Measurement report: Molecular characterization of organic aerosol in coastal environments using offline FIGAERO-I-CIMS

Abstract. Organic aerosol (OA), as a key component of particulate matter, exerts significant impacts on public health and the environment. However, understanding of molecular characterization of OA under diverse environments remains limited. This study employed offline FIGAERO-I-CIMS (Filter Inlet for Gases and Aerosols coupled with iodide-adduct Chemical Ionization Mass Spectrometry) to analyze the molecular composition of OA in PM_2.5 samples collected from a coastal city (urban and seaside sites) in Southeast China during spring 2024. A total of 737 and 768 CHOX compounds were identified at the urban and seaside sites, respectively. CHO compounds dominated in signal intensity (>70 %) at both sites, while CHON were more abundant at the urban site and S-containing compounds at the seaside site. The weighted effective oxygen numbers (urban 0.82, seaside 0.85) indicated higher oxidation levels in coastal compounds. Seaside CHOX exhibited lower unsaturation, reduced aromaticity, and higher oxidation states. Categorization showed that urban OA was more influenced by aromatic compounds, whereas seaside OA contained higher proportions of aged aliphatic compounds. Two distinct pollution episodes were selected to investigate CHOX evolution. Case 1 (local accumulation) showed enhanced CHON signals through NO₃·-initiated nighttime oxidation that promoted Aliphatic Nitrates formation, whereas Case 2 (marine air masses) showed increased proportions of Aliphatic-like O-rich CHOX compounds (28 % to 39 %) via aqueous-phase processing probably under high humidity. These findings advance our understanding of OA molecular characteristics and chemical evolution under different environmental conditions.