INTO THE MED: Searching for Microplastics from Space to Deep-Sea

Empirical Lagrangian parametrization for wind-driven mixing of buoyant particles at the ocean surface

“…Using these parametrizations we calculate the vertical equilibrium profiles of buoyant particles within the ML as a function of the particle rise velocities, the 10 m wind speed and the MLD. Buoyant particles are found below the ML (Pieper et al, 2019;Choy et al, 2019;Egger et al, 2020), but diffusive mixing at such depths is likely not due to wind-driven turbulent mixing and therefore goes beyond the scope of this study. We test two methods for solving stochastic differential equations and consider vertical diffusion coefficient profiles based on the K-profile parameterization (KPP) model (Large et al, 1994) and Kukulka et al (2012), which was extended by Poulain (2020).…”

“…For all stations the wind speed was recorded and the MLD was determined from conductivity-temperature-depth (CTD) data based on a temperature threshold (de Boyer Montégut et al, 2004). The majority of the samples were collected in the North Atlantic (Kukulka et al, 2012;Kooi et al, 2016b;Pieper et al, 2019) and in regions with a relatively shallow MLD. Since wind-driven turbulent mixing is not expected to influence the concentration depth profile below the MLD, we do not consider any measurements collected below 73 m. Measurements were collected with surface wind speeds up to 10.7 m s −1 , with the majority of sampled concentrations being collected for u 10 = 3.4-7.9 m s −1 (535 of the 741 data points).…”

“…In contrast, low-buoyancy particulates (w rise = 0.0003 m s −1 ) are typically not sampled in neuston nets (Kooi et al, 2016b), likely in part due to smaller particulate sizes. Pieper et al (2019) filtered samples collected via Niskin bottles with a 0.8 µm filter and thus was able to filter out smaller particulates with lower rise velocities.…”

“…where C f,i and C m,i are the binned normalized field measurement and modeled concentration within depth bin i. Model evaluation for the low-buoyancy particles is not possible with the available field measurements as low-buoyancy particles are typically too small to be sampled with neuston nets, and the Pieper et al (2019) dataset alone is too small.…”

“…6084/m9.figshare.3427862.v1, Kooi et al, 2016a). For the field data from Kukulka et al (2012), Pieper et al (2019), Egger et al (2020) and Amaral-Zettler (unpublished data), please contact the corresponding authors of the respective studies.…”