Evaluation of the TMO and TOT methods for OC and EC measurements and their characteristics in PM2.5 at an urban site of Korea during ACE-Asia

Daily fine particle (PM2.5) samples, collected during 2001 ACE-Asia at an urban site of Gwangju, Korea, were analyzed for their organic and elemental carbon (OC and EC) concentrations using thermal–optical transmittance (TOT) and thermal manganese dioxide oxidation (TMO) protocols. The goals of this paper were to evaluate the difference in the OC and EC measurements using two different carbon speciation protocols, and investigate the characteristics of carbonaceous particles in relation to the origins of air parcels.The TMO and TOT as operated in the NIOSH method, have been shown to provide equivalent results for total carbon (TC), with a slope of 0.95 and R2R2 of 0.995, but the TMO provides a 9% higher OC and 20% lower EC than the TOT protocol. Importantly, however, combining our results with those of previous comparison studies, between IMPROVE thermal–optical reflectance (TOR) and NIOSH TOT [Chow et al., 2001. Aerosol Science and Technology 34, 23–34], and between TMO and TOR [Fung et al., 2002. Journal of the Air and Waste Management Association 52, 1333–1341], together demonstrate that the nature of the collected samples, in terms of loading, OC/EC ratio and particle composition can significantly affect the analytical results.The origins of air mass pathways arriving at the sampling site, calculated using the HYSPLIT backward trajectory model, can be mostly classified into five types: northwestern China (Type I, i.e. representing clean condition); western/or southwestern marine (Type II); southern marine (Type III); northwestern China, with Asian dust particles (Type IV); and northeastern China, passing through northern Korea (Type V). When an air mass originating from northeastern China passed through the Korean peninsula (Type V), the OC concentrations were the most elevated, with a higher OC/EC ratio (2.7), and accounting for 20.7% of PM2.5 mass on average. However, when the air mass originated from the northwestern China desert regions during Asian dust events (Type IV), the average OC and EC concentrations were 4.6 and 2.1 μg C m–3, respectively, accounting for only 6.7% and 3.1% of the PM2.5 mass, which was due to the huge dust input, resulting in a much elevated PM2.5 mass. Also, a lower black carbon (BC)/carbon monoxide (CO) slope was observed for the air masses originating from the northwestern Chinese regions, which had traveled through Chinese coastal industrial areas and over the Yellow Sea (Types I and IV), than in those originating from marine (Types II and III) or northeastern Chinese (Type V) regions, which had then passed over the Korean peninsula prior to reaching the measurement site.