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 PM2.5 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 to carbon content (Oeff/C) ratios (urban 0.82, seaside 0.85) indicated the highly oxidized nature of 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) exhibited enhanced CHON signals attributable to increased organonitrate formation under elevated NOx levels, whereas Case 2 (marine air masses) showed enhanced CHO signals and a higher CHOX oxidation state likely resulting from intensified aqueous/heterogeneous reactions under humid conditions. These findings advance our understanding of OA molecular characteristics and chemical evolution under different environmental conditions.