Characterization of Tibetan soil as a source or sink of atmospheric

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Environmental Processes

Characterization of Tibetan soil as a source or sink of atmospheric persistent organic pollutants: Seasonal shift and impact of global warming Jiao Ren, Xiao-ping Wang, Ping Gong, and Chuanfei Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b00698 • Publication Date (Web): 01 Mar 2019 Downloaded from http://pubs.acs.org on March 3, 2019

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Environmental Science & Technology

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Characterization of Tibetan soil as a source or sink of atmospheric

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persistent organic pollutants: Seasonal shift and impact of global

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warming

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Jiao Ren1,3, Xiaoping Wang1, 2, 4*, Ping Gong1,2, Chuanfei Wang1,2 1

Key Laboratory of Tibetan Environment Changes and Land Surface Processes,

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Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing,

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100101, China

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2

Sciences, Beijing, 100101, China

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3

Research Institute of Transition of Resource-Based Economics, Shanxi University of Finance and Economics, Taiyuan 030006, Shanxi, China

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CAS Center for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of

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University of Chinese Academy of Sciences, Beijing 100049, China

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* Corresponding author address:

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E-mail: [email protected]

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Tel: +86-10-84097120

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Fax: +86-10-84097079

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ACS Paragon Plus Environment


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Abstract

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Background soils are reservoirs of persistent organic pollutants (POPs). After decades

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of reduced primary emissions, it is now possible that the POPs contained in these

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reservoirs are being remobilized because of climate warming. However, a

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comprehensive investigation into the remobilization of POPs from background soil on

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the largest and highest plateau on Earth, the Tibetan Plateau (TP), is lacking. In this

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study, a sampling campaign was carried out on the TP at three background sites with

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different land cover types (forest, meadow and desert). Field measurements of the

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air–soil exchange of POPs showed that previous prediction using empirical models

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overestimated the values of the soil–air partitioning coefficient (KSA), especially for

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chemicals with KOA > 9. The direction of exchange for γ-HCH, HCB and PCB-28

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overlapped with the air–soil equilibrium range, but with a tendency for volatilization.

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Their emission fluxes were 720, 2935 and 538 pg m−2 day−1, respectively, and were

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similar in extent to those observed for background Arctic soil in Norway. Nam Co and

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Ngari are also permafrost regions, and most chemicals at these two sites exhibited

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volatilization. This is the first result showing that permafrost can also emit POPs.

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Seasonally, we found that chemicals tended to be re-emitted from soils to the

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atmosphere in winter and deposited from the air to the soil in summer. This finding is

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opposite to most previous results, possibly because of the higher air–soil

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concentration gradient caused by the prevailing transport of POPs in summer. Climate

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warming exerts a strong influence on air–soil exchange, with an increase of 1°C in

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ambient temperature likely leading to an increase of Tibetan atmospheric inventories

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of POPs by 60%–400%.

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Introduction

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Organochlorine pesticides (OCPs) and polychlorinated biphenyls (PCBs) are classic

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persistent organic pollutants (POPs), characterized by environmental persistence,

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semivolatility, high toxicity, and adverse effects on ecosystems and humans.

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Long-range atmospheric transport (LRAT) has resulted in a ubiquitous distribution of

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POPs, which has attracted concern globally.1 Over the last two decades, regional and

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international regulatory actions have been implemented to reduce the primary sources

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of POPs.2, 3 However, studies have found that existing reservoirs of POPs, such as

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water bodies and soils, are becoming “secondary sources” that release legacy POPs

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back into the environment.4-6 The key process controlling the global cycling and

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redistribution patterns of POPs is their air–surface exchange.7, 8

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Soils are the primary reservoir for POPs in the terrestrial environment, making a

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major contribution to their global inventory.9, 10 Results of air–soil exchange display

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significant regional differences, driven by various factors including emission sources,

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atmospheric transport, temperature, land-cover types and soil properties.5, 11 Heavily

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polluted soils in urban,12, 13 agricultural

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regarded as secondary sources of POPs. By contrast, some remote soils are possessing

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the capacity to receive POPs via atmospheric deposition (i.e., as sinks). 5, 7 However,

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with the warming of the climate, net volatilization from soils in the European

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background region has been observed via in-situ air–soil measurements.19 Similarly,

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the potential remobilization of POPs has also been observed in the remote Arctic20 and

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Antarctic21, highlighting the perturbations of climate change on the re-emission of

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POPs from remote background soil.

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Rising temperatures will enhance the mobility of POPs in the environment and

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increase their volatilization from surface media.6, 22 In the Antarctic, for example, an

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increase in temperature of 1°C resulted in an increase in atmospheric PCB levels by

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21%–45%.21 Meanwhile, climate warming also increases the content of organic

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carbon in the soil and hinders the remobilization of POPs to the atmosphere by

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and industrial areas17, 18 are generally

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reducing their fugacity in soil.21 Therefore, the response of the cycling of POPs to

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climate change varies significantly in different regions, and relevant research on this

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aspect is limited.

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As one of the most sensitive regions to global change, the Tibetan Plateau (TP), often

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referred to as Earth’s “third pole”, is generally regarded as a final sink for POPs.

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Wang et al.23 assessed the air–soil exchange status of POPs across the TP by

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combining annually averaged air and soil concentration data, which provided an

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overview of exchange directions but could not represent the real and in-situ air–soil

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interface exchange patterns. As a remote region, the TP mainly receive POPs via

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LRAT, with air masses dominated by maritime air from the Indian Ocean in the

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summer and continental air from central Asia in the winter. The LRAT of POPs also

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exhibits obvious seasonal change (high concentration in monsoon season, and low

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concentration in non-monsoon season).24-26 Tibetan soils are mainly permafrost, with

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the surface soil layer on top of the permafrost thawing in summer, and refreezing in

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winter. This raised questions whether the soil on the TP is volatizing POPs to the

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atmosphere and, whether the role played by soil (i.e., source or sink) shows seasonal

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changes. Moreover, previous studies haven’t given enough attention to the measured

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soil–air partitioning coefficient (KSA) under cold temperature and on air-permafrost

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interface.21,

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The TP has experienced significant warming in the past several decades;28, 29 but

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meanwhile, it remains unclear regarding the quantities of POPs that will be

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evaporated under the present and future climate change scenarios. Thus, to conduct

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in-situ monitoring and make future predictions regarding the interface exchange of

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atmospheric POPs in Tibetan soils is very essential, as doing so will provide a more

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accurate and comprehensive understanding of the role played by remote background

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soil in the global fate of POPs.

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In this study, soil fugacity samplers were employed to investigate the in-situ air–soil

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exchange of POPs on the TP. Three sites (forest, meadow and desert), representing the

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dominant land-cover types of the TP, were chosen, among which the latter two sites

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Actually, KSA is crucial in assessing the environmental fate of POPs.

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were in permafrost regions (soil at or below 0°C for at least two consecutive years).30

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The objectives of this study were to (i) assess the in-situ seasonal patterns of the

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air–soil exchange of POPs (e.g., directions and fluxes) over different land-cover types;

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(ii) derive the air–soil partitioning coefficients in the TP environment and clarify the

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relevant controlling factors; and (iii) estimate the extent of the TP serving as a

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secondary source of POPs under present temperatures and future warming conditions.

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2. Materials and methods

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2.1 Site description and sampling design

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Given the vastness of the TP (> 2.5 × 106 km2), it contains a diverse range of land

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cover types across its different regions, meaning the status of the air–surface exchange

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of POPs may also vary within the TP. Thus, three background sites, with different

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land-cover types, were chosen in this study; namely, Lulang (29.77°N, 94.73°E; 3300

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m; southeastern TP; forest), Nam Co (30.77°N, 90.96°E; 4730 m; central TP; alpine

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meadow), and Ngari (33.39°N, 79.70°E; 4270 m; western TP; alpine desert). A map of

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the sampling sites is provided in Figure 1, and further detailed information is provided

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in the supporting information (SI, Table S1). Along with the changes in land cover

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from east to west, the underlying soils also vary, with the content of soil organic

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carbon (SOC) typically ranging from 24% ± 8.1% at Lulang, to 1.5% ± 0.4% at Nam

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Co, and then to 0.7% ± 0.2% at Ngari. Moreover, the annual soil temperature at Nam

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Co and Ngari is around 0°C, categorizing them as permafrost sites (Table S1).

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At each site, two plots within 100 m from each other were selected for duplicate

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surface air and soil sampling. The surface air equilibrated with soil was collected

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using a soil fugacity sampler proposed by Cabrerizo et al.31, which has been

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previously used in both temperate32,

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sampler consisted of a stainless steel plate with a large surface area of 1 m2, located 3

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cm above the soil surface. The air was sampled using a polyurethane foam (PUF; 3

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cm diameter × 7.5 cm) with a slow flow rate of 4 L min−1 and a 5-day sampling period

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in this study, obtaining average air volume of about 30 m3. The main advantage of this

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and polar regions20,

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21

. The soil fugacity


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sampler is that the sampled gas phase was in contact with the soil for sufficient time

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to reach equilibrium, thus allowing for an accurate determination of the soil fugacity

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of POPs.21 Simultaneously, active air samplers (AAS) were deployed at all three sites

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to measure the ambient air concentrations at a height of 1.5 m. A final air volume of

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~600 m3 was sampled and the gas-phase POPs were collected on a PUF plug (6 cm

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diameter × 7.5 cm) in the AAS. After air sampling, topsoil was taken at each plot by

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gently collecting the uppermost 1 cm of soil under the soil fugacity sampler.

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For all three sites, two sampling periods (summer and winter) were designed to

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monitor the seasonal variations of air–soil exchange. Details for each sampling period

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are provided in Table S2. In total, 46 surface air, 40 soil, and 18 ambient air samples

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were collected. The samples were stored in freezers at −20°C until analysis.

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2.2 Sample extraction, cleanup and analysis

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Both air and soil samples were analyzed in this study; the details of sample extraction

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and cleanup have been published previously.23,

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Soxhlet-extracted using dichloromethane (DCM) for 24 h and spiked with a mixture

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of recovery surrogates [2,4,6-trichlorobiphenyl (PCB-30) and mirex; PCB-30 is used

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for lower molecular weighted (MW) OCPs and PCBs, while mirex is used for heavy

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MW PCBs].19,

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consisting of anhydrous sodium sulfate, deactivated alumina, and deactivated silica

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gel, then further cleaned using gel permeation chromatography (containing 6 g

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Bio-beads SX3). For soil samples, concentrated sulfuric acid was used to remove

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remaining lipids and waxes. Finally, the samples were concentrated in 100 µL of

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dodecane containing known quantities of pentachloronitrobenzene and PCB-209 as

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internal standards. POPs were analyzed on a gas chromatograph with an ion-trap mass

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spectrometer (GC-MS, Finnigan Trace GC/PolarisQ) operating under an MS-MS

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mode. Further information on the chromatographic conditions is provided in Text S1.

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The target compounds in the present study included: hexachlorocyclohexanes (HCHs,

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including α-HCH, β-HCH, γ-HCH and δ-HCH), hexachlorobenzene (HCB), DDTs

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(o,p'-DDE, p,p'-DDE, o,p'-DDT, and p,p'-DDT) and six indicator PCBs (PCB-28,

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Briefly, each sample was

The extract was first purified on a chromatography column

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PCB-52, PCB-101, PCB-153, PCB-138, and PCB-180).

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2.3 Quality assurance/quality control

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All analytical procedures were monitored using strict quality assurance/quality control

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measures. Prior to field sampling, PUF was pre-cleaned by Soxhlet extraction using

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DCM for 16 h. Fourteen field blanks for PUFs and six procedural blanks were set to

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monitor the possible contamination during transport, storage, and analysis. Blanks

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were extracted and analyzed along with the samples in the same way. The individual

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POP concentrations in these blanks are given in Table S3 and only trace amounts of

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POPs were detected in blanks at low concentrations. The method detection limits

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(MDLs) were defined as the mean blank concentration plus three times its standard

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deviation; the derived MDLs for air and soil samples are given in Table S4. The

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recoveries ranged from 68% to 105% for PCB 30 and 74% to 113% for mirex. The

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concentrations reported here were recovery and blank corrected.

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2.4 Calculation of fugacity

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The ambient air fugacity (fa, Pa) and soil fugacity (fs, Pa) were calculated by32 fa =10-12 CA RT/MW

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(1)

and fs =10-12 CSA RT/MW,

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(2)

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respectively, where CA is the measured ambient air concentration at 1.5 m height (pg

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m−3), R is the gas constant (8.314 Pa m3 mol−1 K−1), T is the air temperature (K), MW

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is the chemical molecular weight (g mol−1), and CSA (pg m−3) is the gas-phase surface

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air concentration measured by the soil fugacity sampler.

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Results and discussion

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3.1 Atmospheric concentrations

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The ambient air concentrations of OCPs and PCBs (pg m−3) at each sampling site and

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in each sampling period are listed in Table S5, and a summary of the minimum,

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maximum and mean concentrations is provided in Table 1. In the present study, HCB 7



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and DDTs were the most dominant chemicals in the atmosphere, with concentration

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ranges of 3.1–80 pg m−3 and below the detection limit to 52 pg m−3, respectively,

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followed by HCHs (1.1–18 pg m−3) and PCBs (0.3–33 pg m−3). These values are

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roughly comparable to previous reports for the atmosphere in the TP region35-37 (Table

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S6). From a global perspective, HCB was slightly lower or comparable to the

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circumpolar average of 56 pg m−3 and its abundance in the air indicated the

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remoteness of the TP.38 The average value of DDTs in this study was around tens of

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pg m−3, which is similar to the value for European alpine regions (0.5–24.2 pg m−3)39,

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but one or two orders of magnitude higher than those reported for the remote

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Antarctic40 and Arctic41 (Table S6). This may have been caused by the closer

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proximity of the TP to DDT source regions (India), and thus the greater vulnerability

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to Indian-monsoon-driven atmospheric transport.

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Regarding the seasonal variations of atmospheric concentrations, OCPs in the summer

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generally displayed higher levels compared to those in the winter (Table S7), except

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for HCB. Temperature is usually regarded as a primary controller during the transport,

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deposition/volatilization and degradation processes, and thus a strong influence on the

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atmospheric concentrations of POPs.21,

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temperatures during sampling periods was wide, i.e., −8.6°C to 14.4°C. The

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temperature dependence of atmospheric POP concentrations was tested, and the

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results showed significant positive correlations between the temperature gradient and

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concentration increases for α-HCH, γ-HCH, o,p'-DDT and p,p'-DDT (Figure S1; p
0.05). This can be considered as evidence

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of re-evaporation from soil surfaces. On the other hand, seasonal LRAT can also result

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in the seasonality of the atmospheric concentrations over the TP.25 Given all this, both

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the atmospheric transport and local volatilization may have had a combined effect on

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the ambient concentrations of POPs in the present study, and the seasonality of

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secondary sources needs to be clarified further.

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3.2 Concentrations in soil and relationship with SOC

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In this study, the range of ambient

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The full dataset for OCP and PCB concentrations in soils (pg g−1 dw) is available as

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Table S8, and a summary of descriptive statistical data is provided in Table 1.

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Concentrations of HCHs, HCB, DDTs and PCBs in the soils were in the range from

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BDL to 784 pg g−1 dw, BDL to 2813 pg g−1 dw, 85 to 5321 pg g−1 dw, and 7.0–239 pg

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g−1 dw, respectively. The dominant species were p,p'-DDT, p,p'-DDE, o,p'-DDT,

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β-HCH, and PCB-28. These concentration ranges and compositional profiles are in

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line with those of Tibetan soils observed by previous studies.23, 42

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Among the three sampling sites, Lulang displayed the highest soil concentrations of

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OCPs and PCBs (Figure S2), which is comparable to those of the Pyrenees in

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Europe43 (Table S9). Meanwhile, Nam Co was cleanest, with lowest concentrations

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that were similar to the values reported for the remote Antarctica soil44 (Table S9).

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Given organic matter plays a key role in binding POPs in soils, this discrepancy may

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largely have been due to the differences in SOC between the sampling sites.

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Significant positive correlations between CS and SOC were found for all investigated

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compounds (Figure S3; p < 0.05), with R2 ranging from 0.09 (DDTs), 0.27 (PCBs) to

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0.82 (HCHs). Such correlation has also been observed in other background soils,

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demonstrating the common role of SOC as a descriptor of the burdens of POPs in

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soils.45,

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concentrations, such as the soil type, SOC quality, compound structure, and so on.9, 47

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In comparison with the seasonal variations in ambient air concentrations, there were

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no statistically significant differences in soil concentrations of POPs in summer and

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winter (Table S10; p > 0.05). This was probably due to the higher fugacity capacity in

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soils than that of the atmosphere;10, 20 thus, the soil concentrations did not change

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significantly with the seasons.

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3.3 Soil–air partitioning coefficient

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The KSA is crucial in assessing the interface exchange of POPs and their

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environmental fate, which describes the equilibrium partitioning between the air and

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the soil. Due to the difficulties in measuring the air–soil equilibrium in the field, most

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previous studies have predicted KSA using its empirical relationship with the

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Additionally, other factors may also exert an influence on soil

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octanol-air partitioning coefficient (KOA).27,

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involved in such a calculation process. In the present study, by sampling the air

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equilibrated with the soil using the soil fugacity sampler, accurate values of in-situ

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KSA (L kg−1) in each sampling period were obtained using KSA-real =106

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48-49

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CS CSA

However, there is great uncertainty

,

(3)

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where CS is the soil concentration (pg g−1 dw), and CSA is the gaseous concentration

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(pg m−3; Table S11) that has been equilibrated with the soil measured by the soil

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fugacity sampler. CSA was calculated as the average of the two samplers’

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measurements at each site. The obtained KSA values in the present study spanned

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several orders of magnitude (Table S12), with the minimum value found for γ-HCH

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(log KSA-real = 4.6) and the maximum for p,p′-DDT (log KSA-real = 8.8). Overall, the

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KSA-real values at Lulang were significantly higher than those of Ngari and Nam Co

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(Table S12), which might have been due to the richer content of SOC in the forest

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soils.

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To identify the primary environmental factors affecting the soil–air partitioning of

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POPs on the TP, the influence of temperature (T) and SOC quantity on KSA-real was

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investigated separately. γ-HCH, HCB, o,p′-DDE and PCB-28 exhibited significant

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positive correlations between log KSA-real and 1/T (Figure 2), which is in accordance

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with other studies.27, 33 However, a negative correlation was found for p,p′-DDT. This

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indicated that the effect of temperature on KSA-real in the present study was

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compound-specific. Significant correlations between log KSA-real and log SOC were

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found for most target compounds (Figure 2). These results confirmed that the air–soil

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partitioning of POPs in the TP region also depends on the temperature and soil

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properties.27

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Accurate knowledge of the KSA is crucial for modeling the environmental fate of

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POPs at the regional and global scale. Due to a lack of measured data under field

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conditions, KSA estimated by predictive methods are often used.5, 48 We compared the

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measured KSA-real values with those predicted from the most frequently-used equation

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derived by Hippelein and Mclachlan:50 10


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KSA-Predicted =0.411fOC ρs KOA,

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(4)

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where fOC is the fraction of organic carbon in soil, ρs is the soil density (kg L−1) and

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KOA is the octanol–air partition coefficient. As shown in Figure 3, good agreement

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was found between the measured and predicted KSA for the compounds with log KOA

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< 9 (α-HCH, γ-HCH and HCB). However, with the increase of log KOA, the gap

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between the predicted and measured KSA showed a tendency to become wider, which

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would cause an overestimation of log KSA by 0.2–4 orders of magnitude (Figure 3). A

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similar result was observed for higher chlorinated PCBs and PBDEs by Ddgrendele et

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al.19, which was explained by the slow diffusion across air–soil boundary layers and

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lack of equilibrium for these compounds with log KOA > 9. This should be taken with

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caution when using the prediction method for KSA to assess the air–soil exchange

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status, in particular for regions with a cold climate. Under lower temperatures, the

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temperature-corrected log KOA would be higher, and the difference between the

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KSA-Predicted and its true value (KSA-real) would be larger, thus resulting in inaccurate

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estimation of air–soil fugacity gradients and the environmental fate of POPs.

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In Figure 3, if we take the data from the polar regions (KSA-real values in the Arctic20

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and Antarctic21 obtained by the same fugacity sampler; the green points in Figure 3)

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into account, we find all data in Figure 3 exhibit a significant correlation between the

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measured KSA-real values and temperature-corrected KOA (R2 = 0.30, p < 0.001; the

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equations for calibrating KOA can be found in previous studies51-54). The correlation

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was derived as follows:

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logKSA-real =0.45 × logKOA + 1.96.

(5)

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Given the fitting equation for KSA was derived from the measured values in the

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Tibetan plateau, the Arctic and the Antarctic, these studies covered the air temperature

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ranging from -9 to 17°C.20, 21 Thus, we suggest the usage of this formula for relatively

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cold regions, with temperature around -10~15°C. This equation provides a new

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empirical formula for obtaining a more realistic KSA, especially for cold regions and

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chemicals with KOA > 9. By using an overestimated KSA-predicted, previous studies have 11



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suggested that Tibetan soils generally serve as sinks of POPs.23 Additionally, Li et al.5

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estimated that soils in most parts of the Arctic are sinks of high chlorinated PCBs,

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based on the traditional predicted KSA (i.e., logKSA-predicted = 9–10 for PCB 101).

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However, based on the above-obtained equation (5), the log KSA-real can only reach

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6–7 for PCB 101, indicating that Li et al.5 largely overestimated the retention

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capacities of cold and remote soils for hydrophobic compounds, and the region

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previously regarded as a “final sink” might only act as a temporary or transient sink.

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Therefore, the role of cold and remote soils as secondary sources of POPs may be

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underestimated and should be re-evaluated.

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3.4 Air–soil exchange direction

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Soil fugacity samplers hold the advantage that the fugacity in soils can be measured

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directly, thus facilitating more reliable results with respect to the direction of air–soil

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exchange.31, 32 In this study, fs and fa were significantly correlated at all three sampling

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sites (p < 0.05; Figure S4), with R2 values of 0.47 (Lulang), 0.64 (Nam Co) and 0.58

312

(Ngari), respectively. This close association implied that the atmospheric levels of

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OCPs and PCBs near the ground were likely controlled by re-emissions from soils.

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To assess the direction of air–soil exchange, the fugacity ratio (fs / fa) has been widely

315

used. When fs / fa > 1, there is net volatilization from the soil into air, whereas values
0.05; Table S10). As a consequence of the coupled

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air–soil interface exchange (deposition in summer), emission–transport–deposition

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events would occur in summer due to the prevailing transport of POPs and higher

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air–soil concentration gradient at Lulang. Whereas, in winter, the LRAT from South

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Asia is weakened, which results in lower atmospheric levels of POPs in the air, and

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thus those POPs previously deposited into Tibetan soils would have a higher tendency

374

to volatilize back to the air. This explains the seasonality in exchange directions

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observed in this study, demonstrating that pollutant sources and atmospheric transport

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exert significant influence on the air–soil exchange process on the TP. This implies

377

that previous thinking regarding the air–soil interface exchange of POPs in the Arctic

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and Antarctic regions, where inputs of POPs strongly rely on LRAT, might be

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incomprehensive; the role played by soils in these regions in the accumulation of

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POPs might also exhibit seasonality.

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3.5 Extent of TP soil as a secondary source

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Quantification of air–soil exchange fluxes is essential to further understand the extent

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of soils as a secondary source/sink of POPs.7, 57 Based on a flux estimation method in 14


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the literature (Text S2),58 the annual average air–soil exchange fluxes for individual

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POPs were calculated and presented in Table S13. We found that the exchange fluxes

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for DDTs and higher chlorinated PCBs in the present study were generally around or

387

lower than 1 pg m−2 h−1, showing a dynamic exchange towards an air–soil equilibrium.

388

Only α-HCH, γ-HCH, HCB and PCB-28 exhibited relatively higher volatilization

389

fluxes, ranging from −10 to 31, 2.7 to 99, 35 to 242, and −3.7 to 82 pg m−2 h−1, with

390

average values of 9.2, 30, 122 and 22 pg m−2 h−1, respectively. These fluxes are one or

391

two orders of magnitude lower than those reported in heavily-polluted regions

392

(460–630 pg m−2 h−1 for γ-HCH at an industrial site in the Mediterranean48; 101–5263

393

pg m−2 h−1 for HCB in urban soils of Nepal59), but slightly higher than those observed

394

at European background sites19 (0.09–0.52 and 0.14–2.18 pg m−2 h−1 for α-HCH and

395

HCB, respectively). Combined with the higher fs / fa ratios of HCHs and HCB, this

396

suggests that Tibetan soils are indeed acting as secondary sources for these more

397

volatile OCPs.

398

With respect to the air–soil exchange fluxes of POPs for the Arctic and Antarctic

399

regions, their values have seldom been reported, with only the air–soil exchange

400

directions of POPs evaluated in previous studies. This makes it difficult to compare

401

the strength of the TP as a secondary source to those of the polar regions. Recently,

402

using the same soil fugacity sampler, Casal et al.20 investigated the in-situ

403

air–soil/snow exchange of POPs at an Arctic site (69°N) in northern Norway. To

404

make a comparison, we estimated the air–soil fugacity gradients and exchange fluxes

405

in the Arctic environment by using the reported POP concentrations and temperatures

406

from their study. The calculated emission fluxes for α-HCH, γ-HCH and HCB ranged

407

from 3.7 to 43, −22 to 84, and −61 to 1911 pg m−2 h−1, respectively; while the

408

volatilization fluxes for PCBs were lower than 1 pg m−2 h−1 for the Arctic (northern

409

Norway). Overall, these exchange fluxes are in a good agreement with the current

410

Tibetan flux results. The same order of magnitude in the emission fluxes for the TP

411

and Arctic demonstrated that their intensity as a secondary source for POPs is

412

comparable.

15



413

3.6 Climate warming and re-emission of POPs from the TP

414

For POPs with a higher tendency towards volatilization, i.e., HCHs, HCB and

415

PCB-28, their concentrations in the surface air (CSA) equilibrated with the soil

416

displayed significant correlations with the air temperature (Table S14). Significant

417

increasing of CSA with an increase in temperature was found for all three sampling

418

regions in this study. From the slope, we can expect that an increase of 1°C in

419

temperature would cause an increase in HCH levels in the near-ground atmosphere by

420

60%–303% at Lulang (forest; Figure S6 and Table S14), 70%–76% at Nam Co

421

(grassland), and 67–466% at Ngari (desert; Table S14). Such tight relationships

422

between the surface air concentrations and temperature suggests that the re-emission

423

of POPs from the soil of the TP is sensitive to climate warming. In a previous study

424

conducted in the Antarctic by Cabrerizo et al.21, an increase of 1°C in temperature was

425

found to cause a 21%–45% increase in PCB concentrations. Compared with this

426

Antarctic study, the volatilization of POPs in the current study for the TP is more

427

distinct, indicating an obvious secondary-source role.

428

During the past 50 years, the temperature increase over the TP has been 0.3–0.4°C per

429

decade, which is in excess of twice the average rate for the Northern Hemisphere in

430

the same period.60 Similarly, the current rate of warming on the Antarctic Peninsula is

431

an increase of about 1°C during a period of two decades. Although the warming rate

432

between the TP and Antarctic is similar, the responses of SOC stocks to an increase in

433

temperature in the TP and Antarctic regions are very different. With a 1°C temperature

434

rise, the SOM in soils of the Antarctic can increase by 0.02% to 0.07%;21 whereas, for

435

the TP, a stable level of SOM was found,61 since the loss of SOM due to the

436

accelerated decomposition associated with warming was counterbalanced by the gain

437

in SOM due to increased vegetation productivity.61 Cabrerizo et al.21 suggested that

438

increased SOM can hinder the volatilization of POPs from Antarctic soil, making it a

439

sink of PCBs. However, based on the probable constant SOM and continuous

440

warming during the next two decades, we predict that Tibetan soil (especially

441

grassland and desert) will continue to be a secondary source of POPs (HCHs, HCB 16


Page 16 of 29

Page 17 of 29


442

and PCB-28). With the thawing and shrinking of permafrost in response to climate

443

warming, the accumulation and release of POPs from permafrost are also needed to

444

investigate in the future.

445 446

Acknowledgements. This study was supported by the Strategic Priority Research

447

Program of the Chinese Academy of Sciences, the Pan-Third Pole Environment Study

448

for a Green Silk Road (Pan-TPE) (XDA2004050202), and the National Natural

449

Science Foundation of China (41671480 and 41807456).

17



450

References:

451

(1) Beyer, A.; Mackay, D.; Matthies, M.; Wania, F.; Webster, E., Assessing long-range transport potential of persistent organic pollutants. Environ. Sci. Technol. 2000, 34, (4), 699-703. (2) Becker, S.; Halsall, C. J.; Tych, W.; Kallenborn, R.; Schlabach, M.; Mano, S., Changing sources and environmental factors reduce the rates of decline of organochlorine pesticides in the Arctic atmosphere. Atmos. Chem. Phys. 2012, 12, (9), 4033-4044. (3) Takazawa, Y.; Takasuga, T.; Doi, K.; Saito, M.; Shibata, Y., Recent decline of DDTs among several organochlorine pesticides in background air in East Asia. Environ. Pollut. 2016, 217, 134-142. (4) Stemmler, I.; Lammel, G., Cycling of DDT in the global environment 1950-2002: World ocean returns the pollutant. Geophys. Res. Lett. 2009, 36. (5) Li, Y. F.; Harner, T.; Liu, L. Y.; Zhang, Z.; Ren, N. Q.; Jia, H. L.; Ma, J. M.; Sverko, E., Polychlorinated Biphenyls in Global Air and Surface Soil: Distributions, Air-Soil Exchange, and Fractionation Effect. Environ. Sci. Technol. 2010, 44, (8), 2784-2790. (6) Ma, J. M.; Hung, H. L.; Tian, C.; Kallenborn, R., Revolatilization of persistent organic pollutants in the Arctic induced by climate change. Nature Climate Change 2011, 1, (5), 255-260. (7) Nizzetto, L.; Macleod, M.; Borga, K.; Cabrerizo, A.; Dachs, J.; Di Guardo, A.; Ghirardello, D.; Hansen, K. M.; Jarvis, A.; Lindroth, A.; Ludwig, B.; Monteith, D.; Perlinger, J. A.; Scheringer, M.; Schwendenmann, L.; Semple, K. T.; Wick, L. Y.; Zhang, G.; Jones, K. C., Past, Present, and Future Controls on Levels of Persistent Organic Pollutants in the Global Environment. Environ. Sci. Technol. 2010, 44, (17), 6526-6531. (8) Cui, S.; Fu, Q.; Li, Y. F.; Ma, J.; Tian, C.; Liu, L.; Zhang, L., Modeling the air-soil exchange, secondary emissions and residues in soil of polychlorinated biphenyls in China. Scientific Reports 2017, 7, (1), 221. (9) Meijer, S. N.; Ockenden, W. A.; Sweetman, A.; Breivik, K.; Grimalt, J. O.; Jones, K. C., Global distribution and budget of PCBs and HCB in background surface soils: Implications or sources and environmental processes. Environ. Sci. Technol. 2003, 37, (4), 667-672. (10) Dalla Valle, M.; Jurado, E.; Dachs, J.; Sweetman, A. J.; Jones, K. C., The maximum reservoir capacity of soils for persistent organic pollutants: implications for global cycling. Environ. Pollut. 2005, 134, (1), 153-164. (11) Ruzickova, P.; Klanova, J.; Cupr, P.; Lammel, G.; Holoubek, I., An assessment of air-soil exchange of polychlorinated biphenyls and organochlorine pesticides across Central and Southern Europe. Environ. Sci. Technol. 2008, 42, (1), 179-185. (12) Chakraborty, P.; Zhang, G.; Li, J.; Sivakumar, A.; Jones, K. C., Occurrence and sources of selected organochlorine pesticides in the soil of seven major Indian cities: Assessment of air–soil exchange. Environ. Pollut. 2015, 204, 74-80.

452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492

18


Page 18 of 29

Page 19 of 29

493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536


(13) Tasdemir, Y.; Salihoglu, G.; Salihoglu, N. K.; Birgül, A., Air–soil exchange of PCBs: Seasonal variations in levels and fluxes with influence of equilibrium conditions. Environ. Pollut. 2012, 169, (15), 90-97. (14) Bidleman, T. F.; Leone, A. D., Soil-air exchange of organochlorine pesticides in the Southern United States. Environ. Pollut. 2004, 128, (1-2), 49-57. (15) Kurt-Karakus, P. B.; Bidleman, T. F.; Staebler, R. M.; Jones, K. C., Measurement of DDT fluxes from a historically treated agricultural soil in Canada. Environ. Sci. Technol. 2006, 40, (15), 4578-4585. (16) Bidleman, T. E.; Leone, A. D.; Wong, F.; Van, V. L.; Szeto, S.; Ripley, B. D., Emission of legacy chlorinated pesticides from agricultural and orchard soils in British Columbia, Canada. Environmental Toxicology & Chemistry 2006, 25, (6), 1448-1457. (17) Wang, Y.; Luo, C.; Wang, S.; Cheng, Z.; Li, J.; Zhang, G., The abandoned e-waste recycling site continued to act as a significant source of polychlorinated biphenyls: An in situ assessment using fugacity samplers. Environ. Sci. Technol. 2016, 50, (16), 8623. (18) Cetin, B.; Yurdakul, S.; Keles, M.; Celik, I.; Ozturk, F.; Dogan, C., Atmospheric concentrations, distributions and air-soil exchange tendencies of PAHs and PCBs in a heavily industrialized area in Kocaeli, Turkey. Chemosphere 2017, 183, 69. (19) Degrendele, C.; Audy, O.; Hofman, J.; Kučerik, J.; Kukučka, P.; Mulder, M. D.; Přibylová, P.; Prokeš, R.; Šáňka, M.; Schaumann, G. E., Diurnal Variations of Air-Soil Exchange of Semivolatile Organic Compounds (PAHs, PCBs, OCPs, and PBDEs) in a Central European Receptor Area. Environ. Sci. Technol. 2016, 50, (8), 4278-4288. (20) Casal, P.; Castro-Jiménez, J.; Pizarro, M.; Katsoyiannis, A.; Dachs, J., Seasonal soil/snow-air exchange of semivolatile organic pollutants at a coastal arctic site (Tromsø, 69°N). Sci. Total Environ. 2018, 636, 1109-1116. (21) Cabrerizo, A.; Dachs, J.; Jones, K. C., Climatic and biogeochemical controls on the remobilization and reservoirs of persistent organic pollutants in Antarctica. Environ. Sci. Technol. 2013, 47, (9), 4299-4306. (22) Ma, J. M.; Cao, Z. H., Quantifying the Perturbations of Persistent Organic Pollutants Induced by Climate Change. Environ. Sci. Technol. 2010, 44, (22), 8567-8573. (23) Wang, X. P.; Sheng, J. J.; Gong, P.; Xue, Y. G.; Yao, T. D.; Jones, K. C., Persistent organic pollutants in the Tibetan surface soil: Spatial distribution, air-soil exchange and implications for global cycling. Environ. Pollut. 2012, 170, 145-151. (24) Wang, X. P.;; Crispin, H.; Garry, C.; Zhiyong, X.; Baiqing, X.; Zhen, Z.; Yonggang, X.; Ralf, E.; Jones, K. C., Accumulation of perfluoroalkyl compounds in tibetan mountain snow: temporal patterns from 1980 to 2010. Environ. Sci. Technol. 2014, 48, (1), 173-181. (25) Sheng, J. J.; Wang, X. P.;; Gong, P.; Joswiak, D. R.; Tian, L.; Yao, T.; Jones, K. C., Monsoon-driven transport of organochlorine pesticides and polychlorinated biphenyls to the tibetan plateau: three year atmospheric monitoring study. 19



537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580

Environ. Sci. Technol. 2013, 47, (7), 3199-208. (26) Wang, X.; Ren, J.; Gong, P.; Wang, C.; Xue, Y.; Yao, T.; Lohmann, R., Spatial distribution of the persistent organic pollutants across the Tibetan Plateau and its linkage with the climate systems: a 5-year air monitoring study. Atmos. Chem. Phys. 2016, 16, (11), 6901-6911. (27) Daviemartin, C. L.; Hageman, K. J.; Chin, Y. P.; Rougé, V.; Fujita, Y., Influence of Temperature, Relative Humidity, and Soil Properties on the Soil–Air Partitioning of Semivolatile Pesticides: Laboratory Measurements and Predictive Models. Environ. Sci. Technol. 2015, 49, (17), 10431-10439. (28) Yang, K.; Wu, H.; Qin, J.; Lin, C. G.; Tang, W. J.; Chen, Y. Y., Recent climate changes over the Tibetan Plateau and their impacts on energy and water cycle: A review. Global Planet. Change 2014, 112, 79-91. (29) Liu, X. D.; Chen, B. D., Climatic warming in the Tibetan plateau during recent decades. International Journal of Climatology 2015, 20, (14), 1729-1742. (30) Ran, Y. H.; Xin, L.; Cheng, G. D.; Zhang, T. J.; Wu, Q. B.; Jin, H. J.; Rui, J., Distribution of Permafrost in China: An Overview of Existing Permafrost Maps. Permafrost & Periglacial Processes 2012, 23, (4), 322-333. (31) Cabrerizo, A.; Dachs, J.; Barcelo, D., Development of a Soil Fugacity Sampler for Determination of Air-Soil Partitioning of Persistent Organic Pollutants under Field Controlled Conditions. Environ. Sci. Technol. 2009, 43, (21), 8257-8263. (32) Cabrerizo, A.; Dachs, J.; Jones, K. C.; Barcelo, D., Soil-Air exchange controls on background atmospheric concentrations of organochlorine pesticides. Atmos. Chem. Phys. 2011, 11, (24), 12799-12811. (33) Cabrerizo, A.; Dachs, J.; Moeckel, C.; Ojeda, M. J.; Caballero, G.; Barcelo, D.; Jones, K. C., Factors Influencing the Soil-Air Partitioning and the Strength of Soils as a Secondary Source of Polychlorinated Biphenyls to the Atmosphere. Environ. Sci. Technol. 2011, 45, (11), 4785-4792. (34) Meijer, S. N.; Halsall, C. J.; Harner, T., .; Peters, A. J.; Ockenden, W. A.; Johnston, A. E.; Jones, K. C., Organochlorine pesticide residues in archived UK soil. Environ. Sci. Technol. 2001, 35, (10), 1989-1995. (35) Gong, P.; Wang, X. P.; Xue, Y. G.; Sheng, J. J.; Gao, S. P.; Tian, L. D.; Yao, T. D., Influence of atmospheric circulation on the long-range transport of organochlorine pesticides to the western Tibetan Plateau. AtmRe 2015, 166, (3), 157-164. (36) Ren, J.; Wang, X.; Wang, C.; Gong, P.; Yao, T., Atmospheric processes of organic pollutants over a remote lake on the central Tibetan Plateau: implications for regional cycling. Atmos. Chem. Phys. 2017, 17, (2), 1401-1415. (37) Wang, C. F.; Wang, X. P.; Gong, P.; Yao, T., Long-term trends of atmospheric organochlorine pollutants and polycyclic aromatic hydrocarbons over the southeastern Tibetan Plateau. Sci. Total Environ. 2017, 624, 241. (38) Su, Y. S.; Hung, H.; Blanchard, P.; Patton, G. W.; Kallenborn, R.; Konoplev, A.; Fellin, P.; Li, H.; Geen, C.; Stern, G.; Rosenberg, B.; Barrie, L. A., Spatial and seasonal variations of hexachlorocyclohexanes (HCHs) and hexachlorobenzene (HCB) in the Arctic atmosphere. Environ. Sci. Technol. 2006, 40, (21), 20


Page 20 of 29

Page 21 of 29

581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624


6601-6607. (39) Kirchner, M.; Jakobi, G.; Körner, W.; Levy, W.; Moche, W.; Niedermoser, B.; Schaub, M.; Ries, L.; Weiss, P.; Antritter, F., Ambient Air Levels of Organochlorine Pesticides at Three High Alpine Monitoring Stations: Trends and Dependencies on Geographical Origin. Aerosol & Air Quality Research 2016, 16, (3), 738-751. (40) Bigot, M.; Muir, D. C.; Hawker, D. W.; Cropp, R.; Dachs, J.; Teixeira, C. F.; Bengtson, N. S., Air-seawater exchange of organochlorine pesticides in the Southern Ocean between Australia and Antarctica. Environ. Sci. Technol. 2016, 50, (15), 8001. (41) Bossi, R.; Vorkamp, K.; Skov, H., Concentrations of organochlorine pesticides, polybrominated diphenyl ethers and perfluorinated compounds in the atmosphere of North Greenland. Environ. Pollut. 2016, 217, (2), 4-10. (42) Tao, S.; Wang, W. T.; Liu, W. X.; Zuo, Q. A.; Wang, X. L.; Wang, R.; Wang, B.; Shen, G. F.; Yang, Y. H.; He, J. S., Polycyclic aromatic hydrocarbons and organochlorine pesticides in surface soils from the Qinghai-Tibetan plateau. J. Environ. Monit. 2011, 13, (1), 175-181. (43) Grimalt, J. O.; van Drooge, B. L.; Ribes, A.; Vilanova, R. M.; Fernandez, P.; Appleby, P., Persistent organochlorine compounds in soils and sediments of European high altitude mountain lakes. Chemosphere 2004, 54, (10), 1549. (44) Zhang, Q.; Chen, Z.; Li, Y.; Wang, P.; Zhu, C.; Gao, G.; Xiao, K.; Sun, H.; Zheng, S.; Liang, Y., Occurrence of organochlorine pesticides in the environmental matrices from King George Island, west Antarctica. Environ. Pollut. 2015, 206, 142-149. (45) Sweetman, A. J.; Valle, M. D.; Prevedouros, K.; Jones, K. C., The role of soil organic carbon in the global cycling of persistent organic pollutants (POPs): Interpreting and modelling field data. Chemosphere 2005, 60, (7), 959-72. (46) Nam, J. J.; Gustafsson, O.; Kurt-Karakus, P.; Breivik, K.; Steinnes, E.; Jones, K. C., Relationships between organic matter, black carbon and persistent organic pollutants in European background soils: Implications for sources and environmental fate. Environ. Pollut. 2008, 156, (3), 809-817. (47) Cabrerizo, A.; Dachs, J.; Moeckel, C.; Ojeda, M. J.; Caballero, G.; Barcelo, D.; Jones, K. C., Ubiquitous Net Volatilization of Polycyclic Aromatic Hydrocarbons from Soils and Parameters Influencing Their Soil-Air Partitioning. Environ. Sci. Technol. 2011, 45, (11), 4740-4747. (48) Odabasi, M.; Cetin, B., Determination of octanol-air partition coefficients of organochlorine pesticides (OCPs) as a function of temperature: application to air-soil exchange. J. Environ. Manage. 2012, 113, (1), 432-439. (49) Wang, Y.; Luo, C.; Wang, S.; Liu, J.; Pan, S.; Li, J.; Ming, L.; Zhang, G.; Li, X., Assessment of the air-soil partitioning of polycyclic aromatic hydrocarbons in a paddy field using a modified fugacity sampler. Environ. Sci. Technol. 2015, 49, (1), 284-91. (50) Hippelein, M.; McLachlan, M. S., Soil/air partitioning of semivolatile organic compounds. 1. Method development and influence of physical-chemical 21



625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662

properties. Environ. Sci. Technol. 1998, 32, (2), 310-316. (51) Xiao, H., Li, N.Q., Wania, F., Compilation, evaluation, and selection of physical-chemical property data for alpha-, beta-, and gamma-hexachlorocyclohexane. J. Chem. Eng. Data. 2004, 49, 173-185. (52) Shoeib, M., Harner, T., Using measured octanol-air partition coefficients to explain environmental partitioning of organochlorine pesticides. Environ. Toxicol. Chem. 2002, 21, 984-990. (53) Harner, T., Bidleman, T.F., Measurements of octanol-air partition coefficients for polychlorinated biphenyls. J. Chem. Eng. Data. 1996, 41, 895-899. (54) Paasivirta, J., Sinkkonen, S.I., Environmentally relevant properties of all 209 polychlorinated biphenyl congeners for modeling their fate in different natural and climatic conditions. J. Chem. Eng. Data. 2009, 54, 1189-1213. (55) Obrist, D.; Agnan, Y.; Jiskra, M.; Olson, C. L.; Colegrove, D. P.; Hueber, J.; Moore, C. W.; Sonke, J. E.; Helmig, D., Tundra uptake of atmospheric elemental mercury drives Arctic mercury pollution. Nature 2017, 547, (7662), 201. (56) Schuur, E. A. G.; Mcguire, A. D.; Sch?Del, C., .; Grosse, G., .; Harden, J. W.; Hayes, D. J.; Hugelius, G., .; Koven, C. D.; Kuhry, P., .; Lawrence, D. M., Climate change and the permafrost carbon feedback. Nature 2015, 520, (7546), 171-179. (57) Koblizkova, M.; Ruzickova, P.; Cupr, P.; Komprda, J.; Holoubek, I.; Klanova, J., Soil Burdens of Persistent Organic Pollutants: Their Levels, Fate, and Risks. Part IV. Quantification of Volatilization Fluxes of Organochlorine Pesticides and Polychlorinated Biphenyls from Contaminated Soil Surfaces. Environ. Sci. Technol. 2009, 43, (10), 3588-3595. (58) Komprda, J.; Komprdová, K.; Sáňka, M.; Možný, M.; Nizzetto, L., Influence of Climate and Land Use Change on Spatially Resolved Volatilization of Persistent Organic Pollutants (POPs) from Background Soils. Environ. Sci. Technol. 2013, 47, (13), 7052-7059. (59) Pokhrel, B.; Gong, P.; Wang, X.; Chen, M.; Wang, C.; Gao, S., Distribution, sources, and air-soil exchange of OCPs, PCBs and PAHs in urban soils of Nepal. Chemosphere 2018, 200, 532-541. (60) Chen, D.; Chen, D.; Chen, D.; Chen, D.; Chen, D., Assessment of past, present and future environmental changes on the Tibetan Plateau (in Chinese with English abstract). Chin. Sci. Bull. 2015, 60, (32), 3025-3035. (61) Yang, Y.; Fang, J.; Pete, S.; Tang, Y.; Chen, A.; Chengjun, J. I.; Huifeng, H. U.; Rao, S.; Tan, K.; Jinsheng, H. E., Changes in topsoil carbon stock in the Tibetan grasslands between the 1980s and 2004. Global Change Biol. 2010, 15, (11), 2723-2729.

22


Page 22 of 29

Page 23 of 29


663

Catalogue of tables and figures:

664

Graphic abstract

665

Figure 1. Map of the three air–soil sampling sites on the Tibetan Plateau used in this

666

study.

667

Table 1. Summary of OCP and PCB concentrations in the ambient air (CA; pg m−3),

668

surface air (CSA; pg m−3) and soil (CS; pg g−1 dw).

669

Figure 2. Significant influence of ambient temperature and soil organic carbon content

670

(SOC) on KSA for selected compounds.

671

Figure 3. Comparison of measured and predicted LogKSA values.

672

Figure 4. Surface-to-air fugacity ratios (fs / fa) at Ngari, Nam Co and Lulang, with

673

different land cover. Dashed lines indicate the equilibrium interval (−1.2 to 0.53).

674

23



675

TOC Art:

676

24


Page 24 of 29

Page 25 of 29


677 678 679

Figure 1. Map of the three air–soil sampling sites on the Tibetan Plateau used in this study.

25



Page 26 of 29

680

Table 1. Summary of OCP and PCB concentrations in the ambient air (CA; pg m−3), surface air (CSA; pg m−3) and soil (CS; pg g−1

681

dw). CA

Lulang

Nam Co

Ngari

CSA

CS

mean

SD

min

max

mean

SD

min

max

mean

SD

min

max

HCHs

8.4

5.5

2.3

18

39

29

4.0

82

410

178

197

784

HCB

15

14

3.1

46

419

243

64

816

1150

730

463

2813

DDTs

17

16

2.3

50

19

13

5.9

44

2095

1251

622

5321

PCBs

5.7

11

1.3

33

46

37

4.3

118

106

59

33

239

HCHs

8.3

6.2

1.1

16

42

33

2.5

95

37

20

BDL

56

HCB

20

6.1

12

29

293

180

72

649

225

138

BDL

367

DDTs

22

21

BDL

52

6.8

4.7

BDL

20

485

401

85

1224

PCBs

3.7

5.6

0.30

15

59

32

8.7

128

25

12

7.0

43

HCHs

9.9

6.5

4.2

17

93

66

18

227

23

14

BDL

63

HCB

70

12

59

80

447

238

225

1202

116

60

50

258

DDTs

18

14

6.8

38

48

38

BDL

110

1360

737

459

2969

PCBs

15

8.6

4.4

24

144

124

BDL

437

48

33

12

122

682

26


Page 27 of 29


683

684 685 686

Figure 2. Significant influence of ambient temperature and soil organic carbon content (SOC) on KSA for selected compounds

27



687 688

Figure 3. Comparison of measured and predicted LogKSA values. All measured

689

data of KSA from our study (Lulang, Nam Co, and Ngari) are included in the

690

figure for the TP region (red points). KSA values in the Antarctic and the Arctic

691

were from references20, 21 (green points). KOA values have been corrected to local

692

temperature using empirical equations.51-54

28


Page 28 of 29

Page 29 of 29

693 694 695 696


Figure 4. Surface-to-air fugacity ratios (fs / fa) at Ngari, Nam Co and Lulang, with different land cover. Dashed lines indicate the equilibrium interval (−1.2 to 0.53). 29


Characterization of Tibetan soil as a source or sink of atmospheric

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