We use a high-resolution cosmological dark matter-only simulation to study the orbital trajectories of haloes and subhaloes in the environs of isolated hosts. We carefully tally all apsis points and use them to distinguish haloes that are infalling for the first time from those that occupy more evolved orbits. We find that roughly 21 per cent of resolved subhaloes within a host’s virial radius are currently on first infall, and have not yet reached their first orbital pericentre; roughly 44 per cent are still approaching their first apocentre after infall. For the range of host masses studied, roughly half of all accreted systems were pre-processed prior to infall, and about 20 per cent were accreted in groups. We confirm that the entire population of accreted subhaloes – often referred to as ‘associated’ subhaloes – extends far beyond the virial radii of their hosts, with roughly half currently residing at distances that exceed ≈1.2 × r200. Many of these backsplash haloes have gained orbital energy since infall, and occupy extreme orbits that carry them well past their initial turnaround radii. Such extreme orbits are created during the initial accretion and dissolution of loosely bound groups, but also through penetrating encounters between subhaloes on subsequent orbits. The same processes may also give rise to unexpectedly abrupt losses of orbital energy. These effects combine, giving rise to a large variation in the ratio of sequent apocentres for accreted systems. We find that, within two virial radii from host centres, the concentrations of first-infall haloes are remarkably similar to those of isolated field haloes, whereas backsplash haloes, as well as systems that were pre-processed, are considerably more concentrated.
<p>Investigation of the transport and distribution of atmospheric concentrations of microplastic (MP) particles is an important challenge, since MP may have a negative impact on human health and ecosystems. When considering particle shape, most of the atmospheric transport models assume only spherical particles, whereas MP particles cover a wide range of observed shapes. Non-spherical particles experience a larger drag in the atmosphere, which leads to a reduction of their settling velocity, hence longer atmospheric residence times. Here we study gravitational settling of one of the dominant microplastic shapes &#8211; fibers. To reduce the difference between model output and ground-based measurements, we have implemented a parameterization of the shape correction in the gravitational settling scheme of the Lagrangian transport model FLEXPART.</p><p>We have determined model sensitivity to the shape correction to explore its impact on particles transport for a range of scenarios.&#160; This was done with a statistical comparison of 3D fields of mass concentration and deposition patterns of shape-corrected and non-corrected parameterization schemes. Using the model output, we quantified average horizontal transport distances and atmospheric residence times for spheres and fibers of different sizes and aspect ratios in different climatic regions and for different release heights of the MP particles.</p>
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