Historical Black Carbon Reconstruction from the Lake Sediments of

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Characterization of Natural and Affected Environments

Historical Black Carbon Reconstruction from the Lake Sediments of the Himalayan - Tibetan Plateau Bigyan Neupane, Shichang Kang, Pengfei Chen, Yulan Zhang, Kirpa Ram, Dipesh Rupakheti, Lekhendra Tripathee, Chhatra MAni Sharma, Zhiyuan Cong, Chaoliu Li, Juzhi Hou, Min Xu, and Poonam Thapa Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 17 Apr 2019 Downloaded from http://pubs.acs.org on April 17, 2019

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Environmental Science & Technology

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Historical Black Carbon Reconstruction from the Lake Sediments of

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the Himalayan - Tibetan Plateau

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Bigyan Neupane1, 2, Shichang Kang1, 2, 3*, Pengfei Chen1, Yulan Zhang1, Kirpa Ram4, 5,

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Dipesh Rupakheti1, Lekhendra Tripathee1, Chhatra Mani Sharma1,6, Zhiyuan Cong3,4, Chaoliu

5

Li3,4, Juzhi Hou3,4, Min Xu1, Poonam Thapa1, 2

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1State

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Resources, Chinese Academy of Sciences, Lanzhou 730000, China

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2University

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3CAS

Key Laboratory of Cryospheric Science, Northwest Institute of Eco-Environment and

of Chinese Academy of Sciences, 100049, Beijing, China

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

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4Key

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Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China

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5Institute

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Varanasi-221005, India

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6Central

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*Corresponding author

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Prof. Dr. Shichang Kang

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State Key Laboratory of Cryospheric Science, Northwest Institute of Eco-Environment and

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Resources, Chinese Academy of Sciences, Lanzhou 730000, China.

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Email: [email protected]

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Tel.: +86-0931-4967368

Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of

of Environment and Sustainable Development, Banaras Hindu University,

Department of Environmental Science, Tribhuvan University, Kirtipur 44618, Nepal

1 ACS Paragon Plus Environment


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ORCID Number

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Bigyan Neupane - 0000-0002-2713-5269

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Shichang Kang - 0000-0003-2115-9005

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Yulan Zhang - 0000-0003-1839-4987

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Kirpa Ram - 0000-0003-1147-4634

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Dipesh Rupakheti - 0000-0001-5436-4086

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Lekhendra Tripathee - 0000-0001-6210-5105

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Chhatra Mani Sharma - 0000-0003-0714-7411

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Chaoliu Li -0000-0003-2092-2435

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Juzhi Hou - 0000-0002-8512-5739

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Abstract

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Black Carbon (BC) is one of the major drivers of climate change and its measurement in

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different environment is crucial for better understanding of long-term trend in Himalayan-

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Tibetan Plateau (HTP) as climate warming has intensified in the region. We present

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measurement of BC concentration from six lake sediments in HTP to reconstruct historical BC

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deposition since the preindustrial era. Our results show an increasing trend of BC, concurrent

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with increased anthropogenic emission pattern after the commencement of an industrialization

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era during the 1950s. Also, sedimentation rates and glacier melt strengthening influenced the

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total input of BC into the lake. Source identification, based on char and soot composition of

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BC, suggests biomass burning emission as a major contributor to BC, which is further

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corroborated by open fire occurrence events in the region. The increasing BC trend continues

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to the recent years indicating an increasing BC emission, mainly from South Asia.

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Keywords: Black carbon, Lake sediments, Historical trend, Long-range transport, Himalayan-

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Tibetan Plateau



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1. Introduction

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Black Carbon (BC), a kind of specific carbonaceous aerosols, is produced during incomplete

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combustion of fossil fuel and biomass 1-3. BC aerosols are predominantly found in small size

49

(< 2.5µm) and has retention time in the atmosphere for about one week and thus, can even

50

reach to remote sites via long-range transport 4, 5.

51

The role of BC as a pollutant has been redefined to its importance as a driver of global warming

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in recent years

53

warming 8. BC not only has potential to absorb solar radiation and contribute to atmospheric

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warming but also can accelerate glacier melting process owing to albedo reduction after its

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deposition onto the glacier surfaces

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has been globally distributed and evident in sediments, soils, loess and glaciers 11. Atmospheric

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transport is the main route for an increase of contaminants loading over remote atmospheres

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and lakes 12, 13 where precipitation removes atmospheric pollutants and subsequently, deposits

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in lake sediments

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environmental changes, past climate and depositional history of various chemical constituents

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15.

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The Himalayan-Tibetan Plateau (HTP) are the hotspots to appraise BC concentration due to

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their geographical location bordering some of the largest sources of BC on a global scale 16.

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The HTP is also called “the third pole” with an area of around 2,500,000 km2 and situated at

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an average elevation more than 4,000 m 17 can exert a high impact on climate regionally and

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globally. The plateau is also one of the regions with concentrated glacier

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urbanization and economic development in Asia

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consumption of fossil fuels and biomasses, have been attributed to the major sources of BC 20

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in the region. Although HTP is comparatively cleaner and still represent pristine environment,

6, 7

which is considered to be the second largest contributor of the climate

14.

7, 9, 10.

Via atmospheric and fluvial transport, BC aerosol

Thus, lake sediments serve as one of the archives to study long-term

19,

18.

The rapid

an increasing demand for energy led


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recent studies have highlighted that atmospheric pollutants, including BC, can be transported

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across the Himalayas and into the inland HTP 21. BC being chemically inert in environment

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and resistant to microorganisms in sediment 22, it can serve as a reliable indicator to study its

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sources and transport as well climate in the past. Therefore, in this context, lake sediments in

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the HTP serve as archive to study historical and long-term BC variation in the past. Earlier

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studies have well reported the historical records of BC deposition from sediment records in the

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HTP

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marginal seas

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increasing emissions in south Asia while a decrease in European emission led to a general

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reduction of BC emission in those regions. An earlier study, based on measurement of BC in

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Nam Co Lake sediment in the HTP inferred low BC concentration and related it with

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geographic characteristics viz. remote location and high altitude 4; however, BC flux started to

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increase after the 1960s. In addition, a few studies have focused on measurement of

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atmospheric BC in snow/ice and lakes from the northern slopes of the HTP. Hence, historical

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BC reconstruction from the lake sediments from northern as well as southern slope of HTP is

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essential to better understand the paleo-BC records in this region.

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In this study, we aim to quantify depositional history of BC from the sediment cores obtained

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from six lakes across the HTP region. Among them, four lakes (Qiangyong Co, Ranwu,

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Tanglha, and Lingge Co) are located in the Tibetan Plateau (TP) in the northern slope of the

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HTP and the two lakes (Gokyo and Gosainkunda) are situated in Nepal-Himalayas in the

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southern slope of the HTP. We have investigated the temporal trend of BC concentration and

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its deposition flux in these lake sediments. In addition, we have also measured char and soot

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carbon (low and high-temperature combustion products of emissions, respectively), fire history

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to correlate BC emissions with those measured in lakes sediments. Further, we also compared

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our results with BC concentration observed in lakes over different parts of the globe. Therefore,

2, 4, 21, 23,

Europe 28.

12, 24,

USA

25,

Arctic

26,

coastal northern China

27

and eastern China

These studies have reported an increasing BC concentration owing to



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this study provides detailed insights into the state of BC contamination in high mountain lakes

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and revealing its link from the emission, transport and ultimately deposited into the lakes.

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2. Materials and Methods

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2.1. Sampling Site Description

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The details on the location of the lakes, investigated in this study and other relevant information

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have been presented in Figure 1 and Table 1. Among the six studied lakes, the two,

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Gosainkunda and Gokyo, are located in the southern slope of Nepal-Himalayas and rest four

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lakes are from the TP in the northern slopes of the Himalayas (Qiangyong Co in the southern

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TP, Ranwu in the southeast TP whereas Lingge Co and Tanglha in the central TP). These lakes

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cover an important geographical region of TP and Himalayas. In addition, these regions are

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under the influence of South Asian summer monsoon (June-September) and westerlies during

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non-monsoon months 4, 23, 29. Riverine inputs of BC are largely controlled by the snow-ice melt

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from the surrounding mountains in Lake Ranwu. Qiangyong Co is a small lake and also have

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contribution from glacier melt discharge having an outlet. All these lakes are located far from

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the urban location and thus, have minimal anthropogenic impacts from emissions from the

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vicinity, except Gosainkunda which is impacted from direct anthropogenic activities.

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2.2. Sediment Collection and Chronology Dating

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A total of six lake sediment cores were drilled from the deep basin of these lakes during 2008

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– 2017 using a gravity coring system having a 6 cm inner diameter polycarbonate tube. The

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cores were sliced in the field at intervals of 0.5 cm except for Lingge Co 30 and Ranwu which

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were sliced at 1cm interval, stored in plastic bags, and kept frozen until analysis. The sediment

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cores were brought to lab and chronology was constructed by measuring radionuclide (210Pb)

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using γ-ray spectrometry (HPGe, ORTECGWL) at the Key Laboratory of Tibetan Environment 6 ACS Paragon Plus Environment

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Changes and Land Surface Process, Chinese Academy of Sciences, Beijing, China. Most of

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the sediment records provide a temporal coverage spanning more than 150 years and dating

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back to the mid-1800 A.D.

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especially in recently deposited sediment layers in the past 150 ~ 200 years. The rates of

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sedimentation are expected to vary during this period. Where this has occurred, unsupported

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210Pb activity varies in a complicated way with depth where the profile (plotted logarithmically)

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will be non-linear and methodology employed for calculating dates is known as Constant Rate

125

of Supply (CRS) model

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sedimentation rates. A constant decrease in

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Supporting Information (Figure S1) which provides a base for the reliability of the core dating

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and is acceptable. More information on chronology dating has been described elsewhere30.

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2.3. Sediment Pretreatment and BC Determination

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In this study, a method by Han, et al.

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been successfully adopted by Cong et al. 4 for Nam Co sediments in the TP. Briefly, The freeze-

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dried samples were grinded into powder with size < 0.074mm using agate mortar and about

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0.10-0.15 g samples were weighed and transferred into 50 ml centrifuge tube. For removal of

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carbonates, silicates, and metal oxides, 10 ml HCl (2N) was added into the tube and digested

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for 24 hours at room temperature. The supernatants were removed and rinsed with ultrapure

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water. Then, 15 ml mixture of HCl (6N) and HF (48% v/v) in the ratio of 1:2 were added to the

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residue and kept aside for 24 hours at room temperature for pretreatment and rinsed thoroughly

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with MilliQ subsequently. Further, the residue was treated with HCl (4N) at 60°C overnight to

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get rid of fluoride formed, which was then centrifuged to remove the supernatant liquid. The

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residue was rinsed with pure water of 18.2 M-cm until the rinsed water became neutral.

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Finally, the residual solid was diluted with 200 ml ultrapure water and filtered through a 47

31.

210Pb

dating technique has been widely used geochronometer,

In this study, CRS model was employed to calculate age and

32

210Pb

activity with increasing depth is evident

has been used for sediment pretreatment which has



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mm quartz fiber (Whatman, pore size 0.4 µm) filter, using a pump to ensure even distribution

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on their surface. The filters were dried in an oven at 40°C for BC analysis.

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The quartz filters were analyzed for BC using a Sunset carbon analyzer (Tigard, USA) at the

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State Key Laboratory of Cryospheric Science, CAS. IMPROVE-A protocol with Thermal

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Optical Reflectance (TOR) has been widely used for determination of BC in sediments4, 21, 23,

147

32.

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BC, based on temperature protocol 33. A punch of 1.5 cm2 from the filter was introduced into

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an oven and analyzed following the IMPROVE-A protocol, where the filters were heated to

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140, 280, 480 and 580 °C in pure Helium (He) to determine the four Organic carbon (OC)

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fractions (OC1, OC2, OC3 and OC4) following heating in an oxidizing atmosphere (2% O2+

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98% He, v/v) at 580, 740 and 840 °C to determine Elemental Carbon (EC) fractions EC1, EC2,

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and EC3, respectively. Flattening of carbon signal defines the residence time of each heating

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step. The pyrolyzed carbon fraction (OPC) was determined when reflectance of the laser light

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returned to its initial value after O2 was introduced to the analysis. In this method, OC is defined

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as the sum of four OC fractions and OPC whereas BC is defined as the sum of three EC fraction

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(EC1 + EC2 + EC3) minus OPC.

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We have performed repeat measurements of BC in a few lake sediments (n=5) to ensure

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reproducibility of the measurement. The reproducibility of BC measurement reported as

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relative percentage deviation is better than 8%. In addition, we have also analyzed eight

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standard reference material (marine sediment, NIST SRM-1941b) to assess accuracy of

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measurements for a better quality control. The measured BC concentration (12.2 ± 0.7 mg g-1,

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n = 8) in our lab compares very well in SRM-1941b (Table S1 SI) with the reported value of

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(12.8 ± 1.4 mg g-1) by Han et al.

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measurement. The details of method used to determine BC flux in our study are described in

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Cong et al. 4 (S1 SI).

In addition, this method is also capable of differentiating char and soot, the two subtypes of

11

yielding as an average accuracy of 5.5% for the


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2.4. MODIS Fire Data and BC Emission Inventory

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The temporal and geographical fire counts were obtained by Moderate Resolution Imaging

169

Spectroradiometer

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(https://firms.modaps.eosdis.nasa.gov/). Thirteen years’ fire counts, from 2003 to 2015, were

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considered in this study between 5-55°N and 65-135°E geographical region which are possible

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fire sources in the vicinity impacting BC in the lake sediments. The available BC emission data

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from wildfire (2003 to 2014) was retrieved from the inventory of Department of Environmental

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Science, Peking University (PKU; inventory.pku.edu.cn/).

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3. Results and Discussion

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3.1. BC Concentration

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BC concentration in the lake sediments from the southern slope ranged between 0.04 – 64.5

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mg g-1 with an average of 15.50 ± 22.67 mg g-1 and that in the sediments from four lakes in the

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northern slope ranged between 0.14 – 2.58 mg g-1 with an average of 1.28 ± 0.62 mg g-1. The

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BC concentrations in the TP lakes are comparable with the Fennoscandian Arctic lakes (0.52-

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5.1 mg g-1) 26. The level of BC from the studied lakes compared with other lakes around the

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region and globe is depicted in Table 1 and the BC concentrations from each sediment core

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layer in all lakes has been presented in Table S3 SI. Except for Lake Ranwu and Lake

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Qiangyong Co, the concentration levels in the northern slope compare well with previous

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studies from Nam Co (average: 0.74 mg g-1) 4, Qinghai (North TP) (average: 0.46 mg g-1) 21,

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and Pumoyum Co. (average: 0.97 mg g-1) 23. BC concentrations in the lake sediments from the

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northern and southern slopes of Himalayas are lower compared to lakes in other parts of China.

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For instance, BC concentration in Daihai Lake in north China and Taihu Lake in east China

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ranged from 0.52-4.9 mg g-1 and 0.43-1.95 mg g-1, respectively. This higher concentrations

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could have been attributed to increased emissions from anthropogenic activities 23. Further, BC

(MODIS)

onboard

Aqua

and

Terra

satellites



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concentrations in five high altitude Slovenian lakes ranged between 1 - 11 mg g-1 24 and Lake

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West Pine Pond, New York state ranged 0.68 – 8.00 mg g-1 14. The lower BC concentration in

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the HTP lakes might be due to its geographical characteristics i.e. high altitude as well as the

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relatively pristine environment with sparse population. Nevertheless, Lake Gosainkunda, a

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holy lake where a large number of Hindu and Buddhist devotees pay their religious tribute each

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summer from over the Indian subcontinent

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with others in HTP. Further, the pilgrims devote sacred food into the lake and also believe to

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purifying themselves by taking a bath in the lake which might have caused a disturbance in the

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sediment layers 35. The concentration of pollution in this lake has been also reported by Kang

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et al. 30, wherein unusual high level of Hg was observed. Since, the drilling site was situated in

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a short distance from the religious activity site, it is quite possible that Lake Gosainkunda is

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also likely influenced by such anthropogenic activities.

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The top-down core profile for the BC concentration is shown in Figure 2(a). Except for Lake

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Gokyo and Lake Ranwu, BC records showed relatively less variations and have no specific

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trends before the 1900s. It shows a gradual increasing trend after the 1940s and had a significant

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increment after the 1950s. Though limited numbers of sediment samples were obtained after

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2000, BC concentration indicate an increasing trend in recent decades. Following Bond et al.

208

16

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BC profile in the lakes is in an agreement with the BC emissions from South Asia, China, and

210

the Middle East showing an growth around the 1950s. During this era, rapid economic

211

development in these regions took place along with the establishment of industries and fuel

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combustion which lead to an increase in the emission of pollutants including BC.

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3.2. BC Deposition Flux and Historical Trend

34,

demonstrated higher concentration compared

reconstruction of BC emission inventory for 1850-2000 (Figure S2), this increasing trend in


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BC concentration reflects the level of contamination in the lake from the pollutants but might

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not give detailed information about the actual input as dilution of detrital matter, and varying

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water contents that affects the BC concentration 4, 36, 37. In this scenario, deposition fluxes have

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been calculated to get real variations of BC input and are shown in Table 1. The BC flux in the

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southern slope lakes ranged between 0.02 – 5.78 g m-2y-1 with an average of 1.40 ± 1.83 g m-

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2y-1.

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average of 2.98 ± 3.33 g m-2y-1. Figure 2(a, b) shows the temporal trend of BC fluxes, where

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until the 1950s, fluxes were relatively constant and could be attributed to the background level,

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i.e., without perturbations of anthropogenic activities. However, after the mid-1950s, fluxes

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started to increase gradually in most of the lakes except in Gosainkunda and Tanglha. In

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contrast, Gokyo in the southern slope and Lingge Co and Ranwu in the northern slope of HTP

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displayed a remarkable acceleration.

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The trends in BC deposition fluxes are different from the BC concentration. It is noticeable

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here that both the profiles of BC flux and sedimentation rate exhibit similar pattern with depth

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i.e., an increase of BC flux with increased sedimentation rate and vice-versa (Figure S3).

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Higher BC deposition flux corresponds to the higher sedimentation rates pointing out the high

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pollutant input in the lake. Also, Fang et al. 28 confirmed the relative magnitude of the average

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BC depositional flux to be consistent with their sedimentation rates. However, Lake Ranwu

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(Figure 2 b) displayed a higher flux than other lakes, and it is in an agreement with high

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sedimentation rate. This lake is formed by glacial debris with the inflow of glacier melt and

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has been expanding recently due to accelerated glacier melt. From 1980 to 2005, the lake has

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expanded by 3.48 km2 (29.79 to 33.27 km2) and it is attributed to the considerable input of

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glacier melting 38. This is an indication that the accelerated glacier melt might introduce more

237

sediments that could influence the BC flux in the lakes. Lake Qiangyong from TP is a proglacial

238

lake where glacial melting and retreat aid directly to the glacial melt supply

Similarly, the flux in the northern slope ranged between 0.01 – 10.46 g m-2y-1 with an

39.

This glacial 11

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melt directly makes a channel in the lake which is plausible mechanism for the BC input from

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glaciers. Since the lake is less than one kilometer from the glacier terminal

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water supply to the lake is the glacier meltwater, especially during the summer

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decrement of BC flux was observed since the decades of mid - 1980 to recent years. Qiangyong

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is a small lake where the sediment layers possibly undergo disturbances in the top layers which

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might have altered the depositional flux.

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Lake Lingge Co is a stable lake with a surface area over 100 km2 and incorporates large

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terrestrial catchment 42. Therefore, pollutants deposited in the catchment can make their way

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into the lake 43. Catchment erosion plays as a catalyst by not only bringing more pollutant into

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the lake but is also a significant contributor to lake sediments 44. We analyzed the mean annual

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precipitation record from five meteorological stations surrounding the Lingge Co region during

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1962-2011. The precipitation at all five stations showed an increasing trend during the recent

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decades (Figure S4) and corresponds well with the increased sedimentation rate and the BC

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flux (Figure S3) which might play a role in increased erosion from the catchment to the lake.

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Thus, besides more BC deposition directly into the lake due to more anthropogenic emission,

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additional BC could have been delivered via erosion from the catchment areas into the lake.

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Lake Gokyo is a moraine-dammed glacier lake, mainly supplied by the summer glacier melt

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though there is no direct surface linkage with the glaciers. Seepage and streams draining from

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the Ngozumpa glacier on the southern slopes of Mt. Cho Oyu connect a series of lakes

258

inferring to the glacier melt contribution to the deposition of BC in the high altitude lakes

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draining the BC from glaciers. Lake Gosainkunda and Lake Gokyo are situated in a close

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geographic proximity but the mean flux ranged (3.55±1.17 g m-2y-1) and (0.08±0.04 g m-2y-1)

261

respectively. Lake Gokyo is situated in a region with minimal anthropogenic activities except

262

from tourism. Thus, the probable route of pollutants is via long range transportation of aerosols

263

and monsoon precipitation with air masses traversing through India and Nepal

40

and the main 41.

46.

The

45

Lake 12

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Gosainkunda, on the other hand, is situated in the region where sources of pollution are biomass

265

burning, vehicle emission, tourism and agricultural activities on daily basis besides long range

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transport 47. This could also possibly contribute to the high BC flux in the lake as compared to

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Lake Gokyo. As mentioned in section 3.1, the high number of pilgrim’s flow as well as daily

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local activity into the lake might have altered the depositional pattern due to which no variation

269

in the flux was observed. However, BC flux trend is similar to the Hg flux reported earlier in

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Lake Gosainkunda30.

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Majority of investigated lakes displayed an increasing trend of flux after the mid-1950s (Figure

272

2b). Our study sites being close to the South Asian source regions, it is likely to receive the

273

pollutants from this rapidly developing region, where BC emission has continuously increased

274

since the 1950s 16. Despite the increasing emission, fluxes from these investigated lakes are

275

significantly lower than reported from other remote lakes in the world.

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3.3. Possible Source Region

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The investigated lakes are located in the remote pristine region with few local emissions and

278

minimal anthropogenic activities. The increasing BC in this region is likely to be transported

279

from the surrounding region of the HTP due to atmospheric circulation and deposition.

280

According to the study by Kang et al.

281

increase of mercury (Hg) since the 1950s was evident and was attributed to the influence of

282

anthropogenic pollutants, especially after World War II, via the long-range transport of

283

contaminants from South Asia. Hg is released during forest and grassland fire as well as

284

burning of field crop residue

285

atmospheric Hg include emissions from wildfires

286

important repercussion for the fate, transport, and deposition 50; especially gas phase Hg which

287

undergoes long-range transport 51. Also, as a by-product of fossil fuel combustion, Hg emitted

48.

30

conducted in the same lakes of the region, a vivid

However, similar to BC emission, natural sources of 49.

Thus, biomass combustion plays an



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by the combustion activities result in the accumulation in sediments during atmospheric fallout

289

52.

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and Hg, they don’t show a good correlation. Therefore, it is possible that a part of Hg might

291

have been lost or transformed during transport and/or deposition processes. However, it is very

292

difficult to assess the loss of Hg by such processes.

293

The Gosainkunda and Gokyo lakes are in the proximity to the Indo-Gangetic Plain (IGP), South

294

Asia which is regarded as the most populated and polluted regions of the world. A recent study

295

have shown an equal contribution from biomass burning and fossil fuel combustion in the

296

Himalayas reflecting the sources of BC from the IGP 53. Modeling the origin of anthropogenic

297

BC suggested South Asian sources is predominant contributing about 50% and 80% of the BC

298

concentration in the monsoon and non-monsoon season respectively, over the TP 54. Real-time

299

observation and simulation studies have revealed that BC aerosols from South Asia and north

300

India were transported to the Himalayas in monsoon and non-monsoon seasons respectively

301

with relatively lower contribution of BC from northwest India and central Asia 55. Cong et al.

302

4

303

Southeast Asia, thus, the increasing concentration and flux in the lakes could be related to long-

304

range transport. These observations are also supported by a study of BC in Pumoyum Co in the

305

TP

306

beginning of 20th century and after 2000 increment in flux was related to enhanced South Asian

307

emission. Nam Co lake located in the inland TP was influenced by the transport of pollutants

308

from Indian sub-continent during south Asian monsoon and the westerly wind traversing

309

through Nepal, India and Pakistan during non-monsoon 4, 56. Other previous studies have also

310

shown the existence of BC over the South Asian region; IGP and southern slopes of the

311

Himalayas 53, 57-59 justifying the atmospheric BC can be transported via the air mass movement

312

to the receptor locations.

Although, both biomass burning and fossil fuel combustion results in the emission of BC

mentioned that the atmospheric BC has its presence in Africa, East China, and South and

23

wherein an increasing concentration has followed European emission during 19th and


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However, emissions from open fires could not be ruled out as many atmospheric compounds

314

including BC are released during open fire combustion 60. HTP region is significantly prone to

315

be affected by the open fires, especially during January to June

316

burning emission, especially from south and south-east Asia, can influence BC over southern

317

edge of TP 63. In addition, local biomass burning contribution is also a significant source of BC

318

in the inland TP 53, especially burning of yak dung for cooking and heating 64. Local biomass

319

sourced BC emission in HTP are capable of transport to adjacent regions influencing high

320

altitude glacial regions

321

focus on South and Southeast Asia as this region is dominated by the presence of aerosols and

322

gases pollutants from these fire activities 66. Figure 3 shows the total number of fire events in

323

each year from 2003 to 2015 in the region as mentioned in section 2.4. The maximum number

324

of fire occurrence was observed in the pre-monsoon. The seasons are defined as: pre-monsoon

325

or summer (March-May), monsoon (June-September), post-monsoon (Oct-Nov) and winter

326

(December-February) in the HTP region. (March – May). The wind pattern over the HTP is

327

dominated by South Asian summer monsoon (June – September) and westerlies during non-

328

monsoon seasons 4, 23, 29. Biomass burning emissions have severely contaminated the glaciers

329

on the southern side of the Himalayas during pre-monsoon season due to the stronger emission

330

sources and short transport distance 67. Unlike Polar and high latitude regions, HTP is situated

331

in the downwind of intense biomass burning regions as suggested by high concentrations of

332

biomass burning tracer

333

biomass burning in the downwind regions which are very common during pre-monsoon season

334

(Figure 3). Earlier studies have suggested that these aerosols are transported to HTP under

335

prevailing meteorological conditions such as winds and mountain valley circulation

336

Putero et al. 57 pointed out the influence of open fires in South Asia and its role in seasonal and

337

inter-annual BC variability over the southern Himalayas where 56% of the polluted days were

65.

67.

61, 62.

An intense biomass

Therefore, we have investigated the fire events with a particular

The emission sources include forest fires in the Himalayas and

53, 68.



Page 16 of 31

338

directly linked with the open fires events over central and south Asia. It further states that 78%

339

of the fires had reached over the southern Himalayas region with a significant BC contribution

340

during dry season. Also, You et al. 67 provided an evidence of increasing fire activites in South

341

Asia utilizing levoglucosan as a biomarker of biomass burning. To further evaluate, we

342

analyzed the fire occurrences of three years (2003-2005) in pre-monsoon (dry season) (Figure

343

S5) which showed an unprecedented open fire events in the south and Southeast Asia. The BC

344

emission from 2003 to 2014 from biomass burning (wildfire) in the region is discussed in

345

section 2.4 and is also shown in Figure 4. The total emission during this period was 1.18 × 109

346

g BC/km2/month. Our study follows the results of Putero et al. 57 and You et al. 67 where the

347

open fire in the south and south-east Asian region was prominent with the possible transport of

348

BC emitted during the fire events into the lakes.

349

3.4. Source Indication Based on Temperature Protocol

350

BCLT-TOR (low temperature) and BCHT-TOR (high temperature) are the two fractions of BC (char

351

and soot, respectively) based on the temperature protocol. The ratio of two BC fractions

352

provides a meaningful information about sources of BC aerosol 11. BCLT-TOR is defined as the

353

difference of BC liberated at 580 ºC in He/O2 condition and pyrolyzed carbon fraction whereas

354

BCHT-TOR is the sum of BC released at 740 and 840 ºC 4. Generally, BCLT-TOR represents

355

combustion debris from biomass and fossil fuel while retaining the original structural

356

information of the source material and BCHT-TOR is formed through the condensation of gas

357

emitted from combustion 4, 23.

358

BCLT-TOR to BCHT-TOR ratio varied from 0.20 to 5.43 with a mean value of 2.23 ± 1.82 from the

359

lakes in the southern Himalayas whereas it varied from 0.13 to 4.78 with a mean value of 1.61

360

± 1.18 in the TP lakes. The mean BCLT-TOR to BCHT-TOR ratio and its temporal trend as well as

361

comparison with studies in the TP are shown in Figure 5. Our result indicates BCLT-TOR as the


Page 17 of 31


362

dominant constituent of BC demonstrating biomass burning to be a major contributor to BC in

363

the lakes and in agreement with the studies conducted in lake sediments in the TP

364

Temporally, BCLT-TOR to BCHT-TOR ratio decreased gradually in Gosainkunda, Lingge Co and

365

Tanglha sediments (Figure 5 (b)) reflecting the energy usage from low temperature to high-

366

temperature combustion. This pattern is consistent with Nam Co in the region 4. Soot represents

367

high-temperature combustion and have submicron particle size distribution. Therefore, it can

368

be efficiently transported and dispersed in the atmosphere both on a regional and global scale

369

69

370

deposition of soot fraction of BC in BCLT-TOR to BCHT-TOR ratio.

371

Our study supports the proposition that sediments can be used to infer the transport of pollutants

372

and reconstruct historical BC concentration trend and emission pattern. The similar trend of

373

BC concentration in all the HTP lakes suggests similar emission sources and transport pattern

374

as well as representing the impact of increasing contribution from biomass burning from south

375

Asia, especially after industrialization. However, local BC emission sources (e.g., the majority

376

of the population rely on biofuel for heating and cooking) could not be ignored over the HTP

377

region. Identifying BC sources is crucial for better understanding of emission, transport

378

mechanism as well as its light absorption properties. The booming South Asian economy leads

379

to an increased energy consumption and emission of BC exerting implications on radiative

380

forcing and climate change in the HTP region. This pursues for the attention in the formulation

381

of an effective mitigation strategies for BC emission sources in south Asian region.

382

Acknowledgments

383

This work was supported by the Chinese Academy of Sciences (XDA20040501, QYZDJ-

384

SSWDOC039), the National Natural Science Foundation of China (41603129, 41701074, and

385

41705132) and the State Key Laboratory of Cryospheric Science (SKLCS-ZZ-2017). Bigyan

4, 23.

therefore indicating the lakes are influenced by atmospheric transport of pollutants and



Page 18 of 31

386

Neupane is supported by UCAS Scholarship for International PhD students. Kirpa Ram thanks

387

Banaras Hindu University for granting the study leave and Chinese Academy of Science for

388

international visiting scholar support under PIFI (2018VCC0005) program. We would like to

389

thank the three reviewers for their critical comments and valuable suggestions which helped us

390

to improve the paper.


Page 19 of 31


391

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392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438

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590 591


organic carbon, black carbon, and polycyclic aromatic hydrocarbons in rural low-altitude and remote high-altitude lakes. Journal of environmental monitoring 2011, 13, (5), 1316-1326.

592



Page 24 of 31

593 594 595 596

Figure 1. Location map showing the lake sampling sites in Nepal Himalayas and Tibetan Plateau (TP). The location of other lakes in the TP (for BC comparison) are shown for reference.


Page 25 of 31


597 598 599

Figure 2(a). Temporal variability of BC concentration (mg/g) and flux (g m-2y-1) of the sediment cores from the Himalayas (i, ii) and the Tibetan Plateau (iii – vi).

600



Page 26 of 31

601 602 603

Figure 2(b). Temporal variability of BC flux (g m-2y-1) of the sediment cores from the HTP. The BC flux data are presented separately in order to avoid overlap in BC fluxes.

604


Page 27 of 31


605

606 607

Figure 3. Time series of monthly number of open fire occurrence events for the regions (5ºN -

608

65ºE, 55ºN - 135ºE) considered from 2003 to 2015 in this study. The fire count data is acquired

609

from MODIS, Aqua and Terra satellites.

610



Page 28 of 31

611 612

Figure 4. BC emission from 2003 to 2014 from biomass burning (wildfire) in the regions

613

(5°N – 65°E, 55°N – 135°E), acquired from Peking University (PKU) Inventory

614

(inventory.pku.edu.cn/).

615


Page 29 of 31


616 617

Figure 5. BCLT-TOR and BCHT-TOR ratio in the six lakes from the Himalayas and the Tibetan

618

Plateau (TP), (a) depicts the mean BCLT-TOR to BCHT-TOR ratio and compared with other lakes

619

in the TP (in shaded portion). Error bar represents one standard deviation, (b) Trend in BCLT-

620

TOR and

BCHT-TOR ratios in six lakes. 29 ACS Paragon Plus Environment


Page 30 of 31

621

Table 1. BC concentrations (mg g−1) and fluxes (g m−2 yr−1) in lake sediment cores from the

622

HTP compared with other lakes around the world. The method employed in this study and

623

other parts of TP is TOR - IMPROVE

Lakes

Location

Elevation

Latitude

Longitude

(m.a.s.l)

(N)

(E)

Age (AD)

BC

BC Flux

concentration

(Mean±SD)

Reference

(Mean±SD) Gokyo

Gosainkunda

Ranwu

Qiangyong Co

Tanglha

Lingge Co

Nepal

Nepal

TP

TP

TP

TP

4750

4390

3800

4866

5152

5051

4722

27º57.063’

28º5.717’

29º26.433’

28º53.409’

32º54.209’

33º49.85’

86º41.414’

85º39.017’

96º47.783’

90º13.558’

91º57.162’

88º36.15’

1853 - 2005

0.04 – 0.30

0.02 – 0.21

(0.16 ± 0.07)

(0.08±0.04)

9.06 – 64.5

1.35 – 5.78

(42.45±16.70)

(3.55±1.17)

1.15 – 2.05

1.16 – 10.46

(1.53±0.21)

(6.42±2.61)

1.53 – 2.58

0.86 – 1.83

(2.03±0.30)

(1.37±0.26)

0.81 – 1.27

0.23 – 0.60

(0.97±0.12)

(0.39±0.1)

0.14 – 0.63

0.01 – 0.14

(0.27±0.14)

(0.06±0.03)

1857-2009

0.49-1.09

0.12-0.44

4

1895 - 2010

1883 - 2015

1922 - 2011

1890 - 2011

1869 - 2011

This study

ˮ

ˮ

ˮ

ˮ

ˮ

Nam Co

TP

Pumoyum Co

TP

1860-2010

0.46-1.48

0.09-0.61*

23

Qinghai(north)

TP

1857-2009

0.49-1.09

0.12-0.44

21

Qinghai(south)

TP

1775-2003

0.39-0.61

0.22-0.36

21

Fennoscandian Arctic**

Northern Finland

144-679

1830-2012

0.52-5.1

0.02-0.5

26

West Pine Pond***

New York State

484

1835-2005

0.6-8

0.26-0.77

14

Slovenian Alpine Lakes**

Alps, Slovenia

1383-2150

1800-1998

1-11

0.3-1.3

24

Engstlen**

Alps, Switzerland

1850

1963-2008

1.5-3.3

2.1-7.4

70

Stora Frillingen**

Aspvreten,

1000-2005

1.82-2.95

0.05-0.40

36

Sweden Flux in mg cm-2y-1, ** CTO-375 method, ***TOT-STN method

*

624


Page 31 of 31

625


Graphical Abstract

626 627 628 629


Historical Black Carbon Reconstruction from the Lake Sediments of

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