Characteristics and Relationships between Indoor and Outdoor PM_(2.5) in Beijing: A Residential Apartment Case Study

In order to understand the characteristics and relationships between indoor and outdoor PM_(2.5) during the heating period of 2014 in Beijing, the investigation of PM_(2.5) and associated species including organic and elemental carbon (OC/EC), water soluble ions, metal elements and trace organic matter (OM) were undertaken at a residential apartment. The average PM_(2.5) concentration was 55.2 ± 47.3 μg m^(-3) for indoor and 100.4 ± 82.1 μg m^(-3) for outdoor, and the indoor PM_(2.5) was found to be mainly from outdoors. OM and (NH_4)_2SO_4 were the dominated components of PM_(2.5), accounted for 71.5% in indoor PM_(2.5) and 52.4% in outdoor PM_(2.5), followed by fine soil and NH_4NO_3 (23.7% and 27.9%). The polycyclic aromatic hydrocarbons (PAHs) concentration was 187.3 ng m^(-3) and 387.0 ng m^(-3), and the phthalic acid esters (PAEs) concentration reached 1054.2 ng m^(-3) and 515.3 ng m^(-3), for indoor and outdoor, respectively. Hexachlorobenzene (HCB) only existed indoors (5.5 ng m^(-3)). HCB and most PAEs in indoor PM_(2.5) were dominated by indoor sources whereas other species were greatly influenced by outdoor sources especially during the pollution period.