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.

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

Sodium nitrate as an important inorganic component can be chemically formed from the reactions of nitrogen oxides and nitric acid (HNO3) with sea salt in atmosphere. Organic acids contribute a significant fraction of photochemical formed secondary organics that can condense on the preexisting nitrate-containing particles. Atmospheric particles often include a complex mixture of nitrate and secondary organic materials accumulated within the same individual particles. Here we studied the hygroscopicity of aerosol particles composed of sodium nitrate and glutaric acid (GA) by using a pulsed RH controlling system and a rapid scan vacuum FTIR spectrometer (PRHCS-RSVFTIR). The water content in the particles and efflorescence ratios of both NaNO3 and GA at ambient relative humidity (RH) as a function of time were obtained from the rapid-scan infrared spectra with a sub-second time resolution. Our study showed that both NaNO3 and GA crystallized at 44.1% RH during two different RH control processes (stepwise and pulsed processes). It was found that the addition of GA could suppress the efflorescence of NaNO3 during the dehumidifying process. In addition, the mixed NaNO3/GA particles release HNO3 during the dehumidifying and humidifying cycles. These findings are important in further understanding the role of interactions between water-soluble dicarboxylic acids and nitrates on hygroscopicity and environmental effects of atmospheric particles.

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

The hygroscopicity and phase transition of the mixed aerosol particles are significantly dependent upon relative humidity (RH) and interactions between particle components. Although the efflorescence behavior of particles has been studied widely, the crystallization behavior of each component in the particles is still poorly understood. Here, we study the hygroscopicity and crystallization behaviors of internally mixed ammonium sulfate (AS)/glutaric acid (GA) aerosols by a vacuum FTIR spectrometer coupled with a RH-controlling system. The mixed AS/GA aerosols in two different RH control processes (equilibrium and RH pulsed processes) show the fractional crystallization upon dehydration with AS crystallizing prior to GA in mixed particles with varying organic to inorganic molar ratios (OIRs). The initial efflorescence relative humidity (ERH) of AS decreased from ~43% for pure AS particles to ~41%, ~36% and ~34% for mixed AS/GA particles with OIRs of 2:1, 1:1 and 1:2, respectively. Compared to the ERH of 35% for pure GA, the initial ERHs of GA in mixed AS/GA particles were determined to be 31%, 30% and 28% for OIRs of 2:1, 1:1 and 1:2, respectively, indicating that the presence of AS decreased the crystallization RH of GA instead of inducing the heterogeneous nucleation of GA. When the AS fractions first crystallized at around 36% RH in the 1:1 mixed particles, GA remained noncrystalline until 30% RH. For the first time, the crystallization ratios of AS and GA are obtained for the internally mixed particles during the rapid downward RH pulsed process. The crystallization ratio of AS can reach around 100% at around 24% RH for both pure AS and the 1:1 mixed particles, consistent with the equilibrium RH process. It is clear that the RH downward rate did not influence efflorescence behavior of AS in pure AS and AS in mixed particles. In contrast, the crystallization ratio of GA can reach about 90% at 15.4% RH for pure GA particles in excellent agreement with the equilibrium RH process, whereas it is only up to 50% at 16.0% RH in the 1:1 mixed particles during the rapid downward pulsed process lower than that of the equilibrium RH process. Our results reveal that the rapid RH downward rate could inhibit the efflorescence of GA in the mixed droplets.

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

Although the dicarboxylic acid has been reported to react with nitrate for aged internally mixed aerosols in atmosphere, the quantitative nitrate depletion dependent upon composition in particles is still not well constrained. The chemical composition evolutions for malonic acid/sodium nitrate (MA/SN), malonic acid/magnesium nitrate (MA/MN) and malonic acid/calcium nitrate (MA/CN) particles with the organic to inorganic molar ratio (OIR) of 1:1 are investigated by vacuum Fourier transform infrared spectroscopy (FTIR). Upon dehydration, the intensity of the asymmetric stretching mode of COO- group (νas-COO-) increases, accompanying the decrease in OH feather band and COOH band and NO3- band. These band changes suggest malonate salts formation and HNO3 release. The quantitative NO3- depletion data shows that the reactivity of MA-MN is most and that of MA-SN is least. Analysis of the stretching mode of COO- indicates the different bond type between metal cation and carboxylate anion. In addition, water content in particles decreases at the constant RH, implying water loss with the chemical reaction. When the RH changes very quickly, water uptake delay during the humidification process reveals that water mass transport is limited below 37% RH.

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

In this research, we applied a pulsed RH controlling system and a rapid scan vacuum FTIR spectrometer (PRHCS-RSVFTIR) to investigate hygroscopicity of internally mixed (NH4)2SO4(AS)/citric acid (CA) particles. The water content and efflorescence ratio of AS in the particles and ambient relative humidity (RH) as a function of time were obtained with a subsecond time resolution. The hygroscopic behavior of AS aerosols in two different RH control processes (equilibrium and RH pulsed processes) showed that AS droplets crystallize with RH ranging from 42% to 26.5%. It was found that the half-life time ratio between the water content in the CA particles and the gas phase under RH pulsed change was greater than one under low RH conditions (<40% RH), indicating the significant water transfer limitation due to the high viscosity of CA aerosols at low RH, especially at RH<20%. In addition, water diffusion constants between 10-12 m2 s-1 and 10-13 m2 s-1 in micron size CA aerosols were obtained in a sub-second and second timescale. The addition of AS enhanced the water transfer limitation in the mixed aerosols. The efflorescence relative humidity (ERH) of the mixed particles with AS/CA by molar ratio 3:1 was found between 22.7% and 5.9%, which was much lower than AS particles. No efflorescence process was observed for the 1:1 mixed particles, indicating that CA greatly suppressed nucleation of AS. Our results have shown that the PRHCS-RSVFTIR is effective to simulate hygroscopicity and water transport of aerosols under fast variations in RH in atmosphere.

Choosing a proper exchange-correlation functional for the computational catalysis on surface.

To choose a proper functional among the diverse density functional approximations of the electronic exchange-correlation energy for a given system is the basis for obtaining accurate results of theoretical calculations. In this work, we first propose an approach by comparing the calculated ΔE0 with the theoretical reference data based on the corresponding experimental results in a gas phase reaction. With ΔE0 being a criterion, the three most typical and popular exchange-correlation functionals (PW91, PBE and RPBE) were systematically compared in terms of the typical Fischer-Tropsch synthesis reactions in the gas phase. In addition, verifications of the geometrical and electronic properties of modeling catalysts, as well as the adsorption behavior of a typical probe molecule on modeling catalysts are also suggested for further screening of proper functionals. After a systematic comparison of CO adsorption behavior on Co(0001) calculated by PW91, PBE, and RPBE, the RPBE functional was found to be better than the other two in view of FTS reactions in gas phase and CO adsorption behaviors on a cobalt surface. The present work shows the general implications for choosing a reliable exchange-correlation functional in the computational catalysis of a surface.

A DFT study of the structures of Au(x) clusters on a CeO2(111) surface.

Studying the structures of metal clusters on oxide supports is challenging due to their various structural possibilities. In the present work, a simple rule in which the number of Au atoms in different layers of Au(x) clusters is changed successively is used to systematically investigate the structures of Au(x) (x=1-10) clusters on stoichiometric and partially reduced CeO(2)(111) surface by DFT calculations. The calculations indicate that the adsorption energy of a single Au atom on the surface, the surface structure, as well as the Au-Au bond strength and arrangement play the key roles in determining Au(x) structures on CeO(2)(111). The most stable Au(2) and Au(3) clusters on CeO(2)(111) are 2D vertical structures, while the most stable structures of Au(x) clusters (x>3) are generally 3D structures, except for Au(7). The 3D structures of large Au(x) clusters in which the Au number in the bottom layer does not exceed that in the top layer are not stable. The differences between Au(x) on CeO(2)(111) and Mg(100) were also studied. The stabilizing effect of surface oxygen vacancies on Au(x) cluster structures depends on the size of Au(x) cluster and the relative positions of Au(x) cluster and oxygen vacancy. The present work will be helpful in improving the understanding of metal cluster structures on oxide supports.

A density functional theory study of the CH(2)I(2) reaction on Ag(111): Thermodynamics, kinetics, and electronic structures.

Density functional theory calculation was performed to study the adsorption and reaction of CH(2)I(2) on Ag(111). Thermodynamically favorable reactions of CH(2)I(2) on Ag(111) are C-I bond ruptures and CH(2) coupling to form ethylene. The energy barriers for the C-I bond ruptures of chemisorbed CH(2)I(2) on Ag(111) are 0.43-0.48 eV, whereas the activation energy for the C-H bond rupture of chemisorbed CH(2) on Ag(111) is 1.76 eV. The coupling reaction barrier of neighboring chemisorbed CH(2) to form C(2)H(4) on Ag(111) was much less than those of the C-I bond ruptures of CH(2)I(2)(a) and the migration of chemisorbed CH(2) on Ag(111). The adsorption behaviors of different surface species on Ag(111) were well explained in terms of the charge density difference.