Wildfire Smoke Alters Particle Size Distributions Indoors and Outdoors in an Urban Office Building

Wildfire smoke is a growing contributor to urban particulate pollution, yet its size-resolved impacts on outdoor and indoor air quality remain inadequately characterized. This study examined particle size distributions (10 nm - 10 μm) outdoors and indoors in Edmonton, Canada, during the 2025 wildfire season. Here, we combined scanning mobility and optical particle sizers to capture hourly particle concentrations, revealing a shift from ultrafine particles (~21 nm) under background conditions to accumulation-mode particles (~209 nm) during wildfire events. The relationship between particle number concentrations in different size bands and PM2.5 (mass concentration) was examined, indicating strong linear relationships (R2 > 0.85) between PM2.5 and particle number concentrations in the 100-300 nm and 300-1000 nm size ranges. As a result, ambient PM2.5 data obtained from the nearby weather stations are an effective predictor of size-resolved wildfire smoke concentrations for both size bands. Indoor particle measurements indicated that wildfire smoke entered the building, though indoor concentrations remained much lower than outdoors. Regression-based outdoor particle estimates enabled paired indoor-outdoor analysis, revealing positive correlations (r > 0.6) for both particle bands in the 100-300 nm and 300-1000 nm size ranges, with average indoor/outdoor ratios of 0.20 ± 0.07 and 0.18 ± 0.11, respectively. This result indicates substantial attenuation of wildfire smoke particles by the filtration system (MERV 8 + MERV 14), given the high pollutant levels during wildfire season. These findings provide novel insights into size-dependent wildfire smoke infiltration processes and highlight the need for particle-size-resolved models to improve indoor exposure assessments during wildfire episodes.