Multivariable Determinants of Indoor PM2.5 and Infiltration Dynamics in 48 Residential Buildings Across Eight Cities of Northern China: A Seasonal Monitoring and Mixed-Effects Modeling Study

Indoor exposure to fine particulate matter (PM2.5) is a major environmental health concern in Northern China, where severe ambient pollution, coal-based district heating, and diverse residential building stocks coexist. Previous studies have been constrained by small sample sizes, limited geographic coverage, and predominantly bivariate analyses. This study presents a large-scale seasonal monitoring campaign covering 48 residences across eight cities in Northern China (Beijing, Tianjin, Shijiazhuang, Taiyuan, Jinan, Zhengzhou, Xi’an, and Anyang). Paired indoor and outdoor PM2.5 concentrations were measured continuously at 5-min intervals for seven consecutive days in each of four seasons (winter heating, spring transition, summer, and autumn transition), yielding 32,256 hourly observations. Building characteristics, occupant behavior, and meteorological covariates were recorded simultaneously. A multivariable ordinary least squares model with cluster-robust standard errors (R² = 0.879) identified outdoor PM2.5 (β = 0.891, p < 0.001), window-open fraction (β = 0.572, p < 0.001), cooking events (β = 0.049, p < 0.001), and air-purifier operation (β = −0.935, p < 0.001) as the strongest determinants of indoor PM2.5. Building-level infiltration factors (F_inf) averaged 0.28 ± 0.15, with pronounced seasonal and inter-city variability. Outdoor sources contributed 57–69% of indoor PM2.5, peaking during the heating season. Mass balance analysis yielded a median deposition rate of 0.53 h⁻¹ and confirmed that a median air change rate of 0.52 h⁻¹ across all buildings, with window-open periods exceeding 2 h⁻¹. These findings provide robust, multivariable evidence for targeted interventions—including improved envelope airtightness, behavioral guidance on window operation, and expanded air-purifier use—to reduce residential PM2.5 exposure in Northern China.