Abstract
The proportion of ice crystals in clouds can affect cloud albedo and lifetime, impacting the Earth's radiative budget. Ice nucleating particles (INPs) lower the energy barrier of ice nucleation and thus facilitate primary ice formation in the atmosphere. Atmospheric nanoplastics (NPs) have been detected in remote regions far from emission sources, suggesting that they can become airborne and undergo long-range transport in the atmosphere. During the atmospheric residence of NPs, they could catalyse primary ice crystal formation by acting as INPs. In this study, we present results from laboratory experiments in which model NPs composed of polystyrene and polyacrylonitrile were tested for their ice-nucleating ability using the horizontal ice nucleation chamber (HINC) as a function of ice-nucleation temperature and water saturation ratio. The results showed that NPs can be effective INPs under both cirrus and cold mixed-phase cloud conditions. The surface characteristics and wettability of the NPs were analysed via scanning electron images and dynamic vapour sorption measurements, which revealed the freezing mechanism as a combination of deposition nucleation and pore condensation and freezing. The results highlight the need to enumerate and characterise NPs in the atmosphere, given their potential to get scavenged by clouds via primary ice formation in clouds.