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

摘要

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) NO_2 that originated from NO_2 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 (SO_2) triggered N-containing and S-free BrCM formations under high nitrogen oxides (NO_x) 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.