Laboratory Investigation of Nanoplastic Mixing States with Water-Soluble Coatings using Single-Particle Mass Spectrometry

Nanoplastic particles (NPPs) are increasingly recognized as an emerging class of atmospheric aerosol. However, their mixing state with inorganic and organic aerosol components remains poorly constrained, limiting our understanding of their atmospheric lifecycle and environmental fate. Conventional bulk aerosol measurements often obscure particle-to-particle chemical heterogeneity, complicating predictions of NPP transport, deposition, and cloud-relevant properties.

Here, we present an online mass spectrometric approach using an Aerosol Mass Spectrometer (AMS) to resolve the mixing state of NPPs in real time by combining event-trigger single-particle (ETSP) measurements with complementary bulk analysis. This approach extends recent AMS-based efforts for real-time NPP detection from bulk tracers to particle-resolved mixing-state constraints. Controlled laboratory experiments were conducted to simulate atmospheric mixing, in which polystyrene-NPP suspensions were atomized and mixed with representative inorganic and organic constituents, including ammonium nitrate, ammonium sulfate, sodium chloride, and succinic acid. Chemically resolved single-particle mass spectra were analyzed using unsupervised k-means clustering to separate externally mixed particle populations from internally mixed NPP–coating systems.

We identified distinct particle classes characterized by the co-occurrence of polymer fragments (e.g., styrene-related ions) with coating-specific ions (e.g., nitrate markers), enabling the direct differentiation of coated versus uncoated NPPs at the single-particle level. The derived mixing-state index (χ) varied systematically across coating types (ranging from 10% to 40%), indicating a transition from external to partial internal mixing under controlled conditions. Coated NPPs further exhibited distinct vaporization kinetics and temporal ion profiles relative to bare particles, reflecting particle-level interactions between polymer cores and inorganic or organic coatings and providing independent evidence for internal mixing that is not discernible from bulk-averaged spectra.

These results illustrate how the AMS can be effectively leveraged to quantitatively constrain the mixing state of NPPs. This work provides a methodological foundation for identifying polymeric particles within complex atmospheric aerosol matrices and for improving the representation of NPPs in atmospheric transport and lifecycle models, where mixing state is a key but largely unconstrained parameter.