<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>
<p>Microplastic particles (MP), i.e., plastic particles with a size between 1 &#181;m and 1 mm, have been detected in all compartments of the Earth system. While we are beginning to develop a quantitative understanding of the primary emissions of MPs such as tire wear, secondary sources from polluted environmental compartments such as the oceans and arid land surfaces, are currently not understood at all.</p> <p>In this study, we use reported MP concentrations in soils across the world combined with MP enrichment ratios (ER) in wind eroded sediments with respect to the soils (Bullard et al., 2021; Rezaei et al., 2019; Tian et al., 2022) and a population density map to estimate MP resuspension factors (RF) from arid regions.&#160; We then use global, 3-hourly dust emissions at&#160; 0.5<sup>o</sup> x 0.5<sup>o</sup> resolution from FLEXDUST (Groot Zwaaftink et al., 2016, 2017), as a proxy for the spatial and temporal variation of MPs emitted by arid regions. Scaled with the resuspension factors, we estimate the global MP resuspensions. To calculate the uncertainty of our emission model, we conduct a one-thousand-member Monte Carlo simulation with 14 different RF scenarios for each population category, perturbing the MP concentration in soils, the ER and the spatial scale used for the population density.</p> <p>We define, as a reference case, the emissions derived from the average ER, average soil concentrations and 50 km radius population categorization. These MP resuspension emissions are used as input to the Lagrangian atmospheric dispersion model FLEXPART (Pisso et al., 2019; Stohl et al., 2005) to simulate the global atmospheric cycle of resuspended MPs from arid regions. The simulations are driven by ERA5 meteorological fields at 0.5&#176; horizontal resolution and 1-hour temporal resolution. &#160;We simulate the global atmospheric concentration and the deposition of resuspended MPs for different size distributions as well as shapes (spheres, fibers) of MPs. Lastly, we quantify the impact and compare it with observations, to estimate the importance of resuspension from arid regions for global MP abundance.</p>
<p>Gravitational settling is a crucial parameter to study the transport and distribution of atmospheric concentrations, sources, and sinks of particles. Although the settling velocity is highly dependent on the particle shape, most atmospheric transport models assume particles to be spherical, ignoring other geometries. In this study, we focus on the gravitational settling of microplastics (MP) particles, which often deviate strongly from sphericity. For instance, MP fibers can be approximated more closely by cylinders rather than spheres.&#160;</p> <p>Here, we present the results of conducted experiments with extremely elongated MP particles to define their settling velocity. This was done with the settling column and 3D-printed MP particles of different shapes (straight cylinders, half-circled cylinders, and quarter-circled cylinders), lengths, and aspect ratios. The experimental data shows that the parameterization scheme for shape correction proposed by Bagheri and Bonadonna, 2016 is a reliable tool to predict the gravitational settling of fibers considering different types of particle orientation (random, horizontal, and average of both).</p> <p>This scheme was implemented in the gravitational settling scheme of the Lagrangian transport model FLEXPART to eliminate uncertainties regarding the shape of a particle when simulating solid particle transport. As a study case, the mass concentration and deposition 3D fields of MP fibers were estimated according to the global population density to understand the contribution of the individual sites/regions to MP contamination of the atmosphere, land, and World Ocean.</p>
<p>&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; The safe operation of aviation and shipping, particularly in areas of insufficient coverage of automatic meteorological stations in the Arctic requires accurate interpretation of satellite images. Operational detection of fog and low stratus clouds and recognizing of them on the background of snow and ice cover and cloudiness of the upper layer is very important challenge.&#160;</p><p>&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; &#160;The verified images obtained by Aqua and Terra satellites with a scanning radiometer MODIS, which operates in 36 spectral bands, with wavelengths from 0.4 &#181;m to 14.4 &#181;m, were collected.&#160; With the Beam VISAT 5.0 software, which was designed to work with satellite data in raster format, thematic digital techniques of satellite multispectral information, based on difference in the values of the integral brightness of the images, both in optical and far-infrared ranges of the spectrum, have been developed. &#160;These techniques, models of additive color synthesis, improve the quality of interpretation of fogs and low stratus clouds in terms of the complex structure of cloudiness and underlying surface in polar regions. Developed RGB combinations, which are based on the selected MODIS bands are:</p><ol><li>RGB (1.6 &#181;m; 0.8 &#181;m; 0.6 &#181;m)</li> <li>RGB (0.8 &#181;m; 3.9-8.7 &#181;m; 10.8 &#181;m)</li> <li>RGB (0.8 &#181;m; 1.6 &#181;m; 3.9-8.7 &#181;m)</li> <li>RGB ((0-12)-(0-11) &#181;m, (0-11)-(0-3.8) &#181;m, (0-11) &#181;m)</li> </ol><p>&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; Analysis of the obtained images has shown that the developed models of color synthesis help to distinguish the fog/low stratus clouds under different conditions of cloudiness and underlying surface accurately.</p><p>Key words: remote sensing, satellite imagery, additive color synthesis, fog, low stratus clouds, polar regions</p>
<p>The Lagrangian atmospheric transport model, FLEXPART, is used to investigate a broad spectrum of topics within the atmospheric sciences, from the propagation of particles emitted during nuclear accidents to global moisture transport. Since its inception in 1998, FLEXPART has undergone many changes, with its last official release (version 10.4) published in 2019. At the same time, numerous versions have been developed across institutes to cater to various needs. To make it easier to modify FLEXPART, while not having to diverge from the main version and its updates, we introduce a more modular way of organising the source code. Running times are improved by consistent OpenMP parallelisation in all parts of the code, resulting in reasonable scaling behaviour. Alongside the restructuring of FLEXPART, other improvements have been made. For instance, the interpolation errors have been reduced by replacing the traditional FLEXPART-internal terrain-following coordinate system with the option of doing all calculations on the native ECMWF ETA coordinate systems. Accuracy improvements are being investigated by quantifying the conservation quantities of dynamical tracers.</p>
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