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Two semi-continuous Sunset carbon analyzers, with different peak inert mode temperatures (615 and 740℃), were simultaneously operated to measure fine particulate organic carbon (OC) and elemental carbon (EC) using a thermal optical transmittance method at an urban site in Beijing, China, from 16 August to 3 September 2006 during the CAREBeijing 2006 campaign. Excellent agreements were obtained between the collocated semi-continuous carbon analyzers, with slopes of 1.02 (R2 = 0.91) for OC and 1.06 (R2 = 0.93) for EC, resulting in very similar average EC/total carbon (TC) ratios of ∼0.36. These results imply that the different peak inert mode (100% helium) temperatures did not cause significant biases on the semi-continuous OC and EC measurements. However, it was found that the EC/TC ratio was greatly influenced by the accumulated refractory particles remaining on a quartz filter (PM refractory). Fresh quartz filters, with low PM refractory loadings, which is defined as a laser correction factor ≥0.94, gave ∼8-10% lower EC/TC ratios than aged quartz filters with high PM refractory loadings. The linear regression slope between EC and optically measured EC (OPT-EC) was much higher with fresh quartz filters (slope = 1.03, R2 = 0.96) than aged quartz filters (slope = 0.89, R2 = 0.95), suggesting the underestimation of EC on fresh quartz filters by ∼15% compared to those measured on aged quartz filters. Authentic standard humic-like substances (HULIS) on the clean quartz filter showed the highest extent of pyrolyzed organic carbon (POC) formation (47.4% in total detected carbon mass), followed by those on the Asian dust loaded quartz filter (37%) and the refractory urban pollutant loaded quartz filter (34.1%), indicating that the Asian dust and refractory urban pollutant reduced the POC formation from the HULIS. Thus, this study suggested that the PM refractory loading plays an important role in the semi-continuous OC and EC measurements by altering the degree of POC formation in the inert atmosphere.