Reactive Uptake of Dimethylamine by Ammonium Sulfate and Ammonium Sulfate–Sucrose Mixed Particles

Short-chain alkyl amines can undergo gas-to-particle partitioning via reactive uptake by ammonium salts, whose phases have been thought to largely influence the extent of amine uptake. Previous studies mainly focused on particles of single ammonium salt at either dry or wet conditions without any addition of organic compounds. Here we report the uptake of dimethylamine (DMA) by ammonium sulfate (AS) and AS–sucrose mixed particles at different relative humidities (RHs) using an electrodynamic balance coupled with in situ Raman spectroscopy. DMA is selected as a representative of short-chain alkyl amines, and sucrose is used as a surrogate of viscous and hydrophilic organics. Effective DMA uptake was observed for most cases, except for the water-limiting scenario at <5% RH and the formation of an ultraviscous sucrose coating at 10% RH and below. DMA uptake coefficients (γ) were estimated using the particle mass measurements during DMA uptake. Addition of sucrose can increase γ by absorbing water or inhibiting AS crystallization and decrease γ by elevating the particle viscosity and forming a coating layer. DMA uptake can be facilitated for crystalline AS or retarded for aqueous AS with hydrophilic viscous organics (e.g., secondary organic material formed via the oxidation of biogenic volatile organic compounds) present in aerosol particles

Enhanced Sulfate Formation through Synergistic Effects of Chlorine Chemistry and Photosensitization in Atmospheric Particles

Numerous studies have demonstrated that organic photosensitizers from biomass burning can generate oxidants to effectively convert inorganic and organic precursors into secondary aerosols. Particulate chloride ions can be internally mixed with organic photosensitizers in biomass burning particles. In this study, we investigate the impact of the interaction of chlorine chemistry and photosensitization on the oxidative potential of aerosols by utilizing SO2 oxidation to form sulfate as an indicator. Mixed particles of chloride with glyoxal and its reaction products of ammonia of imidazole-2-carboxaldehyde (IC) were studied. Premixed NH4Cl + glyoxal particles have a 4–5 times higher sulfate formation rate than premixed NaCl + glyoxal, particularly at low relative humidity, suggesting the role of photosensitization. Furthermore, the addition of IC resulted in an ∼73-fold increase in sulfate production rate compared to NH4Cl alone. No noticeable sulfate formation was observed in the presence of IC alone, likely due to the high particle acidity in this study (i.e., pH = 2). The kinetic analysis of these particles results yields a reaction rate constant of chloride ions with the triplet state of IC, 3IC*, ∼3 orders of magnitude higher than previously reported values in bulk solution. These findings underscore the significance of the synergetic effect of chlorine chemistry and photosensitization in enhancing atmospheric oxidative capacity

Formation and Transformation of Metastable Double Salts from the Crystallization of Mixed Ammonium Nitrate and Ammonium Sulfate Particles

Ammonium nitrate (AN) and ammonium sulfate (AS) are ubiquitous components of atmospheric aerosols. Thermodynamic models predict formation of pure (AN and AS) and double salts (3AN·AS and 2AN·AS) for the AN/AS system. Because of the high supersaturation at which a droplet crystallizes, metastable crystal formation is possible. In this study, the identity of the crystals formed from the crystallization of equimolar AN/AS mixed droplets was investigated in an electrodynamic balance coupled with a Raman spectroscopic system. Raman spectra of bulk AN/AS double salts possibly formed in this system are first reported for comparison with the single particle Raman results. The double-salt 3AN·AS, not predicted from thermodynamics, was observed in the freshly crystallized single particles. The degree of metastability can be different among several crystallization processes of the same particles. The metastable salt 3AN·AS gradually transformed into stable 2AN·AS, and the rate of such transformation increased with increasing relative humidity. This study illustrates the possibility of occurrence of metastable salts in atmospheric aerosols

Role of the Aerosol Phase State in Ammonia/Amines Exchange Reactions

The exchange reaction of ammonia in (NH4)2SO4 with an amine and the corresponding reverse reaction of amines in aminium sulfates with ammonia were investigated using an electrodynamic balance coupled with a Raman spectrometer. The temporal changes in particle mass, chemical composition, and phase state were simultaneously monitored. When the salt particles were in an aqueous state at elevated relative humidities (RHs), the exchange of ammonia/amine vapors in the particle phase was reversible. The exchange rates of aqueous particles were in general higher than those of their corresponding solid counterparts. An aqueous phase was essential for the effective displacement of ammonia and amines. Aminium salts in different phase states and with different evaporation characteristics showed remarkably different reaction behaviors in ammonia vapor. The less compact amorphous aminium sulfate solids were more susceptible to ammonia exchange than the crystalline solids. The aminium salts in a liquid state exhibited substantial amine evaporation at <3% RH and formed acidic bisulfate. Under ammonia exposure, these acidic aminium droplets underwent both neutralization and displacement reactions. Stable solid salts containing ammonium, aminium, sulfate, and bisulfate were formed and hindered further reactions. The result suggests that ambient aminium sulfates may be acidic. Overall, the phase states of the ammonium and aminium salt particles crucially determine the heterogeneous reaction rates and final product properties and identities

Heterogeneous Reactions of Linoleic Acid and Linolenic Acid Particles with Ozone: Reaction Pathways and Changes in Particle Mass, Hygroscopicity, and Morphology

In this study, an electrodynamic balance (EDB) and a single particle Raman spectroscopic system were used to investigate the heterogeneous reactions of linoleic acid and linolenic acid with ozone under ambient temperatures (22−24 °C) and dry conditions (RH < 5%). Raman characterizations provide evidence that ozone-induced autoxidation, in addition to direct ozonolysis, is a plausible pathway in the reactions between ozone and linoleic acid and linolenic acid particles. Furthermore, the significance of this specific oxidation pathway depends on the ozone concentrations used in the experiment. A low ozone concentration (∼200−250 ppb) with a longer exposure period (20 h) favors autoxidation but an extremely high ozone concentration (∼10 ppm) favors ozonolysis and forces most unsaturated fatty acids to react with ozone in a relatively short period of time. In the low ozone concentration experiments, the mass of the ozone-processed linoleic acid and linolenic acid particles increased by about 2−3% and 10−13%, respectively. In addition, the mass ratios (particle mass at RH ≈ 85% to particle mass at RH < 5%) of the ozone-processed linoleic acid and linolenic acid particles increased by about 2−3% and 3−4%, respectively. The morphology of the pure and ozone-processed linoleic acid and linolenic acid particles are compared, based on imagining and their light scattering patterns

Gas−Particle Partitioning of Alcohol Vapors on Organic Aerosols

Single particle levitation using an electrodynamic balance (EDB) has been found to give accurate and direct hygroscopic measurements (gas−particle partitioning of water) for a number of inorganic and organic aerosol systems. In this paper, we extend the use of an EDB to examine the gas−particle partitioning of volatile to semivolatile alcohols, including methanol, n-butanol, n-octanol, and n-decanol, on levitated oleic acid particles. The measured Kp agreed with Pankow’s absorptive partitioning model. At high n-butanol vapor concentrations (103 ppm), the uptake of n-butanol reduced the average molecular-weight of the oleic acid particle appreciably and hence increased the Kp according to Pankow’s equation. Moreover, the hygroscopicity of mixed oleic acid/n-butanol particles was higher than the predictions given by the UNIFAC model (molecular group contribution method) and the ZSR equation (additive rule), presumably due to molecular interactions between the chemical species in the mixed particles. Despite the high vapor concentrations used, these findings warrant further research on the partitioning of atmospheric organic vapors (Kp) near sources and how collectively they affect the hygroscopic properties of organic aerosols

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

Reactive Uptake of Monoethanolamine by Sulfuric Acid Particles and Hygroscopicity of Monoethanolaminium Salts

CO2 capture plants are a significant source of emission of monoethanolamine (MEA) in the atmosphere. As a potential MEA sink, the heterogeneous uptake of MEA by sulfuric acid (SA) particles can form particulate MEA sulfate (MEAS), changing the hygroscopicity of the particles. We determined the hygroscopicities of MEA salts, including MEAS, at different MEA:sulfate molar ratios over a wide range of relative humidity (RH) using an electrodynamic balance (EDB) and a water activity meter. Other salts, including MEA oxalate, nitrate, and chloride, were studied using the water activity meter. Empirical functions were fitted to the experimentally measured hygroscopicity data of MEA salts. We further investigated the reactive uptake of parts per million-level MEA by SA particles in an EDB. The relative mass change of the levitated particles was the combined result of MEA uptake and changes in particle hygroscopicity due to compositional changes. The measured hygroscopicity was used to analyze the particle composition change during MEA uptake and the uptake kinetics. The uptake coefficients (γMEA) were estimated to be (3.23 ± 0.64) × 10–3 and (9.89 ± 2.62) × 10–4 at 40% and 70% RH, respectively. MEA reactive uptake by acidic particles could be competitive with respect to MEA gas-phase oxidation under high-particle concentration conditions near power plants

Water Activities and Osmotic Coefficients of Aqueous Solutions of Five Alkylaminium Sulfates and Their Mixtures with H₂SO₄ at 25^oC

<div><p>Alkylaminium sulfates are frequently detected in ambient aerosols, and are believed to be important in the nucleation of new particles in the atmosphere, despite the comparatively low gas phase concentrations of amines. In this study, water activities and osmotic coefficients have been measured, using a chilled mirror dew point technique, of aqueous mixtures of sulfuric acid and the following alkylaminium sulfates: methylaminium, ethylaminium, dimethylaminium, diethylaminium, and trimethylaminium sulf-ate. The samples were prepared by mixing solutions of the five corresponding amines and aqueous sulfuric acid and determining the exact aminium to sulfate molar ratios by ion chromatography. The results were correlated using an extended Zdanovskii–Stokes–Robinson equation to enable concentration/water activity rela-tionships to be calculated over the entire composition range from pure aqueous sulfuric acid to pure aqueous aminium sulfate. Water activities and osmotic coefficients for aminium:sulfate ratios of 1:1 (the bisulfate salts) and lower showed great similarity with ammonium bisulfate, but osmotic coefficients for the 2:1 ratio (the aminium sulfates) were significantly larger (and water activities lower) than for ammonium sulfate. These results differ from those obtained in Clegg et al.'s (2013) study. The relative values of the osmotic coefficients, in concentrated solutions, suggest that the numbers of methyl or ethyl groups in the aminium ion may have a stronger lowering effect on water activity than the alkyl chain length.</p><p>Copyright 2015 American Association for Aerosol Research</p></div