Effects of indoor activities and outdoor penetration on PM2.5 and associated organic/elemental carbon at residential homes in four Chinese cities during winter

There is increasing public attention on exposure to PM2.5 and its related health impacts. It is essential to study the pollution levels, sources, and health implications of indoor PM2.5, especially for residential homes, as people tend to spend most of their time indoors. The indoor PM2.5 mass and organic/elemental carbon (OC/EC) during winter and early spring period of 2016–2017 at 68 residential households in four large Chinese cities (i.e. Hong Kong, Guangzhou, Shanghai, and Xi'an) were studied. Average indoor PM2.5 varied by two-fold, lowest in Hong Kong (34.0 ± 14.6 μg m−3) and highest in Xi'an (78.7 ± 49.3 μg m−3), with comparable levels for Guangzhou (47.2 ± 5.4 μg m−3) and Shanghai (50.3 ± 17.9 μg m−3). Lowest air exchange rate (AER, 0.8 ± 0.8 h−1) and PM2.5 indoor/outdoor (I/O) ratio (0.72 ± 0.23) were found for Xi'an households, indicating the limited influence from indoor sources, while importance of indoor PM2.5 sources is signified with the highest PM2.5 I/O ratio (1.32 ± 0.43) identified for Shanghai households. For households in four cities, OC and EC accounted for 29.5%–38.5% and 7.5%–8.9% of the indoor PM2.5 mass, indicating the significance of carbonaceous aerosols. Larger differences between indoor and outdoor OC (2.6–8.4%) than EC (−2.2–1.5%) indicate the presence of indoor OC sources. Decreasing trends of PM2.5 I/O ratio and indoor OC proportion were found as the worsening ambient air quality. On average, 11.8 μg m−3 (23.1%) and 3.02 μg m−3 (18.7%) higher indoor PM2.5 and OC concentrations were identified for households with other indoor combustions (e.g., tobacco smoking, incense burning) compared to those with only cooking activities. For Hong Kong and Shanghai households, increments of 13.2 μg m−3 (54.1%) of PM2.5 and 4.1 μg m−3 (45.4%) of OC were found at households with cooking activities as compared to households with no specific indoor combustion.