Light absorption characteristics of carbonaceous aerosols in two remote stations of the southern fringe of the Tibetan Plateau, China

Atmospheric Environment 143 (2016) 79e85

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Light absorption characteristics of carbonaceous aerosols in two remote stations of the southern fringe of the Tibetan Plateau, China Chaoliu Li a, c, *, Fangping Yan b, e, Shichang Kang b, c, Pengfei Chen b, Zhaofu Hu b, d, €a € e, f Shaopeng Gao a, Bin Qu e, Mika Sillanpa a

Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China State Key Laboratory of Cryospheric Sciences, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000, China c CAS Center for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Sciences, Beijing 100101, China d University of Chinese Academy of Sciences, Beijing 100049, China e Laboratory of Green Chemistry, Lappeenranta University of Technology, Sammonkatu 12, FI-50130 Mikkeli, Finland f Department of Civil and Environmental Engineering, Florida International University, Miami, FL 33174, USA b

h i g h l i g h t s
MAC of BC and WSOC at two remote areas on the Tibetan Plateau were studied.
Remarkable seasonal fluctuation of MACWSOC related to photobleaching was found.
Seasonal fluctuation of MACWSOC should be a general phenomenon at other remote areas.
Contribution of direct radiative forcing (RDF) of WSOC to that of BC were calculated.
RDF caused by WSOC was around 6% and 12% to that of BC at two areas.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 April 2016 Received in revised form 25 July 2016 Accepted 12 August 2016 Available online 15 August 2016

Light absorption characteristics of carbonaceous aerosols are key considerations in climate forcing research. However, in situ measurement data are limited, especially on the Tibetan Plateau (TP) e the Third Pole of the world. In this study, the mass absorption cross section (MAC) of elemental carbon (EC) and water soluble organic carbon (WSOC) of total suspended particles at two high-altitude stations (Lulang station and Everest station) in the Tibetan Plateau (TP) were investigated. The mean MACEC values at 632 nm were 6.85 ± 1.39 m2 g1 and 6.49 ± 2.81 m2 g1 at these two stations, both of which showed little seasonal variations and were slightly higher than those of EC of uncoated particles, indicating that the enhancement of MACEC by factors such as coating with organic aerosols was not significant. The mean MACWSOC values at 365 nm were 0.84 ± 0.40 m2 g1 and 1.18 ± 0.64 m2 g1 at the two stations. Obvious seasonal variations of high and low MACWSOC values appeared in winter and summer, respectively, mainly reflecting photobleaching of light absorption components of WSOC caused by fluctuations in sunlight intensity. Therefore, this phenomenon might also exists in other remote areas of the world. The relative contributions of radiative forcing of WSOC to EC were 6.03 ± 3.62% and 11.41 ± 7.08% at these two stations, with a higher ratio in winter. As a result, both the contribution of WSOC to radiative forcing of carbonaceous aerosols and its seasonal variation need to be considered in radiative forcing related study. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Black carbon Water soluble organic carbon Light absorption The Tibetan Plateau Seasonal variation

1. Introduction * Corresponding author. Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China. E-mail address: [email protected] (C. Li). http://dx.doi.org/10.1016/j.atmosenv.2016.08.042 1352-2310/© 2016 Elsevier Ltd. All rights reserved.

Carbonaceous aerosols include elemental carbon (EC, a term used synonymously with black carbon (BC)) (Petzold et al., 2013) and organic carbon (OC). EC is emitted into the atmosphere solely

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as primary particles from either fossil fuel combustion or biomass burning. OC is emitted either directly or is formed from gaseous precursors as a secondary organic carbon (SOC) (Jenk et al., 2006). The radiative forcing of carbonaceous aerosols influences the radiation budget of the atmosphere and climate forcing, which is one of the largest uncertainties in the climate system (Bond et al., 2013; Gustafsson and Ramanathan, 2016; IPCC, 2013; McComiskey et al., 2008). It is estimated that EC is the second atmospheric agent after carbon dioxide in terms of its climate warming effect in the present-day atmosphere (Bond et al., 2013; Jacobson, 2000). The mass absorption cross section of EC (MACEC) is a fundamental input to models of radiative transfer (Bond and Bergstrom, 2006; Bond et al., 2013; Cui et al., 2016). Measured MACEC values for EC of uncoated particles fall within a relatively narrow range of 7.5 ± 1.2 m2 g1 at 550 nm (Bond and Bergstrom, 2006). It has been shown recently by an increasing amount of evidences that some components of OC also absorb sunlight in addition to its scattering effects (Andreae and Gelencser, 2006; Saleh et al., 2014; Yan et al., 2015). This type of OC is called brown carbon (BrC), which originates mainly from low-temperature combustion processes (e.g., biomass burning) or secondary formation (Andreae and Gelencser, 2006; Chen and Bond, 2010; Kim et al., 2016). Water-soluble organic carbon (WSOC) accounts for large parts of the OC in the atmosphere, especially at remote areas. For instance, WSOC accounts for around 57% of total carbon (TC) in remote sites of Europe (Pio et al., 2007). Some component of WSOC shows strong light absorption in the blue to ultraviolet spectral range, which is called as water soluble organic carbon (WS-BrC) and also causes climate warming (Andreae and Gelencser, 2006; Chakrabarty et al., 2010). However, so far, compared to EC, knowledge of the atmospheric processing of WS-BrC species and variations in their light absorption abilities are still not well investigated, and less studies are conducted on in situ values and seasonal variations in their radiative impacts in remote areas with low aerosol loading and sensitive atmospheric conditions. The Himalayas and the Tibetan Plateau (TP) are among the most remote and highest regions in the world. Warming of the atmosphere over the TP, partly caused by carbonaceous aerosol (i.e., EC), is an important trigger for the evolution of the Asian monsoon (Prell and Kutzbach, 1992; Ramanathan and Carmichael, 2008) and melting of TP glaciers (Menon et al., 2010). Carbonaceous aerosols deposited on glaciers also cause a decrease in glacier albedo and the retreat of glaciers (Qu et al., 2014; Xu et al., 2009), which is closely connected to the fresh water supply for billions of local residents in Asia (Ramanathan et al., 2007). Therefore, studies on carbonaceous aerosols in the TP have generated considerable concern during last several decades. At present, the consensus is that most carbonaceous aerosols on the TP are mainly transported from outside of the TP (e.g., South Asia). However, almost all the previous work has focused specifically on concentrations, sources and spatial and temporal variations of carbonaceous aerosols (especially on EC) in the atmosphere of the TP (Cao et al., 2010; Cong et al., 2015; Ming et al., 2010; Zhao et al., 2013), and few have examined the light absorption characteristics of EC and WS-BrC (Li et al., 2016a), which is an obstacle in precisely evaluating the radiative forcing of carbonaceous aerosols in the atmosphere of the TP. Due to the high elevation of the TP, it is assumed that the radiative forcing contributed by WS-BrC is high, as shown in a previous study (Liu et al., 2014), which revealed that BrC accounted for approximately 20% of the direct radiative forcing from aerosols at the top of the atmosphere. Similarly, another research even suggested that the primary organic aerosol (POA) absorptivity led to ~27% reduction in the amount of the net global average POA cooling (Lu et al., 2015). In this study, therefore, total suspended particles (TSP) were

collected at two remote stations on the TP. Values and seasonal variations of optical characteristics of EC and WSOC of collected aerosols were quantitatively investigated. In addition, the relative contributions of radiative forcing from WSOC and EC were evaluated to provide fundamental information on the optical characteristics of carbonaceous matter, especially WSOC at these two stations. 2. Methods 2.1. Sampling sites TSP samples were collected at the Southeast Tibetan Station for Alpine Environment Observation and research in Lulang (Lulang station) (29 450 58.7700 N, 94 44017.6800 E, 3330 m.a.s.l.) and the Qomolangma Station for Atmospheric and Environmental Observation and Research (Everest station) (28 21040.5200 N, 86 560 55.6700 E, 4276 m.a.s.l.) from August 2014 to August 2015 (Fig. 1, Table 1). The Lulang station is located in a sub-valley of the Yarlung Tsangpo Grand Canyon, a corridor for the warm-humid Indian monsoon to penetrate into the inner part of the TP (Cao et al., 2010). The Everest station is located on the north slope of the middle Himalayas (Ma et al., 2011). Both of these two stations are located on the southern fringe of the TP, far from urban cities or industry centers, and are normally considered as typical remote areas of the Northern Hemisphere and the southern TP that easily receive pollutants transported mainly from South Asia (Fig. 1, Fig. S4). 2.2. Sample collection Because aerosol sampler equipped with air flow meter is easily broken at this high altitude of TP and is hard to run continuously for long time, a stable vacuum pump (VT 4.8, Germany) was used to collect TSP samples on 90 mm pre-burned (550  C, 6 h) quartz fiber filters (Whatman Corp). Although air volume, which passes through each filter, cannot be achieved, it does not influences the objectives of this research (e.g., the OC/EC ratio and relative contributions to radiative forcing between WSOC and EC). Two samples were collected every month, for approximately 7 and 20 days, respectively. Four field blank filters were also collected at each station by exposing the filters in the sampler for 7 and 20 days without pumping. 2.3. Analytical methods The OC and EC concentrations of the samples were analyzed with the standard IMPROVE-A method at 632 nm using the Desert Research Institute (DRI) Model 2001 Thermal/Optical Carbon Analyzer (Atmoslytic Inc., Calabasas, CA, USA) (Chen et al., 2015; Chow et al., 2001). The DOC concentration was measured with a total organic carbon (TOC) 5000A (Shimadzu Corp, Kyoto, Japan) (Li et al., 2016b, 2016c). The major cations (e.g., Kþ) and major anions (e.g., NO 3 ) were measured by Dionex-6000 Ion Chromatograph and Dionex-3000 Ion Chromatograph (Dionex, USA), respectively (Li et al., 2007). The light absorption of WSOC was measured with a spectrophotometer (SpectraMax M5, USA) from 200 nm to 800 nm (Li et al., 2016a). Field blank concentrations of all the measured indexes were far lower than those of samples (Table S1). Detailed information on the measurement method and calculation of light absorption of EC and WSOC and their relative contributions (Cheng et al., 2011; Kirillova et al., 2014a) are shown in the supporting information file.

C. Li et al. / Atmospheric Environment 143 (2016) 79e85

3. Results and discussion 3.1. MACEC OC and EC showed significant relationships at the two stations (R2 ¼ 0.73 (Lulang) and 0.40 (Everest)), which was similar to previous studies (Cong et al., 2015; Zhao et al., 2013). The OC/EC ratios of the two stations were 6.37 ± 1.12 and 8.48 ± 2.67 for Lulang and Everest stations (Table 1), which were respectively lower and comparable to previously reported values (11.41 and 6.7) (Cong et al., 2015; Zhao et al., 2013). The high previously reported ratio of 11.41 at Lulang is caused by too many samples (27) with high OC/ EC ratio (17.67), which were collected during monsoon period compared to other seasons (16, 6 and 16 samples with OC/EC ratio of 6.29, 6.72 and 6.45 for pre-monsoon, post-monsoon and winter, respectively). Therefore, the previous annually average ratio was intensively influenced by monsoon period, which is different to this study. Nevertheless, the low ratio of Lulang station implied that it received a greater contribution of fossil combustion-sourced aerosols and less SOC (Table 1). The EC concentrations of both stations were significantly connected to those of optical attenuation (ATN) (Fig. 2), implying that EC was the main component causing the variation in light absorption of the Carbon Analyzer (Cheng et al., 2011). The average MACEC values of the Lulang station (6.85 ± 1.39 m2 g1) and Everest station (6.49 ± 2.81 m2 g1) were comparable to that of EC of uncoated particles (6.5 m2 g1). It has been proposed that the instrument and method used for the measurement of EC can introduce a variety of artifacts due to factors such as the shadowing of the incident light with increasing filter loading and aerosol scattering effects (Andreae and Gelencser, 2006; Bond and Bergstrom, 2006; Kondo et al., 2009). Here, it is assumed that coating with organic aerosols on EC have little influence on the MACEC of aerosols at the Lulang station because MACEC and OC/EC were poorly related (R2 ¼ 0.03, P > 0.05)

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(Fig. S1A). Correspondingly, the MACEC and OC/EC were significantly related (R2 ¼ 0.53, P < 0.01) for aerosols at the Everest station (Fig. S1B), reflecting the influence of SOC on aerosols at this station. Despite this fact, no remarkable seasonal variations in the MACEC were observed. €m exponent (AAE)WSOC 3.2. MACWSOC and absorption Ångstro The WSOC concentrations of samples at the two studied stations showed significant relationships to OC concentrations (r2 ¼ 0.75 (Lulang) and 0.74 (Everest), P < 0.01), implying an important contribution of OC to WSOC. The WSOC/OC ratios of these two stations were 0.44 ± 0.09 and 0.36 ± 0.13, respectively (Table 1). A significant relationship existed between the attenuation and WSOC concentrations (Fig. 3), indicating a similar light absorption ability of WSOC of the studied stations throughout the year. The AAE330e400nm values of Lulang and Everest stations were 5.39 ± 1.22 and 4.64 ± 1.15, respectively, which were lower than those of the source regions of the pollutants, such as the outflow from northern China (6.4 ± 0.6) (Kirillova et al., 2014a) and the United States (7.6 ± 0.5) (Zhang et al., 2013), implying the WSOC of these two stations experienced strong atmospheric process because the AAE decreased as the BrC aged (Forrister et al., 2015). Because light absorption ability of the WSOC decreased exponentially with wavelength increase, AAE330e530nm of Lulang and Everest stations decreased to 4.10 ± 1.36 and 3.75 ± 0.96, respectively, lower than those of AAE330e400nm. In addition, AAE values were inversely related to the MACWSOC at 365 nm (Fig. S2), which was also found for aerosols from wood burning at different temperatures (Chen and Bond, 2010), indicating that WSOC absorption has a stronger spectral dependence than that of EC (Schnaiter et al., 2006). The absorption spectra of the WSOC in this study showed a sharp absorption increase with the wavelength decrease, which was similar to those of megacities such as Beijing, China (Fig. 4).

Fig. 1. Locations of the sampling stations. The red arrows represent the dominant long-range transported air amasses to the studied stations. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Table 1 Comparison of OC/EC, WSOC/TC, MACEC (m2 g1) and MACWSOC (m2 g1) of aerosols in this study with other sites. Sampling site

OC/EC

WSOC/TC

MACEC

MACWSOC

Methods of EC

Reference

Lulang Everest Lulang Everest NCO-P, Nepal European remote area Beijing in winter, China Beijing in summer, China MCOH, Indian Ocean New Delhi, India Urban areas of southeastern United States in summer Urban areas of southeastern United States in winter

6.37 ± 1.12 8.48 ± 2.67 11.41 ± 9.28 6.7(1.91e43.8) 4.8 5.91

0.44 ± 0.09 0.36 ± 0.13

6.85 ± 1.39 6.49 ± 2.81

0.84 ± 0.40 1.18 ± 0.64

8.45 ± 1.71 (E) 9.41 ± 1.92 (E)

1.26 (F) 0.51(F) 0.5 ± 0.2 1.6 ± 0.5 0.31 ± 0.07 0.70 ± 0.07

IMPROVE(TOT) IMPROVE(TOT) IMPROVE(TOR) IMPROVE(TOR) EUSAAR2(TOT) NIOSH(TOT) IMPROVE(TOT) IMPROVE(TOT)

This study This study A B C D E,F E,F G H I I

0.57

A, B, C, D, E, F, G, H, I are adopted from (Zhao et al., 2013), (Cong et al., 2015), (Decesari et al., 2010), (Pio et al., 2007), (Cheng et al., 2011), (Du et al., 2014), (Bosch et al., 2014), (Kirillova et al., 2014b), (Hecobian et al., 2010), respectively.

Therefore, the WSOC obtained here contained a large part of BrC. The MACWSOC values for Lulang and Everest stations at 365 nm were 0.84 ± 0.40 m2 g1 and 1.18 ± 0.64 m2 g1, respectively. The higher value of the Everest station reflected the larger contribution of biomass combustion or SOC from fossil fuel to the aerosols. The MACWSOC values of both stations were lower than those of New Delhi, India (1.6 ± 0.5 m2 g1) (Kirillova et al., 2014b) and winter at Beijing (1.26 m2 g1) (Du et al., 2014) (Table 1), which might reflect the photobleaching of WSOC because of the long-range transport from source regions to the studied station (Forrister et al., 2015). Similarly, the values in this study were higher than that of a remote site in the Indian Ocean (Maldives Climate Observatory at Hanimaadhoo, MCOH) (0.5 ± 0.2 m2 g1) (Bosch et al., 2014) because MCOH is located in remote ocean so that higher ratio of its WSOC was influenced by the aging process during long-range transport from the source region (Lambe et al., 2013; Sun et al., 2011). 3.3. Seasonal variations of MACWSOC and bleaching of WSOC Obvious seasonal variations of the MACWSOC were discovered at both stations (Fig. 5, Table S2), possibly due to effects of photobleaching and chemical bleaching on the WSOC that occurred during transport and aerosol collection processes instead of varied sources of WSOC at different seasons. It is assumed that WSOC at these two stations is derived from

Fig. 2. Dependence of light attenuation measured at 632 nm (ATN) on the EC loading (ECs) at the studied stations.

the same source at different seasons. WSOC of two stations was þ significantly related to NO 3 and K (Fig. S3), both of which have been considered as typical combustion sourced ions in previous studies at these two stations (Cong et al., 2015; Zhao et al., 2013). In addition, their relations were inseparable among different seasons, further indicating common sources of WSOC at different seasons. Backward trajectory analysis demonstrated that air masses of these two stations were mainly transported from the same source region of South Asia for the whole year (Fig. S4). For instance, air masses transported from Middle East and Western TP by typical westerlies and air masses stemmed from the TP accounted for less than 10% at all four seasons at Lulang station. The similar patterns existed for Everest station excepted for monsoon and winter. Therefore, emission of south Asia is the dominant pollution source region for both Lulang and Everest stations, causing same source of WSOC (Cong et al., 2015; Zhao et al., 2013). Furthermore, because aerosols collected at studied stations have been transported for long distance, they were well mixed so that WSOC showed similar sources. This phenomenon is different to previous studies at urban areas that are heavily influenced by the emission of different types of local fuels at different seasons (Du et al., 2014; Hecobian et al., 2010; Kim et al., 2016). For instance, MACWSOC of Beijing, China showed high and low values in winter and summer, respectively, mainly because more biomass was burned in winter (Du et al., 2014). Therefore, photobleaching and chemical bleaching of WSOC

Fig. 3. Dependence of attenuation at 365 nm on the WSOC concentration for aerosols at the Lulang and Everest stations.

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play an important role for the seasonal variations of MACWSOC. Because WSOC is chemically reactive compared to EC in the atmosphere, the long-range transport or the long-term sampling of aerosols cause variations in WSOC compositions (Pio et al., 2007), even decreasing its light absorption ability, which is induced mainly by atmospheric processes (chemical bleaching and photobleaching) (Forrister et al., 2015). For instance, the MACWSOC decreased in around March and reached its lowest values during the monsoon period, then increased again in October (Fig. 5), indicating intensive washing out effect of WSOC or increased SOC contribution (Saleh et al., 2013) because of high humidity and temperature during monsoon period. Correspondingly, WSOC in the winter was mainly originated from more sources for absorbing species (e.g., biomass burning, photochemical reactions). Correspondingly, photochemical oxidation played a large role in other seasons due to relatively intense sunlight. The influence of photobleaching on the decreased MACBrC has also been observed on the evolution of wildfire plumes at high altitude (Forrister et al., 2015). In addition, a previous study at Everest station also proposed that carbonaceous aerosols were transported over long distance from South Asia into the TP at May ahead of Indian monsoon outbreak (Cong et al., 2015), implying potential photobleaching of the WSOC during transport. Despite sunlight did not increase much, concentration of an important oxidant in atmosphere-Ozone in atmosphere of the TP and the Himalayans reached peak level in spring (Cristofanelli et al., 2010; Lin et al., 2015), accelerating the bleaching of WSOC in atmosphere during this period. Therefore, it is reasonable that MACWSOC began decreasing in spring. Another observed phenomenon was that almost all consecutive aerosol samples showed higher MACWSOC values for a short collection period (7 days) than those over a long period (20 days) during the winter and spring (Fig. 5), indicating that long collection time also caused decreased MACWSOC values. This was caused by the chemical bleaching of those components of WSOC with strong light absorption ability by oxidant in atmosphere (e.g., Ozone) because the longer time the aerosols were collected, the larger amount of oxidant passed through the aerosols that loaded on the filter (Forrister et al., 2015). Furthermore, the significant relationship between attenuation and WSOC concentration (Fig. 3) meant MACWSOC value was independent of WSOC concentration, so that the saturation effect is little. The disparity of MACWSOC values

Fig. 4. Variations in MACWSOC from 350 nm to 450 nm in this study and compared to reported values of aerosols in an urban area (winter and summer in Beijing, China) (Cheng et al., 2011) and a typical biomass combustion aerosol (pine) (Chen and Bond, 2010).

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between aerosols collected was not obvious during summer and autumn, because the larger proportion of WSOC experienced intensive photobleaching processes before reaching the studied stations due to intense sunlight. Consequently, the influence of the length of the collection time play a relatively minor role for the decreasing MACWSOC values in summer and autumn. 3.4. Relative contributions of light absorption of WSOC to EC Significant contribution of BrC to light absorption have been found at the top of the atmosphere (Forrister et al., 2015). In this study, relative light absorption caused by WSOC and EC at the studied stations was estimated according to a simplistic absorption-based method (Bosch et al., 2014; Kirillova et al., 2014a). It need to be pointed out that because light absorption of EC may be higher at long wavelength (e.g.700 nm) than at short wavelength 450 nm due to the coating of secondary organic aerosol (Schnaiter et al., 2005), leading to lower contribution of WSOC to EC in this study. The calculated ratios of solar energy absorbed by the WSOC to EC at the Lulang and Everest stations were 6.03 ± 3.62% and 11.41 ± 7.08%, respectively, comparable to those of ambient aerosols for the East Asian outflow (2e10%) (Kirillova et al., 2014a) and seriously polluted Delhi (3e11%) (Kirillova et al., 2014b) and higher than that of remote MCOH (0.5 ± 0.2%) (Bosch et al., 2014). Therefore, the WSOC of aerosols at the Lulang and Everest stations played a similar important role to urban areas in the radiative forcing of the atmosphere on the southern TP, despite the fact that they are generally considered as a remote area. Like that of the

Fig. 5. Seasonal variations in MACWSOC at 365 nm and sunlight radiation at the top of the atmosphere at Lulang station.

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MACWSOC, the contribution of the WSOC on radiative forcing during winter was far higher than that of other seasons (Fig. S5), showing remarkable seasonal variations. 4. Implications The light absorption characteristics and seasonal variations of EC and WSOC in TSP at two remote stations in the southern TP were reported. The MACEC values of these two stations were slightly higher than that of EC of uncoated particles, with minor variations throughout the year. Therefore, the radiative forcing of EC will be underestimated if the referenced MACEC value is directly adopted in radiative forcing studies in these areas. The MACWSOC values of both stations showed obvious seasonal variation patterns, with high and low values appeared in the winter and monsoon, respectively, reflecting atmospheric processes, especially seasonal variations in sunlight strength that strongly influenced the light absorption ability of WSOC. Therefore, our results support the assumption that atmospheric BrC evolves differently from both BC and bulk organic aerosols because of their different sources and the photobleaching of BrC in the atmosphere (Forrister et al., 2015). Due to similar seasonal variations of MACWSOC at both studied stations, it is assumed that this phenomenon may exist at other remote areas of the TP. In addition, because seasonal variations in sunlight intensity at the top of the troposphere are stable and become more obvious at high latitude, the phenomenon found in this study also exists in the atmosphere in other high-altitude regions in the world. If this is the case, both the radiative forcing characteristics of WSOC and its seasonal variations must be considered in related modeling studies for the purpose of precisely evaluating the radiative forcing of WSOC. Another phenomenon we found was that long aerosol collection period cause reduced absorption efficiency of WSOC. Therefore, it is better to reduce the aerosol collection time for an optical-related study of WSOC. Meanwhile, this study has some limitations. Firstly, water-insoluble organic carbon (WIOC) has a stronger light absorbing ability than that of WSOC (Bond and Bergstrom, 2006), which was not involved in this study; thus, the actual contribution of BrC for radiative forcing should be higher than that of WSOC. As discussed in Section 3.3, MACWSOC decreased over long-term sample collection; thus, even MACWSOC of aerosols of seven day collection was lower than the real value in atmosphere. Therefore, the WSOC caused radiative forcing in this study was underestimated. Nevertheless, these are the best data we can achieve so far in the remote and high areas of the TP. Acknowledgements This study was supported by the National Nature Science Foundation of China (41271015, 41225002 and 41421061), State Key Laboratory of Cryospheric Science (SKLCS-ZZ-2008-01 and SKLCSOP-2014-05), Academy of Finland (decision number 268170). The authors acknowledge staff of Lulang station and. Everest Station for sample collecting. The authors want to express their gratitude to € Prof. Orjan Gustafsson, Dr. August Andersson and Dr. Carme Bosch for helping calculating DRF of WSOC to EC. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.atmosenv.2016.08.042. References Andreae, M.O., Gelencser, A., 2006. Black carbon or brown carbon? the nature of light-absorbing carbonaceous aerosols. Atmos. Chem. Phys. 6, 3131e3148.

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