## 2 Materials and Methods
### Air Masses Trajectories Analyses  
The 7-day back-trajectories were analyzed in order to identify dust transfer for the dates with high nutrients concentrations. Some of these dates are analyzed in [30; 35]. We used the model of the international network AERONET (Aerosol Robotic Network) and the HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory) model. The air flow trajectory is a set of points in space, each of them is determined by the coordinates of the Lagrangian particle at a moment in time. Trajectories of air movement can be forward and reverse depending on the method of counting the time along the trajectory. A forward trajectory shows the path that an air particle makes as it moves away from the start point. A negative time step in the AERONET model is used to determine the back-trajectory, i.e., the trajectory characterizes the path that the air particle moved before it reached the monitoring point. Back-trajectories corresponding to the position of the air masses at 0 and 12 h GMT are generated twice a day. The reliability of the atmosphere dynamics data confirmed by a number of papers in which the accuracy of model calculations of forward and backward trajectories of air particles is estimated [36; 37; 38; 39; 40]. Such estimates are performed based on comparing the simulation results with the real trajectories obtained as a result of tracking the movement of special probes or markers. The markers can be various impurities released during the experiment (inert gases) or entering the atmosphere as a result of natural (pollen, volcanic ash) and man-made phenomena (radioactive substances), as well as physical quantities (potential temperature, specific humidity, potential vortex), which persist along the trajectories of the air movement [41].  
The HYSPLIT model is a complete system for computing simple air parcel trajectories, as well as complex transport, dispersion, chemical transformation, and deposition simulations. HYSPLIT continues to be one of the most extensively used atmospheric transport and dispersion models in the atmospheric sciences community. A common application is a back trajectory analysis to determine the origin of air masses and establish source-receptor relationships. HYSPLIT has also been used in a variety of simulations describing the atmospheric transport, dispersion, and deposition of pollutants and hazardous materials. The model calculation method is a hybrid between the Lagrangian approach, using a moving frame of reference for the advection and diffusion calculations as the trajectories or air parcels move from their initial location, and the Eulerian methodology, which uses a fixed three-dimensional grid as a frame of reference to compute pollutant air concentrations (The model name, no longer meant as an acronym, originally reflected this hybrid computational approach). HYSPLIT has evolved over more than 30 years, from estimating simplified single trajectories based on radiosonde observations to a system accounting for multiple interacting pollutants transported, dispersed, and deposited over local to global scales ([[http://ready.arl.noaa.gov/HYSPLIT.php](http://ready.arl.noaa.gov/HYSPLIT.php)]([http://ready.arl.noaa.gov/HYSPLIT.php](http://ready.arl.noaa.gov/HYSPLIT.php)), accessed on 14 May 2021).