A reduced-parameter physics-based transient model for skin oil ozone chemistry

Human-surface ozonation is an important indoor ozone sink and a source of skin-oil oxidation products. Detailed mechanistic models can resolve skin-phase chemistry and transport but generally require poorly constrained parameters and assumptions, whereas simplified steady-state formulations have limited capability to represent transient storage and release processes. To bridge this gap, a reduced-parameter transient model was developed by coupling human-surface ozone uptake, skin-oil storage, skin-oil/air interfacial mass transfer, gas-phase secondary chemistry, empty-chamber background contributions, and ventilation within a unified mass-balance framework. The apparent human-surface ozone reaction probability inferred from 22 occupied-chamber cases involving three subjects had a mean value of (5.58 ± 0.84) × 10^-5. Product related parameters were fitted using continuous transient concentration measurements of nine typical skin oil oxidation products from full time series observations. The model reproduced the observed temporal trends, including the rapid accumulation and subsequent stabilization of 6-MHO and acetone and the delayed increase of 4-OPA. Sensitivity analysis showed that all examined parameter sensitivity indices under ±20% perturbations were below 4%, with the highest sensitivities associated with the apparent human-surface ozone reaction probability (3.50%), the apparent generation coefficient of GA (3.45%) and 6-MHO (2.55%). The proposed framework provides a practical approach for representing transient indoor chemistry and human-surface ozone reactions.