In this study an automatic aerosol generation setup has been developed to have a stable concentration during experiments. All three aforementioned techniques for generating PM have been implemented in this setup to investigate their effect on the sensor response. As shown in Fig. 1, the generator consists of three parts. First part is used to generate aerosol from PM suspended in water. Dry chemical powders firstly mixed in the water and then the mixture is evaporated via a nebulizer. Afterward it passes through a custom-made PM dryer consisting network pipes passing through a cylinder filled silica gel grains. The mixture loses its moisture to the silica grains via diffusion by passing through these pipes. The details of the adopted technique can be found elsewhere (Victor et al. 2012). After that the aerosols comes out from PM chamber via an air pump. Second section shows the system for creating dry aerosol from chemical powders. The powder firstly gets dispersed in a cylinder by via pressurized air. Two small fans are placed inside the cylinder to keep the particle suspended and make the mixture homogeneous. The aerosol then pushed out via a piston controlled by a stepper motor and threaded bar. Third section is used for smoke aerosol generation. The particles are generated from burning of an incense stick and gathered in smoke chamber. Like in the first part, the mixture of PM comes out from chamber via an air pump. All the pumps and stepper motor are controlled by a microcontroller that is connected to computer. The rotation speed of the pumps and stepper motor could be adjusted via user interface in the PC. It is also possible to adjust the speed according to data coming from a sensor placed somewhere to have a constant PM concentration.
The mixture coming out from the generator then gets diluted in the mixing chamber shown in Fig. 2. There is a Peltier module placed in the chamber to control inside temperature, heating or cooling the mixture. RH of inside is adjusted via an electrical mixing valve which controls the proportion of the moisturized or dried fresh air coming from HEPA filter. The humidifier consists of wet cotton and the dryer is filled with silica gel grains. Two different laser-based PM sensors (Sensirion SPS30 and Cubic PM2008) have been utilized here to measure the PM concentration which will be sent to the QCM afterward. One Sensirion SHT30 sensors are placed inside to measure RH and temperature values. There is a central microcontroller which handles controlling the Peltier module and mixing valve, reading the data coming from sensors and sending them to computer. So, all the parameters including temperature, RH, aerosol concentration and production principle could be controlled automatically via a computer at a user-defined value.
The Q-1 QCM model and its driver unit from (OpenQCM, Italy) was employed in this study. The top cover of the sensor in this device which includes the inlet and outlet of the flow is made suitable for liquid measurements, where the inlet is placed far from the QCM’s center. However, for PM-type measurements, and since the center of the QCM is the most sensitive part of it (Huang et al. 2018), the cover design is modified. In the new design, the inlet is located in the center and three outlets placed around of it. Figure 3 shows a comparison of the original and the new top covers.
To ensure the regular flow to the QCM cell, another pump and controller unit placed at the QCM’s outlet to provide suction of the air. The flowrate of the air to the QCM cell is measured by Renesas FS1015 air speed sensor and then the microcontroller adjusts the pump speed to obtain set value for the flowrate according to the feedback from this sensor.
The materials used for particle generation are listed in Table 1. The selection material was pursuant to the aim of the study and also availability. The incense stick was used here for smoke aerosol generation. The size of particles is obtained from laser sensors. For wet particle production, the different types of the used powders were dispersed in the water and evaporated via nebulizer.
Table 1
Type and size of tested particulate matters.
Material | Formula | Size |
Graphite | C | 1 µm |
Graphite | C | 1–5 µm |
Silicon carbide | SiC | 1 µm |
Boron carbide | B4C | 1–3 µm |
Aluminum oxide | Al2O3 | < 1 µm |
Incense stick | - | < 1 µm |
Various coatings have been made for QCM with aim of increasing the PM sensitivity (Olsson et al. 2013; Palomba et al. 2001; Zampetti et al. 2017). The main purpose of the coating is to eliminate bounce of the particulates form the sensor surface and increase sticking efficiency. For the coated sensors, Apiezpn-L grease is used as coating material. The grease is firstly diluted by toluene with mass ratio of 1:10 and the sprayed on the QCM surface via an airbrush, followed by heating up to 45°C in evacuated furnace to make sure all the toluene get evaporated. The obtained coating caused around 50Hz decrease in baseline resonance frequency of QCM. The sensor was tested in both bare and coated mode to investigate the effect of the coating on the sensing performance.
During the tests, the QCM sensor was seen to be highly sensitive to the temperature. Hence before starting the tests it is necessary to wait for a few minutes to reach the equilibrium condition between the temperature of the QCM and air flow. The standard ambient condition for the tests was set 25°C and 50% RH expect of ones in which the effect of temperature and RH were examined. The flow rate of pump providing the flux for QCM firstly was set to 1 liter per minute (lpm) to obtain optimum speed for the highest sticking efficiency of ultra-fine particles (Palomba et al. 2001). However, as a part of this study, to investigate the total performance of QCM the flow rate of the flux is also reduced down to 0.5 lpm when tested with larger particles in order to eliminate bouncing.