Determination of deposition rates from indoor particle dynamics

Particle deposition is a key removal mechanism affecting indoor particle concentrations. Quantifying deposition under real-world conditions remains challenging because measurements are affected by outdoor variability, indoor emissions, air leakage, and ventilation. Reliable characterization of size-dependent deposition is nevertheless essential for accurate modeling of indoor particle dynamics. This study evaluates two approaches for estimating size-resolved deposition loss coefficients in an unoccupied residence with stable mechanical exhaust ventilation. Indoor and outdoor particle number concentrations were measured in a single-storey 63 m2 terraced house equipped with mechanical exhaust ventilation during a one-week campaign in spring 2025. Replacement outdoor air entered through four purpose-installed outdoor air inlet vents in the external walls. During the deposition experiment, the vents were operated without filters, providing an unobstructed outdoor-air pathway. Measurements were conducted under unoccupied conditions to enable controlled assessment of deposition. The particle number concentrations were continuously monitored using a scanning mobility particle sizer (SMPS) and an optical particle counter (OPC). Size-resolved particle deposition loss rates were estimated using two separate methods: (i) dynamic modeling, in which model-predicted indoor number concentrations were fitted to the measured data and (ii) an I/O-ratio method, in which deposition loss rates were calculated for individual particle size bins from time-averaged indoor-to-outdoor (I/O) concentration ratios. Deposition loss coefficients were determined for particle sizes between 0.01 and 2.5 µm. The resulting deposition loss rate curve exhibited a U-shaped dependence on particle diameter, with a minimum at approximately 0.1–0.2 µm. Both methods produced comparable deposition loss coefficients and consistent size-dependent behavior, provided that the sampling duration was sufficient for stable I/O ratios. Deposition loss rates ranged from 0.15 to 0.75 h−1.