Setting threshold values of particle sizes for determination of the appropriate dispersion/deposition model during various atmospheric stability conditions

Pollutant Ground Concentrations of Nonneutrally Buoyant Particles

“…The estimation of D pc is based on the assumption that the origin of particles in a plume that would reach (by turbulent diffusion) the ground level at a certain downwind distance x is at h = 2 σ z (x) , where σ z is the pollutant's standard deviation (of the normal distribution) in the vertical direction, and ±2 σ z (x) refers to the boundaries within which 95% of the aerosols are included at a defined downwind distance. This particle size threshold value for the two utterly different methodologies was shown in Mandel et al . to depend on the atmospheric stability condition, the height of release from the column, and the particle's flow regime, as shown in Equations –.…”

“…The model suggests that the maximal distance that nonneutrally buoyant particles can reach by gravitational settling (denoted by x max , in meters) corresponds to the settling distance of particles with D = D pc (H) , (i.e., the critical particle diameter released from the maximum height of the contaminated column, H) . Consequently, the maximal downwind distance reachable by gravitational settling would be x max = 1,000 (H/ 2 a) 1 /b m downwind from the explosion source . Note that the proposed analysis automatically addresses this criterion, whereby the ground concentration of the nonneutrally buoyant particles for any L > x max would result in a zero value (although a nonzero ground concentration due to turbulent diffusion processes and dry deposition processes may still be encountered at L > x max ).…”

“…Dispersion of considerable fractions of these particles is influenced by gravitation (moderated by drag forces), horizontal air movement (i.e., winds), and atmospheric turbulence. Based on Hanna et al, (11) Mandel et al (10) have shown that for particles of an equivalent diameter D p , the fallout distance x from an elevated source h can be estimated based on Equations (1a) and (1b). Nevertheless, D p is strongly substance-dependent because gravitational settling velocity depends on the combination of diameter and the specific density of the particle.…”

“…The threshold value of the particle size for considering gravitational settling in Gaussian‐type models typically is set to about 10–20 microns. Such a cutoff value is arbitrary and does not take into account the atmospheric conditions and the release height . Moreover, in most cases, the corrections concerning the nonneutrally buoyant particles are based on an average particle size value.…”

“…Such a cutoff value is arbitrary and does not take into account the atmospheric conditions and the release height. (10) Moreover, in most cases, the corrections concerning the nonneutrally buoyant particles are based on an average particle size value. More details can be found in the relevant textbooks.…”