3.2.1. Indoor Particle Concentrations during the Winter Season

An overview of the indoor submicron particle number (PN) concentrations and PM2.5 and PM10 concentrations is presented Tables 4 and 5 (mean ± SD and 95%) and illustrated in Figure 5 for each of the eight Jordanian dwellings investigated in this study. Particle concentration time series are presented in the supplementary material (Figures S1–S8). Indoor particle concentrations (mean ± SD) were also evaluated during the nighttime, when there were no indoor activities reported in the dwellings and the concentrations were observed to be at their lowest levels (Table 6).

**Table 4.** Indoor particle number and mass concentrations (mean ± SD and 95%) during the winter campaign.


**Table 5.** Indoor particle number and mass concentrations (mean ± SD and 95%) during the summer campaign.




**Figure 5.** Overall mean indoor particle concentrations during the measurement period in each dwelling: (**a**) submicron particle number (PN) concentrations measured with the condensation particle counter (CPC 3007) and (**b**) PM2.5 concentrations measured with the DustTrak. The blue bars represent the winter campaign and the orange bars represent the summer campaign.

Submicron PN concentrations were the lowest in apartment A2, which was equipped with an air conditioning (AC) heating/cooling setting and nonsmoking occupants. For example, the overall mean submicron PN concentrations in A2 was approximately 1.6 <sup>×</sup> 104 cm−3. The second lowest PN concentrations were observed in the ground floor apartment GFA2, which was equipped with a central heating system (water radiators) and, periodically, electric heaters. Occupants in GFA2 were nonsmokers. The overall mean submicron PN concentration in GFA2 was approximately double that of A2 at 3.2 <sup>×</sup> 104 cm<sup>−</sup>3.

The highest submicron PN concentrations were measured in duplex apartment D1, with a mean of 1.3 <sup>×</sup> 105 cm−3. This apartment had a kerosene heater and one of the occupants smoked shisha (waterpipe or hookah) on a daily basis. The second highest submicron PN concentrations were observed in houses H1 and H2, with overall mean values of 1.2 <sup>×</sup> 105 cm−<sup>3</sup> and 9.7 <sup>×</sup> 10<sup>4</sup> cm−3, respectively. House H1 was heated by using a natural gas heater and smoking shisha was often conducted by more than one occupant. House H2 was heated with a kerosene heater and cooking activities occurred frequently.

The ground floor apartments, GFA3 and GFA1, showed intermediate submicron PN concentrations among the study sites, with mean concentrations of 6.3 <sup>×</sup> 104 cm−<sup>3</sup> and 5.4 <sup>×</sup> 104 cm−3, respectively. Although occupants in GFA3 heavily smoked tobacco and shisha, the concentrations were lower than those observed in D1 and H1, where shisha was also smoked. The building envelopes of D1 and H1 may be more tightly sealed, with lower infiltration rates compared to GFA3. Furthermore, GFA3 used a natural gas heater and cooking activities were not as frequent. As for GFA1, the heating was a combination of a kerosene heater and a natural gas heater. The cooking activities in GFA1 were minimal and not frequent. Occupants in apartment A1 were nonsmokers. Indoor emission source manipulations were conducted in A1, including various cooking activities and the use of three different types of heating (kerosene heater, natural gas heater, and AC). The overall mean submicron PN concentration in A1 was approximately 4.3 <sup>×</sup> 104 cm<sup>−</sup>3.

For PM2.5 concentrations, the lowest levels were observed not in A2 (highest submicron PN concentrations), but rather in GFA2, with a mean of approximately 29 μg/m3. GFA2 was heated by means of a central heating system and, periodically, with electric heaters. Ground floor apartment GFA1 and apartment A2 exhibited intermediate overall mean PM2.5 concentrations among the study sites, with mean values of 42 μg/m<sup>3</sup> and 44 μg/m3, respectively. As previously discussed, the occupants in GFA1 did not conduct frequent cooking activities and heated their dwelling by means of kerosene and natural gas heaters, whereas A2 was heated via an AC. GFA1 was built in the 1970s, whereas A2 was relatively new (less than 10 years old); therefore, A2 is expected to be a more tightly sealed indoor environment compared to GFA1. However, infiltration rate and air leakage (i.e., blower door) measurements were not conducted for the dwellings in this study.

Apartment A1, in which manipulations of various cooking activities and heating methods were conducted, showed an overall mean PM2.5 concentration of 91 μg/m3. The impact of shisha smoking on PM2.5 concentrations in D1 and H1 was clearly evident, with overall mean PM2.5 concentrations of 131 μg/m<sup>3</sup> and 138 μg/m3, respectively. The influence of a kerosene heater and intense cooking activities in H2 was also evident, with an overall mean PM2.5 concentration of 156 μg/m3. The highest PM2.5 concentrations were recorded in GFA3 (approximately 433 μg/m3), which reflects the frequent shisha and tobacco smoking in this dwelling.

In the absence of indoor activities (Table 6), the submicron PN concentrations were the lowest (approximately 6 <sup>×</sup> 103 cm<sup>−</sup>3) in A1 and A2 and the highest in D1 (approximately 1.3 <sup>×</sup> 104 cm<sup>−</sup>3) and GFA3 (approximately 1.5 <sup>×</sup> 104 cm<sup>−</sup>3). As for the PM2.5 concentrations measured with the DustTrak, the lowest concentrations (approximately 10 μg/m3) were observed in A2 and GFA2 and the highest concentrations were observed in GFA3 (approximately 67 μg/m3). It is important to note that the measured indoor particle concentrations were primarily the result of the transport of outdoor particles indoors via ventilation and infiltration. However, indoor-generated aerosols during the day may still have traces overnight. For example, the dwellings with combustion and smoking activities also had background concentrations higher than other dwellings. Furthermore, differences in background concentrations among dwellings can be due to the geographical location of the dwelling within the city; this might reflect the outdoor aerosol concentrations at a given location [16,48].

#### 3.2.2. Indoor Particle Concentrations: Summer Versus Winter

Indoor aerosol measurements were repeated for three apartments in the summer campaign. We selected a dwelling (H2) that was heated with a kerosene heater and had nonsmoking occupants, a dwelling (GFA2) that was not heated with combustion processes and had nonsmoking occupants, and a dwelling (GFA3) that was heated with a natural gas heater and the occupants were smokers. Although the number of selected indoor environments was fewer in the summer campaign, the measurement period in each dwelling was longer and more extensive than what was measured during the winter campaign.

In general, the observed concentrations during the summer campaign were lower than those observed during the winter campaign (Tables 4 and 5, Figure 5). The overall mean submicron PN concentration during the summer campaign in GFA2 was approximately 1.5 <sup>×</sup> 10<sup>4</sup> cm<sup>−</sup>3, which was about 40% of that during the winter campaign. As for the PM2.5 concentrations, the overall mean during the summer campaign was approximately 30 μg/m3, which was almost the same as that observed during the winter campaign.

The overall mean submicron PN concentrations in GFA3 and H2 were similar (approximately 1.6–1.9 <sup>×</sup> 10<sup>4</sup> cm−3), whereas the corresponding mean PM2.5 concentrations were higher in H2 (approximately 46 μg/m3) compared to GFA3 (approximately 31 μg/m3). The summer/winter ratio for submicron PN concentrations for GFA3 and H2 were 0.3 and 0.2, respectively. The corresponding PM2.5 ratios were approximately 0.1 and 0.3. The primary reason for higher particle concentrations during the winter was the use of fossil fuel combustion for heating (i.e., kerosene and natural gas

heaters). Furthermore, the dwellings during the summer were more likely to be better ventilated than during the winter, when the dwellings had to conserve energy during heating periods.
