*3.1. Comparisons between Di*ff*erent Aerosol Instruments—Technical Notes*

The co-location of different aerosol instruments covering similar size ranges provides a basis to compare concentration outputs as measured through different techniques. First, the PM2.5 and PM10 concentrations reported by the DustTrak can be compared to evaluate the contribution of the submicron fraction to the total PM concentration in Jordanian indoor environments. According to the DustTrak measurements, it was observed that most of the PM was in the submicron fraction as the mean PM10/PM2.5 ratio was 1.03 ± 0.04 (Figure 2). This was somewhat expected as most of the tested indoor activities in this field study were combustion processes (smoking, heating, and cooking) that produce significant emissions in the fine particle range. However, more sophisticated aerosol instrumentation would be needed to verify this finding, such as an aerodynamic particle sizer (APS) and scanning mobility particle sizer (SMPS).

**Figure 2.** Comparison between the PM10 and PM2.5 concentrations measured with the DustTrak.

The DustTrak and SidePak both employ a light-scattering laser photometer to estimate PM concentrations. As such, their output can be compared for the same particle diameter range. In general, the PM2.5 concentrations measured with the DustTrak were lower than the corresponding values measured with the SidePak (Figure 3). This trend was consistent across the measured concentration range from approximately 10 to >1000 μg/m3. The mean SidePak/DustTrak PM2.5 concentration ratio was 2.15 ± 0.48. These differences can be attributed to technical matters related to the internal setup of the instruments and their factory calibrations. For example, the SidePak inlet has an impactor plate with a specific aerodynamic diameter cut point (here chosen as PM2.5), whereas the DustTrak differentiates the particle size based solely on the optical properties of particles.

Following the methodology outlined in Section 2.3, we converted the measured particle number size distributions (via CPC 3007, P-Trak, and AeroTrak) to particle mass size distributions assuming spherical particles of unit density. From integration of the latter, we calculated the PM2.5 and PM10 concentrations. The calculated PM2.5 and PM10 concentrations can be compared with those reported by the DustTrak. The calculated PM2.5 concentrations were found to be less than those reported by the DustTrak (Figure 4). More variability was observed for PM10, with the calculated PM10 both underand overestimating the DustTrak-derived values across the measured concentration range. The mean calculated-to-DustTrak PM2.5 ratio was 0.63 ± 0.58 and that for PM10 was 1.46 ± 1.27.

**Figure 3.** Comparison between the PM2.5 concentrations measured with the DustTrak and SidePak.

**Figure 4.** Comparison between the PM2.5 and PM10 concentrations measured with DustTrak and those calculated using the measured particle number size distributions, assuming spherical particles of unit density.

This brief comparative analysis of the PM concentrations measured by the DustTrak, SidePak, and calculated via measured particle number size distributions illustrates that portable aerosol instruments have limitations and their output is likely to be inconsistent. Relying on a single instrument output may not provide an accurate assessment of PM concentrations. The utilization of an array of portable aerosol instruments can provide lower and upper bounds on PM concentrations in different indoor environments. Calculating PM concentrations from measured particle number size distributions is uncertain in the absence of reliable data on size-resolved particle effective densities for different indoor emission sources.
