OH-initiated Heterogeneous Oxidation of Nanoparticles from Plastic Combustion

LIN KONG, Hongru Shen, Alex K.Y. Lee, Arthur W. H. Chan, Man Nin Chan, The Chinese University of Hong Kong

     Abstract Number: 288
     Working Group: Chemicals of Emerging Concern in Aerosol: Sources, Transformations, and Impacts

Abstract
The widespread use of plastics and their subsequent incineration treatment significantly contribute to environmental pollution, particularly in the form of atmospheric nanoplastic particles (NPP) that can have deleterious effects on human health and ecosystems. Additionally, with the expected increase in intensity and frequency of wildfire activities, NPP emissions from combustion sources are expected to increase. After emitting, NPP can undergo heterogeneous reactions, potentially forming secondary products and changing their physical and chemical properties. However, understanding of the heterogeneous transformation of these NPP remains limited. In this study, we investigated the heterogeneous reactions initiated by OH radicals of NPP from plastic combustion, including polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), and polyethylene terephthalate (PET) in a flow reactor. The chemical composition and size of the NPP were characterized using a scanning mobility particle sizer (SMPS) and a time-of-flight aerosol mass spectrometer (ToF-AMS). Distinct changes of NPP were observed in the AMS mass spectra upon oxidation, for example, the decreased C_xH_y⁺ and increased C_xH_yO_z⁺ signals, and a rise in the O/C ratio and the OS_C at a maximum OH exposure of 2.2 × 10¹¹ molecules cm⁻³ s. We propose that a hydroxyl or a carbonyl functional group was likely added to each repeating unit of NPP during heterogeneous reaction. Furthermore, we employed Pyrolysis Gas Chromatography-Mass Spectrometry (Py-GC-MS) to identify the polymeric backbone alterations and the formation of oxidative compounds due to oxidation. The study also examined the impact of particle size on oxidation kinetics and demonstrated that smaller NPP exhibit higher oxidation rates due to their increased surface area-to-volume ratio. The findings of this study help us understand the atmospheric fate of NPP through heterogeneous oxidation. Our results highlight the evolution of NPP composition in the atmosphere, which should be considered in air quality modeling and health risk assessments.