Bridging Indoors and Outdoors: The Impact of Ambient Aerosols on Indoor Air Quality

LUIS MIGUEL FEIJO BARREIRA, Delun Li, Minna Aurela, Laura Salo, Ville Silvonen, Mohamed Elsayed, Teemu Lepistö, Andrea Wagner, Tina Wahle, Roel Schins, Toni Tykkä, Heidi Hellén, Topi Rönkkö, Sanna Saarikoski, Hilkka Timonen, Finnish Meteorological Institute

     Abstract Number: 171
     Working Group: Chemicals of Emerging Concern in Indoor and Outdoor Aerosol: Sources, Vectors, Reactivity, and Impacts

Abstract
Understanding the relationship between indoor and outdoor air quality is crucial for accurately assessing human exposure to air pollutants. To investigate these interactions, two weeks of simultaneous indoor and outdoor measurements were conducted in an office room at a research facility in Düsseldorf during the spring of 2025. A suite of state-of-the-art instruments, including a soot particle aerosol mass spectrometer (SP-AMS), a proton-transfer reaction mass spectrometer (PTR-MS), and an electrical low-pressure impactor (ELPI+), was used to obtain a detailed characterization of both gas and particle phases. The results showed that outdoor-originating aerosols infiltrated and dominated the indoor environment, even in the presence of mechanical ventilation. Temporal trends in chemical composition revealed that nitrate was the dominant component during the first week, likely due to regional or long-range transport. In the second week, organic aerosols became more prevalent, indicating a shift in source influence or atmospheric processes. Positive Matrix Factorization (PMF) analysis of organic aerosols (OA) identified similar source profiles and mass spectra indoors and outdoors, further confirming the substantial impact of outdoor air on indoor aerosol composition. However, differences in source contributions were observed. Cooking Organic Aerosols (COA), Hydrocarbon-like Organic Aerosols (HOA), and Biomass Burning Organic Aerosols (BBOA) were high outdoors. Indoors, HOA and BBOA were more prevalent. The indoor prevalence of HOA and BBOA, especially during working hours, points to the role of human activities—such as frequent door and window openings—in enhancing air exchange and altering the indoor aerosol profile. Size-resolved particle composition showed both temporal and chemical variability. The results highlight a significant and often underestimated link between outdoor and indoor air pollution, underscoring the importance of accounting for both ambient air quality and occupant behavior when assessing indoor exposure to pollutants.