Current status of the calibration standard materials in the size range of a few nanometers is reviewed. The techniques for aerosolizing the monodisperse nanoparticles are described, and the method for size determination of gasborne nanoparticles is discussed when differential mobility analyzers (DMA) are used. Additionally, some candidates of size standard nanoparticles in the range of a few nanometers such as fullerenes, dendrimers, globular proteins and tetra-alkyl ammonium ions are reported.
This paper reviews the preparation method of polystyrene standard particles and their industrial applications. The polystyrene standard particles are synthesized in water by an emulsion polymerization method followed by refining. They are supplied as standard particles after determination of particle sizes. Although they are used mostly for calibration and verification of instruments for the cleanliness measurement and contamination control including fine particle counters, the application fields extends to an extremely wide range of products, for example semiconductors, micro-machining of liquid crystals, pharmacy, and drinking water. Due to the wide variety of applications, not only the particle size but also the particle number concentrations are becoming more important. Since there exist many JIS standards and regulations based on the polystyrene standard particles, they are now playing a vital role as a nano-scale rule measure in addition to the micro-scale measure.
Reference filters with uniformly deposited particles consisting of known masses of elemental carbon (EC) and organic carbon (OC) were prepared by a spray-drying method and an isokinetic sampling system. The submicrometer carbon particles were produced by nebulizing a carbon black hydrosol and/or a solution of potassium hydrogen phthalate (KHP). Reference filters were analyzed using representative thermal optical analysis methods (TOA) as IMPROVE and NIOSH 5040 methods, and the measured concentrations of TC, EC, and OC were compared with those measured by the gravimetric analysis. As a result, the method reported by the authors (Aerosol Sci. Technol., 41, 284 (2007)) is a simple technique for preparing the reference EC/OC filters in which EC, OC, and mixed EC-OC particles of a controlled size, mass, and chemical composition are deposited uniformly. These reference filters are invaluable in evaluating TOA methods of EC/OC measurement.
The research and development trend, and recent activities in international standardization, of characterization and analysis method for emission behavior of PM10/PM2.5 and nanometer scaled particles from stationary sources such as factory, power plants and waste incinerators was introduced. The international standard method for PM10/PM2.5 emission behavior from stationary sources is discussed in ISO TC146/SC1/WG20. In this committee, three kinds of PM10/PM2.5 separation methods from flue gas are focused upon, and the ISO committee draft, Part 1 “Cascade impactors”, has already been proposed. The subjects and our improved proposal of this draft, and the structure and performance of virtual impactor, which is under discussion as Part 3 of this ISO, were introduced. Furthermore, attention has to be paid to condensable suspended particulate matter (SPM), which may pass through the high-performance dust collectors, because it is a new high risk matter for human health in the future. Condensable SPM is collected by the mass concentration measurement with a dilution method, which is discussed in ISO TC146/SC1/WG21. However, there are many issues that must be solved for successful development of measurement methods of such nanometer scaled particles and the optimal dilutor design.
The aerosol charge neutralizer is an essential device in size distribution measurement of aerosol particles by the differential mobility analyzer (DMA). It is not a common practice to calibrate neutralizers, and charged fractions of particles that passed through an aerosol charge neutralizer are assumed to follow a theoretical distribution, which is represented by the approximate equations by Wiedensohler (1988). This assumption, however, would cause significant error if the actual charged fractions deviated from theory. It is not an easy task, though, to calibrate neutralizers for many users. As part of an effort to provide calibration standards for aerosol particle measurements, we have been building an experiment station for calibration of aerosol charge neutralizers. This paper reports its construction and the method to derive particle size-dependent charged fractions. We also report measurement results of charged fractions of an Am-241 neutralizer at different flow rates, in which the location of the radiation source could be varied. We observed that, at a low flow rate, charged fractions varied when the location of the ion source was changed and deviated from theory as the radiation source was moved toward upstream while all other conditions were maintained unchanged. This indicates that the performance of neutralizers may be influenced by the exact location of the ion source in the housing, especially under low flow-rate conditions.
Methods for calibration of the condensation particle counter (CPC) and characterization of the transfer function of the differential mobility analyzer (DMA), both of which have been developed at AIST, are discussed in this article. In the first part, the methodology for calibration of the detection efficiency of CPCs is explained. It will be used in the calibration service by AIST for CPCs and aerosol electrometers. The procedure is also proposed by us as part of a future ISO standard. In the second part, our method for evaluation of the transfer function of DMAs is discussed. It is based on the work by Martinsson et al. (Aerosol Sci. Technol., 35, 815 (2001)) which allows determination of the sizing resolution, particle losses, and their size dependence and has been improved by us. These calibration and characterization techniques for the CPC and DMA, which are the two crucial instruments for electrical mobility analysis, are expected to provide accuracy and reliability to size distribution measurements of aerosol nanoparticles.
Spectral dependence of aerosol light absorption was measured on the summit of Mt. Fuji (3,776 m msl.) during the summer of 2005. Black carbon (BC) concentration showed diurnal variation with typical pattern of maximum occurring in daytime and minimum in early morning, suggesting the thermally induced transport from the planetary boundary layer (PBL) . The absorption coefficients of aerosols at each wavelength relative to the value of 520 nm showed slopes of about -0.9 against wavelength in log-log plot when the maximum concentrations occurred, which has been reported to be typical for the urban/industrial originated BC. When minimum concentrations of BC occurred, two types of slopes, one with the same slope of maximum concentration cases and the other with much steeper (∼-1.5) slope, were distinguished. The former can be regarded as diluted urban/industrial aerosols originated in the PBL, whereas the latter exhibited the characteristic of soil dust or biomass burning aerosols which are candidates of free-tropospheric aerosols in the Asian-Pacific rim region.
Evaporation rates of chemical components in cigarette main-stream smoke were measured using a denuder system in order to clarify the dividing factor between the vapor and the particle phase. Chemical components passing through a denuder tube were trapped by a tube filled with Tenax TA adsorbent. They were analyzed using thermal desorption system, cooled injection system, gas chromatography and mass spectrometer. It was found that water-soluble components had lower evaporation rates than water-insoluble components, even if they had the same vapor pressure. It was suggested that the water solubility of each chemical component was an important factor for the evaporation property of cigarette main-stream smoke. Furthermore, we measured the filtration efficiency of cigarette filter for some chemical components. It was found that the filtration efficiency of water-soluble components was lower than water-insoluble components when they had the same vapor pressure. It was suggested that the filtration efficiency of each chemical component in cigarette main-stream smoke for cigarette filter was affected by their water solubility, which related to the evaporation property.
The rate of water vapor condensation on a water cluster is determined from the sticking probability of water molecule on the cluster and the collision cross section between the molecule and cluster. We investigate the mechanisms of enhancement of vapor condensation rate on a water cluster ion by comparing it with that on a neutral water cluster by using the data of sticking probabilities and collision cross sections as a function of the water cluster size.