Molecular Characterization of Nitrogen-Containing Compounds in Humic-like Substances Emitted from Biomass Burning and Coal Combustion.

N-containing organic compounds (NOCs) in humic-like substances (HULIS) emitted from biomass burning (BB) and coal combustion (CC) were characterized by ultrahigh-resolution mass spectrometry in the positive electrospray ionization mode. Our results indicate that NOCs include CHON+ and CHN+ groups, which are detected as a substantial fraction in both BB- and CC-derived HULIS, and suggest that not only BB but also CC is the potential important source of NOCs in the atmosphere. The CHON+ compounds mainly consist of reduced nitrogen compounds with other oxygenated functional groups, and straw- and coal-smoke HULIS exhibit a lower degree of oxidation than pine-smoke HULIS. In addition, the NOCs with higher N atoms (N2 and/or N3) generally bear higher modified aromaticity index (AImod) values and are mainly contained in BB HULIS, especially in straw-smoke HULIS, whereas the NOCs with a lower N atom (N1) always have relatively lower AImod values and are the dominant NOCs in CC HULIS. These findings imply that the primary emission from CC may be a significant source of N1 compounds, whereas high N number (e.g., N2-3) compounds could be associated with burning of biomass materials. Further study is warranted to distinguish the NOCs from more sources.

Light absorption and source apportionment of water soluble humic-like substances (HULIS) in PM2.5 at Nanjing, China.

Humic-like substances (HULIS), as important components of brown carbon (BrC), play an important role in climate change. In this study, one-year PM2.5 samples from 2017 to 2018 were collected at Nanjing, China and the water soluble HULIS and other chemical species were analyzed to investigate the seasonal variations, optical properties and possible sources. The HULIS concentrations exhibited highest in winter and lowest in summer. The annual averaged HULIS concentration was 2.61 ± 1.79 µg m-3, accounting for 45 ± 13% of water-soluble organic carbon (WSOC). The HULIS light absorption coefficient at 365 nm (Abs365, HULIS) averagely accounted for 71 ± 19% of that of WSOC, suggesting that HULIS are the main light-absorbing components in WSOC. The annual averaged Ångström absorption exponent and mass absorption efficiency of HULIS at 365 nm were 5.22 ± 0.77 and 1.71 ± 0.70 m2 g-1. Good correlations between HULIS with levoglucosan and K+ suggested biomass burning (BB) influence on HULIS. High concentrations of HULIS and secondary species (e.g., NO3-, SO42-, NH4+, C2O42-) were found in present of high relative humidity, indicating strong aqueous phase secondary HULIS formation. Secondary HULIS produced from anthropogenic and biogenic precursors were quantified based on the positive matrix factorization (PMF) model and the results showed that both fossil (55%) and biogenic (45%) emission sources made great contributions to HULIS. Fossil fuel combustion significantly contributed to HULIS formation throughout the whole year, which were enriched with more secondary HULIS (30%) than primary HULIS (25%). Strongest BB contribution (39%) was found in winter and biogenic SOA contribution (32%) was found in summer. A multiple linear regression (MLR) method was further applied to obtain specific source contributions to Abs365, HULIS and the results showed that strong light-absorbing chromophores were produced from anthropogenic precursors. Our results highlight the anthropogenic SOA and fossil fuels combustion contributions to HULIS in addition to the biggest contributor, BB, in urban area in China.

Particle-Phase Photoreactions of HULIS and TMIs Establish a Strong Source of H2O2 and Particulate Sulfate in the Winter North China Plain.

During haze periods in the North China Plain, extremely high NO concentrations have been observed, commonly exceeding 1 ppbv, preventing the classical gas-phase H2O2 formation through HO2 recombination. Surprisingly, H2O2 mixing ratios of about 1 ppbv were observed repeatedly in winter 2017. Combined field observations and chamber experiments reveal a photochemical in-particle formation of H2O2, driven by transition metal ions (TMIs) and humic-like substances (HULIS). In chamber experiments, steady-state H2O2 mixing ratios of 116 ± 83 pptv were observed upon the irradiation of TMI- and HULIS-containing particles. Correspondingly, H2O2 formation rates of about 0.2 ppbv h-1 during the initial irradiation periods are consistent with the H2O2 rates observed in the field. A novel chemical mechanism was developed explaining the in-particle H2O2 formation through a sequence of elementary photochemical reactions involving HULIS and TMIs. Dedicated box model studies of measurement periods with relative humidity >50% and PM2.5 ≥ 75 µg m-3 agree with the observed H2O2 concentrations and time courses. The modeling results suggest about 90% of the particulate sulfate to be produced from the SO2 reaction with OH and HSO3- oxidation by H2O2. Overall, under high pollution, the H2O2-caused sulfate formation rate is above 250 ng m-3 h-1, contributing to the sulfate formation by more than 70%.

Atmospheric HULIS and its ability to mediate the reactive oxygen species (ROS): A review.

Atmospheric humic-like substances (HULIS) are not only an unresolved mixture of macro-organic compounds but also powerful chelating agents in atmospheric particulate matters (PMs); impacting on both the properties of aerosol particles and health effects by generating reactive oxygen species (ROS). Currently, the interests of HULIS are intensively shifting to the investigations of HULIS-metal synergic effects and kinetics modeling studies, as well as the development of HULIS quantification, findings of possible HULIS sources and generation of ROS from HULIS. In light of HULIS studies, we comprehensively review the current knowledge of isolation and physicochemical characterization of HULIS from atmospheric samples as well as HULIS properties (hygroscopic, surface activity, and colloidal) and possible sources of HULIS. This review mainly highlights the generation of reactive oxygen species (ROS) from PMs, HULIS and transition metals, especially iron. This review also summarized the mechanism of iron-organic complexation and recent findings of OH formation from HULIS-metal complexes. This review will be helpful to carry out the modeling studies that concern with HULIS-transition metals and for further studies in the generation of ROS from HULIS-metal complexes.

Effects of aerosol water content and acidity on the light absorption of atmospheric humic-like substances in winter.

Atmospheric humic-like substances (HULIS) could affect regional climate due to their strong light-absorbing capacity. Daily fine particulate matter (PM2.5) samples were collected from December 18, 2016 to January 8, 2017 at an urban site in Chongqing, Southwest China. The mean concentration of HULIS in terms of carbon (HULIS-C) was 6.4 ± 3.4 µg m-3, accounting for 72% of water-soluble organic carbon. The mass absorption efficiency at 365 nm (MAE365) and absorption Ångström index (AAE) of atmospheric HULIS were 2.8 ± 0.30 m2 g-1 C and 4.6 ± 0.37, respectively. Good correlations between the light absorption coefficients of HULIS at 365 nm (Abs365) and the concentrations of K+, elemental carbon, NO3-, and NH4+ were observed, with correlation coefficients higher than 0.83, indicating that biomass burning and secondary formation were potential sources of light-absorbing HULIS, as evidenced by abundant fluorescent components related to less-oxygenated HULIS. Comparing the changes in Abs365 values, concentrations of major water-soluble inorganic ions and carbonaceous compounds in PM2.5, and environmental factors during the clean and pollution periods, we found that extensive biomass burning during the pollution period contributed significantly to the increase of Abs365 values. Moreover, the aerosol pH during the pollution period was close to 4, and NO2 concentration and aerosol water content were about 1.6 and 2.7 times higher than those during the clean period, respectively, which were favorable to form secondary HULIS through aqueous phase reactions in the presence of high NOx, resulting in an evident increase in its light absorption. Knowledge generated from this study is critical for evaluating the regional radiative forcing of brown carbon in southwest China.

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.

Size-Resolved Redox Activity and Cytotoxicity of Water-Soluble Urban Atmospheric Particulate Matter: Assessing Contributions from Chemical Components.

Throughout the cold and the warm periods of 2020, chemical and toxicological characterization of the water-soluble fraction of size segregated particulate matter (PM) (<0.49, 0.49−0.95, 0.95−1.5, 1.5−3.0, 3.0−7.2 and >7.2 µm) was conducted in the urban agglomeration of Thessaloniki, northern Greece. Chemical analysis of the water-soluble PM fraction included water-soluble organic carbon (WSOC), humic-like substances (HULIS), and trace elements (V, Cr, Mn, Fe, Ni, Cu, Zn, As, Cd and Pb). The bulk (sum of all size fractions) concentrations of HULIS were 2.5 ± 0.5 and 1.2 ± 0.3 µg m−3, for the cold and warm sampling periods, respectively with highest values in the <0.49 µm particle size fraction. The total HULIS-C/WSOC ratio ranged from 17 to 26% for all sampling periods, confirming that HULIS are a significant part of WSOC. The most abundant water-soluble metals were Fe, Zn, Cu, and Mn. The oxidative PM activity was measured abiotically using the dithiothreitol (DTT) assay. In vitro cytotoxic responses were investigated using mitochondrial dehydrogenase (MTT). A significant positive correlation was found between OPmDTT, WSOC, HULIS and the MTT cytotoxicity of PM. Multiple Linear Regression (MLR) showed a good relationship between OPMDTT, HULIS and Cu.

[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.

Optical properties, molecular characterizations, and oxidative potentials of different polarity levels of water-soluble organic matters in winter PM2.5 in six China's megacities.

Humic-like substances (HULIS) accounted for a great fraction of water-soluble organic matter (WSOM) in PM2.5, which efficiently absorb ultraviolet (UV) radiation and pose climate and health impacts. In this study, the molecular structure, optical properties, and oxidative potential (OP) of acid- and neutral-HULIS (denoted as HULIS-a, and HULIS-n, respectively), and high-polarity WSOM (HP-WSOM) were investigated in winter PM2.5 collected at six China's megacities. For both carbon levels and optical absorption coefficients (babs_365), HULIS-a/HULIS-n/HP-WSOM showed significant spatial differences. For each city, the carbon levels and babs_365 follow a similar order of HULIS-n > HULIS-a > HP-WSOM. Besides, the babs_365 of HULIS-n and HULIS-a showed the same order of Harbin > Beijing ≈ Wuhan > Xi'an > Guangzhou > Chengdu, while HP-WSOM exhibited an order of Wuhan > Chengdu > Xi'an > Harbin > Beijing > Guangzhou. Both HULIS-a and HULIS-n were abundant in aromatic and aliphatic compounds, whereas HP-WSOM was dominated by a carboxylic acid group. The OP (in unit of nmol H2O2 µg-1C) followed the order of HP-WSOM > HULIS-a > HULIS-n in all the cities. The OPs of HULIS-a, HULIS-n, and HP-WSOM in Harbin and Beijing were much higher than those of other cities, attributing to the high contribution from biomass burning. Highly positive correlations between reactive oxygen species (ROS) of HULIS-a and MAE365 were obtained in Chengdu, Wuhan, and Harbin, but ROS of HULIS-n had stronger correlation with MAE365 in Harbin, Chengdu, and Xi'an.

Molecular characterization of nitrogen-containing organic compounds in fractionated atmospheric humic-like substances (HULIS) and its relationship with optical properties.

Diverse nitrogen-containing organics are important components of humic-like substances (HULIS) in the atmosphere. In this study, organic components in particulate matter (PM) samples representing multiple sources were separated by successive solvent fractionation, which were then analyzed by mass spectrometric and optical instruments. The CHON compounds were eluted and clustered into the Low-polar, Medium-polar, and High-polar fractions, and discrepancies of the polar-fractions were particularly reflected by molecular descriptors such as aromaticity, oxygen content and molecular weight. In addition, the results from the light-absorbing parameters (i.e., MAE365 and SUVA254) underscored the importance of the Low-polar and High-polar fractions on optical absorption properties. The Low-polar fraction accounted for 40% of the cumulative SUVA254 values, suggesting significant content of ultraviolet-absorbing organics. The High-polar fraction contributed 52% of the cumulative MAE365 values, indicating abundant light absorption capacity and efficiency. Significant improvements were made on statistical analysis of multidimensional data by a combination of the molecular descriptors and optical parameters. Molecular structures, including condensed aromatic, lignin-like, and aliphatic compounds observed in distinct electrospray ionization modes, were found as main contributors to the light absorption capacity and the abundances of fluorophores in individual polar-fractions. Differential contributions of molecular characteristics on types and abundances of fluorophores were further found among the samples of multiple sources. Conclusions obtained from this successive solvent fractionation experiment could promote development of the pretreatment method for exploring the potential light-absorbing organics, which also provide insights into the emission sources of organics that are related to specific light absorption and fluorescence properties.

pH modifies the oxidative potential and peroxide content of biomass burning HULIS under dark aging.

Humic-like substances (HULIS) account for a major redox-active fraction of biomass burning organic aerosols (BBOA). During atmospheric transport, fresh acidic BB-HULIS in droplets and humid aerosols are subject to neutralization and pH-modified aging process. In this study, solutions containing HULIS isolated from wood smoldering emissions were first adjusted with NaOH and NH3 to pH values in the range of 3.6-9.0 and then aged under oxic dark conditions. Evolution of HULIS oxidative potential (OP) and total peroxide content (equivalent H2O2 concentration, H2O2eq) were measured together with the changes in solution absorbance and chemical composition. Notable immediate responses such as peroxide generation, HULIS autoxidation, and an increase in OP and light absorption were observed under alkaline conditions. Initial H2O2eq, OP, and absorption increased exponentially with pH, regardless of the alkaline species added. Dark aging further oxidized the HULIS and led to pH-dependent toxic and chemical changes, exhibiting an alkaline-facilitated initial increase followed by a decrease of OP and H2O2eq. Although highly correlated with HULIS OP, the contributions of H2O2eq to OP are minor but increased both with solution pH and dark aging time. Alkalinity-assisted autoxidation of phenolic compounds and quinoids with concomitant formation of H2O2 and other alkalinity-favored peroxide oxidation reactions are proposed here for explaining the observed HULIS OP and chemical changes in the dark. Our findings suggest that alkaline neutralization of fresh BB-HULIS represents a previously overlooked peroxide source and pathway for modifying aerosol redox-activity and composition. Additionally, these findings imply that the lung fluid neutral environment can modify the OP and peroxide content of inhaled BB-HULIS. The results also suggest that common separation protocols of HULIS using base extraction methods should be treated with caution when evaluating and comparing their composition, absorption, and relative toxicity.

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.

A preliminary study on humic-like substances in particulate matter in Malaysia influenced by Indonesian peatland fires.

In this paper, ambient total suspended particulates (TSP) with a focus on humic-like substances (HULIS) are characterized based on intensive ground-based field samplings collected in Malaysia during non-haze and haze periods caused by peatland fires on the Indonesian island of Sumatra. Furthermore, concentrations of water-soluble organic carbon (WSOC) and carbon content of HULIS (HULIS-C) were determined, and fluorescence spectra of the HULIS samples were recorded by excitation emission matrix (EEM) fluorescence spectroscopy. The concentrations of WSOC and HULIS-C over the entire period ranged from 4.1 to 24 and 1.3 to 18 µgC m-3, respectively. The concentrations of WSOC and HULIS-C during the peatland fire-induced strong haze periods were over 4.3 and 6.1 times higher, respectively, than the average values recorded during the non-haze periods. Even during the light haze periods, the concentrations of WSOC and HULIS-C were significantly higher than their averages during the non-haze periods. These results indicate that peatland fires induce high concentrations of WSOC, particularly HULIS-C, in ambient TSP at receptor sites. EEM fluorescence spectra identified fulvic-like fluorophores at the highest intensity level in the EEM fluorescence spectra of the haze samples. A peak at excitation/emission (Ex/Em) ≈ (290-330)/(375-425) nm is also observed at high intensity, though this peak is normally associated with marine humic-like fluorophores. It is shown that a peak at Ex/Em ≈ (290-330)/(375-425) nm is not derived from marine sources only; furthermore, peatland fires are shown to be important contributors to HULIS around this peak.

Concentrations, optical properties and sources of humic-like substances (HULIS) in fine particulate matter in Xi'an, Northwest China.

Humic-like substances (HULIS) are ubiquitous in the atmospheric environment, which affects both human health and climate. We present here the mass concentration and optical characteristics of HULIS isolated from aerosol samples collected in Xi'an, China. Both mass concentration and absorption coefficient (Abs365) of HULIS show clear seasonal differences, with the highest average in winter (3.91 µgC m-3 and 4.78 M m-1, respectively) and the lowest in summer (0.65 µgC m-3 and 0.55 M m-1, respectively). The sources of HULIS_C and light absorption of HULIS were analyzed by positive matrix factorization (PMF) and four major sources were resolved, including secondary formation, biomass burning, coal burning, and vehicle emission. Our results show that secondary formation (i.e., gas-to-particle conversion from e.g., photochemical oxidation) was the major contributor to both HULIS_C (50%) and light absorption (55%) of HULIS in summer, biomass burning and coal burning were major sources of HULIS_C (~70%) and light absorption (~80%) of HULIS in winter. It is worth noting that biomass burning and coal burning had higher contribution to HULIS light absorption (47% in spring, 37% in summer, 73% in fall, and 77% in winter) than their corresponding contribution to HULIS_C concentration (41% in spring, 37% in summer, 54% in fall, and 69% in winter). However, vehicle emission had lower contribution to HULIS light absorption (26% in spring, 8% in summer, 18% in fall, and 11% in winter) than to HULIS_C concentration (24% in spring, 13% in summer, 28% in fall, and 18% in winter). These results suggest that HULIS from biomass burning and coal burning have higher light absorption ability than from vehicle emission.

Diurnal evolutions and sources of water-soluble chromophoric aerosols over Xi'an during haze event, in Northwest China.

Atmospheric brown carbon and their chemical behavior potentially impacts the climate and air quality. Due to lack of researches on the atmospheric chromophores by using online experimental instrument, so using the offline EEM approaches to study their types, sources and chemical processes. In this study, PILS-EEM-TOC system (Particle into liquid sampler coupled with excitation-emission matrix and total organic carbon) was developed in order to distinguish the hourly evolutions and sources of water-soluble chromophoric organic matters in atmospheric fine particles. The results suggested that the sources of atmospheric chromophores in winter were primary combustion (~90%) and coal burning, followed by biomass burning and cooking emissions in Xi'an (Northwest China). These atmospheric chromophores decay under the combined action of solar radiation and atmospheric oxidants. Meanwhile, the secondary chromophores were mainly highly-oxygenated humic-like substance (HULIS), produced by atmospheric oxidation reactions with the highest peak in the afternoon. The partly secondary chromophores can also be generated through the Maillard-like reaction in the morning, which depends on the relative humidity of the atmosphere. These findings made a deeper understanding of the sources and transformation of atmospheric brown carbon aerosols.

Light absorption properties and molecular profiles of HULIS in PM2.5 emitted from biomass burning in traditional "Heated Kang" in Northwest China.

Humic-like substances (HULIS) in PM2.5 emitted from biomass burning (BB), including maize cob, wheat straw, maize straw, wood branch, and wood, in a traditional "Heated Kang" were investigated. The relative abundances, optical properties, chemical functional groups, and molecular components in HULIS were characterized using total organic carbon (TOC) analyzer, ultraviolet-visible spectroscopy (UV-vis), Fourier-transform infrared spectroscopy (FT-IR), and Fourier-transform ion cyclotron resonance mass spectrometer (FT-ICR/MS), respectively. The emission factors (EF) of HULIS-C (in term of carbon weight, EFHULIS-C) from BB were in the range of 0.83 to 5.17 g/kg fuel, with a mean value of 1.93 ± 1.31 g/kg fuel. The HULIS-C accounted for 15.0-37.8% and 9.1-12.6% of fractions in organic carbon (OC) and PM2.5, respectively, suggesting that BB is an important emission source of atmospheric HULIS. The FT-IR spectra showed BB HULIS mainly contain O-containing, aliphatic CH, and aromatic CC functional groups. The presences of carboxyl group and OH band demonstrated the uniqueness of maize straw and wood burning. Moreover, the higher ratio of CH3 and -CH2 groups could be used to distinguish the wood branches from the maize cob. CHO and CHON were much dominant in BB HULIS, which accounted for 44.6-47.6% and 50.1-54.2%, respectively, to the total molecular mass. The positive correlation between MAE365 and AAE in term of number concentration of CHNO implied that the CHNO species could greatly influence on the light absorption properties of the BB HULIS. The CHO and S-containing compounds (i.e., CHNOS and CHOS, that is CHNOS+CHOS) showed weak light absorbances of the BB HULIS. The BB HULIS from maize straw had relatively high molecular weight in comparison to that in other BB emissions. The highest and lowest aromaticity were seen on the wood burning and maize cob, respectively.

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.

Investigation of the Acid-Mediated Photosensitized Reactions of Amphiphilic α-Keto Acids at the Air-Water Interface Using Field-Induced Droplet Ionization Mass Spectrometry.

The photochemistry of α-keto acids has been of great interest due to its implications in atmospheric and prebiotic chemistries. α-Keto acids with long alkyl chains are amphiphilic in nature, and they tend to partition at the air-water interface of atmospheric water droplets and add to the complexity of the chemistries therein. The air-water interface is a unique environment that plays a vital role in overall atmospheric processes. However, existing studies mostly focus on the photochemistry of α-keto acids in the bulk solution and neglect the reactions that occur at the interface. In this study, using the field-induced droplet ionization mass spectrometry methodology that is capable of selectively sampling amphiphilic molecules that reside at the air-water interface, we show that the acid-mediated photochemistry of 2-oxooctanoic acid and 2-oxoheptoic acid is highly different from those of previously reported reactions in the bulk and contributes to the formation of humic-like substances (HULIS). This work emphasizes the uniqueness of the photochemistry at the air-water interface. We anticipate that studies of atmosphere-relevant photochemistry at the air-water interface will be an avenue rich with opportunities.