## 4 Discussion
Although gravimetric measurements may be used to determine a source- or environment-specific calibration for a research study, the approach is not practical for routine monitoring in homes. A key objective of continuous monitoring--to activate controls--can be achieved if the monitor reliably and clearly responds to sources that account for the majority of particles in the home even if responses are not quantitative. We conducted this study by assuming that indoor PM\({}_{2.5}\) emission sources exist, that is, from frying pork in a pan and smoking, with consideration of Korean life style [20] and relatively highsmoking prevalence [34]. We determined that our linear model obtained from the indoor test for each single-aerosol type showed excellent performance (R2 = 0.98 or 0.92 for ESCORTAIR; 0.94 or 0.90 for PA), compared to FEM responses. However, the responses of ESCORTAIR as well as PA were relatively less precise but good (R2 = 0.81, 0.87) for monitoring in urban traffic hotspots suggesting that IRMs need a site-specific calibration with a reference method before they are used [13; 32; 33]. The shape of the response curve can be related to the type of OEM sensors integrated with the monitor and relative humidity. Because we did not have information about the internal conversion factors (count to mass concentration for OPC type or light intensity to mass concentration for photometer), using GRIMM or BAM, known as US FEM, we tried to obtain our own correction factors for usage of ESCORTAIR or PA in urban indoor or outdoor settings. The limitations of our study should be noted. First, the sample size in this study was relatively small. However, in our tests, we determined a PM\({}_{2.5}\) range of 10 to 3000 \(\mu\)g/m\({}^{3}\) including both indoor and outdoor measurement. This range ensures that the concentration distributions would not be systematically biased. Our PM\({}_{2.5}\) concentrations might not be representative of each sampling season or area as a result of spatial-temporal variations. Additional experiments are needed to understand the stability of our correction factor in different seasons and/or in other locations, that is, in industrial or rural areas of Korea. Extending the sampling periods for each season and location would ensure that our results are more representative. Furthermore, in future studies, measurements of the wind direction and speed are expected to provide improved correction factors between the IRM and FEM methods. As we mentioned in the method section, we checked flow rate for our RGMs or FEMs prior to our experiment. For IRMs, we have considered measuring flow rate but due to its open wide inlet and very low flow rate, we could not connect it to our mass flowmeter properly. Instead, especially, before our outdoor test, we operated 5 ESCORTAIRs and 5 PAs simultaneously and checked measurement errors between devices. Then, we selected 2 of them which provide best outcomes, compared to FEM. A preparation of QC/QA test program for massive products of IRMs are recommended. And for ESCORTAIR, like PA, application of weather proof design is suggested. In addition, future studies may be necessarily conducted to obtain site-specific correction factors including at coal power plants or in rural areas. In this study, the performances of the IRM with RGM and that of the FEM operated with a high QCQA program were compared with one-hour monitoring intervals at national PM\({}_{2.5}\) monitoring site. This makes this study unique compared to previous studies, which were mostly conducted with one-day interval gravimetric methods at ordinary sampling sites. Despite the growing public interest in reducing personal exposure levels to PM\({}_{2.5}\) in Korea, IRM monitoring still faces challenges in terms of providing real-time concentration information.