Abstract
Abstract. Due to rising or descending air and due to gravity, aerosol particles carry out a complicated, chaotic motion and move downwards on average. We simulate the motion of aerosol particles with an atmospheric dispersion model called the Real Particle Lagrangian Trajectory (RePLaT) model, i.e., by solving Newton's equation and by taking into account the impacts of precipitation and turbulent diffusion where necessary, particularly in the planetary boundary layer. Particles reaching the surface are considered to have escaped from the atmosphere. The number of non-escaped particles decreases with time. The short-term and long-term decay are found to be exponential and are characterized by escape rates. The reciprocal values of the short-term and long-term escape rates provide estimates of the average residence time of typical particles, and of exceptional ones that become convected or remain in the free atmosphere for an extremely long time, respectively. The escape rates of particles of different sizes are determined and found to vary in a broad range. The increase is roughly exponential with the particle size. These investigations provide a Lagrangian foundation for the concept of deposition rates.
Highlights
There are several Eulerian and Lagrangian dispersion models that simulate and forecast the movement of air pollutants in the atmosphere by using meteorological data
Lagrangian particle-tracking models can be divided into two classes: models in which an artificial mass is assigned to any particle and this mass depends on time, and models that follow “real particles” with fixed, realistic size and density
While far from the surface, on large scales, the effect of turbulent diffusion is typically negligible, this process plays an important role in the dispersion of particles in the planetary boundary layer (PBL)
Summary
There are several Eulerian and Lagrangian dispersion models that simulate and forecast the movement of air pollutants in the atmosphere by using meteorological data. Lagrangian particle-tracking models can be divided into two classes: models in which an artificial mass is assigned to any particle and this mass depends on time (we refer to them in this paper as “ghost” or “computational” particles), and models that follow “real particles” with fixed, realistic size and density The latter ones (such as PUFF, Searcy et al, 1998 and VAFTAD, Heffter and Stunder, 1993) are typically designed to predict the dispersion of volcanic ash as quickly as possible. 4. The results obtained for the deposition dynamics and for the dependence of the escape rate on particle radius, both in the free atmosphere and in the boundary layer, are given in Sect.
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