Beyond Bulk Metrics: A Molecular-Level Framework Reveals How Transport and Partitioning Shape Indoor Organic Aerosols

A precise understanding of indoor particle dynamics requires moving beyond traditional mass- or number-based metrics, which lack the detailed molecular composition information needed to identify key processes governing organic aerosols (OAs). Here, we developed and applied a novel molecular-level framework based on nontarget, high-resolution mass spectrometry analysis of OA composition from multiple sites. This framework resolves the impacts of transport and partitioning, as demonstrated by characterizing particles simultaneously collected from the living room, kitchen, and outdoors of an apartment. Our analysis revealed that outdoor transport was the primary source of living room OA and that the indoor/outdoor ratios of specific components differ from those of bulk particle mass, implying compound-specific transport behaviors. Furthermore, semivolatile organic compounds were found to partition from indoor surfaces into aerosols, as evidenced by benzalkonium chloride cleaning experiments. These results demonstrate that transport and surface-gas partitioning critically shape OA composition at the molecular level, highlighting the limitations of conventional mass- and number-based approaches in capturing the drivers of compositional change and exposure-relevant indoor OA dynamics. More importantly, our framework provides a transferable approach with broad applicability beyond this specific indoor environment, enabling future investigations of OA evolution across various atmospheric settings.