Chemical characteristics of PM2.5 and organic aerosol source analysis during cold front episodes in Hong Kong, China

In this study, we investigate the influence of long-range transport (LRT) episodes brought in by cold front on the concentration levels of PM2.5, major aerosol constituents, organic tracers, and PM2.5 source characteristics in Hong Kong, China. PM2.5 samples were collected during January–March 2004 and January–March 2005 and analyzed for major constituents and organic tracer species. Synoptic weather conditions and characteristics of common air pollutants were used to categorize the sampling days to three groups, i.e., groups mainly affected by local emissions or regional transport (RT) or cold front LRT. Concentrations of PM2.5 mass and its major constituents during cold-front days were lower than those during RT-dominated periods but higher than those during local emissions-dominated periods. Source apportionment using chemical mass balance (CMB) indicates that vehicular exhaust was a significant primary OC source of mainly local emissions, making average contributions of 1.82, 1.50, and 2.39 μg C m− 3 to OC in the local, LRT, and RT sample groups, respectively. During cold front periods, primary OC concentrations attributable to biomass burning and coal combustion were approximately triple and double, respectively, those during periods dominated by local emissions. Suspended dust, a minor primary OC source (0.24–0.40 μg C m− 3), also showed increased contribution during cold fronts. The unexplained OC by CMB (i.e., total OC minus apportioned primary OC), an approximate indicator for secondary OC, was a significant fraction of OC (> 48%) and its mass concentration was much higher in the cold front LRT and RT sample groups (6.37 and 9.48 μg C m− 3) than in the local sample group (3.8 μg C m− 3). Source analysis as well as tracer concentration variation shows that biomass burning OC and water soluble organic carbon (WSOC) were correlated, suggesting biomass burning as a significant contributor to WSOC.