Comparative Evaluation of Secondary Organic Aerosols in PM2.5 in Shenzhen Using Multiple Methodologies

Accurate quantification secondary organic aerosols (SOA) in ambient PM_2.5 is crucial for addressing the current challenges in visibility improvement and further exploring the climate impacts of SOA. In this study, we conducted synchronous measurements of PM_2.5 components in Shenzhen from November 1 to December 15, 2022, utilizing a suite of multiple online instruments, including high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS), semi-continuous OC/EC carbon aerosol analyzer, a monitor for aerosols and gases (MARGA), and a continuous multi-metals monitor (Xact-625). Three methods were employed for SOA in PM_2.5 quantification, including AMS-PMF, Online-PMF, and the EC (elemental carbon) tracer method. The three methods yielded generally consistent SOA mass concentrations of 4.2 ± 3.2 μg m^-3, 3.6 ± 2.6 μg m^-3 and 3.5 ± 2.4 μg m^-3, respectively. A strong correlation (r = 0.95) was found between AMS-PMF and Online-PMF results. While the EC tracer method showed lower consistency with AMS-PMF and Online-PMF, with correlation coefficients r of only 0.82 and 0.79, respectively. The AMS-PMF method determined that SOA accounted for 61.3 % of the organic mass (33.7% for more oxygenated organic aerosols, MO-OOA, and 27.6% for less oxygenated organic aerosols, LO-OOA), and the Online-PMF method estimated 57.0 %. However, the EC tracer method estimated only 50.5 %, primarily due to the higher uncertainty associated with SOA quantification in this method. The daily SOA variations from all three methods showed consistent peaks in the afternoon (14:00 ~ 15:00), and a significant rise at night. These patterns were attributed to increased photochemical activity in the afternoon and changes in boundary layer height at night. These analyses further support the reliability of the SOA quantification results in this study, encompassing both MO-OOA and LO-OOA.