Elemental carbon (EC) has been widely used as a tracer to estimate secondary organic carbon (SOC) from ambient EC and OC data. Key to the EC tracer method is to determine a proper primary OC / EC ratio at the observation site. We examine here a method that derives this ratio by seeking the minimum correlation between SOC and EC. This method has a clear quantitative criterion for the ratio derivation and is demonstrated to have superior accuracy over the commonly used approaches for SOC estimation.