Optical properties and potential sources of water-soluble and methanol-soluble organic aerosols in Taipei, Taiwan

The optical properties and sources of brown carbon (BrC) have been poorly constrained in climate models due to the variability of spatiotemporal characteristics, impeding the accurate understanding of its impact on air quality and climate. In this study, daily PM2.5 samples, which were collected from January to November 2021 in urban Taipei, Taiwan, and seasonal variations of optical properties of water-soluble and methanol-soluble organic carbon (WSOC and MSOC) were evaluated. The light absorption coefficients at 365 nm (Abs365) of both extracts, which strongly correlated with WSOC and MSOC mass concentrations, displayed distinct seasonal variations with the highest in winter and the lowest in summer. The Absorption Ångström Exponent of WSOC and MSOC ranged from 4.16 to 7.75 and 4.03–6.83, with averages of 6.05 ± 0.56 and 5.29 ± 0.61, respectively. The mass absorption efficiency (MAE365), which normalizes the Abs365 of both extracts to the mass of WSOC and MSOC, showed significant seasonal difference with the high MAE365, WSOC of 0.96 ± 0.29 m2 g−1 in winter and the lowest in summer of 0.49 ± 0.07 m2 g−1, whereas contrasting with the largest MAE365, MSOC of 0.99 ± 0.46 m2 g−1 in summer and the lowest in winter of 0.66 ± 0.28 m2 g−1, respectively. Fossil fuel combustion, such as traffic emission, and biomass burning, such as crematorium, were identified to be important contributors to light-absorbing substances. The estimated fractional radiative forcing by WSOC and MSOC to elemental carbon was most significant during winter (8.15 ± 3.77%) and spring (13.90 ± 4.38%), respectively, which may greatly affect the atmospheric photochemistry and climate. This study suggests that the impact of BrC in Taiwan on the local and regional air quality and climate is non-negligible.