Contribution of outdoor-originating particles, indoor-emitted particles and indoor secondary organic aerosol (SOA) to residential indoor PM2.5 concentration: A model-based estimation

• Different sources contributions to indoor PM2.5 were estimated with a model.
• The estimated ourdoor contribution was 54%–96%.
• The estimated indoor contribution was 4%–45%.
• SOA contribution was less than 3%.

Indoor particles represent a combination of outdoor-originating particles, indoor-emitted particles and indoor secondary organic aerosol (SOA). It is important to distinguish the contributions of different particle sources to indoor PM2.5 (particulate matter with an aerodynamic diameter of 2.5 μm or less) because of the differences in concentration, composition, toxicity and management approaches among these sources. The objective of this study is to estimate the extent to which outdoor-originating particles, indoor-emitted particles and SOA contribute to the residential indoor PM2.5 concentration in Beijing, China. Based on measurements of reactive organic gases (ROGs), their concentrations in 90 residential rooms and an experimental verification of the O3 surface removal rate, a mass-balance model was employed to estimate the different sources' contributions to the indoor PM2.5 concentration. The results show that SOA contribute little (less than 3%) to the indoor PM2.5 concentration. Outdoor-originating particles contribute the most to indoor PM2.5 concentration, with a contribution of 54%–63% when windows are closed and over 92% when windows are open. The indoor-emitted particles contribution varies over a wide range, from approximately 4% when windows are open to 37%–46% when windows are closed. The outdoor PM2.5 concentration and PM2.5 deposition rate are the most important two factors affecting indoor PM2.5 concentration. Air exchange rate, cooking and smoking emission strength can clearly also change the indoor PM2.5 concentration. The effect of the PM2.5 penetration factor is very small.