Humic-like Substances (HULIS) in Ship Engine Emissions: Molecular Composition Effected by Fuel Type, Engine Mode, and Wet Scrubber Usage.

Humic-like substances (HULIS), known for their substantial impact on the atmosphere, are identified in marine diesel engine emissions obtained from five different fuels at two engine loads simulating real world scenarios as well as the application of wet sulfur scrubbers. The HULIS chemical composition is characterized by electrospray ionization (ESI) ultrahigh resolution mass spectrometry and shown to contain partially oxidized alkylated polycyclic aromatic compounds as well as partially oxidized aliphatic compounds, both including abundant nitrogen- and sulfur-containing species, and clearly different to HULIS emitted from biomass burning. Fuel properties such as sulfur content and aromaticity as well as the fuel combustion efficiency and engine mode are reflected in the observed HULIS composition. When the marine diesel engine is operated below the optimum engine settings, e.g., during maneuvering in harbors, HULIS-C emission factors are increased (262-893 mg kg-1), and a higher number of HULIS with a shift toward lower degree of oxidation and higher aromaticity is detected. Additionally, more aromatic and aliphatic CHOS compounds in HULIS were detected, especially for high-sulfur fuel combustion. The application of wet sulfur scrubbers decreased the HULIS-C emission factors by 4-49% but also led to the formation of new HULIS compounds. Overall, our results suggest the consideration of marine diesel engines as a relevant regional source of HULIS emissions.

pH-Dependent Chemical Transformations of Humic-Like Substances and Further Cognitions Revealed by Optical Methods.

Humic-like substances (HULIS) are macromolecular complex groups in water-soluble organic compounds (WSOC). pH is a crucial factor that influences the chemical transformations of HULIS in atmospheric particles, but this has been rarely investigated, especially under varying pH conditions. This study attempted to unveil the chemical transformation mechanisms of HULIS under a range of pH conditions using optical methods. The pH-dependent light absorption and fluorescence properties of HULIS were comprehensively analyzed; the acidity coefficient (pKa) of HULIS in relation to chemical structures was determined, and the hypothetical chemical transformation mechanisms of HULIS with increasing pH were analyzed by optical characterizations. The results suggested that pH greatly impacted the light absorption and fluorescence efficiencies of HULIS in both winter and summer seasons, and pKa was an important inflection point. The pKa of HULIS ranged from 3.5 to 8.0 in winter and 6.4 to 10.0 in summer. The acidic/basic groups were identified as -OH or -NH2 substituted quinolines, carboxylic aromatics, and pyridines. The pH-sensitive species accounted for about 6% and 21% of HULIS-C (carbon concentrations of HULIS) in winter and summer, respectively. The varying optical spectra with increasing pH might result from charge transfer or complex reactions with HULIS deprotonation.

Biological and Nonbiological Sources of Fluorescent Aerosol Particles in the Urban Atmosphere.

Online detection of bioaerosols based on the light-induced fluorescence (LIF) technique is still challenging due to the complexity of bioaerosols and the external/internal mixing with nonbiological fluorescent compositions. Although many lab studies have measured the fluorescence properties of the biological and nonbiological materials, there is still a scarcity of knowledge of the sources of fluorescent aerosol particles (FAP) in the ambient atmosphere. Here, we fill this gap by combining the online measurement of an LIF-based instrument (wideband integrated bioaerosol sensor, WIBS, 0.8-20 µm) with the measurements of typical biological matter and the compositions related to major nonbiological FAP from May to July in the megacity Beijing. We find that fungal spores and pollen are widely observed in all types of FAP using a WIBS. Bacteria are suggested to be associated with the fine mode FAP (excitation/emission: 280 nm/310-400 nm; 0.8-3 µm). The FL-B and -BC particles (emission in 420-650 nm) contributing the most to FAP are strongly associated with humic-like substances, dust, burning and combustion emissions, and secondary organic aerosols (SOA). This study provides a guide for interpreting individual FAP measured by LIF instruments and points to the applicability of online LIF instruments to characterize nonbiological compositions including SOA.

Increase of Fluorescence of Humic-Like Substances in Interaction with Cd(II): a Photoinduced Charge Transfer Approach.

Described is the enhancement of fluorescence intensity due to the interaction of a humic-like substance (HLS 1%) extracted from process water (PW) and Cd(II) ions in aqueous solution. Using Canonical Polyadic/Parallel Factor Analysis (CP/PARAFAC), two main components were seen that contributed to fluorescence, the first one increased it and the second one kept it constant in both static and dynamic fluorescence studies. Two-dimensional FTIR analysis indicated that the interaction of HLS 1% and Cd(II) ions occurred in the following order of affinity with the groups: C-O bonds in polysaccharides > C-O bonds in carboxylic acid. The results obtained suggest that the increase in fluorescence intensity and lifetime suggest a photoinduced charge transfer (PCT) between Cd(II) ions and carboxylic acid groups present in HLS 1%.

Comparison of three different wastewater sludge and their respective drying processes: Solar, thermal and reed beds - Impact on organic matter characteristics.

Drying process aims at minimising the volume of wastewater sludge (WWS) before disposal, however it can impact sludge characteristics. Due to its high content in organic matter (OM) and lipids, sludge are mainly valorised by land farming but can also be considered as a feedstock for biodiesel production. As sludge composition is a major parameter for the choice of disposal techniques, the objective of this study was to determine the influence of the drying process. To reach this goal, three sludges obtained from solar, reed beds and thermal drying processes were investigated at the global and molecular scales. Before the drying step the sludges presented similar physico-chemical (OM content, elemental analysis, pH, infrared spectra) characteristics and lipid contents. A strong influence of the drying process on lipids and humic-like substances contents was observed through OM fractionation. Thermochemolysis-GCMS of raw sludge and lipids revealed similar molecular content mainly constituted with steroids and fatty acids. Molecular changes were noticeable for thermal drying through differences in branched to linear fatty acids ratio. Finally the thermal drying induced a weakening of OM whereas the solar drying led to a complexification. These findings show that smooth drying processes such as solar or reed-beds are preferable for amendment production whereas thermal process leads to pellets with a high lipid content which could be considered for fuel production.

Influence of haze pollution on water-soluble chemical species in PM2.5 and size-resolved particles at an urban site during fall.

To investigate the influence of haze on the chemical composition and formation processes of ambient aerosol particles, PM2.5 and size-segregated aerosol particles were collected daily during fall at an urban site of Gwangju, Korea. During the study period, the total concentration of secondary ionic species (SIS) contributed an average of 43.9% to the PM2.5, whereas the contribution of SIS to the PM2.5 during the haze period was 62.3%. The NO3- and SO42- concentrations in PM2.5 during the haze period were highly elevated, being 13.4 and 5.0 times higher than those during non-haze period, respectively. The PM, NO3-, SO42-, oxalate, water-soluble organic carbon (WSOC), and humic-like substances (HULIS) had tri-modal size distributions peaks at 0.32, 1.0, and 5.2µm during the non-haze and haze periods. However, during the non-haze period they exhibited dominant size distributions at the condensation mode peaking at 0.32µm, while on October 21 when the heaviest haze event occurred, they had predominant droplet mode size distributions peaking at 1.00µm. Moreover, strong correlations of WSOC and HULIS with SO42-, oxalate, and K+ at particle sizes of <1.8µm indicate that secondary processes and emissions from biomass burning could be responsible for WSOC and HULIS formations. It was found that the factors affecting haze formation could be the local stable synoptic conditions, including the weak surface winds and high surface pressures, the long-range transportation of haze from eastern China and upwind regions of the Korean peninsula, as well as the locally emitted and produced aerosol particles.

Compositional and optical characteristics of aqueous brown carbon and HULIS in the eastern Indo-Gangetic Plain using a coupled EEM PARAFAC, FT-IR and 1H NMR approach.

This study provides insights into the fluorophoric composition of aqueous brown carbon (BrCaq) and chemically-separated humic-like substances (HULIS): neutral HULIS (HULIS-n; at pH = 7) and acidic HULIS (HULIS-a; at pH = 2) on a seasonal and day-night basis in the eastern Indo-Gangetic Plain (IGP), India. A coupled approach including excitation-emission matrix (EEM) fluorescence and parallel factor analysis (PARAFAC) model, Fourier-transformed infrared spectroscopy (FT-IR) and proton nuclear magnetic resonance (1H NMR) spectroscopy was employed to understand the links between structural, compositional and fluorophoric characteristics of BrCaq and HULIS fractions. HULIS fluorophores (HULISfluoro) with varying oxidation states transported from the northwest IGP were dominant during biomass burning seasons (post-monsoon and winter), while protein-like fluorophores (PRLISfluoro) from marine emissions showed large contributions during summer. HULIS-n moieties were mostly primary in nature with higher conjugation, while HULIS-a were associated with secondarily formed and aged species with a larger contribution from degradation products. A substantial presence of tyrosine-like proteins in both chemically-separated HULIS fractions indicated that atmospheric HULIS is not entirely humic or fulvic-like in the eastern IGP. Finally, the dominance of H-C-O groups across seasons suggested consistent fossil fuel signatures along with season-specific influence of photodegradable cellulose from marine organisms in the summer and biomass burning in the post-monsoon and winter.

Oxidation of hydrochar produced from byproducts of the sugarcane industry for the production of humic-like substances: Characterization and interaction study with Cu(II).

Humic-like substances (HLS) are molecules extracted in an alkaline medium from different materials that have not been subjected to the natural process of humification that occurs in the soil. HLS have the potential to be used as organic fertilizers due to their ability to incorporate micronutrients such as Cu(II) and Co(II); in addition, they represent an alternative for the remediation of contaminated areas due to their high affinity for metals. HLS can be extracted from hydrochar (HC) but only with low yields of approximately 5%. Therefore, the present study aimed to increase the amount of HLS extracted from the HC produced from byproducts of the sugarcane industry through the oxidation of HC with HNO3. HLS extracted from oxidized and unoxidized HC were characterized by CHNS analysis and 13C CPMAS NMR. The interaction between HLS and Cu(II) was studied by molecular fluorescence quenching (EEM-PARAFAC) and applying the Ryan and Weber complexation model. The oxidation of HC with HNO3 allowed high yields of extracted HLS of above 80%. The oxidation carried out with 30% HNO3 for 2 h showed the best result, since the HLS30%(2h) were extracted with a very high yield (88.3%) in a short period of time. Oxidation promoted a decrease in HLS aromaticity and an increase in oxygen and nitrogen groups. HLS showed high affinity for Cu(II), as evidenced by the high logK values (between 5.5 and 5.9). HLS extracted from oxidized HCs showed higher complexation capacity due to the greater incorporation of the oxygenated groups promoted by oxidation, which are fundamental during the interaction with metallic cations. Therefore, the oxidation of HC substantially increased the production of HLS, representing a big advance for the production of carbonaceous materials with higher added value from byproducts of the sugarcane industry produced on a large scale in Brazil.

[Characteristics of Organic Matter Composition and Oxidation Potential in Road Dust in Winter in Xi'an].

Roads are the main places where urban people are exposed to atmospheric particulate matter from outdoor activities, and certain oxidatively active substances contained in road particulate matter are important components that induce the generation of reactive oxygen species (ROS), which in turn endanger human health. Here, we explored the characteristics of organic matter composition in water-soluble (WSM) and methanol-soluble fractions (MSM) of road dust in Xi'an and its oxidation potential (OP). Additionally, we investigated the organic fractions and their distribution based on parallel factor analysis (PARAFAC) and analyzed the correlation between organic matter types and OP. The results showed that the water-insoluble fraction of road dust in Xi'an contained more chromophoric organic matter with an average total concentration of (4.71±1.27)×104 R.U., which was 12 times higher than that of WSM[(3.96±1.10)×103 R.U.], of which low-oxidizing humic-like substances (HULIS) were the main organic matter (34.8%-43.7% of the total organic matter). The results of cluster analysis showed that the important sources of organic matter in road dust in Xi'an were fuel combustion and industrial production. The mean value of dust oxidative toxicity was (0.34±0.08) pmol·(min·µg)-1, with the water-insoluble fraction providing 70% of the total oxidative toxicity of dust particles, which was 2.4 times higher than the water-soluble fraction. The main precursors of oxidative toxicity of dust particles were metal elements, and special types of organic substances were also one of the important oxidative toxicity precursors, among which chromophore organic matter was the main cause of OP production in the WSM fraction (r=0.35, P<0.01), and protein-like organic matter and highly oxidized HULIS in WSM may have been the main two types of organic substances for OP production. However, there was no significant correlation between organic matter concentration in MSM and water-insoluble OP (OPTotal-OPWSM) (r=-0.04, P>0.1), so the oxidative toxicity of the water-insoluble particulate matter fraction was mainly generated from non-organic matter.

Optical source apportionment of aqueous brown carbon (BrC) on a daytime and nighttime basis in the eastern Indo-Gangetic Plain (IGP) and insights from 13C and 15N isotopic signatures.

This study reports day-night and seasonal variations of aqueous brown carbon (BrCaq) and constituent humic-like substances (HULIS) (neutral and acidic HULIS: HULIS-n and HULIS-a) from the eastern Indo-Gangetic Plain (IGP) of India during 2019-2020. This is followed by the application of the receptor model positive matrix factorization (PMF) for optical source apportionment of BrCaq and the use of stable isotopic ratios (δ13C and δ15N) to understand atmospheric processing. Nighttime BrCaq absorption and mass absorption efficiencies (MAE) were enhanced by 40-150 % and 50-190 %, respectively, compared to the daytime across seasons, possibly as a combined effect from daytime photobleaching, dark-phase secondary formation, and increased nighttime emissions. MAE250 nm/MAE365 nm (i.e., E2/E3) ratios and Angstrom Exponents revealed that BrCaq and HULIS-n were relatively more aromatic and conjugated during the biomass burning-dominated periods while BrCaq and HULIS-a were comprised mostly of non-conjugated aliphatic structures from secondary processes during the photochemistry-dominated summer. The relative radiative forcing of BrCaq with respect to elemental carbon (EC) was 10-12 % in the post-monsoon and winter in the 300-400 nm range. Optical source apportionment using PMF revealed that BrCaq absorption at 300, 365 and 420 nm wavelengths in the eastern IGP is mostly from biomass burning (60-75 %), followed by combined marine and fossil fuel-derived sources (24-31 %), and secondary processes (up to 10 %). Source-specific MAEs at 365 nm were estimated to be the highest for the combined marine and fossil fuel source (1.34 m2 g-1) followed by biomass burning (0.78 m2 g-1) and secondary processing (0.13 m2 g-1). Finally, δ13C and δ15N isotopic analysis confirmed the importance of summertime photochemistry and wintertime NO3--dominated chemistry in constraining BrC characteristics. Overall, the quantitative apportionment of BrCaq sources and processing reported here can be expected to lead to targeted source-specific measurements and a better understanding of BrC climate forcing in the future.

[Pollution Characterizations and Oxidative Potentials of Water-Soluble Organic Matters at Different Polarity Levels in Winter PM2.5 Over Xi'an].

Atmospheric fine particulate matter (PM2.5) can produce reactive oxygen species (ROS), which have adverse effects on health. Acidic, neutral, and highly polar water-soluble organic matter (WSOM) is an important component of ROS in organic aerosols. PM2.5 samples were collected in winter 2019 in Xi'an City to deeply explore the pollution characteristics and health risks of WSOM components with different polarity levels. The results showed that the concentration of WSOM in PM2.5 in Xi'an was (4.62±1.89) µg·m-3, humic-like substances (HULIS) were an important part of WSOM (78.81%±10.50%), and the proportion of HULIS was higher in haze days. The concentration levels of three WSOM components with different polarities in haze and non-haze days were:neutral HULIS (HULIS-n)>acidic HULIS (HULIS-a)>highly-polarity WSOM(HP-WSOM) and HULIS-n>HP-WSOM>HULIS-a. The oxidation potential (OP) was measured using the 2',7'-dichlorodihydrofluorescein (DCFH) method. It was found that the law of OPm in haze and non-haze days was HP-WSOM>HULIS-a>HULIS-n, and the characteristic of OPv was HP-WSOM>HULIS-n>HULIS-a. During the whole sampling period, OPm was negatively correlated with the concentrations of the three components of WSOM. The OPm of HULIS-n (R2=0.8669) and HP-WSOM (R2=0.8582) in haze days were highly correlated with their respective concentrations. The OPm of HULIS-n, HULIS-a, and HP-WSOM in non-haze days were strongly dependent on their respective component concentrations.

Efficient removal of U(VI) in acidic environment with spent coffee grounds derived hydrogel.

In this study, humic-like substances (HLSs) was extracted from spent coffee grounds (SCGs), and it together with poly acrylic acid (PAA), was used for the first time to synthesize hydrogel material, namely HLSs/PAA gel, by one-step radical polymerization. Its maximum theoretical sorption capacity toward U(VI) at pH 3.00 was 661.01 mg/g, and it could decrease the concentration of U(VI) in acidic actual groundwater from 0.2537 to 0.0003 mg/L, showing that the gel had excellent U(VI) removal efficiency in acidic environment. The SEM characterization of HLSs/PAA gel showed that its macroporous network structure maintained well after the sorption process, indicating that the gel had excellent acid-resistant property. Moreover, the gel exhibited excellent anti-interference performance in the interfering ions effect experiment. The gel integrates the merits of excellent U(VI) sorption properties, stability and anti-interference performance in acidic environment, and has promising application prospects in the remediation of acidic uranium wastewater.

Formation of Recalcitrant Compounds during Anaerobic Digestion of Thermally Pre-Treated Sludge: A Critical Macromolecular and Structural Study.

Thermal hydrolysis, when used as pre-treatment, enhances the anaerobic digestion of sewage sludge; moreover, due to the high temperature normally applied, undesirable recalcitrant compounds via Maillard reactions may also be formed. However, although the appearance of these recalcitrant compounds is widely reported, more information on the formation, structure, and fate of these compounds is still needed. This study was focused on understanding the amount and whereabouts of such compounds during the anaerobic digestion process with thermal pre-treatment in soluble and total phase and advance in its structural identification by analyzing their infrared (IR) spectra. It was found that, even with the improved methane production and COD degradation, at 165 °C for 30 min, humic-like compounds are formed which could not be degraded at the anaerobic digestion step. These compounds account for 25% of the original sludge. Infrared spectroscopy proved to be a powerful technique, permitting their differentiation from the natural humic-like compounds. This research provides new information about the structure of melanoidins at every stage of the thermal hydrolysis pre-treatment and how they contribute to the dissolved organic nitrogen.

Microbial inoculants reshape structural distribution of complex components of humic acid based on spectroscopy during straw waste composting.

This study aimed to explore the mechanism of actinomycetes inoculation to promote humification based on spectroscopy during straw waste composting. Results showed that inoculating actinomycetes could significantly increase the humification index and humification ratio, which were 2.53% and 21.79% respectively (P < 0.05). A spectroscopic analysis suggested that actinomycetes promoted the relative content of complex components of humic acid and reshaped the structural distribution of two sub fluorescence peaks in it. Furthermore, variance partitioning analysis demonstrated that compared with the intensity, the high-quality uniform distribution of fluorescence peaks had a greater contribution to the improvement of humification. In addition, structural equation model further verified that actinomycetes inoculation promoted the transformation of fulvic acid to humic acid, and then promoted the formation of humic acid. Therefore, actinomycetes inoculation can promote the humification of straw compost by reshaping the complex components of humic acid.

Characterization of DOM Released from Bacteria in Response to Chlorine in Water Based on Indicator Bacteria E. coli.

In this study, Escherichia coli (E. coli) was used as an indicator bacterium treated with five different concentrations of chlorine (0.1; 0.5; 1.0; 2.0, and 5.0 mg/L) and without chlorine (0.0 mg/L) to evaluate the changes in the DOM characteristics. The dissolved organic carbon (DOC) concentration initially increased along with the chlorine concentrations and decreased after 24 hours (0.0 and 0.1 mg/L) and 168 hours (0.5; 1.0; 2.0 and 5.0 mg/L). Ultra-violet absorbance at 260 nm (UV260) showed that the absorbance decreased for control without chlorine (0.0 mg/L) and 0.1 mg/L chlorine, while increased for other concentrations of chlorine within 120 hours. The DOC and UV260 results indicated that the high concentrations of chlorine initiated high contents of DOM which contained more humic-like molecules than the DOM released from E. coli without chlorine. Fluorescence excitation-emission matrix (EEM) analysis suggested that the DOM released from E. coli without chlorine enriched with protein-like substances, whereas the fulvic-like and humic-like substances more intensified in the DOM for the high concentrations of chlorine (>1.0 mg/L). The molecular weight distribution of DOM showed that the intensity of high molecular weight substances and polydispersity increased along with chlorine concentration and contact time, whereas the low molecular weight substances were relatively higher in the DOM for control without chlorine. The obtained results of this study would be useful for a better understanding of the variation of DOM during treatment and could be used as an important reference for optimizing the operation condition of the water treatment plants.

Are fireworks a significant episodic source of brown carbon?

We hypothesize that firework events involving the combustion of charcoal fuel, organic binders, metal salts, and cellulose-based wrapping material could be significant transient sources of aerosol brown carbon (BrC). To test this, we couple high time-resolution (1 min) measurements of black carbon (BC) and BrC absorption from a 7-wavelength aethalometer with time-integrated (12-24 h) measurements of filter extracts, i.e., UV-visible, fluorescence, and Fourier-transformed infrared (FT-IR) signatures of BrC, total and water-soluble organic carbon (OC and WSOC), ionic species, and firework tracer metals during a sampling campaign covering the Diwali fireworks episode in India. In sharp contrast to BC, BrC absorption shows a distinct and considerable rise of 2-4 times during the Diwali period, especially during the hours of peak firework activity, as compared to the background. Fluorescence profiles suggest enrichment of humic-like substances (HULIS) in the firework plume, while the enhancement of BrC absorption in the 400-500 nm range suggests the presence of nitroaromatic compounds (NACs). Considerable contributions of WSOC and secondary organics to OC (44.1% and 31.2%, respectively) and of the water-soluble fraction of BrC to total BrC absorption (71.0%) during the Diwali period point toward an atmospherically processed, polar signature of firework-related BrC, which is further confirmed by FT-IR profiles. This aqueous BrC exerts a short-lived but strong effect on atmospheric forcing (12.0% vis-à-vis BC in the UV spectrum), which could affect tropospheric chemistry via UV attenuation and lead to a stabilization of the post-Diwali atmosphere, resulting in enhanced pollutant build-up and exposure.

Identification of the key biochemical component contributing to disinfection byproducts in chlorinating algogenic organic matter.

Disinfection byproducts (DBPs) remains an ongoing issue because of their widespread occurrence and toxicity. Various organic substances in Algogenic organic matter (AOM) can produce DBPs in the chlorination process. To provide specific suggestions for the targeted removal of DBP precursors in AOM, the main biochemical components in AOM were qualitatively and quantitatively analyzed. An accurate model for predicting the DBP formation potentials (DBPFPs) of AOM was herein developed based on the dissolved organic carbon of the five main biochemical components in AOM and the DBPFPs of their corresponding surrogates. The contributions of each biochemical component to the three DBP species were evaluated, and the key components were identified. The results showed that lipids, proteins, carbohydrates, humic acid-like substances, and fulvic acid-like substances were the main biochemical components in AOM. Thereof, proteins (71.2 ± 2.1%) and carbohydrates (53.1 ± 2.1%) were the major contributor to the carbon content in intracellular organic matter and extracellular organic matter, respectively. The contribution results of biochemical components to the formation of DBPs showed that proteins were the key contributor to DBPs, suggesting that the targeted removal of proteins before the chlorination process would effectively reduce DBPs from AOM.

Sorption of Pb(II) onto biochar is enhanced through co-sorption of dissolved organic matter.

Biochar plays an important role in controlling migration of pollutants in soils. However, little information is available on the interactions between soil-derived dissolved organic matter (DOM), biochar and soluble metal species. The aim of this work was to present the adsorption process of soil DOM by biochar (corn straw biochar produced at 700 °C) and to determine whether co-sorption of DOM would change the affinity for Pb(II). The adsorption rates of biochar and biochar + DOM for Pb(II) were best fitted with a pseudo-second order kinetic model, and the equilibrium adsorption isotherm data agreed well with both the Langmuir and Freundlich models. Adsorption of DOM to biochar reached equilibrium after 15 h with an uptake of 52% of the supplied DOM. We used fluorescence excitation-emission matrices (EEMs) with parallel factor (PARAFAC) analysis to demonstrate that protein-like, fulvic acid-like and humic acid-like substances were the primary constituents of the DOM, which were quenched over time in the presence of biochar. Synchronous fluorescence spectra indicated that the protein-like structures were the predominant fluorescence substances in DOM. Two-dimensional correlation spectroscopy (2D-COS) showed the binding of DOM to biochar resulted in the quenching of fluorescence in the order: protein-like substances > humic-like substances (280 > 355 nm). Data supports the notion that DOM can increase the adsorption capacity of biochar for metal-ions.

Aqueous-phase oxidation of syringic acid emitted from biomass burning: Formation of light-absorbing compounds.

Syringic acid is a methoxyphenol model compound derived from biomass burning, and its photooxidation processes have important effects on atmospheric chemistry. However, its aqueous-phase photochemistry remains unclear. In this study, we systematically report the photooxidation of syringic acid induced by OH radicals in the aqueous phase. Employing the relative rate technique, the bimolecular rate constant for syringic acid reaction with OH radicals was acquired to be (1.1 ± 0.3) × 1010 M-1 s-1. Notably, colored products were formed as the reaction progressed. Furthermore, the UV-vis and fluorescence spectra confirmed the formation of light-absorbing organic species, and the results agreed well with previous results on atmospheric and natural humic-like substances (HULIS). The photooxidation products were detected by high performance liquid chromatography mass spectrometry (HPLC/MS), and a possible reaction mechanism was proposed. The aqueous-phase reaction of syringic acid would undergo functionalization process forming a hydroxylation product that enhances the degree of oxidation of aqueous secondary organic aerosol (aqSOA), and goes through dimerization process by C-C or C-O coupling of phenoxy radicals which may conduce to the formation of HULIS. These findings suggest that the photooxidation of syringic acid is an important pathway for highly oxygenated phenolic aqSOA formation, providing a secondary source for HULIS in a liquid phase or in deliquescent particles surrounded by a layer of water.

Assessment of oxidative potential by hydrophilic and hydrophobic fractions of water-soluble PM2.5 and their mixture effects.

Transition metals (TMs) (e.g. copper (Cu) and iron (Fe)) and certain organic compounds are known active constituents causing oxidative potential (OP) by inhaled ambient fine particulate matter (PM2.5) in lung fluid. Humic-like substances (HULIS), isolated from atmospheric PM2.5, are largely metal-free and contain mixtures of organics that are capable of complexing TMs. TMs and HULIS co-exist in the water-extractable part of PM2.5. In this work, we used a solid phase extraction procedure to isolate the water-soluble TMs in the hydrophilic fraction (HPI) and HULIS in the hydrophobic fraction (HPO) and carried out this isolation procedure to a set of 32 real-world PM2.5 samples collected in Beijing and Hong Kong, China. We quantified two OP endpoints, namely hydroxyl radical formation (denoted as OP•OH) and ascorbic acid depletion (denoted as OPAA), by the two fractions separately and combined, as well as by the bulk water-soluble aerosols. OP•OH and OPAA were well-correlated in both separate fractions and their combined mixtures or bulk water-soluble aerosols. OP by HPI far exceeded that by HPO. On a per unit PM2.5 mass basis, the Hong Kong samples on average had a higher OPAA and OP•OH than the Beijing samples due to more water-soluble Cu. For HPI, Cu was a dominant OP•OH and OPAA contributor (>80%), although water-soluble Fe was present at a concentration approximately one order of magnitude higher. Suppression effects on OP•OH were observed through comparing the OP of the bulk water-soluble aerosol with that of HPI. Our work reveals the importance of monitoring PM2.5 chemical compositions (especially water-soluble redox active metals). Furthermore, we demonstrate the need to consider metal-organic interactions when evaluating the aggregate OP by PM2.5 from individual components or apportioning OP by PM2.5 to specific chemical components.