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The East Siberian Arctic Shelf holds large amounts of inundated carbon and methane (CH4). Holocene warming by overlying seawater, recently fortified by anthropogenic warming, has caused thawing of the underlying subsea permafrost. Despite extensive observations of elevated seawater CH4 in the past decades, relative contributions from different subsea compartments such as early diagenesis, subsea permafrost, methane hydrates, and underlying thermogenic/free gas to these methane releases remain elusive. Dissolved methane concentrations observed in the Laptev Sea ranged from 3 to 1,500 nM (median 151 nM; oversaturation by ∼3,800%). Methane stable isotopic composition showed strong vertical and horizontal gradients with source signatures for two seepage areas of δ13C-CH4 = (-42.6 ± 0.5)/(-55.0 ± 0.5) ‰ and δDCH4 = (-136.8 ± 8.0)/(-158.1 ± 5.5) ‰, suggesting a thermogenic/natural gas source. Increasingly enriched δ13C-CH4 and δD-CH4 at distance from the seeps indicated methane oxidation. The Δ14C-CH4 signal was strongly depleted (i.e., old) near the seeps (-993 ± 19/-1050 ± 89‰). Hence, all three isotope systems are consistent with methane release from an old, deep, and likely thermogenic pool to the outer Laptev Sea. This knowledge of what subsea sources are contributing to the observed methane release is a prerequisite to predictions on how these emissions will increase over coming decades and centuries. [ABSTRACT FROM AUTHOR]

Methyl bromide (CH3Br) emitted from plants constitutes a natural source of bromine to the atmosphere, and is a component in the currently unbalanced global CH3Br budget. In the stratosphere, CH3Br contributes to ozone loss processes. Studies of stable isotope composition may reduce uncertainties in the atmospheric CH3Br budget, but require well-constrained isotope fingerprints of the source end members. Here we report the first measurements of stable bromine isotopes (δ81Br) in CH3Br from abiotic plant emissions. Incubations of both KBr-fortified pectin, a ubiquitous cell-stabilizing macromolecule, and of a natural halophyte (Salicornia fruticosa), yielded an enrichment factor (ε) of −2.00±0.23‰ (1σ, n =8) for pectin and −1.82±0.02‰ (1σ, n =4) for Salicornia (the relative amount of the heavier 81Br was decreased in CH3Br compared to the substrate salt). For short incubations, and up to 10% consumption of the salt substrate, this isotope effect was similar for temperatures from 30 up to 300°C. For longer incubations of up to 90h at 180°C the δ81Br values increased from −2‰ to 0‰ for pectin and to −1‰ for Salicornia. These δ81Br source signatures of CH3Br formation from plant matter combine with similar data for carbon isotopes to facilitate multidimensional isotope diagnostics of the CH3Br budget. [ABSTRACT FROM AUTHOR]

Abstract: Compound-specific Cl-isotope analysis was performed on the persistent and bioaccumulating compound tris-(4-chlorophenyl)methane (4,4′,4″-TCPMe, referred to as TCPMe in this study) to elucidate whether its main source is natural or anthropogenic. Blubber from the Baltic grey seal (Halichoerus grypus) was extracted by continuous acetonitrile partitioning, and the TCPMe was isolated from the extract by preparative-capillary gas chromatography. Chlorine isotope analysis was subsequently performed by sealed-tube combustion in conjunction with thermal-ionization mass spectrometry (TIMS). The δ 37Cl of TCPMe was −3.5±0.5‰, similar to the previously reported δ 37Cl of technical grade p,p′-DDT (referred to as DDT in this study). The data is not consistent with a putative marine natural source of TCPMe, as enzymatic (biotic) production is reported to give values of δ 37Cl<−10‰. The δ 37Cl–TCPMe data thus supports the hypothesis that TCPMe is produced as a byproduct during DDT synthesis and is released to the environment through the same pathways as DDT. It is also consistent with tris-(4-chlorophenyl)methanol as the primary biotransformation product of TCPMe. [ABSTRACT FROM AUTHOR]

Compound-specific chlorine isotope analysis (CSIA–Cl) is promising as a novel and powerful method for monitoring in situ degradation of organochlorines in the environment and for source fingerprinting purposes. In order to apply CSIA–Cl in field studies of polychlorinated biphenyls (PCBs), the chlorine isotopic composition (δ37Cl) of individual PCB congeners in source materials must be known. In the present study, we determined δ37Cl of 18 congeners isolated from three widely produced technical mixtures. All congeners provided δ37Cl ranging between −1.9‰ and −3.5‰. Although the comparable products Aroclor 1242 (−2.0‰ to −2.5‰) and Clophen A30 (−1.9‰ to −3.0‰) were synthesized by different industries, they provided similar δ37Cl for the same type of congeners. On the contrary, the more chlorinated congeners present in Aroclor 1254 (−2.1 to −3.5‰) were more 37Cl depleted compared to Aroclor 1242 manufactured by the same company. Overall, δ37Cl of PCB congeners decreased by −0.26 ‰ for each additional chlorine atom. [Copyright &y& Elsevier]

Compound-specific chlorine-isotope analysis (CSIA-Cl) of 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (p,p′-DDT) and 1,1-dichloro-2,2-bis(p-chlorophenyl)ethene (p,p′-DDE) in blubber from Baltic Grey seal (Halichoerus grypus) was performed in order to investigate if a kinetic isotope effect (KIE) could be observed concomitant to environmental degradation of DDT. The δ37Cl of p,p′-DDT and p,p′-DDE were −0.69±0.21‰ and −2.98±0.57‰ (1σ, n =3), respectively. Both samples were enriched relative to the hypothesized initial isotope composition (−4.34‰), thus indicating a composite KIE associated with the degradation mechanisms pertaining to DDT. An isotope fractionation factor for degradation of dichloromethane, from the literature, was adapted and modified for use in the calculation of DDT degradation. A subsequent simplified Rayleigh distillation model of the DDT chlorine-isotope composition yielded an estimated fraction (f) of 7±2% of released DDT presently remaining as undegraded compound in the environment. The consistency between the result of the Rayleigh model (f ∼7%) and the use of the DDT/(DDT+DDE) ratio as a measure of DDT degradation (∼10% undegraded DDT) suggests that the KIE of DDT degradation may be significant, and that the novel approach of CSIA-Cl may be a valuable tool for degradation/persistence studies of lipophilic organochlorines in the environment. [Copyright &y& Elsevier]

Polychlorinated dibenzo-p-dioxins (PCDDs) of high concentrations in a ball clay deposit from the Mississippi Embayment were found to be consistent with a natural abiotic and non-pyrogenic origin by investigation with bulk radiocarbon analysis, compound-specific chlorine isotope analysis (CSIA-δ37Cl) of octachlorodibenzo-p-dioxin (OCDD), and black carbon (BC) analysis. The conventional radiocarbon date of total organic carbon from a depth of approximately 10 m in three parallel cores ranged from 14 700 years to >48 000 years, indicating that the strata with elevated levels of PCDDs have remained isolated from recent anthropogenic input in these >40 Ma old clay sediments. The CSIA-δ37Cl of OCDD yielded a δ37Cl of -0.2‰, which is significantly higher than the postulated range for biotic chlorination by chloroperoxidase enzymes, -11 to -10‰, and falls within the known range for abiotic organochlorines, -6 to +3‰. The absence of correlations between concentrations of PCDDs and corresponding pyrogenic black carbon (BC), together with estimations of BC sorptive loadings and the absence of polychlorinated dibenzofurans (PCDFs), suggest that vegetation fires did not form these ball-clay PCDDs. Results from this study indicate that the high levels of the toxic and carcinogenic PCDDs found in kaolinite-bearing clays may result from natural abiotic formation via in situ surface-promoted reactions on the clay mineral, including a so-far unknown organic precursor, rather than being the result of anthropogenic contamination. [ABSTRACT FROM AUTHOR]

Abstract: Chlorine isotope fractionation during preparative capillary gas chromatography (pcGC) was investigated using 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) as a model compound for semi-volatile organochlorine (OCl) molecules. Chlorine isotope analysis by thermal ionization mass spectrometry revealed no significant alteration of the chlorine isotope composition when the whole peaks were collected in pcGC (δ37Cl −3.2‰ versus −3.6‰ for the unprocessed DDT, ±0.5‰ SD). However, distinct isotope fractionations were measured for the front (δ37Cl −5.1‰) and tail (δ37Cl −1.8‰) segments of partially collected samples. Isolation of individual OCls by pcGC enables accurate off-line chlorine isotope analysis, and thus facilitates the investigation of naturally occurring OCls. [Copyright &y& Elsevier]

Improved sensitivity in the analysis of stable chlorine isotopes of organochlorines (δ37Cl-OCl) has been established using sealed tube combustion in conjunction with thermal ionization mass spectrometry (TIMS). TIMS of chlorine isotopes was performed on < 85 nmol of Cl with an achievable precision of < 0.25‰ for pure inorganic chloride samples and 0.46‰ for chloride liberated from organochlorines (OCls). This makes possible significant reductions in the overall sample size requirement, as compared to the techniques of gas source stable isotope ratio mass spectrometry (SIRMS). Yields in excess of 99% were demonstrated in the dechlorination of <0.14 μmol 1,1, l- trichloro-2,2-bis(p-chlorophenyl)ethane (DDT), and the overall yield, including purification of liberated chloride, was 86-97%. The accuracy of TIMS in the measurement of chlorine isotopes derived from OCls was con- finned by analysis of a DDT sample previously analyzed with SIRMS. Using the described method for TIMS, the DDT sample gave a bulk chlorine isotope ratio of δ37Cl -4.42 ± 0.46‰ (1σ). The reported value from SIRMS analysis is -4.34 ± 0.25‰ indicating the conformity of the two methods. [ABSTRACT FROM AUTHOR]

According to ecological theory, two species cannot occupy the same niche. Using nitrogen isotope analyses (δ15N) of amino acids, we tested the extent to which two sympatric deposit‐feeding amphipods, Monoporeia affinis and Pontoporeia femorata, partition their trophic resources. We found that trophic position (TP) and resynthesis index (∑V; a proxy for degradation status of ingested material prior to assimilation by the consumer) differ between species. The surface‐feeding M. affinis had higher TP and intermediate ∑V, both pointing to a large contribution of metazoans in its diet. P. femorata, which feeds in the subsurface layers, had lower TP and a bimodal distribution of the ∑V values, supporting previous experimental evidence of a larger feeding niche. We also evaluated whether TP and ∑V values have consequences for amphipod fecundity and embryo viability and found that embryo viability in M. affinis was negatively linked to TP. Our results indicate that the amino acid‐δ15N data paired with information about reproductive status are useful for detecting differences in the trophic ecology of sympatric amphipods. [ABSTRACT FROM AUTHOR]

Rationale The environmental occurrence of chlorinated acetic acids (CAAs) has been extensively studied, but the sources and transport are still not yet fully understood. A promising approach for source apportionment and process studies is the isotopic characterization of target compounds. We present the first on-line stable chlorine isotope analysis of CAAs by use of gas chromatography/quadrupole mass spectrometry (GC/qMS). Methods Following approved procedures for concentration analysis, CAAs extracted into MTBE were methylated to GC-amenable methyl esters (mCAAs). These mCAAs were then analyzed by GC/qMS for their stable chlorine isotope composition using a sample/standard-bracketing approach (CAA standards in the range δ37Cl −6.3 to −0.2 ‰, Standard Mean Ocean Chloride). Results Cross-calibration of the herein presented method with off-line reference methods (thermal ionization and continuous-flow GC isotope ratio mass spectrometry; TI-MS and CF-GC/IRMS, respectively) shows good agreement between the methods (regression slope for GC/qMS vs reference method data sets: 0.92 ± 0.29). Sample amounts as small as 10 pmol Cl can herewith be analyzed with a precision of 0.1 to 0.4 ‰. Conclusions This method should be useful for environmental studies of CAAs at ambient concentrations in precipitations (<0.06 to 100 nmol L-1), surface waters (<0.2 to 5 nmol L-1) and soil (<0.6 to 2000 nmol kg-1 dry soil) where conventional off-line methods cannot be applied. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The widespread use of chlorinated phenols (CPs) as a wood preservative has led to numerous contaminated sawmill sites. However, it remains challenging to assess the extent of in situ degradation of CPs. We evaluated the use of compound-specific chlorine and carbon isotope analysis (Cl- and C-CSIA) to assess CP biotransformation. In a laboratory system, we measured isotopic fractionation during oxidative 2,4,6- trichlorophenol dechlorination by representative soil enzymes (C. fumago chloroperoxidase, horseradish peroxidase, and laccase from T. versicolor). Using a mathematical model, the validity of the Rayleigh approach to evaluate apparent kinetic isotope effects (AKIE) was confirmed. A small but significant Cl-AKIE of 1.0022 ± 0.0006 was observed for all three enzymes, consistent with a reaction pathway via a cationic radical species. For carbon, a slight inverse isotope effect was observed (C-AKIE = 0.9945 ± 0.0019). This fractionation behavior is clearly distinguishable from reported reductive dechlorination mechanisms. Based on these results we then assessed degradation and apportioned different types of technical CP mixtures used at two former sawmill sites. To our knowledge, this is the first study that makes use of two-element CSIA to study sources and transformation of CPs in the environment. [ABSTRACT FROM AUTHOR]

Tropospheric methyl bromide (CH3Br) is the largest source of bromine to the stratosphere and plays an important role in ozone depletion. Here, the first stable bromine isotope composition (δ81Br) of atmospheric CH3Br is presented. The δ81Br of higher concentration Stockholm samples and free air subarctic Abisko samples suggest a source/background value of −0.04±0.28‰ ranging up to +1.75±0.12‰. The Stockholm δ81Br versus concentration relationship corresponds to an apparent isotope enrichment factor of −4.7±3.7‰, representing the combined reaction sink. This study demonstrates the scientific potential of atmospheric δ81Br measurements, which in the future may be combined with other isotope systems in a top-down inverse approach to further understand key source and sink processes of methyl bromide. [ABSTRACT FROM AUTHOR]

A site in mid-western Sweden contaminated with chlorinated solvents originating from a previous dry cleaning facility, was investigated using conventional groundwater analysis combined with compound-specific isotope data of carbon, microbial DNA analysis, and geoelectrical tomography techniques. We show the value of this multidisciplinary approach, as the different results supported each interpretation, and show where natural degradation occurs at the site. The zone where natural degradation occurred was identified in the transition between two geological units, where the change in hydraulic conductivity may have facilitated biofilm formation and microbial activity. This observation was confirmed by all methods and the examination of the impact of geological conditions on the biotransformation process was facilitated by the unique combination of the applied methods. There is thus significant benefit from deploying an extended array of methods for these investigations, with the potential to reduce costs involved in remediation of contaminated sediment and groundwater. [ABSTRACT FROM AUTHOR]

South Asia is a global hotspot for the climate and health impact of black and brown carbon aerosols (BC and BrC, respectively). However, the climate impact is troubled by large uncertainties, including the optical properties of these components. In this poster, we present a comparison of the mass absorption cross-section (MAC) of BC in air (PM10) and in rainwater for year-round samples collected at a South Asian receptor site located in the Maldives. We find that the MAC of BC in rainwater is almost twice as high (13.3 ± 4.2 m2/g, 678nm) as for PM10 aerosols (7.2 ± 2.6 m2/g). A possible explanation is the elevated organic carbon (OC) to BC ratios observed in the rain samples, contributing to a coating-enhancement effect.In addition to BC, we also investigated the MAC of water-soluble BrC in PM10 (0.42 ± 0.4 m2/g, at 365nm). For the BrC we observed a clear trend of the MAC with respect to air mass history, with higher values for samples with air originating over the South Asian landmass. This trend is less clear for BC MAC, possibly indicating a longer atmospheric lifetime of BC MAC. The washout ratio of BC was significantly lower than for OC and inorganic ions, implying that BC was removed less efficiently from the atmosphere. The wet deposition of BC is 3 times higher during the NE monsoon than SW monsoon. Taken together, this study shows that BC may be more efficient in absorbing light in cloud droplets (while not considering droplet scattering) and that the atmospheric fate of BC and BrC may be very different in the South Asian outflow. [ABSTRACT FROM AUTHOR]

The East Siberian Arctic Shelf (ESAS) has a large inventory of buried carbon and methane (CH4) as part of and below subsea permafrost. Warming of this system by geothermal heat and by overlying seawater throughout the Holocene, recently ammended by Anthropocene warming, has brought subsea permafrost towards the point of thaw. There have been extensive observations of strongly elevated CH4 concentrations in ESAS seawater over the recent decade. However, there are still large uncertainties from what part of the subsea system the released methane originates, and also about the large-scale geographical variations. This study presents methane observations from the 2014 SWERUS-C3 cruise along an east-west transect of the outer East Siberian Sea (ESS). Surface ocean methane concentrations were regularly above the atmospheric equilibrium concentration, with seawater concentrations in a ESS hotspot region reaching up to 460 nM. The observations include 13C stable isotope observation in an active ebullition site. Understanding the sources and spatial distribution of the methane releases will aid in understanding the composition and functioning of the subsea permafrost/methane system and help us move toward predictive capacity of how the methane evasions will develop into the future. [ABSTRACT FROM AUTHOR]

Pleistocene ice complex permafrost deposits contain roughly a quarter of the organic carbon (OC) stored in permafrost (PF) terrain. When permafrost thaws, its OC is remobilized into the (aquatic) environment where it is available for degradation, transport or burial. Aquatic or coastal environments contain sedimentary reservoirs that can serve as archives of past climatic change. As permafrost thaw is increasing throughout the Arctic, these reservoirs are important locations to assess the fate of remobilized permafrost OC. We here present compound-specific deuterium (δ2H) analysis on leaf waxes as a tool to distinguish between OC released from thawing Pleistocene permafrost (ice complex deposits; ICD) and from thawing Holocene permafrost (from near-surface soils). Bulk geochemistry (%OC; δ13C; %total nitrogen, TN) was analyzed as well as the concentrations and δ2H signatures of long-chain n-alkanes (C21 to C33) and mid-to long-chain n-alkanoic acids (C16 to C30) extracted from both ICD-PF samples (n = 9) and modern vegetation and Ohorizon (topsoil-PF) samples (n = 9) from across the northeast Siberian Arctic. Results show that these topsoil-PF samples have higher %OC, higher OC= TN values and more depleted δ13C-OC values than ICD-PF samples, suggesting that these former samples trace a fresher soil and/or vegetation source. Whereas the two investigated sources differ on the bulk geochemical level, they are, however, virtually indistinguishable when using leaf wax concentrations and ratios. However, on the molecular isotope level, leaf wax biomarker δ2H values are statistically different between topsoil PF and ICD PF. For example, the mean δ2H value of C29 n-alkane was -246±13% (mean ± SD) for topsoil PF and -280±12% for ICD PF. With a dynamic isotopic range (difference between two sources) of 34 to 50 %; the isotopic fingerprints of individual, abundant, biomarker molecules from leaf waxes can thus serve as endmembers to distinguish between these two sources. We tested this molecular δ2H tracer along with another source-distinguishing approach, dual-carbon (δ13C-Δ14C) isotope composition of bulk OC, for a surface sediment transect in the Laptev Sea. Results show that general offshore patterns along the shelf-slope transect are similar, but the source apportionment between the approaches vary, which may highlight the advantages of either. This study indicates that the application of δ2H leaf wax values has potential to serve as a complementary quantitative measure of the source and differential fate of OC thawed out from different permafrost compartments. [ABSTRACT FROM AUTHOR]

Methyl chloride and methyl bromide (CH 3 Cl and CH 3 Br) are the largest natural sources of chlorine and bromine, respectively, to the stratosphere, where they contribute to ozone depletion. We report the anthropogenic production of CH 3 Cl and CH 3 Br during breadbaking, and suggest this production is an abiotic process involving the methyl ester functional groups in pectin and lignin structural polymers of plant cells. Wide variations in baking styles allow only rough estimates of this flux of methyl halides on a global basis. A simple model suggests that CH 3 Br emissions from breadbaking likely peaked circa 1990 at approximately 200 tonnes per year (about 0.3% of industrial production), prior to restrictions on the dough conditioner potassium bromate. In contrast, CH 3 Cl emissions from breadbaking may be of similar magnitude as acknowledged present-day CH 3 Cl industrial emissions. Because the mechanisms involve functional groups and compounds widely found in plant materials, this type of methyl halide production may occur in other cooking techniques as well. [ABSTRACT FROM AUTHOR]

A system was developed for collecting from the ambient atmosphere the methyl halides CH3Cl and CH3Br in quantities sufficient for chlorine and bromine isotope analysis. The construction and operation of the novel cryogenic collection system (cryosampler) and sample purification system developed for this task are described. This study demonstrates the capability of the cryosampler by quantifying the CH3Cl and CH3Br collected from atmospheric samples and the nonfractionating bromine isotope fingerprint of CH3Br from synthetic air samples of controlled composition. An optimized cryosampler operation time of 4 h at a flow rate of 15 L min−1 is applied to yield the nearly 40 ng required for subsequent δ81Br-CH3Br analyses. The sample purification system is designed around a packed column gas chromatography-quadropole-mass spectrometry (GCqMS) system with three additional cryotraps and backflushing capacity. The system's suitability was tested by observing both the mass recovery and the lack of Δ81Br isotope fractionation induced during sample purification under varying flow rates and loading scenarios. To demonstrate that the entire system samples and dependably delivers CH3Br to the isotope analysis system without inducing isotope fractionation, diluted synthetic air mixtures prepared from standard gases were processed through the entire system, yielding a Δ81Br-CH3Br of +0.03‰ ± 0.10‰ relative to their starting composition. Finally, the combined cryosampler-purification and analysis system was applied to demonstrate the first-ever δ81Br-CH3Br in the ambient atmosphere with two samples collected in the autumn of 2011, yielding −0.08‰ ± 0.43‰ and +1.75‰ ± 0.13‰ versus standard mean ocean bromide for samples collected at a suburban Stockholm, Sweden, site. [ABSTRACT FROM AUTHOR]

Two-dimensional compound-specific isotope analysis (2D-CSIA), combining stable carbon and chlorine isotopes, holds potential for monitoring of natural attenuation of chlorinated ethenes (CEs) in contaminated soil and groundwater. However, interpretation of 2D-CSIA data sets is challenged by a shortage of experimental Cl isotope enrichment factors. Here, isotope enrichments factors for C and Cl (i.e., εC and εCl) were determined for biodegradation of tetrachloroethene (PCE) and trichloroethene (TCE) using microbial enrichment cultures from a heavily CE-contaminated aquifer. The obtained values were εC = -5.6 ± 0.7‰ (95% CI) and εCl = -2.0 ± 0.5‰ for PCE degradation and εC = -8.8 ± 0.2‰ and εCl = -3.5 ± 0.5‰ for TCE degradation. Combining the values for both εC and εCl yielded mechanism-diagnostic εCl/εC ratios of 0.35 ± 0.11 and 0.37 ± 0.11 for the degradation of PCE and TCE, respectively. Application of the obtained εC and εCl values to a previously investigated field site gave similar estimates for the fraction of degraded contaminant as in the previous study, but with a reduced uncertainty in assessment of the natural attenuation. Furthermore, 16S rRNA gene clone library analyses were performed on three samples from the PCE degradation experiments. A species closely related to Desulfitobacterium aromaticivorans UKTL dominated the reductive dechlorination process. This study contributes to the development of 2D-CSIA as a tool for evaluating remediation strategies of CEs at contaminated sites. [ABSTRACT FROM AUTHOR]

Chlorinated ethenes (CEs) are ubiquitous groundwater contaminants, yet there remains a need for a method to efficiently monitor their in situ degradation. We report here the first field application of combined stable carbon and chlorine isotope analysis of tetrachloroethene (PCE) and trichloroethene (TCE) to investigate their biodegradation in a heavily contaminated aquifer. The two-dimensional Compound Specific Isotope Analysis (2D-CSIA) approach was facilitated by a recently developed gas chromatography-quadrupole mass spectrometry (GCqMS) method for δ37Cl determination. Both C and Cl isotopes showed evidence of ongoing PCE transformation. Applying published C isotope enrichment factors (ϵC) enabled evaluation of the extent of in situ PCE degradation (11-78%). We interpreted C and Cl isotopes using a numerical reactive transport model along a 60-m flow path. It revealed that combined PCE and TCE mass load was dechlorinated by less than 10%, and that cis-dichloroethene was not further dechlorinated. Furthermore, the 2D-CSIA approach allowed estimation of Cl isotope enrichment factors ϵCl (-7.8 to -0.8‰) and characteristic ϵCl/ϵC values (0.42-1.12) for reductive PCE dechlorination at this field site. This investigation demonstrates the benefit of 2D-CSIA to assess in situ degradation of CEs and the applicability of Cl isotope fractionation to evaluate PCE and TCE dechlorination. [ABSTRACT FROM AUTHOR]

Black carbon (BC) aerosols from incomplete combustion of biomass and fossil fuel exert asignificant influence on the climate system in South Asia, effecting in excess of 1.2 billionpeople. Models—seeking to advise mitigation policy—are constantly challenged inreproducing observations of seasonally varying BC concentrations in the densely populatedregions of South Asia e.g., over the Indo-Gangetic Plain (IGP). The systematicunderestimation of the absorption aerosol optical depth over South Asia by a factor of 2-3in climate models relative to observation-based estimates illustrates the currentlylarge uncertainties. This uncertainty may be related to several different factors,including inaccurate estimates of relative source contributions and emissions ofBC. As part of the South Asian Pollution Experiment 2016 (SAPOEX-16), we investigatedwintertime BC and its dual carbon-isotopic signature (δ13C/Δ14C) - diagnosed sources fromregional and large-scale background receptor sites — the Bangladesh Climate Observatory atBhola Island (BCOB), strategically located in the outflow region of the IGP to intercept theintegrated IGP emission signal and the remote Indian ocean-based Maldives ClimateObservatory at Hanimaadhoo Island (MCOH) to provide an integrated footprint of SouthAsia, respectively. Our results of "top-down" radiocarbon measurements of atmospheric BCfrom two regional-receptor sites show that fossil fuel combustion produces 55±3% (Δ14C:-440 ± 6 per mil) of BC emitted from South Asia. In contrast, the δ13C signalsfor BCOB-BC (∼ -27.2 ± 0.2) are more depleted relative to that of MCOH-BC(∼ −25 ± 0.2). Using a Bayesian statistical approach, we resolve the isotopicsignatures into three source classes namely biomass, coal and liquid fossil fuel.We find the liquid fossil fuel component of BC emissions is 49 ± 5% in the IGPversus the larger South Asian footprint of 23 ± 9%. Our observation is in starkcontrast to previous "Bottom-up" emission inventory-based modelling estimatessuggesting over ∼ 60% BC from biomass burning in the IGP. This finding brings outthe previously unknown sharp contrast in regional emission sources of BC fromanthropogenic combustion in South Asia. Observationally-constrained insights onBC sources allows for a more credible scientific underpinning of policy aimedat mitigating the acuteness and scale of ramifications due to BC in South Asia. [ABSTRACT FROM AUTHOR]