Atmospheric organic aerosols: online molecular characterization and environmental impacts

Organic aerosols (OA) play critical roles in atmospheric chemistry, air quality, climate forcing, and public health. However, their chemical complexity, comprising thousands of compounds with a wide range of volatilities, functionalities, and oxidation states, poses substantial challenges for comprehensive characterization and impact assessment. Advances in high-resolution mass spectrometry, particularly when coupled with specialized inlets such as the Filter Inlet for Gases and Aerosols (FIGAERO) and Extractive Electrospray Ionization (EESI), have enabled real-time molecular-level analysis of both gas- and particle-phase organics. These developments have substantially improved insights into OA composition, physicochemical properties, sources, and formation pathways. This review critically assesses recent progress in widely used analytical techniques for molecular characterization of OA and their applications in ambient air, emission sources, and indoor environments. Parameterizations of key OA properties, including volatility, viscosity, and hygroscopicity based on molecular data are summarized. Recent findings on secondary organic aerosol (SOA) formation mechanisms, including homogeneous oxidation, heterogeneous processing, and gas-particle partitioning, are discussed. In addition, the review highlights molecular-marker-based advances in source apportionment and examines the role of OA in new particle formation and its implications for climate and health. Finally, future research directions to improve molecular-level understanding of OA and its environmental impacts are proposed.