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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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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] 17
Tel: +86-10-84097120
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Fax: +86-10-84097079
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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
14-16
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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. 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
46
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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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
284
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
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(Ngari), respectively. This close association implied that the atmospheric levels of
313
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
369
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
372
Asia is weakened, which results in lower atmospheric levels of POPs in the air, and
373
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
375
observed in this study, demonstrating that pollutant sources and atmospheric transport
376
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
379
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
382
Quantification of air–soil exchange fluxes is essential to further understand the extent
383
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
385
POPs were calculated and presented in Table S13. We found that the exchange fluxes
386
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.
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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
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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).
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References:
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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
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TOC Art:
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Figure 1. Map of the three air–soil sampling sites on the Tibetan Plateau used in this study.
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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
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684 685 686
Figure 2. Significant influence of ambient temperature and soil organic carbon content (SOC) on KSA for selected compounds
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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
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Page 29 of 29
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Environmental Science & Technology
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
ACS Paragon Plus Environment