# Suitability of Low-Cost Sensors for Submicron Aerosol Particle Measurement
## 3 Results and Discussion
### Influence of Aerosol Heating on Measurement Capability of Low-Cost Sensors under Indoor Conditions  
Due to the strong influence of air humidity on particle size, aerosol measurements with low-cost sensors must take place under constant conditions. Therefore, it is necessary to control the relative humidity for indoor aerosol monitoring and keep it at a low level. This can be achieved by aerosol heating in the previously described low-cost copper heating construction (cf. Section 2.2). The relation between the heating voltage and the specific electrical energy has already been explored (cf. Figure 2).  
The SDS011 low-cost sensor was combined with a heating stage (cf. Figure 1) to investigate its influence on aerosol measurements. For this purpose, continuous measurement is carried out with this low-cost unit. The NaCl aerosol (nebulizer concentration at 0.25 g\(\cdot\)L\({}^{-1}\)) is preconditioned to approx. 70% humidity by two packed columns (cf. Section 3.2). After the PM value becomes stationary, the heating stage is turned on and operates for a period of 5 min at one energy input (up to 14 kWh/m\({}^{3}\)). Figure 8 shows the PM results for the operating points at approximately 6, 8 and 10 kWh/m\({}^{3}\). In addition, the heating time is marked (gray area).  
Figure 8: PM2.5 values measured over time with the SDS011 low-cost sensor at a relative humidity of 70% and a specific electrical energy of 6, 8 and 10 kWh/m\({}^{3}\).  
Figure 7: PM2.5 values for particle-free air depending on the relative humidity measured with the low-cost sensor SDS011, nebulizer concentration 0.25 g\(\cdot\)L\({}^{-1}\).  
The recorded PM2.5 value before the heating corresponds to the data from Section 3.2. A significant abrupt drop in PM2.5 can be observed after starting the heating process, after which the value remains at a constant level. Further heating no longer influences the particle concentration. After the heating element has been switched off, the PM value is still the same as previously and remains constant for a period of time. Subsequently, the PM2.5 value rises again and rapidly returns to its initial value. The time interval between the heating end and PM recovery is hereafter referred to as residence time. Studies on the lower and higher power inputs show the similar behavior of the particle concentrations. The PM2.5 value measured during the heating phase and afterwards is the same in all experiments and remains at approx. 100 \(\mathrm{\SIUnitSymbolMicro m}^{-3}\). The curves in Figure 8 only differ in the time until the PM drops after the start of the heating process and the duration of the residence time. For low-energy inputs, the influence of the heating appears to be time-delayed, while for higher inputs it occurs nearly instantly. The residence time increases with increasing specific electrical energy.  
The results show that the heating stage fulfills its purpose of reducing the effect of humidity on particle size and therefore improves the measurement of aerosol-particle mass concentration. It is surprising that the PM level instantaneously drops and remains at the same value regardless of heating time. It is probable that a critical stable humidity is reached inside the heating stage, which leads to the abrupt changes in the PM values. The increasing residence time for higher specific electric energy is presumably caused by residual heat that is still being emitted into the aerosol even after the heating section has been switched off. The greater the input energy, the longer this effect lasts. The observed effects must be further investigated and the influence of the initial humidity and the volume flow on the constant PM level and the residence time must be clarified. Based on the current results, the heating stage seems applicable for both low and high power consumption. In this context, an energy-optimized indoor-air-quality measurement guide can be developed that coordinates measurements and heating intervals.