3.3.1. Indoor Particle Size Distributions in the Absence of Indoor Activities

The mean particle number and mass size distributions for each dwelling in the absence of indoor activities during the winter campaign are presented in Figure S9. Significant differences in the mean particle number and mass size distributions were observed among the eight dwellings. Based on the number size distributions, the submicron PN concentration was the lowest (approximately 6 × 10<sup>3</sup> cm−3, with a corresponding PM2.5 of 5 μg/m3) in dwellings A1 and A2 and the highest in GFA3 (approximately 1.5 <sup>×</sup> 104 cm<sup>−</sup>3, with a corresponding PM2.5 of 12 <sup>μ</sup>g/m3) and D1 (approximately 1.3 <sup>×</sup> 104 cm−3, with a corresponding PM2.5 of 8 <sup>μ</sup>g/m3). The mean submicron PN concentration was between 9 <sup>×</sup> 103 cm−<sup>3</sup> and 10<sup>4</sup> cm−<sup>3</sup> and the mean PM2.5 was 7–9 <sup>μ</sup>g/m3 in the remainder of the dwellings. It should be noted that GFA3 had the highest submicron PN concentration, whereas H2 had the highest PM2.5 concentration (approximately 13 μg/m3). Differences between the PN and PM concentrations among the eight dwellings is an indicator of variability in the shape and magnitude of the aerosol size distributions, as illustrated in Figure S9.

The coarse PN concentrations were the lowest in A1 (approximately 0.4 cm<sup>−</sup>3, with a corresponding PMcoarse of 0.9 μg/m3) and D1 (approximately 0.4 cm−3, with a corresponding PMcoarse of 1.3 μg/m3) and the highest was in H2 (approximately 5.2 cm<sup>−</sup>3, with a corresponding PMcoarse of 39.9 μg/m3) and the second highest was in H1 (approximately 2.5 cm<sup>−</sup>3, with a corresponding PMcoarse of 17.3 μg/m3). As for A2, GFA1, and GFA3, the coarse PN concentrations were approximately 0.9 cm−<sup>3</sup> for each of the dwellings, but the corresponding PMcoarse was about 6.3, 3.5, and 5.6 μg/m3, respectively. The similarity in the coarse PN concentrations, compared to the differences observed for the PMcoarse concentrations, in these dwellings is an indication of differences in the coarse size fraction of the indoor particle size distributions. This likely reflects differences in indoor emission sources of coarse particles among the dwellings. For example, H2 had the highest coarse PN and PM concentrations which could be explained by the existence of pets (more than two cats), in addition to the geographical location of this dwelling, which was close to an arid area in southeast Amman, where dust events and coarse particle resuspension are common.
