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We directly measure the Fe(iii)-catalyzed SO2 conversion in single microdroplet trapped and levitated with a gradient-force aerosol optical tweezer. There was a significant acceleration in the formation of SO2 to sulfate conversion on the surface of the aerosol microdroplets.

Accurately measuring the pH of atmospheric aerosols is a prerequisite for understanding the multiphase chemistry that profoundly affects the environment and climate systems. Despite the advancements of experimental techniques for in situ pH measurements in aerosols, current studies are limited to measuring the static pH of aerosol microdroplets with an unperturbed composition. This steady-state scenario, however, deviates from the real-world aerosols undergoing atmospheric aging reactions, specifically, those characterized with a spontaneous displacement of strong bases (or acids) with high volatility. Here, we introduce a continuous and in situ measurement of aerosol pH by using a 4-mercaptopyridine-functionalized silver nanoparticle probe and surface-enhanced Raman spectroscopy. We find that the ammonium depletion─a spontaneous displacement of ammonium by dicarboxylic acid salts─continuously acidifies aerosol water over time. The decaying trends of pH in the aerosols under various humidity conditions can be unified with a universal exponential function. Such an exponentially decaying function further indicates that the ammonium depletion reaction is a self-limiting process. Our technique can be applied to study the dynamic change of aerosol acidity during the complex atmospheric aging processes, toward elucidating their implications on atmospheric chloride, nitrate, and ammonium cycles.

Artemisia argyi is a widely distributed and inexpensive plant resource, and study on its chemical compositions and biological activities will provide an important basis for its food applications and pharmaceutical developments. In this study, fourteen known guaiane-type sesquiterpenes (1-14), four known eudesmane-type sesquiterpenes (15-18), two known germacranolide-type sesquiterpenes (19, 20), and eight other types of terpenoids (20-28) were isolated from the leaves of A. argyi by polyamide and ODS CC and HPLC. The structures of all compounds are determined by 1 D NMR (1H-NMR、13C-NMR) and literature comparison. Among them, compounds 1 and 8 were isolated from Chinese folk medicine A. argyi for the first time. Besides, the LPS-induced RAW264.7 cell model has been evaluated the anti-inflammatory activities in vitro by the Griess reagent. The results indicated that the guaianolide sesquiterpenoids obtained from A. argyi have an excellent ability to inhibit NO production, especially Argyin A, a guaianolide sesquiterpenoid with isovaleryloxy substitution.

Acidity is a defining property of atmospheric aerosols that profoundly affects environmental systems, human health, and climate. However, directly measuring the pH of aerosol microdroplets remains a challenge, especially when the microdroplets' composition is nonhomogeneous or dynamically evolving or both. As a result, a pH measurement technique with high spatiotemporal resolution is needed. Here, we report a spatiotemporally resolved pH measurement technique in microdroplets using spontaneous Raman spectroscopy. Our target sample was the microdroplets comprising sodium chloride and oxalic acid─laboratory surrogates of sea spray aerosols and water-soluble organic compounds, respectively. Our measurements show that the chloride depletion from the microdroplets caused a continuous increase in pH by ∼0.5 units in 2 hours. Meanwhile, the surface propensity of chloride anions triggers a stable pH gradient inside a single droplet, with the pH at the droplet surface lower than that at the core by ∼ 0.4 units. The uncertainties arising from the Raman detection limit (±0.08 pH units) and from the nonideal solution conditions (-0.06 pH units) are constrained. Our findings indicate that spontaneous Raman spectroscopy is a simple yet robust technique for precise pH measurement in aerosols with high spatiotemporal resolution.

Depletion of chloride, nitrate, or ammonium from atmospheric aerosols represents a unique class of displacement reactions in which strong acids (HCl and HNO

Atmospheric aerosols consisting of organic and inorganic components may undergo liquid–liquid phase separation (LLPS) and liquid–solid phase transitions during ambient relative humidity (RH) fluctuation. However, the knowledge of dynamic phase evolution processes for mixed organic–inorganic particles is scarce. Here we present a universal and visualized observation of LLPS, efflorescence and deliquescence transitions as well as hygroscopic growth of laboratory-generated mixed 1, 2, 6-hexanetriol / ammonium sulfate (AS) particles with different organic–inorganic mole ratios (OIR = 1:4, 1:2, 1:1, 2:1 and 4:1) with high time resolution (0.5 s) using an optical microscope operated with a video camera. The optical images suggest that an inner AS solution phase is surrounded by an outer organic-rich phase after LLPS for all mixed particles. The LLPS mechanism for particles with different OIRs is found to be distinct; meanwhile, multiple mechanisms may dominate successively in individual particles with a certain OIR, somewhat inconsistently with previously reported observations. More importantly, another phase separation in the inner AS solution phase, defined as secondary LLPS here, is observed for OIR = 1:1, 1:2 and 1:4 particles. The secondary LLPS may be attributed to the formation of more concentrated AS inclusions in the inner phase and becomes more obvious with decreasing RH and increasing AS mole fraction. Furthermore, the changes in size and number of AS inclusions during LLPS are quantitatively characterized, which further illustrate the equilibrium partitioning process of organic and inorganic components. These experimental results have significant implications for the revelation of complex phase transitions of internally mixed atmospheric particles and evaluation of liquid–liquid and liquid–solid equilibria in thermodynamic models.

Severe winter haze events in Beijing and North China Plain are characterized by rapid production of sulfate aerosols with unresolved mechanisms. Oxidation of SO

The hygroscopic behaviors and phase changes of inorganic aerosols have been widely explored, but little is known on the hygroscopicity of soluble carbonates. The hydrated states of solid Na2CO3 particles in an air environment remain largely unclear. In this work, the hygroscopic growth, hydrated form transformations, and influence of internal Li2CO3 on phase transitions of Na2CO3 particles are investigated in linear and pulsed relative humidity (RH) changing modes by the vacuum Fourier transform infrared (FTIR) technique. For pure Na2CO3, aqueous droplets effloresced to a mixture of anhydrous Na2CO3 and Na2CO3·H2O with the initial efflorescence relative humidity (ERH) of 50.8%, probably concerning the formation of Na2CO3·10H2O in the conversion from aqueous to anhydrous Na2CO3. A reverse process is presented during the three-stage deliquescence transition beginning at ∼60.1% RH; i.e., anhydrous Na2CO3 transforms into aqueous Na2CO3 and Na2CO3·10H2O in stage I, Na2CO3·10H2O dissolves to aqueous Na2CO3 in stage II, and Na2CO3·H2O dissolves into aqueous Na2CO3 in stage III. For internally mixed Na2CO3/Li2CO3 particles, a double salt, LiNaCO3, is found in mixed crystalline phases for the first time, leading to the eutonic composition with Na2CO3. The experimental observations point to the excess of LiNaCO3 and complete consumption of Na2CO3 in eutonic composition formation, which results in the absence of Na2CO3 hydrates during phase transitions. The results provide key data for model simulations of hygroscopic properties and phase transitions of Na2CO3 as well as mixed soluble carbonates.

Water-uptakes of pure sodium carbonate (Na2CO3), pure β-alanine and internally mixed β-alanine/Na2CO3 aerosol particles with different mole ratios are first monitored using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) technique. For pure Na2CO3 aerosol particles, combining the absorptions at 877 and 1422 cm−1 with abrupt water loss shows the efflorescence relative humidity (ERH) of 62.9%–51.9%. Upon humidifying, solid Na2CO3 firstly absorbs water to from Na2CO3·H2O crystal at 72.0% RH and then deliquesces at 84.5% RH (DRH). As for pure β-alanine particles, the crystallization takes place in the range of 42.4%–33.2% RH and becomes droplets at ~ 88.2% RH. When β-alanine is mixed with Na2CO3 at various mole ratios, it shows no efflorescence of Na2CO3 when β-alanine to Na2CO3 mole ratio (OIR) is 2:1. For 1:1 and 1:2 β-alanine/Na2CO3 aerosols, the ERHs of Na2CO3 are 51.8%–42.3% and 57.1%–42.3%, respectively. While β-alanine crystal appears from 62.7% RH for 2:1 and 59.4% RH for both 1:1 and 1:2 particles and lasts to driest state. On hydration, the DRH is 44.7%–75.2% for Na2CO3 with the OIR of 1:1 and 44.7%–69.0% for 1:2 mixture, and those of β-alanine are 74.8% for 2:1 mixture and 68.9% for two others. After the first dehumidification–humidification, all the water contents decrease despite of constituent fraction. And at ~ 92% RH, the remaining water contents are 92%, 89% and 82% at ~ 92% RH, corresponding to OIR of 2:1, 1:1 and 1:2 mixed system, respectively.

The high NO

ConspectusAerosols are ubiquitous in the atmosphere and profoundly affect climate systems and human health. To gain more insights on their broad impacts, we need to comprehensively understand the fundamental properties of atmospheric aerosols. Since aerosols are multiphase, a dispersion of condensed matter (solid particles or liquid droplets, hereafter particles) in gas, partitioning of volatile matter between the condensed and the gas phases is one defining characteristic of aerosols. For example, water content partitioning under different relative humidity conditions, known as aerosol hygroscopicity, has been extensively investigated in the past decades. Meanwhile, partitioning of volatile organic or inorganic components, which is referred to as aerosol volatility, remains understudied. Commonly, a bulk solution system is treated as a single phase, with volatility mainly determined by the nature of its components, and the composition partitioning between solution and gas phase is limited. Aerosols, however, comprise an extensive gas phase, and their volatility can also be induced by component reactions. These reactions occurring within aerosols are driven by the formation of volatile products and their continuous partitioning into the gas phase. As a consequence, the overall aerosol systems exhibit prominent volatility. Noteworthily, such volatility induced by reactions is a phenomenon exclusively observed in the multiphase aerosol systems, and it is trivial in bulk solutions due to the limited extent of liquid-gas partitioning. Take the chloride depletion in sea salt particles as an example. Recent findings have revealed that chloride depletion can be caused by reactions between NaCl and weak organic acids, which release HCl into the gas phase. Such a reaction can be described as a strong acid displaced by a weak acid, which is hardly observed in bulk phase. Generally, this unique partitioning behavior of aerosol systems and its potential to alter aerosol composition, size, reactivity, and other physicochemical properties merits more attention by atmospheric community.This Account focuses on the recent advancements in the research of component reactions that induce aerosol volatility. These reactions can be categorized into four types: chloride depletion, nitrate depletion, ammonium depletion, and salt hydrolysis. The depletion of chloride or nitrate can be regarded as a displacement reaction, in which a strong acid is displaced by a weak acid. Such a reaction releases highly volatile HCl or HNO3 into the gas phase and leads to a loss of chloride or nitrate within the particles. Likewise, ammonium depletion is a displacement reaction in which a strong base is displaced by a weak base, resulting in release of ammonia and substantial changes in aerosol hygroscopicity. In addition, aerosol volatility can also be induced by salt hydrolysis in a specific case, which is sustained by the coexistence of proton acceptor and hydroxide ion acceptor within particles. Furthermore, we quantitatively discuss these displacement reactions from both thermodynamic and kinetic perspectives, by using the extended aerosol inorganic model (E-AIM) and Maxwell steady-state diffusive mass transfer equation, respectively. Given the ubiquity of component partitioning in aerosol systems, our discussion may provide a new perspective on the underlying mechanisms of aerosol aging and relevant climate effects.

The hygroscopicity and phase state of atmospheric aerosols, depending on relative humidity (RH), dominate their impacts on global climate and heterogeneous atmospheric chemistry. It is challenging to achieve full course measurements on hygroscopic mass growth factors (MGFs), deliquescence relative humidity (DRH), efflorescence relative humidity (ERH) and efflorescence kinetics of aerosols at different ambient RH, which concerns the gas, liquid and solid phases. The combination of a rapid scan vacuum FTIR and a RH pulsed controlling system allowed us to directly and synchronously determine ambient RH and MGFs of aerosols, as well as nucleation rates in the efflorescence process with a subsecond time resolution. With an excellent signal-to-noise ratio and high time resolution spectra, the vacuum FTIR method allowed for real-time in situ measurements of water partitioning between gas and particle phases as it changed with RH, as well as the ratio between the number of crystallized aerosol droplets and the total number of particles during the efflorescence of aerosols. The hygroscopicity and efflorescence kinetics of NaCl and (NH4)2SO4 aerosols were studied in linear RH change mode and in pulsed RH change mode. The measured MGF values, DRH and ERH agreed well with theoretical data from the Extended Aerosol Inorganic Model (EAIM) and literature for both the linear and pulsed RH change modes. In addition, the pulsed RH mode also provided both heterogeneous and homogeneous nucleation rates for NaCl and (NH4)2SO4 aerosols. There were significant advantages of the vacuum FTIR method combined with the pulsed RH controlling technique. First, the RH was continuously changed and was determined in real-time with a time resolution of 0.12 s in the pulsed mode. During the RH change process, the water content in the aerosols was measured synchronously with the same time resolution, allowing for high-efficiency measurements of MGFs, ERH and DRH. The deviations of measured MGFs from the EAIM predictions in the RH range of 75–55% were less than ±3.3%. Second, FTIR spectroscopy was sensitive to phase transitions, which provided quantitative information on the ratio of the number of aerosols transformed from droplets to solid particles at ERH, allowing for the measurement of nucleation kinetics during the efflorescence of aerosols.

Sodium nitrate as an important inorganic component can be chemically formed from the reactions of nitrogen oxides and nitric acid (HNO3) with sea salt in atmosphere. Organic acids contribute a significant fraction of photochemical formed secondary organics that can condense on the preexisting nitrate-containing particles. Atmospheric particles often include a complex mixture of nitrate and secondary organic materials accumulated within the same individual particles. Here we studied the hygroscopicity of aerosol particles composed of sodium nitrate and glutaric acid (GA) by using a pulsed RH controlling system and a rapid scan vacuum FTIR spectrometer (PRHCS-RSVFTIR). The water content in the particles and efflorescence ratios of both NaNO3 and GA at ambient relative humidity (RH) as a function of time were obtained from the rapid-scan infrared spectra with a sub-second time resolution. Our study showed that both NaNO3 and GA crystallized at 44.1% RH during two different RH control processes (stepwise and pulsed processes). It was found that the addition of GA could suppress the efflorescence of NaNO3 during the dehumidifying process. In addition, the mixed NaNO3/GA particles release HNO3 during the dehumidifying and humidifying cycles. These findings are important in further understanding the role of interactions between water-soluble dicarboxylic acids and nitrates on hygroscopicity and environmental effects of atmospheric particles.

Achieving the desired selective transformations of the very stable CO2 into useful chemicals is quite important for the development of economically and environmentally sustainable synthetic methods. Herein, mass spectrometric experiments and quantum-chemical calculations have identified that ScNH+ reacts quite efficiently with CO2 under thermal collision conditions to exclusively yield ScO+ and isocyanic acid (HNCO). This is a novel reaction type in CO2 activation reactions mediated by gas-phase ions. In this reaction, the C═N double bond has also been formed for the first time in the gas phase. The mechanism of "migratory insertion" is proposed. Coupled with the previously reported reaction of Sc+ with NH3, HNCO can be synthesized under mild conditions from NH3 and CO2 in quite simple reactions. The mechanistic information gained in this gas-phase model reaction can offer fundamental insights relevant to corresponding processes and further guide on how to design brand new catalysts.

The air/liquid interface of aqueous methanol solutions, one of the earliest benchmark liquid interfaces ever studied by surface-selective sum frequency generation vibrational spectroscopy (SFG-VS),...

Water-soluble organic acid salts are important components of atmospheric aerosols. Despite their importance, it is still not clear how water-soluble organic acid salts influence interactions between aerosols and water vapor in the atmosphere. In this study, the hygroscopic behaviors and chemical compositions of aerosol particles containing water-soluble organic acid salt ((CH2) n(COONa)2, n = 0, 1, 2) and (NH4)2SO4 were measured using in situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). The ammonium depletion due to release of gaseous NH3 was found in mixed aerosols composed of (CH2) n(COONa)2 ( n = 1, 2) and (NH4)2SO4 upon dehydration. The ammonium loss could modify the aerosol composition, resulting in the formation of corresponding organic acid and monosodium dicarboxylate in mixed particles with high and low (NH4)2SO4 content, respectively. Due to the weaker hydrolysis of oxalate anions, the ammonium depletion was not observed for the Na2C2O4/(NH4)2SO4 mixtures. The changes in the particle composition led to the decreased water uptake upon hydration as compared to that upon dehydration. Our findings reveal that interactions between water-soluble organic acid salts and (NH4)2SO4 in aqueous aerosols may affect the repartition of NH3 between the condensed and gas phases, thus modifying composition and physicochemical properties of aerosols as well as relevant chemical processes.

In this paper, the factors determining the spectroscopic characteristics of vsym–B(OH)4− band including coupling effect, hydrogen bonding effect, and direct contact effect in Li[B(OH)4] solutions are investigated by using ab initio calculation. The coupling effect between the liberations of water and [B(OH)4−] has a larger effect on vsym–B(OH)4− in solvent-shared ion pair (SIP) and monodentate contact ion pair (MCIP), but the smaller effect in bidentate contact ion pair (BCIP). Water molecule tends to hydrate to the middle position between the first sphere of B(OH)4− and outer–sphere of [Li(H2O)4+] and has a different effect on vsym–B(OH)4− in ion pairs. The direct contact effect and polarization effect lead to 19.7 cm−1 red shift of vsym–B(OH)4− in MCIP, and 0.4 cm−1 blue shift in BCIP. The association process in Li[B(OH)4] solution was also introduced by using Raman spectral evolution of vsym –B(OH)4− in the dehydration process.

The hygroscopicity and phase transition of the mixed aerosol particles are significantly dependent upon relative humidity (RH) and interactions between particle components. Although the efflorescence behavior of particles has been studied widely, the crystallization behavior of each component in the particles is still poorly understood. Here, we study the hygroscopicity and crystallization behaviors of internally mixed ammonium sulfate (AS)/glutaric acid (GA) aerosols by a vacuum FTIR spectrometer coupled with a RH-controlling system. The mixed AS/GA aerosols in two different RH control processes (equilibrium and RH pulsed processes) show the fractional crystallization upon dehydration with AS crystallizing prior to GA in mixed particles with varying organic to inorganic molar ratios (OIRs). The initial efflorescence relative humidity (ERH) of AS decreased from ~43% for pure AS particles to ~41%, ~36% and ~34% for mixed AS/GA particles with OIRs of 2:1, 1:1 and 1:2, respectively. Compared to the ERH of 35% for pure GA, the initial ERHs of GA in mixed AS/GA particles were determined to be 31%, 30% and 28% for OIRs of 2:1, 1:1 and 1:2, respectively, indicating that the presence of AS decreased the crystallization RH of GA instead of inducing the heterogeneous nucleation of GA. When the AS fractions first crystallized at around 36% RH in the 1:1 mixed particles, GA remained noncrystalline until 30% RH. For the first time, the crystallization ratios of AS and GA are obtained for the internally mixed particles during the rapid downward RH pulsed process. The crystallization ratio of AS can reach around 100% at around 24% RH for both pure AS and the 1:1 mixed particles, consistent with the equilibrium RH process. It is clear that the RH downward rate did not influence efflorescence behavior of AS in pure AS and AS in mixed particles. In contrast, the crystallization ratio of GA can reach about 90% at 15.4% RH for pure GA particles in excellent agreement with the equilibrium RH process, whereas it is only up to 50% at 16.0% RH in the 1:1 mixed particles during the rapid downward pulsed process lower than that of the equilibrium RH process. Our results reveal that the rapid RH downward rate could inhibit the efflorescence of GA in the mixed droplets.

The hygroscopicity of aerosols is dependent upon their chemical composition. When their chemical compositions are altered, the water content in aerosols often changes, which may further modify phase behaviour. However, the study of phase behaviour dependence on chemical reactions is still limited. In this work, internally mixed sodium pyruvate (SP)/ammonium sulfate (AS) droplets were studied using an in-situ ATR-FTIR spectrometer. FTIR spectral analysis showed that solid sodium sulfate (SS) formed during the dehydration process, indicating a chemical reaction between SP and AS. In addition, the water content decreased after a dehydration-hydration process despite organic salt (SS) to inorganic salt (AS) mole ratios (OIRs) During the second relative humidity (RH) cycle, the water content remained constant, however, the efflorescence relative humidity (ERH) was lower than that in the first dehydration. The crystal relative humidities (CRHs) of SS are 66.7–53.1%, 66.0–58.2%, 62.2–57.1% and 49.6–43.6% for OIRs of 3:1, 2:1, 1:1 and 1:3, respectively, suggesting the crystallization of SS was favoured by higher SP content. For 2:1 OIRs, the solid SS was the greatest and an excess of either SP or AS blocked the solid SS formation. At a constant 80% RH, depletion of reagents was ∼0.97, and water loss was ∼0.6 in ∼40 min. After 90 min, solid SS formed. The chemical reaction was faster than water loss; furthermore, water loss from the chemical reaction led to solid SS above the ERH of pure SS particles (∼75% RH). When the RH changed rapidly, the reaction was slow and solid SS decreased.

As an important component of atmospheric aerosols, water is profoundly related with aerosol hygroscopicity and provides a medium for atmospheric heterogeneous reactions. The quantitative analysis of water content in aerosol droplets is instrumental to understanding atmospheric chemistry, as well as to addressing the related environmental issues, such as air pollution and climate change. Fourier transform infrared (FTIR) spectroscopy has been widely adopted to quantify the amount of water content in atmospheric aerosols, which is based on the absorbance of OH functional group in proportion to water content. However, in the OH stretching vibration band around 3400 cm−1, spectral distortions may occur, making a quantitative analysis impossible. In addressing this issue, here we put forth a model to simulate the FTIR absorption of hemispherical water droplets, along with a quantitative description of the spectral distortion. Our model prediction was benchmarked with the microscopic-FTIR experiments conducted on sodium sulfate droplets, and good agreements between theoretical and experimental results were found. We observed that the absorbance spanning across the mid-wavenumber infrared region increases with water absorption coefficients; while such an increasing trend was not seen in the 3400 cm−1 band. We speculate that the spectral saturated absorption is related to the absorption coefficient of water and the ratio of the projected area of droplets to the aperture area. In addition, the effects of droplet size and number density on the absorption spectra were investigated. The waveband range of the saturated absorption broadens with an increase in droplet radius. On the other hand, as the number density of water droplets increases, the absorption at 3400 cm−1 is enhanced, and the characteristic peak of condensed water becomes increasingly sharper, asymptotic to the typical infrared spectra of water collected by the pressing method.

The formation mechanism of aerosol sulfate during wintertime haze events in China is still largely unknown. As companions, SO2 and transition metals are mainly emitted from coal combustion. Here, we argue that the transition metal-catalyzed oxidation of SO2 on aerosol surfaces could be the dominant sulfate formation pathway and investigate this hypothesis by integrating chamber experiments, numerical simulations and in-field observations. Our analysis shows that the contribution of the manganese-catalyzed oxidation of SO2 on aerosol surfaces is approximately one to two orders of magnitude larger than previously known routes, and contributes 69.2% ± 5.0% of the particulate sulfur production during haze events. This formation pathway could explain the missing source of sulfate and improve the understanding of atmospheric chemistry and climate change., Sulfate aerosols are an important component of wintertime haze events in China, but their production mechanisms are not well known. Here, the authors show that transition metal-catalyzed oxidation of SO2 on aerosol surfaces could be the dominant sulfate formation pathway in Northern China.

The Hofmeister effect of inorganic ions to precipitate proteins has been used to understand the coagulation phenomenon in colloid and protein science. Herein, for the first time, this effect is studied on the hygroscopicity of aerosols using ATR-FTIR spectroscopy. The representative Hofmeister salts (MgSO4, KCl, NH4NO3) and amino acid (glycine) with different amino acid/salt molar ratios (ASRs) are mixed and atomized into micrometer-sized particles. For mixed kosmotrope (MgSO4)/glycine and chaotrope (NH4NO3)/glycine with an ASR of 1:1, both ERHs (efflorescence relative humidities) and DRHs (deliquescence relative humidities) are absent. However, for the mixtures of glycine and neutral salt (KCl), no DRH is observed while 66.2 and 61.4% ERH of glycine is detected for mixtures with ASRs of 1:1 and 1:3, respectively, which is similar to pure glycine. For the mixture of NH4NO3/glycine with an ASR of 1:3, ERH and DRH are found to be 15.4 and 32.2% RH, less than that of pure NH4NO3. Further, interactions between glycine-salt and/or water is also studied in the mixtures during hydration and dehydration. Water-mediated ion-glycine interaction is detected based on the two glycine bands merging into one band. Glycine-SO42- interaction is present for glycine/sulfate in all ASRs, while glycine-NO3- interaction is only seen for 1:3 glycine/NH4NO3 mixtures during hydration. This work opens a window to understand the Hofmeister effect on the hygroscopicity of atmospheric aerosols.

Atmospheric aerosols consisting of organic and inorganic components may undergo liquid-liquid phase separation (LLPS) and liquid-solid phase transitions during ambient relative humidity (RH) fluctuation. However, the knowledge of dynamic phase evolution processes for mixed organic-inorganic particles is scarce. Here we present a universal and visualized observation on LLPS, efflorescence and deliquescence transitions as well as hygroscopic growth of mixed 1, 2, 6-hexanetriol/ammonium sulfate (AS) particles with different organic-inorganic mole ratios (OIR = 1:4, 1:2, 1:1, 2:1 and 4:1) with the high time resolution (0.5 s), using an optical microscope with a video camera. The optical images suggest that an inner AS solution phase is surrounded by an outer organic-rich phase after LLPS for all mixed particles. The LLPS mechanism for particles with different OIRs differs, meanwhile, multiple mechanisms may dominate successively in individual particles with a certain OIR, somewhat inconsistent with earlier observations by literature. More importantly, another phase separation in inner AS solution phase, defined as secondary LLPS here, is observed for OIR = 1:1, 1:2 and 1:4 particles. The secondary LLPS may be attributed to the formation of more concentrated AS inclusions in the inner phase, and becomes more obvious with decreasing RH and increasing AS mole fraction. Furthermore, the changes in size and amount of AS inclusions during LLPS are quantitatively characterized, which further illustrate the equilibrium partitioning process of organic and inorganic components. The experimental results have significant implications for revelation of complex phase transitions of internally mixed atmospheric particles and evaluation of liquid-liquid and liquid-solid equilibria in thermodynamic models.

Tropospheric aerosols are usually complex mixtures of inorganic and organic components, which show non-ideal behavior in hygroscopicity, mass transfer, and partitioning between gas and aerosols. In this study, we applied a novel approach based on a combination of a pulse RH controlling system and a rapid scan vacuum FTIR spectrometer to investigate the mass transfer limit of magnesium sulfate/glutaric acid (GA) mixture aerosol particles. The liquid water band area of the aerosols is used to reveal the mass transfer limit during the rapid pulse RH downward and upward processes. Partitioning equilibrium between the aerosol particles and water gas phase is observed at the higher RH range (73-50%). When the RH is lower than 40%, there is a hysteresis for the liquid water content changing with the RH, indicating the limited water mass transfer in the aerosols.

The heterogeneous reactions of sea salt single droplets with the mixture of O

An increasing amount of research has investigated whether direct contact ion pairs (CIP) exist in magnesium nitrate solutions. In this work, the relationship between the concentration and microstructure, as well as the details of the ion pair structure in magnesium nitrate solutions were studied by Raman spectroscopy, molecular dynamics (MD) simulations, and density functional theory (DFT) calculations. Component analysis showed that solvent-shared ion pairs (SIPs) and free hydrated ions were the dominant species in dilute solution. SIPs gradually transformed into contact ion pairs as the concentration increased. Complex structures and CIPs were the main species when WSR

Model of aerosols and cloud formation processes requires extensive data on hygroscopic properties of water soluble organic species at lower troposphere relevant temperatures, which are ubiquitously found in atmospheric aerosols. Recent field studies (Kerminen et al., 1998) have revealed that sea salt aerosols can be efficiently chemically processed by water soluble carboxylic acids and lead to chloride depletion and formation of carboxylic acid salts. In this study, we report hygroscopic properties of four carboxylic acid salts, namely, sodium formate, sodium acetate, sodium oxalate and sodium succinate at temperatures commonly encountered in the lower troposphere ranging from 295.8 K to 270.0 K for the first time via attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). Deliquescence relative humidity (DRH) and efflorescence relative humidity (ERH) of these carboxylic salts as a function of temperature were derived using both changes in water-to-solute (WSR) data and changes in band positions and shapes of carboxylate asymmetric and symmetric stretch modes. Hygroscopic data were also fitted by polynomial and corresponding coefficients were obtained. Our results have shown that these carboxylic acid salts exhibit strong temperature dependence and they would deliquesce at a lower humidity and are more likely to remain deliquesced when they are elevated in the troposphere. Since all these carboxylic acid salts present very different hygroscopic behaviors at lower temperatures from that of NaCl, chemically processed sea salt aerosols by carboxylic acids are expected to have fairly different phase state, size, and water content, consequently affecting their reactivity, lifetime, and ability to serve as cloud condensation nuclei.

Rate constants for the reactions of ozone with 1-octen-3-ol, 1-nonen-3-ol and 1-nonen-4-ol have been determined at 298±1K and atmospheric pressure for the first time. The experiments were performed in a 100-L FEP Teflon film bag using absolute rate method; the rate constants were (1.91±0.19)×10-17, (1.89±0.20)×10-17, and (0.83±0.08)×10-17cm3/(molecule·sec) for 1-octen-3-ol, 1-nonen-3-ol, and 1-nonen-4-ol, respectively. The rate constants have been compared with those of unsaturated alcohols structural homologs, and used to estimate the reaction reactivity. The electronegativity of carbon-carbon double bond was calculated by atomic charges analysis. The calculated results show that the electronic effect of the lone pair electrons of hydroxyl oxygen is the main cause of the difference in rate coefficient. According to the obtained rate constants, the atmospheric lifetimes of studied unsaturated alcohols were also estimated, which indicates that the reaction with ozone is an important loss pathway in the atmosphere, especially in polluted areas.

Although the dicarboxylic acid has been reported to react with nitrate for aged internally mixed aerosols in atmosphere, the quantitative nitrate depletion dependent upon composition in particles is still not well constrained. The chemical composition evolutions for malonic acid/sodium nitrate (MA/SN), malonic acid/magnesium nitrate (MA/MN) and malonic acid/calcium nitrate (MA/CN) particles with the organic to inorganic molar ratio (OIR) of 1:1 are investigated by vacuum Fourier transform infrared spectroscopy (FTIR). Upon dehydration, the intensity of the asymmetric stretching mode of COO– group (νas-COO−) increases, accompanying the decrease in OH feather band and COOH band and NO3− band. These band changes suggest malonate salts formation and HNO3 release. The quantitative NO3− depletion data shows that the reactivity of MA-MN is most and that of MA-SN is least. Analysis of the stretching mode of COO– indicates the different bond type between metal cation and carboxylate anion. In addition, water content in particles decreases at the constant RH, implying water loss with the chemical reaction. When the RH changes very quickly, water uptake delay during the humidification process reveals that water mass transport is limited below 37% RH.

The confocal Raman microscopy is utilized to investigate the liquid-liquid phase separation (LLPS) of mixed magnesium sulfate/glutaric acid (MgSO4/GA) droplets deposited on a hydrophobic polytetrafluoroethylene (PTFE) substrate and a hydrophilic quartz substrate. Raman spectra collected from different regions of the mixed droplets provide detailed information of component distributions for MgSO4 and GA. During the dehydration process, the MgSO4/GA mixed particles show the initial liquid-liquid phase separation between 85% and 80% relative humidity (RH) on both the hydrophobic and hydrophilic substrates. For the droplets deposited on the two substrates, the inner phase of droplets is dominated by aqueous MgSO4, which is surrounded by a rich GA organic layer due to the surface tension effects. In addition, the crystallization of GA could be observed in the organic aqueous phase while it is inhibited in the inner MgSO4 phase due to the effects of gel formation of MgSO4 at low RH. The Raman spectra reveal that with decreasing RH the morphology of the mixed droplet evolves from a uniform droplet to the structure of LLPS with the GA crystallizing in the outer layer and MgSO4 gel formed in the inner phase. These findings contribute to the further understanding of the role of interactions between inorganic salts and organic acids on the morphological evolution and environmental effects of atmospheric aerosols under ambient RH conditions.

Sulfate is one of the most important components in the aerosol due to its key role in air pollution and global climate change. Recent work has suggested that reactive nitrogen chemistry in aqueous water can explain the missing source of sulfate in the aqueous water. Herein, we have mapped out the energy profile of the oxidization process of SO2 leading from NO2 and two feasible three-step mechanisms have been proposed. For the oxidation of HOSO2− and HSO3− by the dissolved NO2 in weakly acidic and neutral aerosol (pH ≤ 7), the main contribution to the missing sulfate production comes from the oxidation of HOSO2−. The whole process is a self-sustaining process. For the oxidation of SO32− in alkaline aerosol (pH > 7), the third step - decomposition step of H2O or hydrolysis of SO3 step which are two parallel processes are the rate-limiting steps. The present results are of avail to better understand the missing source of sulfate in the aerosol and hence may lead to better science-based solutions for resolving the severe haze problems in China.

Internal mixed aerosol particles in atmosphere often take the drastic physical property changes owing to interactions between compositions. Hygroscopicity and phase transition behavior of mixed particles composed of glycine and NaNO 3 with different organic to inorganic molar ratios (OIRs) have been investigated by using Fourier transform infrared attenuated total reflection spectroscopy (ATR-FTIR). It is found that pure glycine particles effloresce when the relative humidity (RH) is ∼64.5% and do not deliquesce on humidifying. The efflorescence and deliquescence point for pure NaNO 3 particles are ∼54% RH and ∼91.9% RH, respectively. For the internal mixed glycine/NaNO 3 particles, there exhibit various hygroscopicities and solid-aqueous phase transitions dependent upon the OIR. When the OIRs are 2:1 and 1:1, NaNO 3 particles cannot crystallize. When the OIRs are 1:2, 1:4 and 1:8, crystalline ratios are ~31%, ~45% and 72%, NaNO 3 effloresce at ∼44.7% RH, ~35.2% RH and ~35% RH, respectively. For all studied mixture particles, glycine can crystallize. For OIRs of 2:1, 1:1, 1:2, 1:4 and 1:8, the crystalline fractions of glycine are ~78%, 89%, 93%, 94% and 95%, respectively, corresponding to the ERH range of 74% − 42%, 73% − 35.9%, 65.5% − 19.2%, 35.2% − 8.1% and 35% − 3.8%. It shows that both NaNO 3 and glycine crystals are favored by more enriched NaNO 3 . On hydration, the DRHs of NaNO 3 for mixtures with OIR of 1:2, 1:4 and 1:8 are ~87.4%, ~87% and 85.4%, respectively. Glycine in mixtures deliquesces at ~99% (incomplete deliquescence), ~98.3%, ~97.3%, ~93.3%, ~85.4% RH, respectively, with decreasing OIR. The results suggest the positive effect of NaNO 3 on the deliquescence of glycine and NaNO 3 .

In recent years, brown carbon aerosols, as important contributors to light absorption and climate forcing by aerosols, have been forefront in the field of atmospheric research. Aqueous brown carbon can be formed through the aqueous reaction of glyoxal (GX) with glycine (Gly). GX-Gly mixtures exhibit changes in their optical properties in the ultraviolet and near visible regions, which can be monitored with ultraviolet/visible and fluorescence spectroscopy. In this study, we quantified the absorption and excitation-emission matrix spectra during the formation of aqueous brown carbon, which was generated from GX-Gly mixtures. The formation of brown carbon was further evidenced using several optical parameters, including absorption coefficient, absorption Angstrom exponents, mass absorption coefficient, effective quantum yields and fluorescence lifetime values. The results of hydrogen peroxide oxidation photolysis revealed the probable removal processes of the atmospheric aqueous brown carbon. The fluorescence lifetime values of the brown carbon samples were less than 10 ns. Liquid chromatography combined with mass spectrometry analysis was used to investigate the probable chemical composition of the brown carbon samples from GX-Gly mixtures.

Despite increased awareness of the roles played by atmospheric amines and carboxylic acids in an atmospheric environment, current knowledge of heterogeneous reactions between atmospheric amines and carboxylic acids, especially how the reaction rates vary with different ambient conditions including temperature, relative humidity (RH), and reactant concentration, remains very scarce. Here, the kinetics of the heterogeneous reaction of n-butylamine (BA) with succinic acid (SA) were studied via a flow system combined with attenuated total reflection Fourier-transformed infrared spectroscopy (ATR-FTIR) over a wide range of temperatures (263-295 K), BA concentrations (0.145-32.5 ppm), thin film thicknesses (0.05-0.15 μm), and RHs (0-75%) under atmospheric pressure conditions for the first time. Pseudo-first-order rate constant (kapp) and overall reactive uptake coefficient (γ) values were derived according to the changes in absorbance from the peak located near 1634 cm-1, which can be assigned to the -COO- antisymmetric stretch (νas(-COO-)). The results show that both the kapp and γ values display very strong temperature dependence, and low temperatures promote the reaction. According to the kapp values as a function of temperature, the activation energy for the heterogeneous reaction is estimated to be -71.9 kJ mol-1. Such a phenomenon indicates that physisorption of BA at the surface of the SA thin film is probably the rate-limiting step for the overall reaction. Meanwhile, the heterogeneous reaction of SA with BA follows the Langmuir-Hinshelwood mechanism, which confirms that kapp is largely dominated by the surface reaction over the bulk phase reaction. In addition, with increasing RH, both the kapp and γ values increase considerably, indicating that the presence of water vapor has a synergistic effect on the reaction. Water uptake results also show that the hygroscopic behavior of the thin film is greatly enhanced after BA exposure.

Heterogeneous reactions play an essential role in the research area of formation processes of secondary aerosols. In this study, heterogeneous reactions of NH3/SO2/NO2 with α-Fe2O3 particles at different relative humidities (RHs) were investigated by using a gas-flow system combined with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The results show that the presence of NH3 leads to the rapid conversion of SO2 to sulfate as a result of the proton transfer function and neutralization on the α-Fe2O3 particles in dry condition. NH3 can also promote the hydrolysis of NO2 dimers to HONO formation and result in the formation of more sulfate on the particle surface in humid conditions. On the other hand, water vapor plays both positive and negative roles in the uptake of SO2 during the reaction processes. When the RH increases from 0% to 62%, the formation rate of sulfate and uptake coefficient for SO2 in the initial stage increase about 3-fold. However, due to the inhibition effect of water layer between trace gases and active surface, the above values of reaction kinetic decrease about 21% when the RH increases from 62% to 72%. Our results may provide insight into the effects of NH3 and RH for the converting SO2 into sulfate on α-Fe2O3 particles, which supply basic data for the formation of secondary particles in field studies of atmospheric environment.