The concentrations of elemental carbon (EC) and organic carbon (OC) of particulate matter (PM)2.5 were investigated at coastal, urban, and inland sites in central Taiwan during October 1999 and March 2000, and the carbon contents were analyzed using a total organic carbon analyzer (TOC). The results showed that the average EC concentrations at the various sites were from 2.7–4.4 μ g/m3, while the mean OC concentrations were from 6.9–12.3 μ g/m3. Both higher EC and OC levels were found in inland areas. Good correlations of OC and EC were found in coastal and urban areas, indicating that OC was mainly from the primary combustion emissions, whereas OC was weakly correlated with EC at the inland sites, suggesting that the contributions of OC were unrelated combustion sources (i.e., secondary organic carbon). The optimal Deming regression was used to estimate the ratio of OC/EC, leading to calculate the amount of secondary organic aerosols in this study. On average, secondary organic carbon constituted 13%–48% of the total organic carbon for PM2.5 with high levels observed in inland areas. Moreover, higher secondary OC concentration was observed at low wind speed, high relative humidity and high oxidants sum (O3 + NO2) levels atmosphere.
