Generation of Internally Mixed Insoluble and Soluble Aerosol Particles to Investigate the Impact of Atmospheric Aging and Heterogeneous Processing on the CCN Activity of Mineral Dust Aerosol

Heterogeneous reactions of trace gases with mineral dust aerosol not only impact the chemical balance of the atmosphere but also the physicochemical properties of the dust particle and the ability of the particle to act as a cloud condensation nuclei (CCN). Recent field studies have shown that carbonate minerals are preferentially associated with nitrates whereas aluminum silicates (i.e., clay minerals) are preferentially associated with sulfates. To better understand how this association can impact the climate effects of mineral dust particles, we have measured the CCN activity of a number of pure and internal mixtures of aerosols relevant to these recent field studies. The CCN activity of CaCO ₃ -Ca(NO ₃ ) ₂ aerosol, simulating the activity of mineral dust aerosol that has been partially processed by nitrogen oxides in the atmosphere, is significantly enhanced relative to CaCO₃ aerosol of the same diameter. Similar results are obtained for a clay mineral, kaolinite, internally mixed with (NH ₄ ) ₂ SO ₄ . For example, at 0.3% supersaturation, a 200 nm particle containing a soluble nitrate or sulfate component is 2 to 4 times more active than an unreacted particle. The results presented here show that when determining the contribution of mineral dust aerosol to the overall impact of the aerosol indirect effect on radiative forcing, changes in chemical composition due to atmospheric processing cannot be ignored.     

Cloud condensation nuclei activity and droplet formation of primary and secondary organic aerosol mixtures

Understanding the mixing behavior of anthropogenic primary and biogenic secondary organic aerosol (POA and SOA) is important for characterizing their interactions with water vapor. The following work expands upon previous studies and investigates cloud condensation nuclei (CCN) activity and droplet kinetics of α-pinene SOA formed in an environmental chamber and mixed with diesel or motor oil-diesel fuel POA. The changes in the aerosol mixing are similar to previously published work but this study provides new CCN activity and droplet information. The CCN activity of the unmixed aerosol systems are measured separately; κ = 0.15, 0.11, 0.022 for α-pinene SOA, diesel POA and motor oil-diesel fuel POA, respectively. In the α-pinene SOA + diesel POA mixture, the CCN activity, characterized by κ-hygroscopicity, decreases from κ = 0.15 to 0.06 after an initial injection of the POA but increases to κ = 0.12. The increase in CCN activity occurs after particle collision (coagulation and wall-loss) rates dominate aerosol processes in the chamber. The α-pinene SOA + motor oil-fuel POA does not readily mix and the CCN activity of the complex system increases with time (from κ = 0.022 to 0.10). An empirical equation using unit mass resolution (UMR) AMS data of two different ion fragments reasonably predicts CCN activity of the POA and SOA mixtures. CCN measurement may be a promising tool to gain additional insight into the complex mixtures of organic aerosol and subsequent interactions with water vapor.

Copyright © 2018 American Association for Aerosol Research     

Mapping the Operation of the DMT Continuous Flow CCN Counter

This work thoroughly analyzes a new commercial instrument for measuring Cloud Condensation Nuclei (CCN), the Droplet Measurement Technologies Cylindrical Continuous-Flow Streamwise Thermal Gradient CCN Chamber (CFSTGC). This instrument can measure CCN concentrations at supersaturations from 0.06% to 3% (potentially up to 6%), at a 1 Hz sampling rate that is sufficient for airborne operation. Our analysis employs a fully coupled numerical flow model to simulate the water vapor supersaturation, temperature, velocity profiles and CCN growth in the CFSTGC for its entire range of operation (aerosol sample flow rates 0.25–2.0 L min ^{? 1} , temperature differences 2–15 K and ambient pressures 100–1000 mb). The model was evaluated by comparing simulated instrument responses for calibration aerosol against actual measurements from an existing CCN instrument. The model was used to evaluate the CCN detection efficiency for a wide range of ambient pressures, flow rates, temperature gradients, and droplet growth kinetics. Simulations overestimate the instrument supersaturation when the thermal resistance across the walls of the flow chamber is not considered. We have developed a methodology to determine the thermal resistance and temperature drop across the wetted walls of the flow chamber, by combining simulations and calibration experiments. Finally, we provide parameterizations for determining the thermal resistance, the instrument supersaturation and the optimal detection threshold for the optical particle counter.     

Characterization of an Ambient Coarse Particle Concentrator Used for Human Exposure Studies: Aerosol Size Distributions,Chemical Composition,and Concentration Enrichment

The goals of the experiments described herein involve determining in real time the size, concentration enrichment, and chemical composition of coarse-mode (<2.5 μm) and fine-mode (>2.5 μm) particles within the nonconcentrated and concentrated flows of a coarse particle concentrator used for human exposure studies. The coarse particle concentrator was intended to concentrate ambient particles in the PM_10–2.5 size range before sending them into a human exposure chamber. The aerodynamic size and chemical composition of particles in the upstream and downstream flows of the concentrator were monitored with an aerosol time-of-f1ight mass spectrometer (ATOFMS) for fixed time intervals over the course of three days. Based on the ATOFMS results, it was found that there was no change in the composition of the ten major particle types observed in the upstream and downstream flows of the concentrator under normal operating conditions. Furthermore, no new particle types were detected downstream that were not detected upstream of the concentrator. A characterization of the aerosol chemical composition and its dependence on sampling conditions is also discussed. Aerosol size distributions were measured with three aerodynamic particle-sizing (APS) instruments sampling simultaneously from different regions of the concentrator. The APS size distributions were used to scale ATOFMS data and measure the ambient concentration factors for the coarse particle concentrator and the exposure chamber. The average concentration factor (ratio of inlet number concentration to the outlet number concentration) for the particle concentrator was 60 + 17 for the 2.5–7.2 μm size range before dilution and transport to the exposure chamber. It was observed that not only were coarse particles being concentrated but fine (<2.5 μm) particles were being concentrated as well, with concentration factors ranging from 2–46 for aerodynamic particle sizes from 0.54–2.5 μm.     

Integrating Cloud Condensation Nuclei Predictions with Fast Time Resolved Aerosol Instrumentation to Determine the Hygroscopic Properties of Emissions Over Transient Drive Cycles

The physical and chemical properties of aerosols emitted from vehicles can vary in composition under different driving conditions. Thus, characterizing ephemeral changes in aerosol cloud condensation nuclei (CCN) activity and apparent hygroscopicity for vehicle-testing procedures conducted over transient drive cycles can be challenging. To evaluate CCN activity of these emitted aerosols, a closure method integrating traditional CCN measurements with fast time resolved aerosol instrumentation typically used to measure engine exhaust was utilized. Calibration of the method predicted activation diameters, D_d, within 10% and 15% of D_d derived from Köhler theory for two stable sources, aerosolized ammonium sulfate and α-pinene secondary organic aerosol, respectively. It was then applied to a transient source to estimate the effect of six different ethanol and iso-butanol gasoline blends on the hygroscopic properties of emissions downstream a gasoline direct injection light duty passenger vehicle over transient drive cycles. To describe the CCN activity, a single hygroscopicity parameter, kappa, was used. Results indicate low CCN activity with kappa ranging between ~0.002 and 0.06.

Copyright 2015 American Association for Aerosol Research     

Using Raman Microspectroscopy to Determine Chemical Composition and Mixing State of Airborne Marine Aerosols over the Pacific Ocean

Chemical composition and mixing state of aerosols collected over an 11,000 km latitudinal cruise in the Pacific Ocean are reported here as determined by a new application of Raman spectroscopy. The Raman microspectroscopy technique employs a Raman spectrometer coupled to an optical microscope to identify the chemical composition and internal mixing state of single particles. By analyzing multiple particles in a collected ensemble, the degree of external mixing of particles was also determined. To lend context to the Pacific aerosol population sampled, atmospheric aerosol concentration, and the critical supersaturation required for the aerosols to activate as cloud condensation nuclei, and chlorophyll a concentration in the underlying water (a metric for phytoplankton biomass in the ocean) were also obtained. Our results indicate that long chain organic molecules were prevalent in the marine aerosol samples throughout the cruise, including during coastal and open ocean locations, in both hemispheres, and in the seasons of autumn and spring. Long chain organic compounds tended to be present in internal mixtures with other organic and inorganic components. Although variations in the fraction of aerosols activated as CCN were observed, no simple correlation between organics and CCN activation was found. According to our measurements, marine aerosol in the Pacific Ocean may be generally characterized as multicomponent aerosol containing and often dominated by a high organic fraction. Our results suggest that the prevalence of organics and the high degree of internal mixing of aerosol must be accounted for in accurate modeling of the role of marine aerosols in cloud formation and climate.

Copyright 2014 American Association for Aerosol Research     

New Particle Formation and Growth in an Isoprene-Dominated Ozark Forest: From Sub-5 nm to CCN-Active Sizes

Particle Investigations at a Northern Ozarks Tower: NOx, Oxidant, Isoprene Research (PINOT NOIR) were conducted in a Missouri forest dominated by isoprene emissions from May to October 2012. This study presents results of new particle formation (NPF) and the growth of new particles to cloud condensation nuclei (CCN)-active sizes (～100 nm) observed during this field campaign. The measured sub-5 nm particles were up to ～20,000 cm^?3 during a typical NPF event. Nucleation rates J₁ were relatively high (11.0 ± 10.6 cm^?3 s^?1), and one order of magnitude higher than formation rates of 5 nm particles (J₅). Sub-5 nm particle formation events were observed during 64% of measurement days, with a high preference in biogenic volatile organic compounds (BVOCs)- and SO₂-poor northwesterly (90%) air masses than in BVOCs-rich southerly air masses (13%). About 80% of sub-5 nm particle events led to the further growth. While high temperatures and high aerosol loadings in the southerly air masses were not favorable for nucleation, high BVOCs in the southerly air masses facilitated the growth of new particles to CCN-active sizes. In overall, 0.4–9.4% of the sub-5 nm particles grew to CCN-active sizes within each single NPF event. During a regional NPF event period that took place consecutively over several days, concentrations of CCN size particles increased by a factor of 4.7 in average. This enhanced production of CCN particles from new particles was commonly observed during all 13 regional NPF events during the campaign period.

Copyright 2014 American Association for Aerosol Research     

Deposition and Clearance Models of Ni Compounds in the Mouse Lung and Comparisons with the Rat Models

Experimental data from inhalation studies in mice were used to develop mathematical models of deposition, clearance, and retention kinetics in the respiratory tract for inhaled Ni compounds (high temperature (green) NiO,Ni₃S₂, and NiSO₄.6H₂O) in the mouse lung. For deposition, a new model was developed using the experimental data on nasal deposition and lung morphometry by Phalen (1991). Three major mechanisms of airway deposition, including impaction, sedimentation, and diffusion, were considered in the deposition model. Because of the differences in physiological and ventilation conditions, it was found that mice have a lower alveolar deposition fraction than rats when exposed to the same Ni compounds. In the development of a clearance model, a single compartment model in the lung was used and a general assumption was made that the clearance of the insoluble and moderately soluble Ni compounds (high temperature (green) NiO and Ni₃S₂, respectively) depends highly on the volume of retained particles in the lungs. As for the highly soluble Ni compound (NiSO₄.6H₂O), the clearance rate coefficient was assumed to depend on the retained particle mass and total alveolar surface. The retention half time, however, was found to increase with the lung burden for high temperature (green) NiO and NiSO₄.6H₂O particles, but to decrease with the lung burden for Ni₃S₂ particles. The retention half times for high temperature (green) NiO and Ni₃S₂ particles in mice are shorter than in rats, whereas the retention half time for NiSO₄.6H₂O particles is the same for both species.     

CCN activation of oxalic and malonic acid test aerosols with the University of Vienna cloud condensation nuclei counter

The cloud droplet activation of monodisperse laboratory aerosols consisting of single organic and inorganic substances as well as a mixture of several substances was investigated using the University of Vienna cloud condensation nuclei counter (CCNC). The CCNC operates on the principle of a static thermal diffusion chamber. Water vapour supersaturations can be set in the range from 0.1% to 2%. Aqueous solutions of oxalic acid and malonic acid as well as solutions of inorganic compounds (NaCl and (NH₄)₂SO₄) were nebulized in a Collison atomizer and then passed through a closed-loop differential mobility particle spectrometer to produce monodispersed particles. An internally mixed aerosol consisting of ammonium sulphate, oxalic acid and malonic acid with relative concentrations resembling those found in cloud water at a mountain station [Löflund, Kasper-Giebl, Schuster, Giebl, Hitzenberger, Reischl et al. (2002) Atmos. Environ. 36, 1553] was also investigated for cloud condensation nuclei (CCN) activation. All these particles were activated at supersaturations expected from Köhler theory. Oxalic and malonic acid particles are therefore expected to be good atmospheric CCN both as pure particles and as internally mixed particles containing other chemical compounds.     

Coagulation Mechanism in Wet Spinning of Fibres

Spinning baths are divided into three types as a function of the coagulating (desolvating) power: hard, soft, and ultrasoft. The concentration of precipitant c_p in hard spinning baths is two times higher than the critical (threshold) precipitation concentration c_cr, i.e., c_p > 2c_cr. Precipitation takes place with formation of a boundary line (precipitation front) which coincides with the critical supersaturation line — spinodal. The fibre is characterized by large fibril size and high porosity. Soft baths are realized when the concentration of precipitant in the bath is less than two and more than one critical precipitation concentration, i.e., c_cr > c_p > 2c_cr. Precipitation takes place according to a frontal mechanism with initial formation of a liquid segment in the spun fibre. In spinning into soft baths, the optimum conditions are created for formation of fibres with a small-fibril structure and elevated physicomechanical indexes. At a concentration of precipitant below c_cr but above c_e (equilibrium), the precipitant accumulates on the fibre and precipitation takes place over the entire volume, without formation of a precipitation front. The fibre obtained is characterized by high porosity. __________ Translated from Khimicheskie Volokna, No. 4, pp. 26–31, July–August, 2005.     

First Surface Measurement of Cloud Condensation Nuclei over Kanpur,IGP: Role of Long Range Transport

Measurements have been carried out for cloud condensation nuclei (N _CCN, number concentration at 0.38% average depleted supersaturation, SS) and submicron aerosol (N _CN), using a CCN (cloud condensation nuclei) counter (Droplet Measurement Technology) and Scanning Mobility Particle Sizer (TSI), respectively, for a large number of days in each season of the year 2008 and 2009 at Kanpur, North India. Aerosol chemical composition was also measured for 3 days and 3 nights during November–December 2009. N_CCN was generally much higher than observed at similar environments elsewhere except in Chinese cities. Due to higher loading of CCN the supersaturation depletion correction is applied. Significant intraseasonal variability was observed in N_CCN and CCN/CN ratio (N _CCN /N _CN), due to different air masses coming from north–west, east, and central parts of India. The CCN concentrations at 0.38% and CCN/CN ratio for the year 2008 varied between 10,043–12,107 cm^?3 and 0.12–0.30 in winter season and 5942–7184 cm^?3 and 0.07–0.15 in premonsoon season, respectively. For 2009, it varied between 10,518–13,029 cm^?3 and 0.28–0.53 in winter season and 3596–8040 cm^?3 and 0.20–0.28 in postmonsoon season, respectively. Higher CCN/CN ratio was observed during winter season when the air mass came from north–west, central, and eastern landmass of India. This was most likely due to relatively high accumulation mode particle concentration and large number of forest fires observed in those regions. As expected, polluted continental air masses lead to a significant increase in CCN concentrations over the winter months, most likely due to increased anthropogenic activities, i.e., increased fuel usage, large biomass burning coupled with lower mixed boundary layers. A closure study was performed by application of Köhler theory, utilizing chemical composition, and size distribution measured by SMPS. CCN concentrations were predicted for 3 days and 3 nights and these values were compared with measured CCN values at 0.13, 0.33, and 0.64% SS. In the present closure study, CCN values were slightly overpredicted to the extent of 21% ± 18%.

Copyright 2012 American Association for Aerosol Research     

Opposed Migration Aerosol Classifier (OMAC)

A new differential mobility classifier is described. This instrument classifies aerosol particles in a channel flow between porous (or screen) electrodes. The aerosol enters the channel parallel to the porous electrodes, while a larger, particle-free cross-flow enters through one of the porous electrodes and exits through the opposing porous electrode. A potential difference between the electrodes causes charged particles to migrate upstream, against the cross flow. Particles whose migration velocity, v_m exactly balances the cross-flow velocity, v, are transmitted directly along the length of the channel with the minor aerosol flow. Particles whose migration velocities differ from the cross-flow velocity deposit on or pass through the porous electrodes. In the limit of nondiffusive particles, the probability of transmission of particles with migration velocities different from v* _m = –v matches the triangular transfer function of the classical differential mobility analyzer (DMA). Monte Carlo simulations of the transmission of diffusive particles reveals that the resolution of this Opposed Migration Aerosol Classifier (OMAC) remains close to the same ideal, nondiffusive limit of the DMA to much lower voltages than those required by the DMA to achieve equivalent resolution. This extended range enables development of a Nano Opposed Migration Aerosol Classifier (nOMAC) with the same dynamic range of mobilities as the DMA but in a much smaller package. The lower voltage operation also enables operation at low pressure without the loss of dynamic range that DMAs suffer and at higher peak resolutions than are possible with DMAs. Furthermore, the classification method can be applied to gravitational, centrifugal, thermophoretic, and other separation fields, or to separations of particles in liquids.     

The calibration of a timbrell aerosol spectrometer

An experimental method for the calibration of a Timbrell aerosol spectrometer is described and the influence of the aerosol and winnowing air flow rates on performance investigated. The Timbrell spectrometer size classifies airborne particles according to their aerodynamic diameters, winnowing them in a recirculating flow of clean air while they fall under the influence of gravity in a horizontal settling chamber. Size separation takes place under Stokesian conditions and the particles deposit on to a series of microscope slides. The results of the calibration with monodisperse polymer latex microspheres and polydisperse spherical particles of polyvinyl acetate show that the instrument achieved acceptable size resolution (12%) at an aerosol sampling flow rate of 1 cm³ min⁻¹, with a ratio of aerosol to winnowing flow rate of 6.7 × 10⁻³. Under the conditions used the instrument was shown to be most sensitive for size classifying particles in the range 4–12 μm aerodynamic diameter. The Timbrell aerosol spectrometer is a useful device both as a reference method for the determination of aerodynamic diameters of airborne particles and as a means of collecting such particles for subsequent examination by other aerosol analysis equipment.     

Hydrolysis of Organonitrate Functional Groups in Aerosol Particles

Organonitrate (ON) groups are thought to be important substituents in secondary organic aerosols (SOAs). Model simulations and laboratory studies indicate a large fraction of ON groups in aerosol particles, but much lower quantities are observed in the atmosphere. Hydrolysis of ON groups in aerosol particles has been proposed recently to account for this discrepancy. To test this hypothesis, we simulated formation of ON molecules in a reaction chamber under a wide range of relative humidity (RH) (0 to 90%). The mass fraction of ON groups (5 to 20% for high-NO_x experiments) consistently decreased with increasing RH, which was best explained by hydrolysis of ON groups at a rate of 4 day^?1 (lifetime of 6 h) for reactions under RH greater than 20%. In addition, we found that secondary nitrogen-containing molecules absorb light, with greater absorption under dry and high-NO_x conditions. This work provides the first evidence for particle-phase hydrolysis of ON groups, a process that could substantially reduce ON group concentration in atmospheric SOAs.

Copyright 2012 American Association for Aerosol Research     

Validation of computation method to predict aerodynamic diameter of particles with rough surface

Two instruments, namely the Aerosizer LD and the Particle Size Distribution Analyser (PSDA) 3603, were used to measure aerodynamic diameters of spray dried bovine serum albumin (BSA) particles with different surface corrugation (D_S). The measured aerodynamic diameters were compared with the values calculated using the theoretical model previously developed by our group. The model-calculated aerodynamic diameters have been shown to agree with the measured values for corrugated and less cohesive BSA particles with D_S varying between 2.18 and 2.41. For smoother and more cohesive BSA particles with D_S of 2.06 the measured and calculated values differed by 27% (compared to the value measured by PSDA 3603) and 115% (compare to the value measured by Aerosizer). This variation resulted from the difficulty to completely disperse the more cohesive particles during measurement. Hence, the model can be used to estimate aerodynamic diameter in situations when direct measurement fails due to unavailability of equipment, limitations or difficulty in dispersing powders into their individual particles in the aerosol.     

Effect of pressure on the precipitation and sol-phase aqueous polymerization of methyl methacrylate

Effect of pressure (atmospheric to 120 kg/cm²) on the K₂S₂O₈–Na₂S₂O₄-initiated aqueous polymerization of methyl methacrylate has been studied at 25°C. When the concentrations of the redox initiator are so adjusted as to obtain the separating polymer phase as a coarse coagulum, the conversion, rate, and molecular weight of polymerization tend to rise initially with increase of pressure up to a certain value and fall subsequently to a limiting value. However, these parameters fall monotonously with an increase in pressure when the polymer phase separates out as a fine colloid at a lower concentration of the initiator. The initial rise in rate is consistent with an increase in k_p and or a decrease in k_t under high pressure; the ultimate fall in rate may be due to a decrease in the diffusion of monomer from the aqueous phase to the growing polymer radical site. The fall in the molecular weight with pressure is explained on the basis of enhanced monomer transfer. In the colloidal range the pressure dependence trend is related to the stability of the colloidal phase. The rate is proportional to the square root of the product of K₂S₂O₈–Na₂S₂O₄ and varies linearly as the first power of the monomer concentration as also observed under normal pressure conditions. The MWD values of the polymers are ca. 2.5 and do not change with applied pressure.     

Ignition Characteristics of Zr/BaCrO4 Pyrolant

Zirconium (Zr) and barium chromate (BaCrO₄) is a pyrolant of high energy content. Zr has a high and BaCrO₄ a low sensitivity. The mixture of the two components has been investigated with respect to its ignition characteristics. The ignition delay time of the pyrolant decreases with an increasing concentration of Zr particles. That means, since the concentration of Zr is proportional to its total surface area S, the ignition delay time decreases with increasing S. An ignition delay time, τ_ig, is obtained adding τ_p and τ_c, where τ_p is the physical ignition delay time and τ_c is the chemical ignition delay time. τ_p does not change when the concentration of Zr increases. However, τ_c decreases with an increasing concentration of Zr, resulting in an increase of its surface area S. It is expected that Zr easily reacts with barium chromate.     

Design and Operation of a Pressure-Controlled Inlet for Airborne Sampling with an Aerodynamic Aerosol Lens

Two pressure-controlled inlets (PCI) have been designed and integrated into the Aerodyne Aerosol Mass Spectrometer (AMS) inlet system containing an aerodynamic aerosol lens system for use in airborne measurements. Laboratory experiments show that size calibration and mass flow rate into the AMS are not affected by changes in upstream pressure (P ₀ ) of the PCI as long as the pressure within the PCI chamber (P _PCI ) is controlled to values lower than P ₀ . Numerous experiments were conducted at different P _PCI , P ₀ , and AMS lens pressures (P _Lens ) to determine particle transmission efficiency into the AMS. Based on the results, optimum operating conditions were selected which allow for constant pressure sampling with close to 100% transmission efficiency of particles in the size range of ～ 100–700 nm vacuum aerodynamic diameter (d _va ) at altitudes up to ～ 6.5 km. Data from an airborne field study are presented for illustration.     

Intermediate Air Filters for General Ventilation Applications: An Experimental Evaluation of Various Filtration Efficiency Expressions

Neither the European standard nor the US standard for classification of intermediate class filters comprises testing of filter performance with respect to ultrafine particles (UFPs) or particles of the most penetrating size (MPPS). This could turn out to be a major lack in classification standards since UFPs have been pointed out as a serious health hazard. In this study, fractional efficiencies of eight new full-scale bag filters and twenty-three new filter medium samples were determined. The influence of air velocity and aerosol type was investigated, and correlations between efficiencies for UFPs (EF_UFPs), MPPS-sized particles (EF_MPPS) and 0.4 μm-sized particles (EF_0.4μm) were established. The tested bag filters were challenged by four aerosol types: a neutralized atomized oil aerosol, the same oil aerosol but non-neutralized, a non-neutralized thermally generated oil smoke, and a “natural” indoor aerosol. The tests were carried out at different air velocities through the filter medium, ranging between 0.08 m/s and 0.22 m/s. The relationships that were observed between EF_UFPs, EF_MPPS, and EF_0.4μm appeared to be linear within the observed filtration efficiency ranges. These relationships were similar regardless of the test aerosol type used, but somewhat different for glass fiber filters than for charged synthetic filters. Generally, EF_MPPS was 10–20% lower than EF_0.4μm. The influence of air velocity variations on the size resolved efficiency was determined. The glass fiber filters showed practically the same fractional efficiencies regardless of whether the test aerosol was neutralized or not. However, the charged synthetic filters showed substantially lower efficiencies when tested with the non-neutralized aerosol compared to the case when the aerosol was neutralized.

Copyright 2013 American Association for Aerosol Research     

Contribution of the edge effect to physical adsorption in micropores of activated carbons

The adsorption process in activated carbon micropores can be viewed as a two-dimensional condensation on micropore walls at a critical condensation pressure, p_c, which evolves into volume filling. p_c on a surface of micropore walls is calculated in respect to a critical condensation pressure on the reference nonporous surface, p_c^ref. Although theoretical treatment predicts that p_c^ref should be a parameter of adsorption isotherms, experimental isotherm equations are expressed in terms of the bulk saturation pressure, p_s. Experimental values of p_s/p_c^ref for a graphitized carbon black, which is often considered as a model of activated carbon surfaces, range from 37 to 4000 for typical adsorbates and are by no means close to unity. It is proposed that this apparent discrepancy between theory and experiments has its origin in the edge effects: carbon blacks are modeled by semi-infinite slabs, whereas the magnitudes of thickness and diameters of micropore walls are of the order of a nanometre. Factors associated with edge effects reduce the adsorption energies on surfaces of finite particles resulting in shifts of condensation pressures to higher values. For particles with diameters about 1.4 and 1.8 nm, adsorption energies decrease to ≈0.72ε₁*^∞ and ≈0.81ε₁*^∞, respectively, where ε₁*^∞ is the energy of adsorption on an infinite plane, and p_c approaches p_s. This phenomenon substantiates the application of p_s in adsorption equations and the success of the Dubinin equations probably owes much to this fact.