Chemical Fates of Oils Deposited on Indoor Surfaces

Abstract

The COVID-19 pandemic has substantially increased the public awareness of indoor environments. Despite the global effort in reducing primary indoor air pollutants, especially emissions related to solid fuel combustion, there remains insufficient monitoring and understanding of secondary pollutant generation via indoor chemical reactions. In particular, the multiphase oxidation that occurs between airborne species and reactive material on indoor surfaces is an important contributor that can also impact occupant health and wellbeing. Transported from outside, ozone is the most important indoor oxidant. It rapidly reacts with unsaturated lipids that are commonly found on indoor surfaces contaminated by cooking oils and human skin lipids. However, such reactions have not been comprehensively characterized, especially in complex indoor environments. Here, I initially studied the heterogeneous ozonolysis of a pure representative lipid (triolein) in a controlled reactor. With mass spectral and quantitative NMR methods, our results indicate that triolein decays rapidly with ozone exposure, with stable secondary ozonides (SOZs) the major condensed-phase products. Known as the Criegee mechanism, the reaction products are strongly dependent on ambient relative humidity. Specifically, the SOZ molar yield peaks at ~80% under dry conditions, regardless of the ozone mixing ratio, whereas water vapor significantly reduces its formation and favors the release of volatile organic compounds (VOCs). This is due to water scavenging the Criegee Intermediates, and the resulting α-hydroxyhydroperoxides (α-HHPs) decompose into aldehydes and reactive H2O2. A kinetic multilayer model (KM-GAP) which implements this set of chemical reactions accurately simulates the yields of major products under indoor relevant conditions. After studying fundamental mechanisms, I characterized the chemical fate of commercial cooking oil on genuine indoor surfaces. While SOZs are the major products when oils are exposed to air, low-ozone dark locations lead to the slow formation of hydroperoxides, which cannot be explained by the Criegee mechanism. Additionally, indoor direct sunlight drives lipid peroxidation whose products are potential toxins. Overall, my indoor sampling studies indicate that autoxidation and/or photooxidation mechanisms of unsaturated oils also play an important role in indoor oxidative surface chemistry, depending on their rates relative to ozonolysis.