Estimation of diurnal and seasonal variations of primary and secondary organic carbon concentrations using highly time resolved PM2.5 measurements

Recently, several investigations have been carried out to identify secondary organic carbon (SOC) contributions to total organic carbon (TOC) mass concentrations. Since direct SOC measurements cannot be made due to limitations of current measurement techniques, alternatively, empirical models based on measured particulate matter (PM) and trace gas data are used to estimate SOC concentrations. In this study, the measurement took place in Seoul from January to December 2010. The amounts of carbonaceous and ionic species of PM2.5 (Particles less than 2.5 micrometers in diameter) were measured hourly with a Sunset Laboratory semi-continuous OC/elemental carbon (EC) analyzer and an Ion chromatography analyzer, respectively. The amounts of primary organic carbon (POC) and SOC were estimated using an EC tracer method and multiple regression analysis based on hourly PM2.5 data. Both methods, which use tracer species of POC and SOC formations, estimated that SOC concentrations account for about 20% of TOC concentrations. The SOC concentrations estimated by the multiple regression analysis are comparable to those derived from the EC tracer method with the small differences in magnitude. Seasonal variations of POC, TOC, and EC concentrations show peaks in autumn and winter whereas SOC concentrations are enhanced in summer and autumn. Diurnal variations of the estimated SOCs show peaks at around 15:00 local time (LT) while POC, TOC and EC concentrations show peaks at morning and midnight. In conclusion, temporal characteristics of POC, which account for large portion of TOC, show good agreement with those of TOC and EC. Both seasonal and diurnal characteristics of SOC concentrations, however, are unveiled to be different from those of POC and TOC. POC and TOC concentrations were directly affected by primary sources while SOC peaks were found in ozone rich conditions with enhanced atmospheric radiation.