Deliquescence Relative Humidity Research Articles

Complex Hygroscopic Behavior of Ambient Aerosol Particles Revealed by a Piezoelectric Technique.

Understanding the complex interactions between atmospheric aerosols and water vapor in subsaturated regions of the atmosphere is crucial for modeling and predicting aerosol-cloud-radiation-climate interactions. However, the microphysical mechanisms of these interactions for ambient aerosols remain poorly understood. For this study, size-resolved samples were collected from a high-altitude, relatively clean site situated in the Western Ghats of India during the monsoon season, in order to study background and preindustrial processes as a baseline for climate functioning within the context of the most polluted region of the world. Measurements of humidity-dependent mass-based growth factors, hygroscopicity, deliquescence behavior, and aerosol liquid water content (ALWC) were made by a novel approach using a quartz crystal microbalance based on a piezo-electric sensor. The climate-relevant fine-mode aerosols (≤2.5 μm) exhibited strong size-dependent variations in their interactions with water vapor and contributed a high fraction of ALWC. Deliquescence occurred for relatively large aerosols (diameter >180 nm) but was absent for smaller aerosols. The deliquescence relative humidity for ambient aerosols was significantly lower than that of pure inorganic salts, suggesting a strong influence of organic species. Our study establishes an improved approach for accurately measuring aerosol water uptake characteristics of ambient aerosols in the subsaturated regime, aiding in the assessment of radiative forcing effects and improving climate models.

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.

Deliquescence‐induced release of ethyl formate and chlorine dioxide from their precursors‐loaded sachets for in‐package fumigation of spotted wing drosophila (Drosophila suzukii Matsumura)

AbstractThe toxicity and environmental concerns of methyl bromide (MB) have been a significant impetus for research into environmentally benign fumigant alternatives. Ethyl formate (EF) and chlorine dioxide (ClO2) are FDA‐approved agents with potential as alternatives to MB. However, both are chemically unstable and pose considerable end‐use challenges. In this study, we developed moisture‐activated sachets for EF and ClO2 to facilitate their end‐use deployment for in‐package fumigation. The sachets were loaded with EF or ClO2 metastable precursors, citric acid (CA), and a deliquescent salt carrier (e.g., CaCl2 or NaCl). When placed in an elevated relative humidity environment, these combinations activate the release of EF or ClO2 gases, separately. The deliquescence lowering phenomenon and its effect on fumigants release rate were investigated when mixing CA with CaCl2 or NaCl. The mutual deliquescence relative humidity for CA–CaCl2 and CA–NaCl mixtures were 40% and 63%, respectively. CaCl2 enhanced the release rate of fumigants from their sachets, whereas NaCl resulted in a slower release rate. EF and ClO2 sachets were evaluated for fumigation of different life stages of spotted wing drosophila (SWD). Complete control of SWD adults was achieved after exposure to 10.1 mg·h/L EF, while eggs, larval, and pupal stages were more tolerant toward EF vapour and required 110.2 mg·h/L EF to achieve control. ClO2 at low concentrations was more effective than EF against all SWD life stages, excluding pupae. Complete mortality was achieved after fumigating SWD adults, eggs, and larvae with 0.73, 2.39, and 1.21 mg·h/L ClO2, respectively. These results indicated that both EF and ClO2 sachets could be employed safely for EF and ClO2 in‐packaging fumigation to control produce pests.

Studying Deliquescent Relative Humidity Lowering in Mixtures of Five Fertilizer Ingredients Using Mixture Experiments and a Special Cubic Equation

Studying Deliquescent Relative Humidity Lowering in Mixtures of Five Fertilizer Ingredients Using Mixture Experiments and a Special Cubic Equation

Thermodynamic modeling of calcium or magnesium chloride, chlorate, and perchlorate ternary mixtures deliquescence at Mars-relevant temperatures

The definition of habitability on Mars is intimately linked to the stability of liquid water on the surface or near sub-surface. Brines provide the best pathway to stabilize liquid water, and form through deliquescence where a solid salt crystal transitions into an aqueous solution when exposed to a humid atmosphere. In a typical brine controlled by temperature and water relative humidity, ternary mixtures represent the best potential liquid brines. Here we modeled the deliquescence relative humidity (DRH) and the eutonic relative humidity (RH) of ternary salt mixtures. Chloride, chlorate, and perchlorate were modeled with either calcium or magnesium as the cation at temperatures ranging from 223 to 273 K. For the calcium ternary mixtures, the main salt composition precipitating at the DRH was dominated by calcium chloride, and by magnesium perchlorate in the magnesium ternary system. The hydration state of the precipitating salts systematically increased as temperature decreased. The eutonic RH for the calcium mixtures ranged from 14.24% at 273 K and increased to 43.54% by the coldest temperature of 223 K. The eutonic RH for the magnesium mixtures was significantly higher than the calcium counterpart, at 49.76% at 273 K and increased to 53.09% by 223 K. Calcium perchlorate was the predominate eutonic precipitate for the calcium mixtures, and magnesium chlorate for the magnesium mixtures., This study shows that ternary mixtures bring a slight improvement to the stability of brines on Mars compared to single salts or binary mixtures.

On the Chemical Composition and Hygroscopicity of Aerosols Deposited on the Insulators of Italian Power Lines

The reliability of the national power grid is a key issue in modern society. Atmospheric aerosols are the main cause of the reduction in the performance of insulators and the increase in the possibility of flashovers, resulting in power line failures. Under high ambient humidity, the water-soluble compounds of atmospheric aerosols collected on the insulators’ surface can dissociate in ions and form a conductive layer, which may lead to flashover events. With a view to investigating the processes that drive these phenomena, the chemical composition of aerosol deposits on insulators in Italy was determined by ion chromatography analysis and thermos-optical and X-ray techniques. In addition, a synthetic aerosol with the same analyzed chemical composition was generated in a laboratory and deposited on PTFE filters and glass specimens allowing us to determine the deliquescence and crystallization relative humidity and the conductive effect in an aerosol exposure chamber. The results evidenced the presence of a hazardous inorganic ion layer, which generates a sharp phase transition of the aerosol deposit as a function of the ambient relative humidity; this layer poses a dangerous threat to the reliability of the power grid, increasing the probability of flashover events where the conductive layer facilitates the flow of electrical current across the insulator surface, potentially causing power outages or damage to the power lines.

Significant impact of water-soluble organic matter on hygroscopicity of fine particles under low relative humidity condition

Uncertainties in estimating the hygroscopicity of bulk aerosols under conditions of low relative humidity (RH) or below the deliquescent RH (DRH) of aerosols remain to be significant, mainly due to the presence of water-soluble organic matter (WSOM). To quantify the contributions of WSOM to aerosol hygroscopicity and associated uncertainties, a field campaign was conducted to measure the hygroscopic growth curve (f(RH)) of bulk aerosols online, dominant chemical compositions in PM2.5 online and offline, and size distributions of the dominant chemical compositions offline during the dry and wet seasons of 2019–2020 in urban Guangzhou of south China. Based on the measured f(RH), the hygroscopicity parameter (κ) of bulk aerosols (κ-f(RH)) exhibits a logarithmic increase with increasing RH until RH reaches 69 %. Beyond this threshold, κ-f(RH) increases very slowly with further increase of RH, reaching 0.32 ± 0.04 during the dry season and 0.31 ± 0.05 during the wet season. The κ of WSOM (κ-WSOM) was further estimated to be 0.22 ± 0.03 and 0.13 ± 0.04 in the dry and wet seasons, respectively, when RH > 69 %. WSOM significantly affects κ-f(RH) by retarding the deliquescence process of aerosols and altering the mass ratio of water-soluble inorganic salts (WSIS) to WSOM within the size range of 0.4–0.9 μm, especially under low RH conditions (<60 %). The κ-f(RH) under low RH conditions was revised based on the logarithmic regression equation between RH and the ratio of measured κ-f(RH) to estimated κ-f(RH>69%). f(RH) of WSIS and WSOM were then corrected using the revised κ-f(RH) under low RH conditions, which showed 22–31 % lower values than those produced by the IMPROVE formulas.

Chemical composition-dependent hygroscopic behavior of individual ambient aerosol particles collected at a coastal site

Abstract. This study investigated the hygroscopic behavior of individual ambient aerosol particles collected at a coastal site of Jeju Island, South Korea. The size of the particles changes along with the phase transitions during humidification and dehydration processes, and the chemical compositions of the particles were determined by optical microscopy and scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM-EDX), respectively. Of the 39 particles analyzed, 24 were aged sea spray aerosols (SSAs), with diverse mixing ratios of Cl− and NO3-. The ambient SSAs exhibited multiple deliquescence and efflorescence transitions that were dominantly influenced by NaCl, NaNO3, MgCl2, Mg(NO3)2, and organic species covering the surface of the aged SSAs. For Cl-rich SSAs with X(Na,Mg)Cl&gt;0.4, although some particles showed very slow water uptake at low relative humidity levels (RH ≃30 %), two major transitions were observed during the humidification process. The first was at RH ≃63.8 %, regardless of their chemical compositions, which is the mutual deliquescence relative humidity (MDRH) level; and the second was at RH 67.5 % to 73.5 %, depending on their chemical compositions, which are the final deliquescence relative humidity (DRH) levels. During the dehydration process, the Cl-rich SSAs showed single-stage efflorescence at RH 33.0 % to 50.5 %, due to simultaneous heterogeneous crystallization of inorganic salts. For Cl-depleted SSAs with X(Na,Mg)Cl&lt;0.4, two prompt deliquescence transitions were observed during the humidification process. The first was at MDRH 63.8 %, and the second was at RH 65.4 % to 72.9 %. The mutual deliquescence transition was more distinguishable for Cl-depleted SSAs. During the dehydration process, step-wise transitions were observed at efflorescence RH levels (ERH 24.6 % to 46.0 % and 17.9 % to 30.5 %), depending on their chemical compositions. Additionally, aged mineral particles showed partial or complete phase changes with varying RH due to the presence of SSAs and/or NO3- species. In contrast, non-reacted mineral and Fe-rich particles maintained their size during the entire hygroscopic process. The mixture particles of organic and ammonium sulfate (AS) exhibited lower deliquescence and efflorescence RH levels compared to pure AS salt, highlighting the impact of organic species on the hygroscopic behavior of AS. These findings emphasize the complexity of atmospheric aerosols and the importance of considering their composition and mixing state when modeling their hygroscopic behavior and subsequent atmospheric impacts.

Direct observation for relative-humidity-dependent mixing states of submicron particles containing organic surfactants and inorganic salts

Abstract. Aerosol mixing state plays an important role in heterogeneous reactions and cloud condensation nuclei (CCN) activity. Organic surfactants could affect aerosol mixing state through bulk–surface partitioning. However, the mixing state of surfactant-containing particles remains unclear due to the lack of direct measurements. Here, direct characterizations of the mixing state for 20 kinds of submicron particles containing inorganic salts (NaCl and (NH4)2SO4) and atmospheric organic surfactants (organosulfates, organosulfonates, and dicarboxylic acids) were conducted upon relative humidity (RH) cycling by environmental scanning electron microscopy (ESEM). As the RH increased, the surfactant shells inhibited water diffusion being exposed to the inorganic core, leading to notably increased inorganic deliquescence RH (88.3 %–99.5 %) when compared with pure inorganic aerosol. Meanwhile, we directly observed an obvious Ostwald ripening process (that is, the growth of larger crystals at the expense of smaller ones) in 6 out of 10 NaCl–organic surfactant systems. As a result of water inhibition by the organic surfactant shell, Ostwald ripening in all systems occurred at RH above 90 %, which were higher than the reported RH range for pure NaCl measured at 27 ∘C (75 %–77 %). As RH decreased, eight systems underwent liquid–liquid-phase separation (LLPS) before efflorescence, showing a strong dependence on the organic molecular oxygen-to-carbon ratio (O:C). Quantitatively, LLPS was always observed when O:C≤0.43 and was never observed when O:C&gt;∼0.57. Separation RH (SRH) of inorganic salt–organic surfactant mixtures generally followed the trend of (NH4)2SO4 &lt; NaCl, which is consistent with their salting-out efficiencies reported in previous studies. Solid-phase separations were observed after efflorescence for systems without LLPS. Our results provide a unique insight into the consecutive mixing processes of the inorganic salt–organic surfactant particles, which would help improve our fundamental knowledge of model development on radiative effect.

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.

Considerations for realistic atmospheric environments: An application to corrosion testing

Measured salt compositions in dust collected over roughly the last decade from surfaces of in-service stainless-steel alloys at four locations around the United States are presented, along with the predicted brine compositions that would result from deliquescence of these salts. The salt compositions vary greatly from ASTM seawater and from laboratory salts (i.e., NaCl or MgCl2) commonly used on corrosion testing. The salts contained relatively high amounts of sulfates and nitrates, evolved to basic pH values, and exhibited deliquescence relative humidity values (RH) higher than seawater. Additionally, inert dust in components were quantified and considerations for laboratory testing are presented. The observed dust compositions are discussed in terms of the potential corrosion behavior and are compared to commonly used accelerated testing protocols. Finally, ambient weather conditions and their influence on diurnal fluctuations in temperature (T) and RH on heated metal surfaces are evaluated and a relevant diurnal cycle for laboratory testing a heated surface has been developed. Suggestions for future accelerated tests are proposed that include exploration of the effects of inert dust particles on atmospheric corrosion, chemistry considerations, and realistic diurnal fluctuations in T and RH. Understanding mechanisms in both realistic and accelerated environments will allow development of a corrosion factor (i.e., scaling factor) for the extrapolation of laboratory-scale test results to real world applications.

Cold-end corrosion caused by hygroscopic ammonium chloride in thermal conversion of biomass and waste

Optimization of energy recovery and efficient use of the energy in flue gases is of high importance for the economy of power plants. The fuel quality and additives to mitigate corrosion may dramatically affect cold-end deposits and corrosion. Ammonium chloride may form in the cold-end of the boiler if the flue gases contain both HCl(g) and NH3(g). Ammonia may be present in the cold-end when using ammonium-based additives or operating the boiler at low load leading to an NH3(g) slip. Ammonium chloride is a hygroscopic salt, which can cause corrosion of pre-heaters and flue gas cleaning equipment NH3(g) and HCl(g) can also pass through filters and form NH4Cl(s) after the baghouse filters. In this work, the hygroscopic properties of NH4Cl were determined by measuring the conductivity of the salt during decrease of temperature at various water vapor concentrations (10–35 vol%). The corrosiveness of NH4Cl salt on carbon steel was studied at 25 vol% water vapor in the temperature range of 70–160 °C. The cross-sections were analyzed using SEM-EDX, and corrosion rates were determined from panorama SEM images by analyzing the material loss in about 50 000 points. The material loss was also determined gravimetrically after removing the corrosion products with citric acid. The work revealed that hygroscopic NH4Cl is highly corrosive on carbon steel when water is absorbed. Water uptake and corrosion occurred at relative humidities well below the deliquescence relative humidity and depended on the temperature. An empirical equation for water uptake by NH4Cl was determined for water vapor concentrations between 10 and 35 vol% H2O. As corrosion proceeded, chlorine was enriched close to the carbon steel surface due to the formation of iron chlorides. The results can be used to avoid conditions typical for NH4Cl deposit build-up and create strategies to prevent corrosion.

Influence of Surface-Active Substances and Substrates on the Wettability of Individual Aerosol Particles during Condensation by Environmental Scanning Electron Microscopy

The formation of liquid cloud droplets from aerosol particles in the Earth atmosphere is still under debate particularly because of the difficulties to quantify the importance of bulk and surface effects in these processes. Recently, single-particle techniques have been developed to access experimental key parameters at the scale of individual particles. Environmental scanning electron microscopy (ESEM) has the advantage to provide in situ monitoring of the water uptake of individual microscopic particles deposited on solid substrates. In this work, ESEM was used to compare droplet growth on pure ammonium sulfate (NH4)2SO4 and mixed sodium dodecyl sulfate/ammonium sulfate (SDS/(NH4)2SO4) particles and to explore the role of experimental parameters, such as the hydrophobic-hydrophilic character of the substrate, on this growth. With hydrophilic substrates, the growth on pure salt particles was strongly anisotropic, but this anisotropy was suppressed by the presence of SDS. With hydrophobic substrates, it is the wetting behavior of the liquid droplet that is impacted by the presence of SDS. The wetting behavior of the pure (NH4)2SO4 solution on a hydrophobic surface shows a step-by-step mechanism that can be attributed to successive pinning-depinning phenomena at the triple-phase line frontier. Unlike the pure (NH4)2SO4 solution, the mixed SDS/(NH4)2SO4 solution did not show such a mechanism. Therefore, the hydrophobic-hydrophilic character of the substrate plays an important role in the stability and dynamics of the liquid droplets' nucleation by water vapor condensation. In particular, hydrophilic substrates are not suited for the investigation of the hygroscopic properties (deliquescence relative humidity (DRH) and hygroscopic growth factor (GF)) of particles. Using hydrophobic substrates, data show that the DRH of (NH4)2SO4 particles is measured within 3% accuracy on the RH and their GF could indicate a size-dependent effect in the micrometer range. The presence of SDS does not seem to modify the DRH and GF of (NH4)2SO4 particles. This study shows that the water uptake on deposited particles is a complex process but, once carefully taken into account, ESEM is a suitable technique to study them.

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.

Constraining the Effect of Surfactants on the Hygroscopic Growth of Model Sea Spray Aerosol Particles

The cloud condensation nuclei activation of sea spray aerosol (SSA) is tightly linked to the hygroscopic properties of these particles and is defined by their physical and chemical properties. While hygroscopic sea salt in SSA strongly influences particle water uptake, the marine-derived components that make up the organic fraction of SSA constitute a complex mixture, and their effect on hygroscopic growth is unknown. To constrain the effect of organic compounds and specifically surface-active compounds that adsorb on particle interfaces, particle hygroscopic growth studies were performed on laboratory-generated model sea salt/sugar particles. For sea salt/glucose particles, ionic surfactants facilitated water uptake at low relative humidity (RH), increasing the particle growth factor (GF) by up to 7.61%, and caused a reduction in the deliquescence relative humidity (DRH), while nonionic surfactants had a minimal effect. Replacing glucose with polysaccharide laminarin in sea salt/sugar/surfactant particles caused a reduction in GF at low RHs and minimized the effect of ionic surfactants on the DRH. At RHs above the DRH, the addition of anionic or nonionic surfactants caused a decrease in GF for both sea salt/glucose and sea salt/laminarin particles. The addition of cationic surfactants, however, did not have a dampening effect on water uptake of sea salt/sugar particles and even showed a GF increase of up to 3.7% at 90% RH. An increase in the complexity of the sugar dampens the water uptake for particles containing nonionic surfactants but increases the water uptake for cationic surfactants. The cloud activation potential for 100 nm particles analyzed in this study is higher for ionic surfactants and decreases with an increase in surfactant molecular size when particle interfacial tension is considered. The surfactant effect on the hygroscopic growth and cloud activation potential of the particles containing sea salt/sugar is dependent on the surfactant ionicity and molecular size, the particle size and interfacial tension, and the interactions between inorganic salt and organic species under different RH conditions.

Regional characteristics of fine aerosol mass increase elucidated from long-term observations and KORUS-AQ campaign at a Northeast Asian background site

Northeast Asia has suffered from severe PM2.5 pollution and the exact mechanisms have yet to be fully understood. Here, we investigated the transformation processes of submicron aerosols using a 4-year data set obtained at Jeju, a Northeast Asian background site. The diurnal-cycle constrained empirical orthogonal function analysis of nanoparticle size–number distribution distinguished 2 modes: burst of nucleation–Aitken particles and increase in accumulation mode particles, representing “new particle formation and growth” and “PM2.5 mass increase,” respectively. In these events, aerosol and meteorological characteristics changed progressively over several days, revealing that the PM2.5 mass increase is an episodic event occurring on a regional scale. The increase in PM2.5 mass was accompanied by an increase in aerosol liquid water content, which correlated well with SO4−2 and NO3, and a decrease in incoming solar radiation (−14.1 Wm−2 day−1) constituting a positive feedback. The “transport/haze” episode of KOREA–U.S. Air Quality campaign corresponds to “PM2.5 mass increase,” during which the vertical evolution of particles demonstrates that nanoparticles ≥3.5 nm were entrained into the shallow boundary layer upon vertical mixing and converted to accumulation-mode particles ≥0.3 μm at relative humidity (RH) exceeding the deliquescence RH of secondary inorganic aerosol (SIA). Coincidently, at ground, the coating thickness of refractory black carbon (rBC) (48 ± 39 nm) and SIA concentration increased. Furthermore, the diameter of rBC (180–220 nm)-containing particle in core–shell configuration linearly increased with PM2.5 mass, reaching 300–400 nm at PM2.5 ≥ 40 μg m−3. This observational evidence suggests that the thick coating of rBCs resulted from the active conversion of condensable gases into the particulate phase on the rBC surface, thereby increasing the mass of the accumulation-mode aerosol. Consequently, this result complies with the strategy to reduce primary emissions of gaseous precursors for SIA and particulates such as rBC as a way to effectively mitigate haze pollution as well as climate change in Northeast Asia.

Deliquescence of NaCl Confined in Nanoporous Silica.

The deliquescence behavior of salt nanocrystals is different from that of bulk crystals. Here, we report the first systematic measurements of the deliquescence relative humidity (DRH) of sodium chloride crystals confined in various nanoporous silica materials with pore diameters ranging from 8 to 89 nm. Deliquescence humidities were determined by water vapor sorption measurements. In comparison to the DRH of bulk NaCl crystals (75.3% RH), the DRH decreases from 73 to 58% as the pore size decreases from 89 to 8 nm. In contrast, according to literature data, the DRH of NaCl aerosol nanoparticles increases with decreasing crystal size. A thermodynamic model approach, based on the combined use of an ion-interaction model, the Laplace equation, and the Kelvin equation, is used to calculate the influence of the confinement in nanopores on the solid-liquid and liquid-vapor phase equilibria. These calculations reveal that the main reason for the decrease in the DRH in nanopores is the concave curvature of the liquid-vapor interface that is formed during deliquescence. The same model approach shows that the increase in DRH of nanosized aerosol particles is due to the convex curvature of the liquid-vapor interface, whereas the effect of increases in solubility with decreasing crystal size is small.

Thermodynamic modelling of perchlorate/chloride and perchlorate/chlorate deliquescence at Mars-relevant temperatures

Perchlorate (ClO4−) salts were discovered on Mars and are known to absorb water vapor from the atmosphere and deliquesce into the aqueous phase. Other species such as chlorides (Cl−) and chlorates (ClO3−) were also identified; these species can affect the deliquescence of perchlorates. Here we generate phase diagrams of perchlorate/chloride and perchlorate/chlorate binary mixtures for K, Na, Mg and Ca in the temperature range 223–273 K. Using a new approach based on thermodynamic modelling of evaporation, we determined the deliquescence relative humidity (the minimum relative humidity at which a salt converts into a liquid by absorbing atmospheric water vapor) and the eutonic relative humidity (the minimum relative humidity at which two salts are in equilibrium with liquid) for binary salt mixtures. Our modelling results show that the deliquescence relative humidity values of all salt mixtures is always lower than that of each individual end-member salt at a fixed temperature, typically a few percent lower. The closer the eutonic is to one of the end-member, the smaller the decrease in relative humidity compared to the pure pole. Thus, only eutonics which are far from both poles exhibit a significant drop in relative humidity. Moreover, the eutonic relative humidity always increases with decreasing temperature, which does not favor liquids in the dry and cold Martian environment. Finally, the increased stability of water ice at the lowest temperatures always reduces or even eliminates the stability of liquids. Therefore, the favorable temperature and relative humidity conditions under which binary salt liquid mixtures exist are generally not significantly improved compared to single salts.

Combined effects of high relative humidity and ultraviolet irradiation: Enhancing the production of gaseous NO2 from the photolysis of NH4NO3

Free radicals and nitrogen-containing species produced by nitrate photolysis can affect various atmospheric chemical processes, and thereby the photochemical behavior of atmospheric nitrate aerosols has been attracting much attention. However, the photolysis mechanism of NH4NO3 and its products under different atmospheric conditions remain unclear. In this study, the effects of relative humidity (RH), pH, NH3, ultraviolet (UV) light intensity and halogen ions (Cl−, Br− and I−) on the photolysis of particulate NH4NO3 have been investigated through a flow tube reactor. The results show that RH can significantly enhance the production of gaseous NO2 from the photolysis of NH4NO3 when RH is higher than its deliquescence RH, but almost no NO2 is generated under dry conditions. Under high RH and UV light, the main product of NH4NO3 photolysis is NO2, rather than NO and HONO, and another main species HNO3 which mainly comes from the hydrolysis of product NO2 in the gas path was detected. Almost no NO2 and HNO3 are produced under high RH without UV light or low RH with UV light, showing the combined effect of high RH and UV irradiation on the photolysis of NH4NO3. In addition, under high RH, the lower the pH and the stronger the light intensity, the higher the NO2 production. Furthermore, surprising yields of NO and HONO are detected in the presence of halogen ions, especially in the presence of I−, indicating the important role of halogen ion in the nitrate photolysis. These results provide new insights into the photolysis of atmospheric nitrate aerosols, and may contribute to elucidating the formation and migration of atmospheric nitrate aerosols and the potential mechanisms of the occurrence and evolution of atmospheric pollution and ozone pollution.

Saturated salt solutions in showcases: humidity control and pollutant absorption

The sustainable, passive climatisation of display cases using saturated salt solutions is rarely applied nowadays. These solutions adjust the atmospheric humidity to the specific deliquescence relative humidity (DRH) of the salt. Practical problems like the risk of spilling or ‘creeping’ of salts can be overcome. The DRH of suitable salts does not depend significantly on temperature. Solutions were Oddy-tested to check for corrosive emissions of the acids corresponding to the salt anions. Only magnesium chloride failed in accordance with thermodynamic calculations of the HCl vapour pressure. Solutions can absorb water-soluble pollutants like acids or aldehydes from the atmosphere, as has successfully been demonstrated for formaldehyde in chamber experiments. The alkaline potassium carbonate solution not only dissolves acids and aldehydes but also reacts with them chemically. Research needs to further study the absorption of pollutants are outlined. A revival of the saturated salt solution would make museum displays more sustainable.