Comparison of secondary organic aerosol estimation methods

Abstract

To investigate the secondary organic aerosol (SOA) formation in Beijing, fine particle (PM2.5) samples were collected at an urban site (Peking University, PKU) and a rural site (Yufa) during CAREBEIJING 2008 summer intensive field campaign. Several approaches were used to estimate SOA concentrations, including tracer-yield method, non-primary organic carbon method (receptor model, Chemical Mass Balance model), non-biomass burning water soluble organic carbon (WSOC) method and EC tracer OC/EC ratio method. To develop non-biomass burning WSOC method, simulation of typical Chinese biomass burning was conducted to obtain the important parameters. The average WSOC/OC ratio in Chinese biomass burning is 0.48±0.04. This ratio can be used in non-biomass burning WSOC method to estimate SOC of China. The results from different methods all indicated secondary formation has become major contributor to organic aerosols in Beijing, accounting for 50% or more of the total OC. A closure study was made by combination of CMB model and tracer-yield method. Five primary sources, including vegetative detritus, biomass burning, coal burning, gasoline engines and diesel engines, and four secondary organic aerosols derived from isoprene, α-pinene, β-caryophyllene and toluene were apportioned. To the current knowledge, about 20%~27% of the OC sources still remain unknown. Applicability of these SOC methods in China was tested by comparing the different methods. Tracer-yield method, CMB model and EC tracer OC/EC ratio method can be used to estimate SOC in China. However, non-biomass burning WSOC method can only be used to estimate water-soluble SOC. Uncertainty analysis was conducted to help researchers to determine the proper method to estimate SOC in China. Tracer-yield method underestimates total SOC, because it can only estimate SOCs from several precursors. Similarly, non-biomass burning WSOC method also underestimate SOC. Non-primary OC method overestimates SOC due to unapportioned primary OC, especially in urban area where the particle sources are complicated. The uncertainty of EC-tracer OC/EC ratio method is mainly from the primary OC/EC ratio. The largest overestimation and underestimation of single point value can be 54% and 64%.