[Relationship between indoor and outdoor levels of black carbon in residential houses in urban areas of Beijing in 2016].

Objective: To investigate the black carbon (BC) pollution in the indoor air of typical residential houses in urban areas of Beijing, and to explore the relationship between indoor and outdoor BC concentrations as well as the main influencing factors.
Methods: The indoor and outdoor PM_(2.5) samples were collected simultaneously from 33 apartments in the urban areas of Beijing during both the heating season (January to March) and the non-heating season (June to August) in 2016. Subsequently, optical method were employed to analyze BC concentrations in PM_(2.5)samples. The Spearman correlation coefficient (r_s) and the indoor/outdoor (I/O) ratio of BC concentrations were both calculated to characterize the relationship between indoor and outdoor BC concentrations. The factors may influence indoor BC pollution was collected through a questionnaire, including the basic characteristics of the residential buildings and households, smoking, cooking, window opening behavior, the use of air conditioner or air purifier and so on. Additionally, a linear mixed-effects model or multiple linear regression model was applied to identify the main factors influencing the I/O ratio.
Results: The(M(P25, P75)) concentrations of indoor and outdoor BC for season-pooled analysis were2.84 (2.59, 3.26)μg/m~3 and 3.08 (2.90, 3.63)μg/m~3, respectively. There were significant seasonal differences in both indoor and outdoor concentrations (P<0.05), with higher levels observed during the heating season compared to the non-heating season. There was a strong correlation between indoor and outdoor BC (r_s=0.74). The correlation during the heating season (r_s=0.78) was stronger than that during the non-heating season (r_s=0.44). The ■ of I/O ratio was 0.90±0.11, with 93.5%(29/31)and 86.7%(26/30) of I/O ratios being less than 1 during the heating season and non-heating season, respectively. Statistical analysis also showed that outdoor BCconcentrations were significantly higher than indoors (P<0.05). In season-pooled analysis, the result of the linear mixed-effects model showed that window opening duration was the most important factor affecting the I/O ratio, explaining 21.3%of the total variation. The I/O ratio increased with longer window opening duration. In season-specific analysis, the characteristics of residential buildings (including building age and floor level) and window opening duration were the main factors affecting the I/O ratio during the heating season and non-heating season, respectively in 2016.
Conclusion: Residents in the urban areas of Beijing experienced relatively high indoor levels of BCpollution, but lower than the outdoor concentration during the same period in 2016. The window opening and the characteristics of residential buildings were the most important factors affecting the I/O ratio of BC.