[Pollution Characteristics and Chromophore Types of Brown Carbon in Xi'an]

To explore the optical characteristics and chemical composition of atmospheric brown carbon (BrC) in Xi'an, particulate phase and gas phase atmospheric samples were collected using an atmospheric particulate sampler and adsorbent, and the samples were analyzed using an ultraviolet-visible spectrophotometer and a three-dimensional (3D) fluorescence photometer. The absorption and fluorescence properties of BrC were analyzed using the parallel factor analysis (PARAFAC) method to obtain type and compositional information. The results show that at a wavelength of 365 nm, the absorbances of the BrC particulate and the gas phases were (13.8±7.9) Mm^-1 and (8.0±3.1) Mm^-1, with proportions of 63% and 37%, respectively. No significant correlation was found between the absorbance of the gas and particulate phases. PARAFAC results show that in winter, atmospheric BrC in Xi'an is composed of humic-like and protein-like chromophores, with different proportions in the gas and particulate phases. Humic-like and protein-like chromophores are dominant in the particulate phase (41% and 36%, respectively), while the gas phase mainly contains phenolic chromophores (accounting for 78%). These results reveal that gas phase BrC may be an important factor contributing to positive radiative forcing in the atmosphere as well as an important atmospheric component that participates in atmospheric photochemical reactions.