Indoor/outdoor relationships for PM_2.5and associated carbonaceous pollutants at residential homes in Hong Kong - Case study

Six residences were selected (two roadside, two urban, and two rural) to evaluate the indoor-outdoor characteristics of PM2.5(aerodynamic diameter <2.5 μm) carbonaceous species in Hong Kong during March and April 2004. Twenty-minute-averaged indoor and outdoor PM2.5concentrations were recorded by DustTrak samplers simultaneously at each site for 3 days to examine diurnal variability of PM2.5mass concentrations and their indoor-to-outdoor (I/O) ratios. Daily (24-h average) indoor/outdoor PM2.5samples were collected on pre-fired quartz-fiber filters with battery-powered portable mini-volume samplers and analyzed for organic and elemental carbon (OC, EC) by thermal/ optical reflectance (TOR) following the Interagency Monitoring of Protected Visual Environments (IMPROVE) protocol. The average indoor and outdoor concentrations of 24 h PM2.5were 56.7 and 43.8 μg/m3, respectively. The short-term PM2.5profiles indicated that the penetration of outdoor particles was an important contributor to indoor PM2.5, and a household survey indicated that daily activities were also sources of episodic peaks in indoor PM2.5. The average indoor OC and EC concentrations of 17.1 and 2.8 μg/m3, respectively, accounted for an average of 29.5 and 5.2%, respectively, of indoor PM2.5mass. The average indoor OC/EC ratios were 5.8, 9.1, and 5.0 in roadside, urban, and rural areas, respectively; while average outdoor OC/EC ratios were 4.0, 4.3, and 4.0, respectively. The average I/O ratios of 24 h PM2.5, OC, and EC were 1.4, 1.8, and 1.2, respectively. High indoor-outdoor correlations (r2) were found for PM2.5EC (0.96) and mass (0.81), and low correlations were found for OC (0.55), indicative of different organic carbon sources indoors. A simple model implied that about two-thirds of carbonaceous particles in indoor air are originated from outdoor sources.