1. System Preparation

1. Make sure to use new or well cleaned particle transport tubing and connectors to assemble the system. Ensure that the chamber walls are cleaned and particle-free (refer to the cleaning methods at the end of the protocol).
2. To remove potential background particles, connect a filtered dry air flow (5-10 L/min) directly to the mixing chamber (without installing the funnel between positions a and b de Figure 1), for at least 30 min.
3. Measure the particle number concentration in the measurement chamber using the SMPS according to manufacturer's protocol. If the concentration is below 10 #/cm³ after three scans, then consider the environment clean. Note that the flow rate may slow when measuring using the SMPS.
4. Stop the air flow and close the sampling tube outlets and the exit tube outlet with a plastic or rubber stopper (position c de Figure 1) to prevent ambient particles from entering the system.
5. Prepare and warm up the measurement instruments (SMPS and OPC) and particle samplers for microscopic analysis.

2. Material Preparation 

1. Store test materials in a well-controlled environment with respect to temperature and humidity. This is very important for ensuring repeatable results in follow-up experiments.
2. Weigh the powder carefully using an analytical balance or high-precision balance, in a well-ventilated space (e.g., laboratory hood).
   Note: Weights of 250-500 mg TiO_2, SiO₂, ZnO and CeO₂ nanoparticle powders were tested, and these generally proved to be sufficient for at least 30 min of stable aerosolization. However, the appropriate amount strongly depends on the powder type and may vary significantly for bulk materials, cement or organic powders.
3. Fix the aerosol generator vertically, and feed the powder from the top opening of the aerosol generator using a properly cleaned laboratory funnel. Prior to the experiment, rinse the funnel with water and dry out by filtered air, to remove any dust deposition on the inner wall. Gently tap the funnel to ensure all the powder particles are fed into the process. Do not shake the funnel hard in order to avoid significant loss of the material to early aerosolization.
   1. Make sure that the majority of the powder particles reach the bottom of the generator, rather than falling on the surrounding sloping walls. Gently tap the side walls of the generator to move deposited powder particles down to the bottom.
   2. Alternatively, use a long funnel that directly deposits the powder particles at the bottom of the generator. For safety reasons, conduct these operations under a ventilation hood or inside a negative pressure chamber.
4. As unsticky material may slip through the opening at the bottom of the generator, use a 2 mm diameter needle to temporarily block the opening before feeding in the powder.
5. Remove the funnel and close the top and bottom openings of the generator in order to avoid particle emission during transfer.

3. Aerosolization

1. Install the aerosol generator, remove the blocks on the inlet and outlet tubing to the funnel, connect its bottom to filtered air supply and its top exit to the mixing chamber (positions a and b respectively in Figure 1), and attach it vertically with a metallic scaffold.
2. Remove the blocks on the setup exit (position c de Figure 1).
3. Switch on the aerosolization flow. Slowly increase the rate from 0 to 0.3-0.5 L/min using the flow tuner. Do not move to high flow rates too quickly-the aim is to reach a flow rate that can provide stable aerosol generation for at least 30 min.
   1. To achieve this, do not consume powder quantity significantly during this period of stable aerosolization. As an empirical rule, use a fluidized-bed height of about 1 cm (denoted by H de Figure 1) for generating a robust aerosol flow while maintaining a stable concentration over a relatively long period. If the energy put into the aerosolization process is too strong then the material will be rapidly used up, failing to sustain a steady aerosol generation during the rest of the experiment. Note that the flow rate range can vary for different powders; the values mentioned above were used for the abovementioned nanopowders tested.
4. Switch on the dilution flow. Slowly increase the rate from 0 to 2 L/min. The total dilution flow needed is determined by the sampling equipment. The SMPS, OPC, and the mini-sampler used in the system presented here require a total flow of 1.6-1.8 L/min.

4. Characterization

1. Start the online measurement instruments simultaneously (here, the SMPS and OPC) as soon as the aerosolization and dilutions flows are introduced.
   Note: If a stable state of aerosolization is achieved, the aerosol particle number concentration and the size distribution should become stable after approximately 30 min. Use measurements starting from this time point for the comparison of aerosol properties under different conditions (e.g., humidity) and using different powders. Analyze the results from 10 consecutive SMPS scans to calculate average concentrations and size distributions.
2. Once the aerosolization is stable, turn on the pump connected to the TEM sampler to start sampling the airborne particles. Use a flow rate of 0.3 L/min using the TEM grid coated with holey carbon film. The thin film on the grid may be damaged if the flow rate is too high. Detailed information on the use of the sampler is available ²¹. Typically, the sampling process lasts for about 3 min.
   1. Vary the sampling duration according to the different particle concentrations, and approximate by considering a moderate surface coverage of the TEM grid by particle deposition (e.g., 50%). Thick depositions may modify particle morphology due to on-site agglomeration.

5. Post-sampling Operations and Clean-up 

1. After finishing the measurements, switch off the dilution flow and then the aerosolization flow.
2. Disconnect the aerosol generator from the system, block its top and bottom openings, and transfer it to the cleaning space. Clean in a well-ventilated cleaning facility or an enclosed space, especially if hazardous materials have been treated.
3. Disperse powder residues with water or organic solvents, depending on the hydrophilicity of the particle surface. Pour the solution off into chemical containers for safe recycling. After long experiments, test powders tend to stick firmly onto the glass wall and do not dissolve easily. If this occur, use acids or bases together with an ultrasonic cleaner to dissolve sticky materials.
4. In order to remove any moisture content left on the wall and to thoroughly dry the inside of the generator, pass dry air through it for at least 1 hr. Make sure there are no flames or ignition sources when working with organic solvents, and ensure good ventilation of the space.
5. Disconnect particle transport tubing and connectors. Rinse them with water or solvents. Wipe the inner walls of the mixing and measurement chambers with a wet paper tissue or cloth. Dry them in an open space for at least a day or with dry air flow for 1 hr prior to next experiment.
6. Regularly clean the SMPS impactor (if used).