Sodium Chloride Aerosol Research Articles

Laboratory Characterization of Modified Tapered Element Oscillating Microbalance Samplers

Laboratory tests with generated aerosols were conducted to test the efficacy of two recent design modifications to the well-established tapered element oscillating microbalance (TEOM) continuous particulate matter (PM) mass monitor. The two systems tested were the sample equilibration system-equipped TEOM monitor operating at 30 °C, which uses a Nafion dryer as part of the sample inlet, and the differential TEOM monitor, which adds a switched electrostatic precipitator and uses a self-referencing algorithm to determine “true PM mass.” Test aerosols included ammonium sulfate, ammonium nitrate, sodium chloride, copper (II) sulfate, and mixed aerosols. Aerosols were generated with an atomizer or a vibrating orifice generator and were equilibrated in a 450-L slow flow chamber before being sampled. Relative humidity in the chamber was varied between 10 and 90%, and step changes in humidity were executed while generating aerosol to test the response of the instruments. The sample equilibration system-equipped TEOM monitor does reduce, but not totally eliminate, the sensitivity of the TEOM mass monitor to changes in humidity. The differential TEOM monitor gives every indication of being a very robust technique for the continuous real-time measurement of ambient aerosol mass, even in the presence of semi-volatile particles and condensable gases.

Reactive Uptake of Nitric Acid into Aqueous Sodium Chloride Droplets Using Real-Time Single-Particle Mass Spectrometry

The kinetics and rate-limiting mechanism of reactive uptake of nitric acid into submicron, deliquescent sodium chloride aerosols are determined. The reaction was performed in a flow tube with a real-time single-particle mass spectrometer connected to the outlet to determine the chloride-to-nitrate ratio of the transformed particles. Pseudo-first-order conditions were maintained by keeping the nitric acid pressure constant, as measured with a chemiluminescence analyzer. Experiments were performed with 100−220 nm diameter droplets at ca. 80% relative humidity and nitric acid concentrations between 60 and 380 ppb. Rate constants and uptake coefficients were determined from time-dependent changes in the chloride concentration of individual particles that traversed the reactor. The initial reactive uptake coefficient for 100 nm droplets was found to be 4.9 ± 2.7 × 10-3 and, to a first approximation, independent of the nitric acid concentration. The uptake coefficient also was found to increase linearly with increasing droplet size over the size range studied. These dependencies suggest that transport phenomena (diffusion of HNO3(g) to the droplet surface, accommodation of HNO3 into the droplet, diffusion of HNO3(aq) and HCl(aq) inside the droplet, and transport of HCl across the droplet surface into the gas phase) do not limit the reaction rate. Instead, reactive uptake of HNO3(g) is limited by formation of molecular HCl in the aqueous phasea thermodynamically controlled process. The results are consistent with related measurements that show uniform chloride/nitrate/sodium mole ratios in submicron-size aerosols, a large uptake coefficient for aerosols in the low micron-size range, and decreasing chloride-to-nitrate ratios (because of transport limitations) in supermicron-size aerosols.

Performance of a New Commercial Electrical Mobility Spectrometer

A commercial electrical mobility spectrometer (Grimm 5.4-900 DMA and Grimm 5.403 CPC) was examined with respect to size, concentration and time-dependent responses of the CPC, the transfer function of the DMA, as well as the performance of the data inversion algorithm. For comparison with previously published results, the responses were also measured for two other commercial CPCs (TSI Model 3010 and TSI Model 3022). All measurements were performed using sodium chloride aerosol produced by an atomizer, except for size-dependent counting efficiency measurements, where an evaporation and condensation method was used to produce particles down to a size of 2 nm. The concentration-dependent CPC counting efficiency was measured against a reference instrument working in the single particle count mode following dilution. The CPC time response was evaluated for monodisperse aerosol by measuring the step response using a solenoid valve technique. The transfer function of the Grimm DMA 5.4-900 was determined between 10 and 100 nm with two verified identical DMAs in series following the approach of Fissan et al. Measured tandem-DMA responses were compared to simulated responses based on the diffusion-broadened transfer function of Stolzenburg to define the factors influencing resolution. Finally the data inversion algorithm was tested by comparison with other devices.

Brownian coagulation of polydisperse aerosols in the transition regime

Brownian coagulation of aerosols in the transition regime was studied at Knudsen numbers ranging from 0.8 to 5.5. Coagulation rates were obtained from the change in total number concentration of polydisperse oleic acid and the sodium chloride aerosols with geometric mean diameters between 25 and 150 nm in a closed chamber at atmospheric pressure. Particle size distributions and concentrations were measured by scanning mobility particle sizer (TSI) and condensation nuclei counter (TSI), respectively. The experimental coagulation rates took deposition rates (diffusional wall loss rates) into account, because coagulation and diffusion deposition occur simultaneously in a closed chamber. In addition, to obtain the accurate coagulation rates of the experiments, two approaches were used and compared with each other. The experimental results were observed to follow the theoretical results of Dahneke and Fuchs, which showed that coagulation coefficients increased just a little in the transition regime.

Effect of air humidity on the evolution of permeability and performance of a fibrous filter during loading with hygroscopic and non-hygroscopic particles

A model was developed to predict the permeability and performance (i.e., the product of the permeability by the number of aerosol particles caught per unit surface) of a fibrous filter in dynamic regime. Besides the mass loading of the aerosol particles, this model took into account the structural characteristics of the filter, the size of the loading particles and the filtration velocity. An experimental filtration study was carried out to obtain the necessary input data for a comprehensive estimation of some relevant parameters of the model and to verify it. This study was performed with sodium chloride and alumina aerosols in the size range of 0.5– 1.3 μm and 0.8– 6 μm , respectively, and filtration velocities of 2, 5 and 15 cm/s . The relative air humidity was controlled in the range of 32–90%. It was observed that the permeability and the performance of tested filter were not only influenced by the size of aerosol particle and mass loading of particles but also by the particle hygroscopicity and air humidity, through the interdependence of this quantities with cake particle density. Furthermore, the filter was found to be more efficient when it operated at high humidities, except for sodium chloride particles at humidities above the deliquescent point.

A closure study of aerosol mass concentration measurements: comparison of values obtained with filters and by direct measurements of mass distributions

We compare measurements of aerosol mass concentrations obtained gravimetrically using Teflon coated glass fiber filters and by integrating mass distributions measured with the differential mobility analyzer–aerosol particle mass analyzer (DMA–APM) technique (Aerosol Sci. Technol. 36 (2002) 227). The DMA–APM technique measures the distribution of particle mass as a function of mobility size for particles of arbitrary shape and composition (Relationship between particle mass and mobility, and between aerodynamic and mobility size distributions for diesel exhaust particles, Environ. Sci. Technol., 2003). Because DMA–APM measurements are made on particles suspended in the air, data are not affected by volatilization or adsorption that can affect the accuracy of the filter measurements. We show that the average ratio of the filter to DMA–APM mass concentrations for laboratory-generated dioctyl sebacate (DOS) and sodium chloride (NaCl) aerosols is 1.14±0.28, and they are well correlated ( R 2>0.97). For diesel exhaust aerosols from an engine operating at 75% load, the two techniques agreed well with the average ratio of 0.98±0.20. When the engine was operated at a low (10%) load, mass concentrations measured with the filter were 2.13±0.54 times higher than values measured with the DMA–APM. We believe that the higher filter loading may be due to the adsorption of condensable vapors, which are emitted at higher rates under low engine load conditions. Measurements in which the condensable organics were removed with a catalytic stripper show much better agreement between the filter and DMA–APM, which support the hypothesis that vapor adsorption leads to artificially high filter data for low-load measurements. We conclude that the DMA–APM technique can be used to evaluate the accuracy of filter samples that may be affected by sampling artifacts, and to measure mass distributions with high time resolution for sub-0.5 μm aerosols.

Particle size dependent response of aerosol counters

During an international workshop at the Institute for Experimental Physics of the University of Vienna, Austria, which was coordinated within the Committee on Nucleation and Atmospheric Aerosols (IAMAS-IUGG), 10 instruments for aerosol number concentration measurement were studied, covering a wide range of methods based on various different measuring principles. In order to investigate the detection limits of the instruments considered with respect to particle size, simultaneous number concentration measurements were performed for monodispersed aerosols with particle sizes ranging from 1.5 to 50 nm diameter and various compositions. The instruments considered show quite different response characteristics, apparently related to the different vapors used in the various counters to enlarge the particles to an optically detectable size. A strong dependence of the 50% cutoff diameter on the particle composition in correlation with the type of vapor used in the specific instrument was found. An enhanced detection efficiency for ultrafine hygroscopic sodium chloride aerosols was observed with water operated systems, an analogous trend was found for n-butanol operated systems with nonhygroscopic silver and tungsten oxide particles.

Impacts of venturi turbulent mixing on the size distributions of sodium chloride and dioctyl-phthalate aerosols

The purpose of this investigation was to determine if turbulent mixing created by the ejector causes change in the size distribution of particles passing through it. The results using dry solid sodium chloride (NaCl) particles show no discernible difference in the geometric mean diameter and the geometric standard deviation of particles passing through the ejector. Similar results were found for the dioctyl-phthalate (DOP) particles. In other words, there was no apparent shift in the center and spread (measured by the geometric standard deviation) of the particle size distributions. The ratio of the total number concentrations before and after the ejector indicates a dilution ratio of approximately 20 applies equally for both DOP and NaCl particles. The result suggests the dilution capability of this ejector was not affected by the particle composition. When the variability of the particle counts (in coefficient of variation, CV) in individual size ranges was examined, we found that the bin-to-bin variability was much greater for DOP than for NaCl particles. Although the first and second moments of a distribution (i.e., the geometric mean diameter and geometric standard deviation) were not altered significantly for the DOP particles by the ejector, the skewness measure indicated otherwise. The ejector may modify the particle size distribution of deformable particles such as DOP. The results suggest that it may be difficult to precisely characterize the size distribution of particles using the ejector technique if the particle is not solid. This result has an implication for the use of the ejector as a dilution tool, and one needs to know that the particles to be diluted with such a device are reasonably dry and/or non-deformable under the turbulent condition inside the ejector.

THE EFFECT OF SURFACTANTS ON THE DELIQUESCENCE OF SODIUM CHLORIDE

This study investigated the deliquescence of sodium chloride aerosol with surfactants at a retention time of 4.24 sec. Two surfactants used in this study, glutaric acid and pyruvic acid, are found in atmospheric aerosol and have a high hydrophile-lipophile balance (HLB) value. The experimental system consisted of a relative humidity conditioner, a tandem differential mobility analyzer (TDMA) and a scanning mobility particle sizing (SMPS) system. Results obtained from the observation of TDMA presented the deliquescence point of sodium chloride aerosol at 75% RH. In addition, the growth size of sodium chloride aerosol was observed to be79.47nm and thegrowth ratio was 77.94%, when theinitial sizeof aerosol was 101.82nm. Surfactants were internally mixed with sodium chloride at six different weight fractions, i.e. 2.5, 5, 10, 20, 40, and 60% by weight of surfactants in dry aerosol. Both surfactants apparently decreased the deliquescence point of sodium chloride, in which the lowest deliquescence point appeared at about 71% RH when the weight fraction of surfactants is 60% by weight of surfactants in dry aerosol. Moreover, the smallest size of sodium chloride aerosol with surfactants was 142.7nm at 60% by weight of pyruvic acid in dry aerosol. Our results further demonstrate that the deliquescence point and size of sodium chloride aerosol with surfactants are related to the weight fraction of surfactants.

Analysis of Sulfuric Acid Aerosols by Laser-Induced Breakdown Spectroscopy and Laser-Induced Photofragmentation

Laser-induced breakdown spectroscopy and laser photofragmentation spectroscopy are investigated as potential approaches to the detection and quantification of the sulfuric acid aerosols resulting from the oxidation of dimethylsulfide in air. The former, direct technique, where aerosols are introduced in the plasma formed by focusing the fundamental frequency of a Nd:YAG laser in air, and atomic sulfur emission is measured in the region 180 nm, gives a limit of detection of 165 ppbv for a 15 min integration time, which is inadequate for our purposes. The second, indirect approach, based on the photofragmentation, with 193 nm photons, of the compound resulting from interaction between sodium chloride and sulfuric acid aerosols, gives a detection limit of 46.5 ppbv in 10 s measuring time. With this method, a complete “titration curve” for sulfuric acid aerosols can be obtained in “quasi” real-time.

Differences in Detected Fluorescence Among Several Bacterial Species Measured with a Direct-Reading Particle Sizer and Fluorescence Detector

Naturally-contained fluorescing chemicals (such as riboflavin or NADPH) can be used to detect the presence of biological organisms. A new instrument from TSI Incorporated measures fluorescence of particles using an ultraviolet laser operating at an excitation wavelength of 355 nm. We have employed this instrument (Model 3312 Ultraviolet Aerodynamic Particle Sizer (tm) Spectrometer) to assess the degree of fluoresence associated with a variety of biological aerosols. Nonfluorescent and fluorescent latex sphere and sodium chloride aerosols were first used to assure proper operation of the instrument and to obtain correct instrument settings. Biological aerosols were then generated by combining organisms with double distilled and filtered water in a Collison nebulizer operated at low pressure. After passage through a charge neutralizer and dilution with humidified air (45%RH), the aerosol was measured downstream for both particle size and fluorescence distributions. Bacterial aerosols generated include Bacillus subtilis subsp. niger (spores and vegetative cells), Staphylococcus epidermidis, Eschericia coli, and Mycobacterium abscessus (a surrogate for M. tuberculosis). Cladosporium spp. fungal spores were also evaluated, and the effect of heat treatment on fluorescence was tested using B. subtilis spores. For each test the percentage of organisms that produced a fluorescence signal above a threshold was recorded. The organisms demonstrated considerable differences in percent fluorescence, ranging from means of 11% for S. epidermidis to 44% for B. subtilis spores. Vegetative cells of B. subtilis were generally less fluorescent (mean of 33%) than the spores, while the highest level of fluorescence was associated with heat-treated spores (averaging about 75%). This instrument has some potential for use in settings where immediate detection of biological organisms is important. Work remains to be done on understanding the effect on fluorescence of organism viability, presence of nonbiological particles, and interferences from mixtures.

Electrostatic N-95 respirator filter media efficiency degradation resulting from intermittent sodium chloride aerosol exposure.

The effects of intermittently loading small masses of sodium chloride aerosol on the filtration efficiency of N-95 filtering facepiece respirators was investigated. The National Institute for Occupational Safety and Health (NIOSH) certifies that N-95 respirators must provide at least 95 percent filtration efficiency against a sodium chloride aerosol challenge as per the respirator certification (42 CFR 84) test criteria. N-95 respirators are specified for protection against solid and water-based particulates (i.e., non-oil aerosols). New N-95 respirators from three different manufacturers were loaded with 5 +/- 1 mg of sodium chloride aerosol one day a week, over a period of weeks. Aerosol loading and penetration measurements were performed using the TSI 8130 Filter Tester. Respirators were stored uncovered on an office desktop outside the laboratory. To investigate environmental and temporal effects of filters being stored without sodium chloride exposure, control respirators were stored on the desk for various lengths of time before being initiated into weekly testing. For all manufacturers' respirators, the controls showed similar initial penetrations on their day of initiation (day zero) to those of the study samples on day zero. As the controls were tested weekly, they showed similar degradation rates to those of the study samples. Results show that some of the manufacturers' models had penetrations of greater than 5 percent when intermittently exposed to sodium chloride aerosol. It is concluded that intermittent, low-level sodium chloride aerosol loading of N-95 respirators has a degrading effect on filter efficiency. This reduction in filter efficiency was not accompanied by a significant increase in breathing resistance that would signal the user that the filter needs to be replaced. Furthermore, it was noted that the effect of room storage time prior to initial exposure was much less significant.

Electrostatic respirator filter media: filter efficiency and most penetrating particle size effects.

New electrostatic filter media has been developed for use in 42 CFR 84 negative pressure particulate respirator filters. This respirator filter media was not available for evaluation prior to the change from 30 CFR 11 to 42 CFR 84. Thus, characterization of this filter media is warranted. In this study, the new 42 CFR 84 electrostatic respirator filters were investigated with respect to filter penetration and most penetrating particle size. Three different models of N95 filters, along with one model each of the N99, R95, and P100 class filters were used in this study. First, three of each filter were loaded with a sodium chloride (NaCl) aerosol, and three of each filter were loaded with a dioctyl phthalate (DOP) aerosol to obtain normal background penetration results for each filter. Then, two new filters of each type were dipped in isopropanol for 15 seconds and allowed to dry. This isopropanol dip should reduce or eliminate any electrostatic charge on the fibers of each filter, as reported in the technical literature. These dipped filters, along with controls of each filter type, were tested on a TSI 8160 filter tester to determine the most penetrating particle size. These same filters were then tested against a NaCl aerosol to get final penetration values. Electret filters rely heavily on their electrostatic charge to provide adequate filter efficiencies, and correlations between penetration and a filter's electrostatic characteristics are found in the technical literature. In all six of the filter models tested, filter penetration values increased considerably and the most penetrating particle size noticeably shifted toward larger particles. These results are important in better understanding how these new filter materials perform under various conditions, and they indicate the need for additional research to define environmental conditions that may affect electrostatic filter efficiency.

Aerosol Synergism and Hydrate Formation—A Possible Connection

Earlier experiments using guinea pigs and other laboratory animals have shown that aerosols of sodium chloride potentiate the irritant effects of inhaled sulfur dioxide and formaldehyde. This potentiation was not seen with other vapors such as formic acid and acetic acid; nor was it observed when water-insoluble aerosols were substituted for the sodium chloride. A simple explanation for these results would be that the potentiated vapors dissolved in the hydroscopic particulates and were thereby carried deeper into the respiratory tract. However, such an explanation does not explain the observed difference between potentiated and nonpotentiated vapors. Additionally, the retention time for dissolved vapors is too short to permit the aerosol to transport the dissolved vapor for any appreciable distance. Here it is pointed out that both sulfur dioxide and formaldehyde are hydrated in water solutions to compounds (methylene hydroxide and sulfite, respectively) that have low vapor pressures and are known to decompose only very slowly in water solution. The physical chemistry associated with the formation of these compounds may provide the desired explanation for the observed potentiating effect of inhaled sodium chloride aerosols.

The effects of aging processes on critical supersaturation ratios of ultrafine carbon aerosols

The condensation properties of polydisperse aged ultrafine carbon aerosols (particle diameter<1 μm) have been investigated by means of a variable supersaturation condensation nucleus counter. The critical supersaturation ( S c), as the point, where 50% of all particles have been activated and grew to droplets was compared to the median dry particle diameter for pure carbon aerosols, benzo[a]pyrene-tagged carbon aerosols and external mixtures of the carbon particles with sodium chloride and sulphuric acid aerosols. Additionally, ozone as oxidising gaseous compound was added in some of the experiments. Simple coagulation of pure and benzo[a]pyrene-tagged carbon particles resulted in only slightly lower values for S c due to the increased median particle diameter. The formation of soluble functionalities on the particle surface, i.e. the coagulation with the soluble sodium chloride and sulphuric acid aerosols or the chemical decomposition of benzo[a]pyrene into polar, hydrophilic products due to the reaction with ozone resulted in significant lower values for S c for the modified carbon aerosol. The necessary supersaturations for the increased hydrophilic particles dropped to atmospherically relevant values of 3% after 5 h reaction time (benzo[a]pyrene decomposition) and 15 h (coagulation with soluble particles), respectively.

Cloud condensation nucleus activity of organic compounds: a laboratory study

Critical or activation diameters of laboratory generated organic aerosols composed of succinic acid, adipic acid and glucose were determined. Measurements of sodium chloride and ammonium sulfate aerosols were performed for comparison. Our experimental approach involved producing single component aerosol particles of a known size, and measuring the fraction of aerosol number concentrations (CN) that act as CCN at several supersaturations. The particle diameter ( D 50) at which the CCN/CN ratio of 0.50 is reached is defined as the critical, or activation, diameter. These experimentally derived diameters are compared with the theoretical critical diameter ( D C ). The results indicate that highly water-soluble organic compounds exhibit critical diameters that approach that of ammonium sulfate.

Fast in situ sizing technique for single levitated liquid aerosols

A fast in situ sizing technique for single levitated aerosol particles is presented. It makes use of the analysis of fast-Fourier-transformed vertically polarized Mie scattering patterns from single liquid aerosols levitated in a Paul-trap-type electrodynamic balance. The technique is shown to give reliable results for a test sphere of known physical properties. A first application to aqueous sodium chloride aerosol particles that changed size on humidity variation is presented, yielding good agreement with the full Mie theory.

Observations on gaseous and aerosols components of the atmosphere and their relationships

Measurements of sulphur dioxide, ozone, ammonia, and soluble inorganic components of the atmospheric aerosol were made at a site in central southern England. Ammonia, ozone, and nitrate aerosol in winter were shown to exhibit significant diurnal variation. Ozone showed a typical diurnal variability, the magnitude of which was dependent upon wind speed. The lower night-time ozone concentrations at lower wind speeds were attributed to depletion inside nocturnal boundary layers by dry deposition. Ammonia, in contrast, showed a different behaviour, whereby the diurnal cycle was more pronounced at higher wind speeds, indicating that the cycle was unlikely to be the result of dry deposition at night. Ammonia concentrations showed a temperature dependence and the diurnal cycle of ammonia at this site appears to be the result of a temperature-driven emission signal. Of the total reduced nitrogen, NHx (NHx = NH3 + NH4 + aerosol), the phase was dominant and it is likely that more than 60% of the boundary layer NHx is in this phase. The loss term of ammonia by reaction with acid sulphate aerosol is likely to be greater than that by dry deposition on a UK scale. Nitrate aerosol showed a positive correlation with sodium aerosol, once the effect of mutual correlations with sulphate and ammonium were removed. This correlation effect, in combination with evidence of a marine-oriented directional dependence of nitrate aerosol, and negative non sea-salt chloride aerosol from the same ‘marine’ sector, shows the potential importance of the formation of sodium nitrate aerosol from reaction of dinitrogen pentoxide, or possibly nitric acid or nitrogen dioxide with sodium chloride aerosol. It is likely that this provides the major route of nitrate into rain, not the scavenging of nitric acid vapour. Aerosol sulphate, nitrate, and ammonium have been measured at Harwell since 1954. Sulphate aerosol increased up until 1976 and has declined subsequently. Nitrate aerosol has increased over the whole period, whereas ammonium aerosol follows a similar pattern to that of sulphate, but with an equivocal direction of trend after 1976. Sulphate, nitrate and ammonium aerosol all show a similar statistically significant seasonality. A historical inventory of ammonia emissions shows a clear correlation with ammonium aerosol.

Aerosol Loading Performance of Electret Filter Media

In this study various types of flat sheet electret filter media including tribocharged polypropylene/acrylic, corona charged polypropylene, fibrillated electret film, and new advanced electret media have been compared with mechanical filter media (fiber glass) using aerosol filtration tests designed for particulate respirators. The number of layers, and thus the total mass and pressure drop, of the media were varied while the filter area exposed to the aerosol and the aerosol flow rate was held constant (constant face velocity). Tests included challenges with sodium chloride and dioctyl phthalate (DOP) aerosols per NIOSH 42 CFR 84 specifications for particulate respirator certification. Initial filtration efficiency and pressure drop and the same during loading were determined. Results revealed significant differences in filtration performance and loading behavior among the various media. It is concluded that respirators can be designed for all nine 42 CFR 84 classes using electret filter media without sacrificing low pressure drop, light weight, or user comfort.

Aerosol Loading Performance of Electret Filter Media

In this study various types of flat sheet electret filter media including tribocharged polypropylene/acrylic, corona charged polypropylene, fibrillated electret film, and new advanced electret media have been compared with mechanical filter media (fiber glass) using aerosol filtration tests designed for particulate respirators. The number of layers, and thus the total mass and pressure drop, of the media were varied while the filter area exposed to the aerosol and the aerosol flow rate was held constant (constant face velocity). Tests included challenges with sodium chloride and dioctyl phthalate (DOP) aerosols per NIOSH 42 CFR 84 specifications for particulate respirator certification. Initial filtration efficiency and pressure drop and the same during loading were determined. Results revealed significant differences in filtration performance and loading behavior among the various media. It is concluded that respirators can be designed for all nine 42 CFR 84 classes using electret filter media without sacrificing low pressure drop, light weight, or user comfort.