The PM2.5-Bound Polycyclic Aromatic Hydrocarbon Behavior in Indoor and Outdoor Environments, Part III: Role of Environmental Settings in Elevating Indoor Concentrations of Benzo(a)pyrene

This study aims to investigate the impact of indoor sources and outdoor air on indoor PM2.5-bound benzo(a)pyrene, with a focus on identifying emission sources and understanding the influence of environmental variables. For this purpose, we collected indoor and outdoor data on PM2.5-bound PAHs, inorganic gaseous pollutants, trace metals, ions, radon, and meteorological parameters, resulting in a comprehensive dataset of 100 variables from an urban site in Belgrade, Serbia. We applied seven regression tree ensemble algorithms to interrelate the variables alongside six metaheuristic optimization algorithms to refine model accuracy and robustness. Subsequently, we explained the best-performing model locally using Shapley additive explanations and clustered variables with similar impacts into distinct groups. These groups were systematically characterized, defining them as environmental settings that shape benzo(a)pyrene dynamics. The setting resulting in the highest indoor benzo(a)pyrene concentrations (197% to 297% relative to the expected value) was dominated by outdoor emissions associated with residential heating and traffic (up to 140%) and indoor source identified as cooking. This integrated approach uniquely enables a quantitative assessment of the contributions from both indoor and outdoor emission sources to pollutant concentrations in indoor spaces, underscoring the importance of both in shaping indoor air quality. Unlike traditional source apportionment methods that assume linear source mixing, our approach integrates nonlinear interactions and contextual variables, such as meteorological conditions and outdoor pollutants, to better capture indoor air quality dynamics. The results also highlight the need for further studies to explore broader contextual factors and refine source attribution in complex urban settings.