Single Droplet Tweezer Revealing the Reaction Mechanism of Mn(II)-Catalyzed SO2 Oxidation.

Sulfate aerosol is one of the major components of secondary fine particulate matter in urban haze that has crucial impacts on the social economy and public health. Among the atmospheric sulfate sources, Mn(II)-catalyzed SO2 oxidation on aerosol surfaces has been regarded as a dominating one. In this work, we measured the reaction kinetics of Mn(II)-catalyzed SO2 oxidation in single droplets using an aerosol optical tweezer. We show that the SO2 oxidation occurs at the Mn(II)-active sites on the aerosol surface, per a piecewise kinetic formulation, one that is characterized by a threshold surface Mn(II) concentration and gaseous SO2 concentration. When the surface Mn(II) concentration is lower than the threshold value, the reaction rate is first order with respect to both Mn(II) and SO2, agreeing with our traditional knowledge. But when surface Mn(II) concentration is above the threshold, the reaction rate becomes independent of Mn(II) concentration, and the reaction order with respect to SO2 becomes greater than unity. The measured reaction rate can serve as a tool to estimate sulfate formation based on field observation, and our established parametrization corrects these calculations. This framework for reaction kinetics and parametrization holds promising potential for generalization to various heterogeneous reaction pathways.

Compositional evolution for mixed aerosols containing gluconic acid and typical nitrate and the effect of multiply factors on hygroscopicity.

The aging process of atmospheric aerosols usually leads to a mixture of inorganic salts and organic compounds of anthropogenic origin. In organic compounds, polyhydroxy organic acids are important components, however, the study on composition and hygroscopic properties of the mixture containing inorganics and polyhydroxy organic acids is scanty. In this study, gluconic acid, the proxy of polyhydroxy organic acids, is mixed with the representative nitrate (Mg(NO3)2, Ca(NO3)2) to form aerosols. ATR-FTIR and optical microscopy are employed to study the component changes and hygroscopicity as a function of relative humidity. As relative humidity fluctuates, the FTIR-ATR spectra display that the internal mixed gluconic acid (CH2(CH)4(OH)5COOH) and nitrate can react to release acidic gases, forming relevant gluconate and further affecting the hygroscopicity. The specific presentation is particles cannot be recovered to their original size after the dehydration-hydration process and there will be some disparities in GF for mixed particles. For the gluconic acid-Ca(NO3)2/Mg(NO3)2 mixtures with molar ratios of 1:1, higher degree of reaction resulting in the production of large amounts of gluconate should be responsible to the lower hygroscopicity compared to ZSR model. For 1:2 gluconic acid-nitrate mixed systems (with higher nitrate content), the hygroscopicity of mixtures are higher than the ZSR prediction. A possible reason could be 'salt-promoting effect' on the organic fractions of the surplus inorganic salt in the mixture. These data can improve the chemical composition list evaluation, in turn hygroscopic properties and phase state of atmospheric aerosol, and then the climate effect.

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.

Heterogeneous uptake of NO2 by sodium acetate droplets and secondary nitrite aerosol formation.

The high NO3- concentration in fine particulate matters (PM2.5) during heavy haze events has attracted much attention, but the formation mechanism of nitrates remains largely uncertain, especially concerning heterogeneous uptake of NOX by aqueous phase. In this work, the heterogeneous uptake of NO2 by sodium acetate (NaAc) droplets with different NO2 concentrations and relative humidity (RH) conditions is investigated by microscopic Fourier transform infrared spectrometer (micro-FTIR). The IR feature changes of aqueous droplets indicate the acetate depletion and nitrite formation in humid environment. This implies that acetate droplets can provide the alkaline aqueous circumstances caused by acetate hydrolysis and acetic acid (HAc) volatilization for nitrite formation during the NO2 heterogeneous uptake. Meanwhile, the nitrite formation will exhibit a pH neutralizing effect on acetate hydrolysis, further facilitating HAc volatilization and acetate depletion. The heterogeneous uptake coefficient increases from 5.2 × 10-6 to 1.27 × 10-5 as RH decreases from 90% to 60% due to the enhanced HAc volatilization. Furthermore, no obvious change in uptake coefficient with different NO2 concentrations is observed. This work may provide a new pathway for atmospheric nitrogen cycling and secondary nitrite aerosol formation.

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 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.

Hygroscopicity measurement of sodium carbonate, ß-alanine and internally mixed ß-alanine/Na2CO3 particles by ATR-FTIR.

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

Hygroscopicity and Compositional Evolution of Atmospheric Aerosols Containing Water-Soluble Carboxylic Acid Salts and Ammonium Sulfate: Influence of Ammonium Depletion.

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

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

We report a new method to investigate water transport kinetics in aerosol particles by using rapid scan FTIR spectroscopy combined with a custom-built pulse relative humidity (RH) control system. From real time in situ measurements of RH and composition using high time resolution infrared spectroscopy (0.12 s for one spectrum), and through achieving a high rate of RH change (as fast as 60% per second), we are able to investigate the competition between the gas and condensed phase diffusive transport limits of water for particles with mean diameter ∼3 µm and varying phase and viscosity. The characteristic time (τ) for equilibration in particle composition following a step change in RH is measured to quantify dissolution timescales for crystalline particles and to probe the kinetics of water evaporation and condensation in amorphous particles. We show that the dissolution kinetics are prompt for crystalline inorganic salt particles following an increase in RH from below to above the deliquescence RH, occurring on a timescale comparable to the timescale of the RH change (<1 s). For aqueous sucrose particles, we show that the timescales for both the drying and condensation processes can be delayed by many orders of magnitude, depending on the viscosity of the particles in the range 101 to 109 Pa s considered here. For amorphous particles, these kinetics are shown to be consistent with previous measurements of mass transfer rates in larger single particles. More specifically, the consistency suggests that fully understanding and modelling the complex microphysical processes and heterogeneities that form in viscous particles may not be necessary for estimating timescales for particle equilibration. A comparison of the kinetics for crystalline and amorphous particles illustrates the interplay of the rates of gas and condensed phase diffusion in determining the mass transport rates of water in aerosols.

Crystal Nucleation and Crystal Growth and Mass Transfer in Internally Mixed Sucrose/NaNO3 Particles.

Secondary organic aerosols (SOA) can exist in a glassy or semisolid state under low relative humidity (RH) conditions, in which the particles show nonequilibrium kinetic characteristics with changing ambient RH. Here, we selected internally mixed sucrose/NaNO3 droplets with organic to inorganic molar ratios (OIRs) of 1:8, 1:4, 1:2, and 1:1 as a proxy for multicomponent ambient aerosols to study crystal nucleation and growth processes and water transport under a highly viscous state with the combination of an RH-controlling system and a vacuum Fourier transform infrared (FTIR) spectrometer. The initial efflorescence RH (ERH) of NaNO3 decreased from ∼45% for pure NaNO3 droplets to ∼38.6 and ∼37.9% for the 1:8 and 1:4 sucrose/NaNO3 droplets, respectively, while no crystallization of NaNO3 occurred for the 1:2 and 1:1 droplets in the whole RH range. Thus, the addition of sucrose delayed the ERH and even completely inhibited nucleation of NaNO3 in the mixed droplets. In addition, the crystal growth of NaNO3 was suppressed in the 1:4 and 1:8 droplets most likely due to the slow diffusion of Na+ and NO3- ions at low RH. Water uptake/release of sucrose/NaNO3 particles quickly arrived at equilibrium at high RH, while the hygroscopic process was kinetically controlled under low RH. The half-time ratio between the liquid water content and the RH was used to describe the mass transfer behavior. For the 1:1 droplets, no mass limitation was observed with the ratio approaching to 1 when the RH was higher than 53%. The ratio increased 1 order of magnitude under an ultraviscous state with RH ranging from 53 to 15% and increased a further 1 order of magnitude at RH < 15% under a glassy state.

A Relative Humidity Pulse Approach to Observe Mass Transfer Processes Controlled by Bulk and Surface for Aerosols.

With combination of a pulse relative humidity (RH) controlling system and rapid scan vacuum FTIR technique, dynamic hygroscopicity of aerosol can be studied during pulse RH process. The time-resolved FTIR spectra can provide both water content of aerosols and water vapor amount of the aerosol ambient in sub-second time resolution. Experiments were performed on sodium nitrate, magnesium sulfate and magnesium nitrate aerosols. By comparing their hygroscopicity in pulse RH process and quasi-equilibrium state, for sodium nitrate aerosols, under time resolution of 0.12 s, we didn't see water transfer delay between aerosols and ambient environment. For magnesium sulfate aerosols, after gel formation, the water transfer speed is limited by the aerosol bulk phase. While for aged magnesium nitrate aerosols, non-soluble species generated and formed a film on the surface of aerosol particles, which slow down the water exchange rate between aerosols and ambient environment. This method turned out to be an efficient and convenient tool to elucidate the water transfer process controlled by bulk and surface for aerosols.

Study on the Water Transfer of Magnesium Acetate Aerosols Led by the Rapid and Slow Change of Relative Humidity.

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

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

A novel approach based on a combination of a pulse RH controlling system and a rapid scan vacuum FTIR spectrometer (PRHCS-RSVFTIR) was utilized to investigate dynamic hygroscopicity of two atmospheric aerosols: ammonium sulfate ((NH4)2SO4) and magnesium sulfate (MgSO4). In this approach, rapid-scan infrared spectra of water vapor and aerosols were obtained to determine relative humidity (RH) in sample cell and hygroscopic property of aerosols with a subsecond time resolution. Heterogeneous nucleation rates of (NH4)2SO4 were, for the first time, measured under low RH conditions (<35% RH). In addition, studies of MgSO4 aerosols revealed that water mass transport may be limited by different processes depending on RH values (surface limited at 40% < RH < 52% and bulk phase limited at RH < 40%). Furthermore, we are also the first to report water diffusion constants in micron size MgSO4 aerosols at very low RH values. Our results have shown that the PRHCS-RSVFTIR is well-suited for determination of hygroscopicity of atmospheric aerosols and water transport and nucleation kinetics of liquid aerosols.

Micro-Raman observation on the H2PO4(-) association structures in a supersaturated droplet of potassium dihydrogen phosphate (KH2PO4).

The efflorescence of an individual KH(2)PO(4) droplet on Teflon substrate was investigated by micro-Raman spectroscopy. With the decrease of relative humidity (RH) from 98.0% to 73.0%, the KH(2)PO(4) droplet lost water gradually and entered into supersaturated state, which was reflected by the area ratio between the water stretching band to the sum of ν(s)-PO(2) and ν(s)-P(OH)(2) bands of the H(2)PO(4)(-) (A(H(2)O)/(A((ν(s)-PO(2))+A(ν(s)-P(OH)(2))))). In 1.0 mol l(-1) KH(2)PO(4) solution, the ν(s)-P(OH)(2) and ν(s)-PO(2) bands appeared at 877 and 1077 cm(-1). In the KH(2)PO(4) droplet, the two bands shifted to 894 and 1039 cm(-1) at 98.0% RH, to 899 and 1031 cm(-1) at 89.6% RH, and then to 904 and 997 cm(-1) at 73.0% RH. Moreover, the aggregation process between the H(2)PO(4)(-) ions was observed from the spectral characteristic of the ν(s)-P(OH)(2) band in the concentration process, including the transitions of the H(2)PO(4)(-) ions from monomer in bulk solutions (0.5-1.0 mol l(-1)) to possible dimers at 98.0% RH and then further to oligomers in the droplet with the RH decrease, which were indicated by the blueshift of the ν(s)-P(OH)(2) band and its full width at half-height as a function of the RH. When the RH reached at 72.0%, the anhydrous crystal was obtained. A strong peak appeared at 928 cm(-1), implying that the four oxygen atoms of the H(2)PO(4 (-) were all hydrogen bonding through the bridge hydrogen atoms to get the extensive hydrogen-bonded network structure of the H(2)PO(4)(-) association, leading to the symmetric increase of the H(2)PO(4)(-) ion from C(2v) in dilute solution to quasi-T(d) in the anhydrous crystal.

Micro-Raman observation on the HPO4(2-) association structures in an individual dipotassium hydrogen phosphate (K2HPO4) droplet.

A single K(2)HPO(4) droplet with size of ∼50 µm on a Teflon substrate was forced to enter into the supersaturated state by decreasing the relative humidity (RH), allowing accurate control over the concentration of the solute within a droplet of a nanogram. The K(2)HPO(4) solutions from dilute (0.1-1.0 mol·L(-1) bulk) to concentrated state (a droplet from RH 98.2% to 25.1%) were studied through micro-Raman spectroscopy in the spectral region of about 200-4000 cm(-1). The area ratio between the water stretching band to the sum of the ν(1)-PO(3), ν(2)-POH, and ν(4)-PO(3) bands of the HPO(4)(2-) at various RHs was used to describe the dehydration behavior of a microsized single K(2)HPO(4) droplet in dehumidifying process. The peak position of the v(1)-PO(3) band for the 1 mol·L(-1) bulk solution appeared at 991 cm(-1) and moved to 986 cm(-1) at 98.2% RH, to 978 cm(-1) at 70.2% RH, and then to 964 cm(-1) at 30.0% RH for a droplet, accompanying an increase of the full width at half-height (fwhh) of this peak from 16.3 to 17.2, 22.2, and then to 24.2 cm(-1), indicating transition of the HPO(4)(2-) anions from monomers to dimers/trimers/oligomers and then to polyanions with chain structures in the K(2)HPO(4) solutions. After 25.1% RH, the solid was proved to be K(2)HPO(4)·3H(2)O according to the Raman spectral features. Furthermore, the O-H stretching envelope of a K(2)HPO(4) droplet showed that the intensity ratios of the strong hydrogen bonding component (3255 cm(-1)) to the weak one (3417 cm(-1)) and the cage-like water (2925 cm(-1)) to the weak one (3417 cm(-1)) were sensitive to the HPO(4)(2-) association structures, which can be used to understand the effects of dimers/trimers/oligomers and chain structures of the HPO(4)(2-) associations on the hydrogen bonding of water molecules.

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.

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.

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.

Heterogeneous reactions of isoprene and ozone on α-Al2O3: The suppression effect of relative humidity.

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