Measurements of the Hygroscopic and Deliquescence Properties of Organic Compounds of Different Solubilities in Water and Their Relationship with Cloud Condensation Nuclei Activities

The initial phase (solid or aqueous droplet) of aerosol particles prior to activation is among the critical factors in determining their cloud condensation nuclei (CCN) activity. Single-particle levitation in an electrodynamic balance (EDB) was used to measure the phase transitions and hygroscopic properties of aerosol particles of 11 organic compounds with different solubilities (10−1 to 102 g solute/100 g water). We use these data and other literature data to relate the CCN activity and hygroscopicity of organic compounds with different solubilities. The EDB data show that glyoxylic acid, 4-methylphthalic acid, monosaccharides (fructose and mannose), and disaccharides (maltose and lactose) did not crystallize and existed as metastable droplets at low relative humidity (RH). Hygroscopic data from this work and in the literature support earlier studies showing that the CCN activities of compounds with solubilities down to the order of 10−1 g solute/100 g water can be predicted by standard Köhler theory with the assumption of complete dissolution of the solute at activation. We also demonstrate the use of evaporation data (or efflorescence data), which provides information on the water contents of metastable solutions below the compound deliquescence RH that can be extrapolated to higher dilutions, to predict the CCN activity of organic particles, particularly for sparingly soluble organic compounds that do not deliquesce at RH achievable in the EDB and in the hygroscopic tandem differential mobility analyzer

Responses of Ammonium Sulfate Particles Coated with Glutaric Acid to Cyclic Changes in Relative Humidity: Hygroscopicity and Raman Characterization

Atmospheric particles, which may have an organic coating, exhibit cyclical phase changes of deliquescence and crystallization in response to changes in the ambient relative humidity (RH). Here, we measured the hygroscopicity and Raman spectra of solid ammonium sulfate ((NH4)2SO4) particles initially coated with water-soluble glutaric acid in two consecutive cycles of deliquescence and crystallization utilizing an electrodynamic balance. (NH4)2SO4 particles with glutaric acid coating (49 wt % glutaric acid) had different hygroscopicity and morphology in the two cycles. Once the particles deliquesced, the dissolution of the solid (NH4)2SO4 core and the glutaric acid coating formed mixed (NH4)2SO4−glutaric acid solution droplets, which was confirmed by Raman characterization. Coating studies with either deliquescence or crystallization measure ments, or one complete cycle of these two measurements may not fully assess the effects of the organic coatings on aerosol hygroscopicity. We also present an analysis on the kinetic and chemical effects of organic coating on aerosol hygroscopicity. Glutaric acid coating does not impede the evaporation and condensation rates of water molecules compared to the rates of (NH4)2SO4 particles in the two cycles. The coating likely affects the hygroscopicity of aerosol particles through dissolution and its chemical interactions with (NH4)2SO4

Effects of Relative Humidity and Particle Phase Water on the Heterogeneous OH Oxidation of 2‑Methylglutaric Acid Aqueous Droplets

Organic aerosols can exist as aqueous droplets, with variable water content depending on their composition and environmental conditions (e.g., relative humidity (RH)). Recent laboratory studies have revealed that oxidation kinetics in highly concentrated droplets can be much slower than those in dilute solutions. However, it remains unclear whether aerosol phase water affects the formation of reaction products physically and/or chemically. In this work, we investigate the role of aerosol phase water on the heterogeneous chemistry of aqueous organic droplets consisting of 2-methylglutaric acid (2-MGA), measuring the reaction kinetics and the reaction products upon heterogeneous OH oxidation over a range of RH. An atmospheric pressure soft ionization source (direct analysis in real time, DART) coupled with a high-resolution mass spectrometer is used to obtain real-time molecular information on the reaction products. Aerosol mass spectra show that the same reaction products are formed at all measured RH. At a given reaction extent of the parent 2-MGA, the aerosol composition is independent of RH. These results suggest the aerosol phase water does not alter reaction mechanisms significantly. Kinetic measurements find that the effective OH uptake coefficient, γ_eff, decreases with decreasing RH below 72%. Isotopic exchange measurements performed using aerosol optical tweezers reveal water diffusion coefficients in the 2-MGA droplets to be 3.0 × 10^–13 to 8.0 × 10^–13 m² s^–1 over the RH range of 47–58%. These values are comparable to those of other viscous organic aerosols (e.g., citric acid), indicating that 2-MGA droplets are likely to be viscous at low humidity. Smaller γ_eff at low RH is likely attributed to the slower diffusion of reactants within the droplets. Taken together, the observed relationship between the γ_eff and RH is likely attributed to changes in aerosol viscosity rather than changes in reaction mechanisms

Importance of Unimolecular HO₂ Elimination in the Heterogeneous OH Reaction of Highly Oxygenated Tartaric Acid Aerosol

Oxygenated organic molecules are abundant in atmospheric aerosols and are transformed by oxidation reactions near the aerosol surface by gas-phase oxidants such as hydroxyl (OH) radicals. To gain better insights into how the structure of an organic molecule, particularly in the presence of hydroxyl groups, controls the heterogeneous reaction mechanisms of oxygenated organic compounds, this study investigates the OH-radical initiated oxidation of aqueous tartaric acid (C₄H₆O₆) droplets using an aerosol flow tube reactor. The molecular composition of the aerosols before and after reaction is characterized by a soft atmospheric pressure ionization source (Direct Analysis in Real Time) coupled with a high-resolution mass spectrometer. The aerosol mass spectra reveal that four major reaction products are formed: a single C₄ functionalization product (C₄H₄O₆) and three C₃ fragmentation products (C₃H₄O₄, C₃H₂O₄, and C₃H₂O₅). The C₄ functionalization product does not appear to originate from peroxy radical self-reactions but instead forms via an α-hydroxylperoxy radical produced by a hydrogen atom abstraction by OH at the tertiary carbon site. The proximity of a hydroxyl group to peroxy group enhances the unimolecular HO₂ elimination from the α-hydroxylperoxy intermediate. This alcohol-to-ketone conversion yields 2-hydroxy-3-oxosuccinic acid (C₄H₄O₆), the major reaction product. While in general, C–C bond scission reactions are expected to dominate the chemistry of organic compounds with high average carbon oxidation states (OS_C), our results show that molecular structure can play a larger role in the heterogeneous transformation of tartaric acid (OS_C = 1.5). These results are also compared with two structurally related dicarboxylic acids (succinic acid and 2,3-dimethylsuccinic acid) to elucidate how the identity and location of functional groups (methyl and hydroxyl groups) alter heterogeneous reaction mechanisms

Chemical Transformation of Methanesulfonic Acid and Sodium Methanesulfonate through Heterogeneous OH Oxidation

Methanesulfonic acid (CH₃SO₃H, MSA) is one of the major organosulfur acids formed from the photochemical oxidation of dimethyl sulfide (DMS) produced by oceanic phytoplankton. MSA can react with metal halides (e.g., sodium chloride) in ambient aerosols to form methanesulfonate salts (e.g., sodium methanesulfonate, CH₃SO₃Na). While the formation processes of MSA and its salts are reasonably well understood, their subsequent chemical transformations in the atmosphere are not fully resolved. MSA and its salts accumulate near the aerosol surface due to their surface activities, which make them available to heterogeneous oxidation at the gas–aerosol interface by oxidants such as hydroxyl (OH) radicals. In this work, the compositional changes of aerosol comprised of MSA and its sodium salt (CH₃SO₃Na) are measured following heterogeneous OH oxidation. An aerosol flow tube reactor is coupled with a soft atmospheric pressure ionization source (Direct Analysis in Real Time, DART) and a high-resolution mass spectrometer at a relative humidity (RH) of 90%. DART-aerosol mass spectra reveal that MSA and CH₃SO₃Na can be detected as methanesulfonate ion (CH₃SO₃^–) with minimal fragmentation in the negative ionization mode. Kinetic measurements show that OH oxidation with MSA and CH₃SO₃Na has an effective OH uptake coefficient of 0.45 ± 0.14 and 0.20 ± 0.06, respectively, revealing that MSA reacts with OH radical faster than its sodium salt. One possibility for the difference in reactivity of these two compounds is that CH₃SO₃Na is more hygroscopic than MSA. The increase in the coverage of water molecules at the surface of CH₃SO₃Na might reduce the reactive collision probability between CH₃SO₃^– and OH radicals, resulting in a smaller reaction rate. MSA and CH₃SO₃Na dissociate to form CH₃SO₃^–, which tends to fragment into formaldehyde (HCHO) and a sulfite radical (SO₃^•–) upon oxidation. Formaldehyde partitions back to the gas phase owing to its high volatility, and SO₃^•– can initiate a series of chain reactions involving various inorganic sulfur radicals and ions in the aerosol phase. Overall, the fragmentation and SO₃^•–-initiated chemistry are the major processes controlling the chemical evolution of MSA and its sodium salt aerosols during heterogeneous OH oxidation

Evidence for High Molecular Weight Nitrogen-Containing Organic Salts in Urban Aerosols

High molecular weight (Mw) species were observed at substantial intensities in the positive-ion mass spectra in urban Shanghai aerosols collected from a single-particle time-of-flight mass spectrometer (in the m/z range 250−500) during three separate periods over 2007−2009. These species correlate well with the CN− mass signal, suggesting that C−N bonds are prevalent and that the observed high-Mw species are potentially nitrogen-containing organic salts. Anti-correlation with the ambient O3 concentration suggests that photochemical oxidants are not involved directly in the formation of these species. The Mannich reaction, among amines (or ammonia), formaldehyde, and carbonyls with an adjacent, acidic proton, is proposed as a plausible pathway leading to these organic salts. Although the high-Mw species observed in the single-particle mass spectra appear to be nitrogen-containing organics, further chemical confirmation is desired to verify if the proposed Mannich reaction can explain the formation of these high-Mw species in regions where ammonia, amines, and carbonyls are prevalent

Significant Conversion of Organic Sulfur from Hydroxymethanesulfonate to Inorganic Sulfate and Peroxydisulfate Ions upon Heterogeneous OH Oxidation

Hydroxymethanesulfonate (CH2(OH)­SO3–, HMS) has been found to be an important organosulfur compound in atmospheric aerosols. However, its atmospheric fate is largely uncertain. In this work, we investigated the heterogeneous OH oxidation of HMS using an oxidation flow reactor. In particular, we examined the rate and chemistry of the sulfur conversion from its organic form to its inorganic counterparts upon oxidation by quantifying HMS and inorganic sulfur species (i.e., sulfate (SO42–) and peroxydisulfate (S2O82–) ions) by using ion chromatography. Kinetic data show that OH oxidation of HMS can proceed efficiently with an effective OH uptake coefficient, γeff of 0.35 ± 0.03. Upon oxidation, the formation of SO42– and S2O82– can explain the aerosol sulfur conversion. An aerosol sulfur mass closure was also obtained by quantifying the amount of HMS, SO42–, and S2O82– before and after oxidation. Kinetic model simulations show that reaction kinetics and product formation can be well explained by a series of aerosol-phase chain reactions initiated by the sulfite radical anion (SO3•–) under our experimental conditions. Overall, our results highlight a significant conversion of organic sulfur from HMS to inorganic sulfur species, including S2O82– upon heterogeneous OH oxidation

Measurements of Isoprene-Derived Organosulfates in Ambient Aerosols by Aerosol Time-of-Flight Mass SpectrometryPart 2: Temporal Variability and Formation Mechanisms

Organosulfate species have recently gained attention for their potentially significant contribution to secondary organic aerosol (SOA); however, their temporal behavior in the ambient atmosphere has not been probed in detail. In this work, organosulfates derived from isoprene were observed in single particle mass spectra in Atlanta, GA during the 2002 Aerosol Nucleation and Characterization Experiment (ANARChE) and the 2008 August Mini-Intensive Gas and Aerosol Study (AMIGAS). Real-time measurements revealed that the highest organosulfate concentrations occurred at night under a stable boundary layer, suggesting gas-to-particle partitioning and subsequent aqueous-phase processing of the organic precursors played key roles in their formation. Further analysis of the diurnal profile suggests possible contributions from multiple production mechanisms, including acid-catalysis and radical-initiation. This work highlights the potential for additional SOA formation pathways in biogenically influenced urban regions to enhance the organic aerosol burden

Measurements of Isoprene-Derived Organosulfates in Ambient Aerosols by Aerosol Time-of-Flight Mass Spectrometry - Part 1: Single Particle Atmospheric Observations in Atlanta

Organosulfate species have recently been identified as a potentially significant class of secondary organic aerosol (SOA) species, yet little is known about their behavior in the atmosphere. In this work, organosulfates were observed in individual ambient aerosols using single particle mass spectrometry in Atlanta, GA during the 2002 Aerosol Nucleation and Characterization Experiment (ANARChE) and the 2008 August Mini-Intensive Gas and Aerosol Study (AMIGAS). Organosulfates derived from biogenically produced isoprene were detected as deprotonated molecular ions in negative-ion spectra measured by aerosol time-of-flight mass spectrometry; comparison to high-resolution mass spectrometry data obtained from filter samples corroborated the peak assignments. The size-resolved chemical composition measurements revealed that organosulfate species were mostly detected in submicrometer aerosols and across a range of aerosols from different sources, consistent with secondary reaction products. Detection of organosulfates in a large fraction of negative-ion ambient spectra − ca. 90−95% during ANARChE and ∼65% of submicrometer particles in AMIGAS − highlights the ubiquity of organosulfate species in the ambient aerosols of biogenically influenced urban environments