Characterization and source apportionment of atmospheric organic and elemental carbon during fall and winter of 2003 in Xi'an, China

Continuous observation of atmospheric organic and elemental carbon (OC, EC) were conducted at Xi'an during high pollution seasons from September 2003 to February 2004. PM2.5 samples were collected on pre-fired quartz-fiber filters with battery-powered mini-volume samplers every day and PM10 samples were collected every third days. Three types of source samples (i.e., coal-combustion, motor vehicle exhaust, and biomass burning) were also collected during ambient sampling period. Ambient and source samples were analyzed for OC and EC by thermal/optical reflectance (TOR) following the Interagency Monitoring of Protected Visual Environments (IMPROVE) protocol. The average PM2.5 OC concentrations in fall and winter were 34.1±18.0 μg m-3 and 61.9±33.2 μg m-3, respectively, while EC were 11.3±6.9 μg m-3 and 12.3±5.3 μg m-3, respectively. Most of OC and EC were associated with fine particle (PM2.5) mode. The OC and EC levels at Xi'an are higher than most urban cities in Asia. The OC and EC in fall were found to be strongly correlated (R2>0.9), with moderate correlation in winter (R2=0.66). The carbonaceous aerosol accounted for 48.8±10.1% of the PM2.5 during fall and 45.9±7.5% during winter. Average OC/EC ratio was 3.3 in fall and 5.1 in winter with individual OC/EC ratios constantly exceeding 2.0. Elevated OC/EC ratios were found during heating seasons with increased coal combustion. The contribution of secondary organic carbon was not significant during winter. The time series of OC and EC showed periodic variability. Traffic contributes 5 and 7 day peaks in the spectrum, precipitation appears as a 10 day periodicity and biomass burning can be identified as a 24 day periodicity. Total carbon (TC) was apportioned by absolute principal component analysis (APCA) using the 8 carbon fraction data (OC1, OC2, OC3, OC4, EC1, EC2, EC3, and OP [a pyrolyzed carbon fraction]). TC attributes 73% to gasoline exhaust, 23% to diesel exhaust, and 4% to biomass burning during fall. However, TC attributes 44% each to gasoline exhaust and coal burning, 9% to biomass burning, and 3% to diesel exhaust during winter.