Analysis of indoor secondary organic aerosol formation near occupants in a classroom using computational fluid dynamics simulations

Secondary organic aerosol (SOA) forms indoors when ozone reacts with terpenes, generating a range of low- and semi-volatile compounds, over 50% of which partition into the particle phase. This study investigated the formation of SOA in indoor spaces under heterogeneous thermal conditions resulting from the combined effects of HVAC systems and heat emitted by human occupants. The core of this study involved integrating the volatility basis set (VBS) model with computational fluid dynamics (CFD) simulations. The resulting VBS-CFD framework was used to simulate SOA formation from ozone-terpene reactions, with terpenes originating from human emissions. Model accuracy was assessed using experimental data from previous measurement studies and a material balance model. Results indicate that semi-volatile compound concentrations are substantially higher near occupants compared to ambient levels, while SOA concentrations are lower near humans due to temperature gradients. The study results further revealed notable spatial variability in SOA concentrations under both cooling and heating scenarios, despite maintaining a consistent average indoor temperature. These findings highlight the important role of semi-volatile compounds in influencing particle concentrations near occupants, with over 50% of these compounds potentially contributing to aerosol formation—and thereby increasing human exposure to indoor aerosols.