## 1 Introduction  
Ultrafine particles (PM\({}_{2.5}\)) having a diameter of 2.5 \(\upmu\)m or below are known to cause respiratory diseases in certain vulnerable social groups such as the elderly and infants [1]. In particular, smaller particles are more likely to be toxic and can penetrate deeply into the lung, thus inducing serious health issues, including respiratory and cardiovascular diseases, reproductive and central nervous system dysfunction, and even cancer [2, 3]. Therefore, it is crucial to real-time monitor the air quality in industrial areas, where numerous businesses emit airborne pollutants, as well as in residential and commercial areas, which have high population densities. Particulate matter concentrations are provided to the public on a daily basis [4]. Accordingly, pollutant emission standards are becoming stricter worldwide in order to reduce the effects of particulate matter on air pollution [5]. In particular, dust concentration must be monitored in real time at large air-pollutant-emitting facilities such as coal-fired power plants [6]. In general, the light-transmission method is used to measure the dust concentration in the chimney of a coal-fired power plant. This method involves measuring the degree of change in light intensity owing to foreign substances such as dust in the exhaust gas when light is transmitted from a light source installed in one side of the chimney to the detector installed on the opposite side; this light-intensity change is then converted into a concentration [7]. Coal-fired power plants are also investing in the reduction of air pollutants. The emission concentration of discharged dust is strictly monitored by a telemonitoring system (TMS) using the light-transmission method, which is used in conjunction with an electrostatic precipitator [8]; in general, the accuracy of dust concentration measurements is determined via the error rates of the semi-automatic method and light transmission method used by the TMS. Since the semi-automatic method utilizes a sampler to ensure that the sampling nozzle and flow direction within a power-plant chimney are aligned parallel to each other, it is important to minimize the effect of exhaust gas flow during isokinetic sampling [9]. Particles gain an inertial force as they move along the flow direction, and the degree of isokinetic sampling is determined according to free-stream and sampling flow velocity [10]. Figure 1a shows the super-isokinetic sampling of particles when the velocity inside the sampler is faster than that of the free stream, which results in greater inertia of the particles. By contrast, Figure 1c shows the case of sub-isokinetic sampling of particles along the flow arising from the slower velocity of the particles inside the sampler compared with the particles in the free stream. As shown in Figure 1b, an accurate particle concentration can be obtained only from isokinetic sampling [11]. Therefore, typical sampling of an aerosol can be performed, and pollutants can be effectively controlled by developing an isokinetic sampler [12]. Figure 1: _Cont._  
The aspiration ratio, i.e., the ratio of the particle number concentration measured in a free stream to the particle number concentration measured at the sampling probe inlet, is one of the indicators for evaluating the accuracy of isokinetic sampling of an aerosol. An aspiration ratio closer to 1 indicates that the sampling probe is effectively performing isokinetic sampling [13]. The geometric structure of a sampling probe can significantly impact the particle distribution [14]. The major causes of distortion of the aerosol in the sampling nozzle area include the formation of vena contracta as a result of the impact against the nozzle wall owing to turbulence, gravity sedimentation, and electrostatic interaction [15].