David O. De Haan

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. Measured elemental ratios are consistent with imidazoles and oligomers being major reaction products, while effective aerosol densities suggest extensive reactions take place within minutes. These reactions may be a source of the light-absorbing, nitrogen-containing oligomers observed in urban and biomass-burning aerosol particles.

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. Measurements of aerosol density and time-dependent AMS signal changes are consistent with the formation of oligomers by each compound during the drying process. Unlike glyoxal, which forms acetal oligomers, exact mass AMS data indicates that the majority of methylglyoxal oligomers are formed by aldol condensation reactions, likely catalyzed by pyruvic acid, formed from methylglyoxal disproportionation. Our measurements of evaporation fractions can be used to estimate the global aerosol formation potential of glyoxal and methylglyoxal via self-reactions at 1 and 1.6 Tg C yr(-1), respectively. This is a factor of 4 less than the SOA formed by these compounds if their uptake is assumed to be irreversible. However, these estimates are likely lower limits for their total aerosol formation potential because oxidants and amines will also react with glyoxal and methylglyoxal to form additional low-volatility products.

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. These results differ from those of glyoxal, which, lacking the methyl group, cannot access the enol structure leading to aldol condensation. For methylglyoxal, the product from nucleophilic attack at the aldehyde rather than the ketone was favored. Our results help explain some of the observed differences between methylglyoxal and glyoxal, in particular the different array of oligomers formed.

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 0.25 M, and UV-vis and fluorescence spectroscopy monitoring. Glycolaldehyde-glycine mixtures produced the most intense absorbance. In carbonyl compound reactions with AS, methylamine, or AS/glycine mixtures, product absorbance followed the order methylglyoxal > glyoxal > glycolaldehyde > hydroxyacetone. Absorbance extended into the visible, with a wavelength dependence fit by absorption Ångstrom coefficients (Å(abs)) of 2 to 11, overlapping the Å(abs) range of atmospheric, water-soluble brown carbon. Many reaction products absorbing between 300 and 400 nm were strongly fluorescent. On a per mole basis, amines are much more effective than AS at producing brown carbon. In addition, methylglyoxal and glyoxal produced more light-absorbing products in reactions with a 5:1 AS-glycine mixture than with AS or glycine alone, illustrating the importance of both organic and inorganic nitrogen in brown carbon formation. Through comparison to biomass burning aerosol, we place an upper limit on the contribution of these aqueous carbonyl-AS-amine reactions of ≤ 10% of global light absorption by brown carbon.

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. In the present study, the complex refractive indices of model secondary organic aerosol formed by aqueous phase reactions between the α-dicarbonyls glyoxal and methylglyoxal and the primary amines glycine and methylamine have been determined. The reaction products exhibit significant absorption in the visible, and refractive indices are similar to those for light absorbing species isolated from urban aerosol. However, the optical properties are different from the values used in models for secondary organic aerosol, which typically assume little to no absorption of visible light. As a result, the climatic cooling effect of such aerosols in models may be overestimated.

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. We find these particles are in a semisolid or glassy state based upon their behavior when exposed to mechanical pressure as well as their flow behavior. Further, we find that these aqSOA particles are poor depositional ice nuclei, in contrast to previous studies on simple mixtures of glassy organics. Additionally, we have studied the effect of ammonium sulfate on the phase, morphology, and ice nucleation behavior of the aqSOA. We find that the plasticizing effect of ammonium sulfate lowers the viscosity of the aqSOA, allowing the ammonium sulfate to effloresce within the aqSOA matrix. Upon humidification, the aqSOA matrix liquefies before it can depositionally nucleate ice, and the effloresced ammonium sulfate can act as an immersion mode ice nucleus. This change in the mode of nucleation is accompanied by an increase in the overall ice nucleation efficiency of the aqSOA particles.

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. Our results confirm that formaldehyde and, by extension, other small aldehydes are likely to be incorporated into imidazole ions in the presence of glyoxal and primary amines in clouds and aqueous aerosol. This is a new mechanism of aerosol formation by formaldehyde, the most abundant aldehyde in the atmosphere. The amount of aerosol formed will depend on the amounts of glyoxal and amines present.

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. The evaporation of water from a droplet was found to trigger aldehyde reactions that increased residual particle volumes by a similar extent in room-temperature experiments, regardless of whether AS, methylamine, or glycine was present. The production of organic aerosol volume was highest from droplets containing glyoxal, followed by similar production from methylglyoxal or hydroxyacetone. Significant organic aerosol production was observed for glycolaldehyde, acetaldehyde, and formaldehyde only at elevated temperatures in thermogravimetric experiments. In many experiments, the amount of aerosol produced was greater than the sum of all solutes plus nonvolatile solvent impurities, indicating the additional presence of trapped water, likely caused by increasing aerosol-phase viscosity due to oligomer formation.

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. There is also relatively good agreement between the calculated activation barriers and the estimated rate constants for the hydration reaction. The computational approach also predicted that two of the trimers would have a small but appreciable equilibrium concentration (>0.005 M), and this was confirmed by NMR measurements. Our results suggest that while our computational protocol is reasonable and may be applied to quickly map the energy landscape of more complex reactions, knowledge of the caveats and potential errors in this approach need to be taken into account.

Heterogeneous Glyoxal Oxidation: A Potential Source of Secondary Organic Aerosol

Laboratory studies are described that suggest reactive uptake of glyoxal on particulate containing HNO(3) could contribute to the formation of secondary organic aerosol (SOA) in the upper troposphere (UT). Using a Knudsen cell flow reactor, glyoxal is observed to react on supercooled H(2)O/HNO(3) surfaces to form condensed-phase glyoxylic acid. This product was verified by derivatization and GC-MS analysis. The reactive uptake coefficient, γ, of glyoxal varies only slightly with the pressure of nitric acid, from γ = 0.5 to 3.0 × 10(-3) for nitric acid pressures between 10(-8) and 10(-6) Torr. The data do not show any dependence on temperature (181-201 K) or pressure of glyoxal (10(-7) to 10(-5) Torr). Using the determined reactive uptake kinetics in a simple model shows that glyoxal uptake to supercooled H(2)O/HNO(3) may account for 4-53% of the total organic mass fraction of aerosol in the UT.