Global climate model development: Adding microplastics to the UK Earth System Model

Airborne microplastics, an emerging class of anthropogenic aerosols, are small and lightweight, allowing them to remain suspended in the atmosphere for extended periods of time. Their detection in remote locations (such as Antarctica) and in high-altitude cloud water raises questions about their potential impacts on climate systems. As global climate models do not routinely include airborne microplastics as an aerosol species, the full consequences of microplastics on climate remain uncertain. To investigate these impacts, we have incorporated micro- and nanoplastics (MnP) as a new aerosol species within GLOMAP-mode, the aerosol scheme used in the United Kingdom Chemistry & Aerosols (UKCA) component model of the UK Earth System Model (UKESM). MnP have been implemented in GLOMAP-mode alongside the existing aerosol species of sulfate, black carbon, organic carbon, sea salt and dust.  Microplastics can be emitted into UKESM as both fragments and fibres. MnP are added to the Aitken (5 – 50 nm), accumulation (50 - 500 nm), coarse (> 500 nm) and super-coarse (> 2500 nm) modes. Emissions are sourced from an observationally-derived dataset with global spatial coverage. MnP are initially emitted in insoluble modes but can transition to soluble modes through the accumulation of organic material on their surfaces, enabling them to act as CCN. Airborne microplastics in UKSEM transported and can be removed from the atmosphere by both dry and wet deposition processes. We present preliminary results using the new microplastics scheme, which has been run for both microplastics fragments and fibres. Our novel microplastic scheme is a significant advancement in modelling airborne microplastics and lays the groundwork for understanding their impact on climate and the wider Earth system.