Estimating the outdoor-derived component of indoor fine particulate matter using the Indoor Environmental Concentrations in Buildings with Conditioned and Unconditioned Zones (IECCU) mass-balance model

Understanding the outdoor-derived contribution to indoor fine particulate matter is essential for characterizing exposure and developing effective indoor air-quality management strategies. This study applied a mass-balance model to estimate the seasonal outdoor-derived contribution to indoor PM 2.5 in a two-zone student dormitory in Saint Petersburg, Russia. Indoor PM 2.5 concentrations were measured in two zones, including a living/sleeping area with kitchen facilities and a bathroom/toilet area, while outdoor PM 2.5 data were collected from 13 monitoring stations. The Indoor Environmental Concentrations in Buildings with Conditioned and Unconditioned Zones (IECCU) model simulated the outdoor-infiltration component considering air exchange, particle penetration, interzonal airflow, and deposition parameters. In addition, a sensitivity analysis based on a polynomial regression was conducted to identify the most influential model input parameters. Seasonal outdoor PM 2.5 concentrations ranged from 3.08 µg/m 3 in spring to 4.15 µg/m 3 in winter. Measured indoor PM 2.5 concentrations varied from approximately 22 to 87 µg/m 3 , with indoor-to-outdoor ratios of 8.69–12.10. The modeled outdoor-derived indoor PM 2.5 contribution ranged from 0.19 to 2.25 µg/m 3 , accounting for 0.51–6.24% of measured indoor PM 2.5 depending on season and zone. Sensitivity analysis based on polynomial regression showed that air-exchange flow rate was the most influential parameter, followed by the penetration factor, whereas room volume and deposition rate had smaller effects. The results indicate that indoor sources and particle dynamics play a dominant role in indoor PM 2.5 levels, highlighting the importance of source control, filtration, and ventilation management. Further studies with direct source measurements and independent model validation are recommended.