Brown carbon in fine particles in four typical cities in Northwest China during wintertime: coupling optical properties with chemical processes

Abstract. Brown carbon (BrC) aerosol could impact atmospheric radiative forcing and play a crucial role in atmospheric photochemistry. Most previous studies have predominantly focused on bulk optical properties of ambient BrC from biomass burning emitted primary or secondary BrC aerosol. Few studies have focused on fossil-fuel-influenced BrC aerosol, especially coal combustion emissions. In this study, fine particulate matter (PM_2.5) filter samples were collected synchronously in four capital cities of Northwest China during the winter season (December 2019–January 2020): Lan-zhou (LZ), Xining (XN), Yinchuan (YC), and Urumqi (UR), which are represented as energy-producing and heavy manufacturing cities in China. The aim of the study was to explore the opti-cal 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 were 1.24 ± 0.19 m²/g (XN), 1.19 ± 0.12 m²/g (LZ), 1.07 ± 0.23 m²/g (YC), and 0.78 ± 0.16 m²/g (UR), respectively. The properties of WS-BrC were investigated by an acid-base titration experiment. The MAE365 values in all cities increased with increasing pH values (2–11), while the fluorescent intensities of water extracts fluctuated with corresponding pH values, being stronger at higher acidic and basic conditions. The WS-BrC at YC and LZ were the two most sensitive sites to pH variation, suggest-ing the important contribution of acid/base functional groups. Furthermore, significant photo-enhancement (LZ) or photo-bleaching (YC and UR) phenomena based on coupling bulk chemical properties with light absorption properties were observed in different cities, indicating their sources and/or chemical processes were different among each other.

The sources and chemical processes of WS-BrC were further explored by the combination of par-allel factor analysis (PARAFAC) on excitation-emission matrix of WS-BrC and positive matrix factorization analysis (PMF) on high-resolution mass spectra of water-soluble organic aerosol (OA). Six PARAFAC components including three humic-like substances (LO-HULIS, HO-HULIS1, and HO-HULIS2), two protein-like (PLS) or phenol-like substances, and one undefined substance were obtained. Four PMF factors including a water-soluble primary OA (WS-POA), a less oxi-dized oxygenated OA that associated with coal combustion-induced WSOA (LO-OOA), and two highly oxidized oxygenated OAs respectively from photochemical oxidation and aqueous-phase oxidation transformations (HO-OOA1 and HO-OOA2) were identified. WS-POA was the most important source of light absorption accounting for 30 %–60 % based on multiple linear regression model and was significantly correlated with PLS and LO-HULIS components. The loss of light absorption of WS-POA is accomplished by conversion to LO-OOA and HO-OOAs through photo- or aqueous reactions, where HO-OOAs were significantly correlated with HO-HULIS component. The potential precursors and reaction pathways for WS-BrC in each city are proposed.