WSOC and Its Relationship with BC, Levoglucosan and Transition Metals in the PM2.5 of an Urban Area in the Amazon

Water-soluble organic carbon (WSOC) makes up a large mass fraction of the organic carbon in the aerosol and can influence important cloud processes in the atmosphere. The capacity of WSOC to form metallic complexes with transition metals is well known; however, its influence on the aerosol of urban areas in the Amazon region is not very well known. In this study, we investigated the relationship between WSOC, black carbon (BC), levoglucosan (LEV) and transition metals (Fe, Cu and Mn) present in the PM2.5 (particles with a diameter smaller than 2.5 µm) of an urban environment during the dry season in the central Amazon. Oxalic acid (C2) was used to identify the influence of transition metals on WSOC. The mean mass concentration value of the PM2.5 was 14.72 μg m-3 (2.11-31.68 μg m-3). The WSOC made up 58.34% of the PM2.5 mass, followed by BC (20.28%), and LEV (2.62%). The WSOC showed significant correlation with the transition metals analyzed (> 0.56), especially Mn with C2 (linear coefficient (R2 ) = 0.74). A multiple linear regression with WSOC, BC and LEV showed a strong linear correlation between them (R2 = 0.86), indicating the influence of biomass burning and vehicle traffic on the organic aerosol.

Chemical composition, optical properties, and oxidative potential of water- and methanol-soluble organic compounds emitted from the combustion of biomass materials and coal

Biomass burning (BB) and coal combustion (CC) are important sources of brown carbon (BrC) in ambient aerosols. In this study, six biomass materials and five types of coal were combusted to generate fine smoke particles. The BrC fractions, including water-soluble organic carbon (WSOC), humic-like substance carbon (HULIS-C), and methanol-soluble organic carbon (MSOC), were subsequently fractionated, and their optical properties and chemical structures were then comprehensively investigated using UV–visible spectroscopy, proton nuclear magnetic resonance spectroscopy (1H NMR), and fluorescence excitation–emission matrix (EEM) spectroscopy combined with parallel factor (PARAFAC) analysis. In addition, the oxidative potential (OP) of BB and CC BrC was measured with the dithiothreitol (DTT) method. The results showed that WSOC, HULIS-C, and MSOC accounted for 2.3 %–22 %, 0.5 %–10 %, and 6.4 %–73 % of the total mass of combustion-derived smoke PM2.5, respectively, with MSOC extracting the highest concentrations of organic compounds. The MSOC fractions had the highest light absorption capacity (mass absorption efficiency at 365 nm (MAE365): 1.0–2.7 m2/gC) for both BB and CC smoke, indicating that MSOC contained more of the strong light-absorbing components. Therefore, MSOC may represent the total BrC better than the water-soluble fractions. Some significant differences were observed between the BrC fractions emitted from BB and CC with more water-soluble BrC fractions with higher MAE365 and lower absorption Ångström exponent values detected in smoke emitted from BB than from CC. EEM-PARAFAC identified four fluorophores: two protein-like, one humic-like, and one polyphenol-like fluorophores. The protein-like substances were the dominant components of WSOC (47 %–80 %), HULIS-C (44 %–87 %), and MSOC (42 %–70 %). The 1H-NMR results suggested that BB BrC contained more oxygenated aliphatic functional groups (H-C-O), whereas CC BrC contained more unsaturated fractions (H-C-C= and Ar−H). The DTT assays indicated that BB BrC generally had a stronger oxidative potential (DTTm, 2.6–85 pmol/min/µg) than CC BrC (DTTm, 0.4–11 pmol/min/µg), with MSOC having a stronger OP than WSOC and HULIS-C. In addition, HULIS-C contributed more than half of the DTT activity of WSOC (63.1 % ± 15.5 %), highlighting that HULIS was a major contributor of reactive oxygen species (ROS) production in WSOC. Furthermore, the principal component analysis and Pearson correlation coefficients indicated that highly oxygenated humic-like fluorophore C4 may be the important DTT active substances in BrC.

Measurement report: Source characteristics of water-soluble organic carbon in PM_2.5 at two sites in Japan, as assessed by long-term observation and stable carbon isotope ratio

The sources and seasonal trends of water-soluble organic carbon (WSOC) in carbonaceous aerosols are of significant interest. From July 2017 to July 2019, we collected samples of PM2.5 (particulate matter, aerodynamic diameter<2.5 µm) from one suburban and one rural site in Japan. The average δ13CWSOC was -25.2±1.1 ‰ and -24.6±2.4 ‰ at the suburban site and rural site, respectively. At the suburban site, the δ13CWSOC was consistent with the δ13C of burned C3 plants, and a high correlation was found between WSOC concentrations and non-sea-salt potassium concentrations; these results suggest that the main source of WSOC at this site was biomass burning of rice straw. At the rural site, the average δ13CWSOC was significantly heavier from autumn to spring (-23.9±2.1 ‰) than in summer (-27.4±0.7 ‰) (p<0.01). The δ13CWSOC from autumn to spring was consistent with that of biomass burning of rice straw, whereas that in summer was considered to reflect mainly the formation of secondary organic aerosols from biogenic volatile organic compounds (VOCs). The heaviest δ13CWSOC (-21.3±1.9 ‰) was observed from February to April 2019, which may be explained by long-range transport of C4 plant burning such as corn from overseas. Thus, the present study indicates that δ13CWSOC is potentially useful for elucidating the sources and atmospheric processes that contribute to seasonal variations of WSOC concentration.

Desiccation time and rainfall control gaseous carbon fluxes in an intermittent stream

Droughts are recognized to impact global biogeochemical cycles. However, the implication of desiccation on in-stream carbon (C) cycling is not well understood yet. We subjected sediments from a lowland, organic rich intermittent stream to experimental desiccation over a 9-week-period to investigate temporal changes in microbial functional traits in relation to their redox requirements, carbon dioxide (CO2) and methane (CH4) fluxes and water-soluble organic carbon (WSOC). Concurrently, the implications of rewetting by simulated short rainfalls (4 and 21 mm) on gaseous C fluxes were tested. Early desiccation triggered dynamic fluxes of CO2 and CH4 with peak values of 383 and 30 mg C m−2 h−1 (mean ± SD), respectively, likely in response to enhanced aerobic mineralization and accelerated evasion. At longer desiccation, CH4 dropped abruptly, likely because of reduced abundance of anaerobic microbial traits. The CO2 fluxes ceased later, suggesting aerobic activity was constrained only by extended desiccation over time. We found that rainfall boosted fluxes of CO2, which were modulated by rainfall size and the preceding desiccation time. Desiccation also reduced the amount of WSOC and the proportion of labile compounds leaching from sediment. It remains questionable to which extent changes of the sediment C pool are influenced by respiration processes, microbial C uptake and cell lysis due to drying-rewetting cycles. We highlight that the severity of the dry period, which is controlled by its duration and the presence of precipitation events, needs detailed consideration to estimate the impact of intermittent flow on global riverine C fluxes.

Measurement report: Molecular composition, optical properties, and radiative effects of water-soluble organic carbon in snowpack samples from northern Xinjiang, China

Water-soluble organic carbon (WSOC) in the cryosphere has an important impact on the biogeochemistry cycling and snow–ice surface energy balance through changes in the surface albedo. This work reports on the chemical characterization of WSOC in 28 representative snowpack samples collected across a regional area of northern Xinjiang, northwestern China. We employed multimodal analytical chemistry techniques to investigate both bulk and molecular-level composition of WSOC and its optical properties, informing the follow-up radiative forcing (RF) modeling estimates. Based on the geographic differences and proximity of emission sources, the snowpack collection sites were grouped as urban/industrial (U), rural/remote (R), and soil-influenced (S) sites, for which average WSOC total mass loadings were measured as 1968 ± 953 ng g−1 (U), 885 ± 328 ng g−1 (R), and 2082 ± 1438 ng g−1 (S), respectively. The S sites showed the higher mass absorption coefficients at 365 nm (MAC365) of 0.94 ± 0.31 m2 g−1 compared to those of U and R sites (0.39 ± 0.11 m2 g−1 and 0.38 ± 0.12 m2 g−1, respectively). Bulk composition of WSOC in the snowpack samples and its basic source apportionment was inferred from the excitation–emission matrices and the parallel factor analysis featuring relative contributions of one protein-like (PRLIS) and two humic-like (HULIS-1 and HULIS-2) components with ratios specific to each of the S, U, and R sites. Additionally, a sample from site 120 showed unique pollutant concentrations and spectroscopic features remarkably different from all other U, R, and S samples. Molecular-level characterization of WSOC using high-resolution mass spectrometry (HRMS) provided further insights into chemical differences among four types of samples (U, R, S, and 120). Specifically, many reduced-sulfur-containing species with high degrees of unsaturation and aromaticity were uniquely identified in U samples, suggesting an anthropogenic source. Aliphatic/protein-like species showed the highest contribution in R samples, indicating their biogenic origin. The WSOC components from S samples showed high oxygenation and saturation levels. A few unique CHON and CHONS compounds with high unsaturation degree and molecular weight were detected in the 120 sample, which might be anthraquinone derivatives from plant debris. Modeling of the WSOC-induced RF values showed warming effects of 0.04 to 0.59 W m−2 among different groups of sites, which contribute up to 16 % of that caused by black carbon (BC), demonstrating the important influences of WSOC on the snow energy budget.

Organic Molecular Tracers in PM2.5 at Urban Sites during Spring and Summer in Japan: Impact of Secondary Organic Aerosols on Water-Soluble Organic Carbon

To understand the characteristics of secondary organic aerosols (SOAs) and estimate their impact on water-soluble organic carbon (WSOC) in urban areas in Japan, we measured 17 organic tracers using gas chromatography–mass spectrometry from particulate matter with an aerodynamic diameter smaller than 2.5 μm collected at five urban sites in Japan during spring and summer. Most anthropogenic, monoterpene-derived, and isoprene-derived SOA tracers showed meaningful correlations with potential ozone in both these seasons. These results indicate that oxidants play an important role in SOAs produced during both seasons in urban cities in Japan. WSOC was significantly affected by anthropogenic and monoterpene-derived SOAs during spring and three SOA groups during summer at most of the sites sampled. The total estimated secondary organic carbons (SOCs), including mono-aromatic, di-aromatic, monoterpene-derived, and isoprene-derived SOCs, could explain the WSOC fractions of 39–63% in spring and 46–54% in summer at each site. Notably, monoterpene-derived and mono-aromatic SOCs accounted for most of the total estimated SOCs in both spring (85–93%) and summer (75–82%) at each site. These results indicate that SOAs significantly impact WSOC concentrations during both these seasons at urban sites in Japan.

Molecular Speciation of Size Fractionated Particulate Water-Soluble Organic Carbon by Two-Dimensional Nuclear Magnetic Resonance (NMR) Spectroscopy

Particulate matter is associated with increased morbidity and mortality; its effects depend on particle size and chemical content. It is important to understand the composition and resultant toxicological profile of particulate organic compounds, the largest and most complex fraction of particulate matter. The objective of the study was to delineate the nuclear magnetic resonance (NMR) spectral fingerprint of the biologically relevant water-soluble organic carbon (WSOC) fraction of size fractionated urban aerosol. A combination of one and two-dimensional NMR spectroscopy methods was used. The size distribution of particle mass, water-soluble extract, non-exchangeable organic hydrogen functional types and specific biomarkers such as levoglucosan, methane sulfonate, ammonium and saccharides indicated the contribution of fresh and aged wood burning emissions, anthropogenic and biogenic secondary aerosol for fine particles as well as primary traffic exhausts and pollen for large particles. Humic-like macromolecules in the fine particle size range included branched carbon structures containing aromatic, olefinic, keto and nitrile groups and terminal carboxylic and hydroxyl groups such as terpenoid-like polycarboxylic acids and polyols. Our study show that 2D-NMR spectroscopy can be applied to study the chemical composition of size fractionated aerosols.