Atmospheric microplastic measurements reconciliation with emission estimates: A Lagrangian approach

Microplastics (MPs), defined as particles measuring between 1 μm and 5 mm, originate from the fragmentation of larger plastic materials or are produced intentionally. These particles have attracted considerable attention in aquatic, terrestrial, and marine environments. Despite the pivotal role played by the atmosphere in the global transport and dispersion of MPs, there is a paucity of data concerning their atmospheric abundance and deposition. Additionally, the existing measurements exhibit substantial variability, and global estimates of total MP emissions tend to reflect significant inconsistencies based on the estimation method.

This study seeks to address these gaps by reconciling current measurement data with estimated emission fluxes through a detailed analysis of source-receptor relationships that link emissions, atmospheric concentrations, and deposition measurements. To establish these source-receptor relationships, we employ the Lagrangian particle dispersion model FLEXPART, considering various shapes and sizes of microplastics. Back trajectory analysis is utilized to elucidate the sources of MPs in distinct geographical regions. Three MP emission inventories, estimated with different methods, are combined with the back trajectory output to yield the simulated values at the measurement locations.

For the geographical region of Europe, around 50% of the simulated MP concentration values agree with the measured values within a factor of 10, while only 5% of the simulated deposition is in a 10-factor range of the deposition data. A clear overprediction of the modeled values indicates that the available emissions may be lower, that the transport and scavenging scheme of the model should be reconsidered, or that the measuring and identification methodologies are coming with substantial errors. We emphasize that a uniform sampling protocol should be created to achieve reliable and comparable data, and more efforts should be made to create bottom-up emission inventories based on relevant emission factors and proxies.