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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.

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.

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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.

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.

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.

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.

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.

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 .

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 ammonium salts and organic salts are the important components of atmospheric aerosols. The reaction between the two salts will cause the release of NH3, hence, affecting the budget of NH3 in the atmosphere. Furthermore, the equilibrium partitioning of NH3 in the gas and particle phases plays a key role in the evolution of chemical composition and size of fine atmospheric particles. In this work, the chemical composition evolution processes of mixed particles consisting of four different organic acid salts (sodium oxalate (SO), sodium malonate (SM), sodium succinate (SS), sodium pyruvate (SP)) with ammonium nitrate (AN) were measured by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). The release of NH3 were gained by monitoring the changes in the absorbance of NH4 + in mixed organic acid salts/AN particles under 70% relative humidity (RH). After 127 minutes, the remaining ammonium content in the four mixed aerosols was 84.2%, 71.6%, 57.1%, and 28.7% for SO, SM, SS, and SP mixed with AN, respectively, which is lower than that in pure AN aerosol (97%). This study may help to further understand the conversion process between organic salts and AN in the atmosphere, as well as the NH3 cycles in the atmosphere.

The efflorescence transitions of aerosol particles have been intensively investigated due to their critical impacts on global climate and atmospheric chemistry. In the present study, we present a critical review of efflorescence kinetics focusing on three key issues: the efflorescence relative humidity (ERH) and the influence factors for aerosol ERH (e.g. particle sizes, and temperature); efflorescence processes of mixed aerosols, concerning the effect of coexisting inorganic and organic components on the efflorescence of inorganic salts; homogeneous and heterogeneous nucleation rates of pure and mixed aerosols. Among the previous studies, there are significant discrepancies for measured aerosol ERH under even the same conditions. Moreover, the interactions between organic and inorganic components remain largely unclear, causing efflorescence transition behaviours and chemical composition evolutions of certain mixed systems to be debatable. Thus, it is important to better understand efflorescence to gain insights into the physicochemical properties and characterize observed efflorescence characteristics of atmospheric particles, as well as guide further studies on aerosol hygroscopicity and reactivity.

The chemical compositions in atmospheric aerosols, which often evolve with environmental factors, have significant impact on climate and human health, while our fundamental understanding of chemical process is limited owing to their sensitive to atmospheric conditions. pH and RH are critical chemical factors of aerosols, impacting reaction pathways and kinetics that ultimately govern final components in particles. Herein, we monitored the chemical composition in internally mixed malonic acid/calcium nitrate with the mole ratio of 1:1 as a function of pH and relative humidity (RH). At 30% RH, lower than efflorescence relative humidity (ERH) of pure malonic acid aerosols, malonic acid still exhibits solution feature reflected by IR spectra, which was observed to transform to malonate, along with water loss and nitrate depletion. At another RH of 54% and 80%, the similar chemical process happened with less reaction rate. The response of chemical reaction between malonic acid and calcium nitrate to pH was studied by manipulating the starting pH of the bulk solution through dropping aqueous sodium hydroxide. Due to lower H+ concentration at higher pH, the formation and liberation of HNO3 slow down, as well as water loss. After a down-up RH cycle, the water loss was obvious and grew with the decrease in pH. These measurements are improving our understanding of chemical composition evolution dependent upon pH and RH from a fundamental physical chemistry perspective and are critical for connecting chemistry and climate.

In this research, we applied a pulsed RH controlling system and a rapid scan vacuum FTIR spectrometer (PRHCS-RSVFTIR) to investigate hygroscopicity of internally mixed (NH4)2SO4(AS)/citric acid (CA) particles. The water content and efflorescence ratio of AS in the particles and ambient relative humidity (RH) as a function of time were obtained with a subsecond time resolution. The hygroscopic behavior of AS aerosols in two different RH control processes (equilibrium and RH pulsed processes) showed that AS droplets crystallize with RH ranging from 42% to 26.5%. It was found that the half-life time ratio between the water content in the CA particles and the gas phase under RH pulsed change was greater than one under low RH conditions (

Secondary organic aerosols (SOA) can exist in a glassy or semisolid state under low relative humidity (RH) conditions, in which the particles show nonequilibrium kinetic characteristics with changing ambient RH. Here, we selected internally mixed sucrose/NaNO3 droplets with organic to inorganic molar ratios (OIRs) of 1:8, 1:4, 1:2, and 1:1 as a proxy for multicomponent ambient aerosols to study crystal nucleation and growth processes and water transport under a highly viscous state with the combination of an RH-controlling system and a vacuum Fourier transform infrared (FTIR) spectrometer. The initial efflorescence RH (ERH) of NaNO3 decreased from ∼45% for pure NaNO3 droplets to ∼38.6 and ∼37.9% for the 1:8 and 1:4 sucrose/NaNO3 droplets, respectively, while no crystallization of NaNO3 occurred for the 1:2 and 1:1 droplets in the whole RH range. Thus, the addition of sucrose delayed the ERH and even completely inhibited nucleation of NaNO3 in the mixed droplets. In addition, the crystal growth of NaNO3 wa...

Heterogeneous reactions of SO 2 /O 3 and SO 2 /NO 2 with α-Al 2 O 3 particles at different RHs were investigated using a gas-flow system combined with microscopic Fourier transform infrared (micro-FTIR) spectrometer. The results show that the trace gas O 3 or NO 2 leads to rapid conversion of SO 2 to sulfate on the surface of α-Al 2 O 3 particles in initial stage and then conversion rate decreases in the following stages. The rate of sulfate formation and uptake coefficient (γ) for SO 2 in the two systems as a function of relative humidity (RH) are determined for the first time, which are all strongly enhanced more than seven-fold as the RH increase from 15% to 95% in initial stage for SO 2 /O 3 and SO 2 /NO 2 . Moreover, the γ in the system of SO 2 /O 3 on α-Al 2 O 3 particles is more than 3-fold than that of SO 2 /NO 2 for the similar RH condition. Our results may be broadly applicable to understand the effects of RH and trace gases (e.g., O 3 , NO 2 ) for the converting SO 2 into sulfate on the mineral dust, which supply basic data for atmospheric chemistry modeling studies.

The heterogeneous reactions of soot particles with O 3 and the mixture of O 3 and SO 2 were studied as a function of relative humidities (RHs). The reactions were followed in real time using microscopic Fourier transform infrared (micro-FTIR) spectrometer to obtain kinetic data. The results show that the ketone (C O) group is the main product of the O 3 /soot reaction, and the sulfate is identified on the surface of soot particles in the presence of O 3 /SO 2 . Both reactions are sensitive to RHs and surrounding water significantly promotes the proceeding of the heterogeneous reactions. For the O 3 /soot reaction, the pseudo-first-order rate constant increases from 3.2 × 10 −4 s −1 to 7.1 × 10 −4 s −1 with increasing RH in the range of 1%–82%. When O 3 and SO 2 exist simultaneously during the reaction, the reaction rate and uptake coefficient are all enhanced by about an order of magnitude as the RH increases from 1% to 83%. The high productions of the ketone and sulfate on soot surface are of highly hydrophilic, which play a key role in environmental effect under humid environment. The possible reaction mechanism speculates that products of aromatic carbonyls and dihydrofuran species on soot particles will be more harmful to human health.

A novel approach based on a combination of a pulse relative humidity (RH) controlling system and a rapid scan vacuum FTIR spectrometer was utilized to investigate the hygroscopic property and phase transition of malonic acid (MA) aerosols. By using this approach, both water vapor amount around the aerosols and water content within aerosols with sub-second time resolution were obtained. Based on the features of FTIR absorbing bands, it can be known that the evolution of hydrogen-bonding structures of malonic acid aerosols took place from (H 2 O) n - MA to MA-MA accompanying with phase transition in the dehumidifying process. And in present paper, the stepwise efflorescence of MA aerosols and nucleation rates at different RHs are first reported. Our observation has shown that the efflorescence of MA started at ∼17% RH and the nucleation rates increased with decreasing RH.

Energetic salts provide many advantages over conventional energetic molecular compounds. Their high densities, high heats of formation and low vapor pressures make them an important class of compounds for the development of energetic materials. More and more novel energetic ions are being obtained, but energetic cations are reported to a much lesser extent. Energetic cations develop from simple linear structures to form N-heterocyclic structures. Together with the introduction of various energetic groups, the energetic properties of energetic salts are significantly promoted. The development of energetic cations is becoming the major restriction for improving the performance of energetic salts.

Ozone initiated heterogeneous oxidation of micron-sized oleic acid (OA), linoleic acid (LA), and linolenic acid (LOA) single droplets was investigated using a gas-flow system combined with microscopic Fourier transform infrared (micro-FTIR) spectrometer. The pseudo-first-order rate constant (kapp) and the overall uptake coefficient (γ) are obtained by quantitatively estimating the changes in absorbance area of the CO stretching band at 1710 cm−1, which is assigned to the carboxyl group of the reactant. The overall kinetics is dominated by surface reaction. And the effect of surface adsorption, which is derived from the ozone concentration and particle size effects on reaction kinetics, plays an important role during the reaction. Comparison of the kapp values corresponding to OA, LA and LOA shows the positive correlation between double bonds and reaction rate. In the view of RH effect, both kapp and γ are strongly enhanced by over a factor of three for the LOA/O3 reaction system as the relative humidity (RH) increases from ∼0% to 83%. The LA/O3 reaction system exhibits a weaker RH dependence. In contrast, the kapp and γ of the OA/O3 reaction system are independent of the RH changes. Moreover, the various hygroscopicities of the three acids and corresponding products lead to different reactivities.

The heterogeneous reactions of α-Al

The deliquescence behavior of atmospheric aerosols has significant effects on global climate and atmospheric heterogeneous chemistry but remains largely unclear. The deliquescence kinetics data of micron-sized particles are scarce owing to the difficulty on performing the time-resolved dissolution measurements. In view of this technique bottleneck, an applicable and powerful experimental technique, i. e., vacuum FTIR combining pulsed relative humidity (RH) change technique, is introduced for gaining deliquescence kinetics information of three inorganic salts. For NaCl and (NH4)2SO4 aerosols, a solid-liquid mixing state derived from partial dissolution of NaCl and (NH4)2SO4 crystals is present during deliquescence, and the recrystallization will occur once RH decreases. While for NaNO3 particles, the recrystallization cannot occur as RH decreases owing to the formed amorphous NaNO3 solids after dying. The dissolution rates of NaCl, (NH4)2SO4 and NaNO3 solid particles are calculated, as a first attempt, by the upward pulsed RH mode. The measured rates show a significant dependency on ambient RH with three orders of magnitude. For NaCl particles, the measured J values range from 1.41 × 10−4 to 7.67 × 10−1 s−1 at RH of 73.41–75.15%. The J for (NH4)2SO4 particles is 7.34 × 10−3 to 2.46 × 100 s−1 over the RH range of 77.27%–80.13%. The J values for amorphous NaNO3 solids range from 6.01 × 10−3 to 2.63 × 100 s−1 as RH increases from 71.15% to 73.84%. Our results fill in the dataset of atmospheric models describing the kinetics features of deliquescence and provide an insight into dynamic solid-solution transition for PM2.5 particles.

The efflorescence of mixed Na2SO4/glycerol aerosols on the ZnSe substrate with various mole ratios (Na2SO4/glycerol = 1:1, 1:2, 1:4) has been studied in the relative humidity (RH) linearly decline process, using a situ Fourier transform infrared attenuated total reflection (FTIR-ATR) technique. The crystal ratio at a given RH can be gained by the absorbance of the band at 1132 cm−1, which shows the incomplete nucleation for mixed Na2SO4/glycerol aerosols and the decreased amount of the droplets crystallized at the lowest RH with the glycerol increase. Using the volume fraction of droplets that have yet to crystallize, the heterogeneous nucleation kinetics has been gained. By the Extended Aerosol Inorganics Model (E-AIM), the nucleation rate as the function of solute saturation degree has been gained for various mixed Na2SO4/glycerol aerosols.

In Poria cocos, triterpene compound and polysaccharides are the main compositions. The heteropolysaccharide was identified as a linear chain of β-(1→3)-d-glucan, which has the strong water absorbing capacity. In order to investigate the effect of water on the structure of Poria cocos, which belongs to a kind of Polyporaceae, the Micro-Fourier transform infrared spectroscopy (micro-FTIR) technique has been employed with the ambient relative humidity (RH) increasing. The gained IR spectra are measured and analyzed in detail. Because of strong overlaps between some bands, the differential spectra and band decompositions have been applied to analyze the structural change. IR spectra show the transformation of hydrogen bonds with the RH. The blue shift of the CH2 bending vibration from 1417 to 1424cm(-1) with the increase in RH means that the hydrogen bonds are formed between CH2 groups and water molecules at lower RH and some transferred to weak hydrogen bonds. The further study suggests that the C1OH, C2OH, and C3OH groups from polysaccharide bond formed C1OH⋯H2O, C2OH⋯H2O, and C3OH⋯H2O steady modes, respectively, with water molecules.

FTIR-ATR technology is used to study the efflorescence kinetic of Na2SO4 and mixed Na2SO4/CTAB aerosols. As the RH decreased linearly, the ν3 SO42− band shifts from 1094 cm−1 to 1132 cm−1, suggesting the phase transition of Na2SO4 from solution to crystal phase (III). For pure Na2SO4 aerosols, the ERH is 75.1% RH, whereas the efflorescence point of mixed Na2SO4/CTAB aerosols (74.2%) is lower. By further analysis of IR differential spectra, the ratio of Na2SO4 crystals in mixed aerosols is only 62.7% and the heterogeneous nucleation rate of Na2SO4 in Na2SO4/CTAB mixed aerosols is lower than that in pure Na2SO4 aerosols. They showed that CTAB assembled into reversed micelle and part Na2SO4 droplets are in the core to form core-shell structure, and CTAB shell prevents core Na2SO4 solutions from crystallizing. However, the counter ion Br− for CTAB reversed micelle can interact with Na+ ions, which decreases the crystallization rate of free Na2SO4 droplets and ERH is delayed.

The in situ infrared spectra of sodium nitrate (NaNO3) and mixed NaNO3/glycerol droplets with organic to inorganic molar ratio (OIR) of 1:8, 1:4, 1:2, 1:1, and 2:1 on the ZnSe substrate were collected using the Fourier transform infrared attenuated total reflection (FTIR-ATR) technique in the RH linearly decreasing process. When the efflorescence process occurred in the RH decreasing process, the stochastric transformation from NaNO3 droplets to NaNO3 solid particles resulted in gradually increasing of a new band at 836 cm(-1) and contineously decreasing of an initial band at 829 cm(-1), which were assigned to the v2-NO3(-) mode in crystal phase state and in liquid state, respectively. There were excellent isobesic points between the two bands in the transformation processes, indicating the synchronization between the disappearence of NO3(-) in solutions and the production of NaNO3 crystal. The nucleation ratio, i.e., the amount of the droplets crystallized at a given RH upon the total amount droplets, was obtained by using the absorbance of ν2-NO3(-) band at 836 cm(-1), which was used to calculate the nucleation rates of NaNO3 either for heterogeneous or for homogeneous nucleation process. While the glycerol molecules delayed the efflorescence RHs (ERH) of NaNO3 in the mixed NaNO3/glycerol droplets (OIR = 2:1) to 15%, greatly lower than the ERH for pure NaNO3 droplets at 62.5%, they also greatly suppressed the heterogeneous nucleation rate with increase of the OIR ratio. Two different kinetic mechanisms were suggested in the mixed droplets with OIR = 1:8, 1:4, 1:2, and 1:1, i.e., homogeneous nucleation at higher supersaturation and heterogeneous nucleation at lower supersaturation. For the mixed droplets with 2:1 OIR, they fell into the homogeneous nucleation region completely.

Although nitrates and amino acids are ubiquitous in the atmosphere, the interactions of amino acids with nitrates on the hygroscopicity of aerosols are still not clear. We studied the hygroscopic properties and compositional evolution of three atmospherically relevant species, sodium nitrate (NaNO3), glycine and L-alanine (alanine). The investigations on the NaNO3, glycine, alanine and their internally NaNO3/amino acid mixed particles with molar ratios of 3:1, 1:1 and 1:3 were conducted using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). The mixed NaNO3/glycine droplets with molar ratios of 3:1, 1:1 and 1:3 exhibited efflorescence transition at 52.3%, 62.6% and 63.2%RH, respectively. For the NaNO3/alanine mixed systems with molar ratios of 3:1, 1:1 and 1:3, the efflorescence relative humidity (ERH) was measured to be 50.2%, 59.8% and 69.3%, respectively. These results suggested that in each of the systems studied the addition of amino acids to aerosols could promote the crystallization of mixed droplets. However, the higher deliquescence relative humidity (DRH) with increased fraction of glycine for the mixed NaNO3/glycine (3:1, 79.2%RH; 1:1, 86.3%RH; 1:3, 86.5%RH) also indicated that glycine could inhibit the deliquescence phase transition of mixed systems. In the case of NaNO3/alanine systems, the DRH for 3:1 and 1:3 mixed particles was 78.2% and 82.7% while no deliquescence transition occurred for the 1:1 mixed particles. The hygroscopic properties of mixed NaNO3/amino acids droplets were modified due to the formed amino acid-NaNO3 complex salts from the interactions of amino acids with NaNO3 in aerosols during dehydration process. The discrepancies in water content for the mixed NaNO3/amino acids with molar ratios of 1:1 and 1:3 between dehydration and hydration processes at high RH could be attributed to the crystallization of amino acid-NaNO3 complex salts during dehydration and the residual amino acids.

As a well-known Criegee intermediate (CI) scavenger, SO2 can react with CIs in the gas phase to form sulphuric acid, which can be further converted to sulfate aerosols, which make an important contribution to haze formation. Despite the potential importance, the interaction of SO2 with the CIs produced by heterogeneous oxidation of unsaturated fatty acids remains largely unclear. In this work, the heterogeneous reactions of oleic acid (OA) thin films with O3 and O3/SO2 mixture gases are studied under four different relative humidities (RHs): 5%, 30%, 60% and 80%, using microscopic Fourier transform infrared (micro-FTIR) spectrometer combined with a gas-flow system. A detailed comparison on formation rates of ester and intensity ratios of 1742 cm−1 band (envelope C O of ester) to 1712 cm−1 band (feature C O of carboxyl) between the two reaction systems shows that the addition of SO2 inhibits the conversion of carboxyl to ester at any RH. As the SO2 concentration increases, the formation rates and final relative amount of ester decrease. Based on these data, it can be concluded that SO2 consumes part condensed-phase CIs produced by ozonolysis of OA, resulting in a significant inhibition in ester formation. The present results provide an important atmospheric implication on the reaction between SO2 and condensed-phase CIs, which may be a missing sulfate production pathway in the atmosphere.

The heterogeneous reactions of α-Al2O3 particles with a mixture of ozone (∼50 ppm) and isoprene (∼50 ppm) were studied as a function of relative humidities (RHs). The reactions were monitored in real time through the microscopic Fourier transform infrared (micro-FTIR) spectrometer. The results show that the presence of ozone leads to the rapid conversion of isoprene to carboxylate (COO−) ions on the surfaces of α-Al2O3 particles in the initial stage. The water significantly suppresses the formation of the carboxylate ions. For the isoprene ozonolysis reaction on the α-Al2O3 particles, the reactive uptake coefficient is strongly suppressed by over a factor of 8 when the RH increases from 8% to 89%. The negative correlation between RH with the secondary organic aerosol (SOA) produced by isoprene ozonolysis plays a key role in the actual atmospheric environment under high humidity. Our results may provide insight into the ozonolysis process of biogenic alkenes over mineral aerosol surfaces with the influence of RHs.

The pH value is an important parameter of atmospheric aerosol. It affects the concentration of conjugate acid-base pair through the acid-base equilibrium and thus determines the gas–particle partitioning of acids or bases with volatility. Our recent report shows that there is a substitution of weak base for strong base in the aerosols of internally mixed water-soluble organic acid salt/ammonium sulfate. However, the acidity effect on the substitution process still remains ambiguous. In this work, the aerosols generated from sodium succinate/ammonium sulfate solutions with different pHs were studied in detail by using ATR-FTIR technique. The effects of relative humidity (RH) and acidity (pH) on the composition evolution, hygroscopic property and phase change were monitored. At a constant RH for a given pH, there were continuous depletions of NH4+, COO− and water content accompanying occurrence of (CH2COOH)2 at initial stage, and then followed by Na2SO4 efflorescence, and at last participation of (CH2COOH)2. Lower RH was conductive to faster chemical composition evolution and resultant Na2SO4 crystallization. Higher pH promoted the composition evolution process and solid phase formation process. The consumptions of COO− and NH4+ increased with increasing pH, showing that the dissolution of NH4+ to release H+ in aerosols and NH3 to gas phase led to water loss, in turn, Na2SO4 and succinic acid efflorescence. Water loss was more sensitive to Na2SO4 efflorescence than succinic acid. When a RH cycle was experienced, Efflorescence RHs of sodium succinate/ammonium sulfate aerosols almost kept unchanged and deliquscence RHs increased obviously with pH, while no deliquescence for pH 7.62. To our knowledge, it is the first time to investigate the pH effect on chemical process about composition evolution.

Sodium nitrate as an important inorganic component can be chemically formed from the reactions of nitrogen oxides and nitric acid (HNO

A combination of vacuum FTIR spectrometer (Vertex 80v, Bruker, German) and novel relative humidity (RH) adjusting equipment,which provides the pressure by pure water vapor, is used to study the hygroscopicity of magnesium acetate (Mg(CH3COO)2) aerosols. The RH can change not only rapidly but also slowly by the RH adjusting equipment. Because the RH is decided by the pure vapor, the real-time RH can be gained by calculating the integrated intensity of a feature band of vapor in an IR spectrum. Such the synchronism between FTIR spectrum and RH canbe ensured. The high-quality spectra of aerosols are obtained and the water peak and feature peaks of Mg(CH3COO)2 are analyzed during the slow and rapid RH changing process. The result shows that the areas of acetate ions and water decreases continuously at constant high RHs. After a slow cycle of RH (1.05×104 minutes), the water area decreases from 1.5 to 1.1, which means that the water content decreases after a cycle of RH. This phenomenon is reported at first up to date. The detailed analysis suggests that the hydrolysis of Mg(CH3COO)2 at high RH produces acetic acid, which was put out from the aerosols owing to the decrease of the pressure around the aerosols droplets. Furthermore, the dynamic hygroscopicity of Mg(CH3COO)2 aerosols is studied by changing RH as a pulse mode. It reveals that there is only water transfer hysteresis and no water loss after a pulse (10 seconds) when the RH is above 70%. Compared to slow process, it can be concluded that the hydrolysis reaction rate is slower than that of a pulse RH. The water transfer limited on rapid process should rise from some species on the surface of aerosols.

Hygroscopicity and volatility of single magnesium acetate (MgAc2) aerosol particles at various relative humidities (RHs) are studied by a single-beam optical tweezers, and refractive indices (RIs) and morphology are characterized by cavity enhanced Raman spectroscopy. Gel formation and volatilization of acetate acid (HAc) in MgAc2 droplets are observed. Due to the formation of amorphous gel structure, water transposition in droplets at RH

A novel approach based on a combination of a pulse RH controlling system and a rapid scan vacuum FTIR spectrometer (PRHCS-RSVFTIR) was utilized to investigate dynamic hygroscopicity of two atmospheric aerosols: ammonium sulfate ((NH4)2SO4) and magnesium sulfate (MgSO4). In this approach, rapid-scan infrared spectra of water vapor and aerosols were obtained to determine relative humidity (RH) in sample cell and hygroscopic property of aerosols with a subsecond time resolution. Heterogeneous nucleation rates of (NH4)2SO4 were, for the first time, measured under low RH conditions (35% RH). In addition, studies of MgSO4 aerosols revealed that water mass transport may be limited by different processes depending on RH values (surface limited at 40%RH52% and bulk phase limited at RH40%). Furthermore, we are also the first to report water diffusion constants in micron size MgSO4 aerosols at very low RH values. Our results have shown that the PRHCS-RSVFTIR is well-suited for determination of hygroscopicity of atmospheric aerosols and water transport and nucleation kinetics of liquid aerosols.

The conformational change of poly(vinyl alcohol) has been studied by Fourier transform infrared spectroscopy at various temperatures in the 4000–400 cm−1 region. The molecular motion and the trans/gauche content are sensitive to the C H, C C stretching modes. FTIR spectra show that the I2920/I2849 decreases from 1.84 to 1.0 with increasing temperature, companying the decrease in I1047/I1095 from 0.78 to 0.58, implying the conformational transition from trans to gauche in alkyl chain. Based on the van’t Hoff relation, the enthalpies and entropies have been calculated in different temperatures, which are 4.61 kJ mol−1 and 15.23 J mol−1 K−1, respectively, in the region of 80–140 °C. From the C O stretching mode and O H band, it can be concluded that the intermolecular hydrogen bonds decrease owing to elevating temperature, which leads to more gauche conformers.

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

Secondary organic aerosols (SOA) can exist in a glassy or semisolid state under low relative humidity (RH) conditions, in which the particles show nonequilibrium kinetic characteristics with changing ambient RH. Here, we selected internally mixed sucrose/NaNO

We report a new method to investigate water transport kinetics in aerosol particles by using rapid scan FTIR spectroscopy combined with a custom-built pulse relative humidity (RH) control system. From real time in situ measurements of RH and composition using high time resolution infrared spectroscopy (0.12 s for one spectrum), and through achieving a high rate of RH change (as fast as 60% per second), we are able to investigate the competition between the gas and condensed phase diffusive transport limits of water for particles with mean diameter ~3 mm and varying phase and viscosity. The characteristic time (t) for equilibration in particle composition following a step change in RH is measured to quantify dissolution timescales for crystalline particles and to probe the kinetics of water evaporation and condensation in amorphous particles. We show that dissolution kinetics are prompt for crystalline inorganic salt particles following an increase in RH from below to above the deliquescence RH, occurring on a timescale comparable to the timescale of the RH change (1 to 109 Pa s considered here. For amorphous particles, these kinetics are shown to be consistent with previous measurements of mass transfer rates in larger single particles. More specifically, the consistency suggests that fully understanding and modelling the complex microphysical processes and heterogeneities that form in viscous particles may not be necessary for estimating timescales for particle equilibration. A comparison of the kinetics for crystalline and amorphous particles illustrates the interplay of the rates of gas and condensed phase diffusion in determining mass transport rates of water in aerosol.

Hygroscopicity and volatility of secondary organic aerosol (SOA) are two important properties, which determine the composition, concentration, size, phase state of SOA and thus chemical and optical properties for SOA. In this work, magnesium acetate (Mg(Ac)2) aerosol was used as a simple SOA model in order to reveal relationship between hygroscopicity and volatility. A novel approach was set up based on a combination of a vacuum FTIR spectrometer and a home-made relative humidity (RH) controlling system. The striking advantage of this approach was that the RH and the compositions of aerosols could be obtained from a same IR spectrum, which guaranteed the synchronism between RH and spectral features on a sub-second scale. At the constant RH of 90% and 80% for 3000s, the water content within Mg(Ac)2 aerosol particles decreased about 19.0% and 9.4% while there were 13.4% and 6.0% of acetate loss. This was attributed to a cooperation between volatile of acetic acid and Mg2+ hydrolysis in Mg(Ac)2 aerosols, which greatly suppressed the hygroscopicity of Mg(Ac)2 aerosols. When the RH changed with pulsed mode between ~70% and ~90%, hygroscopicity relaxation was observed for Mg(Ac)2 aerosols. Diffuse coefficient of water in the relaxation process was estimated to be ~5×10-12m2·s-1 for the Mg(Ac)2 aerosols. Combining the IR spectra analysis, the decrease in the diffuse coefficient of water was due to the formation of magnesium hydroxide accompanying acetic acid evaporation in the aerosols.

In this research, we applied a pulsed RH controlling system and a rapid scan vacuum FTIR spectrometer (PRHCS-RSVFTIR) to investigate hygroscopicity of internally mixed (NH

The confocal Raman spectra of mixed phthalic acid/ammonium sulfate (AS) droplets deposited on a polytetrafluoroethylene (PTFE) substrate and a hydrophilic glass substrate are collected. The evolution of the physical state of the mixed droplet deposited on the PTFE substrate at various relative humidities consists of three states: well-mixed liquid state, liquid-liquid phase-separated state, and crystalline state. When the mixed droplets exist in liquid-liquid phase-separated state, the morphologies of the droplets on a PTFE substrate and on a glass slide are totally reverse, that is, an aqueous AS inner phase surrounded by an organic outer phase and an organic inner phase surrounded by an AS outer phase, respectively. We propose that the salting-out effect may induce the diffusion and formation of the organic phase, thus leading to the generation of liquid-liquid phase separation. The surface tension and hydrophobicity/hydrophilicity of substrates influence the spatial distribution of mixed aerosols. The understanding about the evolution of the physical state of mixed droplets and the effect of the substrates are important for probing the formation of atmospheric aerosols' morphology in the dehumidifying process.

The FTIR-ATR spectra of Na 2 SO 4 and mixed Na 2 SO 4 /glycerol (1:1) aerosols are monitored in the dehumidifying and humidifying process. It shows that glycerol suppresses both heterogeneous and homogeneous nucleation of Na 2 SO 4 /glycerol aerosols on the ZnSe substrate, while the water uptake of surrounding glycerol leads to lower deliquescence relative humidity of mixed Na 2 SO 4 /glycerol aerosols. In mixed Na 2 SO 4 /glycerol aerosols, glycerol restrains the formation of contact ion pairs between Na + and SO 4 2− , resulting in the delay of the efflorescence relative humidity. And glycerol molecular conformations transform from γγ to αα type in the dehumidifying process.