Pollution Characteristics and Indoor-Outdoor Correlation Analysis of PM₂.₅ in Northern Chinese Residential Buildings

This study presents a field-based, empirically grounded investigation into the spatiotemporal dynamics and indoor–outdoor (I/O) coupling mechanisms of PM₂.₅ in residential buildings across North China. Concurrent high-resolution (10-min interval) measurements of indoor and outdoor PM₂.₅ mass concentrations were conducted from April to December 2025 across six instrumented residential units—stratified by urban/rural setting, building age, heating infrastructure, and envelope integrity—to capture representative heterogeneity in exposure contexts. Data were acquired using calibrated β-attenuation monitors (BAM-1020, ±2.5 µg/m³ accuracy) with integrated temperature/humidity compensation, and synchronized via GPS time-stamping to ensure temporal alignment.Statistical analysis employed rigorous inferential methods: paired t-tests (α = 0.001, two-tailed) confirmed statistically significant concentration disparities between indoor and outdoor environments (p < 0.001 for all sites), rejecting the null hypothesis of I/O equilibrium. Linear regression modeling (R², slope, intercept, residual diagnostics) quantified infiltration-driven coupling strength, while Pearson correlation coefficients (r) and associated p-values assessed monotonic dependence under varying operational conditions. The observed I/O ratio spanned 0.674–2.673, reflecting pronounced building-specific modulation of infiltration and source dominance. Critically, under window-open conditions, residences with active indoor sources (e.g., cooking, incense burning, biomass space heating) exhibited mean I/O ratios >1.0 (1.32 ± 0.18), whereas under window-closed conditions—where infiltration is suppressed—the same units registered I/O <1.0 (0.87 ± 0.11), indicating net indoor generation outweighing penetration loss. In contrast, source-free dwellings maintained strong linear I/O correlation (r = 0.89–0.95, p < 0.001) across both ambient and haze episodes (PM₂.₅ > 150 µg/m³), with regression slopes consistent with empirically derived infiltration factors (0.62–0.78). Conversely, source-active units displayed statistically significant negative Pearson correlations (r = −0.41 to −0.63, p < 0.001) during source events—demonstrating dynamic decoupling wherein indoor concentrations diverge inversely from outdoor trends due to dominant internal emission fluxes.