The Roles of N, S, and O in Molecular Absorption Features of Brown Carbon in PM_2.5 in a Typical Semi-Arid Megacity in Northwestern China

Brown Carbon (BrC) absorbs light in wavelength of 300–400 nm, and BrC molecule (BrCM) is a fundamental component responsible for aerosol radiative forcing. In this study, Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) coupled with electrospray ionization (ESI) was used to determine methanol extracted BrCM in PM_2.5 collected in Xi'an, China. The absorption of individual BrCM was quantified through partial least square regression (PLSR) method. Results showed that 77.5% and 91.8% of winter and summer BrCMs were weak absorptive. The top BrCMs were responsible for 60.4% and 84.6%, respectively, of the absorbances in summer and winter. The nitrogen (N)-containing organic molecules were identified to be critical components of light-absorbing matters in both of the two seasons, outlining the significance of N chromogenesis in BrC. The top BrCMs were more closely related to -(O)NO₂ that originated from NO₂ engaged reactions in winter, and to -NH that formed in NH₃ reactions in summer. Sulfur (S)-containing functional groups were not chromophoric while sulfur dioxide (SO₂) triggered N-containing and S-free BrCM formations under high nitrogen oxides (NOx) concentration levels and relative humidity (RH) in winter. Hypochromicity of oxygen (O) in BrC was discovered because of the photobleaching of oxidation and weak light-absorbing of highly oxidized molecules.