Optical properties of mountain primary and secondary brown carbon aerosols in summertime.

Brown carbon (BrC) can affect atmospheric radiation due to its strong absorption ability from the near ultraviolet to the visible range, thereby influencing global climate. However, given the complexity of BrC's chemical composition, its optical properties are still poorly understood, especially in mountainous areas. In this study, the black carbon (BC) tracer method is used to explore the light-absorbing properties of primary and secondary BrC at Mount Hua, China during the 2018 summer period. The primary BrC absorption contributes to 10-15% of the total BrC absorption at a wavelength of 370 nm. From the positive matrix factorization analysis, traffic emissions are found to be a major source of primary BrC absorption (44%), followed by industry and biomass-burning emissions (29%). The secondary BrC accounts for 87% of the total BrC absorption at a wavelength of 370 nm, indicating that BrC is dominated by secondary formation. The observation of a higher secondary BrC absorption diurnal pattern at Mount Hua can be affected by secondary BrC in the residual layer after sunrise and the formation of light-absorbing chromophores by photochemical oxidation in the afternoon. The estimated average mass absorption efficiencies of primary and secondary BrC (MAE_pri and MAE_sec, respectively) are 0.4 m2/g and 2.1 m2/g at wavelengths of 370 nm, respectively, indicating a stronger light-absorbing ability for secondary BrC than for primary BrC. There is no significant difference in MAE_pri within a daily variation, but the daytime MAE_sec value is higher than that during the night. Our study shows that secondary BrC is important to light absorption in mountainous areas.