylower in summer but comparable in winter. This indicates meth-anol extracts more organic matters, which have stronger absorp-tion capabilities at longer wavelength, from the summer aerosols.Therefore, the BrC aerosols from summer season may play moreimportant role in solar absorption at SETS, because most solar en-ergy occurs at longer wavelength than ultraviolet wavelength(Levinson et al., 2010).

3.2.3. Radiative effectsOver the solar spectrum ranging from 300 to 700 nm, the fBrC/BC

value of WS-BrC is minor, both in summer (4.2± 1.9%) and winter(3.4± 1.1%) (Table S3). These values are comparable to that at thehigh Himalayas (4± 1%, NCO-P, Nepal) (Kirillova et al., 2016) butlower than that from South Asia such as 8.78± 3.74% at Godavari(Wu et al., 2019) and 40% in the Indo-Gangetic Plain (Srinivas et al.,2016). However, due to the strong wavelength dependence, the fBrC/BC value of WS-BrC increases to 17.6± 7.9% in summer and12.3± 4.2% in winter, over the wavelength range of 300e400 nm(Table S3). For MeS-BrC, its fBrC/BC values are approximately 1e2.5folds larger thanWS-BrC, which are attributed tomore abundant orstronger light absorption capability of MeS-BrC as discussed insection 3.1 and 3.2.1. Similar toWS-BrC, the absorption contributionof MeS-BrC also increases significantly to 32.5± 18.4% in summerand 14.7± 6.9% in winter over the ultraviolet wavelengths (i.e.,300e400 nm).

3.3. Fluorescence properties of BrC

Fluorescence technique is based on the absorption and then re-emission of light by fluorophores. Generally, each fluorophore hasits specific excitation/emission peak in the excitation-emissionmatrix, which can act as fingerprint to identify the chemical com-ponents in atmosphere (Coble, 2007). Recent studies have foundthat BrC shares similar molecular conformation with some fluo-rophores (Laskin et al., 2015), and thus fluorescence technique canbe used as a powerful tool to quickly identify the chemical com-ponents in BrC aerosols (Chen et al., 2016b;Wu et al., 2019; Yan andKim, 2017). In this study, three fluorescence components wereidentified in WS-BrC from summer samples while four fromwintersamples with explained variance (core consistency) of 99.4%(81.5%) and 99.6% (47.9%), respectively (Fig. 2). The first two com-ponents (i.e., C1 and C2) belong to HULIS. The component C1 ismost likely characterized by terrestrially-derived humic acid(Coble, 2007) with similar peak locations ((Ex/Em (Excitation/Emission)¼ ~250/384e412 nm) for both summer and wintersamples. C2 has obvious blue shift in emission wavelength insummer (Ex/Em¼ ~250/450e470 nm) relative to winter (Ex/Em¼ ~250/490e510 nm). The HULIS (i.e., C1 and C2) contributes to67.7± 8.2% (summer) and 47.5± 11.4% (winter) of total fluorescenceintensities (Fig. 3), which are lower than that documented at thesouth slope of Himalayas (i.e., 80.2± 4.1%, Godavari, Nepal) (Wuet al., 2019). Meanwhile, the mass concentration of HULIS is0.83± 0.36 mgCm�3 and 0.82± 0.33 mgCm�3 in summer andwinterseasons, respectively (Fig. 1 and Table S1), which accounts forapproximately half of the corresponding WSOC concentrations.

Besides HULIS, two other fluorescent components (i.e., C3 andC4) are also identified in SETS aerosols (Fig. 2), which refer toprotein-like substance (PRLIS) (Coble, 2007). These PRLISscontribute to 32.3± 7.4% and 52.5± 8.3% of the total fluorescenceintensities in summer and winter, respectively, demonstrating thatPRLIS is also important component of BrC. Given the fact that HULIShas comparable light absorption capability (i.e., MAEHULIS) to WS-BrC, while its concentration is only half of WSOC (Table 1 andFig. 1), indicates that PRLIS and other unknown compounds may beresponsible for near half the absorption of WS-BrC (Fig. 3).

As discussed in section 3.1, the abundances of methanol-solubleorganic carbon are almost twice of WSOC, which underline theimportance of water-insoluble organic aerosols. Therefore, thefluorescence properties of methanol-soluble BrC were alsoanalyzed in this study. By the PARAFAC analysis, three kinds offluorescent components are identified in MeS-BrC in both summerand winter aerosols with explained variance (core consistency) of98.6% (91.0%) and 99.3% (86.1%), respectively. They include oneHULIS (i.e., C2) and two PRLISs (i.e., C3, C5) (Fig. 4) with their cor-responding peak locations in EEMpresented in Table S4. Comparing

Fig. 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 C1and C2 belong to humic-like substances, C3 and C4 are protein-like substances.


the fluorescent components identified in WS-BrC and MeS-BrC,humic-like substance (i.e., C1) in WS-BrC is not found in MeS-BrC.Meanwhile, more protein-like substances are identified in MeS-BrC with the contributions to total fluorescence intensities of76.4± 23.8% in summer and 84.2± 5.9% in winter, which areapproximately 2 and 1.5 times larger than those inWS-BrC. As such,the protein-like substances appear to be more important thanHULIS in the methanol-soluble BrC. Recent studies have found thatmethanol extracts contain abundant amines, proteinaceous mate-rials and organic nitrates (Chen et al., 2016a; Duarte et al., 2004),which are important components of BrC (Huang et al., 2018; Songet al., 2018; Wang et al., 2017; Xie et al., 2019). Therefore, theincreased protein-like substances may be responsible for thestronger light absorption capability of methanol-extracted BrC than

water-soluble BrC in summer (Table 1), which are possibly derivedfrom vegetation emissions, such as bioaerosols. More detaileddiscussions are presented in the following section.

3.4. Source identification of BrC

The light absorption coefficient (Abs) at wavelength 365 nm isdirectly related with the light absorption capability and abundanceof BrC in extraction, which is widely used as proxy of BrC (Chenet al., 2018; Du et al., 2014; Hecobian et al., 2010). Meanwhile,several organic compounds are derived from specific emissionsources, and can be used as tracers to investigate the formation ofatmospheric aerosols. For example, cis-pinic and b-caryophyllinicacid are the well-recognized biogenic volatile organic carbon

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

Fig. 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 ofmethanol-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.



(BVOC) tracers (Stone et al., 2012). cis-Pinic acid is the oxidationproduct of a-pinene by ozone (Christoffersen et al., 1998). The b-caryophyllinic acid is a tracer for b-caryophyllene (sesquiterpenereleased from plant foliage) (Jaoui et al., 2007). Glucose and arabitolare mainly related with primary emissions from plant debris andfungal spores, respectively (Wan et al., 2019). Levoglucosan is aunique indicator for biomass burning emission (Bhattarai et al.,2019; Simoneit et al., 1999).

In summer season, both OC (r2¼ 0.05, P¼ 0.14, Fig. 5a) andWSOC (r2¼ 0.0, P¼ 0.30, Fig. 5b) exhibit no liner relationships withEC, indicating less influence from combustion emissions. However,the Abs value of WS-BrC at 365 nm (AbsWS-BrC) demonstrates goodlinear relationships with the above-mentioned BVOC tracers, i.e., b-caryophyllinic acid (r2¼ 0.58, P< 0.01, Fig. 6a) and cis-pinic acid(r2¼ 0.63, P< 0.01, Fig. 6b), which indicates that secondary for-mations contribute considerably to WS-BrC aerosols at SETS.Furthermore, the AbsWS-BrC is relatively well correlated with b-caryophyllinic acid (r2¼ 0.74, P< 0.01, Figure S2a) and cis-pinic acid(r2¼ 0.74, P< 0.01, Figure S2c) in summer daytime, but no linearrelationships in nighttime (Figure S2b, d). This finding furthersupports that WS-BrC is formed from secondary reactions of BVOC,which are more efficient in daytime with stronger solar irradiationand higher temperature (Hansen and Seufert, 2003). These pre-cursors for BrC’s secondary formations may be released from local

Fig. 5. The relationships between EC and OC (a, d), WSOC (b, e) and MeSOC (c,

and surrounding forest. Wang et al. (2007) have found that thesoutheast TP is one of the strongest BVOC (e.g., monoterpene andisoprene) emission regions in China with the emission intensitymore than 1mgC m�2 hr�1 and 2.5mgC m�2 hr�1, respectively.

The Abs value of MeS-BrC at 365 nm (AbsMeS-BrC) shows no linerrelationships with BVOC tracers (i.e., b-caryophyllinic and cis-pinicacid, Figure S3a, b) in summer but is well correlated with the pri-mary organic tracers for plant debris and fungal spores (i.e.,glucose, r2¼ 0.48, P< 0.01, Fig. 7a and arabitol, r2¼ 0.41, P< 0.01,Fig. 7b). These findings indicate that methanol tends to extractmore bioaerosols, which is consistent with the fluorescence result,that is, more protein-like substances are identified in methanolextract (section 3.3). Bioaerosols (e.g., fungal spores, plant debrisand amino acids) have been frequently documented as importantfluorescent components in atmospheric aerosols (Poehlker et al.,2012; Yue et al., 2019) and could be released from forest at SETS.

During the winter season, the concentrations of OC, WSOC andMeSOC are well correlated with EC (Fig. 5d, e, f), which highlightthe importance of combustion emissions for BrC aerosols at SETS.Moreover, the AbsWS-BrC (r2¼ 0.73, P< 0.01, Fig. 8c) and AbsMeS-BrC(r2¼ 0.53, P< 0.01, Fig. 8d) exhibit strong linear relationships withlevoglucosan, demonstrating obvious influence of biomass burningemissions. The air mass at SETS is dominated by westerly in winterwith 57% coming from northeast India and 43% from Nepal

f) in summer (left column) and winter (right column), respectively, at SETS.

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

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


(Figure S4). According to the observations of MODIS satellite(Figure S4), dense active fire spots are identified in the above-mentioned regions, which demonstrate wide-spread biomassburning activities. According to previous studies, large amounts ofair pollutants significantly buildup in South Asia during winterseason (Lelieveld et al., 2001). These air pollutants could be trans-ported to SETSwithin a few days through the Yarlung Tsangpo Rivervalley under the influence of prevailing westerly winds (Cao et al.,2010). The aerosol vertical profile achieved by Cloud-Aerosol Lidarwith Orthogonal Polarization (CALIOP, Figure S5) during the sam-pling period also proved that the polluted smoke from South Asiacould be transported to SETS.

4. Summary and implications

In this study, both water- and methanol-soluble BrC have beeninvestigated in the southeast TP. The results show that the averagedabundances of MeS-BrC are 3.03± 0.93 mgC m�3 and

3.61± 2.07 mgC m�3 in summer and winter, respectively, which arenearly twice of those of WS-BrC. This suggests that the climateeffect of BrC may be underestimated if only WS-BrC was taken intoconsideration. Previously, MeS-BrC is recognized to exhibit stron-ger light absorption capability than WS-BrC due to the fact thatmore organic matters are extractable by methanol. However, thisstudy finds the optical properties of BrC are more complex over TP,i.e., the averagedMAE values of MeS-BrC in summer are higher thanthose of WS-BrC, while the case in winter is inverse. The seasonalvariations of BrC’s optical properties should be considered whenestimating the radiative forcing of atmospheric aerosols in future.By organic tracer (i.e., glucose and arabitol) and fluorescenceanalysis, this study finds the stronger light absorption of MeS-BrCin summer is mainly attributed to bioaerosol emissions fromvegetation. The solar absorption of MeS-BrC is comparable to thatof black carbon over wavelength 300e400 nm (32.5± 18.4%) insummer, further highlighting that bioaerosols are important light-absorbing compounds in atmosphere. In winter, BrC aerosols are

Fig. 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 insummer (left column) and winter (right column) at SETS.


mainly derived from long-range transport of biomass burningemissions from South Asia, as supported by levoglucosan analysisand satellite data. In general, this study deepens our understandingabout the seasonal variations of BrC’s optical properties and sourcesat the forest representative site over TP, which may benefit theestimation of environmental effect of atmospheric aerosols at high-altitude regions.

Acknowledgments

This study was supported by the Strategic Priority ResearchProgram of the Chinese Academy of Sciences, Pan-Third PoleEnvironment Study for a Green Silk Road (Pan-TPE, XDA20040501),and the National Natural Science Foundation of China (41807389,41625014 and 41877315). We thank the SETS station for logisticaland sampling support. Xin Wan appreciates the financial supportfrom China Postdoctoral Science Foundation (2019T120140).Guangming Wu thanks Xiaolong Zhang from ITPCAS and ShengjieHou from LAPCCAS for the support of sample pretreatment.

Appendix A. Supplementary data

Supplementary data to this article can be found online athttps://doi.org/10.1016/j.envpol.2019.113616.

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