Comparing apples and oranges: Using the MPsizeBase and power law size distribution to extrapolate and inter-compare microplastic concentrations

Studies reporting environmental MP concentration rarely cover the full MP size range of 1 to 5000 µm due to sampling and analytical limitations. However, microplastic (MP) number concentration in the environment increases exponentially with decreasing particle size. This leads to difficulties in the intercomparison of studies, which is critical for environmental and human health risk assessment. Indeed, for the same MP sample, a study observing the small MP fraction (1-300 µm for ex.) will report a higher number concentration than another study observing the large MP fraction (300-5000 µm) of the same sample.

In this presentation, we summarize the current understanding of the MP particle size distribution (PSD), based on the power law model (Segur et al., 2025). We confront the power law model with 90 published MP PSD observations from the literature, compiled in the new MPsizeBase open access database (Sonke et al., 2025). We show that the MP PSD power law slope is influenced by particle shape (fragments, fibers), but does not vary significantly between environmental compartment studied (surface ocean, deep ocean and atmosphere).

We propose simple equations to extrapolate MP concentrations for the limited observed size range to the full MP size range (1 to 5000 µm), or any other sub-size range, for both MP number and mass concentrations. By comparting the observed MP concentrations to the corrected full size range MP concentration, we show that the 90 published studies underestimated MP number concentrations.

The MP number PSD is dominated by small fragments: in the surface ocean, we estimate that 70% of MP particles have a diameter between 1 and 2 µm. Conversely, we also show that the MP mass PSD is dominated by large particles and estimate that, for surface ocean MP, common plankton nets (mesh size 300 - 330 µm) only catch 0.003% of all MP particles (in number), but 94% of MP mass. This indicates the need to express results both in term of numeric and mass concentration. To do so, we provide simple equations to convert a numeric PSD to mass PSD.  

References

Segur, T., Hough, I., Dobiasova, N., Voisin, D., Richon, C., Angot, H., Thomas, J. L., and Sonke, J. E.: Using the power law size distribution to extrapolate and compare microplastic number and mass concentrations in environmental media, Research Square preprint, https://www.researchsquare.com/article/rs-8524083/v1, 2025.

Sonke, J. E., Segur, T., Hough, I., Dobiasova, N., Voisin, D., Yakovenko, N., Margenat, H., Hagelskjaer, O., Abbasi, S., Bucci, S., Richon, C., Angot, H., Thomas, J. L., and Le Roux, G.: MPsizeBase: a database for particle size distributed environmental microplastic data, EarthArXiv, preprint, https://eartharxiv.org/repository/view/10605/, 2025.