Glyoxal-methylglyoxal cross-reactions in secondary organic aerosol formation

pH-Dependent Aqueous-Phase Brown Carbon Formation: Rate Constants and Implications for Solar Absorption and Atmospheric Photochemistry

Aqueous-phase reactions of α-dicarbonyls with amines or ammonium have been identified as important sources of secondary brown carbon (BrC). However, the kinetics of BrC formation and the effects of pH are still not very clear. In this study, the kinetics of BrC formation by aqueous reactions of α-dicarbonyls (glyoxal and methylglyoxal) with ammonium, amino acids, or alkylamines in bulk solution at different pH values are investigated. Our results reveal pH-parameterized BrC production rate constants, kBrCII (m-1 [M]-2 s-1), based on the light absorption between 300 and 500 nm: log10(kBrCII) = (1.0 ± 0.1) × pH - (7.4 ± 1.0) for reactions with glyoxal and log10(kBrCII) = (1.0 ± 0.1) × pH - (6.3 ± 0.9) for reactions with methylglyoxal. The linear slopes closing to 1.0 indicate that BrC formation is governed by the nitrogen nucleophilic addition pathway. Consequently, the absorptivities of the produced BrC increase exponentially with the increase of pH. BrC from reactions with methylglyoxal at higher pH (≥6.5) exhibits optical properties comparable to BrC from biomass burning or coal combustion, categorized as the "weakly" absorbing BrC, while BrC from reactions with methylglyoxal at lower pH (<6.0) or reactions with glyoxal (pH 5.0-7.0) falls into the "very weakly" absorbing BrC. The pH-dependent BrC feature significantly affects the solar absorption ability of the produced BrC and thus the atmospheric photochemical processes, e.g., BrC produced at pH 7.0 absorbs 14-16 times more solar power compared to that at pH 5.0, which in turn could lead to a decrease of 1 order of magnitude in the photolysis rate constants of O3 and NO2.

VOCs hyperspectral imaging: A new insight into evaluate emissions and the corresponding health risk from industries

The harm of VOCs emitted from industries to surrounding atmospheric environment and human health was well known and had received continuous attention. In order to improve the quality of urban atmospheric environment and the living environment of urban residents, a large number of original urban industries had been relocated to economically underdeveloped suburbs, which has significantly deteriorated the atmospheric environment in these areas and brought potential health risks to local vulnerable residents, which is actually an unfair manifestation under the background of economic development and ecological civilization construction. There were many residents near industrial parks, but there was a significant lack of VOCs monitoring equipment and data. At present, the time resolution of the most commonly used in situ method was seriously insufficient, and it was unable to quantify the diffusion/transport process of VOCs. It was urgent to have effective detection methods for industrial VOCs plume concentration and diffusion/transport process. In this study, we proposed a hyperspectral imaging technology, which can realize long-term continuous imaging monitoring on plume concentrations of formaldehyde (HCHO), glyoxal (CHOCHO) and benzaldehyde (C6H5CHO) and their corresponding diffusion processes. The deviation between the imaging and in situ sampling concentrations in the outlet was 4-19 %. The spatial resolution of this technique reached meter level, and the temporal resolution of one pixel was better than 20 s. In this study, we carried out hyperspectral imaging of aldehyde VOCs for a chemical facility, a petrochemical facility and an industrial park containing various types of enterprises in the Yangtze River Delta. The maximum observed concentration of HCHO was 120.44 ± 12.14 ug/m3 with the emission flux of 39.27 ± 3.97 g/h, which was emitted from a petrochemical facility in Shanghai. A diffusion/transport model was established, and we found that the spatial distribution of HCHO, CHOCHO and C6H5CHO for the chemical facility case in Shanghai were all mainly along the southeast-northwest direction during one year. The health risk assessment emphasized that residents within 10 km north of the outlet of the chemical facility in Shanghai should pay more attention to the health risks caused by industrial HCHO emissions. More systematically and comprehensively hyperspectral imaging of VOCs emissions for different types of enterprises and different processes were expected to performed to greatly promote the establishment of a dynamic emission inventory and an effective health risk evaluation system in the future.

New Insights into the Brown Carbon Chromophores and Formation Pathways for Aqueous Reactions of α-Dicarbonyls with Amines and Ammonium

Aqueous-phase reactions of α-dicarbonyls with ammonium or amines have been identified as important sources of secondary brown carbon (BrC). However, the identities of most chromophores in these reactions and the effects of pH remain largely unknown. In this study, the chemical structures, formation pathways, and optical properties of individual BrC chromophores formed through aqueous reactions of α-dicarbonyls (glyoxal and methylglyoxal) with ammonium, amino acids, or methylamine at different pH's were characterized in detail by liquid chromatography-photodiode array-high resolution tandem mass spectrometry. In total, 180 chromophores are identified, accounting for 29-79% of the light absorption of bulk BrC for different reactions. Thereinto, 155 newly identified chromophores, including 76 imidazoles, 57 pyrroles, 10 pyrazines, 9 pyridines, and 3 imidazole-pyrroles, explain additionally 9-69% of the light absorption, and these chromophores mainly involve four formation pathways, including previously unrecognized reactions of ammonia or methylamine with the methylglyoxal dimer for the formation of pyrroles. The pH in these reactions also shows remarkable effects on the formation and transformation of BrC chromophores; e.g., with the increase of pH from 5.0 to 7.0, the light absorption contributions of imidazoles in identified chromophores decrease from 72% to 65%, while the light absorption contributions of pyrazines increase from 5% to 13% for the methylglyoxal + ammonium reaction; meanwhile, more small nitrogen heterocycles transformed into oligomers (e.g., C9 and C12 pyrroles) via reaction with methylglyoxal. These newly identified chromophores and proposed formation pathways are instructive for future field studies of the formation and transformation of aqueous-phase BrC.

Direct emissions of particulate glyoxal and methylglyoxal from biomass burning and coal combustion

Glyoxal (Gly) and methylglyoxal (Mgly) are key precursors globally for secondary organic aerosol (SOA) formation. These two species were often thought to be formed in the atmosphere via photochemical oxidation of organics from biogenic and anthropogenic origins, although few studies have shown their direct emissions. In this study, we report direct emissions of particulate Gly and Mgly from different residential fuels typically used in north China. The emission ratios (ERs) and emission factors (EFs) of particulate Gly and Mgly for biomass burning were approximate 5-fold and 7-fold higher than those for coal combustion, respectively. The large variances in emissions of Gly and Mgly could be attributed to the different combustion processes, which influenced by the fuel types and combustion conditions. The averaged ERs and EFs of particulate Gly and Mgly were about one order of magnitude lower than their gaseous counterparts due to the low Henry's law constant, which was also consistent with the low particle-to-gas ratio of Gly (0.04) and Mgly (0.02). Our results suggest that the direct emissions of Gly and Mgly from emission sources should be considered when estimating the formation of SOA from Gly and Mgly.

Probing Anion-Molecule Complexes of Atmospheric Relevance Using Anion Photoelectron Detachment Spectroscopy

Bimolecular reaction and collision complexes that drive atmospheric chemistry and contribute to the absorption of solar radiation are fleeting and therefore inherently challenging to study experimentally. Furthermore, primary anions in the troposphere are short lived because of a complicated web of reactions and complex formation they undergo, making details of their early fate elusive. In this perspective, the experimental approach of photodetaching mass-selected anion-molecule complexes or complex anions, which prepares neutrals in various vibronic states, is surveyed. Specifically, the application of anion photoelectron spectroscopy along with photoelectron-photofragment coincidence spectroscopy toward the study of collision complexes, complex anions in which a partial covalent bond is formed, and radical bimolecular reaction complexes, with relevance in tropospheric chemistry, will be highlighted.

Detection of excited triplet species from photolysis of carbonyls: Direct evidence for single oxygen formation in atmospheric environment

Excited triplet species play an important role in the photolytic formation of ¹O₂ from carbonyls, but the related mechanism is still uncertain, due to lack of direct evidence. In this study, steady-state and transient photolysis of eleven carbonyls to produce ¹O₂ was investigated. Dicarbonyl displayed greater ¹O₂ production ability than monocarbonyl, while dicarbonyl containing both ketone and carboxyl groups connected by CC bond (i.e., pyruvic acid (PA)) showed the highest ¹O₂ steady-state concentration ([¹O₂]_SS). For the first time, the production of ³PA* from PA with narrow energy gap was confirmed by laser flash photolysis technique and the second-order decay rate constant of ³PA* was 2.78 × 10⁷ M^-1 s^-1. Quenching results verified the dominant contribution of ³PA* to ¹O₂ production from PA. Addition of inorganic salt or increase in solution pH showed negligible effect on ³PA*, but significantly decreased the [¹O₂]_SS of PA by up to two orders of magnitude, due to reduction of hydrate content. Photolysis of methylglyoxal and dimethylamine mixture led to higher content of excited triplet species at pH ≈ 11 and remarkably enhanced [¹O₂]_SS, which was 2.3 times of that from PA and dimethylamine mixture. These findings provide direct evidence for the contribution of transient species from carbonyls or their product to ¹O₂ formation in atmospheric environment.

Free L-Lysine and Its Methyl Ester React with Glyoxal and Methylglyoxal in Phosphate Buffer (100 mM, pH 7.4) to Form Nε-Carboxymethyl-Lysine, Nε-Carboxyethyl-Lysine and Nε-Hydroxymethyl-Lysine

Glyoxal (GO) and methylglyoxal (MGO) are highly reactive species formed in carbohydrate metabolism. N^ε-Carboxymethyllysine (CML) and N^ε-carboxyethyllysine (CEL) are considered to be the advanced glycation end-products (AGEs) of L-lysine (Lys) with GO and MGO, respectively. Here, we investigated the reaction of free L-lysine (Lys) with GO and MGO in phosphate buffer (pH 7.4) at 37 °C and 80 °C in detail in the absence of any other chemicals which are widely used to reduce Schiff bases. The concentrations of Lys, GO and MGO used in the experiments were 0.5, 2.5, 5.0, 7.5 and 10 mM. The reaction time ranged between 0 and 240 min. Experiments were performed in triplicate. The concentrations of remaining Lys and of CML and CEL formed in the reaction mixtures were measured by stable-isotope dilution gas chromatography-mass spectrometry (GC-MS). Our experiments showed that CML and CEL were formed at higher concentrations at 80 °C compared to 37 °C. CML was found to be the major reaction product. In mixtures of GO and MGO, MGO inhibited the formation of CML from Lys (5 mM) in a concentration-dependent manner. The highest CML concentration was about 300 µM corresponding to a reaction yield of 6% with respect to Lys. An addition of Lys to GO, MGO and their mixtures resulted in strong reversible decreases in the Lys concentration up to 50%. It is assumed that free Lys reacts rapidly with GO and MGO to form many not yet identified reaction products. Reaction mixtures of Lys and MGO were stronger colored than those of Lys and GO, notably at 80 °C, indicating higher reactivity of MGO towards Lys that leads to polymeric colored MGO species. We have a strong indication of the formation of N^ε-(hydroxymethyl)-lysine (HML) as a novel reaction product of Lys methyl ester with MGO. A mechanism is proposed for the formation of HML from Lys and MGO. This mechanism may explain why Lys and GO do not react to form a related product. Preliminary analyses show that HML is formed at higher concentrations than CEL from Lys methyl ester and MGO. No Schiff bases or their hydroxylic precursors were identified as reaction products. In their reactions with Lys, GO and MGO are likely to act both as chemical oxidants on the terminal aldehyde group to a carboxylic group (i.e., R-CHO to R-COOH) and as chemical reductors on labile Schiff bases (R-CH=N-R to R-CH₂-NH-R) presumably via disproportionation and hydride transfer. Our study shows that free non-proteinic Lys reacts with GO and MGO to form CML, CEL and HML in very low yield. Whether proteinic Lys also reacts with MGO to form HML residues in proteins remains to be investigated. The physiological occurrence and concentration of HML in biological fluids and tissues and its relation to CML and CEL are elusive and warrant further investigations in health and disease. Chemical synthesis and structural characterization of HML are expected to advance and accelerate the scientific research in this topic.

Autodetachment over Broad Photon Energy Ranges in the Anion Photoelectron Spectra of [O2-]- ( = Glyoxal, Methylglyoxal, or Biacetyl) Complex Anions

Complexes of anion-neutral pairs are prevalent in chemical and physical processes in the interstellar medium, the atmosphere, and biological systems, among others. However, bimolecular anionic species that cannot be described as simple ion-molecule complexes due to their competitive electron affinities have received less attention. In this study, the [O₂-M]^- (M = glyoxal, methylglyoxal, or biacetyl) anion photoelectron spectra obtained with several different photon energies are reported and interpreted in the context of ab initio calculations. The spectra do not resemble the photoelectron spectra of M^- or O₂^- "solvated" by a neutral partner. Rather, all spectra are dominated by near-threshold autodetachment from what are likely transient dipole bound states of the cis conformers of the complex anions. Very low Franck-Condon overlap between the neutral M·O₂ van der Waals clusters and the partial covalently bound complex anions results in low-intensity, broad direct detachment observed in the spectra. The [O₂-glyoxal]^- spectra measured with 2.88 and 3.495 eV photon energies additionally exhibit features at ∼0.5 eV electron kinetic energy, which is more difficult to explain, though there are numerous quasibound states of the anion that may be involved. Overall, these features point to the inadequacy of describing the complex anions as simple ion-molecule complexes.

High Pressure Inside Nanometer-Sized Particles Influences the Rate and Products of Chemical Reactions

The composition of organic aerosol has a pivotal influence on aerosol properties such as toxicity and cloud droplet formation capability, which could affect both climate and air quality. However, a comprehensive and fundamental understanding of the chemical and physical processes that occur in nanometer-sized atmospheric particles remains a challenge that severely limits the quantification and predictive capabilities of aerosol formation pathways. Here, we investigated the effects of a fundamental and hitherto unconsidered physical property of nanoparticles-the Laplace pressure. By studying the reaction of glyoxal with ammonium sulfate, both ubiquitous and important atmospheric constituents, we show that high pressure can significantly affect the chemical processes that occur in atmospheric ultrafine particles (i.e., particles < 100 nm). Using high-resolution mass spectrometry and UV-vis spectroscopy, we demonstrated that the formation of reaction products is strongly (i.e., up to a factor of 2) slowed down under high pressures typical of atmospheric nanoparticles. A size-dependent relative rate constant is determined and numerical simulations illustrate the reduction in the production of the main glyoxal reaction products. These results established that the high pressure inside nanometer-sized aerosols must be considered as a key property that significantly impacts chemical processes that govern atmospheric aerosol growth and evolution.

Importance of Wintertime Anthropogenic Glyoxal and Methylglyoxal Emissions in Beijing and Implications for Secondary Organic Aerosol Formation in Megacities

Atmospheric glyoxal (GLY) and methylglyoxal (MGLY) are key precursors of secondary organic aerosol (SOA). However, anthropogenic emissions of GLY and MGLY and their contribution to surface GLY and MGLY concentrations, as well as the secondary organic aerosol (SOA) formation, are not well quantified. By developing an emission inventory of anthropogenic GLY and MGLY and improving the Community Multiscale Air Quality Model (CMAQ) with SOA formation from irreversible surface uptake of GLY and MGLY, as well as a precursor-origin resolved technique, we quantified the source contributions of GLY and MGLY and their impact on wintertime SOA formation in Beijing, China. The total emissions of GLY and MGLY in Beijing in 2017 are 1.1 × 10⁴ kmol and 7.0 × 10³ kmol, respectively. Anthropogenic primary emissions are found to be the dominant contributor to wintertime GLY and MGLY concentrations (∼74% for GLY and ∼63% for MGLY). Anthropogenic primary emissions of GLY and MGLY contributes to 30% of GLY/MGLY SOA daily average concentration and accounts for up to 45% of nighttime GLY/MGLY SOA in winter. The study suggests that the anthropogenic GLY and MGLY emissions, mainly from gasoline vehicles and cooking, are important for SOA formation and shall be strictly controlled in Chinese megacities.

Glyoxal and Methylglyoxal as E-cigarette Vapor Ingredients-Induced Pro-Inflammatory Cytokine and Mucins Expression in Human Nasal Epithelial Cells

BACKGROUND

Glyoxal (GO), and methylglyoxal (MGO) are among the most toxic compounds emitted by electronic cigarette (E-cig) and regular tobacco cigarette smoke. Airway diseases presented mucus over production as their major pathophysiologic feature. However, the effects of GO and MGO on pro-inflammatory cytokines and mucin expression in human nasal epithelial cells, as well as the underlying signaling pathway, have not yet been studied.

OBJECTIVE

This study is to determine whether GO and MGO induce pro-inflammatory cytokines, and MUC5AC/5B expression via mitogen-activated protein kinase (MAPK)s and nuclear factor-kappa-light-chain-enhancer of activated B cells (NF-κB) signaling pathways.

METHODS

The effect of GO, and MGO on pro-inflammatory cytokines, mucins expression and the signalling pathway of GO and MGO were investigated using water-soluble tetrazolium salt-1, enzyme immunoassays, and immunoblot analysis with specific inhibitors and small interfering RNA.

RESULTS

GO and MGO did not affect cell viability up to 2 mM in human nasal epithelial cells. GO and MGO increased production of pro-inflammatory such as interleukin (IL)-1β and IL-6) and MUC5AC/5B. Additionally, GO and MGO significantly activated extracellular signal-regulated kinase 1/2 (ERK1/2), p38 MAPK, and NF-κB. Whether ERK1/2, p38 MAPK, and NF-κB signaling pathway were involved in GO and MGO-induced production of pro-inflammatory cytokines (IL-1β and IL-6) and MUC5AC/5B, we used specific inhibitors and siRNA transfection. These significantly repressed GO- and MGO-induced expression of pro-inflammatory cytokines (IL-1β and IL-6) and MUC5AC/5B.

CONCLUSIONS

GO and MGO induced pro-inflammatory cytokines and MUC5AC/5B expression via ERK1/2, p38 MAPK, and NF-κB in human nasal epithelial cells. These results suggested that GO and MGO may be involved in mucus hypersecretion-related airway diseases.

Modeling of Carbonyl/Ammonium Sulfate Aqueous Brown Carbon Chemistry via UV/Vis Spectral Decomposition

The proper characterization of aqueous brown carbon (BrC) species, their formation, and their light absorbance properties is critical to understanding the aggregate effect that they have on overall atmospheric aerosol climate forcing. The contribution of dark chemistry secondary organic aerosol (SOA) products from carbonyl-containing organic compounds (CVOCs) to overall aqueous aerosol optical properties is expected to be significant. However, the multiple, parallel pathways that take place within CVOC reaction systems and the differing chromophoricity of individual products complicates the ability to reliably model the chemical kinetics taking place. Here, we proposed an alternative method of representing UV-visible absorbance spectra as a composite of Gaussian lineshape functions to infer kinetic information. Multiple numbers of curves and different CVOC/ammonium reaction systems were compared. A model using three fitted Gaussian curves with magnitudes following first-order kinetics achieved an accuracy within 65.5% in the 205–300-nm range across multiple organic types and solution aging times. Asymmetrical peaks that occurred in low-200-nm wavelengths were decomposed into two overlapping Gaussian curves, which may have been attributable to different functional groups or families of reaction products. Component curves within overall spectra exhibited different dynamics, implying that the utilization of absorbance at a single reference wavelength to infer reaction rate constants may result in misrepresentative kinetics for these systems.

Infrared Ion Spectroscopy of Environmental Organic Mixtures: Probing the Composition of α-Pinene Secondary Organic Aerosol

Characterizing the chemical composition of organic aerosols can elucidate aging mechanisms as well as the chemical and physical properties of the aerosol. However, the high chemical complexity and often low atmospheric abundance present a difficult analytical challenge. Milligrams or more of material may be needed for speciated spectroscopic analysis. In contrast, mass spectrometry provides a very sensitive platform but limited structural information. Here, we combine the strengths of mass spectrometry and infrared (IR) action spectroscopy to generate characteristic IR spectra of individual, mass-isolated ion populations. Soft ionization combined with in situ infrared ion spectroscopy, using the tunable free-electron laser FELIX, provides detailed information on molecular structures and functional groups. We apply this technique, along with quantum mechanical modeling, to characterize organic molecules in secondary organic aerosol (SOA) formed from the ozonolysis of α-pinene. Spectral overlap with a standard is used to identify cis-pinonic acid. We also demonstrate the characterization of isomers for multiple SOA products using both quantum mechanical computations and analyses of fragment ion spectra. These results demonstrate the detailed structural information on isolated ions obtained by combining mass spectrometry with fingerprint IR spectroscopy.

Elevated levels of glyoxal and methylglyoxal at a remote mountain site in southern China: Prompt in-situ formation combined with strong regional transport

The dicarbonyls glyoxal (Gly) and methylglyoxal (Mgly) are key tracers for the oxidation of volatile organic compounds (VOCs) in the atmosphere, but their atmospheric chemistry in remote forest environments is not well understood. A study was carried out during Jul. 31-Nov. 5 of 2016 at the summit of Mt. Tianjing (1690 m.a.s.l.), a remote mountaintop site in southern China, to measure the levels of Gly and Mgly and explore their sources and fate. During the study period, the average mixing ratios of Gly and Mgly were 509 ± 31 pptv and 340 ± 32 pptv, respectively, with the Gly/Mgly ratios averaging 1.8 ± 0.2. Both the dicarbonyl concentrations and the Gly/Mgly ratios were significantly higher than those observed in other background sites. Production yield calculations and meteorological data analysis indicate that high levels of Gly and Mgly observed at the study site were largely a combined result of rapid in-situ formation and regional transport by prevailing winds. On average, in-situ formation from precursors is estimated to account for 67% of the observed Mgly and about 9% of the observed Gly. There were significant changes in Gly and Mgly mixing ratios among different time periods when air masses from different source regions dominated, indicating contribution of regional transport to the observed dicarbonyl mixing ratios at the study site. Biogenic emissions in eastern China and anthropogenic emissions in the Pearl River Delta region were the two main sources responsible for the dicarbonyls observed at the site during most of the sampling period, but large-scale biomass burning in central China was also important in the late autumn, as supported by a backward trajectory analysis of fire spot data and the identification of biomass burning tracers. This study provides insights into the background atmospheric chemistry and the impact of biogenic and anthropogenic sources on the dicarbonyls speciation.

Effect of Titanium Dioxide on Secondary Organic Aerosol Formation

Secondary organic aerosol (SOA), a dominant air pollutant in many countries, threatens the lives of millions of people. Extensive efforts have been invested in studying the formation mechanisms and influence factors of SOA. As promising materials in eliminating air pollutants, the role of photocatalytic materials in SOA formation is unclear. In this study, TiO₂ was employed to explore its impact on SOA formation during the photooxidation of m-xylene with NO _x in a smog chamber. We found that the presence of TiO₂ strongly suppressed SOA formation. The yields of SOA in the photooxidation experiments of m-xylene with NO _x were 0.3-4%, whereas negligible SOA was formed when TiO₂ was added. When ((NH₄)₂SO₄) was introduced as seed particles, the presence of TiO₂ decreased the yields of SOA from 0.3-6% to 0.3-1.6%. The sharply decreased concentrations of reactive carbonyl compounds were the direct cause of the suppression effect of TiO₂ on SOA formation. However, the suppression effect was influenced by the addition of seed particles and the initial concentration of NO _x. Reaction mechanisms of the photocatalysis of m-xylene with and without NO _x were proposed.

Model Behavior: Characterization of Hydroxyacetone at the Air-Water Interface Using Experimental and Computational Vibrational Sum Frequency Spectroscopy

Small atmospheric aldehydes and ketones are known to play a significant role in the formation of secondary organic aerosols (SOA). However, many of them are difficult to experimentally isolate, as they tend to form hydration and oligomer species. Hydroxyacetone (HA) is unusual in this class as it contributes to SOA while existing predominantly in its unhydrated monomeric form. This allows HA to serve as a valuable model system for similar secondary organic carbonyls. In this paper the surface behavior of HA at the air-water interface has been investigated using vibrational sum frequency (VSF) spectroscopy and Wilhelmy plate surface tensiometry in combination with computational molecular dynamics simulations and density functional theory calculations. The experimental results demonstrate that HA has a high degree of surface activity and is ordered at the interface. Furthermore, oriented water is observed at the interface, even at high HA concentrations. Spectral features also reveal the presence of both cis and trans HA conformers at the interface, in differing orientations. Molecular dynamics results indicate conformer dependent shifts in HA orientation between the subsurface (∼5 Å deep) and surface. Together, these results provide a picture of a highly dynamic, but statistically ordered, interface composed of multiple HA conformers with solvated water. These results have implications for HA's behavior in aqueous particles, which may affect its role in the atmosphere and SOA formation.

Atmospheric Aerosols: Clouds, Chemistry, and Climate

Although too small to be seen with the human eye, atmospheric particulate matter has major impacts on the world around us, from our health to global climate. Understanding the sources, properties, and transformations of these particles in the atmosphere is among the major challenges in air quality and climate research today. Significant progress has been made over the past two decades in understanding atmospheric aerosol chemistry and its connections to climate. Advances in technology for characterizing aerosol chemical composition and physical properties have enabled rapid discovery in this area. This article reviews fundamental concepts and recent developments surrounding ambient aerosols, their chemical composition and sources, light-absorbing aerosols, aerosols and cloud formation, and aerosol-based solar radiation management (also known as solar geoengineering).

Potential of Aerosol Liquid Water to Facilitate Organic Aerosol Formation: Assessing Knowledge Gaps about Precursors and Partitioning

Isoprene epoxydiol (IEPOX), glyoxal, and methylglyoxal are ubiquitous water-soluble organic gases (WSOGs) that partition to aerosol liquid water (ALW) and clouds to form aqueous secondary organic aerosol (aqSOA). Recent laboratory-derived Setschenow (or salting) coefficients suggest glyoxal's potential to form aqSOA is enhanced by high aerosol salt molality, or "salting-in". In the southeastern U.S., aqSOA is responsible for a significant fraction of ambient organic aerosol, and correlates with sulfate mass. However, the mechanistic explanation for this correlation remains elusive, and an assessment of the importance of different WSOGs to aqSOA is currently missing. We employ EPA's CMAQ model to the continental U.S. during the Southern Oxidant and Aerosol Study (SOAS) to compare the potential of glyoxal, methylglyoxal, and IEPOX to partition to ALW, as the initial step toward aqSOA formation. Among these three studied compounds, IEPOX is a dominant contributor, ∼72% on average in the continental U.S., to potential aqSOA mass due to Henry's Law constants and molecular weights. Glyoxal contributes significantly, and application of the Setschenow coefficient leads to a greater than 3-fold model domain average increase in glyoxal's aqSOA mass potential. Methylglyoxal is predicted to be a minor contributor. Acid or ammonium - catalyzed ring-opening IEPOX chemistry as well as sulfate-driven ALW and the associated molality may explain positive correlations between SOA and sulfate during SOAS and illustrate ways in which anthropogenic sulfate could regulate biogenic aqSOA formation, ways not presently included in atmospheric models but relevant to development of effective control strategies.

Deciphering the aqueous chemistry of glyoxal oxidation with hydrogen peroxide using molecular imaging

The first in situ molecular imaging study of glyoxal oxidation by hydrogen peroxide leading to the formation of aqueous secondary organic aerosols.

Characteristics of size-segregated carbonaceous aerosols in the Beijing-Tianjin-Hebei region

Mass concentrations of organic carbon (OC) and elemental carbon (EC) in size-resolved aerosols were investigated at four sites (three cities and one country) in the Beijing-Tianjin-Hebei region from September 2009 to August 2011. The size distributions of OC and EC presented large evolutions among rural and urban sites, and among four seasons, with highest peaks of OC and EC in fine mode in urban areas during winter. Geometric mean diameters (GMDs) of OC and EC in fine particles at urban sites during winter were lower than those at rural site mainly due to effects of fine particle coagulation and organic compound repartitioning. Fossil fuel emissions were a dominant source of OC and EC in urban areas, while biomass burning was a major source of OC and EC at rural site. Trajectory clustering and CWT analysis showed that regional transport was an important contributor to OC and EC in Beijing.

Interfacial Tensions of Aged Organic Aerosol Particle Mimics Using a Biphasic Microfluidic Platform

Secondary organic aerosol (SOA) particles are a major component of atmospheric particulate matter, yet their formation processes and ambient properties are not well understood. These complex particles often contain multiple interfaces due to internal aqueous- and organic-phase partitioning. Aerosol interfaces can profoundly affect the fate of condensable organic compounds emitted into the atmosphere by altering the way in which ambient organic vapors interact with suspended particles. To accurately predict the evolution of SOA in the atmosphere, we must improve our understanding of aerosol interfaces. In this work, biphasic microscale flows are used to measure interfacial tension of reacting methylglyoxal, formaldehyde, and ammonium sulfate aqueous mixtures with a surrounding oil phase. Our experiments show a suppression of interfacial tension as a function of organic content that remains constant with reaction time for methylglyoxal-ammonium sulfate systems. We also reveal an unexpected time dependence of interfacial tension over a period of 48 h for ternary solutions of both methylglyoxal and formaldehyde in aqueous ammonium sulfate, indicating a more complicated behavior of surface activity where there is competition among dissolved organics. From these interfacial tension measurements, the morphology of aged atmospheric aerosols with internal liquid-liquid phase separation is inferred.

Carbonaceous aerosol composition over northern China in spring 2012

The organic carbon (OC) and elemental carbon (EC) collected by eight-stage air samplers over northern China during spring 2012 were determined to characterize the spatial variations, size distributions, and sources of carbonaceous aerosols. OC and EC had high concentration levels and spatial heterogeneity. Higher carbonaceous aerosol loadings were found in urban areas, and high concentrations of OC and EC were found in eastern parts of northern China, including Beijing, Taiyuan in Shanxi Province, Yucheng in Shandong Province, Xianghe in Hebei Province, and Shenyang in Liaoning Province. Except the Cele site, OC and EC at all the sites showed a bimodal distribution, peaking in the size of 0.4-0.7 and 4.7-5.8 μm. Carbonaceous aerosols in the fine mode in the urban areas are mostly presented in smaller sizes than those in the rural/regional background areas. For most sites, mass median aerodynamic diameter (MMAD) values in the fine particles for OC were higher than those for EC with the addition of semi-volatile organics. Good correlations between OC and EC in all the cities (5 in North China and 1 in northeast China) may suggest the impact of anthropogenic emissions on carbonaceous aerosols in the above regions.

Aqueous organic chemistry in the atmosphere: sources and chemical processing of organic aerosols

Over the past decade, it has become clear that aqueous chemical processes occurring in cloud droplets and wet atmospheric particles are an important source of organic atmospheric particulate matter. Reactions of water-soluble volatile (or semivolatile) organic gases (VOCs or SVOCs) in these aqueous media lead to the formation of highly oxidized organic particulate matter (secondary organic aerosol; SOA) and key tracer species, such as organosulfates. These processes are often driven by a combination of anthropogenic and biogenic emissions, and therefore their accurate representation in models is important for effective air quality management. Despite considerable progress, mechanistic understanding of some key aqueous processes is still lacking, and these pathways are incompletely represented in 3D atmospheric chemistry and air quality models. In this article, the concepts, historical context, and current state of the science of aqueous pathways of SOA formation are discussed.

Complex chemical composition of colored surface films formed from reactions of propanal in sulfuric acid at upper troposphere/lower stratosphere aerosol acidities

Particles in the upper troposphere and lower stratosphere (UT/LS) consist mostly of concentrated sulfuric acid (40-80 wt %) in water. However, airborne measurements have shown that these particles also contain a significant fraction of organic compounds of unknown chemical composition. Acid-catalyzed reactions of carbonyl species are believed to be responsible for significant transfer of gas phase organic species into tropospheric aerosols and are potentially more important at the high acidities characteristic of UT/LS particles. In this study, experiments combining sulfuric acid (H₂SO₄) with propanal and with mixtures of propanal with glyoxal and/or methylglyoxal at acidities typical of UT/LS aerosols produced highly colored surface films (and solutions) that may have implications for aerosol properties. In order to identify the chemical processes responsible for the formation of the surface films, attenuated total reflectance-Fourier transform infrared (ATR-FTIR) and ¹H nuclear magnetic resonance (NMR) spectroscopies were used to analyze the chemical composition of the films. Films formed from propanal were a complex mixture of aldol condensation products, acetals and propanal itself. The major aldol condensation products were the dimer (2-methyl-2-pentenal) and 1,3,5-trimethylbenzene that was formed by cyclization of the linear aldol condensation trimer. Additionally, the strong visible absorption of the films indicates that higher-order aldol condensation products must also be present as minor species. The major acetal species were 2,4,6-triethyl-1,3,5-trioxane and longer-chain linear polyacetals which are likely to separate from the aqueous phase. Films formed on mixtures of propanal with glyoxal and/or methylglyoxal also showed evidence of products of cross-reactions. Since cross-reactions would be more likely than self-reactions under atmospheric conditions, similar reactions of aldehydes like propanal with common aerosol organic species like glyoxal and methylglyoxal have the potential to produce significant organic aerosol mass and therefore could potentially impact chemical, optical and/or cloud-forming properties of aerosols, especially if the products partition to the aerosol surface.

Heterogeneous chemistry of glyoxal on acidic solutions. An oligomerization pathway for secondary organic aerosol formation

The heterogeneous chemistry of glyoxal on sulfuric acid surfaces has been investigated at various acid concentrations and temperatures, utilizing a low-pressure fast flow laminar reactor coupled to an ion drift-chemical ionization mass spectrometer (ID-CIMS). The uptake coefficient (γ) of glyoxal ranges from (1.2 ± 0.06) × 10(-2) to (2.5 ± 0.01) × 10(-3) for 60-93 wt % H2SO4 at 253-273 K. The effective Henry's Law constant (H*) ranges from (98.9 ± 4.9) × 10(5) to (1.6 ± 0.1) × 10(5) M atm(-1) for 60-93 wt % at 263-273 K. Both the uptake coefficient and Henry's Law constant increase with decreasing acid concentration and temperature. Our results reveal a reaction mechanism of hydration followed by oligomerization for glyoxal on acidic media, indicating an efficient aqueous reaction of glyoxal on hygroscopic particles leading to secondary organic aerosol formation.

Model analysis of secondary organic aerosol formation by glyoxal in laboratory studies: the case for photoenhanced chemistry

The reactive uptake of glyoxal by atmospheric aerosols is believed to be a significant source of secondary organic aerosol (SOA). Several recent laboratory studies have been performed with the goal of characterizing this process, but questions remain regarding the effects of photochemistry on SOA growth. We applied GAMMA (McNeill et al. Environ. Sci. Technol. 2012, 46, 8075-8081), a photochemical box model with coupled gas-phase and detailed aqueous aerosol-phase chemistry, to simulate aerosol chamber studies of SOA formation by the uptake of glyoxal by wet aerosol under dark and irradiated conditions (Kroll et al. J. Geophys. Res. 2005, 110 (D23), 1-10; Volkamer et al. Atmos. Chem. Phys. 2009, 9, 1907-1928; Galloway et al. Atmos. Chem. Phys. 2009, 9, 3331- 306 3345 and Geophys. Res. Lett. 2011, 38, L17811). We find close agreement between simulated SOA growth and the results of experiments conducted under dark conditions using values of the effective Henry's Law constant of 1.3-5.5 × 10(7) M atm(-1). While irradiated conditions led to the production of some organic acids, organosulfates, and other oxidation products via well-established photochemical mechanisms, these additional product species contribute negligible aerosol mass compared to the dark uptake of glyoxal. Simulated results for irradiated experiments therefore fell short of the reported SOA mass yield by up to 92%. This suggests a significant light-dependent SOA formation mechanism that is not currently accounted for by known bulk photochemistry, consistent with recent laboratory observations of SOA production via photosensitizer chemistry.

Light-absorbing oligomer formation in secondary organic aerosol from reactive uptake of isoprene epoxydiols

Secondary organic aerosol (SOA) produced from reactive uptake and multiphase chemistry of isoprene epoxydiols (IEPOX) has been found to contribute substantially (upward of 33%) to the fine organic aerosol mass over the Southeastern U.S. Brown carbon (BrC) in rural areas of this region has been linked to secondary sources in the summer when the influence of biomass burning is low. We demonstrate the formation of light-absorbing (290 < λ < 700 nm) SOA constituents from reactive uptake of trans-β-IEPOX onto preexisting sulfate aerosols as a potential source of secondary BrC. IEPOX-derived BrC generated in controlled chamber experiments under dry, acidic conditions has an average mass absorption coefficient of ∼ 300 cm(2) g(-1). Chemical analyses of SOA constituents using UV-visible spectroscopy and high-resolution mass spectrometry indicate the presence of highly unsaturated oligomeric species with molecular weights separated by mass units of 100 (C5H8O2) and 82 (C5H6O) coincident with the observations of enhanced light absorption, suggesting such oligomers as chromophores, and potentially explaining one source of humic-like substances (HULIS) ubiquitously present in atmospheric aerosol. Similar light-absorbing oligomers were identified in fine aerosol collected in the rural Southeastern U.S., supporting their atmospheric relevance and revealing a previously unrecognized source of oligomers derived from isoprene that contributes to ambient fine aerosol mass.

Organic matrix effects on the formation of light-absorbing compounds from α-dicarbonyls in aqueous salt solution

Aqueous-phase reactions of organic compounds are of general importance in environmental systems. Reactions of α-dicarbonyl compounds in the aqueous phase of atmospheric aerosols can impact their climate-relevant physical properties including hygroscopicity and absorption of light. Less-reactive water-soluble organic compounds may contribute an organic matrix component to the aqueous environment, potentially impacting the reaction kinetics. In this work we demonstrate the effects of organic matrices on the self-reactions of glyoxal (Gly) and methylglyoxal (mGly) in aqueous solutions containing ammonium sulfate. At an organic-to-sulfate mass ratio of 2 : 1, carbohydrate-like matrices resembling oxidized organic aerosol material reduce the rate of formation of light-absorbing products by up to an order of magnitude. The greatest decreases in the reaction rates were observed for organic matrices with smaller, more linear molecular structures. Initial UV-Vis spectra, product studies, relative rate data, acidity changes, and viscosity measurements suggest that shifts in carbonyl equilibria, due in part to (hemi)acetal formation with the matrix, reduce the rate of formation of light-absorbing imidazole and oligomer species.

Formation of semisolid, oligomerized aqueous SOA: lab simulations of cloud processing

Glyoxal, methylglyoxal, glycolaldehyde, and hydroxyacetone form N-containing and oligomeric compounds during simulated cloud processing with small amines. Using a novel hygroscopicity tandem differential mobility analysis (HTDMA) system that allows varied humidification times, the hygroscopic growth (HG) of each of the resulting products of simulated cloud processing was measured. Continuous water uptake (gradual deliquescence) was observed beginning at ∼ 40% RH for all aldehyde-methylamine products. Particles containing ionic reaction products of either glyoxal or glycine were most hygroscopic, with HG between 1.16 and 1.20 at 80% RH. Longer humidification times (up to 20 min) produced an increase in growth factors for glyoxal-methylamine (19% by vol) and methylglyoxal-methylamine (8% by vol) aerosol, indicating that unusually long equilibration times can be required for HTDMA measurements of such particles. Glyoxal- and methylglyoxal-methylamine aerosol particles shattered in Raman microscopy impact-flow experiments, revealing that the particles were semisolid. Similar experiments on glycolaldehyde- and hydroxyacetone-methylamine aerosol found that the aerosol particles were liquid when dried for <1 h, but semisolid when dried for 20 h under ambient conditions. The RH required for flow (liquification) during humidification experiments followed the order methylglyoxal > glyoxal > glycolaldehyde = hydroxyacetone, likely caused by the speed of oligomer formation in each system.

Formation of light absorbing organo-nitrogen species from evaporation of droplets containing glyoxal and ammonium sulfate

In the atmosphere, volatile organic compounds such as glyoxal can partition into aqueous droplets containing significant levels of inorganic salts. Upon droplet evaporation, both the organics and inorganic ions become highly concentrated, accelerating reactions between them. To demonstrate this process, we investigated the formation of organo-nitrogen and light absorbing materials in evaporating droplets containing glyoxal and different ammonium salts including (NH4)2SO4, NH4NO3, and NH4Cl. Our results demonstrate that evaporating glyoxal-(NH4)2SO4 droplets produce light absorbing species on a time scale of seconds, which is orders of magnitude faster than observed in bulk solutions. Using aerosol mass spectrometry, we show that particle-phase organics with high N:C ratios were formed when ammonium salts were used, and that the presence of sulfate ions promoted this chemistry. Since sulfate can also significantly enhance the Henry's law partitioning of glyoxal, our results highlight the atmospheric importance of such inorganic-organic interactions in aqueous phase aerosol chemistry.

Effective Henry's law partitioning and the salting constant of glyoxal in aerosols containing sulfate

The reversible partitioning of glyoxal was studied in simulation chamber experiments for the first time by time-resolved measurements of gas-phase and particle-phase concentrations in sulfate-containing aerosols. Two complementary methods for the measurement of glyoxal particle-phase concentrations are compared: (1) an offline method utilizing filter sampling of chamber aerosols followed by HPLC-MS/MS analysis and (2) positive matrix factorization (PMF) analysis of aerosol mass spectrometer (AMS) data. Ammonium sulfate (AS) and internally mixed ammonium sulfate/fulvic acid (AS/FA) seed aerosols both show an exponential increase of effective Henry's law coefficients (KH,eff) with AS concentration (cAS, in mol kg(-1) aerosol liquid water, m = molality) and sulfate ionic strength, I(SO4(2-)) (m). A modified Setschenow plot confirmed that "salting-in" of glyoxal is responsible for the increased partitioning. The salting constant for glyoxal in AS is K(S)CHOCHO = (-0.24 ± 0.02) m(-1), and found to be independent of the presence of FA. The reversible glyoxal uptake can be described by two distinct reservoirs for monomers and higher molecular weight species filling up at characteristic time constants. These time constants are τ1 ≈ 10(2) s and τ2 ≈ 10(4) s at cAS < 12 m, and about 1-2 orders of magnitude slower at higher cAS, suggesting that glyoxal uptake is kinetically limited at high salt concentrations.

Surfactants from the gas phase may promote cloud droplet formation

Clouds, a key component of the climate system, form when water vapor condenses upon atmospheric particulates termed cloud condensation nuclei (CCN). Variations in CCN concentrations can profoundly impact cloud properties, with important effects on local and global climate. Organic matter constitutes a significant fraction of tropospheric aerosol mass, and can influence CCN activity by depressing surface tension, contributing solute, and influencing droplet activation kinetics by forming a barrier to water uptake. We present direct evidence that two ubiquitous atmospheric trace gases, methylglyoxal (MG) and acetaldehyde, known to be surface-active, can enhance aerosol CCN activity upon uptake. This effect is demonstrated by exposing acidified ammonium sulfate particles to 250 parts per billion (ppb) or 8 ppb gas-phase MG and/or acetaldehyde in an aerosol reaction chamber for up to 5 h. For the more atmospherically relevant experiments, i.e., the 8-ppb organic precursor concentrations, significant enhancements in CCN activity, up to 7.5% reduction in critical dry diameter for activation, are observed over a timescale of hours, without any detectable limitation in activation kinetics. This reduction in critical diameter enhances the apparent particle hygroscopicity up to 26%, which for ambient aerosol would lead to cloud droplet number concentration increases of 8-10% on average. The observed enhancements exceed what would be expected based on Köhler theory and bulk properties. Therefore, the effect may be attributed to the adsorption of MG and acetaldehyde to the gas-aerosol interface, leading to surface tension depression of the aerosol. We conclude that gas-phase surfactants may enhance CCN activity in the atmosphere.

Aqueous-phase secondary organic aerosol and organosulfate formation in atmospheric aerosols: a modeling study

We have examined aqueous-phase secondary organic aerosol (SOA) and organosulfate (OS) formation in atmospheric aerosols using a photochemical box model with coupled gas-phase chemistry and detailed aqueous aerosol chemistry. SOA formation in deliquesced ammonium sulfate aerosol is highest under low-NO(x) conditions, with acidic aerosol (pH = 1) and low ambient relative humidity (40%). Under these conditions, with an initial sulfate loading of 4.0 μg m(-3), 0.9 μg m(-3) SOA is predicted after 12 h. Low-NO(x) aqueous-aerosol SOA (aaSOA) and OS formation is dominated by isoprene-derived epoxydiol (IEPOX) pathways; 69% or more of aaSOA is composed of IEPOX, 2-methyltetrol, and 2-methyltetrol sulfate ester. 2-Methyltetrol sulfate ester comprises >99% of OS mass (66 ng m(-3) at 40% RH and pH 1). In urban (high-NO(x)) environments, aaSOA is primarily formed via reversible glyoxal uptake, with 0.12 μg m(-3) formed after 12 h at 80% RH, with 20 μg m(-3) initial sulfate. OS formation under all conditions studied is maximum at low pH and lower relative humidities (<60% RH), i.e., when the aerosol is more concentrated. Therefore, OS species are expected to be good tracer compounds for aqueous aerosol-phase chemistry (vs cloudwater processing).

Organic constituents on the surfaces of aerosol particles from Southern Finland, Amazonia, and California studied by vibrational sum frequency generation

This article summarizes and compares the analysis of the surfaces of natural aerosol particles from three different forest environments by vibrational sum frequency generation. The experiments were carried out directly on filter and impactor substrates, without the need for sample preconcentration, manipulation, or destruction. We discuss the important first steps leading to secondary organic aerosol (SOA) particle nucleation and growth from terpene oxidation by showing that, as viewed by coherent vibrational spectroscopy, the chemical composition of the surface region of aerosol particles having sizes of 1 μm and lower appears to be close to size-invariant. We also discuss the concept of molecular chirality as a chemical marker that could be useful for quantifying how chemical constituents in the SOA gas phase and the SOA particle phase are related in time. Finally, we describe how the combination of multiple disciplines, such as aerosol science, advanced vibrational spectroscopy, meteorology, and chemistry can be highly informative when studying particles collected during atmospheric chemistry field campaigns, such as those carried out during HUMPPA-COPEC-2010, AMAZE-08, or BEARPEX-2009, and when they are compared to results from synthetic model systems such as particles from the Harvard Environmental Chamber (HEC). Discussions regarding the future of SOA chemical analysis approaches are given in the context of providing a path toward detailed spectroscopic assignments of SOA particle precursors and constituents and to fast-forward, in terms of mechanistic studies, through the SOA particle formation process.

Investigation of aqueous-phase photooxidation of glyoxal and methylglyoxal by aerosol chemical ionization mass spectrometry: observation of hydroxyhydroperoxide formation

Aqueous-phase processing of glyoxal (GLY) and methylglyoxal (MG) produces highly oxygenated, less volatile organic acids that can contribute to SOA formation and aging. In this study, aerosol chemical ionization mass spectrometry (aerosol CIMS) is employed to monitor aqueous-phase photooxidation of GLY and MG. Using iodide (I(-)) as the reagent ion, aerosol CIMS can simultaneously detect important species involved in the reactions: organic acids, peroxides, and aldehydes, so that the reconstructed total organic carbon (TOC) concentrations from aerosol CIMS data agree well with offline TOC analysis. This study also reports the first direct detection of hydroxyhydroperoxide (HHP) formation from the reaction of H(2)O(2) with GLY or MG. The formation of HHPs is observed to be reversible and an estimate of their equilibrium constants is made to be between 40 and 200 M(-1). Results of this study suggest that HHPs can form additional formic acid and acetic acid via photooxidation and regenerate GLY or MG during photooxidation, compensating their loss. HHP formation needs to be further studied for inclusion in aqueous-phase chemical models given that it may affect the aqueous partitioning of carbonyls in the atmosphere.