Secondary Organic Carbon Estimations with Hourly Monitoring Data and the OC/EC Primary Ratio Method

This study examined the hourly monitoring data from 2006 to 2009 collected by the Aerosol Supersite of the Environmental Protection Administration of Taiwan. The OC/EC primary ratio method has been applied to estimate the content of secondary organic carbon (SOC). Results of this study indicated that the monthly concentrations of PM2.5, OC, and EC all remained low in summer but went up in winter. Possible factors were climate-related and influences from continental high pressure systems. The content (24-36%) of SOC in summer was significantly higher than in other seasons, indicating that a great formation of organic carbon in summer. When considering the hourly trend, apparent peaks can be consistently observed in the morning, which may be due to an increase of mobile pollution source and photochemical reactions. (OC/EC)(min) ratio values were calculated based on both hourly and daily concentrations of OC and EC, then annual values (2006 similar to 2009) were 0.20 similar to 1.11 and 0.68 similar to 2.72 for hourly and daily data base respectively. Content of SOC in PM2.5 and OC were estimated to be 16 similar to 23 % and 75 similar to 93 % based on (OC/EC)(min) ratio from hourly data set, and were 11 similar to 18 % and 42 similar to 77 % based on (OC/EC)(min) ratio from daily data set. Results from this study, as well as those from other studies, demonstrated that the OC/EC ratio is dependent upon the sampling method as well as the method of analysis. Furthermore, the daily OC/EC ratio may change, and significant variations may be found even within 24 hours. Taken together, when conducting estimation of SOC, it is important to eliminate the consideration on background concentrations but to take a good advantage of the high temporal resolution of hourly monitoring data in order to estimate SOC using a corrective approach.