Characteristics of indoor and outdoor fine particles in heating period at urban, suburban, and rural sites in Harbin, China.

Concurrent indoor-outdoor fine particulate matter (PM _2.5 ) measurements were conducted at urban, suburban, and rural sites in Harbin, a megacity in the northeast of China. Chemical constituents of indoor-outdoor PM _2.5 were determined. Infiltration factors (F _INF ) of all sites were calculated according to the indoor to outdoor (I/O) ratios of PM _2.5 based on the regression analysis. Linear discriminant analysis (LDA) is applied to determine the indoor-outdoor relationship. Secondary organic carbon (SOC) was calculated on the basis of organic carbon to elemental carbon (OC/EC) ratios. The mean concentrations of indoor and outdoor PM _2.5 were 166.4 ± 32.5 μg/m ³ and 228.4 ± 83.7 μg/m ³ , respectively, during the heating period. OC/EC and potassium ion to elemental carbon (K ⁺ /EC) ratios verified that biomass was an important source in Harbin especially for rural sites. The nitrate to sulfate (NO ₃ ^- /SO ₄ ^2- ) ratio indicates the higher contribution of traffic emissions in urban sites. Cr was the only species that exceeded the guidelines of WHO 2002, which was mainly emitted from coal and oil combustion. SOC/OC and NO ₃ ^- /SO ₄ ^2- ratios, and ion-balanced acidity (the ratio of cation to anion, R _+/- ) showed a large urban-rural and indoor-outdoor difference. The highest SOC/OC ratio was found at urban sites, up to 38.3% for indoors. SOC/OC ratios and R _+/- values of indoor environments were higher, which is attributed to the conducive condition of forming the secondary pollutants during the heating period. The results of LDA indicated that the distributions of the chemical components of PM _2.5 at three sites were statistically dissimilar. Graphical abstract.