Mathematical study of the selective removal of different classes of atmospheric aerosols by coagulation, condensation, and gravitational settling, and the health impact

Abstract

The aim of this paper is to study the scavenging efficiencies of aerosol particles after some given dynamic mechanisms of removal (known as coagulation, condensation, and gravitational settling) as a function of time. In addition, the health impact of aerosols before and after the above dynamic mechanisms is analysed by comparing the respirable dust fractions. The well-known equations of scavenging are applied to eight different classes of atmospheric aerosols: marine background, clean continental background, average background, background and aged urban plume, background and local sources, urban average, urban and freeway, and central power plant. It is found that respirable dust is scavenged with relative difficulty by coagulation, condensation, and gravitational settling. The deposition of particles in the lungs relies on the same basic mechanisms underlying collection in a filter, but the relative importance of each mechanism is quite different. While filtration occurs in a fixed system at a steady flow rate, respiratory deposition occurs in a system of changing geometry, with a flow that changes with time and cycles in direction. This added complexity means that predicting deposition from the basic theory is much more difficult, and we must place greater reliance on experimental data and empirically derived equations. Therefore an understanding of how and where particles deposit in our lungs is necessary for the health hazards posed by aerosols to be evaluated properly. Approximately 10% of the initial volume of respirable aerosol remains after 18 hours of coagulation, condensation and gravitational settling. Gravitational settling is found to be the main mechanism for removal of respirable aerosols.

Keywords:

• Computational fluid dynamics
• Aerosols
• Precipitation scavenging
• Numerical methods
• Health impact

Acknowledgements

The authors express deep gratitude to Department of Mathematics and Department of Construction at Oviedo University for their computational support and useful assistance. Helpful comments and discussion are gratefully acknowledged. This work was partially supported by the basic research grant CN-04–209.