Crystallization Relative Humidity Research Articles

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.

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.

On the Effects of Atmospheric Particles Contamination and Humidity on Tin Corrosion

The effects of hygroscopic atmospheric particles are investigated in relation to the corrosion of tin. Surface insulation resistance test boards were directly contaminated both with ambient particles sampled in the field at Milan, Italy, and with pure saline particles generated in the laboratory. An innovative particle deposition device was used to uniformly coat circular spots on to the test board surfaces. Deliquescence and crystallization of the water-soluble compounds were detected by observing the impedance response to varying relative humidity (RH) conditions with a gradual and continuous ramps. The effects of the adsorption/desorption kinetics and of the temperature on the deliquescence and crystallization RH values were also investigated. Leakage current measurements at 5-V dc highlighted the ability of atmospheric particles to promote corrosion and electrochemical migration at RH levels far below condensing conditions (100% RH).

Crystal nucleation initiated by transient ion-surface interactions at aerosol interfaces.

Particle collisions are a common occurrence in the atmosphere, but no empirical observations exist to fully predict the potential effects of these collisions on air quality and climate projections. The current consensus of heterogeneous crystal nucleation pathways relevant to the atmosphere dictates that collisions with amorphous particles have no effect on the crystallization relative humidity (RH) of aqueous inorganic aerosols because there is no stabilizing ion-surface interaction to facilitate the formation of crystal nuclei. In contrast to this view of heterogeneous nucleation, we report laboratory observations demonstrating that collisions with hydrophobic amorphous organic aerosols induced crystallization of aqueous inorganic microdroplets at high RH, the effect of which was correlated with destabilizing water-mediated ion-specific surface interactions. These same organic aerosols did not induce crystallization once internally mixed in the droplet, pointing toward a previously unconsidered transient ion-specific crystal nucleation pathway that can promote aerosol crystallization via particle collisions.

単一浮遊微小液滴から生成する結晶形態のその場観察

Electrostatic levitation allows a monitoring of solidification behavior of single levitated microdroplet. In this paper, novel designed slim-type electrodynamic balance (EDB) coupled with the microscope is evaluated. It enabled us in situ observation of internal mixing of microdroplet with sufficient spatial and time resolutions. With this instrument, we directly observed the crystal growth from the micodroplet of inorganic salt such as NaCl, NaBr, NaI, KCl, KBr and KI, respectively. The morphologies of produced crystal particles were divided into three types, 1) rectangular solid crystal for KCl or KBr with high crystallization relative humidity (CRH), 2) multi-crystalline particle with crystal habit for NaCl and KI with medium CRH and 3) hollow particles for NaBr and NaI droplets with low CRH. The morphologies of the produced particles were more strongly influenced by CRH rather than supersaturation.

Seasonal behavior of PM2.5 deliquescence, crystallization, and hygroscopic growth in the Po Valley (Milan): Implications for remote sensing applications

Atmospheric aerosols deliquescence and crystallization relative humidity (DRH and CRH) are rarely measured compared to the worldwide number of hygroscopicity measurements; this feature comes from the lack of an efficient method able to capture the whole complexity of chemical composition of aerosols. Despite this, the knowledge of both DRH and CRH are crucial for a correct parameterization of the aerosol hygroscopic growth used in different applications, among which the remote sensing is very important.In this paper, a newly developed technique (direct current conductance method) was applied in an aerosol chamber to Milan PM2.5 samples, to identify aerosol DRH and CRH both during winter and summer. These results were compared with those independently obtained by gravimetric measurements conducted in the chamber using a microbalance. Microbalance data allowed also the determination of the mass hygroscopic growth factor on the collected PM2.5 samples.Results evidenced first a good agreement between the two methods (RMSE=2.7% and 2.3% for DRH and CRH, respectively). Collected data evidenced the hysteresis behavior of ambient particles and variability in both DRH and CRH between the two seasons. Summer samples showed higher DRH and CRH (on average 71.4±1.0% RH and 62.6±1.2% RH, respectively) than the winter ones (on average 55.2±0.7% RH and 46.9±0.6% RH). This behavior was related to the higher content of sulfates during the summer season. Conversely, the mass hygroscopic growth factor at 90% RH was higher for winter samples (2.76±0.06) with respect to the summer ones (1.91±0.11).Since hysteresis behavior affects optical properties of aerosols, when RH conditions are within the loop, the hygroscopic growth factor could be assigned in a wrong way. Thus, the growth factor was calculated within the hysteresis loop for both upper and lower branches: results showed that difference in hygroscopic growth factor could reach up the 24%.

Experimental Measurements of Particulate Matter Deliquescence and Crystallization Relative Humidity: Application in Heritage Climatology

Climate and pollution can lead to materials weathering. In this study, an innovative method is employed to evaluate the hazard for heritage stone substrates induced by the synergic effect of particulate matter (PM) and climate. In this respect, two hazard indicators for stone materials were determined: the time of wetness and the number of dissolution and crystallization cycles. The two indicators were computed by coupling experimental measurements of the PM deliquescence and crystallization relative humidity with climatic data. For the first time, these indicators were estimated based on the PM hygroscopic properties, considering its whole hysteresis loop and its consequent hydration level. The proposed method was applied to PM samples collected in the polluted Po Valley (Milan): the experimental measurements of both PM deliquescence and crystallization relative humidity were performed in an environmental-controlled chamber using an electrical conductivity method. The time of wetness and the number of dissolution and crystallization cycles were then calculated by coupling the PM deliquescence and crystallization relative humidity with climatic data of Milan over the last decade (2003–2013). Results point out that, depending on the seasons, different hazards were identified. In winter, high time of wetness (89 ± 11%) and low number of cycles (3 ± 3 cycles/month) were found. Conversely, summer was characterized by low time of wetness (20 ± 13%) and high number of cycles (11 ± 5 cycles/month). Interestingly, spring and fall resulted the most dangerous seasons for outdoor-exposed stones, since they presented both high time of wetness and number of cycles. Since the two indicators are calculated considering PM properties and climatic data, their values are site-specific, while the method used for their determination is of general application and it can be used for an efficient hazard assessment in a heritage climatology perspective.

Determination of aerosol deliquescence and crystallization relative humidity for energy saving in free-cooled data centers

This study examines an innovative application of the aerosol deliquescence and crystallization determination, for corrosion prevention and energy-saving strategies in free-cooled data centers. Aerosol deliquescence and crystallization were investigated by combining standardized aerosol sampling techniques (i.e. EN-14907) with the assessment of the electrical effects of aerosol, while varying relative humidity within a specially designed aerosol exposure chamber. Aerosol samples collected in the Po Valley (Northern Italy) were analysed; a clearly defined hysteresis cycle (deliquescence and crystallization at 60.5 ± 0.8 and 47.9 ± 0.7 % of RH, respectively) was found. Results were applied to a data center designed for the Italian National Oil and Gas Company, making it possible to identify a critical area for direct free cooling at this data center. As a result, aerosol hydration was avoided (thus preventing aerosol from damaging electrical components) and a large amount of energy saved (using free cooling instead of air-conditioning); the potential energy saving achieved in this way was 79 % (compared to the energy consumption of a traditional air-conditioning system): 215 GWh of energy was saved, and 78 fewer kt of equivalent CO2 was emitted per year. Moreover, in order to evaluate whether a real-time estimation of the aerosol hydration state within a data center could be performed, measured deliquescence and crystallization were compared through simulations performed using three different models: two thermodynamic models for deliquescence and a parametric model for crystallization. The results obtained tend to converge in terms of deliquescence, whereas in the case of crystallization, they failed to effectively simulate experimental aerosol behaviour.

Kinetic Studies of Heterogeneous Reaction of HO2 Radical by Dicarboxylic Acid Particles

The HO2 uptake coefficients (γ) for organic submicron aerosol particles were measured using an aerosol flow tube coupled with a chemical conversion/laser-induced fluorescence technique under ambient conditions (760 Torr and 296 ± 2 K) and different values of relative humidity (RH) (28% and 68%). Determined uptake coefficients for succinic, glutaric, adipic, and pimelic acid aerosol particles at 28% RH were 0.07 ± 0.02, 0.07 ± 0.03, 0.02 ± 0.01, and 0.06 ± 0.03, respectively, whereas the γ values for those particles at 68% RH were 0.18 ± 0.07, 0.15 ± 0.04, 0.06 ± 0.01, and 0.13 ± 0.04, respectively. An increase in γ with increasing RH was observed for all the dicarboxylic acids, suggesting a contribution by water amount in the particle, aqueous phase chemistry, and uptake of HO2–H2O. The anomalously low γ values for adipic acid are likely related to its high crystallization RH and thus provide a new clue that the water amount and/or RH have a significant influence on HO2 uptake.

Phase Transition and Hygroscopic Properties of Internally Mixed Ammonium Sulfate and Adipic Acid (AS-AA) Particles by Optical Microscopic Imaging and Raman Spectroscopy

We studied the phase transition and hygroscopicity of particles of a mixture of two atmospherically relevant species, ammonium sulfate and adipic acid. We conducted extensive investigations on ammonium sulfate–adipic acid (AS-AA) particles deposited on a hydrophobic substrate with the weight percentage (wt%) of AA ranging from 3 to 90%. Crystallization and deliquescence were observed through an optical microscope, which enabled multiple particles to be examined simultaneously. At an initial relative humidity (RH) of 94%, which was the highest RH setting, AA solids were formed from the deposited solution droplets, leading to the presence of mixed-phase particles prior to the complete crystallization recorded between 31% and 42% RH of all mixtures. The complete crystallization RH values were close to that of pure AS, indicating that the AA solids did not promote effective heterogeneous nucleation of AS. When the RH was increased, partial deliquescence in all mixtures was observed at 80% RH, which was attributed to the dissolution of the AS fractions. Full deliquescence was only observed in 3 wt% AA particles at 91% RH. We also used Micro-Raman Spectroscopy to determine the hygroscopicity of mixtures with up to 50 wt% AA. The hygroscopic behavior of the AS fraction in the mixed particles was found to resemble that of pure AS particles. However, there was observable water uptake in the solid particles at about 70% RH in mixtures with up to 70 wt% AA. This early water uptake was more pronounced in mixtures with lower weight percentages of AA (AA less than 30 wt%).

Global distribution of solid and aqueous sulfate aerosols: Effect of the hysteresis of particle phase transitions

The partitioning between solid and aqueous phases of tropospheric sulfate‐ammonium particles is simulated with a global 3‐D chemical transport model (CTM). The simulation explicitly accounts for the hysteresis of particle phase transitions by transporting aqueous sulfate and three solid sulfate forms (namely, ammonium sulfate, letovicite, and ammonium bisulfate). Composition‐dependent deliquescence relative humidities (DRH) and crystallization relative humidities (CRH) are based on recent laboratory data. We find that the solids mass fraction on a sulfate basis is 0.34, partitioned as 93% ammonium sulfate, 6% letovicite, and 1% ammonium bisulfate. The fraction increases with altitude from 0.10 to 0.30 in the boundary layer to 0.60–0.80 in the upper troposphere. The dominance of solids in the upper troposphere arises in part from high sulfate neutralization, reflecting in our simulation a low retention efficiency of NH3 upon cloud freezing. High sulfate neutralization is consistent with the few available observations in the upper troposphere. High acidity with a dominant aqueous phase, however, can occur following volcanic eruptions. Seasonal variation of the solids mass fraction in both the lower and upper troposphere is modulated by emissions of NH3 from the terrestrial biosphere and biomass burning as well as by emissions of dimethylsulfide from the ocean biosphere. The timescale of phase transitions, as driven by changes in relative humidity, varies from 10 to 50 h in the boundary layer to 150–400 h in the upper troposphere. Omission of the hysteresis effect in the CTM by assuming that particle phase follows the lower side of the hysteresis loop increases the solids mass fraction from 0.34 to 0.56. An upper side assumption decreases the fraction to 0.17. Lower and upper side assumptions better approximate particle phase for high and low altitudes, respectively. Fluctuations in the CRH, which can be induced by other constituents in sulfate particles such as minerals or organic molecules, strongly affect the solids mass fraction in the boundary layer but not at higher altitudes. Further studies are needed to determine the effects of large solids mass fraction on heterogeneous chemistry and cirrus cloud formation.

Sensitivity of sulfate direct climate forcing to the hysteresis of particle phase transitions

The effects of solid‐aqueous phase transitions on sulfate direct climate forcing (SDCF) are investigated by using both a column model and a global chemical transport model. Aqueous particles have a larger mass extinction efficiency but a smaller backscattered fraction than their solid counterparts. The column model shows that the hysteresis of the phase transition can result in an uncertainty in the SDCF of 20%. The global chemical transport model explicitly accounts for the relative humidity processing of particles and the associated hysteresis. The model also treats the extent of sulfate neutralization by ammonia. For the anthropogenic sulfate, the base case simulation finds that solid particles contribute 41% of the global burden, 26% of the clear‐sky optical thickness, 31% of the clear‐sky SDCF, and 37% of the full‐sky SDCF, a trend that reflects the correlation of solid particles with clear skies. A perturbation to the model, omitting hysteresis by assuming that all particles are aqueous, results in an overestimate of the SDCF by +8% compared to the base case. A converse assumption that crystallization occurs at the deliquescence relative humidity underestimates the SDCF by −8%. A case that assumes that aqueous particles occur whenever the ambient relative humidity exceeds the crystallization relative humidity biases the SDCF by +5%. A case that includes hysteresis but omits the difference in the fraction of radiation backscattered to space by aqueous compared to solid particles changes the SDCF by +15%. Seasonal and regional differences can be much larger. We recommend that the ratio of the sulfate aerosol optical thickness calculated with versus without consideration of particle hygroscopicity be reported as a standard output of SDCF models to facilitate meaningful intercomparisons among different models.

Mass transfer effects on the hygroscopic growth of ammonium sulfate particles with a water-insoluble coating

Organic coatings have been commonly found in atmospheric particles and can affect the hygroscopicity of atmospheric particles. In this paper, we examine the effect of a water-insoluble coating on the hygroscopic growth and phase transformation properties of ammonium sulfate ((NH 4) 2SO 4) particles using an electrodynamic balance. Octanoic acid was utilized as a model compound for water-insoluble coating. (NH 4) 2SO 4 particles coated with octanoic acid (9 and 34 wt% octanoic acid) were subjected to relative humidity (RH) cycling to effect two cycles of deliquescence and crystallization. The coated particles exhibited very similar hygroscopicity in two cycles. Octanoic acid was likely present on the surface of solid (NH 4) 2SO 4 cores and (NH 4) 2SO 4 solution droplets throughout the RH cycling. Mass transfer effects were found in the deliquescence and evaporation of the heavily coated (34 wt%) particles. The mass transfer of water molecules to and through octanoic acid coating led to a significant delay in the deliquescence and crystallization of the coated particles, an increase in the observed deliquescence RH and a decrease in observed crystallization RH as compared to that of uncoated (NH 4) 2SO 4 particles. A water accommodation coefficient in the order of 10 −3 can explain these observations. When sufficient time was given to ensure equilibrium, the coating had no effect in altering the equilibrium phase transitions and hygroscopic growth of the (NH 4) 2SO 4 particles. It is important to allow sufficient time for equilibration in hygroscopic measurements of aerosol particles coated with organic coatings.

Modeling of Lactose Crystallization and Color Changes in Model Infant Foods

Lactose crystallization and color changes in formulas containing β-lactoglobulin and gelatinized starch were investigated. Model infant formulas were prepared by colyophilization of 3 components (lactose, β-lactoglobulin, and gelatinized starch). A mixture design was used to choose the percentage of each mixture component. These formulas were stored for 3 mo at different relative humidities (RH), ranging from approximately 0 to 94.6%, to study the lactose crystallization and color changes. Crystallization kinetics was studied by gravimetric methods, and lactose state (crystalline vs. amorphous) was verified before and after storage by differential scanning calorimetry. Before storage, lyophilized lactose was amorphous, but during storage it crystallized, depending on the RH. The lactose crystallization RH depended on the quantity of β-lactoglobulin and gelatinized starch, and by increasing these quantities, the crystallization RH increased. For some formulas, the crystallization RH was noted at 3 different RH during storage. The first was noted after 1 d of storage and the second and third were observed later on, showing that crystallization is a time-dependent phenomenon. Nonenzymatic browning was studied in model infant formulas by yellow color changes of samples at 11.3, 43.2, 54.5, and 75.4% RH. In this study, 7 mathematical models were proposed to predict the moisture sorption properties and color changes at different RH, and the models were validated by experimental results.

Crystallization of Aqueous Ammonium Sulfate Particles Internally Mixed with Soot and Kaolinite: Crystallization Relative Humidities and Nucleation Rates

Using optical microscopy, we investigated the crystallization of aqueous ammonium sulfate droplets containing soot and kaolinite, as well as the crystallization of aqueous ammonium sulfate droplets free of solid material. Our results show that soot did not influence the crystallization RH of aqueous ammonium sulfate particles under our experimental conditions. In contrast, kaolinite increased the crystallization RH of the aqueous ammonium sulfate droplets by approximately 10%. In addition, our results show that the crystallization RH of aqueous ammonium sulfate droplets free of solid material does not depend strongly on particle size. This is consistent with conclusions made previously in the literature, based on comparisons of results from different laboratories. From the crystallization results we determined the homogeneous nucleation rates of crystalline ammonium sulfate in aqueous ammonium sulfate droplets and the heterogeneous nucleation rates of crystalline ammonium sulfate in aqueous ammonium sulfate particles containing kaolinite. Using classical nucleation theory and our experimental data, we determined that the interfacial tension between an ammonium sulfate critical nucleus and an aqueous ammonium sulfate solution is 0.064 +/- 0.003 J m(-2) (in agreement with our previous measurements), and the contact angle between an ammonium sulfate critical nucleus and a kaolinite surface is 59 +/- 2 degrees. On the basis of our results, we argue that soot will not influence the crystallization RH of aqueous ammonium sulfate droplets in the atmosphere, but kaolinite can significantly modify the crystallization RH of atmospheric ammonium sulfate droplets. As an example, the CRH50 (the relative humidity at which 50% of the droplets crystallize) ranges from about 41 to 51% RH when the diameter of the kaolinite inclusion ranges from 0.1 to 5 microm. For comparison, the CRH50 of aqueous ammonium sulfate droplets (0.5 microm diameter) free of solid material is approximately 34.3% RH under atmospheric conditions.

Crystallization of Aqueous Inorganic−Malonic Acid Particles: Nucleation Rates, Dependence on Size, and Dependence on the Ammonium-to-Sulfate Ratio

Using an electrodynamic balance, we determined the relative humidity (RH) at which aqueous inorganic-malonic acid particles crystallized, with ammonium sulfate ((NH(4))(2)SO(4)), letovicite ((NH(4))(3)H(SO(4))(2)), or ammonium bisulfate (NH(4)HSO(4)) as the inorganic component. The results for (NH(4))(2)SO(4)-malonic acid particles and (NH(4))(3)H(SO(4))(2)-malonic acid particles show that malonic acid decreases the crystallization RH of the inorganic particles by less than 7% RH when the dry malonic acid mole fraction is less than 0.25. At a dry malonic acid mole fraction of about 0.5, the presence of malonic acid can decrease the crystallization RH of the inorganic particles by up to 35% RH. For the NH(4)HSO(4)-malonic acid particles, the presence of malonic acid does not significantly modify the crystallization RH of the inorganic particles for the entire range of dry malonic acid mole fractions studied; in all cases, either the particles did not crystallize or the crystallization RH was close to 0% RH. Size dependent measurements show that the crystallization RH of aqueous (NH(4))(2)SO(4) particles is not a strong function of particle volume. However, for aqueous (NH(4))(2)SO(4)-malonic acid particles (with dry malonic acid mole fraction = 0.36), the crystallization RH is a stronger function of particle volume, with the crystallization RH decreasing by 6 +/- 3% RH when the particle volume decreases by an order of magnitude. To our knowledge, these are the first size dependent measurements of the crystallization RH of atmospherically relevant inorganic-organic particles. These results suggest that for certain organic mole fractions the particle size and observation time need to be considered when extrapolating laboratory crystallization results to atmospheric scenarios. For aqueous (NH(4))(2)SO(4) particles, the homogeneous nucleation rate data are a strong function of RH, but for aqueous (NH(4))(2)SO(4)-malonic acid particles (with dry organic mole fraction = 0.36), the rates are not as dependent on RH. The homogeneous nucleation rates for aqueous (NH(4))(2)SO(4) particles were parametrized using classical nucleation theory, and from this analysis we determined that the interfacial surface tension between the crystalline ammonium sulfate critical nucleus and an aqueous ammonium sulfate solution is between 0.053 and 0.070 J m(-2).

FTIR-ATR chamber for observation of efflorescence and deliquescence processes of NaClO4 aerosol particles on ZnSe substrate

NaClO4 aerosol particles with diameter of 5–20 microns are deposited on the ZnSe substrate. An FTIR-ATR chamber on the ZnSe substrate is set up to observe the structural changes of NaClO4 aerosol particles at crystallization relative humidity (CRH) and deliquescence relative humidity (DRH). With the decrease of RH (relative humidity) from 96% to 24% in the efflorescence process, the absorbance of O-H stretching band of the aerosol solutions continuously decreases. A sudden decrease of the water peak is observed at RH20%, where solid particles form. Very small amount of residual interfacial water, with two weak peaks at 3602 and 3533 cm−1, can still be resolved in the FTIR-ATR spectra of the solid particles. In the deliquescence process of the same sample, little spectral changes are observed when the value of RH varies from 5% to 29%. Before the abrupt increase of the O-H stretching band at the DRH of about RH46%, a pre-deliquescence process is observed, i.e. in the RH range between 33% and 46%, there is really a slow absorbance increase for the peak area of O-H stretching band. The O-H stretching band shows an arciform O-H envelope, totally different from not only the FTIR-ATR spectra of ClO4 − solutions either under supersaturated state or in diluted state, but also the characteristic of the residually interfacial water discussed in the efflorescence process. Such kind of water is considered as pure water, indicating that small amount of water aggregates in the microspaces of solid aerosol particles due to the capillary cohesion effect. When the RH arrives at RH44%, two weak shoulders at 3384 and 3260 cm−1 can be resolved, and the solid NaClO4 particles begin to be dissolved by increasing capillary cohesion water. The spectral characteristic of the v3-ClO2 −1 band also shows the transition from solid particles to mainly solvated ClO4 −1 ions.

Diurnal variations in the hygroscopic growth cycles of ambient aerosol populations

During August and September of 2002, a relative humidity (RH) scanning tandem differential mobility analyzer system was used to measure the deliquescence/crystallization properties of ambient aerosol populations in southeast Texas. During August, sampling was conducted at a rural site on the Texas A&M campus in College Station, and in September, sampling was conducted at an urban site near the Houston ship channel. Measurements from both sites indicate that there are cyclical changes in the composition of the soluble fraction of the aerosol, which are not strongly linked to the local aerosol source. The observations show that as temperature increases and RH decreases, the hysteresis loop describing the RH dependence of aerosol hygroscopic growth collapses. On the basis of results from other studies that have shown the dominant ions present in aerosols in this region to be ammonium and sulfate, it is proposed that this collapse is due to a decrease in the ammonium to sulfate ratio in the aerosol particles, which coincides with increasing temperature and decreasing RH. This cyclical change in aerosol acidity may influence secondary organic aerosol production and may exaggerate the impact of the aerosol on human health. The compositional changes also result in a daily cycle in crystallization RH that is in phase with that of the ambient RH, which reduces the probability that hygroscopic particles will crystallize in the afternoon when the ambient RH is a minimum.

Deliquescence and crystallization of ammonium sulfate‐glutaric acid and sodium chloride‐glutaric acid particles

In the following, we report the deliquescence relative humidities (DRH) and crystallization relative humidities (CRH) of mixed inorganic‐organic particles, specifically ammonium sulfate‐glutaric acid and sodium chloride‐glutaric acid particles. Knowledge of the DRH and CRH of mixed inorganic‐organic particles is crucial for predicting the role of aerosol particles in the atmosphere. Our DRH results are in good agreement with previous measurements, but our CRH results are significantly lower than some of the previous measurements reported in the literature. Our studies show that the DRH and CRH of ammonium sulfate and sodium chloride only decreased slightly when the mole fraction of the acid was less than 0.4. If other organics in the atmosphere behave in a similar manner, then the DRH and CRH of mixed inorganic‐organic atmospheric particles will only be slightly less than the DRH and CRH of pure inorganic particles when the organic mole fraction is less than 0.4. Our results also show that if the particles contain a significant amount of organics (mole fraction > 0.5) the crystallization relative humidity decreases significantly and the particles are more likely to remain in the liquid state. Further work is needed to determine if other organics species of atmospheric importance have a similar effect.

Mixtures of pollution, dust, sea salt, and volcanic aerosol during ACE‐Asia: Radiative properties as a function of relative humidity

The Ron Brown cruise during ACE‐Asia (March–April 2001) encountered complex aerosol that at times was dominated by marine, polluted, volcanic, and dust aerosols. Average total light scattering coefficients (σsp for Dp <10 μm, relative humidity (RH) = 19%, and λ = 550 nm) ranged from 23 (marine) to 181 Mm−1 (dust). Aerosol hygroscopicity ranged from deliquescent with hysteresis (marine frequently and polluted variably) to hygroscopic without hysteresis (volcanic) to nearly hygrophobic (dust‐dominated). Average deliquescence and crystallization RH were 77 ± 2% and 42 ± 3%, respectively. The ambient aerosol was typically on the upper branch of the hysteresis loop for marine and polluted air masses and the lower branch for dust‐dominated aerosols. Average f(RH = ambient), defined as σsp (RH = ambient)/σsp (RH = 19%), ranged from 1.25 (dust) to 2.88 (volcanic). Average h(RH ∼60%), defined as f(RH)upper branch/f(RH)lower branch, were 1.6, 1.3, 1, and 1.25 for marine, polluted, volcanic, and dust, demonstrating an importance of hysteresis to optical properties. Hemispheric backscatter fraction (b) at ambient RH ranged from 0.077 (marine) to 0.111 (dust), while single scattering albedo (ω) at ambient RH ranged from 0.94 (dust and polluted) to 0.99 (marine).