3.3.2. Overall Mean Indoor Particle Number and Mass Size Distributions

The overall mean particle number and mass size distributions were calculated for each dwelling for the entire winter measurement campaign (Figures 6 and 7). This includes periods with and without indoor activities. In the following section, we will present and discuss the characteristics of the indoor particle number and mass size distributions during different indoor activities. Each dwelling had a unique set of particle number and mass size distributions that reflected the indoor aerosol emission sources associated with the inhabitants' activities, heating processes, and dwelling conditions. For example, among all dwellings, the lowest UFP concentrations were observed in apartment A2 because combustion processes (i.e., cooking using a natural gas stove) were minimal and the indoor space was heated via AC units. GFA2 had the second lowest UFP concentrations because the heating was via water-based central heating and, occasionally, electric heaters. Furthermore, both A2 and GFA2 were nonsmoking dwellings.

**Figure 6.** Mean particle number size distributions calculated for the entirety of the winter measurement campaign at each dwelling: (**a**) apartment A1, (**b**) ground floor apartment GFA1, (**c**) duplex D1, (**d**) ground floor apartment GFA3, (**e**) house H1, (**f**) apartment A2, (**g**) house H2, and (**h**) ground floor apartment GFA2.

**Figure 7.** Mean particle mass size distributions calculated for the entirety of the winter measurement campaign at each dwelling: (**a**) apartment A1, (**b**) ground floor apartment GFA1, (**c**) duplex D1, (**d**) ground floor apartment GFA3, (**e**) house H1, (**f**) apartment A2, (**g**) house H2, and (**h**) ground floor apartment GFA2.

Indoor combustion processes had a pronounced impact on submicron particle concentrations, especially UFPs. For example, the impact of using kerosene heaters was evident in A1, D1, GFA1, and H2. Similarly, the impact of using natural gas heaters was evident in A1, GFA1, GFA3, and H1. Shisha smoking was reported in D1, GFA3, and H1, and the impact can be seen in the high concentrations of UFPs that were measured. D1 never obtained a stable background aerosol concentration during the nighttime likely due to traces of the kerosene heater and shisha smoking.
