Evaluation of factors influencing secondary organic carbon (SOC) estimation by CO and EC tracer methods

Secondary organic carbon (SOC) is known to account for a substantial fraction of fine-mode carbonaceous aerosol. Owing to a limited understanding of SOC formation processes and the absence of direct measurement methods, SOC concentrations are mostly estimated using a tracer-based method utilizing either elemental carbon (EC) or carbon monoxide (CO) as tracers. The performance of these tracer-based methods depends heavily on accurate determination of the (OC/Tracer)_pri value. The minimum R squared (MRS) method is currently recognized as a relatively reasonable tool to determine (OC/Tracer)_pri. This study estimated SOC based on the MRS method with EC and CO as tracers, followed by the Monte Carlo method to analyze quantitatively the effects of measurement uncertainty, emission scenario and sample size on SOC estimates. We report here four major findings: i) the concentration of O₃ was used as an indicator to atmospheric secondary reaction potential, and it was found that the mass proportion of SOC in total OC estimated by CO as the tracer is more consistent with the seasonality of actual secondary reaction potential; ii) the estimation results are highly sensitive to the measurement uncertainty in different emission scenarios, which leads us to conclude that the CO tracer method is more robust than the EC tracer method due to large inherent uncertainties in current EC measurements; iii) oversimplification of emission scenarios has substantial impacts on the estimated SOC value, and careful evaluation of the interdependence between sources should be performed to minimize this bias; and iv) the estimation bias of SOC can be reduced by increasing the sample size, and the tracer method can be expected to generate robust results for sample sizes over 1000. These findings are important in terms of providing a reference to choose appropriate tracers, emission scenarios and sample sizes for robust estimation of SOC in future studies.