Water-soluble Organic Carbon Research Articles

Oxidative potential and source apportionment of size-resolved particles from indoor environments: Dithiothreitol (DTT) consumption and ROS production

Oxidation potential (OP) is an emerging index to measure the toxicity of atmospheric particulate matter (PM), but little research data on indoor PM OP are available. This study focuses on the size-resolved particle OP and source apportionment of PM from indoor environments. Different-size PM samples were gathered from March 6, 2022, to April 18, 2022, from offices, canteens, dormitories, and laboratories using Anderson samplers. Particles from these indoor environments were separated into nine size fractions, and the chemical components (including transition metals, ions, and water-soluble organic carbon (WSOC)) were measured. The water-soluble OP (OPws) of the investigated PM was highest in the dormitory (61.02 nmol min−1 μg−1) and lowest in the canteen (25.58 nmol min−1 μg−1), varying by factors of up to 2.38. Additionally, the particulate Mn (r2 = 0.94), Fe (r2 = 0.93), and Zn (r2 = 0.92) concentrations were closely related to OPws. The OPws distribution in indoor PM samples featured a single peak from 0.7 to 3.2 μm. ROS (·OH) showed the same trend (1.1–3.2 μm) as PM OP, which shows that PM3.2 is the main contributor to adverse health effects among all size fractions of PM due to the strengthened oxidative ability of PM3.2 compared to PM > 3.2. The source apportionment results showed that secondary sources (52.3%), construction sources (28%), vehicle emissions (18.5%), and dust sources (1.2%) were the main sources of indoor PM OP. Multiple path particle dosimetry (MPPD) model tests evidenced that PM3.2 control the lung OPws and ROS (accounting for >71.49% and 68.07%, respectively) deposition because of the high deposition efficiency and metal concentration in the alveolar region. The experimental trend of OP was closely related to that of ROS. Overall, since OPws is directly associated with diverse health effects for PM OP, special attention to PM3.2 impact.

Residential combustion of coal: Effect of the fuel and combustion stage on emissions

Worldwide coal is still used for household heating purposes not only because it is available and cheap but also due to behavioural issues. Regional variability in fuels and combustion appliances make accurate emission estimates from this source hard to achieve. In the present study, gaseous (CO, VOCs, SO2 and NOX) and particulate matter (TSP) emission factors (EFs) were determined for Spanish household coal combustion covering three commercial coals and distinct combustion stages and mimicking usage patterns in real households. TSP samples were analysed to determine water-soluble inorganic ions, metal(loid)s, and organic and elemental carbon (OC and EC). Additionally, the morphology of the emitted particles was also characterised.CO (3.43–169 g kg−1), NOX (1.29–6.00 g kg−1) and SO2 (8.96–22.3 g kg−1) EFs showed no trend regarding the combustion stage or coal type tested. On the other hand, VOC, TSP and EC EFs were higher for the ignition/devolatilisation combustion stage, regardless of the fuel tested. TSP EFs (0.085–1.08 g kg−1) increased with increasing coal volatile matter while the opposite trend was recorded for VOC emissions (0.045–3.39 gC kg−1).TSP carbonaceous matter was dominated by EC while OC represented a small fraction of the particulate mass emitted (less than 8 %wt.). Inorganic compounds composed an important fraction of the TSP samples. Sulphate particulate mass fractions (8.66–22.9 %wt.) appeared to increase with coal S-content. Coal combustion released particles with diverse morphologies, including silicate-rich particles, ferro- and glassy-spheres. This study provides novel emission factors to update emission inventories of residential coal combustion. Additionally, detailed chemical profiles were obtained for source apportionment.

Seasonal Variation and Soil Texture-Related Thinning Effects on Soil Microbial and Enzymatic Properties in a Semi-Arid Pine Forest

Thinning is a practice that reduces competition for available soil resources, thereby promoting vegetation growth and affecting soil, which is involved in important ecosystem processes. Soil quality is directly influenced by various aspects such as ground cover, regional climate, and local microclimate, which can further be modified by forest thinning. In this study, the effect of tree thinning and climate on microbiological and enzymatic soil properties was investigated in an Aleppo pine (Pinus halepensis M.) forest more than a decade after silvicultural treatments. The treatments included were clear-felling (100% of mean basal area (BA) removed), moderate thinning (60% BA removed), and control (no thinning). Soil organic carbon (SOC), water-soluble organic carbon (WSOC), basal soil respiration (BSR), microbial biomass carbon (MBC), soil enzymes (β-glucosidase, acid phosphatase, urease, and dehydrogenase), general soil characteristics, soil temperature and humidity, and precipitation were compared seasonally for over two years by analysis of variance and multivariate analysis. Results showed that the effect of 60% thinning improved soil microbial and enzymatic soil properties with variable results, mainly depending on soil organic matter content and soil texture. SOC, WSOC, and MBC were highly correlated with BSR and enzymatic activities. The main reason for the observed differences was water availability, despite a large seasonal variation. In conclusion, microbial activity was strongly affected by soil characteristics and climate, which in turn were influenced by the silvicultural treatments applied. Moderate thinning can be used as a useful practice to improve soil quality in the Mediterranean area.

Wintertime characteristics of water-soluble organic carbon in PM2.5 during haze and non-haze days in Jinan in the North China Plain

Water-soluble organic carbon (WSOC) makes up a substantial proportion of organic carbon (OC) in fine particles that can disrupt aerosol hygroscopicity, influence global climate change, and harm human health. In the winter of 2014, carbonaceous species and water-soluble inorganic ions (WSII) were analyzed in PM2.5 samples collected from Jinan in the North China Plain. According to the results, the primary components of PM2.5 include SO42−, NO3−, NH4+ (SNA) and organic carbonaceous aerosol. Except for Ca2+, the average concentration of each chemical component was higher on hazy days than on clear days. Secondary aerosol generation probably played a vital role in haze production, as the relative abundance of secondary inorganic ions and secondary organic carbon was higher on haze days compared to non-haze days. The average WSOC concentration during the sampling period was 10.89 ± 4.66 μg m−3, accounting for 0.47 ± 0.14 of OC. The level of WSOC increased from 9.60 ± 3.32 μg m−3 on days without haze to 16.94 ± 5.43 μg m−3 on haze days. Analyses of linear regression and quantitative calculations demonstrated that burning of biomass and the generation of secondary aerosols were essential contributors to WSOC. WSOC increased with relative humidity/aqueous water content, suggesting that secondary WSOC (WSOCsec) may originate from reactions in the aqueous phase, particularly on haze days. The absorption Ångstrom exponents (AAE) were 6.55 ± 1.79 on non-haze days and 6.59 ± 0.90 on haze days. The average mass absorption efficiency (MAE) at 365 nm on haze days (1.07 ± 0.21 m2 g−1) was higher than on non-haze days (0.88 ± 0.33 m2 g−1), indicating the slightly stronger WSOC's light absorption ability during haze times. Four quantitative sources of WSOC were identified using positive matrix factorization modeling: (i) a mixed primary source (21.78%), (ii) biomass burning (22.14%), (iii) a process comparable to NO3− formation (33.17%) and (iv) a process similar to SO42− formation (22.91%). Comparatively, secondary sources (factors 3 and 4) contributed more to WSOC (56.08%) than primary sources (43.92%, factors 1 and 2). Secondary formation contributed more to WSOC during haze conditions.

Water-soluble organic aerosols over South Asia – Seasonal changes and source characteristics

Water-soluble organic carbon (WSOC) has been identified as a key component in atmospheric aerosols due to its ability to act as cloud condensation nuclei (CCN) owing to their highly hygroscopic nature. This paper discusses about the spatio-temporal variability in WSOC mass concentration, sources (primary and secondary contributions), the role of long-range air-mass transport in modulating their abundance, at distinct sectors over South Asia. We found from our observations that, photochemical ageing of primary organic aerosols that are derived from biomass emissions, significantly contribute to the total WSOC budget over South Asia. The wide range of water-soluble compounds released by biomass burning can contribute directly to the WSOC fraction or undergo further atmospheric processing, such as oxidation or ageing, leading to the formation of additional WSOC. WSOC/OC (organic carbon) ratio and the correlation between the WSOC and secondary organic carbon (SOC) are used for assessing the contribution from secondary sources. The three different ratios are used to delineate different source processes; OC/EC (elemental carbon) for source identification, WSOC/OC for long-range atmospheric transport (ageing) and WSOC/SOC to understand the primary and secondary contribution of WSOC. The present investigation revealed that, the primary OC that have undergone significant chemical processing as a result of long-range transport have a substantial influence on WSOC formation over South Asia, especially in Indo Gangetic Plain outflow regions such as southern peninsular and adjacent marine regions. Overall, oxidation and ageing of primary organic aerosols emitted from biomass burning was found to serve as an important source of WSOC over South Asia.

In Situ Surface Tension Measurement of Deliquesced Aerosol Particles.

The surface tension of aerosol particles can potentially affect cloud droplet activation. Hence, direct measurement of the surface tensions of deliquesced aerosol particles is essential but is challenging. Here, we report in situ surface tension measurements based on a novel method that couples a linear quadrupole electrodynamic balance (EDB) with quasi-elastic light scattering (QELS). The EDB-QELS is validated using surface tension measurements of atmospherically relevant inorganic and organic droplets. The surface tension results reasonably agree with the reference values in the range of ∼50-90 mN m-1. We find a significant size dependence for sodium chloride droplets containing surface-active species (sodium dodecyl sulfate) in the size range of ∼5-18 μm. The surface tension increases from ∼55 to 80 mN m-1 with decreased size. Relative humidity (RH)-dependent surface tensions of mixed ammonium sulfate (AS) and polyethylene glycol droplets reveal the onset of liquid-liquid phase separation. Droplets containing water-soluble matter extracted from ambient aerosol samples and 2.3-2.9 M AS exhibit a ∼30% reduction in surface tension in the presence of ∼50 mmol-C L-1 water-soluble organic carbon, compared to pure water (∼72 mN m-1). The approach can offer size-resolved and RH-dependent surface tension measurements of deliquesced aerosol particles.

The Effect of Various Forms of Sulfur on Soil Organic Matter Fractions and Microorganisms in a Pot Experiment with Perennial Ryegrass (Lolium perenne L.).

This article focuses on the agronomic evaluation of the supplementation of mineral NPKMg fertilizers with elemental sulfur, magnesium, potassium, or ammonium sulfates in pot experiments with ryegrass growing in a sandy Arenosol with very low sulfur content. A benefit evaluation was carried out on the basis of biomass production, crop nutritional status, and changes in the content of soil organic matter fractions. Furthermore, the total number of bacteria, nitrogen-fixing bacteria, and fungi was estimated using the qPCR technique in soil samples after 60 days of ryegrass growth. The combined application of NPKMg and sulfur or sulfate fertilizers significantly increased the summary yield of three cuttings of fresh ryegrass biomass in the range of 32.3% to 82.7%. The application, especially in the form of sulfates, significantly decreased the content of free phenolic acids. Furthermore, compared to the control, i.e., soil with NPKMg applied alone, an increase in the content of glomalin-related proteins and a decrease in the amount of water-soluble organic carbon compounds were observed. Neither the number of DNA marker copy numbers of the total bacterial community nor the nitrogen-fixing bacteria were noticeably different. In turn, the total number of genetic markers for fungi was significantly higher in soils with potassium or ammonium sulfates compared to the control soil. The general results suggest that the application of sulfur fertilizers with NPKMg mineral fertilizer can benefit crops and support soil fertility due to the stabilization of aggregates and the decrease in water-soluble organic compounds.

Water-soluble organic carbon (WSOC) from vegetation fire and its differences from WSOC in natural media: Spectral comparison and self-organizing maps (SOM) classification

Vegetation fire frequently occurs globally and produces two types of water-soluble organic carbon (WSOC) including black carbon WSOC (BC-WSOC) and smoke-WSOC, they will eventually enter the surface environment (soil and water) and participate in the eco-environmental processes on the earth surface. Exploring the unique features of BC-WSOC and smoke-WSOC is critical and fundamental for understanding their eco-environmental effects. Presently, their differences from the natural WSOC of soil and water remain unknown. This study produced various BC-WSOC and smoke-WSOC by simulating vegetation fire and used UV–vis, fluorescent EEM-PARAFAC, and fluorescent EEM-SOM to analyze their different features from natural WSOC of soil and water. The results showed that the maximum yield of smoke-WSOC reached about 6600 folds that of BC-WSOC after a vegetation fire event. The increasing burning temperature decreased the yield, molecular weight, polarity, and protein-like matters abundance of BC-WSOC and increased the aromaticity of BC-WSOC, but presented a negligible effect on the features of smoke-WSOC. Furthermore, compared with natural WSOC, BC-WSOC had a greater aromaticity, smaller molecular weight, and more humic-like matters, while smoke-WSOC had a lower aromaticity, smaller molecular size, higher polarity, and more protein-like matters. EEM-SOM analysis indicated that the ratio between the fluorescence intensity at Ex/Em: 275 nm/320 nm and the sum fluorescence intensity at Ex/Em: 275 nm/412 nm and Ex/Em: 310 nm/420 nm could effectively differentiate WSOC of different sources, following the order of smoke-WSOC (0.64–11.38) > water-WSOC and soil-WSOC (0.06–0.76) > BC-WSOC (0.0016–0.04). Hence, BC-WSOC and smoke-WSOC possibly directly alter the quantity, properties, and organic compositions of WSOC in soil and water. Owing to smoke-WSOC having far greater yield and bigger difference from natural WSOC than BC-WSOC, the eco-environmental effect of smoke-WSOC deposition should be given more attention after a vegetation fire.

Soil organic carbon dynamics shift by incorporating wheat straw in paddy soil in China.

The objective of this study was to explore the effects of unfertilized control (CK), mineral NPK fertilizer (NPK), NPK plus medium-rate wheat straw (MSNPK), and NPK plus high-rate wheat straw (HSNPK) on soil organic carbon (SOC) fractions and C-cycle enzymes at distinct depths (0-5, 5-10, 10-20, 20-30, and 30-50cm) in paddy soil. The SOC content at 0-50cm depth ranged from 8.50 to 21.15g kg-1 , following the trend HSNPK>MSNPK>NPK>CK. Correspondingly, the content of water-soluble organic carbon (WSOC), microbial biomass carbon (MBC), particulate organic carbon (POC), and easily oxidizable carbon (EOC) ranged from 0.08 to 0.27g kg-1 , 0.11 to 0.53g kg-1 , 1.48 to 8.29g kg-1 , and 3.25 to 7.33g kg-1 , respectively, and HSNPK exhibited the highest values for these parameters among all treatments and soil depths, with significant differences observed compared to NPK and CK (p<0.05). The cellulase activity of HSNPK was significantly (p<0.05) higher by 6.12%-13.30% compared to the CK at 0-30cm depth; while the activity of invertase and β-glucosidase of HSNPK were significantly (p<0.05) higher by 34.09%-433.43% and 26.61%-130.50%, respectively, compared to the CK at 0-50cm depth. Enzyme activities were significantly (p<0.05) correlated with SOC fractions and the dominant factors driving changes in enzyme activities were WSOC, POC, and EOC. HSNPK was associated with the highest SOC fractions and enzyme activities, indicating that it was the most favorable management practice for promoting soil quality in rice paddy fields.

Light absorption properties of brown carbon aerosol after following the switch from coal to natural gas policy in China: A case study of winter particulate matter over Xi'an, northwestern China

To control haze pollution, the Xi'an city in northwestern China replaced coal by natural gas in industrial and household heating at the end of 2017. Following this change we studied here the light absorption properties of brown carbon (BrC)in atmospheric particulate matter (PM2.5) over Xi'an in 2018. PM2.5 samples were collected from November 15 to December 25, 2018, and the light absorption properties of brown carbon in methanol-soluble organic carbon (MSOC) and water-soluble organic carbon (WSOC) were analyzed. Compared to a previous study from the coal combustion period, the mass absorption efficiency (MAE) increased in the sampling period while the radiation-forced absorption fraction of brown carbon relative to elemental carbon reduced, indicating that the light-absorbing capacity and efficiency of brown carbon has improved. Furthermore, highly positive linear relationships (r > 0.80) occur between the absorption coefficient and typical products of biomass burning and vehicle emission, except for coal combustion (r = 0.57). Additionally, an estimation from the positive matrix factorization and multivariable linear regression model results showed that biomass burning, motor vehicle emission, and secondary source took up approximately 89.53% of AbsWSOC,365. The contribution of coal combustion to light absorption BrC was the lowest (7.36% for AbsMSOC,365 and 10.24% for AbsWSOC,365) in our study, which differs from previous studies in Xi'an. Therefore, the emission source of bulk coal combustion has a relatively small influence on the light absorption of BrC in Xi'an.

Responses of Soil CO2 Emission and Tree Productivity to Nitrogen and Phosphorus Additions in a Nitrogen-Rich Subtropical Chinese Fir Plantation

Nitrogen (N) and phosphorus (P) nutrients have been regularly applied to improve productivity in intensively managed and short-rotation forest plantations in subtropical China. Under the constraint of the national policy of “carbon neutrality”, it is necessary to determine the rational fertilization options by considering both forest productivity and soil CO2 emissions. Past worldwide studies have shown varied responses of forest soil heterotrophic respiration and CO2 emissions to N and P additions. This study designed six treatments with N additions (high level: 15 g N/m2, HN), P (low: 5 g P/m2, LP; high: 15 g P/m2, HP), and their interactions (HNLP and HNHP) to explore the effects of N and P additions on soil CO2 emissions in a P-limited and N-rich Chinese fir plantation (Cunninghamia lanceolata), and we identified the underlying controls using the structural equation model (SEM). The results indicated that LP, HNLP, and HNHP treatments significantly increased soil CO2 emissions in the first four months after treatment and the effects leveled since then. The balance between N and P inputs affected the responses of soil CO2 emissions to P additions. A low P addition significantly increased tree productivity, but the promoting effect gradually declined and was no longer significant after 3 years. Other treatments did not significantly affect tree productivity. The SEM analysis revealed that the promoting effects of P additions on CO2 emission were mainly due to their effects on increasing soil water-soluble organic carbon content and reducing microbial biomass nitrogen content. Considering both soil respiration and tree productivity, this study suggested that LP treatment can effectively balance the N and P nutrients and, in the meantime, maintain relatively low greenhouse gas emissions; thus a low P application level is suggested for N-rich Chinese fir plantations.

Optical properties and molecular composition of wintertime atmospheric water-soluble organic carbon in different coastal cities of eastern China

To evaluate the optical properties and molecular composition of water-soluble organic carbon (WSOC) in the atmosphere of coastal cities, particle samples were collected in Tianjin, Qingdao and Shanghai, three coastal cities in eastern China. Subsequent analysis by ultraviolet visible and fluorescence spectrometer and electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry were performed. The results showed that the concentration levels and light absorption ability of WSOC decreased from the north to south cities, ranking as Tianjin > Qingdao > Shanghai. Three major fluorescent components including less‑oxygenated humic-like substances (52–60 %), highly‑oxygenated humic-like substances (15–31 %) and protein-like substances (17–31 %) were identified in WSOC based on the fluorescence spectroscopy and parallel factor analysis, which might be closely related to anthropogenic emissions and continental sources as well as secondary formation processes. Five subgroups of molecular components were further identified in WSOC, including the predominant CHON compounds (35–43 %), sulfur-containing compounds (i.e., CHONS and CHOS compounds, 24–43 %), CHO compounds (20–26 %) and halogen-containing compounds (1–7 %). Compared to marine air masses influenced samples, WSOC affected by continental air masses exhibited higher light absorption coefficients and generally had a higher degree of aromaticity and unsaturation, as well as contained more molecular formulas of WSOC, especially enriched with sulfur-containing compounds. In contrast, relatively more abundant halogen-containing compounds were identified in the marine air masses influenced samples. Overall, this study provided new insights into the light-absorbing and chemical properties of WSOC in coastal cities, especially under the influences of continental and marine air masses.

Fine aerosol sources at an urban background site in the Eastern Mediterranean (Nicosia; Cyprus): Insights from offline versus online source apportionment comparison for carbonaceous aerosols

A total of 348 daily PM2.5 samples were collected at an urban background site of Nicosia, capital of Cyprus, for one-year period (October 2018–October 2019) to assess the origin and sources of fine PM at the Eastern Mediterranean, a poorly characterized area of the world.The samples were analysed for water soluble ionic species, elemental and organic carbon, carbohydrates and trace metals, the combination of which were utilized to identify pollution sources by applying Positive Matrix Factorization (PMF). Six PM2.5 sources, namely long-range transport (LRT; 38 %), traffic (20 %), biomass burning (16 %), dust (10 %), sea salt (9 %) and heavy oil combustion (7 %), were identified.Despite sampling in an urban agglomeration, the chemical fingerprint of the aerosol is largely dictated by air mass origin rather than local sources. Springtime is characterized by the most elevated particulate levels due to the southerly air masses carrying particles from the Sahara Desert. Northerlies are observed throughout the year but are predominant during summer allowing the LRT source to peak (54 % during summer). Only during winter, due to extensive use of biomass combustion for domestic heating (36.6 % during winter), local sources dominate.A co-located online PMF source apportionment of submicron carbonaceous aerosols (Organic Aerosols, OA; Black Carbon) was performed by the means of an Aerosol Chemical Speciation Monitor (for OA) and an Aethalometer (for BC) for a four-month period. The comparison between the two methodologies allowed to better assess the robustness and limitations of the two methodologies. More specifically, LRT OA and biomass burning BC apportioned by the offline PMF showed a strong consistency with the online apportioned more oxidized oxygenated OA and BCwb, respectively; cross validating these sources. On the other hand, our traffic factor may contain additional hydrocarbon-like OA and BC from fossil fuel sources other than just vehicular emissions. Finally, the offline biomass burning OA source is likely to contain both primary and secondary OA.

Emission and optical characteristics of brown carbon in size-segregated particles from three types of Chinese ships

Brown carbon (BrC) is one of the important light absorption substances that have high light absorption ability under short wavelength light. However, limit studies have focused on the BrC emission from ships. In this study, size-segregated particulate matters (PM) were collected from three different types of ships, light absorption characteristics and size distribution of methanol-soluble BrC and water-soluble BrC in PM from ship exhausts were investigated. Results showed that four-stroke low-power diesel fishing boat (4-LDF) had the highest mass concentrations of methanol-soluble organic carbon (MSOC) and water-soluble organic carbon (WSOC), followed by 2-stroke high-power heavy-fuel-oil vessel (2-HHV), and four-stroke high-power marine-diesel vessel (4-HMV). While 2-HHV had obviously higher light absorption coefficients of methanol-soluble BrC (Abs365,M) and water-soluble BrC (Abs365,W) in unit weight of PM than the other two types of ships. The tested ships presented comparable or higher absorption efficiency of BrC in water extracts (MAE365,W) compared with other BrC emission sources. Majority of BrC was concentrated in fine particles, and the particle size distributions of both Abs365,M and Abs365,W showed bimodal patterns, peaking at 0.43–0.65 µm and 4.7–5.8 µm, respectively. However, different particle size distributions were found for MAE365,M between diesel and heavy fuel oil ships. Besides, different wavelength dependence in particles with different size were also detected. Ship exhaust could be confirmed as a non-ignorable BrC emission source, and complex influencing factor could affect the light absorption characteristics of ship emissions. Particle size should also be considered when light absorption ability of BrC was evaluated.

Characterization of Dissolved Organic Matter Released from Aged Biochar: A Comparative Study of Two Feedstocks and Multiple Aging Approaches.

Dissolved organic matter (DOM) plays important roles in environmental ecosystems. While many studies have explored the characteristics of aged biochar, limited information is available about the properties of DOM derived from aged biochar. In this study, biochar obtained from maize stalk and soybean straw were aged using farmland or vegetable-soil solution, as well as soil solution containing hydrogen peroxide (H2O2). Chemical composition of the extracted DOM from the aged biochar was analyzed via excitation-emission matrix coupled with fluorescence regional integration (FRI) and parallel factor analysis (PARAFAC). Obtained results showed that biochar aged with H2O2-enriched soil solution had higher water-soluble organic carbon, ranging from 147.26-734.13% higher than the controls. FRI analysis revealed fulvic and humic-like organics as the key components, with a considerable increase of 57.48-235.96% in the humic-like component, especially in soybean-straw-aged biochar. PARAFAC identified four humic-like substance components. Concurrently, the aromaticity and humification of the aged-biochar-derived DOM increased, while the molecular weight decreased. These findings suggest that DOM derived from aged biochar, with a high content of humic-like organics, might impact the mobility and toxicity of pollutants in soil.

Triplet-Excited Dissolved Organic Matter Efficiently Promoted Atmospheric Sulfate Production: Kinetics and Mechanisms

Photochemical generation is an important pathway for atmospheric sulfate formation. However, the roles of atmospheric co-existing photosensitive dissolved organic matter (DOM) in sulfate formation are still unclear. The experimental results in this work provide evidence that atmospheric photosensitizers produce active intermediates to oxidize S(IV) into S(VI) under illumination. Quenching experiments of eight photosensitive model compounds (PS) demonstrate that their triplet-excited states (3PS*) dominate sulfate formation for the photosensitizing pathway with a contribution of over 90%, and 1O2 plays an important role in sulfate formation. The results using humic acid (HA) and water-soluble organic carbon in vehicle exhaust particles (WSOC) as representatives of atmospheric photosensitizers further verify that triplet-excited DOM (3DOM*) is the main reactive species for sulfate formation, which is consistent with the results of PS. Our findings provide new insights into the photochemical formation pathways of atmospheric sulfate.

Soil Destabilization Induced by Water Wetting Treatments Simulating Rain Infiltration Processes, Studied via Rheology and Granulometry

A soil depleted of its organic carbon content is typically destabilized, i.e., its capacity to maintain its microstructure intact under various stress conditions weakens, and consequently, landslides and mudflows can be triggered and propagated more easily. In a previous work, we showed with a rheological analysis that the removal of the sole water-soluble organic carbon “destabilized” the slurry very similarly to what occurs with the removal of the vast majority of soil organic carbon. In principle, the water-soluble organic carbon can be dissolved by rainfall, during which water can infiltrate the soil, eventually leaving it either by percolation or evaporation. These two processes are mimicked here with two different soil water wetting procedures. The stability of the treated (wetted) soils is studied with rheological and granulometric experiments. The former run on concentrated suspensions, while the latter run on very diluted ones. Despite this, the results agreed very well, indicating that the two wetting procedures induce the same destabilization of the soil which behaves as the one depleted by the whole water-soluble organic carbon. Our results concluded that a soil destabilized by a wetting procedure, i.e., by a rainfall event, will be more prone to trigger a landslide that will propagate more swiftly and will stop with more difficulties.

Water-soluble organic carbon release from mineral soils and sediments in an irrigated agricultural system

Water interactions with soil and vegetation are greatly altered in agricultural watersheds compared to natural landscapes, which impacts sources and fates of organic carbon (OC). While mineral soil horizons in natural ecosystems primarily act as filters for dissolved organic carbon (DOC) leached from organic surface horizons, tilled soils largely lack an organic horizon and their mineral horizons therefore act as a source for both DOC and sediment to surface waters. Irrigated watersheds highlight this difference, as DOC and total suspended sediment (TSS) concentrations simultaneously increase during the low-discharge irrigation season, suggesting that sediment-associated OC may constitute a significant source of DOC. While water-soluble OC (WSOC) from sediments and soils has been found to be compositionally similar to stream DOC, these contributions remain poorly quantified in agricultural streams. To address this, we conducted abiotic solubilization experiments using sediments (suspended and bed) and soils from an irrigated agricultural watershed in northern California, USA. Sediments (R2 > 0.99) and soils (0.74 < R2 < 0.89) displayed linear solubilization behaviors over the range of concentrations tested. Suspended sediment from the irrigation season exhibited the largest solubilization efficiency (10.9 ± 1.6% TOCsediment solubilized) and potential (1.79 ± 0.26 mg WSOC g−1 dry sediment), followed by suspended sediment from a winter storm, then bed sediment and soils. Successive solubilization experiments increased the total release of WSOC by ∼50%, but most (88–97%) of the solid-phase OC remained insoluble in water. Using these solubilization potential estimates and measured TSS concentrations, we estimated that WSOC from suspended sediment in streams represented 4–7% of the annual DOC export from the watershed. However, field sediment export is much higher than what is represented by suspended sediment in the water column, therefore field-scale contributions from sediments could be much higher than estimated.

Response of soil organic carbon and its fractions to natural vegetation restoration in a tropical karst area, southwest China

The dynamics of soil organic carbon (SOC) and its fractions are important for evaluating the vegetation restoration effect and carbon cycle of the ecosystem. Here, SOC fractions, including light fraction organic carbon (LFOC), heavy fraction organic carbon (HFOC), and labile organic carbon (LOC) fractions (including water-soluble organic carbon, WSOC, readily oxidizable organic carbon, ROC, particulate organic carbon, POC, and microbial biomass carbon, MBC), were investigated at four soil depths under five restoration stages in a tropical karst area in southwest China. This study showed that the content of SOC and its fractions significantly increased with vegetation restoration and decreased with increasing soil depth at each restoration stage (p &amp;lt; 0.05). Additionally, LFOC was more sensitive to vegetation restoration, whereas HFOC was the main storage form of SOC. The LOC fractions in the surface soil layer were significantly higher than those in the lower, and the percentages of some LOC fractions (POC/SOC and MBC/SOC) significantly decreased with increasing soil depth, indicating that SOC was more stable in the lower layer than in the surface layer. Correlation analysis showed that SOC was significantly and positively correlated with its fractions. Moreover, SOC and its fraction were positively correlated with soil chemical factors (TN, TP, AP, AK, ECa, EMg, NH4+-N, and NO3–-N) and negatively correlated with bulk density (BD) at a significant level (p &amp;lt; 0.05). Moreover, redundancy analysis showed that the 12 soil physicochemical factors explained 70.99% of the variation in SOC and its fractions, with AK, NH4+-N, and BD being the main factors, explaining 19.38, 17.24, and 10.52% of the variation, respectively. The structural equation mode analysis showed that soil properties, above-ground biomass, and litterfall explained most of the variation in SOC (59%), LFOC (79%), HFOC (81%), and LOC (61%). Soil properties and above-ground biomass significantly affected SOC, LFOC, and HFOC content mainly through indirect effects, while the total phosphorus content of the litterfall could directly and significantly affect SOC, LFOC, and HFOC content. NH4+-N and AK of soil factors had direct effects on LFOC and LOC accumulation, respectively. This study provides a valuable perspective for estimating carbon sink potential and constructing carbon sink models in karst areas.

Chemical Characterization and Source Apportionment of PM10 Using Receptor Models over the Himalayan Region of India

This study presents the source apportionment of coarse-mode particulate matter (PM10) extracted by 3 receptor models (PCA/APCS, UNMIX, and PMF) at semi-urban sites of the Indian Himalayan region (IHR) during August 2018–December 2019. In this study, water-soluble inorganic ionic species (WSIIS), water-soluble organic carbon (WSOC), carbon fractions (organic carbon (OC) and elemental carbon (EC)), and trace elements of PM10 were analyzed over the IHR. Nainital (62 ± 39 µg m−3) had the highest annual average mass concentration of PM10 (average ± standard deviation at 1 σ), followed by Mohal Kullu (58 ± 32 µg m−3) and Darjeeling (54 ± 18 µg m−3). The annual total ∑WSIIS concentration order was as follows: Darjeeling (14.02 ± 10.01 µg m−3) &gt; Mohal-Kullu (13.75 ± 10.21 µg m−3) &gt; Nainital (10.20 ± 6.30 µg m−3), contributing to 15–30% of the PM10 mass. The dominant secondary ions (NH4+, SO42−, and NO3−) suggest that the study sites were strongly influenced by anthropogenic sources from regional and long-range transport. Principal component analysis (PCA) with an absolute principal component score (APCS), UNMIX, and Positive Matrix Factorization (PMF) were used for source identification of PM10 at the study sites of the IHR. All three models showed relatively similar results of source profiles for all study sites except their source number and percentage contribution. Overall, soil dust (SD), secondary aerosols (SAs), combustion (biomass burning (BB) + fossil fuel combustion (FFC): BB+FFC), and vehicular emissions (VEs) are the major sources of PM10 identified by these models at all study sites. Air mass backward trajectories illustrated that PM10, mainly attributed to dust-related aerosols, was transported from the Thar Desert, Indo-Gangetic Plain (IGP), and northwestern region of India (i.e., Punjab and Haryana) and Afghanistan to the IHR. Transported agricultural or residual burning plumes from the IGP and nearby areas significantly contribute to the concentration of carbonaceous aerosols (CAs) at study sites.