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Adsorption-Driven Cloud Droplet Activation of Fresh and Aged Polypropylene Particles
Summary
Scientists found that tiny plastic particles in the air can help form clouds and potentially affect weather patterns. When these plastic particles get weathered by sunlight and air pollution, they become much better at attracting water droplets to form clouds compared to fresh plastic particles. This matters because microplastics are now everywhere in our atmosphere, and understanding how they change weather and climate could help us better predict environmental changes that affect human health and agriculture.
Micro- and nanoplastics (MNPs) have been detected in atmospheric deposition and cloud samples, suggesting that they may act as cloud condensation nuclei (CCN) and/or ice-nucleating particles (INPs), which is likely the main pathway through which atmospheric microplastics impact climate. Laboratory studies report immersion freezing temperatures for various microplastics between −15.1 °C (Ganguly 2019) and −23.2 °C (Seifried 2024), comparable to those of mineral dust, the dominant global source of atmospheric INPs. Immersion freezing IN activity also requires CCN activity as immersion freezing occurs in existing cloud droplets. Atmospheric aging may further enhance the CCN or IN activity of MNPs, consistent with laboratory evidence and observations showing that microplastics in cloud samples are predominantly aged, as indicated by increased hydroxyl functionalization identified by FTIR analysis (Wang 2023). Despite growing evidence for their potential role in cloud microphysics, no parameterization currently exists to represent the CCN or IN activity of microplastics in cloud models, largely due to limited understanding of the underlying activation mechanisms. Here, we use hybrid Grand Canonical Monte Carlo–molecular dynamics simulations to investigate cloud droplet growth and activation on fresh and atmospherically aged crystalline polypropylene (PP) surfaces. Our results show that aged PP activates as CCN via an adsorption-driven mechanism, whereas fresh, non-oxidized PP does not activate under the simulated conditions. Activation on aged surfaces proceeds through (1) dropletwise adsorption of water nanoclusters at active sites, (2) cluster growth, and (3) coalescence into a continuous multilayer of water. Model calculations based on adsorption nucleation theory (Laaksonen 2015) indicate that activation occurs at slightly higher critical supersaturations than for mineral dust, while the critical radius is smaller than for illite, Saharan dust, or Arizona Test Dust. These findings provide mechanistic insight into CCN activation on aged microplastics and the model calculation provides a first approach to develop parameterizations of microplastics for cloud microphysics schemes. Ganguly, M.; Ariya, P.A. ACS Earth and Space Chemistry 2019, 3, 1729–1739.Seifried, T.M.; Nikkho, S.; Morales Murillo, A.; Andrew, L.J.; Grant, E.R.; Bertram, A.K. Environmental Science & Technology 2024, 58, 15711–15721.Wang, Y.; Okochi, H.; Tani, Y.; Hayami, H.; Minami, Y.; Katsumi, N.; Takeuchi, M.; Sorimachi, A.; Fujii, Y.; Kajino, M.; et al. Environmental Chemistry Letters 2023, pp. 1–8.Laaksonen, A. The Journal of Physical Chemistry A 2015, 119, 3736–374.