Distinguishing Primary and Secondary Tracers to Quantify Naphthalene and Methylnaphthalene Contributions to Secondary Organic Aerosol in the Pearl River Delta

Naphthalene and methylnaphthalene (Nap and MN) are the most abundant polycyclic aromatic hydrocarbons (PAHs) and are important precursors of secondary organic aerosol (SOA) in the atmosphere. 1.2-Phthalic acid (1,2-PhA) and 4-methylphthalic acid (4-MPhA) are usually treated as tracers of SOA from Nap and MN. However, the two tracers also have primary sources, and directly using the tracers to estimate SOA would lead to an overestimation. In this study, we conducted a one-year synchronous observation of the two-ring PAH SOA (SOA2-rings) tracers at nine sites in the Pearl River Delta (PRD) region. We measured and filtered the suitable emission characteristics of SOA2-rings tracers for biomass burning, coal combustion, industrial processes and vehicle exhaust sources. Then, we developed a method to distinguish 1,2-PhA and 4-MPhA from primary emissions and secondary formation. The average proportions of 1,2-PhApri and 4-MPhApri in 1,2-PhA and 4-MPhA were 26.7% and 29.2%, respectively. The direct application of measured 1,2-PhA for estimating SOA2-rings would lead to an overestimation exceeding 30% in the PRD. Furthermore, we estimated SOA2-rings using the separated 1,2-PhAsec and 4-MPhAsec by the tracer-based method. The average contribution of MN to SOA was around three times that of Nap. In addition, when combined with monocyclic aromatic SOA (SOA1-ring) and biogenic SOA, the contributions of SOA1-ring (21%) and SOA2-rings (25%) to total SOA were comparable. SOA2-rings was even the largest contributor to total SOA (~44%) in winter. This study revealed that whether to separate the SOA2-rings tracers for primary emissions and secondary formation is essential in SOA estimation and highlighted that two-ring PAHs make a significant contribution to SOA in the PRD.