Aqueous Aerosol Research Articles

Solvation, Surface Propensity, and Chemical Reactions of Solutes at Atmospheric Liquid-Vapor Interfaces.

surface is covered by oceans, a large number of liquid aerosol particles fill the air, and clouds hold a tiny but critical fraction of Earth's water in the air to influence our climate and hydrology, enabling the lives of humans and ecosystems. The surfaces of these liquids provide the interface for the transfer of gases, for nucleation processes, and for catalyzing important chemical reactions. Coupling a range of spectroscopic tools to liquid microjets has become an important approach to better understanding dynamics, structure, and chemistry at liquid interfaces. Liquid microjets offer stability in vacuum and ambient pressure environments, thus also allowing X-ray photoelectron spectroscopy (XPS) with manageable efforts in terms of differential pumping. Liquid microjets are operated at speeds sufficient to allow for a locally equilibrated surface in terms of water dynamics and solute surface partitioning. XPS is based on the emission of core-level electrons, the binding energy of which is selective for the element and its chemical environment. Inelastic scattering of electrons establishes the probing depth of XPS in the nanometer range and thus its surface sensitivity.In this Account, we focus on aqueous solutions relevant to the surface of oceans, aqueous aerosols, or cloudwater. We are interested in understanding solvation and acid dissociation at the interface, interfacial aspects of reactions with gas-phase reactants, and the interplay of ions with organic molecules at the interface. The strategy is to obtain a link between the molecular-level picture and macroscopic properties and reactivity in the atmospheric context.We show consistency between surface tension and XPS for a range of surface-active organic species as an important proof for interrogating an equilibrated liquid surface. Measurements with organic acids and amines offer important insight into the question of apparent acidity or basicity at the interface. Liquid microjet XPS has settled the debate of the surface enhancement of halide ions, shown using the example of bromide and its oxidation products. Despite the absence of a strong enhancement for the bromide ion, its rate of oxidation by ozone is surface catalyzed through the stabilization of the bromide ozonide intermediate at the interface. In another reaction system, the one between Fe2+ and H2O2, a similar intermediate in the form of highly valent iron species could not be detected by XPS under the experimental conditions employed, shedding light on the abundance of this intermediate in the environment but also on the constraints within which surface species can be detected. Emphasizing the importance of electrostatic effects, we show how a cationic surfactant attracts charged bromide anions to the interface, accompanied by enhanced oxidation rates by ozone, overriding the role of surfactants as a barrier for the access of gas-phase reactants. The reactivity and structure at interfaces thus result from a subtle balance between hygroscopic and hydrophobic interactions, electrostatic effects, and the structural properties of both liquids and solutes.

Influence of selected cationic, anionic, and nonionic surfactants on hygroscopic growth of individual aqueous coarse mode aerosol particles

Influence of selected cationic, anionic, and nonionic surfactants on hygroscopic growth of individual aqueous coarse mode aerosol particles

Deconvolution of Systemic Pharmacokinetics Predicts Inhaled Aerosol Dosimetry of Nicotine

Absorption of inhaled compounds can occur from multiple sites based on upper and lower respiratory tract deposition, and clearance mechanisms leading to differential local and systemic pharmacokinetics. Deriving inhaled aerosol dosimetry and local tissue concentrations for nose-only exposure in rodents and inhaled products in humans is challenging. In this study we use inhaled nicotine as an example to identify regional respiratory tract deposition, absorption fractions, and their contribution toward systemic pharmacokinetics in rodents and humans. A physiologically based pharmacokinetic (PBPK) model was constructed to describe the disposition of nicotine and its major metabolite, cotinine. The model description for the lungs was simplified to include an upper respiratory tract region with active mucociliary clearance and a lower respiratory tract region. The PBPK model parameters such as rate of oral absorption, metabolism and clearance were fitted to the published nicotine and cotinine plasma concentrations post systemic administration and oral dosing. The fractional deposition of inhaled aerosol in the upper and lower respiratory tract regions was estimated by fitting the plasma concentrations. The model predicted upper respiratory tract deposition was 63.9% for nose-only exposure to nicotine containing nebulized aqueous aerosol in rats and 60.2% for orally inhaled electronic vapor product in humans. A marked absorption of nicotine from the upper respiratory tract and the gastrointestinal tract for inhaled aqueous aerosol contributed to the differential systemic pharmacokinetics in rats and humans. The PBPK model derived dosimetry shows that the current aerosol dosimetry models with their posteriori application using independent aerosol physicochemical characterization to predict aerosol deposition are insufficient and will need to consider complex interplay of inhaled aerosol evolutionary process. While the study highlights the needs for future research, it provides a preliminary framework for interpreting pharmacokinetics of inhaled aerosols to facilitate the analysis of in vivo exposure-responses for pharmacological and toxicological assessments.

Dependence of the Intensity of Emission Lines of Chemical Elements on Laser Pulse Duration in the Method of Filament-Induced Breakdown Spectroscopy of Aqueous Aerosol

Dependence of the Intensity of Emission Lines of Chemical Elements on Laser Pulse Duration in the Method of Filament-Induced Breakdown Spectroscopy of Aqueous Aerosol

Spectral Analysis of Aqueous Aerosol by Femtosecond Pulse Laser-Induced Breakdown Method

Spectral Analysis of Aqueous Aerosol by Femtosecond Pulse Laser-Induced Breakdown Method

Homogeneous Freezing Events Sampled in the Tropical Tropopause Layer

AbstractHigh‐frequency (20 Hz) aircraft measurements of tropical tropopause layer (TTL) cirrus and humidity are used to examine recent homogeneous freezing ice nucleation events. Several localized regions with peak ice concentrations greater than 10 cm−3 are documented. These high ice concentrations are almost certainly generated by homogeneous freezing of aqueous aerosols. The high‐frequency data reveals considerable structure in the cloud properties and humidity at along‐track scales as short as 8.5 m. Simulations of vapor depletion by growing ice crystals are used to assess the time since nucleation in these high ice concentration patches. Comparisons between observed and simulated dependence of supersaturation on ice concentration indicate the nucleation events occurred within minutes (sometimes only a few minutes) of the aircraft sampling. The infrequency and short ages of the events suggest that the high ice concentrations do not persist for long times, but they may still be important for TTL cirrus microphysical properties by providing a source of ice crystals that can lead to extensive, persistent clouds with lower ice concentrations. We have also used the extensive TTL measurements provided by the high‐altitude aircraft campaigns to evaluate the threshold supersaturation for homogeneous freezing. Peak supersaturations observed are consistent with recent laboratory experiments indicating that ice saturation ratios of about 1.75–1.95 are required for homogeneous freezing of aqueous aerosols at temperatures of 185–200 K.

Individual adsorption of low volatility pheromones: Amphiphilic molecules on a clean water-air interface.

Environmental conditions can alter olfactory scent and chemical communication among biological species. In particular, odorant molecules interact with aerosols. Thermodynamics variables governing the adsorption from air to water surface of bombykol, the most studied pheromone, and of three derivative molecules, bombykal, bombykoic acid, and bombykyle acetate, are computed by steered and un-biased molecular dynamics in order to compare the role of their polar head group on adsorption on aqueous aerosols. When adsorbed, the molecule center of mass stands at about 1.2 Å from the interface and oscillates on the same length scale, trapped in an energy well. Gibbs energy of adsorption and desorption time of bombykol are found to be 9.2 kBT and 59µs, respectively. The following ordering between the molecules is observed, reading from the more to the least adsorbed: bombykoic acid > bombykol > bombykoic acetate > bombykal. It originates from a complex interplay of entropy and enthalpy. The entropy and enthalpy of adsorption are discussed in the light of structural arrangement, H-bonding, and hydrophilic tail positioning of the molecules at the interface. Our results show that, when dispersed in the air, pheromones adsorb on aqueous aerosols. However, the individual residence time is quite short on pure water surfaces. Aerosols can, therefore, only have a decisive influence on chemical communication through collective effects or through their chemical composition that is generally more complex than that of a pure water surface.

Large contribution of fossil-derived components to aqueous secondary organic aerosols in China

Incomplete understanding of the sources of secondary organic aerosol (SOA) leads to large uncertainty in both air quality management and in climate change assessment. Chemical reactions occurring in the atmospheric aqueous phase represent an important source of SOA mass, yet, the effects of anthropogenic emissions on the aqueous SOA (aqSOA) are not well constrained. Here we use compound-specific dual-carbon isotopic fingerprints (δ13C and Δ14C) of dominant aqSOA molecules, such as oxalic acid, to track the precursor sources and formation mechanisms of aqSOA. Substantial stable carbon isotope fractionation of aqSOA molecules provides robust evidence for extensive aqueous-phase processing. Contrary to the paradigm that these aqSOA compounds are largely biogenic, radiocarbon-based source apportionments show that fossil precursors produced over one-half of the aqSOA molecules. Large fractions of fossil-derived aqSOA contribute substantially to the total water-soluble organic aerosol load and hence impact projections of both air quality and anthropogenic radiative forcing. Our findings reveal the importance of fossil emissions for aqSOA with effects on climate and air quality.

Sulfur radical formation from the tropospheric irradiation of aqueous sulfate aerosols

The sulfate anion radical (SO4•-) is known to be formed in the autoxidation chain of sulfur dioxide and from minor reactions when sulfate or bisulfate ions are activated by OH radicals, NO3 radicals, or iron. Here, we report a source of SO4•-, from the irradiation of the liquid water of sulfate-containing organic aerosol particles under natural sunlight and laboratory UV radiation. Irradiation of aqueous sulfate mixed with a variety of atmospherically relevant organic compounds degrades the organics well within the typical lifetime of aerosols in the atmosphere. Products of the SO4•- + organic reaction include surface-active organosulfates and small organic acids, alongside other products. Scavenging and deoxygenated experiments indicate that SO4•- radicals, instead of OH, drive the reaction. Ion substitution experiments confirm that sulfate ions are necessary for organic reactivity, while the cation identity is of low importance. The reaction proceeds at pH 1-6, implicating both bisulfate and sulfate in the formation of photoinduced SO4•-. Certain aromatic species may further accelerate the reaction through synergy. This reaction may impact our understanding of atmospheric sulfur reactions, aerosol properties, and organic aerosol lifetimes when inserted into aqueous chemistry model mechanisms.

Experimental study on the imidazoles of aqueous secondary organic aerosol formed from reaction of methylglyoxal and ammonium sulphate

Imidazole and its derivatives are the main heterocyclic nitrogen-containing brown carbon constituents in aerosol particles. Imidazoles of aqueous secondary organic aerosol (aqSOA) particles formed from the reaction of methylglyoxal and ammonium sulphate ((NH4)2SO4) are detected by vacuum ultraviolet photoionization aerosol mass spectrometer (VUV-PIMS) in real-time and verified off-line with optical spectrometry in current study. The experimental results showed that the single photon ionization mass spectrum of aqSOA particles measured by VUV-PIMS emerged the molecular ion peaks at m/z = 68, 82 and 110. While, the extraction solution of aqSOA particles detected by off-line techniques demonstrated that the characteristic peaks at 217 and 282 nm appeared in the UV–Vis spectrum; the strong emission in 405–480 nm with maximum emission wavelength at 435 nm contained in the 3D fluorescence spectra; and the chemical shift peaks of hydrogen atoms on N–H, CC and CN bond on imidazole ring and methyl group arised in the 1H-NMR spectrum. In view of these spectral information, 4-methyl-imidazole and 4-methyl-imidazole-2-carbaldehyde are determined as the principal products of the aqueous reaction between methylglyoxal and (NH4)2SO4. The possible reaction mechanism leading to the generation of imidazole compounds are also discussed. These will help to understand the formation process of imidazole compounds in atmospheric aerosol particles, and provide experimental basis for the model mechanism study of aqSOA particles.

The role of source emissions in sulfate formation pathways based on chemical thermodynamics and kinetics model

Sulfate is a major PM2.5 constituent and poses a significant threat to ecosystems and human health, which has attracted lots of attention to the sulfate formation mechanism. In recent years, there has been great scientific interest in the multiphase oxidation of SO2 in aqueous aerosol particles. Many factors are involved in the reaction process, including precursor SO2, oxidants/catalysts, and aerosol acidity, which are three channels closely related to the source emission. The conjoint analysis of source emissions and sulfate aqueous formation can provide a scientific basis for designing effective strategies, though the related research is extremely limited. Here, we applied an improved solute strength-dependent chemical Thermodynamics & Kinetics model (for aqueous pathway contribution) and the Partial Target Transformation-Positive matrix factor model (for source apportionment) to explore the role of source emission in sulfate aqueous formation. The results indicated H2O2 aqueous oxidation was the dominant pathway (65.9 %), and secondary nitrate source may grow together with sulfate formation from H2O2 pathway. H2O2 and TMI pathways were related to higher SOR (sulfur oxidation rate). TMI pathway was significant in summer (54.6 %) and increased with secondary sources and vehicle exhaust. NO2 pathway was more significant at low secondary source and high coal combustion (higher contribution of NO2 pathway appeared in winter, 24.7 %). While high formation rate of the O3 pathway always occurred at low source levels. Coal combustion and vehicle exhaust showed obvious effects on sulfate aqueous formation. Notably, aerosol acidity is a significant factor related to sources and plays a key role in sulfate formation. The result also suggested aerosol pH may be more important than the amounts of substances involved in the oxidation reaction. The findings in this work can provide useful information for better understanding sulfate aqueous formation and offer a scientific basis for designing strategies for air pollution control and sulfate mitigation.

Suppression of the phenolic SOA formation in the presence of electrolytic inorganic seed

Phenolic compounds are largely attributed to wildfire gases and rapidly react with atmospheric oxidants to form persistent phenoxy free radicals, which influence atmospheric chemistry and secondary organic aerosol (SOA) formation. In this study, phenol or o-cresol was photochemically oxidized under various conditions (NOx levels, humidity, and seed conditions) in an outdoor photochemical reactor. Unexpectedly, SOA growth of both phenols was suppressed in the presence of salted aqueous aerosol compared to non-seed SOA. This discovery is different from the typical SOA formation of aromatic or biogenic hydrocarbons, which show noticeably higher SOA yields via organic aqueous reactions. Phenol, o-cresol, and their phenolic products (e.g., catechols) are absorbed in aqueous aerosol and form phenoxy radicals via heterogeneous reactions under sunlight. The resulting phenoxy radicals are redistributed between the gas and particle phases. Gaseous phenoxy radicals quickly react with ozone to form phenyl peroxide radicals and regenerated through a NOx cycle to retard phenol oxidation and SOA formation. The explicit oxidation mechanisms of phenol or o-cresol in the absence of aqueous phase were derived including the Master Chemical Mechanism (MCM v3.3.1) and the path for peroxy radical adducts originating from the addition of an OH radical to phenols to form low volatility products (e.g., multi-hydroxy aromatics). The resulting gas mechanisms of phenol or o-cresol were, then, applied to the Unified Partitioning Aerosol Phase Reaction (UNIPAR) model to predict SOA formation via multiphase partitioning of organics and aerosol-phase oligomerization. The model well simulated chamber-generated phenolic SOA in absence of wet-inorganic seed, but significantly overestimated SOA mass in presence of wet seed. This study suggests that heterogeneous chemistry to form phenoxy radicals needs to be included to improve SOA prediction from phenols. The suppression of atmospheric oxidation due to phenoxy radicals in wet inorganic aerosol can explain the low SOA formation during wildfire episodes.

Persistence of Monoterpene-Derived Organic Peroxides in the Atmospheric Aqueous Phase

Organic peroxides are essential reactive species formed during the atmospheric oxidation of organic compounds and can effectively partition into aqueous aerosols and cloud droplets, yet their aqueous-phase fate in aerosol and cloud water remains poorly characterized. Here, we report the first investigation of the isomer-resolved aqueous-phase chemical behaviors of a large suite of organic peroxides in dissolved monoterpene secondary organic aerosol (SOA). We find that about half of individual peroxides undergo noticeable hydrolysis, while the other half mainly react via nonhydrolysis unimolecular or bimolecular pathways. The chemical complexity of organic peroxides, as well as the effects of coexisting organic compounds in dissolved SOA, results in diverse chemical behaviors of individual peroxides and a complicated dependence on the water content, pH, and even organic concentration in the aqueous phase. Compared to the previously investigated α-substituted hydroperoxides that undergo hydrolysis on timescales of minutes to a few hours, the vast majority of organic peroxides studied herein are significantly more persistent in the aqueous phase, with decay rate constants of 1.8 × 10–6 to 2.4 × 10–4 s–1 and the corresponding e-folding lifetimes of 1.1–153 h under atmospherically relevant conditions, which would have distinctly different impacts on atmospheric chemistry and human health.

Influence of urban heat islands on seasonal aerosol acidity and aerosol liquid water content in humid subtropical Hong Kong, South China

Aerosol acidity governs many important atmospheric processes that modulate the composition and mass concentrations of atmospheric aerosols. Temperature and RH are two key drivers of aerosol acidity. Increasing levels of urbanization in humid subtropical South China have led to the frequent occurrence of urban heat islands (UHIs) in the region. However, the effects that UHIs have on aerosol acidity in cities in South China are currently unknown. In this study, we investigated the effects of UHIs on aerosol acidity and aerosol liquid water concentrations at two urban sites with different land use characteristics in Hong Kong, South China, in winter and summer from 2011 to 2019. Our results showed that the UHI effect resulted in both urban sites having significantly higher temperatures and lower RH than the refence rural site. Higher UHI intensities were observed in winter, but RH differences between the urban and rural sites were the greatest in summer. Temperature and RH changes associated with the UHI effect resulted in urban aerosols having aerosol liquid water concentrations that were up to 6 to 7 times lower than rural aerosols, and urban aerosols being more acidic than rural aerosols by as much as 1.16 pH units. While the pH differences between urban and rural aerosols correlated with temperature and RH differences induced by the UHI effect, these correlations were season-dependent due to differences in the winter and summer aerosol compositions. In addition, although the UHI was most intense in winter, pH differences between urban and rural aerosols were the largest in summer. Changes in temperature, RH, and aerosol liquid water concentrations linked to the UHI effect were found to lead to substantial reductions (by as much as 38%) in the partitioning of semi-volatile basic and acidic species NH3 and HNO3 to the aerosol phase. Our results indicate that changes in the aerosol acidity, aerosol liquid water concentrations, and gas-aerosol partitioning associated with UHIs have significant implications for aqueous urban aerosol chemistry in Hong Kong. Since UHIs will intensify in many cities in South China in the future due to their increasing levels of urbanization and a warming climate, changes in urban aerosol acidity and liquid water concentrations induced by intensifying UHIs need to be considered when modeling urban aerosol composition and levels in the region.

Aqueous secondary organic aerosol formation attributed to phenols from biomass burning

Biomass burning emits large quantities of phenols, which readily partition into the atmospheric aqueous phase and subsequently may react to produce aqueous secondary organic aerosol (aqSOA). For the first time, we quantitatively explored the influence of phenols emitted from biomass burning on aqSOA formation in the winter of Beijing. A typical haze episode associated with significant aqSOA formation was captured. During this episode, aqueous-phase processing of biomass burning promoted aqSOA formation was identified. Furthermore, high-resolution mass spectrum analysis provided molecular-level evidence of the phenolic aqSOA tracers. Estimation of aqSOA formation rate (RaqSOA) with compiled laboratory kinetic data indicated that biomass-burning phenols can efficiently produce aqSOA at midday, with RaqSOA of 0.42 μg m−3 h−1 accounting for 15 % of total aqSOA formation rate. The results highlight that aqSOA formation of phenols contributes the haze pollution. This implies the importance of regional joint control of biomass burning to mitigate the heavy haze.

Coupled Interfacial and Bulk Kinetics Govern the Timescales of Multiphase Ozonolysis Reactions.

Chemical transformations in aerosols impact the lifetime of particle phase species, the fate of atmospheric pollutants, and both climate- and health-relevant aerosol properties. Timescales for multiphase reactions of ozone in atmospheric aqueous phases are governed by coupled kinetic processes between the gas phase, the particle interface, and its bulk, which respond dynamically to reactive consumption of O3. However, models of atmospheric aerosol reactivity often do not account for the coupled nature of multiphase processes. To examine these dynamics, we use new and prior experimental observations of aqueous droplet reaction kinetics, including three systems with a range of surface affinities and ozonolysis rate coefficients (trans-aconitic acid (C6H6O6), maleic acid (C4H4O4), and sodium nitrite (NaNO2)). Using literature rate coefficients and thermodynamic properties, we constrain a simple two-compartment stochastic kinetic model which resolves the interface from the particle bulk and represents O3 partitioning, diffusion, and reaction as a coupled kinetic system. Our kinetic model accurately predicts decay kinetics across all three systems, demonstrating that both the thermodynamic properties of O3 and the coupled kinetic and diffusion processes are key to making accurate predictions. An enhanced concentration of adsorbed O3, compared to gas and bulk phases is rapidly maintained and remains constant even as O3 is consumed by reaction. Multiphase systems dynamically seek to achieve equilibrium in response to reactive O3 loss, but this is hampered at solute concentrations relevant to aqueous aerosol by the rate of O3 arrival in the bulk by diffusion. As a result, bulk-phase O3 becomes depleted from its Henry's law solubility. This bulk-phase O3 depletion limits reaction timescales for relatively slow-reacting organic solutes with low interfacial affinity (i.e., trans-aconitic and maleic acids, with krxn ≈ 103-104 M-1 s-1), which is in contrast to fast-reacting solutes with higher surface affinity (i.e., nitrite, with krxn ≈ 105 M-1 s-1) where surface reactions strongly impact the observed decay kinetics.

Targeting Atmospheric Oxidants Can Better Reduce Sulfate Aerosol in China: H2O2 Aqueous Oxidation Pathway Dominates Sulfate Formation in Haze.

Particulate sulfate is one of the most important components in the atmosphere. The observation of rapid sulfate aerosol production during haze events provoked scientific interest in the multiphase oxidation of SO2 in aqueous aerosol particles. Diverse oxidation pathways can be enhanced or suppressed under different aerosol acidity levels and high ionic strength conditions of atmospheric aerosol. The importance of ionic strength to sulfate multiphase chemistry has been verified under laboratory conditions, though studies in the actual atmosphere are still limited. By utilizing online observations and developing an improved solute strength-dependent chemical thermodynamics and kinetics model (EF-T&K model, EF is the enhancement factor that represents the effect of ionic strength on an aerosol aqueous-phase reaction), we provided quantitative evidence that the H2O2 pathway was enhanced nearly 100 times and dominated sulfate formation for entire years (66%) in Tianjin (a northern city in China). TMI (oxygen catalyzed by transition-metal ions) (14%) and NO2 (14%) pathways got the second-highest contributions. Machine learning supported the result that aerosol sulfate production was more affected by the H2O2 pathway. The collaborative effects of atmospheric oxidants and SO2 on sulfate aerosol production were further investigated using the improved EF-T&K model. Our findings highlight the effectiveness of adopting target oxidant control as a new direction for sustainable mitigation of sulfate, given the already low SO2 concentrations in China.

Aqueous ·OH Oxidation of Highly Substituted Phenols as a Source of Secondary Organic Aerosol.

Biomass burning (BB) releases large quantities of phenols (ArOH), which can partition into cloud/fog drops and aerosol liquid water (ALW), react, and form aqueous secondary organic aerosol (aqSOA). While simple phenols are too volatile to significantly partition into particle water, highly substituted ArOH partition more strongly and might be important sources of aqSOA in ALW. To investigate this, we measured the ·OH oxidation kinetics and aqSOA yields for six highly substituted ArOH from BB. Second-order rate constants are high, in the range (1.9-14) × 109 M-1 s-1 at pH 2 and (14-25) × 109 M-1 s-1 at pH 5 and 6. Mass yields of aqSOA are also high, with an average (±1σ) value of 82 (±12)%. ALW solutes have a range of impacts on phenol oxidation by ·OH: a BB sugar and some inorganic salts suppress oxidation, while a nitrate salt and transition metals enhance oxidation. Finally, we estimated rates of aqueous- and gas-phase formation of SOA from a single highly substituted phenol as a function of liquid water content (LWC), from conditions of cloud/fog (0.1 g-H2O m-3) to ALW (10 μg-H2O m-3). Formation of aqSOA is significant across the LWC range, although gas-phase ·OH becomes dominant under ALW conditions. We also see a generally large discrepancy between measured and modeled aqueous ·OH concentrations across the LWC range.

Characterization of Imidazole Compounds in Aqueous Secondary Organic Aerosol Generated from Evaporation of Droplets Containing Pyruvaldehyde and Inorganic Ammonium

Imidazole compounds are important constituents of atmospheric brown carbon. The imidazole components of aqueous secondary organic aerosol (aqSOA) that are generated from the evaporation of droplets containing pyruvaldehyde and inorganic ammonium are on-line characterized by an aerosol laser time-of-flight mass spectrometer (ALTOFMS) and off-line detected by optical spectrometry in this study. The results demonstrated that the laser desorption/ionization mass spectra of aqSOA particles that were detected by ALTOFMS contained the characteristic mass peaks of imidazoles at m/z = 28 (CH2N+), m/z = 41 (C2H3N+) and m/z = 67 (C3H4N2+). Meanwhile, the extraction solution of the aqSOA particles that were measured by off-line techniques showed that the characteristic absorption peaks at 217 nm and 282 nm appeared in the UV-Vis spectrum, and the stretching vibration peaks of C-N bond and C=N bond emerged in the infrared spectrum. Based on these spectral information, 4-methyl-imidazole and 4-methyl-imidazole-2-carboxaldehyde are identified as the main products of the reaction between pyruvaldehyde and ammonium ions. The water evaporation accelerates the formation of imidazoles inside the droplets, possibly owing to the highly concentrated environment. Anions, such as F−, CO32−, NO3−, SO42− and Cl− in the aqueous phase promote the reaction of pyruvaldehyde and ammonium ions to produce imidazole products, resulting in the averaged mass absorption coefficient (<MAC>) in the range of 200–600 nm of aqSOA increases, and the order of promotion is: F− > CO32− > SO42− ≈ NO3− ≈ Cl−. These results will help to analyze the constituents and optics of imidazoles and provide a useful basis for evaluating the formation process and radiative forcing of aqSOA particles.

Photolysis by UVA-visible light and thermal degradation of TNT in aqueous solutions according to aerodynamic thermal breakup droplet ionization mass spectrometry

The mechanism of photolytic degradation of 2-4-6-trinitrotoluene (TNT) in an aqueous solution (initial concentration 102 mg L−1) by UVA-visible light (>320 nm) was investigated by aerodynamic thermal breakup droplet ionization mass spectrometry. For photolysis, a DRK-120 mercury-quartz lamp was used. The final and intermediate products of the photolysis reaction were compared with known products of TNT transformation in the environment. The first step in the photolysis process under our conditions was found to be the association of TNT molecules into a fairly stable dimer. The TNT dimer formation proceed through the formation of a Meisenheimer complex. Thermal degradation of the TNT dimer in microdroplets within a heated capillary used as a flow reactor was studied next. It was shown that the degradation of the TNT dimer in microdroplets occurs very quickly (2.1 ms) at heating (up to 200–250 °C). Of course, heating an aqueous aerosol requires energy, but the transformation of TNT can be controlled using a combination of photo- and thermal treatments. We hope that these results will help to reduce the toxicity of TNT contaminants and remove nitroaromatic compounds from water at ambient conditions. In this work, the TNT dimer formed at the first stage is transformed with the loss of NO2 fragments or water. In addition, further polymerization of TNT is observed.