Is atmospheric aerosol an aerosol? comments on the article by Jaenicke

The title of a recent article "Is atmospheric aerosol an aerosol?" by Jaenicke caught my attention. This took me back in time, when I was just beginning my career in this field. The first definition I came across or rather assumed was: "anything solid or liquid suspended in the air is aerosol". According to this definition birds and aeroplanes were also aerosols! Thinking of birds and aeroplanes as aerosols wasn't intuitive hence I had to search for a more refined definition. A better one that I came across (well I don't remember from where) was: "any thing suspended in the air and doesn't have self-propelling mechanism is aerosol". This definition implied that mosquitoes are not aerosols but bacteria and virus are. However, this new definition didn't help me win an argument with my friend Neeraj (one of the authors of this blog) who held an opinion that water and ice clouds are aerosols as well, whereas I held the opinion that they are not. We concluded arguments by accepting that clouds are special cases of aerosols and if not explicitly mentioned, atmospheric aerosols mean liquid and solid particles suspended in the air and have aerodynamic diameter between 1e-3 and 1 µm.

Technically aerosols are defined as colloid of air and solid/liquid particles, where air is the dispersion medium and particles are in dispersed phase. The word aerosols brings-in naturally the interaction between particles and air. When particles are not in the air, for example particles collected on filter papers, they are no more aerosols. Hence aerosol is a state of particles rather than the particle itself. This is the reason why most pollution and chemistry related studies report them as particulate matter (PM) because these studies require collecting them on filter papers, while most climate related studies report particles as aerosols since particles are climate modulator as long they are in the air.

Coming back to Jaenicke's article, he starts with questioning the very definition of atmospheric aerosol as colloid. Schmauss and Wigand were probably the first to define atmosphere as colloid of air and particles. The word colloid itself was coined in the year 1875 by Gerber to describe a pseudosolution prepared by Selmi (reference in Jaenicke, 2008) . Jaenicke sees a reason to question this definition because colloid implies a stable state, homogeneity and monodispersed size distribution. Hardly any of these is true for atmospheric aerosols. On the other hand there are properties such as surface-to-volume ratio, interactions between nearby particles, multiple scattering, etc that support their definition as colloid. Jaenicke concludes that atmospheric aerosols can be considered colloid in dynamic equilibrium.

The crux of the paper is not the discussion on the definition of aerosol but modeling aerosol concentration in the atmosphere under the framework of dynamic equilibrium. Dynamic equilibrium by its nature results in highly variable aerosol concentration. Which in turn requires better temporal resolution for measuring them. In this discussion, Jaenicke highlights quite a few gaps in our knowledge about atmospheric aerosols.


References

Jaenicke, R. (2008). Is atmospheric aerosol an aerosol?-a look at sources and variability. Faraday Discuss 137, 235-243.


Art work: courtesy Malkaush
Edited to update non-functional links on 27 August 2017.