Benzene Polycarboxylic Acids Research Articles

Unraveling the Distribution of Black Carbon in Chinese Forest Soils Using Machine Learning Approaches

AbstractBlack carbon (BC) is a highly persistent yet poorly understood component of forest soil carbon reservoirs, while its inventory, distribution, and determining factors in forest soils on a large geographic scale remain unclear. Here, we characterized soil BC across 68 Chinese forest sites using benzene polycarboxylic acid method and developed machine learning (ML) models to predict and interpret potential impacts of soil organic matter (SOM) properties, soil physiochemical properties, meteorological conditions, wildfire history, and microbial diversity on BC. Results revealed that SOM properties were the most critical in predicting BC, complemented by the negative impact of mean annual temperature and alkaline mineral composition. The superior prediction accuracy for BC with higher condensed aromaticity (more benzene hexa‐ and penta‐carboxylic acid monomers) likely results from its simpler sources and greater resistance to transformation. This study introduces an effective ML model for predicting and interpreting soil BC inventory to better understand BC cycling.

Optical characterization of black carbon-derived DOM: Implication for the fluorescence detection of fuel combustion products in marine waters

Among pollutants released from shipping, black carbon (BC), also known as soot carbon, is of great interest due to its impacts on climate, air quality, human health and ecosystems. BC emitted from ships may enter marine waters and partially transfer to the seawater dissolved phase. In this study, we investigated the optical properties (absorbance and fluorescence) of dissolved organic matter (DOM) derived from BC particles (DOMBC) emitted by ships, which were compared to those of DOMBC of other origins (diesel-powered industrial machine, biomass burning, urban dust), and to terrestrial and marine DOM (DOMTER, DOMMAR). Ship and diesel DOMBC displayed higher ratios of fluorescence maximum intensity to dissolved organic carbon concentration (Fmax/DOC), higher specific UV absorbance at 254 nm (SUVA254), and lower fluorescence emission wavelengths than the other tested materials. The parallel factor analysis (PARAFAC)-derived fluorophores of the ship and diesel DOMBC exhibited significant correlations with the concentration of dissolved black carbon (DBC), determined using the benzenepolycarboxylic acid (BPCA) method. Based on these results, we propose the Combustion indeX (COX), to help detecting and tracking ship/fuel combustion pollutions in marine waters.

Biochar data into structure: A methodology for generating large-scale atomistic representations

A well-defined methodology for constructing appropriate atomistic representations of biochar will aid in visualizing the structural features and elucidating biochar behavior with molecular dynamics (MD) simulations. Such knowledge will facilitate engineering biochars tailored to specific applications. To achieve this goal, we adapted modeling strategies applied in coal science by employing multi-cross-polarization 13C nuclear magnetic resonance, ultimate analysis, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy to identify functional groups. Helium density, surface area, and porosity were used to assess structural features. Biochar's aromatic cluster size distribution was proposed based on data from the benzene polycarboxylic acid method. The computational framework reduces bias by incorporating chemical information derived from density functional theory, reactive MD simulations, and advanced characterization data. The construction approach was successfully applied to cellulose biochars produced at four temperatures, obtaining independent representations with a relative error on the atomic contents of <10 % for oxygen and nitrogen and <5 % for carbon and hydrogen. The atomistic representations were validated using X-ray diffraction, electron spin resonance data, and laser desorption/ionization Fourier-transform ion cyclotron resonance-mass spectrometry. The code will assist others in overcoming structural creation barriers and enable the utilization of the generated structures for further simulations.

Preferential extraction of degraded organic matter and mineral protection of aromatic structures based on molecular marker analysis

The humic fractions were sequentially extracted from soils to study the heterogenous properties of soil organic matter (SOM). However, the bulk characteristics of these samples, such as elemental compositions and functional groups, could not fully reveal their diverse compositions. We sequentially extracted humic acids (HAs) from a sediment, and analyzed its molecular markers (benzene polycarboxylic acids (BPCAs), lignin-derived phenols, free lipids and bound lipids) to illustrate the diverse compositions of SOM. Our results suggested that the investigated HAs were derived from terrestrial C3 plants as reflected by the range of their δ13C values (−27.08 ‰ in HA1 to −27.85 ‰ in HA6), more specifically, non-woody tissue of angiosperm and belowground part as suggested by lignin and lipid markers. With the increasing times of extraction, the relative abundances of lignin-derived phenols, free lipids, and bound lipids increased, while those of the condensed aromatics (as indicated by BPCAs) decreased. We also observed that with the increasing times of extraction, the carbon preference index (CPI) increased, the ratios of acids to aldehydes of vanilly units (Ad/Al)v and δ13C of HAs decreased, suggesting that the extensively degraded organic compositions were selectively extracted because of their favored dissolution. Our results emphasize that the complete extraction of organic compositions is essential to ensure reliable analysis on SOM properties and turnover. The distribution of individual BPCAs suggested that the highly condensed aromatics were preferentially extracted. These might be attributed to the less condensed aromatics were more strongly associated with mineral particles, which is important for the protection of aromatic carbons in the environment.

Dual-carbon isotope analysis of benzene polycarboxylic acids for tracking black carbon across different environments

Black carbon (BC) is ubiquitous in the environment and is a significant component of the refractory carbon pool. However, its sources, biogeochemical processes, and environmental residence times are poorly understood. One important reason is that the technical protocols for isolating/analyzing BC vary across different matrices, thereby bringing inconsistency among studies on BC in different environment media. Benzene polycarboxylic acids (BPCA), produced by nitric acid oxidation of the condensed aromatic structure that is common in BC and BC-like substances, has been proven to be a useful molecular proxy for tracking BC in the Earth's surface. Here, we introduce a new technical approach for the compound-specific dual-carbon isotope (δ13C and Δ14C) analysis of BPCA. The BPCA compounds are isolated using preparative liquid chromatography, in which trifluoroacetic acid is used in the mobile phase instead of phosphoric acid. This allows complete removal of solvent from the isolated BPCA fraction, so that conversion of BPCA isolates into CO2 can be done by conventional oxidation methods for off-line Δ14C analysis with accelerator mass spectrometry. Liquid chromatography (void column)-stable carbon isotope ratio mass spectrometry is employed to measure δ13C of the isolated BPCA, leaving as much BPCA carbon amount as possible for 14C analysis. Blockage of oxidation chamber due to incomplete oxidation of other organic acids in the samples is avoided thanks to preliminary isolation of BPCA. We tested the approach by applying it to solid and dissolved BC species in a suite of environmental reference materials, including maize char, coal, riverine natural organic matter (NOM 2R101N), marine sediment (SRM 1941b), urban dust (SRM 1649b), and coke-polluted soils. We assessed the offsets of BPCA-δ13C and BPCA-Δ14C values caused by procedural carbon contaminations to correct measured carbon isotopic values. Paired δ13C-Δ14C data of individual BPCA were plotted to validate and demonstrate the ability of this dual carbon isotope technique in tracking BC across different environmental matrices. We observed slightly different δ13C-Δ14C signatures among individual BPCA, indicating that the less condensed BC may have younger apparent radiocarbon ages.

Quantification and isotopic characterization of benzene polycarboxylic acids (BPCA)-derived black carbon in deep oceanic sediments: Towards assessing pyrogenic inputs from marine sources

Methodologies based on benzene polycarboxylic acids (BPCA) selectively target the polymeric aromatic fraction of black carbon (BC) and are considered adequate to quantify pyrogenic inputs in environmental samples such as soils, lakes, and marine dissolved organic carbon. However, the usefulness of these methodologies to quantify BPCA-derived BC in deep-sea sediments has not been fully evaluated. In this manuscript we describe and validate a procedure to quantify BPCAs in deep oceanic sediments with very low organic carbon content. The resulting analytical procedure has produced reproducible quantitative data for BPCAs over a period of 10 months (coefficient of variation, CV = 6.4 − 6.6%). The stable carbon isotopes (δ13C) of BC_BPCA have been characterized using an LC Isolink™-irMS system with an accuracy better than 0.5‰. The quantitative and isotopic composition of several marine sediments has been characterized to investigate the relative contributions of marine/diagenetic and continental/pyrogenic sources to the BC accumulated in oceanic sediments from different contexts, ranging from upwelling systems to remote oceanic locations. Overall, a significant fraction of the sedimentary BC is of marine origin and should be considered in inventories of pyrogenic materials accumulated in the world oceans. However, the continental/pyrogenic sources can be largely dominant in marine settings with large inputs of pyrogenic materials.

Insight into Relationship between the Products Distribution and Molecular Properties in Carboxylation between Benzene Polycarboxylic Acids and CO2

Insight into Relationship between the Products Distribution and Molecular Properties in Carboxylation between Benzene Polycarboxylic Acids and CO2

Using the benzenepolycarboxylic acid (BPCA) method to assess activated biochars and their PFAS sorption abilities

Activated carbon (AC) has important industrial and environmental applications as it has excellent abilities to sorb contaminants such as per- and polyfluoroalkyl substances (PFAS). Current research aims to develop activated biochars (AB) from renewable biomass to replace AC that is produced from fossil feedstock. Both AC and AB are primarily comprised of condensed aromatic carbon (ConAC), the component that is the focus of this study. ConAC is characterized to determine its relationship with biochar activation conditions and PFAS sorption, which are understudied at present.Benzenepolycarboxylic acid (BPCA) markers for ConAC were quantified in steam-activated biochars (AB-Steam) and carbon dioxide-activated biochars (AB-CO2) prepared from waste timber at different temperatures (800, 850, 900 °C) and molar ratios of feedstock-carbon:steam (0.50 – 1.25). A non-activated biochar was also included as a reference. ConAC relative to total organic carbon content was higher in AB-Steam than in AB-CO2 (92 ± 2 % vs. 81 ± 11%). The ratio of benzenehexa- (B6CA) to benzenepentacarboxylic (B5CA) acids revealed that AB-Steam also had larger ConAC clusters than AB-CO2. These findings provide novel evidence that steam activation is more effective than CO2 activation in creating ConAC.To assess how ConAC impacts AB sorption abilities, AB-Steam were used to remediate PFAS from contaminated soils. The observed strong correlations between ConAC content and sorption of long-chain PFAS suggest the importance of hydrophobic interactions between PFAS tails and ConAC. Poor correlations for short-chain PFAS, on the other hand, indicated the existence of electrostatic repulsion interactions between PFAS head groups and ConAC. Collectively, these results explain the great ability of AB-Steam to sorb PFAS from contaminated soils (up to 100% remediation). More broadly, this work demonstrates that the BPCA method can be a valuable tool to assess the quality of biochars and other carbonaceous sorbents in relation to their production conditions or contaminant sorption abilities.

Variable aging and storage of dissolved black carbon in the ocean

During wildfires and fossil fuel combustion, biomass is converted to black carbon (BC) via incomplete combustion. BC enters the ocean by rivers and atmospheric deposition contributing to the marine dissolved organic carbon (DOC) pool. The fate of BC is considered to reside in the marine DOC pool, where the oldest BC 14C ages have been measured (>20,000 14C y), implying long-term storage. DOC is the largest exchangeable pool of organic carbon in the oceans, yet most DOC (>80%) remains molecularly uncharacterized. Here, we report 14C measurements on size-fractionated dissolved BC (DBC) obtained using benzene polycarboxylic acids as molecular tracers to constrain the sources and cycling of DBC and its contributions to refractory DOC (RDOC) in a site in the North Pacific Ocean. Our results reveal that the cycling of DBC is more dynamic and heterogeneous than previously believed though it does not comprise a single, uniformly "old" 14C age. Instead, both semilabile and refractory DBC components are distributed among size fractions of DOC. We report that DBC cycles within DOC as a component of RDOC, exhibiting turnover in the ocean on millennia timescales. DBC within the low-molecular-weight DOC pool is large, environmentally persistent and constitutes the size fraction that is responsible for long-term DBC storage. We speculate that sea surface processes, including bacterial remineralization (via the coupling of photooxidation of surface DBC and bacterial co-metabolism), sorption onto sinking particles and surface photochemical oxidation, modify DBC composition and turnover, ultimately controlling the fate of DBC and RDOC in the ocean.

The presence of free benzenepolycarboxylic acids (BPCAs) may result in the overestimation of dissolved black carbon in aqueous samples

Dissolved black carbon (DBC) is the condensed aromatic fraction of dissolved organic matter produced during the thermal alteration of organic material (e.g., fire). DBC concentrations are often determined using the benzenepolycarboxylic acid (BPCA) method where condensed aromatic structures are oxidized into BPCA molecular markers for quantification. However, BPCA molecules have been recently identified in fire-affected surface waters and leachates of heated soils and wildfire ash. If they survive the sample preparation and analytical procedures, the presence of these “free” BPCAs in water may result in an overestimation of DBC concentrations in aqueous samples. To assess the potential impact of free BPCAs on DBC quantification, we spiked ultrapure water, salt water, and organic matter solutions with BPCA standards and treated them as environmental samples being analyzed for DBC. Each BPCA standard was recovered in detectable amounts, with the most-substituted BPCAs having lower percent recoveries than less-substituted BPCAs. Spiked organic matter solutions had significantly higher calculated DBC concentrations than their unamended counterparts only when the conversion factor used included less substituted BPCAs. Overall, our results show that DBC quantification could be impacted by free BPCAs in aqueous samples, but the degree of impact is largely dependent upon the properties of the individual BPCA molecular marker.

Dosage- and site-dependent retention of black carbon and polycyclic aromatic hydrocarbons in farmland soils via long-term biochar addition

Biochar, a soil conditioner containing significant amounts of polycyclic aromatic hydrocarbons (PAHs), has gained widespread popularity in agricultural practices due to its advantages in improving soil fertility and carbon sequestration. While biochar may increase soil black carbon (BC) and PAH contents, the quantitative accumulation of BC and PAHs in different soil environments under varying biochar addition dosages remains poorly understood. Here, we investigated the content and composition of black carbon (evaluated using benzene polycarboxylic acids, BPCAs) and PAHs in soils treated with different biochar addition dosages from two long-term experimental farmlands in Ningxia (5-year) and Shandong (7- and 11-year), China. Results showed that increasing cumulative biochar dosage caused elevated contents of black carbon and PAHs, accompanied by decreases in their retention efficiencies. Contrasting retention was observed between sites, with the Shandong site characterized by higher retention efficiencies of BPCAs and lower retention efficiencies of PAHs, possibly owing to its higher temperature, more sandy soil texture, less irrigation, and lower sunlight intensity. Despite both black carbon and PAHs originating from biochar and sharing similar condensed aromatic structures, there was no significant correlation between the contents of black carbon and PAHs, indicating distinct behaviors and fates of these compounds. These findings emphasize the importance of optimizing biochar addition dosages and considering site-specific environmental factors for effective soil black carbon sequestration through biochar application.Graphical

Artificial formation of benzene polycarboxylic acids during sample processing of black carbon analysis: the role of organic carbon amount

Black carbon is also known as pyrogenic carbon formed by partial combustion of organic material under limited oxygen supply. It occurs along a continuum from original organic slightly charred material to highly aromatic combustion residues such as charcoal, graphite, and soot. Black carbon is extensively studied in various environments due to its ubiquity. It is also important for the biochar community because it can specifically trace the stable polycondensed part of biochar. Different methods have been adopted for black carbon determination; among them using benzene polycarboxylic acids (BPCA) as molecular markers for the polycondensed aromatic moieties of charred materials. However, different researchers have shown interferences from organic matter during BPCA analysis. Therefore, the aim of this work was to assess if artificial formation of BPCA occurs in soil samples when the organic carbon load exceeds 5–10 mg. For this purpose, we conducted black carbon analysis of different soil samples with varying TOC contents of up to 20 mg. In addition, organic matter-rich plant materials were used as a black carbon-free control (leaves of Ivy and Beech, leaves/needles of Spruce and needles of Thuja). To exclude the high-pressure digestion as source of artificial black carbon formation, a comparison between the conventional and a microwave-assisted extraction (MAE) oxidation process was included. Our results show that for soil samples, no artificial BPCA formation occurred at least up to 20 mg of total organic carbon. Higher sample weights are unrealistic for BPCA analysis of soils using current methodology. Therefore, our results clearly demonstrate that there is no artificial BPCA formation during properly performed black carbon analysis of soil samples. On the contrary, for some samples, BPCA contents tended to decrease with increasing sample weight, and thus increasing amount of TOC. In contrast, for plant samples, artificial BPCA formation of up to 3 g kg−1 occurred when more plant material equivalent to 10 mg total organic carbon was used. However, there was no amount dependence of artificial BPCA formation. The reason for artificial BPCA formation was not the high-pressure digestion, as microwave-assisted digestion showed comparable results. However, for real-world analysis, this artificial BPCA formation is not relevant because such high soil sample weights cannot be used. Nevertheless, when using organic-rich material such as peat and charred materials, the samples should contain less than 10 mg of total organic carbon.Graphical

Occurrence of Benzene Polycarboxylic Acids in Ash and Streamwater after the Cameron Peak Fire

Occurrence of Benzene Polycarboxylic Acids in Ash and Streamwater after the Cameron Peak Fire

An Upper Pleistocene and Holocene Black Carbon‐Related Fire Record From the SW Balkans

AbstractLake sediments are unique archives of human environment interactions. Lake Prespa is one of the oldest lakes in Europe, lying in the southwestern Balkans and thus on a possible dispersal route of anatomically modern humans from Africa. In this study, we investigated the effects of climate, vegetation and human activity on fire over the last 92,000 years in this region. Sediment samples were taken from Lake Prespa, and nearby Lake Ohrid for comparison of the regional relevance, and analyzed for benzene polycarboxylic acids as markers for burned organic carbon residues (black carbon, BC). Peaking contents of BC (up to 1 g BC kg−1 sediment) coincided with warm and humid phases, when forests expanded at marine isotope stages (MIS) 5 and 1. During the colder and more arid climates of MIS 4 and 2, BC contents were lowered by a factor of 10, with a distinct minimum during the Last Glacial Maximum (0.07 g BC kg−1). The ratio of pentacarboxylic acid to mellitic acid (B5CA/B6CA) declined from 1.2 at MIS 5 to values of 0.3 at MIS 2, confirming a change in fire regime. Overall, BC contents peaked at cycles of solar radiation, vegetation composition and fuel availability, and thus correspond to the BC signal of other environmental archives. However, in the Late Holocene and as a result of human sedentary settlement, BC production increased, independent of decreasing insolation forcing.

Deciphering sources and processing of dissolved black carbon in coastal seas

AbstractDissolved black carbon (DBC) is the largest known slow‐cycling organic carbon pool in the ocean; yet, its sources and processing in the coastal seas remain poorly understood. To address this knowledge gap, we conducted a study in the East China Sea and South Yellow Sea to quantify and molecularly characterize DBC using benzene polycarboxylic acids (DBCBPCA) and ultra‐high resolution mass spectrometry (DBCFT), respectively. The concentration of DBCBPCA was 0.70–1.9 μmol C L−1 and exhibited a significant negative correlation with salinity. Significant correlations were also observed between salinity and molecular characters (oxygen‐to‐carbon ratio; double bond equivalent; molecular weight) of DBCFT. These findings collectively suggest that mixing processes primarily control the concentration and composition of DBC. A two‐end‐member mixing model revealed that riverine input and offshore water contributed ~ 22% and 63%, respectively, to the DBCBPCA in the study region. Additionally, atmospheric deposition and potentially other unidentified sources contributed at least ~ 15% DBCBPCA, assuming no removal. Furthermore, ~ 70% of DBCFT in the coastal seawater could be found in the river water, and aerosols, which was in line with the information from DBCBPCA. Our study provides the first comprehensive assessment of both the sources and influential processes of DBC in coastal seas, highlighting the importance of additional sources, such as atmospheric deposition, and emphasizing the need to identify previously unknown sources of DBC.

Multi-molecular 14C evidence for mineral control on terrestrial carbon storage andexport.

Compound- and compound class-specific radiocarbon analysis of source-diagnostic 'biomarker' molecules has emerged as a powerful tool to gain insights into terrestrial carbon cycling. While most studies thus far have focused on higher plant biomarkers (i.e. plant leaf-wax n-alkanoic acids and n-alkanes, lignin-derived phenols), tracing paedogenic carbon is crucial given the pivotal role of soils in modulating ecosystem carbon turnover and organic carbon (OC) export. Here, we determine the radiocarbon (14C) ages of glycerol dialkyl glycerol tetraethers (GDGTs) in riverine sediments and compare them to those of higher plant biomarkers as well as markers of pyrogenic (fire-derived) carbon (benzene polycarboxylic acids, BPCAs) to assess their potential as tracers of soil turnover and export. GDGT Δ14C follows similar relationships with basin properties as vegetation-derived lignin phenols and leaf-wax n-alkanoic acids, suggesting that the radiocarbon ages of these compounds are significantly impacted by intermittent soil storage. Systematic radiocarbon age offsets are observable between the studied biomarkers, which are likely caused by different mobilization pathways and/or stabilization by mineral association. This article is part of the Theo Murphy meeting issue 'Radiocarbon in the Anthropocene'.

A novel method for producing benzene polycarboxylic acids by electrochemical oxidation of Zhaotong lignite in aqueous NaCl solution

The development of a coal-based synthetic route to produce benzene polycarboxylic acids (BPCAs) is of great importance for the highly efficient utilization of lignites. In this paper, aqueous NaCl electrolytic system was used to oxidize Zhaotong lignite to prepare BPCAs. The electrochemical oxidation of lignite in aqueous NaCl solution could produce more BPCAs than that in aqueous NaOH solution. The aqueous NaCl electrolytic system could in-suit produce a stable OCl−, which was synthesized by the combination reaction between Cl2 and OH− generated in the anode or cathode, respectively. The in-suit produced OCl− would degrade the organic structures of the lignite dispersing in the electrolyte to generate BPCAs. The formation of BPCAs could be greatly affected by current density, electrolysis time and the addition amount of NaCl in the electrolytic system, which resulted from that the factors played an important role in the generation of OCl−. The coal related model compounds including anthracene and phenanthrene were used to investigate the electrochemical oxidation mechanism of the lignite. The results indicated that the aromatic ring structures in the lignite were attacked by O2− from the OCl− to afford BPCAs.

Fire history of the western Amazon basin recorded by multiple pyrogenic carbon proxies

Charcoal abundance has been used as a fire proxy to reconstruct human settlement and climate records, but analyses of pyrogenic carbon (PyC) have become more widespread. To compare these proxies, fire records spanning the past 3000 y of Western Amazon Lakes Ayauchi, Gentry and Parker region soil and sediments were determined by visually identified charcoal, chemical-thermal oxidation (CTO) and benzene polycarboxylic acid (BPCA) markers. Charcoal represented the smallest portion of PyC and, with its irregular distribution, it likely indicated only local or large fires. PyC via CTO concentrations were highest and most uniform, suggesting lower sensitivity in detecting fire. BPCA-derived PyC distributions in soils were consistent with more human occupation and higher temperature fires in the Lake Ayauchi region and, in lake sediments, BPCA-derived PyC bore the closest resemblance to patterns of climate and human influence. Thus, BPCAs were more sensitive indicators of fire over regional scales. The combined information from multiple fire proxies revealed a history of frequent fires in western Amazon. Lake Ayauchi region had greater fire frequency and higher temperature fires than the Gentry/Parker region, suggesting greater human occupation. The greater fire frequency in wetter periods and regions of western Amazon, suggest a major influence of human activity on PyC production over the past 3000 years. Each fire proxy examined in this study had strengths and weaknesses and the use of a combination of fire proxies can provide a fuller picture of fire history than any single approach.

Compound-specific radiocarbon analysis of benzene polycarboxylic acids for source apportionment of polyaromatic organic matter in ambient aerosols

Polyaromatic organic matter (POM) is an important group of pollutants and light absorbers in ambient aerosols, which consists of a wide range of chemicals with fused benzene rings. POM in ambient aerosols is mainly derived from the incomplete combustion of fossil fuel and biomass. Source apportionment of POM is crucial for advising efficient mitigation of anthropogenic emissions, but that is a challenge due to the complicated composition of POM and ambient aerosols. Benzene polycarboxylic acids (BPCAs) have recently been introduced as molecular markers of atmospheric POM. Compound-specific radiocarbon analysis of BPCAs is expected to be a powerful tool for apportioning fossil sources and contemporary (i.e., biomass burning) sources of POM in ambient aerosols, yet this application is still lacking. We developed a method for radiocarbon analysis of BPCAs substituted with 3–6 carboxylic groups (B6CA, B5CA, B4CAs and B3CAs). BPCAs were isolated with preparative liquid chromatography with high recoveries (≥85%). The method is validated with reference materials with fossil and contemporary radiocarbon signatures. Successful radiocarbon analysis of BPCAs was achieved for these reference materials after correcting for the presence of average blanks (B6CA: 1.2 ± 0.2 μg, B5CA: 2.3 ± 0.6 μg, individual B4CAs: 2.7 ± 0.2 μg and individual B3CAs: 6.9 ± 0.7 μg). Source apportionment of POM based on radiocarbon analysis of BPCAs was performed for an urban dust reference material, as well as bulk aerosols and water-soluble, methanol-soluble fractions of ambient aerosols from a megacity of southern China. The results show varied contributions of fossil and contemporary sources among different POM fractions.

Inducing Inorganic Carbon Accrual in Subsoil through Biochar Application on Calcareous Topsoil

Biochar amendments add persistent organic carbon to soil and can stabilize rhizodeposits and existing soil organic carbon (SOC), but effects of biochar on subsoil carbon stocks have been overlooked. We quantified changes in soil inorganic carbon (SIC) and SOC to 2 m depth 10 years after biochar application to calcareous soil. The total soil carbon (i.e., existing SOC, SIC, and biochar-C) increased by 71, 182, and 210% for B30, B60, and B90, respectively. Biochar application at 30, 60, and 90 t ha-1 rates significantly increased SIC by 10, 38, and 68 t ha-1, respectively, with accumulation mainly occurring in the subsoil (below 1 m). This huge increase of SIC (mainly CaCO3) is ∼100 times larger than the inorganic carbon present in the added biochar (0.3, 0.6, or 0.9 t ha-1). The benzene polycarboxylic acid method showed that the biochar-amended soil contained more black carbon particles (6.8 times higher than control soil) in the depth of 1.4-1.6 m, which provided the direct quantitative evidence for biochar migration into subsoil after a decade. Spectral and energy spectrum analysis also showed an obvious biochar structure in the biochar-amended subsoil, accompanied by a Ca/Mg carbonate cluster, which provided further evidence for downward migration of biochar after a decade. To explain SIC accumulation in subsoil with biochar amendment, the interacting mechanisms are proposed: (1) biochar amendment significantly increases subsoil pH (0.3-0.5 units) 10 years after biochar application, thus forming a favorable pH environment in the subsoil to precipitate HCO3-; and (2) the transported biochar in subsoil can act as nuclei to precipitate SIC. Biochar amendment enhanced SIC by up to 80%; thus, the effects on carbon stocks in subsoil must be understood to inform strategies for carbon dioxide removal through biochar application. Our study provided critical knowledge on the impact of biochar application to topsoil on carbon stocks in subsoil in the long term.