Correction to Brown Carbon from Photo-Oxidation of Glyoxal and SO in Aqueous Aerosol.

[This corrects the article DOI: 10.1021/acsearthspacechem.3c00035.

Brown Carbon from Photo-Oxidation of Glyoxal and SO in Aqueous Aerosol.

Aqueous-phase dark reactions during the co-oxidation of glyoxal and S(IV) were recently identified as a potential source of brown carbon (BrC). Here, we explore the effects of sunlight and oxidants on aqueous solutions of glyoxal and S(IV), and on aqueous aerosol exposed to glyoxal and SO. We find that BrC is able to form in sunlit, bulk-phase, sulfite-containing solutions, albeit more slowly than in the dark.

Identifying a sublingual triangle as the ideal site for assessment of sublingual microcirculation.

The sublingual mucosa is a commonly used intraoral location for identifying microcirculatory alterations using handheld vital microscopes (HVMs). The anatomic description of the sublingual cave and its related training have not been adequately introduced. The aim of this study was to introduce anatomy guided sublingual microcirculatory assessment.

Kinetics, Products, and Brown Carbon Formation by Aqueous-Phase Reactions of Glycolaldehyde with Atmospheric Amines and Ammonium Sulfate.

Glycolaldehyde (GAld) is a C water-soluble aldehyde produced during the atmospheric oxidation of isoprene and many other species and is commonly found in cloudwater. Previous work has established that glycolaldehyde evaporates more readily from drying aerosol droplets containing ammonium sulfate (AS) than does glyoxal, methylglyoxal, or hydroxyacetone, which implies that it does not oligomerize as quickly as these other species. Here, we report NMR measurements of glycolaldehyde's aqueous-phase reactions with AS, methylamine, and glycine.

Elucidating an Atmospheric Brown Carbon Species-Toward Supplanting Chemical Intuition with Exhaustive Enumeration and Machine Learning.

Brown carbon (BrC) is involved in atmospheric light absorption and climate forcing and can cause adverse health effects. Understanding the formation mechanisms and molecular structure of BrC is of key importance in developing strategies to control its environment and health impact. Structure determination of BrC is challenging, due to the lack of experiments providing molecular fingerprints and the sheer number of molecular candidates with identical mass.

Brown Carbon Production by Aqueous-Phase Interactions of Glyoxal and SO.

Oxalic acid and sulfate salts are major components of aerosol particles. Here, we explore the potential for their respective precursor species, glyoxal and SO, to form atmospheric brown carbon via aqueous-phase reactions in a series of bulk aqueous and flow chamber aerosol experiments. In bulk aqueous solutions, UV- and visible-light-absorbing products are observed at pH 3-4 and 5-6, respectively, with small but detectable yields of hydroxyquinone and polyketone products formed, especially at pH 6.

Methylglyoxal Uptake Coefficients on Aqueous Aerosol Surfaces.

In order to predict the amount of secondary organic aerosol formed by heterogeneous processing of methylglyoxal, uptake coefficients (γ) and estimates of uptake reversibility are needed. Here, uptake coefficients are extracted from chamber studies involving ammonium sulfate and glycine seed aerosol at high relative humidity (RH ≥ 72%). Methylglyoxal uptake coefficients on prereacted glycine aerosol particles had a strong dependence on RH, increasing from γ = 0.

Nitrogen-Containing, Light-Absorbing Oligomers Produced in Aerosol Particles Exposed to Methylglyoxal, Photolysis, and Cloud Cycling.

Aqueous methylglyoxal chemistry has often been implicated as an important source of oligomers in atmospheric aerosol. Here we report on chemical analysis of brown carbon aerosol particles collected from cloud cycling/photolysis chamber experiments, where gaseous methylglyoxal and methylamine interacted with glycine, ammonium, or methylammonium sulfate seed particles. Eighteen N-containing oligomers were identified in the particulate phase by liquid chromatography/diode array detection/electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry.

Brown Carbon Production in Ammonium- or Amine-Containing Aerosol Particles by Reactive Uptake of Methylglyoxal and Photolytic Cloud Cycling.

The effects of methylglyoxal uptake on the physical and optical properties of aerosol containing amines or ammonium sulfate were determined before and after cloud processing in a temperature- and RH-controlled chamber. The formation of brown carbon was observed upon methylglyoxal addition, detected as an increase in water-soluble organic carbon mass absorption coefficients below 370 nm and as a drop in single-scattering albedo at 450 nm. The imaginary refractive index component k reached a maximum value of 0.

Maillard Chemistry in Clouds and Aqueous Aerosol As a Source of Atmospheric Humic-Like Substances.

The reported optical, physical, and chemical properties of aqueous Maillard reaction mixtures of small aldehydes (glyoxal, methylglyoxal, and glycolaldehyde) with ammonium sulfate and amines are compared with those of aqueous extracts of ambient aerosol (water-soluble organic carbon, WSOC) and the humic-like substances (HULIS) fraction of WSOC. Using a combination of new and previously published measurements, we examine fluorescence, X-ray absorbance, UV/vis, and IR spectra, complex refractive indices, (1)H and (13)C NMR spectra, thermograms, aerosol and electrospray ionization mass spectra, surface activity, and hygroscopicity. Atmospheric WSOC and HULIS encompass a range of properties, but in almost every case aqueous aldehyde-amine reaction mixtures are squarely within this range.

Free energy map for the co-oligomerization of formaldehyde and ammonia.

Density functional theory calculations, including Poisson-Boltzmann implicit solvent and free energy corrections, are applied to construct a free energy map of formaldehyde and ammonia co-oligomerization in aqueous solution at pH 7. The stepwise route to forming hexamethylenetetramine (HMTA), the one clearly identified major product in a complex mixture, involves a series of addition reactions of formaldehyde and ammonia coupled with dehydration and cyclization reactions at key steps in the pathway. The free energy map also allows us to propose the possible identity of some major peaks observed by mass spectroscopy in the reaction mixture being the result of stable species not along the pathway to HMTA, in particular those formed by intramolecular condensation of hydroxyl groups to form six-membered rings with ether linkages.

Secondary organic aerosol formation during evaporation of droplets containing atmospheric aldehydes, amines, and ammonium sulfate.

Reactions of carbonyl compounds in cloudwater produce organic aerosol mass through in-cloud oxidation and during postcloud evaporation. In this work, postcloud evaporation was simulated in laboratory experiments on evaporating droplets that contain mixtures of common atmospheric aldehydes with ammonium sulfate (AS), methylamine, or glycine. Aerosol diameters were measured during monodisperse droplet drying experiments and during polydisperse droplet equilibration experiments at 75% relative humidity, and condensed-phase mass was measured in bulk thermogravimetric experiments.

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.

Heterogeneous ice nucleation on simulated secondary organic aerosol.

In this study, we have explored the phase behavior and the ice nucleation properties of secondary organic aerosol made from aqueous processing (aqSOA). AqSOA was made from the dark reactions of methylglyoxal with methylamine in simulated evaporated cloud droplets. The resulting particles were probed from 215 to 250 K using Raman spectroscopy coupled to an environmental cell.

Brown carbon formation by aqueous-phase carbonyl compound reactions with amines and ammonium sulfate.

Reactions between small water-soluble carbonyl compounds, ammonium sulfate (AS), and/or amines were evaluated for their ability to form light-absorbing species in aqueous aerosol. Aerosol chemistry was simulated with bulk phase reactions at pH 4, 275 K, initial concentrations of 0.05 to 0.

Glycolaldehyde monomer and oligomer equilibria in aqueous solution: comparing computational chemistry and NMR data.

A computational protocol utilizing density functional theory calculations, including Poisson-Boltzmann implicit solvent and free energy corrections, is applied to study the thermodynamic and kinetic energy landscape of glycolaldehyde in solution. Comparison is made to NMR measurements of dissolved glycolaldehyde, where the initial dimeric ring structure interconverts among several species before reaching equilibrium where the hydrated monomer is dominant. There is good agreement between computation and experiment for the concentrations of all species in solution at equilibrium, that is, the calculated relative free energies represent the system well.

Optical properties of the products of α-dicarbonyl and amine reactions in simulated cloud droplets.

Secondary organic aerosol makes up a significant fraction of the total aerosol mass, and a growing body of evidence indicates that reactions in the atmospheric aqueous phase are important contributors to aerosol formation and can help explain observations that cannot be accounted for using traditional gas-phase chemistry. In particular, aqueous phase reactions between small organic molecules have been proposed as a source of light absorbing compounds that have been observed in numerous locations. Past work has established that reactions between α-dicarbonyls and amines in evaporating water droplets produces particle-phase products that are brown in color.

Thermodynamics and kinetics of imidazole formation from glyoxal, methylamine, and formaldehyde: a computational study.

Density functional theory calculations, including Poisson-Boltzmann implicit solvent and free energy corrections, are applied to study the mechanism of experimentally observed imidazole formation from the reaction of glyoxal and methylamine in solution. Our calculations suggest that a diimine species is an important intermediate in the reaction. Under acidic conditions, we find that the diimine acts as a nucleophile in attacking the carbonyl group of either formaldehyde or glyoxal to first generate an acyclic enol intermediate, which then goes on to close the ring and form imidazoles.

Formation of nitrogen-containing oligomers by methylglyoxal and amines in simulated evaporating cloud droplets.

Reactions of methylglyoxal with amino acids, methylamine, and ammonium sulfate can take place in aqueous aerosol and evaporating cloud droplets. These processes are simulated by drying droplets and bulk solutions of these compounds (at low millimolar and 1 M concentrations, respectively) and analyzing the residuals by scanning mobility particle sizing, nuclear magnetic resonance, aerosol mass spectrometry (AMS), and electrospray ionization MS. The results are consistent with imine (but not diimine) formation on a time scale of seconds, followed by the formation of nitrogen-containing oligomers, methylimidazole, and dimethylimidazole products on a time scale of minutes to hours.

Gas-phase water-mediated equilibrium between methylglyoxal and its geminal diol.

In aqueous solution, aldehydes, and to a lesser extent ketones, hydrate to form geminal diols. We investigate the hydration of methylglyoxal (MG) in the gas phase, a process not previously considered to occur in water-restricted environments. In this study, we spectroscopically identified methylglyoxal diol (MGD) and obtained the gas-phase partial pressures of MG and MGD.

Secondary organic aerosol formation by self-reactions of methylglyoxal and glyoxal in evaporating droplets.

Glyoxal and methylglyoxal are scavenged by clouds, where a fraction of these compounds are oxidized during the lifetime of the droplet. As a cloud droplet evaporates, the remaining glyoxal and methylglyoxal must either form low-volatility compounds such as oligomers and remain in the aerosol phase, or transfer back to the gas phase. A series of experiments on evaporating aqueous aerosol droplets indicates that over the atmospherically relevant concentration range for clouds and fog (4-1000 microM), 33 +/- 11% of glyoxal and 19 +/- 13% of methylglyoxal remains in the aerosol phase while the remainder evaporates.

Thermodynamics and kinetics of methylglyoxal dimer formation: a computational study.

Density functional theory (B3LYP//6-311+G*) calculations, including Poisson-Boltzmann implicit solvent and free energy corrections, are applied to study the hydration of methylglyoxal and the subsequent formation of dimeric species in solution. Our calculations show that, unlike glyoxal, fully hydrated species are not thermodynamically favored over their less hydrated counterparts, nor are dioxolane ring species the thermodynamic sink, which is in agreement with experimental data. Instead, we find that aldol condensations are the most favored oligomerization reactions for methylglyoxal.

Secondary organic aerosol-forming reactions of glyoxal with amino acids.

Glyoxal, the simplest and most abundant alpha-dicarbonyl compound in the atmosphere, is scavenged by clouds and aerosol, where it reacts with nucleophiles to form low-volatility products. Here we examine the reactions of glyoxal with five amino acids common in clouds. When glyoxal and glycine, serine, aspartic acid or ornithine are present at concentrations as low as 30/microM in evaporating aqueous droplets or bulk solutions, 1,3-disubstituted imidazoles are formed in irreversible second-order reactions detected by nuclear magnetic resonance (NMR), aerosol mass spectrometry (AMS) and electrospray ionization mass spectrometry (ESI-MS).

Uptake of glyoxal by organic and Inorganic aerosol.

The uptake of glyoxal by a variety of organic and inorganic aerosol types was examined in a Teflon chamber. Rapid glyoxal uptake was observed for all liquid-phase aerosols at all relative humidity levels tested (< 5 to 50% RH). Even for aerosol with known water content, Henry's Law cannot predict glyoxal uptake: H* > (3 +/- 1.

Oligomer formation in evaporating aqueous glyoxal and methyl glyoxal solutions.

Glyoxal and methyl glyoxal are common secondary atmospheric pollutants, formed from aromatic and terpene precursors. Both compounds are extremely water-soluble due to dihydrate formation and partition into cloudwater. In this work, FTIR-ATR and mass measurements indicate that both compounds remain primarily in the condensed phase due to oligomer formation when aqueous solution droplets are evaporated, regardless of concentration (> or = 1 mM) or, for glyoxal, droplet evaporation rate.