Vacuum FTIR Observation on the Dynamic Hygroscopicity of Aerosols under Pulsed Relative Humidity

Key Roles of Bulk Viscosity and Acidity on Liquid-Liquid Phase Separation of Atmospheric Organic-Inorganic Mixed Aerosols

Liquid-liquid phase separation (LLPS) and the resulting particle morphologies in atmospheric organic-inorganic mixed aerosols are key regulators of aerosol chemistry and climate forcing. However, the influence of coexisting viscous water-soluble organic compounds (WSOCs) on the LLPS behavior in complex multicomponent aerosol systems remains poorly understood. In this study, we introduced three representative WSOCs, i.e., sucrose (SUC), glycerol (GLY), and citric acid (CA), to increase the bulk viscosity of a model LLPS system composed of 1,2,6-hexanetriol (HXT) and ammonium sulfate (AS). Using microscopic imaging techniques and viscosity model predictions, we examined the impact of mass transfer limitations on LLPS. As WSOC fractions increased, both the phase separation relative humidity (SRH) and the efflorescence relative humidity (ERH) progressively decreased. For the HXT/AS/SUC and HXT/AS/CA mixed systems with molar ratios of 1:1:0.5 and 1:1:0.75, LLPS was completely suppressed, although efflorescence still occurred. In the 1:1:1 mixtures, neither LLPS nor efflorescence was observed. In contrast, the addition of GLY caused minimal changes to phase transitions due to its minor effect on the aqueous-phase viscosity. Additionally, reducing bulk acidity, along with the transformation of CA into its salts, alleviated molecular transport limitations, leading to increased SRH and ERH values for the HXT/AS/CA mixtures. These findings are critical for advancing high-resolution phase state modeling of multicomponent aerosols and assessing the atmospheric implications of particle morphologies in the presence or absence of LLPS.

Atmospheric Hydroxyl Radical Route Revealed: Interface-Mediated Effects of Mineral-Bearing Microdroplet Aerosol

Hydroxyl radical (·OH) plays a crucial role in atmospheric chemistry, regulating the oxidative potential and aerosol composition. This study reveals an unprecedented source of ·OH in the atmosphere: mineral dust-bearing microdroplet aerosols. We demonstrate that Kaolin clay particles in microdroplet aerosols trigger rapid ·OH production upon solar irradiation, with rates reaching an order of at least 10-3 M s-1. This production rate is several orders of magnitude higher than that of the bulk phase (2.4 × 10-11 M s-1) and previously known pathways. On this basis, the surface-based interfacial ·OH production rate is estimated to be 8.9 × 10-5 mol m-2 s-1 at the air-water-solid interface of 1 μm sized aerosol particles. The enhanced ·OH formation is attributed to the unique features of air-water-solid interfaces, where the lifespan of photoinduced holes was significantly increased due to the presence of strong electric fields at the air-water interface. We further investigated the impacts of various environmental factors and aerosol properties on ·OH production, including light intensity, relative humidity, particle size, and pH. Our findings provide new insights into atmospheric photochemical processes mediated by mineral dust-bearing microdroplet aerosols, which are important contributors to ·OH source in the atmosphere. This work advances our understanding of atmospheric interfacial chemistry and its profound and lasting implications for air quality and climate.

Spectral Characteristics of Unsaturated and Supersaturated Inorganic Aerosols: Insights into Deliquescence Kinetics

The deliquescence phase transition of atmospheric aerosols is crucial for radiative forcing and atmospheric chemistry. However, the deliquescence kinetics of micrometer-sized aerosols, especially the formation and evolution of surface solution films, remain poorly understood. In this study, IR spectral characteristics were employed for the first time to quantify the solute concentration evolution in surface solution films. At a constant relative humidity (RH) of ∼65%, solution films on NaCl crystals exhibited a very low solute concentration (3.06 ± 0.18 mol/L), comparable to aqueous NaCl droplets above 90% RH. These films reached saturation at ∼74% RH, i.e., the deliquescence RH of NaCl, maintaining a nearly constant saturation level during deliquescence. In contrast, amorphous NaNO3 solids showed supersaturated solution films before deliquescence. Following deliquescence, the saturation level of solution phases increased due to faster solid dissolution rates than liquid water condensation. These findings address knowledge gaps in the complex nonequilibrium dissolution processes of crystalline or amorphous atmospheric aerosols.

Role of WSOCs and pH on Ammonium Nitrate Aerosol Efflorescence: Insights into Secondary Aerosol Formation

Efflorescence of ammonium nitrate (AN) aerosols significantly impacts atmospheric secondary aerosol formation, climate, and human health. We investigated the effect of representative water-soluble organic compounds (WSOCs) (sucralose (SUC), glycerol (GLY), and citric acid (CA) on AN:WSOC aerosol efflorescence using vacuum Fourier transform infrared spectroscopy. Combining efflorescence relative humidity (ERH) measurements, heterogeneous nucleation rates, and model predictions, we found that aerosol viscosity, correlating with molecular diffusion, effectively predicted ERH variations among the AN:WSOC aerosols. WSOCs with higher viscosity (SUC and CA) hindered efflorescence, while GLY with a lower viscosity showed a minor effect. At a low AN:CA molar ratio (10:1), CA promoted ERH, likely due to CA crystallization. Increasing the droplet pH inhibited AN:CA aerosol efflorescence. In contrast, for AN:SUC and AN:GLY aerosols, efflorescence is pH-insensitive. With the addition of trivial sulfate, AN:SUC droplets exhibited two-stage efflorescence, coinciding with ammonium sulfate and AN efflorescence. Given the atmospheric abundance, the morphology, phase, and mixing state of nitrate aerosols are significant for atmospheric chemistry and physics. Our results suggest that AN:WSOCs aerosols can exist in the amorphous phase in the atmosphere, with efflorescence behavior depending on the aerosol composition, viscosity, pH, and the cation and anion interactions in a complex manner.

A new apparatus and the relevant method to retrieve IR spectra of solutes from the corresponding aqueous solutions

An apparatus and relevant approach to obtaining IR spectra of solutes from the corresponding aqueous solution were developed. In the experiment, aqueous solutions were converted into aerosols using an ultrasonic or a pneumatic device. Subsequently, water in the nebulized solution is completely gasified under a high-speed flow and low vacuum environment. Via this process, the aqueous solution changes into a mixture of a solute or solutes and gaseous water, whose single-beam IR spectra are collected. Then, the newly developed RMF (retrieving moisture-free IR spectrum) method and the relevant approach described in our recent papers have been adopted to treat the resultant single-beam sample spectrum. As a result, the spectral contribution of the vibrational-rotational peaks of gaseous water can be removed or significantly attenuated, and IR spectra of solutes can be obtained. The approach shows an obvious advantage in retrieving the IR spectrum of volatile solutes from its aqueous solution. This capability is showcased by obtaining IR spectra of isopropanol and ethyl acetate successfully. IR spectra of these compounds can be retrieved even if the concentration of the solute is below 10 wt%. Moreover, atomization via ultrasonic/pneumatic methods offers a mild way to gasify solutes whose boiling points are remarkably higher than that of water. This advantage is manifested by acquiring IR spectra of 1-butanol and 1,2-propanediol in the gaseous phase under ambient conditions.

Investigating the hygroscopicities of calcium and magnesium salt particles aged with SO2 using surface plasmon resonance microscopy

The hygroscopicities of calcium and magnesium salts strongly affect the environment and climate, but the aging products of these salts at high relative humidities (RHs) are still poorly understood. In this study, surface plasmon resonance microscopy (SPRM) was used to determine the hygroscopic growth factors (GFs) of Ca(NO₃)₂ and Mg(NO₃)₂ separately or mixed with galactose at different mass ratios at different RHs before and after aging. For all particles, the measured GFs showed no indication of deliquescence across the range of RHs tested, and overall hygroscopicity was clearly lower after than before aging. The Ca(NO₃)₂ and Mg(NO₃)₂ GFs at 90 % RH were 1.80 and 1.66, respectively, before aging and 1.33 and 1.42, respectively, after 4 h aging, meaning aging decreased the GFs by 26.11 % and 14.46 %, respectively. Aging decreased the hygroscopicity because insoluble or sparingly soluble substances (CaSO₃, CaSO₄, MgSO₃) formed and strongly changed the overall hygroscopicity. For bicomponent aerosols with different mass ratios, the GFs (calculated using the Zdanovskii-Stokes-Robinson method) of the other components except galactose at 90 % RH after 1 h aging were all lower, respectively, than the measured GFs of pure Ca(NO₃)₂ and Mg(NO₃)₂ after aging for 1 h, especially with the mass ratio of 1:2, their GFs have decreased by 14.63 % and 7.50 %, respectively. Subsequently, Ion chromatograms indicated that the peak area ratio of SO₄^2- to NO₃^- ratios were higher for the aged bicomponent particles than aged single-component particles, possibly because adding galactose improved the gas-liquid state stability during drying after the aging process and therefore promoted nitrate consumption and sulfate formation. The results indicated that organic components may play important roles in heterogeneous reactions between trace gases and multicomponent aerosols and should be considered in evaluating the impacts on submicron aerosol composition of high atmospheric SO₂ concentrations at high humidities.

Strong Acids or Bases Displaced by Weak Acids or Bases in Aerosols: Reactions Driven by the Continuous Partitioning of Volatile Products into the Gas Phase

Aerosols 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.

Hygroscopicity and mass transfer limit of mixed glutaric acid/MgSO4/water particles

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.

A review of efflorescence kinetics studies on atmospherically relevant particles

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.

Subsecond measurement on deliquescence kinetics of aerosol particles: Observation of partial dissolution and calculation of dissolution rates

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.

Hygroscopic Growth and Phase Transitions of Na2CO3 and Mixed Na2CO3/Li2CO3 Particles: Influence of Li2CO3 on Phase Transitions of Na2CO3 and Formation of LiNaCO3

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.

Kinetics study of heterogeneous reactions of O3 and SO2 with sea salt single droplets using micro-FTIR spectroscopy: Potential for formation of sulfate aerosol in atmospheric environment

The heterogeneous reactions of sea salt single droplets with the mixture of O₃ and SO₂ were studied in real time using microscopic Fourier transform infrared (micro-FTIR) spectrometer. Chemical conversion of SO₂ to sulfate and consumption of gaseous HCl occur on the surface of droplets in the presence of O₃. The sulfate formation rate and the uptake coefficient are obtained by quantitatively estimating the changes in absorbance area of the sulfate stretching band. In order to further establish a mechanistic framework, we observed the reaction kinetics versus ambient relative humidities (RHs) and droplet sizes. In the view of RH effect, sulfate formation rates are enhanced by about a factor of two on the MgCl₂ and ZnCl₂ single droplets with increasing RH ranges. High RH is favorable for the sulfate formation because water vapor can trap and activate more gas molecules on the interface of the single droplet. The values of uptake coefficient increase slightly with an increase in single droplet size for the two reaction systems, indicating that the effect of surface adsorption dominates the reactions. Considering the existence of combined pollution with high concentrations of trace gases and sea salt aerosols, as expected in coastal regions, the formation micro-mechanism of sulfate revealed in this work should be incorporated into air quality models to improve the prediction of sulfate concentrations.

Effect of waiting time on the water transport kinetics of magnesium sulfate aerosol at gel-forming relative humidity using optical tweezers

With the loss of water, the amorphous gel states in aqueous magnesium sulfate (MgSO₄) aerosol forms and results in nonequilibrium dynamics, owing to the extended time scales for diffusive mixing. The mass transfer resistance in MgSO₄ aerosol droplets during evaporation or condensation is investigated using aerosol optical tweezers (AOTs) coupled with Raman spectroscopy. In addition, the kinetics of water transport during hydration and dehydration after different waiting time is studied. With the cyclic change of the relative humidity (RH) below gel-forming, the waiting time is varied to examine the effect of the duration of drying and humidifying on water transport kinetics during subsequent hydration and dehydration process. Apparent diffusion coefficients (D_ap) of water molecules in the gel state after different waiting time are obtained. The results indicate that the duration of drying will affect water transport kinetics for subsequent humidifying process due to the different structure and composition in MgSO₄ aerosol droplet at different ambient humidities. However, the duration of humidifying has little effect on water transport kinetics for subsequent drying process below gel-forming RH.

Influence of relative humidity on SO2 oxidation by O3 and NO2 on the surface of TiO2 particles: Potential for formation of secondary sulfate aerosol

The heterogeneous reactions of SO₂/O₃ and SO₂/NO₂ with TiO₂ particles were studied as a function of relative humidities (RHs). An in situ microscopic Fourier transform infrared (micro-FTIR) spectrometer was used to monitor the reaction kinetics. Rapid conversion of SO₂ to sulfate occurs on the surface of TiO₂ particles in the presence of O₃ or NO₂, which is sensitive to RHs. For unreacted (fresh) particles, the uptake coefficients for SO₂ in initial stage are both obviously enhanced over four times with the increasing RH from ~4% to ~85%. Moreover, the uptake coefficient in the system of SO₂/O₃ is about 40% higher than that of SO₂/NO₂ on TiO₂ particles at the similar RH conditions. For TiO₂ after exposure to SO₂/O₃ or SO₂/NO₂ (sulfated) particles, the uptake coefficients for SO₂ in moisture absorption stage are all higher than that on fresh particles in initial stage at the similar RH, indicating rapid mixture gases adsorption with particle hygroscopic growth. The high production of the secondary sulfate for heterogeneous reaction of mixture gases on TiO₂ surface from arid region to humid region provides new insights for better understanding the severe haze under the humid condition.

Measuring hygroscopicity of internally mixed NaNO3 and glutaric acid particles by vacuum FTIR

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.

Hygroscopic behavior and fractional crystallization of mixed (NH4)2SO4/glutaric acid aerosols by vacuum FTIR

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.

Observing HNO3 release dependent upon metal complexes in malonic acid/nitrate droplets

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 (ν_as-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.

Vacuum FTIR study on the hygroscopicity of magnesium acetate aerosols

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.

Hygroscopicity of internally mixed particles composed of (NH4)2SO4 and citric acid under pulsed RH change

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 (<40% RH), indicating the significant water transfer limitation due to the high viscosity of CA aerosols at low RH, especially at RH<20%. In addition, water diffusion constants between 10^-12 m² s^-1 and 10^-13 m² s^-1 in micron size CA aerosols were obtained in a sub-second and second timescale. The addition of AS enhanced the water transfer limitation in the mixed aerosols. The efflorescence relative humidity (ERH) of the mixed particles with AS/CA by molar ratio 3:1 was found between 22.7% and 5.9%, which was much lower than AS particles. No efflorescence process was observed for the 1:1 mixed particles, indicating that CA greatly suppressed nucleation of AS. Our results have shown that the PRHCS-RSVFTIR is effective to simulate hygroscopicity and water transport of aerosols under fast variations in RH in atmosphere.

Kinetically Determined Hygroscopicity and Efflorescence of Sucrose-Ammonium Sulfate Aerosol Droplets under Lower Relative Humidity

Organic aerosols will likely form in semisolid, glassy, and high viscous state in the atmosphere, which show nonequilibrium kinetic characteristics at low relative humidity (RH) conditions. In this study, we applied optical tweezers to investigate the water transport in a sucrose/(NH₄)₂SO₄ droplet with high organic to inorganic mole ratio (OIR). The characteristic time ratio between the droplet radius and the RH was used to describe the water mass transfer difference dependent on RH. For OIR greater than 1:1 in sucrose/(NH₄)₂SO₄ droplets, the characteristic time ratio at low RH (<∼30% RH) was two orders magnitude greater than that at high RH (>∼60%). We also coupled vacuum FTIR spectrometer and a high-speed photography to study the efflorescence process in sucrose/(NH₄)₂SO₄ droplets with low OIR. The crystalline fraction of (NH₄)₂SO₄ was used to understand efflorescence behavior when the RH was linearly decreasing with a velocity of 1.2% RH min^-1. Because of suppression of (NH₄)₂SO₄ nucleation by addition of sucrose, the efflorescence relative humidity (ERH) of (NH₄)₂SO₄ decrease from the range of ∼48.2% to ∼36.1% for pure (NH₄)₂SO₄ droplets to from ∼44.7% to ∼25.4%, from ∼43.2% to ∼21.2%, and from ∼41.7% to ∼21.1% for the mixed droplets with OIR of 1:4, 1:3, and 1:2, respectively. No crystallization was observed when the OIR is higher than 1:1. Suppression of (NH₄)₂SO₄ crystal growth was also observed under high viscous sucrose/(NH₄)₂SO₄ droplets at lower RH.

A rapid scan vacuum FTIR method for determining diffusion coefficients in viscous and glassy aerosol particles

The effect of glassy formation on water transport in sucrose aerosol droplets is evaluated from characteristic time in a vacuum FTIR experiment.