Performance of a Capillary Dilution System for High-Concentration Sampling of Ultrafine Aerosols
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To characterize particle properties, a proper conditioning of aerosol samples is required for most aerosol measurement systems. Ultrafine particles are often generated in high concentrations, resulting in the need for dilution prior to measurement. Reasons for this sample dilution include the prevention of condensation of volatile compounds onto the particle surface, the suppression of chemical reactions and simply to bring the particle concentration below the maximum tolerable value for the measuring instruments. A number of different approaches are taken to dilute samples. In this article, a new type of dilution system designed for submicron aerosols with high concentrations is presented and its performance compared with design expectations. The new dilution instrument is based on a capillary/filter technique. Small capillary transports a tiny proportion of the sample and the greater part will be filtered and used as diluting gas. The shuffling between the two parts takes place inside a filter. Therefore, the dilution ratio is determined by the diameter of the capillary and the flow rate. With this principle, the dilution process is realized in a single step and on a fast time scale. Calibration measurements showed a linear relation between the dilution ratio and the control parameters within ± 2%. The dilution ratio was found to be independent of particle diameter in the size range between 10 and 400 nm. The dilution ratio scan for the proposed system can be varied continuously from moderate to very high dilutions (from 1:10 to 1:103); this range could be extended with minor modifications. The proposed instrument was built on a small and portable scale. The described dilution system opens a wide range of applications for particle sampling.
KeywordsUltrafine aerosols Concentration Distribution Capillary Dilution Efficiency
One of the common uses of aerosol counters is to evaluate the concentration of various particle sizes. Characterization of the sizes of ultrafine particles (diameter smaller than or equal to 100 nm) is commonly done using an electrical classifier equipped with a differential mobility analyzer and a condensation particle counter (Knutson and Whitby 1975). Condensation particle counter can detect aerosol concentration of 1000 particles cm−3 or greater. However, aerosol number concentrations in dusty applications greatly exceed the working range of most current condensation particle counters (CPC) making it necessary to dilute the aerosol prior to sampling. Reducing the concentration of particles in an aerosol is known as dilution. Dilution is a mechanical or physical removal of aerosol particles (or increasing the clean air volume). This is achieved when the aerosol passes through a filter medium, which reduces the number of particles in the aerosol. A variety of clean media are available for use, such as filter paper, activated carbon, diatomaceous earth, and cartridges. Since different media have varying efficiencies, at varying size ranges, the user must select the suitable media for the required particle removal (Rushton et al. 2008).
The ease, speed and cost of dilution make the technique frequently the first used to judge the success of ultrafine particle (UFPs) generation. The use of traditional diluter is still having several inherent drawbacks. Sample dilution might be used to prevent adsorption or condensation of volatile compounds onto the particle surface, to suppress particle coagulation or chemical reactions which change the aerosol composition (Cheng et al. 2002). Conditioning and dilution of dusts are very important for a representative measurement. The design of the sampling and dilution system determines largely what is measured later. It serves to reduce the concentration in the raw dust to a concentration which can be handled by the measurement system (Hueglin et al. 1996). Moreover, the measuring range and life of instruments can be extended by sample dilution. Typically, a good dilution method should not change the particle size distribution.
A number of different approaches are taken to dilute samples. One of them is the use of a particle ejector or simply ejector (Abdul-Khalek et al. 1998) to achieve a rapid dilution. Passing particles through the ejector environment may alter the sizes or the size distribution through a number of processes such as particle breakup, condensation, agglomeration and coagulation. A porous tube diluter also provides control over the dilution process (Ranade et al. 1976; Newton et al. 1980). It has the advantage over the ejector diluter that it does not have problems related to plugging of the flow channels and losses are minor even for submicron particles. The control of the flow is not as straightforward as with the ejector diluter. The cavity diluter captures a fixed volume of aerosol into a cavity before mixing with particle-free air. The venturi operates by drawing a known fraction of particle-free sheath air to mix with an aerosol sample (Yoon et al. 2005). The capillary diluter which uses an aerosol capillary to dilute with filtered air from the original air sample. However, some of these systems suffer from high-diffusion losses for the nanometer-sized particles and thus are not suitable for studies in the nano-size range (Hueglin et al. 1997; Helsper et al. 1998). Often, the objective of sampling and dilution is to obtain the properties of the aerosol as it is at the measurement location. Additionally, a dilution method enables the observation with well-established techniques for measuring particle number concentrations or number size distributions (Hueglin et al. 1997).
The overall aim of this paper is to contribute to existing knowledge regarding methods for diluting and measuring characteristics of ultrafine particles. The dilution system proposed and used in this study is designed to give very fast dilution of the dust with dry air and carried it out in one step. The dilution system studied here differs from the aforementioned systems in two ways: (i) the dilution flow is a user-selectable, set by a capillary orifice, and operated in a closed loop; (ii) The sample inlet is adjustable by a valve to provide balanced flow with the dry air. This system’s performance is evaluated for the transmission efficiency of UFPs aerosols with sizes between 10 and 400 nm. Finally, the dilution effectiveness is determined at different conditions (flow rates, capillary diameter, particle concentration distributions) for submicron particles.
2 Diluter Design
3 Experimental Evaluation
In practice, flow rates are measured and the dilution ratio δ is calculated using Eq. 1. The calibration curves show an excellent linear dependence (R2 > 0.9874) between the flow rate and the dilution ratio (Fig. 6). The error of the calibration curves was within ± 2%. The standard deviation of these data is 1.76%. This discrepancy might be explained by inaccuracies in the fabrication of the capillary, particle diffusion loss, coagulation, the pressure drop across the filter and differences between upstream and downstream pressures. The actual performance of the diluter was also compared with design expectations. At the selected operating condition (5 LPM dilution air flow), the accessible dilution ratios cover the range from performance of the TSI diluter. Because flow measurements were more accurate than the sample measurements, it is likely that actual sample dilution was more accurate than ± 5%. It is concluded that flow dilution is an accurate indicator of sample dilution in this device.
The proposed diluter concept provides a uniform dilution ratio for particle sizes ranging from 30 to 300 nm and preserve the initial shape of the concentration distributions.
The dilution ratios were found independent of the particle concentration distributions.
The dilution ratio increases with the decreasing in diameter of the capillary.
The dilution range can easily be extended by varying the flow rate in the upstream and capillary.
The particle size distributions obtained with the capillary dilution system are reliable and compare well with the average measured by TSI device system.
That the proposed dilution system can be very helpful in many areas of aerosol measurement and research.
The authors would like to acknowledge the Institut de Recherche Robert-Sauvé en Santé et en Sécurité du Travail (IRSST) for financial support.
Compliance with ethical standards
Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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