| Summary: | Fine particulate matter (PM<sub>2.5</sub>) was simultaneously collected from the indoor and outdoor environments in urban area of Jinan in North China from November to December 2018 to evaluate the characteristics and sources of indoor PM<sub>2.5</sub> pollution. The concentrations of indoor and outdoor PM<sub>2.5</sub> were 69.0 ± 50.5 µg m<sup>−3</sup> and 128.7 ± 67.9 µg m<sup>−3</sup>, respectively, much higher than the WHO-established 24-h standards for PM<sub>2.5</sub>, indicating serious PM<sub>2.5</sub> pollution of indoor and outdoor environments in urban Jinan. SO<sub>4</sub><sup>2−</sup>, NO<sub>3</sub><sup>−</sup>, NH<sub>4</sub><sup>+</sup>, and organic carbon (OC) were the predominant components, which accounted for more than 60% of the PM<sub>2.5</sub> concentration. The total elemental risk values in urban Jinan for the three highly vulnerable groups of population (children (aged 2–6 years and 6–12 years) and older adults (≥70 years)) were nearly 1, indicating that exposure to all of the elements in PM<sub>2.5</sub> had potential non-carcinogenic risks to human health. Further analyses of the indoor/outdoor concentration ratios, infiltration rates (F<sub>INF</sub>), and indoor-generated concentration (C<sub>ig</sub>) indicated that indoor PM<sub>2.5</sub> and its major chemical components (SO<sub>4</sub><sup>2−</sup>, NO<sub>3</sub><sup>−</sup>, NH<sub>4</sub><sup>+</sup>, OC, and elemental carbon) were primarily determined by outdoor pollution. The lower indoor NO<sub>3</sub><sup>−</sup>/SO<sub>4</sub><sup>2−</sup> ratio and F<sub>INF</sub> of NO<sub>3</sub><sup>−</sup> relative to the outdoor values were due to the volatility of NO<sub>3</sub><sup>−</sup>. Positive matrix factorization (PMF) was performed to estimate the sources of PM<sub>2.5</sub> using the combined datasets of indoor and outdoor environments and revealed that secondary aerosols, dust, cement production, and coal combustion/metal smelting were the major sources during the sampling period.
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