Efflorescence Relative Humidity Research Articles

Studies of the Crystallization and Dissolution of Individual Suspended Sodium Chloride Aerosol Particles.

Aerosols transform between physical phases, as they respond to variations in environmental conditions. There are many industries that depend on these dynamic processes of crystallization and dissolution. Here, a single particle technique (an electrodynamic balance) is used to explore the crystallization and dissolution dynamics of a model system, sodium chloride. The physical and environmental factors that influence the dynamics of crystal formation from a saline droplet (whose initial radius is ∼25 μm) and the kinetics of water adsorption onto dried particles are examined. The drying relative humidity (RH) is shown to impact the physical properties of the dried particle. When a saline droplet is injected into an airflow at an RH close to the efflorescence RH (ERH, 45%), an individual single crystal forms. By contrast, when a compositionally equivalent saline droplet is injected into dry air (RH ∼ 0%), a salt crystal made of multiple crystalline particles is formed. Subsequent to crystallization, the crystal shape, morphology, and surface area were all found to affect the dissolution dynamics of the dried particle. Additionally, we report that the difference between the deliquesce RH and environmental RH significantly impacts the dissolution time scale.

Study on the Mass transfer and hygroscopic behavior of glucose / ammonium sulfate aerosol droplets

Organic aerosols can form semisolid state, glassy state and high viscous state in the atmosphere, which makes aerosols show nonequilibrium kinetic characteristics following the loss of water due to the extended timescales for diffusive mixing. In this study, aerosol optical tweezers (AOTs) and confocal Raman spectroscopy are utilized to investigate the mass transfer of water and the hygroscopicity of internally mixed glucose/ammonium sulfate aerosol droplets with different organic/inorganic molar ratios (OIRs). The characteristic time ratio between the droplet radius and the RH is calculated to describe the mass transfer of water in the droplets. The results shown that the characteristic time ratio of the mixed glucose/ammonium sulfate aerosol droplets is apparently lower than that of pure glucose droplets. And the characteristic time ratio of the mixed droplets decreases with the increase of ammonium sulfate. The hygroscopicity of mixed glucose/ammonium sulfate aerosol droplets is greatly affected by high viscosity organic matter, and the glassy state of glucose suppresses the crystallization of ammonium sulfate in the system. Compared with the pure ammonium sulfate droplets, the efflorescence relative humidity (ERH) of the mixed glucose/ammonium sulfate droplets is delayed and the deliquescence relative humidity (DRH) is advanced. The ERH of the mixed aerosol droplets with molar ratios of 1:4, 1:3 and 1:2 is 35 ± 2%, 32 ± 2.5% and 30 ± 1.5% RH, respectively. And the DRH of the mixed aerosol droplets is 78 ± 1.5%, 76 ± 2% and 74 ± 2.5% RH, respectively. These results improve the understanding of the physical and chemical properties of super viscous organic/inorganic mixed aerosols, and might have important implications for atmospheric chemistry.

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.

Evaluating Leaf Wettability and Salt Hygroscopicity as Drivers for Foliar Absorption.

The objective of this study was to evaluate the rate of foliar absorption of magnesium (Mg) salts with different deliquescence and efflorescence relative humidity values (DRH and ERH, also known as point of deliquescence (POD) and point of efflorescence (POE), respectively) when supplied to leaves of model plants with different wettability properties. For this purpose, a greenhouse pot experiment was conducted with lettuce (very wettable), broccoli (highly unwettable) and leek (highly unwettable). Foliar sprays contained 0.1% surfactant plus 100 mM Mg supplied as MgCl2·6H2O, Mg(NO3)2·6H2O or MgSO4·7H2O. Leaf Mg concentrations were determined 1 and 7 days after foliar application. Anion concentrations were also measured in lettuce where a significant foliar Mg absorption was detected. Leaf wettability, leaf surface free energy and fertilizer drop deposit appearance onto the foliage were assessed. It is concluded that despite including a surfactant in the spray formulation, leaf wettability plays a major role in foliar Mg absorption.

Metal Oxide Particles as Atmospheric Nuclei: Exploring the Role of Metal Speciation in Heterogeneous Efflorescence and Ice Nucleation

Mineral dust can indirectly impact climate by nucleation of atmospheric solids, for example, by heterogeneously nucleating ice in mixed-phase clouds or by impacting the phase of aerosols and clouds through contact nucleation. The effectiveness toward nucleation of individual components of mineral dust requires further study. Here, the nucleation behavior of metal oxide nanoparticle components of atmospheric mineral dust is investigated. A long-working-distance optical trap is used to study contact and immersion nucleation of ammonium sulfate by transition-metal oxides, and an environmental chamber is used to probe depositional ice nucleation on metal oxide particles. Previous theory dictates that ice nucleation and heterogeneous nucleation of atmospheric salts can be impacted by several factors including morphology, lattice match, and surface area. Here, we observe a correlation between the cationic oxidation states of the metal oxide heterogeneous nuclei and their effectiveness in causing nucleation in both contact efflorescence mode and depositional freezing mode. In contrast to the activity of contact efflorescence, the same metal oxide particles did not cause a significant increase in efflorescence relative humidity when immersed in the droplet. These experiments suggest that metal speciation, possibly as a result of cationic charge sites, may play a role in the effectiveness of nucleation that is initiated at particle surfaces.

Development of the low-temperature hygroscopicity tandem differential mobility analyzer (Low-T HTDMA) and its application to (NH4)2SO4 and NaCl particles

Influence of water vapor on aerosol particles depends on both relative humidity (RH) and temperature (T). Both temperature and RH should simultaneously be regulated to investigate physical properties of aerosol particles. We developed a low-temperature hygroscopicity tandem differential mobility analyzer (low-T HTDMA), which can be operated in the range of −20 °C ≤ T ≤ 22.5 °C (room temperature), and 0% < RH ≤ 90% (T > 0 °C) or slightly lower than ice saturation RH (T ≤ −10 °C). Temperature was controlled using a chest freezer, refrigerated liquid circulators, fans, and heat exchangers. The employment of heat exchangers allowed the operation of the instrument without immersing the whole setup into liquid coolant. Aerosol particles were humidified using a Shirasu porous glass (SPG) humidifier to facilitate efficient humidification. The performance of the low-T HTDMA was tested using (NH4)2SO4 and NaCl particles. The hygroscopic growth factors of these particles agreed well with previous studies. Deliquescence RH (DRH) of both (NH4)2SO4 and NaCl particles also agreed well with the literature. Compared with the exiting data, efflorescence RH (ERH) measured by the low-T HTDMA exhibited minimal temperature dependence. For instance, the ERH values were 42% (T = −20 °C), 42% (T = −10 °C), and 43% (T = 22.5 °C) for NaCl particles. We suggest that further independent laboratory studies will need to be conducted for experimentally constraining thermodynamic properties of atmospherically important chemical species for T < 0 °C.

Efflorescence kinetics of aerosols comprising internally-mixed ammonium sulfate and water-soluble organic compounds

Understanding the efflorescence process of atmospheric aerosols is a key step towards a complete description of their phase transition dynamics. Atmospheric aerosols comprise internally mixed organic and inorganic components, and the composition of the condensed phase strongly affects the overall efflorescence dynamics. Although the kinetics of efflorescence has been thoroughly investigated for aerosols containing inorganic salts, the influence of non-ideal mixing with water-soluble organic compounds (WSOCs) remains considerably controversial. In this work, we measure the efflorescence process of ammonium sulfate (AS) aerosol droplets mixed with four types of WSOCs – sucrose, glycerol, malonic acid, and citric acid – by using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The absorbance band of ammonium ions (δ-NH4+ at 1441 cm−1) in the aqueous phase was used to calculate the crystallization ratio of AS and the heterogeneous nucleation rate. Adding WSOCs lowers the characteristic efflorescence relative humidity (ERH), broadens the ERH ranges, and reduces the terminal crystallization ratio of AS aerosol droplets. Amongst the various WSOCs studied here, the inhibiting effect on AS efflorescence decreases in the following order: sucrose > citric acid ≈ malonic acid > glycerol, mainly attributed to the mass transfer limitation caused by the viscosity of WSOCs. Sucrose and glycerol can completely suppress the crystallization of AS droplets at organic to inorganic ratio (OIR) of 1:1 and 4:1, respectively. This distinct observation suggests that atmospheric aerosols may skip efflorescence and evolve into a glassy state at low RH conditions.

Role of barriers in the airborne spread of virus-containing droplets: A study based on high-resolution direct numerical simulations

State-of-the-art direct numerical simulations are exploited to study the role of barriers on the airborne spread of virus-containing droplets. Our study is motivated by recent findings pointing to the key role of turbulence in dictating the final fate of virus-containing droplets in violent human exhalations. Here, all active scales of motion have been explicitly taken into account, including their interplay with the droplet evaporation process occurring once droplets are emitted in a drier ambient air, and accounting for the time-varying droplet inertia due to the water loss via evaporation. We show that barriers commonly used to mitigate the airborne spread of the virus cause nontrivial dynamical effects influencing the final reach of the virus-containing droplets, not always being beneficial to this aim. These conclusions do depend on the relative humidity of the ambient condition, and in particular whether the ambient humidity is above or below the so-called efflorescence relative humidity. Our findings provide a physically based answer to the question on how effective barriers are to protect people from airborne virus transmission in indoor environments.

Drying of virus-containing particles: modelling effects of droplet origin and composition

Background and purposeVirus-containing aerosol droplets emitted by breathing, speech or coughing dry rapidly to equilibrium with ambient relative humidity (RH), increasing in solute concentration with effects on virus survival and decreasing in diameter with effects on sedimentation and respiratory uptake. The aim of this paper is to model the effect of ionic and macromolecular solutes on droplet drying and solute concentration.MethodsDeliquescence-efflorescence concepts and Kohler theory were used to simulate the evolution of solute concentrations and water activity in respiratory droplets, starting from efflorescence data on mixed NaCl/KCl aerosols and osmotic pressure data on respiratory macromolecules.ResultsIn NaCl/KCl solutions total salt concentrations were shown to reach 10-13 M at the efflorescence RH of 40-55%, depending on the K:Na ratio. Dependence on K:Na ratio implies that the evaporation curves differ between aerosols derived from saliva and from airway surfaces. The direct effect of liquid droplet size through the Kelvin term was shown to be smaller and restricted to the evolution of breath emissions. Modelling the effect of proteins and glycoproteins showed that salts determine drying equilibria down to the efflorescence RH, and macromolecules at lower RH.ConclusionDifferences in solute composition between airway surfaces and saliva are predicted to lead to different drying behaviour of droplets emitted by breathing, speech and coughing. These differences may influence the inactivation of viruses.

Evolution of Sea Spray Aerosol Particle Phase State Across a Phytoplankton Bloom

Sea spray aerosol (SSA) represents the largest flux of aerosol mass annually, impacting Earth’s climate and the hydrological cycle by acting as cloud condensation and ice nuclei. The phase state (viscosity, η) of aerosol particles is important for aerosol impacts on climate and air quality, but the viscosity of SSA and chemical and biological drivers have not been evaluated. We measured variability in the relative humidity (RH)-dependent phase states of SSA (with a peak number-weighted aerodynamic diameter of ∼0.2 μm) based on particle bounce factors (BFs) and calculated glass transition temperatures and η of the organic components in SSA (ηorg) from online molecular composition measurements during an induced phytoplankton bloom. The results demonstrate that the SSA BFs and ηorg vary throughout the bloom, with the bulk of the SSA number concentration as liquidlike below the deliquescence and efflorescence RH of sea salt prior to phytoplankton growth on the basis of a measured BF of ≤0.2 and an estimated ηorg of <102 Pa·s. In contrast, the organic components in SSA exhibit viscous semisolid-like phase states over the same RH range during the peak of phytoplankton growth (BF∼0.8 and estimated ηorg∼103 Pa·s). Including the η of the aqueous inorganic salt, under the assumption that it is internally and homogeneously mixed with the organic components, significantly reduces the estimated η of the SSA mixture. The differences in SSA phase state across the bloom appear to be partially driven by specific biological processes in the seawater that alter the average molecular properties of the organic compounds in SSA (e.g., molar mass and hygroscopicity) depending upon the stage of the phytoplankton bloom. These findings have implications for aerosol processing, particularly in the biologically active coastal marine environment, including potential influences on ice nucleation and gas–particle partitioning.

The development and evaluation of a sequential aerosol-water measurement system

Aerosol water content (AWC) is associated with aerosol formation, aerosol–cloud interaction, and atmospheric visibility degradation; however, few studies have directly measured the mass of water in aerosols. Studies on AWC have employed modeling techniques [e.g., ISORROPIA II and the Extended Aerosol Inorganics Model (E-AIM) III)], the thermal-ramp Karl-Fisher method, and conducted volume-based measurements by using a hygroscopic tandem differential mobility analyzer (H-TDMA). The current study developed a sequential aerosol-water measurement system (SAWMS) that sequentially collects particles with up to 10 filters and measures AWC immediately after the collection. With this system, the loss of semi-volatile species was lower, and contamination during sample transport was reduced. The AWC measurement results were validated by comparing them with modeling results obtained in this study and the results obtained in previous studies. The measurement results for three aerosolized salts—NaCl, NH4NO3, and (NH4)2SO4—were found to be in favorable agreement with the corresponding modeling results and results from previous studies. The relationship between aerosol mass change (AMC) and relative humidity (RH) was investigated. The deliquescence RH (DRH) was 75%, 63%, and 81% for NaCl, NH4NO3, and (NH4)2SO4, respectively, whereas efflorescence RH was observed only for NaCl at 40%–50% RH. The AWC of mixed inorganic salts exhibited weak mutual DRH at 45%–50% RH. By contrast, the AWC obtained using ISORROPIA II and E-AIM III exhibited a gradual rather than an abrupt change of RH upon deliquescence. The measured AWC values were more similar to the values obtained with ISORROPIA II than to the values obtained with E-AIM III. To compare our mass-based measurements with the volume-based measurements obtained in previous studies using H-TDMAs, we derived the relationships of hygroscopicity parameter and growth factor with AMC. We discovered that the aforementioned relationships were comparable. In summary, the developed SAWMS integrates a sequential aerosol collection device with a gas chromatograph equipped with a thermal conductivity detector; the system performs well in the quantification of AWC and provides an additional chance of the filter for further chemical analysis.

Viscosity and phase state of aerosol particles consisting of sucrose mixed with inorganic salts

Abstract. Research on the viscosity and phase state of aerosol particles is essential because of their significant influence on the particle growth rate, equilibration times, and related evolution of mass concentration as well as heterogeneous reactions. So far, most studies of viscosity and phase state have been focused on organic aerosol particles, yet data on how viscosity can vary when the organic materials are mixed with inorganic salts remain scarce. Herein, using bead-mobility and poke-and-flow techniques, we quantified viscosities at 293 ± 1 K for binary mixtures of organic material / H2O and inorganic salts / H2O, as well as ternary mixtures of organic material / inorganic salts / H2O over the atmospheric relative humidity (RH) range. Sucrose as the organic species and calcium nitrate (Ca(NO3)2) or magnesium nitrate (Mg(NO3)2) as the inorganic salts were examined. For binary sucrose / H2O particles, the viscosities gradually increased from ∼ 3 × 10−2 to ≳1 × 108 Pa s as RH decreased from ∼ 75 % to ∼ 25 %. Compared with the results for the sucrose / H2O particles, binary Ca(NO3)2/H2O and Mg(NO3)2/H2O particles showed drastic enhancements to ≳1 × 108 Pa s at low RH close to the efflorescence RH. For ternary mixtures of sucrose / Ca(NO3)2 / H2O or sucrose / Mg(NO3)2 / H2O, with organic-to-inorganic mass ratios of 1:1, the viscosities of the particles gradually increased from ∼ 3 × 10−2 to greater than ∼ 1 × 108 Pa s for RH values from ∼ 75 % to ∼ 5 %. Compared to the viscosities of the Ca(NO3)2/H2O particles, higher viscosities were observed for the ternary sucrose / Ca(NO3)2 / H2O particles, with values increased by about 1 order of magnitude at 50 % RH and about 6 orders of magnitude at 35 % RH. Moreover, we applied a thermodynamics-based group-contribution model (AIOMFAC-VISC, Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients Viscosity) to predict aerosol viscosities for the studied systems. The model predictions and viscosity measurements show good agreement within ∼ 1 order of magnitude in viscosity. The viscosity measurements indicate that the studied mixed organic–inorganic particles range in phase state from liquid to semi-solid or even solid across the atmospheric RH range at a temperature of 293 K. These results support our understanding that organic / inorganic / H2O particles can exist in a liquid, semisolid, or even a solid state in the troposphere.

Impact of ambient relative humidity and acidity on chemical composition evolution for malonic acid/calcium nitrate mixed particles

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.

Hygroscopicity of Hofmeister Salts and Glycine Aerosols-Salt Specific Interactions.

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.

Hygroscopic property of inorganic salts in atmospheric aerosols measured with physisorption analyzer

Hygroscopicity is capable of significantly affecting atmospheric visibility, radiative forcing, and cloud condensation nuclei process. Herein, a physisorption analyzer was adopted to determine the hygroscopicity of inorganic salt particles in atmospheric aerosols (e.g., NaNO3, NaClO4, MgSO4, K3PO4, K2HPO4, and KH2PO4) at 298 K. By regulating the ambient relative humidity (RH), qualitative and quantitative information of the mentioned particles in humidifying and dehumidifying cycles was acquired. The deliquescence relative humidity (DRH) and efflorescence relative humidity (ERH) were measured, and supersaturation was observed and recorded during the dehumidification process. The water-to-salt molar ratios (WSR) of the mentioned inorganic salts depending on RH were determined, and the hygroscopic results of NaNO3 and NaClO4 particles were well consistent with the results achieved with other methods or by theoretical prediction. This study fitted WSR-to-RH equations suitable for NaNO3, NaClO4, MgSO4, K3PO4, and K2HPO4 particles. The adsorption heats for monolayer water molecules in the mentioned inorganic salt particles are calculated. The present study lays a solid foundation for gaining insights into the effects of RH on atmospheric aerosols.

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.

Nano-hygroscopicity tandem differential mobility analyzer (nano-HTDMA) for investigating hygroscopic properties of sub-10 nm aerosol nanoparticles

Abstract. Interactions between water and nanoparticles are relevant for atmospheric multiphase processes, physical chemistry, and materials science. Current knowledge of the hygroscopic and related physicochemical properties of nanoparticles, however, is restricted by the limitations of the available measurement techniques. Here, we present the design and performance of a nano-hygroscopicity tandem differential mobility analyzer (nano-HTDMA) apparatus that enables high accuracy and precision in hygroscopic growth measurements of aerosol nanoparticles with diameters less than 10 nm. Detailed methods of calibration and validation are provided. Besides maintaining accurate and stable sheath and aerosol flow rates (±1 %), high accuracy of the differential mobility analyzer (DMA) voltage (±0.1 %) in the range of ∼0–50 V is crucial for achieving accurate sizing and small sizing offsets between the two DMAs (&lt;1.4 %). To maintain a stable relative humidity (RH), the humidification system and the second DMA are placed in a well-insulated and air conditioner housing (±0.1 K). We also tested and discussed different ways of preventing predeliquescence in the second DMA. Our measurement results for ammonium sulfate nanoparticles are in good agreement with Biskos et al. (2006b), with no significant size effect on the deliquescence and efflorescence relative humidity (DRH and ERH, respectively) at diameters down to 6 nm. For sodium sulfate nanoparticles, however, we find a pronounced size dependence of DRH and ERH between 20 and 6 nm nanoparticles.

Hygroscopic properties and compositional evolution of internally mixed sodium nitrate-amino acid aerosols

Abstract 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.

Relative Humidity History Affects Hygroscopicity of Mixed Particles of Glyoxal and Reduced Nitrogenous Species.

The relative humidity (RH) history that manifests the cycling of dehydration (water evaporation) and hydration (water uptake) may affect particle-phase reactions, products from which have strong influences on the physical properties and thus climatic effects of atmospheric particles. Using single-trapped particles, we show herein hygroscopic growths of mixed particles with reactive species undergoing three types of RH cycles, simulating different degrees of particle-phase reactions in the atmosphere. The reactive species are the widely known α-dicarbonyl glyoxal (GLY), and five reduced nitrogenous species, ammonium sulfate (AS), glycine (GC), l-alanine (AL), dimethylamine (DMA), and diethylamine (DEA). The results showed that the mixed particles after reactions generally had altered efflorescence relative humidity (ERH) and deliquescence relative humidity (DRH) values and reduced hygroscopic growths at moderately high RH (>80%) conditions. For example, with an additional slow drying step, the mean mass growth factors at 90% RH during dehydration dropped from 2.56 to 2.02 for GC/GLY mixed particles and from 2.45 to 1.23 for AL/GLY mixed particles. The reduced hygroscopicity with more RH cycling will thus lead to less efficient light scattering of the mixed particles, thereby resulting in less cooling and exacerbating direct heating due to light absorption by the products formed.

Corrosion Behavior of 304 Stainless Steel Exposed to a Simulated Salt Lake Atmosphere

The corrosion behavior of stainless steel exposed to a simulated salt lake atmosphere has been investigated by analyzing the evolution of surface morphologies and corrosion products, the initiation and development of pits, and the electrochemical characteristics. The results indicated that (Mg6Fe2(OH)16(CO3)(H2O)4.5)0.25, a layered double hydroxide, has been detected for the first time in the corrosion products formed on stainless steel exposed to a simulated salt lake atmosphere. The specimens exposed to MgCl2 deposit conditions were corroded more severely than those exposed to NaCl deposit conditions, which was attributed to the differences in the deliquescence relative humidity and efflorescence relative humidity values of MgCl2 and NaCl. In addition, a special corrosion morphology consisting of a concentric circle of yellowish material was observed on the specimens exposed to MgCl2 deposit conditions, which was attributed to the formation of Mg(OH)2, inhibiting the diffusion and migration of OH− ions to the anode region. The maximum pit depth followed a power function with respect to corrosion time. The corrosion mechanism of stainless steel exposed to a simulated salt lake atmosphere is also discussed.