Mass balance model for estimating indoor and outdoor contributions to residential PM_2.5 and indoor source composition...

Indoor-sourced PM_2.5 plays an important role in shaping indoor air quality. Although previous studies have investigated its sources and physicochemical characteristics, there are still gaps in developing effective approaches to quantify the contribution of indoor-source PM_2.5 to total indoor PM_2.5 levels and compositions, both with and without indoor air purification. To fill in this knowledge gap, a mass balance model was developed and validated across 43 residences in Shanghai. 48-hour PM_2.5 samples were simultaneously collected from bedrooms and outside of these homes. PM_2.5 Samples were analyzed for mass concentration and chemical composition, including organic matter (OM), elemental carbon (EC), and 19 other elements. Among these components, aluminum and silicon exhibited the highest outdoor-to-indoor ratios and were selected as tracers for outdoor infiltration. Using these variables as inputs, the mass balance model estimated that indoor sources contributed 63 ± 17 % of total indoor PM_2.5 mass concentration. Our model further characterized composition profile of indoor-sourced PM_2.5, identifying OM as the dominant component (69 %) with consistent contributions across homes (R² = 0.97). When applied under air filtration condition, the model revealed a substantial shift in indoor-sourced PM_2.5 composition profile, with OM increasing from 69 % to 92 % and EC decreasing from 4 % to 0.2 %. The indoor and outdoor contributions to residential PM_2.5 and indoor source composition profiles quantified by our model may help develop air quality management strategies.