Seasonal characteristics of indoor and outdoor fine particles and their metallic compositions in Nanjing, China

Frequent hazy weather across China has increased public concern about the health consequences of indoor and outdoor fine particulate matter (PM2.5) and absorbed metal components. However, few studies have been conducted on the multi-seasonal characteristics of indoor PM2.5 and its metallic composition in China. To fill this research gap, we selected a playground and classroom at the Nanjing Foreign Language School as outdoor and indoor sample sites, respectively. Samples were collected with a portable particulate monitor for four seasons (2015-2016) to obtain real-time PM2.5 records and were further analyzed by inductively coupled plasma mass spectrometry (ICP-MS) to characterize seven typical metallic elements. Considerable seasonal variability was observed in outdoor PM2.5 levels. The highest real-time outdoor PM2.5 mass concentrations were observed in winter (148.6 ± 52.3 μg/m3), and the lowest was observed in summer (15.1 ± 6.6 μg/m3). Due to the protective effects of the enclosed system, the indoor PM2.5 levels in the classroom did not synchronously vary with the outdoor levels, except in autumn. This result was inconsistent with prior findings regarding the indoor-outdoor relationships at urban residences and might be associated with unique characteristics of indoor sources in classrooms, which highlights the importance of this study. Moreover, indoor-sourcing PM2.5 had lower metallic compositions than outdoor-originating PM2.5. Pb was the dominant metal component in both indoor and outdoor PM2.5 in all seasons with the range of 14.93-109.14 ng/m3. Overall, metal contents in Nanjing were lower than other Chinese cities; however, the government cannot neglect the associated dangers because China has relatively lax environmental quality standards.