Light Absorption by Brown Carbon in the Southeastern United States is pH-dependent

Phase State Regulates Photochemical HONO Production from NaNO3/Dicarboxylic Acid Mixtures

Field observations of daytime HONO source strengths have not been well explained by laboratory measurements and model predictions up until now. More efforts are urgently needed to fill the knowledge gaps concerning how environmental factors, especially relative humidity (RH), affect particulate nitrate photolysis. In this work, two critical attributes for atmospheric particles, i.e., phase state and bulk-phase acidity, both influenced by ambient RH, were focused to illuminate the key regulators for reactive nitrogen production from typical internally mixed systems, i.e., NaNO3 and dicarboxylic acid (DCA) mixtures. The dissolution of only few oxalic acid (OA) crystals resulted in a remarkable 50-fold increase in HONO production compared to pure nitrate photolysis at 85% RH. Furthermore, the HONO production rates (PHONO) increased by about 1 order of magnitude as RH rose from <5% to 95%, initially exhibiting an almost linear dependence on the amount of surface absorbed water and subsequently showing a substantial increase in PHONO once nitrate deliquescence occurred at approximately 75% RH. NaNO3/malonic acid (MA) and NaNO3/succinic acid (SA) mixtures exhibited similar phase state effects on the photochemical HONO production. These results offer a new perspective on how aerosol physicochemical properties influence particulate nitrate photolysis in the atmosphere.

Molecular-level transformations of biomass burning-derived water-soluble organic carbon during dark aqueous OH oxidation: Insights from absorption, fluorescence, high-performance size exclusion chromatography and high-resolution mass spectrometry analysis

Biomass burning (BB) releases large amounts of water-soluble organic carbon (WSOC), which would undergo heterogenous oxidation processes that induce transformations in both molecular structures and compositions within BB WSOC. This study designed an aqueous oxidation initiated by OH radicals in the absence of light for WSOC extracted from smoke particles generated by burning of corn straw and fir wood. The BB WSOC was comprehensively characterized using a combination of UV-visible spectra, excitation-emission matrix fluorescence in conjunction with parallel factor analysis (EEM-PARAFAC), high-performance size exclusion chromatography (HPSEC), and high-resolution mass spectrometry (HRMS) analyses. Over the course of oxidation, both chromophores and fluorophores exhibited gradual decreases. Moreover, EEM-PARAFAC revealed a preferential degradation of larger-sized protein-like/phenol-like organic matters, accompanied by the accumulation and/or formation of humic-like substances in aged BB WSOC. HPSEC analysis showed notable changes in molecular weight (MW) distributions for both types of BB WSOC during oxidation. Specifically, high MW species (>1 kDa) displayed a tendency to form along with oxidation, possibly attributed to the formation of assemblies via intermolecular weak forces. After oxidation, evidence of CHO compound degradation and enrichment/formation of CHON compounds was observed for both types of BB WSOC. Remarkably, the resistant, degraded and produced molecules for BB WSOC were dominated by CHO (38-73 %) and lignin-like molecules (41-47 %), suggesting diverse responses to oxidation within these two groups. Furthermore, polyphenols experienced selective degradation, while CHON, aliphatic and poly-aromatic molecules tended to form during the oxidation process for both types of BB WSOC. In summary, this study provides a comprehensive understanding of the molecular-level transformations undergone by BB WSOC during dark aqueous OH oxidation. The findings significantly contribute to our insights into atmospheric evolution of BB WSOC, thereby playing a crucial role in accurately assessing their effects within climate models and informing policy decisions.

Evaluating the pH-dependence of DOM absorbance, fluorescence, and photochemical production of singlet oxygen

The protonation state of dissolved organic matter (DOM) impacts its structure and function in natural and engineered environmental systems, including DOM's ability to absorb light and form photochemically produced reactive intermediates (PPRI). However, the impacts of pH on DOM optical properties and PPRI formation have largely been evaluated separately, with less information being available on their interrelationship as a function of pH for the same set of samples. It is also unclear whether the impact of pH on optical spectra and associated optical surrogates for molecular size (e.g., E2 : E3) of DOM isolates is representative of the behavior of whole water samples. To address these knowledge gaps, spectral pH titrations were performed for seven humic substance and natural organic matter isolates, three whole water samples, and three model compounds. Comparison of the fractional and differential absorption and fluorescence spectra between DOM isolates, whole water samples, and model compounds revealed similar spectral features between all samples. The results show that spectral features observed for DOM isolates also occur for whole water samples, which suggests that there is overlap in the types of chromophores present in DOM isolates and whole waters. Although results from model compounds overlapped with DOM, especially in the ultraviolet region of the spectrum, no model compound replicated DOM's pH dependence perfectly. By measuring apparent quantum yields of singlet oxygen (ΦΔ), we show that aquatic DOM isolates exhibit a different pH-dependence (ΦΔ ∝ pH-1) than soil-derived humic acid isolates (ΦΔ ∝ pH). For aquatic DOM isolates, ΦΔ values measured at different pH were not correlated to apparent fluorescence quantum yields (Φf), suggesting that pH impacts singlet and triplet excited state DOM dynamics in different ways. In contrast, the proportional relationship between Φf and ΦΔ with increasing pH for soil humic acid isolates suggests that pH impacts singlet and triplet excited DOM in these samples in a similar fashion.

Investigating the molecular weight distribution of atmospheric water-soluble brown carbon using high-performance size exclusion chromatography coupled with diode array and fluorescence detectors

Atmospheric brown carbon (BrC) contain amounts of organic species, but their molecular weight (MW) distributions is still poorly understood. This study applied high-performance size exclusion chromatography (HPSEC) coupled with a diode array detector (DAD) and fluorescence detector (FLD) to characterize the MW distributions of typical chromophores and fluorophores within water-soluble BrC. The investigation focused on the spring season, encompassing both typical urban and rural aerosols. Our results showed that chromophores (at 254 and 365 nm), and humic-like and protein-like fluorophores identified by excitation-emission matrix parallel factor analysis (EEM-PARAFAC) within BrC were broadly distributed along the MW continuum (∼50-20,000 Da). This suggests that BrC mainly comprises complex chromophores and fluorophores with heterogeneous molecular sizes. High-MW (HMW, >1 kDa) species (66%-74%) dominated the chromophores at 254 and 365 nm. However, the latter chromophores were enriched with more HMW species. This result suggested that the HMW chromophores might contribute more to BrC absorption at longer wavelengths. The PARAFAC-derived fluorescent components also exhibited different MW distributions. Three humic-like substances (HULIS) were all dominated by HMW fractions (51%-74%), but protein-like fluorescent component (PLOM) enriched low-MW (LMW, <1 kDa) species (60%-66%). Furthermore, the molecular size (i.e., weight-averaged and number-averaged MW) and the ratios between HMW and LMW species decreased in the order highly-oxygenated HULIS > less-oxygenated HULIS > PLOM, indicating that the fluorophores with longer Em were generally related to larger MW. To our knowledge, this is the first report on the molecular size of individual fluorescent components within aerosol BrC. The results obtained here enhanced our knowledge of heterogeneous composition, complex physicochemical properties, and potential atmospheric fates of aerosol BrC.

Charge transfer interactions exist in extracellular polymeric substances: Comparison with natural organic matter

Extracellular polymeric substances (EPS) and natural organic matter (NOM) are widely present in the environment. While the molecular basis of NOM's optical properties and reactivity after treatment with sodium borohydride (NaBH₄) has been successfully explained by the charge transfer (CT) model, the corresponding structure basis and properties of EPS remain poorly understood. In this work, we investigated the reactivity and optical properties of EPS after NaBH₄ treatment, comparing them to the corresponding changes in NOM. After reduction, EPS exhibited optical properties and a reactivity with Au³⁺ similar to NOM, manifesting an irreversible loss of visible absorption (≥70%) associated with blue-shifted fluorescence emission (8-11 nm) and a lower rate of gold nanoparticles formation (decreasing by ≥ 32%), which can be readily explained by the CT model as well. Furthermore, the absorbance and fluorescence spectra of EPS were solvent polarity dependent, contrary to the superposition model. These findings contribute to an original understanding of the reactivity and optical properties of EPS and facilitate further cross-disciplinary studies.

Imaging of pH distribution inside individual microdroplet by stimulated Raman microscopy

Aerosol microdroplets as microreactors for many important atmospheric reactions are ubiquitous in the atmosphere. pH largely regulates the chemical processes within them; however, how pH and chemical species spatially distribute within an atmospheric microdroplet is still under intense debate. The challenge is to measure pH distribution within a tiny volume without affecting the chemical species distribution. We demonstrate a method based on stimulated Raman scattering microscopy to visualize the three-dimensional pH distribution inside single microdroplets of varying sizes. We find that the surface of all microdroplets is more acidic, and a monotonic trend of pH decreasing is observed in the 2.9-μm aerosol microdroplet from center to edge, which is well supported by molecular dynamics simulation. However, bigger cloud microdroplet differs from small aerosol for pH distribution. This size-dependent pH distribution in microdroplets can be related to the surface-to-volume ratio. This work presents noncontact measurement and chemical imaging of pH distribution in microdroplets, filling the gap in our understanding of spatial pH in atmospheric aerosol.

Highly Acidic Conditions Drastically Alter the Chemical Composition and Absorption Coefficient of α-Pinene Secondary Organic Aerosol

Secondary organic aerosols (SOA), formed through the gas-phase oxidation of volatile organic compounds (VOCs), can reside in the atmosphere for many days. The formation of SOA takes place rapidly within hours after VOC emissions, but SOA can undergo much slower physical and chemical processes throughout their lifetime in the atmosphere. The acidity of atmospheric aerosols spans a wide range, with the most acidic particles having negative pH values, which can promote acid-catalyzed reactions. The goal of this work is to elucidate poorly understood mechanisms and rates of acid-catalyzed aging of mixtures of representative SOA compounds. SOA were generated by the ozonolysis of α-pinene in a continuous flow reactor and then collected using a foil substrate. SOA samples were extracted and aged by exposure to varying concentrations of aqueous H₂SO₄ for 1-2 days. Chemical analysis of fresh and aged samples was conducted using ultra-performance liquid chromatography coupled with photodiode array spectrophotomety and high-resolution mass spectrometry. In addition, UV-vis spectrophotometry and fluorescence spectrophotometry were used to examine the changes in optical properties before and after aging. We observed that SOA that aged in moderately acidic conditions (pH from 0 to 4) experienced small changes in composition, while SOA that aged in a highly acidic environment (pH from -1 to 0) experienced more dramatic changes in composition, including the formation of compounds containing sulfur. Additionally, at highly acidic conditions, light-absorbing and fluorescent compounds appeared, but their identities could not be ascertained due to their small relative abundance. This study shows that acidity is a major driver of SOA aging, resulting in a large change in the chemical composition and optical properties of aerosols in regions where high concentrations of H₂SO₄ persist, such as upper troposphere and lower stratosphere.

Mobility of water-soluble aerosol organic matters (WSAOMs) and their effects on soil colloid-mediated transport of heavy metal ions in saturated porous media

Water-soluble aerosol organic matters (WSAOMs) produced by biomass pyrolysis/burning can penetrate subsurface environment, and are anticipated to have a profound effect on the fate of contaminants in aquatic ecosystems. Herein, WSAOMs derived from corn straw (CS-WSAOMs) and pinewood sawdust (PW-WSAOMs) pyrolysis at 300-900 °C were utilized to investigate their mobility characteristics and impacts on the transport of heavy metal ions (i.e., Cd²⁺) in saturated quartz sand with or without soil colloids. This study clearly demonstrated that WSAOMs in subsurface systems exhibited high mobility, which increased as WSAOMs molecular sizes decreased and hydrogen-bond interactions between WSAOMs and sand grains declined. WSAOMs significantly improved heavy metal (i.e., Cd²⁺) and soil colloid-mediated Cd²⁺ mobility in the porous media, which stemmed from the increased binding affinities of colloids toward metal ions and the high mobility of WSAOMs. Interestingly, in terms of the mobility and colloid-facilitated transport of Cd²⁺, WSAOMs from higher pyrolysis temperatures exhibited enhanced effects; meanwhile, the PW-WSAOMs demonstrated stronger effects than the CS-WSAOMs. The trends were mainly attributed to the differences in the metal-binding affinities (e.g., cation-π interactions) and transport abilities of WSAOMs, as well as diverse Cd²⁺ adsorption capacities of colloids induced by various WSAOMs.

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.

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 (pK_a) 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 pK_a was an important inflection point. The pK_a 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.

Effects of pH on light absorption properties of water-soluble organic compounds in particulate matter emitted from typical emission sources

Water-soluble organic compounds (WSOC) have a significant impact on aerosol radiative forcing and climate change, and there is considerable uncertainty in predicting and mitigating their climate and environmental effects. Here, the effects of pH on the light absorption properties of WSOC in particulate matter from different typical emission sources and ambient aerosols were systematically investigated using UV-vis spectrophotometer. pH (2-10) had an important impact on the light absorption properties of WSOC. The absorption, aromaticity, and the light absorption capacity of WSOC increased significantly with increasing pH for all samples. The difference absorbance spectra (∆absorbance) showed that the change of light absorption properties with pH was related to the deprotonate of carboxyl and phenolic groups resonating with aromatic and conjugated systems, with the most likely structures being carboxylic acids and phenols. Coal combustion and summer samples exhibited much higher susceptibility of light absorption properties to pH variation (increased by 27.0% and 65.9% relative to the pH 2 level, respectively). Absorption indices of almost all samples were significantly correlated with pH, indicating that the light absorption properties of WSOC may be quantitatively related to pH. The pH-dependent light absorption properties may have profound implications for evaluating the climate impacts of aerosol WSOC such as radiative forcing.

Enhanced Nitrite Production from the Aqueous Photolysis of Nitrate in the Presence of Vanillic Acid and Implications for the Roles of Light-Absorbing Organics

A prominent source of hydroxyl radicals (^•OH), nitrous acid (HONO) plays a key role in tropospheric chemistry. Apart from direct emission, HONO (or its conjugate base nitrite, NO₂^-) can be formed secondarily in the atmosphere. Yet, how secondary HONO forms requires elucidation, especially for heterogeneous processes involving numerous organic compounds in atmospheric aerosols. We investigated nitrite production from aqueous photolysis of nitrate for a range of conditions (pH, organic compound, nitrate concentration, and cation). Upon adding small oxygenates such as ethanol, n-butanol, or formate as ^•OH scavengers, the average intrinsic quantum yield of nitrite [Φ(NO₂^-)] was 0.75 ± 0.15%. With near-UV-light-absorbing vanillic acid (VA), however, the effective Φ(NO₂^-) was strongly pH-dependent, reaching 8.0 ± 2.1% at a pH of 8 and 1.5 ± 0.39% at a more atmospherically relevant pH of 5. Our results suggest that brown carbon (BrC) may greatly enhance the nitrite production from the aqueous nitrate photolysis through photosensitizing reactions, where the triplet excited state of BrC may generate solvated electrons, which reduce nitrate to NO₂ for further conversion to nitrite. This photosensitization process by BrC chromophores during nitrate photolysis under mildly acidic conditions may partly explain the missing HONO in urban environments.

Spectral characteristics of dissolved organic carbon derived from biomass-pyrogenic smoke (SDOC) in the aqueous environment and its solubilization effect on hydrophobic organic pollutants

Dissolved organic carbon derived from biomass-pyrogenic smoke (SDOC) can be transported and deposited with atmospheric aerosols, enter aqueous environments, and possibly alter aqueous chemistry and quality. However, the characteristics of SDOC in aqueous environments and their effects on the fate of hydrophobic organic pollutants are poorly understood. In this study, we found that the emitted SDOC is 7.2∼19.6 wt.% of biochar retained in situ after biomass pyrolysis, and the emitted SDOC is approximately 1-3 orders of magnitude greater than dissolved organic carbon (DOC) released from biochar in a short term, which indicates that SDOC is a more important source of DOC in aqueous environments relative to biochar-released DOC after a biomass burning/pyrolysis event. The characteristics of SDOC in aqueous environments are dominated by the <1000 Da fraction, which accounts for >96 wt.% of bulk SDOC. In comparison with DOC in biochar, natural water, and soil, the S_275-295 value of SDOC (0.037-0.053) is significantly greater, further indicating that SDOC has a smaller molecular size. Moreover, fluorescence EEM suggests that a fluorescence component located at the Ex/Em of 205/310 nm and the combinational ranges of fluorescence index (1.28-2.28), humification index (0.07-0.80), and biological index (1.16-1.72) can be used to identify SDOC from DOC in other media. Solubilization experiments indicated that SDOC (20 mg/L) improved the solubility of hydrophobic pollutants (pyrene and triclocarban) by 2-6 folds in aqueous environments, which potentially enhances the mobility of pollutants and enlarges the potential risk region. This study indicates that SDOC may cause a severe harm to aqueous environments in addition to the atmosphere. The results have profound implications for comprehensive assessments of the environmental effects of SDOC while promoting its identification and elucidating its behavior in aqueous environments.

Insight into binding characteristics of copper(II) with water-soluble organic matter emitted from biomass burning at various pH values using EEM-PARAFAC and two-dimensional correlation spectroscopy analysis

The metal-binding characteristics of water-soluble organic matter (WSOM) emitted from biomass burning (BB, i.e., rice straw (RS) and corn straw (CS)) with Cu(II) under various pH conditions (i.e., 3, 4.5, and 6) were comprehensively investigated. Two-dimensional correlation spectroscopy (2D-COS) and excitation-emission matrix (EEM) -PARAFAC analysis were applied to investigate the binding affinity and mechanism of BB WSOM. The results showed that pH was a sensitive factor affecting binding affinities of WSOM, and BB WSOMs were more susceptible to bind with Cu(II) at pH 6.0 than pH 4.5, followed by pH 3.0. Therefore, the Cu(II)-binding behaviors of BB WSOMs at pH 6.0 were then investigated in this study. The 2D-absorption-COS revealed that the preferential binding with Cu(II) was in the order short and long wavelengths (237-239 nm and 307-309 nm) > moderate wavelength (267-269 nm). The 2D-synchronous fluorescence-COS results suggested that protein-like substances generally exhibited a higher susceptibility and preferential interaction with Cu(II) than fulvic-like substances. EEM-PARAFAC analysis demonstrated that protein-like (C1) substances had a greater complexation ability than fulvic-like (C2) and humic-like (C3) substances for both BB WSOM. This indicated that protein-like substances within WSOM played dominant roles in the interaction with Cu(II). As a comparison, RS WSOM generally showed stronger complexation capacity than CS WSOM although they exhibited similar chemical properties and compositions. This suggested the occurrence of heterogeneous active metal-binding sites even within similar chromophores for different WSOM. The results enhanced our understanding of binding behaviors of BB WSOM with Cu(II) in relevant atmospheric environments.

Insight to Microbial Fe(III) Reduction Mediated by Redox-Active Humic Acids with Varied Redox Potentials

Redox-active humic acids (HA) are ubiquitous in terrestrial and aquatic systems and are involved in numerous electron transfer reactions affecting biogeochemical processes and fates of pollutants in soil environments. Redox-active contaminants are trapped in soil micropores (<2 nm) that have limited access to microbes and HA. Therefore, the contaminants whose molecular structure and properties are not damaged accumulate in the soil micropores and become potential pollution sources. Electron transfer capacities (ETC) of HA reflecting redox activities of low molecular weight fraction (LMWF, <2.5) HA can be detected by an electrochemical method, which is related to redox potentials (E_h) in soil and aquatic environments. Nevertheless, electron accepting capacities (EAC) and electron donating capacities (EDC) of these LMWF HA at different E_h are still unknown. EDC and EAC of different molecular weight HA at different E_h were analyzed using electrochemical methods. EAC of LMWF at -0.59 V was 12 times higher than that at -0.49 V, while EAC increased to 2.6 times when the Eh decreased from -0.59 V to -0.69 V. Afterward, LMWF can act as a shuttle to stimulate microbial Fe(III) reduction processes in microbial reduction experiments. Additionally, EAC by electrochemical analysis at a range of -0.49--0.59 V was comparable to total calculated ETC of different molecular weight fractions of HA by microbial reduction. Therefore, it is indicated that redox-active functional groups that can be reduced at E_h range of -0.49--0.59 are available to microbial reduction. This finding contributes to a novel perspective in the protection and remediation of the groundwater environment in the biogeochemistry process.

Spectroscopic insight into the pH-dependent interactions between atmospheric heavy metals (Cu and Zn) and water-soluble organic compounds in PM2.5

Taking Cu and Zn as examples, the pH-dependent interactions between atmospheric heavy metals (AHMs) and water-soluble organic compounds (WSOCs) in PM_2.5 were analyzed by a combination of UV-vis absorption, Fourier transform infrared (FTIR) spectroscopy and excitation-emission matrix (EEM) fluorescence spectroscopy coupled with parallel factor analysis (PARAFAC). We found metal-H ion exchange, complexation and electrostatic adsorption might occur between AHMs and WSOCs, and were generally enhanced with the increase of pH. Furthermore, these interactions were strengthened with the stepwise addition of [Cu²⁺] (from 0 to 500 μmol·L^-1), but had a relatively slight change with the stepwise addition of [Zn²⁺] (from 0 to 500 μmol·L^-1) generally. This indicated that the above interactions depended on the types and the concentrations of AHMs. Carboxyl, hydroxyl, carbonyl and aromatic structures of WSOCs were the major binding sites with AHMs. Humic acid-like substances were the dominant components of WSOCs binding with AHMs. The ratios of the apparent fluorescence quantum yields of the low and the high conjugation fractions of WSOCs (Q_ExL/H) declined by more than 28% as adding [Cu²⁺], indicating the formers had more strong complexing capacity with AHMs. AHMs might significantly impact the light absorption capacity and the wavelength dependence of WSOCs. The humification index (HIX_em) declined more than 15% as adding [Cu²⁺] at pH 5.6 and 7.5, indicating AHMs might weaken the oxidation capacity of WSOCs. These results implied the interactions between AHMs and WSOCs might play a profound role in atmospheric environment, human health, and global climate change.

Effect of Relative Humidity on Secondary Brown Carbon Formation in Aqueous Droplets

Atmospheric brown carbon (BrC) is a significant contributor to particulate light absorption. Reactions between small aldehydes and reduced nitrogen species have been shown to produce secondary BrC in atmospheric droplets. These reactions can be substantially accelerated upon droplet evaporation. Despite aqueous droplets undergoing continuous water evaporation and uptake in response to the surrounding relative humidity (RH), secondary BrC formation in these droplets under various RH conditions remains poorly understood. In this work, we investigate BrC formation from reactions of two aqueous-phase precursors, glyoxal and methylglyoxal, with ammonium sulfate or glycine in aqueous droplets after drying at a range of RH (30-90%). Our results illustrate, for the first time, that BrC production varies as a function of RH. For all four chemical reaction systems being investigated, mass absorption efficiencies (MAE, m²/g C) of aqueous aerosol products (from 270 to 512 nm wavelength range) generally increase with reducing RH to reach a maximum at ∼55-65% RH and subsequently decrease, caused by further drying. Chemical characterization using high-resolution aerosol mass spectrometry shows that the formation of nitrogen-containing organic species also follows a similar variation with RH. Our observations reveal that the acceleration of BrC production from evaporation of water may be diminished by other factors, such as limited particle-phase water content, phase transition, and volatility of reactants and products. Overall, our results highlight that intermediate RH conditions in the atmosphere may be more efficient in secondary BrC formation, indicating that the effect of RH needs to be included in atmospheric models for a more accurate representation of light-absorbing aerosol formation in aqueous droplets.

Quantitative determination of redox-active carbonyls of natural dissolved organic matter

Natural dissolved organic matter (DOM) is ubiquitous in environment and plays an important role in numerous environmental processes. Although the molecular basis of the reactivity of DOM remains poorly understood due to its extreme complexity, redox-active carbonyls (aromatic ketones/aldehydes and quinones) within DOM are believed vitally important. Except the rough determination of total carbonyls (including non-redox active -COOR) based on inflexible ¹³C chemical shift range by expensive and time-consuming solid-state nuclear magnetic resonance (NMR), there is no ready method to quantify redox-active carbonyls in DOM. Here we show that after treatment with sodium borohydride (NaBH₄) by selectively eliminating redox-active carbonyls, quenched fluorescence of carbon quantum dots (CD) by DOM recovered dramatically, and displayed a good linear relationship between redox-active carbonyls detected and DOM concentration (R² ≥ 0.977), thus allowing first quantitative determination of the redox-active carbonyls of DOM. Eight DOM isolates present 0.59%-0.90% redox-active carbonyls by the current method. And this method is robust from coexisting proteins and salts. This method could provide better or equal instructive results compared with solid-state NMR for total carbonyls or electrochemical method for electron-accepting capacities (EAC). Our results provide the underlying structural basis of many important geochemical processes that mediated by DOM. We posit that this method could apply to other complex molecular systems such as the atmospheric aerosols and extracellular polymeric substances (EPS), too.

Emerging investigator series: critical review of photophysical models for the optical and photochemical properties of dissolved organic matter

Optical measurements (absorbance and fluorescence) are widely used to track dissolved organic matter (DOM) quantity and quality in natural and engineered systems. Despite many decades of research on the optical properties of DOM, there is a lack of understanding with regards to the underlying photophysical model that is the basis for these optical properties. This review both summarizes advances to date on the photophysical properties of DOM and seeks to critically evaluate the photophysical models for DOM optical properties. Recent studies have refined the quantitative understanding of DOM photophysical properties such as excited state lifetimes and energies, rates of different photophysical processes, and quantum yields. Considering fundamental models, more clarity is needed on whether DOM photophysical processes are due to a superposition of non-interacting components (superposition model), or whether a portion of optical signals can be ascribed to electronically interacting moieties, for example in the form of electron donor-acceptor complexes (charge transfer model). Multiple studies over more than two decades have provided evidence for the charge transfer model. Questions have been raised, however, about the broad applicability of the charge transfer model. The charge transfer and superposition model are critically reviewed in light of this current research. Recommendations are given for future studies to help clarify the accuracy of these competing photophysical models.

The Acidity of Atmospheric Particles and Clouds

Acidity, defined as pH, is a central component of aqueous chemistry. In the atmosphere, the acidity of condensed phases (aerosol particles, cloud water, and fog droplets) governs the phase partitioning of semi-volatile gases such as HNO3, NH3, HCl, and organic acids and bases as well as chemical reaction rates. It has implications for the atmospheric lifetime of pollutants, deposition, and human health. Despite its fundamental role in atmospheric processes, only recently has this field seen a growth in the number of studies on particle acidity. Even with this growth, many fine particle pH estimates must be based on thermodynamic model calculations since no operational techniques exist for direct measurements. Current information indicates acidic fine particles are ubiquitous, but observationally-constrained pH estimates are limited in spatial and temporal coverage. Clouds and fogs are also generally acidic, but to a lesser degree than particles, and have a range of pH that is quite sensitive to anthropogenic emissions of sulfur and nitrogen oxides, as well as ambient ammonia. Historical measurements indicate that cloud and fog droplet pH has changed in recent decades in response to controls on anthropogenic emissions, while the limited trend data for aerosol particles indicates acidity may be relatively constant due to the semi-volatile nature of the key acids and bases and buffering in particles. This paper reviews and synthesizes the current state of knowledge on the acidity of atmospheric condensed phases, specifically particles and cloud droplets. It includes recommendations for estimating acidity and pH, standard nomenclature, a synthesis of current pH estimates based on observations, and new model calculations on the local and global scale.

Assessing Redox Properties of Natural Organic Matters with regard to Electron Exchange Capacity and Redox-Active Functional Groups

Redox processes in groundwater play an important role in bioavailability, toxicity, and mobility of redox-active elements and contaminants. A recent study has demonstrated that low-molecular-weight fraction (LMWF) of humic substances with great number of redox-active functional groups (RAFGs) exhibits great reducing capacity. However, whether LMWF of natural organic matter (NOM) exhibits high redox capacity still remains unclear. Therefore, this study extracted Pahokee peat NOM (PPNOM) and Leonardite NOM (LNOM) from soils, and then LMWFs in these NOMs were collected using a dialysis method. Electron exchange capacities (EEC) and RAFGs of LMWF NOMs at different Eh were analyzed using a novel electrochemical method and a three-dimensional excitation emission fluorescence (3DEEM) spectroscopy. We found that the reducing capacity in LMWF PPNOM was approximately 5-6 times higher than the bulk NOM, while only 7.8% LMWF PPNOM was accounted for in the bulk NOM. An increasing in EEC (EAC + EDC, where EAC is the electron accepting capacity and EDC is the electron donating capacity) of LMWF PPNOM and LNOM with Eh reduced from −0.49 V to −0.69 V. Additionally, an obvious increase in fluorescent intensities of quinone-like fluorophores before and after being reduced LMWF LNOM is responsible for high EAC of LMWF LNOM. These findings provide a better understanding of relationship between RAFGs Eh in LMWF of NOM, further helping in predicting and protection of groundwater environment and fate of transformation and transport for redox-active contaminants in groundwater.

Nitrite-Mediated Photooxidation of Vanillin in the Atmospheric Aqueous Phase

Nitrite (NO₂^-) and its conjugate acid, nitrous acid (HNO₂), have long been recognized as a ubiquitous atmospheric pollutant as well as an important photochemical source of hydroxyl radicals (·OH) and reactive nitrogen species (·NO, ·NO₂, ·N₂O₃, etc.) in both the gas phase and aqueous phase. Although NO₂^-/HNO₂ plays an important role in atmospheric chemistry, our understanding on its role in the chemical evolution of organic components in atmospheric waters is rather incomplete and is still in dispute. In this study, the nitrite-mediated photooxidation of vanillin (VL), a phenolic compound abundant in biomass burning emissions, was investigated under pH conditions relevant for atmospheric waters. The influence of solution pH, dissolved oxygen, and ·OH scavengers on the nitrite-mediated photooxidation of VL was discussed in detail. Our study reveals that the molecular composition of the products is dependent on the molar ratio of NO₂^-/VL in the solution and that nitrophenols are the major reaction products. We also found that the light absorbance of the oxidative products increases with increasing pH in the visible region, which can be attributed to the deprotonation of the nitrophenols formed. These results contribute to a better understanding of methoxyphenol photooxidation mediated by nitrite as a source of toxic nitrophenols and climatically important brown carbon in atmospheric waters.

Modelling the absorption properties of polycyclic aromatic hydrocarbons and derivatives over three European cities by TD-DFT calculations

While brown carbon is a strongly-light-absorbing type of organic aerosol that is capable of significant regional radiative forcing, it has been neglected from climate models, which results in differences between model predictions and measured data. This also results from uncertainty regarding the relationship between the chemical composition of brown carbon and its optical properties. Herein, here was utilized a time-dependent density functional theory (TD-DFT) approach to model the "real-world" absorption of thirty polycyclic aromatic hydrocarbons (PAHs) and twenty-five derivatives (ten nitro-PAHs and fifteen oxygenated-PAHs) present in the atmosphere over three Southern European cities (Porto, Florence and Athens). These data were corrected both for "real-world" experimental concentration of these molecules over these cities, and for their theoretical fluorescence yield. These results indicate that the absorption of the molecules more relevant for climate forcing are at ~330, ~360 and ~440 nm. Furthermore, the absorption is explained mainly by PAH and oxygenated-PAH molecules, while nitro-PAHs provide only negligible contributions. Porto should be the city to be most affected by radiative forcing induced by these molecules, while Florence and Athens appear to be similarly affected. Finally, these models also demonstrate that absorption at ~330 nm is explained by both PAH and oxygenated-PAH molecules, while absorption at ~360 and ~440 nm is only attributed to oxygenated-PAHs. More specifically, from the fifty-five studied molecules, only coronene (a PAH), 1,8-naphthalic anhydride, 6-H-benzo[cd]pyrene-6-one and 7H-benz[de]anthracence-7-one (three oxygenated-PAHs) provide relevant contributions to radiative forcing.

Monitoring the kinetics of reactions between natural organic matter and Al(III) ions using differential absorbance spectra

This study examined the kinetics of the binding of Al(III) ions by natural organic matter (NOM) exemplified by Suwannee River humic acid (SRHA). This processes was studied for a 5-8 pH range and environmentally relevant concentrations of the system components. Al(III)-NOM interactions were quantified using differential absorbance spectra whose intensity and shape depended on pH and reaction time. In all cases the differential spectra had four bands with maxima located at 245, 275, 320, 380 nm. These bands were assigned to the engagement of the carboxylic-like and/or phenolic-like groups, as well as electrostatic gel in NOM. Several parameters of the absorbance spectra (e.g., spectral slopes of log-transformed spectra in wavelength range 260-270 and 350-400 nm, ΔS_260-270 and ΔS_350-400 respectively) were linearly correlated (R² = 0.98) with concentrations of carboxylic-like groups and total NOM-bound Al(III) ions predicted based on the NICA-Donnan model. The binding of Al(III) ion by NOM at all pHs was modeled assuming the presence of three kinetically distinct sites. This study demonstrates that differential absorbance spectroscopy can be used to quantify the kinetics and mechanisms of NOM-metal ions interactions and monitor them in practically important system including water treatment operations.

Combined Effects of pH and Borohydride Reduction on Optical Properties of Humic Substances (HS): A Comparison of Optical Models

The combined effects of pH and borohydride reduction on the optical properties of a series of humic substances and a lignin model were examined to probe the molecular moieties and interactions that give rise to the observed optical properties of these materials. Increasing the pH from 2 to 12 produced significantly enhanced absorption across the spectra of all samples, with distinct spectral responses observed over pH ranges attributable to the deprotonation of carboxylic acids and phenols. Borohydride reduction substantially attenuated the broadband absorption enhancements with pH, clearly indicating that the loss of absorption due to ketone/aldehyde reduction is coupled with the pH-dependent increase in absorption due to deprotonation of carboxylic acids and phenols. These results cannot be easily explained by a superposition of the spectra of independently absorbing chromophores (superposition model) but are readily interpretable within a charge transfer (CT) model. Changes of fluorescence emission with pH for both untreated and borohydride reduced samples suggest that a pH-dependent structural reorganization of the HS may also be influencing the fluorescence emission. Independent of optical model, these results demonstrate that chemical tests targeted to specific moieties can identify distinct structural differences among HS sources as well as provide insight into the molecular moieties and interactions that produce the observed optical and photochemical properties.

Sources, compositions, and optical properties of humic-like substances in Beijing during the 2014 APEC summit: Results from dual carbon isotope and Fourier-transform ion cyclotron resonance mass spectrometry analyses

Humic-like substances (HULIS) are a class of high molecular weight, light-absorbing compounds that are highly related to brown carbon (BrC). In this study, the sources and compositions of HULIS isolated from fine particles collected in Beijing, China during the 2014 Asia-Pacific Economic Cooperation (APEC) summit were characterized based on carbon isotope (¹³C and ¹⁴C) and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analyses, respectively. HULIS were the main light-absorbing components of water-soluble organic carbon (WSOC), accounting for 80.2 ± 6.1% of the WSOC absorption capacity at 365 nm. The carbon isotope data showed that HULIS had a lower non-fossil contribution (53 ± 4%) and were less enriched with ¹³C (-24.2 ± 0.6‰) relative to non-HULIS (62 ± 8% and -20.8 ± 0.3‰, respectively). The higher relative intensity fraction of sulfur-containing compounds in HULIS before and after APEC was attributed to higher sulfur dioxide levels emitted from fossil fuel combustion, whereas the higher fraction of nitrogen-containing compounds during APEC may have been due to the relatively greater contribution of non-fossil compounds or the influence of nitrate radical chemistry. The results of investigating the relationships among the sources, elemental compositions, and optical properties of HULIS demonstrated that the light absorption of HULIS appeared to increase with increasing unsaturation degree, but decrease with increasing oxidation level. The unsaturation of HULIS was affected by both sources and aging level.

Brown Carbon Aerosol in Urban Xi'an, Northwest China: The Composition and Light Absorption Properties

Light-absorbing organic carbon (i.e., brown carbon or BrC) in the atmospheric aerosol has significant contribution to light absorption and radiative forcing. However, the link between BrC optical properties and chemical composition remains poorly constrained. In this study, we combine spectrophotometric measurements and chemical analyses of BrC samples collected from July 2008 to June 2009 in urban Xi'an, Northwest China. Elevated BrC was observed in winter (5 times higher than in summer), largely due to increased emissions from wintertime domestic biomass burning. The light absorption coefficient of methanol-soluble BrC at 365 nm (on average approximately twice that of water-soluble BrC) was found to correlate strongly with both parent polycyclic aromatic hydrocarbons (parent-PAHs, 27 species) and their carbonyl oxygenated derivatives (carbonyl-OPAHs, 15 species) in all seasons ( r² > 0.61). These measured parent-PAHs and carbonyl-OPAHs account for on average ∼1.7% of the overall absorption of methanol-soluble BrC, about 5 times higher than their mass fraction in total organic carbon (OC, ∼0.35%). The fractional solar absorption by BrC relative to element carbon (EC) in the ultraviolet range (300-400 nm) is significant during winter (42 ± 18% for water-soluble BrC and 76 ± 29% for methanol-soluble BrC), which may greatly affect the radiative balance and tropospheric photochemistry and therefore the climate and air quality.

The Case Against Charge Transfer Interactions in Dissolved Organic Matter Photophysics

The optical properties of dissolved organic matter influence chemical and biological processes in all aquatic ecosystems. Dissolved organic matter optical properties have been attributed to a charge-transfer model in which donor-acceptor complexes play a primary role. This model was evaluated by measuring the absorbance and fluorescence response of organic matter isolates to changes in solvent temperature, viscosity, and polarity, which affect the position and intensity of spectra for known donor-acceptor complexes of organic molecules. Absorbance and fluorescence spectral shape were largely unaffected by these changes, indicating that the distribution of absorbing and emitting species was unchanged. Overall, these results call into question the wide applicability of the charge-transfer model for explaining organic matter optical properties and suggest that future research should explore other models for dissolved organic matter photophysics.

Contribution of Quinones and Ketones/Aldehydes to the Optical Properties of Humic Substances (HS) and Chromophoric Dissolved Organic Matter (CDOM)

The molecular basis of the optical properties of chromophoric dissolved organic matter (CDOM) and humic substances (HS) remains poorly understood and yet to be investigated adequately. This study evaluates the relative contributions of two broad classes of carbonyl-containing compounds, ketones/aldehydes versus quinones, to the absorption and emission properties of a representative suite of HS as well as a lignin sample. Selective reduction of quinones to hydroquinones by addition of small molar excesses of dithionite to these samples under anoxic conditions produced small or negligible changes in their optical properties; however, when measurable, these changes were largely reversible upon exposure to air, consistent with the reoxidation of hydroquinones to quinones. With one exception, estimates of quinone content based on dithionite consumption by the HS under anoxic conditions were in good agreement with past electrochemical measurements. In contrast, reduction of ketones/aldehydes to alcohols employing excess sodium borohydride produced pronounced and largely, but not completely, irreversible changes in the optical properties. The results demonstrate that (aromatic) ketones/aldehydes, as opposed to quinones, play a far more prominent role in the optical absorption and emission properties of these HS, consistent with these moieties acting as the primary acceptors in charge-transfer transitions within these samples. As a method, anoxic dithionite titrations may further allow additional insight into the content and impact of quinones/hydroquinones on the optical properties of HS and CDOM.