Characterizing water-soluble brown carbon in fine particles in four typical cities in northwestern China during wintertime: integrating optical properties with chemical processes
Authors: Zhong Miao , Xu Jianzhong , Wang Huiqin , Gao Li , Zhu Haixia , Zhai Lixiang , Zhang Xinghua , Zhao Wenhui 
Published: October 2023

Abstract. Brown carbon (BrC) aerosol could impact atmospheric radiative forcing and
play a crucial role in atmospheric photochemistry. In this study, fine
particulate matter (PM2.5) filter samples were collected synchronously
in four major cities in northwestern China during the winter season (December 2019–January 2020): Lanzhou (LZ), Xining (XN), Yinchuan (YC), and Ürümqi
(UR), which are represented as energy-producing and heavy manufacturing
cities in China. The primary aim of the study is to explore the optical
properties, sources, and chemical processes of water-soluble BrC (WS-BrC).
The average mass absorption efficiency at 365 nm (MAE365) of WS-BrC at
these four cities was 1.24 ± 0.19 m2 g−1 (XN), 1.19 ± 0.12 m2 g−1 (LZ), 1.07 ± 0.23 m2 g−1 (YC), and 0.78 ± 0.16 m2 g−1 (UR). The properties of WS-BrC were further
investigated by an acid–base titration experiment. The results showed that
the MAE365 values in all cities increased with higher pH values
(2–11), while the fluorescence intensities of water extracts fluctuated
with pH values, being stronger under both highly acidic and basic
conditions. The sensitivity to pH variation was most pronounced in the
WS-BrC samples from YC and LZ, indicating the important contribution of
acid or base functional group compounds in these locations. Additionally, the
study revealed significant photo-enhancement (LZ) or photo-bleaching (YC and
UR) phenomena of WS-BrC in different cities. These results suggest that the
sources and/or chemical processes of WS-BrC varied among the cities. The sources and chemical processes of WS-BrC were further explored by a
combination of parallel factor analysis (PARAFAC) on excitation–emission
matrix (EEM) spectra of WS-BrC and positive matrix factorization analysis
(PMF) on high-resolution mass spectra of water-soluble organic aerosol (WSOA).
Six PARAFAC components were identified, including three humic-like
substances (HULIS; two less oxygenated (LO) HULIS and one highly oxygenated (HO) HULIS), two protein-like or
phenol-like substances (PLS), and one undefined substance. Four PMF factors were
identified, including a water-soluble primary OA (WS-POA), a less oxidized
oxygenated OA associated with coal combustion-induced WSOA (LO-OOA), and two
highly oxidized oxygenated OAs resulting from photochemical oxidation and
aqueous-phase oxidation transformations (HO-OOA1 and HO-OOA2). WS-POA was
determined to be the most significant source of light absorption, accounting
for 30 %–60 % based on multiple linear regression models, and it showed
a significant correlation with PLS and LO-HULIS components. The loss of
light absorption of WS-POA was found to occur through its conversion to
LO-OOA and HO-OOAs through photochemical or aqueous reactions, with HO-OOAs
being significantly correlated with the HO-HULIS component. These processes
can be clearly illustrated by integrating optical properties and chemical
composition using a Van Krevelen diagram and an EEM plot.