Characterizing the Aging Rate Constant and Half-life Time of the Atmospheric Polystyrene (PS) Microplastics Particles (MPP)

SAHIR GAGAN, Ruizhe Liu, Sining Niu, Zhenli Lai, Andrew Lambe, Yuzhi Chen, Xingmao Ma, Yue Zhang, Texas A&M University

     Abstract Number: 362
     Working Group: Aerosol Chemistry

Abstract
Microplastics are an emerging class of anthropogenic pollutants that have been found in urban, suburban, and even remote areas. However, much is unknown about the life cycle of atmospheric microplastic particles (MPP), especially with regard to chemical reactivity towards atmospheric oxidants. Characterizing the lifetime of MPP may contribute towards improved understanding of their environmental persistence, health implications, and potential contribution to climate forcing. In this regard, this study provides the first measurement of the oxidation and photolysis lifetimes of polystyrene(PS), a major component of MPP, against hydroxyl radicals(·OH), ozone, and UV photons.

Atomized PS particles were introduced to a Potential Aersosol Mass (PAM) reactor in the presence of ·OH, ozone, and UVC radiation (λ=254nm). A high resolution time-of-flight aerosol mass spectrometer(HR-ToF-AMS) and a scanning electric mobility sizer(SEMS, Brechtel Inc.) were used to quantify the mass concentrations and surface areas of PS particles at different stages of aging. Decay of m/z78 and m/z104, tracer ion for PS was monitored from HR-ToF-AMS to understand the aging of PS. The degree of aging of PS particles by ozonolysis, photolysis, and ·OH oxidation were measured by varying their respective concentrations inside the PAM.

The second order rate constant of PS particle against ·OH under pseudo first order rate assumption, k_PS, is calculated to be 1.4×10^-13cm³molecule^-1s^-1 while the ozonolysis and photolysis rates were negligible. Our results demonstrate that the main degradation pathway for MPP in the atmosphere is likely ·OH oxidation. Assuming an ambient ·OH concentration of 1×10⁶cm^-3, the half-lifetime of microplastic in the atmosphere is estimated to be 84 days.

This lifetime could be enough to transport MPP in upper atmosphere, resulting in long-range transportation. Such parameter also needs to be consider while developing model which simulate long-term transportation processes.