McErlich et al. (2025) Description and evaluation of airborne microplastics in the United Kingdom Earth System Model (UKESM1.1) using GLOMAP-mode

Identification

• Journal: Geoscientific model development
• Year: 2025
• Date: 2025-11-21
• Authors: Cameron McErlich, Felix Goddard, Alex Aves, Catherine Hardacre, Nikolaos Evangeliou, Alan J. Hewitt, Laura E. Revell
• DOI: 10.5194/gmd-18-8827-2025

Research Groups

• School of Physical and Chemical Sciences, University of Canterbury, Christchurch, New Zealand
• Department of Atmospheric and Climate Research (ATMOS), Stiftelsen NILU (former Norwegian Institute for Air Research), Kjeller, Norway
• Met Office, Exeter, Devon, United Kingdom

Short Summary

This study describes the integration and evaluation of airborne microplastics into the United Kingdom Earth System Model (UKESM1.1) using its GLOMAP aerosol scheme. The model simulates global transport and deposition, revealing that smaller, hydrophilic microplastics have longer atmospheric lifetimes and can reach remote regions and the lower stratosphere, though their current contribution to total aerosol burden is minor.

Objective

• To incorporate airborne microplastics as a new aerosol species into the United Kingdom Earth System Model (UKESM1.1) using the Global Model of Aerosol Processes (GLOMAP).
• To simulate the global airborne microplastic loading, transport, and deposition to marine and terrestrial environments.
• To evaluate the model's performance against existing observational data of atmospheric microplastic concentrations and deposition fluxes.

Study Configuration

• Spatial Scale: Global, with a grid resolution of 1.25° latitude × 1.85° longitude, and 85 atmospheric levels extending to 85 km above the surface.
• Temporal Scale: 10-year simulation period from January 2005 to December 2014, following a 12-month spin-up. Microplastic emissions for 2018 were repeated annually.

Methodology and Data

• Models used:
  • United Kingdom Earth System Model (UKESM1.1), specifically the atmosphere-only configuration (UKESM1.1-AMIP).
  • Global Atmosphere 7.1 (GA7.1) science configuration of the Unified Model (UM) for the physical atmosphere.
  • United Kingdom Chemistry and Aerosols (UKCA) model for atmospheric composition, chemistry, and aerosols.
  • Global Model of Aerosol Processes (GLOMAP) for aerosol microphysics, extended to include microplastics as fragments and fibres across eight log-normal size modes (excluding nucleation mode).
  • Lagrangian particle dispersion model FLEXPART, version 11, used for developing the microplastic emissions inventory.
• Data sources:
  • Microplastic emissions inventory: Updated observationally-derived inventory from Evangeliou et al. (2022), based on airborne microplastic deposition measurements from Brahney et al. (2020) in the Western USA (2017-2019). Extrapolated for smaller sizes using a power law distribution (Leusch et al., 2023).
  • Sea-ice extent for emissions: Fifth generation ECMWF atmospheric reanalysis (Hersbach et al., 2020).
  • Observational data for model evaluation: Collated from a Scopus search for airborne microplastics, supplemented by specific studies (Brahney et al., 2020; Kau et al., 2024; Materić et al., 2021).
  • Ancillary data: Greenhouse gas emissions for the historical period (Mulcahy et al., 2023).

Main Results

• Microplastics are successfully implemented into UKESM1.1, represented as fragments and fibres in various size modes, interacting with existing aerosol processes like ageing and deposition.
• Simulated microplastic concentrations are higher over land but show global transport, reaching remote regions such as Antarctica despite no local emissions.
• Atmospheric lifetimes range from approximately 1 hour to 17 days, with smaller, hydrophilic microplastics exhibiting longer lifetimes.
• Microplastics are present throughout the troposphere, and the smallest particles are simulated to reach the lower stratosphere in small numbers.
• Dry deposition is the dominant removal pathway for microplastics (average 61% of total removal). Wet deposition is more significant for smaller, hydrophilic microplastics due to cloud interactions (e.g., 52% loss via nucleation scavenging in the accumulation soluble mode).
• The total global atmospheric microplastic burden is estimated at 1.36 × 10^4 tonnes.
• Current microplastic emissions (73.4 Tg/year) and deposition fluxes (73.26 Tg/year) are higher than previous models, primarily due to the inclusion of larger (100–250 µm) particles; excluding these, emissions are 16 Tg/year.
• Microplastics currently constitute a minor fraction (0.02%) of the total atmospheric aerosol number concentration.
• Model-observation comparisons show general disagreement, with the model often simulating higher concentrations than observed (correlation coefficients r=0.32 for active sampling, r=0.36 for deposition), attributed to uncertainties in emissions, physical processes, and observational limitations.

Contributions

• This study represents the first integration of atmospheric microplastics as an aerosol species into a comprehensive Earth System Model (UKESM1.1), providing a novel framework for global simulations.
• It quantifies the current atmospheric burden, transport, and deposition pathways of microplastics, including their vertical distribution and atmospheric lifetimes.
• The work establishes a foundation for future research to assess the long-term impacts of microplastics on air quality, climate, and human exposure under various emission scenarios.
• It highlights critical uncertainties in microplastic emissions and atmospheric processes, emphasizing the need for standardized observational data to improve model accuracy.

Funding

• Royal Society Te Apārangi, Marsden Fund (grant no. 22-UOC-088)
• Norges Forskningsråd (Airborne Microplastic Detection, Origin, Transport and Global Radiative Impact, Project No. 334086)
• Joint BEIS/Defra Met Office Hadley Centre Climate Programme (GA01101)

Citation

@article{McErlich2025Description,
  author = {McErlich, Cameron and Goddard, Felix and Aves, Alex and Hardacre, Catherine and Evangeliou, Nikolaos and Hewitt, Alan J. and Revell, Laura E.},
  title = {Description and evaluation of airborne microplastics in the United Kingdom Earth System Model (UKESM1.1) using GLOMAP-mode},
  journal = {Geoscientific model development},
  year = {2025},
  doi = {10.5194/gmd-18-8827-2025},
  url = {https://doi.org/10.5194/gmd-18-8827-2025}
}