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Abstract
Aerosol particles deposit onto human body surfaces in indoor environments. However, the relative importance of this pathway is poorly characterized. In this study, an improved three-layer model was developed; it incorporates Brownian and turbulent diffusion, gravitational settling, turbophoresis, thermophoresis, and diffusiophoresis to predict particle deposition velocities onto human body surfaces. The model was preliminarily evaluated with manikin-based experiments, conducted in an 8 m3 stainless steel chamber for particles ranging from 0.01 μm to 5 μm. Both standing and sitting manikins with heat dissipation ranging from 50 w to 100 w were used. Following comparisons with the experimental results, the model was used to estimate particle deposition velocities onto the body surfaces of standing and sitting humans for three normal scenarios (transition season, summer, and winter). For particles from 0.01 μm to 3 μm deposition velocities were the highest in summer and the lowest in winter. For particles larger than 3 μm the trend was inversed. The modeled results suggest that direct deposition onto human body for particles ranging from about 0.05 μm to 0.5 μm is a relatively unimportant exposure pathway for standing and sitting human beings. However, for particles smaller than 0.05 μm and larger than 0.5 μm, direct deposition onto standing and sitting human beings may be an important exposure pathway.
Copyright 2013 American Association for Aerosol Research
NOMENCLATURE
| A | = | area of human body surface (m2) |
| C | = | indoor particle concentration (μg·m−3) |
| Cc | = | Cunningham coefficient |
| C∞ | = | particle concentration out of the boundary layer (μg·m−3) |
| C+ | = | dimensionless indoor particle concentration |
| D | = | Brownian diffusivity of the particle (m2·s−1) |
| da | = | particle aerodynamic diameter (μm) |
| dm | = | particle electric mobility diameter (μm) |
| dp | = | particle diameter (μm) |
| dve | = | particle volume equivalent diameter (μm) |
| e+ | = | dimensionless shifted distance of velocity boundary layer |
| hc | = | coefficient of heat convection at the human body surfaces (W·m−2·K−1) |
| J | = | particle mass flux (μg·m−2·s−1) |
| K | = | particle deposition rate (h−1) |
| k | = | skin roughness height (μm) |
| ka | = | air thermal conductivity (W·m−1·K−1) |
| kp | = | particle thermal conductivity (W·m−1·K−1) |
| Kn | = | Knudsen number, 2λ/dp |
| k+ | = | dimensionless skin roughness height |
| mp | = | particle mass (μg) |
| P | = | partial vapor pressure (kPa) |
| Pa | = | air partial vapor pressure (kPa) |
| Psk | = | skin surface partial vapor pressure (kPa) |
| r+ | = | dimensionless particle radius |
| Sc | = | Schmidt number |
| t | = | time (s) |
| ta | = | air temperature (°C) |
| Ta | = | air temperature (K) |
| tsk | = | skin temperature (°C) |
| Tsk | = | skin temperature (K) |
| u* | = | friction velocity (m·s−1) |
| u∞ | = | free stream air speed (m·s−1) |
| V | = | bulk of the chamber (m3) |
| vd | = | particle deposition velocity (m·h−1) |
| vdif | = | particle diffusiophoretic velocity (m·s−1) |
| v+d | = | dimensionless particle deposition velocity |
| vs | = | particle gravitational settling velocity (m·s−1) |
| v+s | = | dimensionless particle gravitational settling velocity |
| vt | = | particle turbophoretic velocity (m·s−1) |
| vth | = | particle thermophoretic velocity (m·s−1) |
| = | dimensionless air wall normal fluctuating velocity intensity | |
| y | = | vertical distance from the skin surface (m) |
| y+ | = | dimensionless vertical distance from the skin surface |
| = | Greek Symbols | |
| ɛp | = | particle eddy diffusivity in the boundary layer (m2·s−1) |
| θ | = | surface inclination angel (°) |
| μ | = | air dynamic viscosity (N·s·m−2) |
| ν | = | air kinetic viscosity (m2·s−1) |
| vt | = | air turbulent viscosity (m2·s−1) |
| ρ | = | air density (kg·m−3) |
| ρp | = | particle density (kg·m−3) |
| τL | = | Lagrangian timescale of the air (s) |
| τp | = | particle relaxation time (s) |
| τ+ | = | dimensionless particle relaxation time |
| χ | = | dynamic shape factor of particles |