An aerosol is a gaseous suspension of solid or liquid particles. In our atmosphere, particles ranging from a few nanometers (nm) to tens of micrometers (µm) in equivalent diameter (Dp) suspend for hours if not days, whereupon they effect atmospheric chemistry, the global radiation budget, cloud processes, even our health profoundly: their size, composition, and concentration determining their local or regional impact.

Observations show that in our atmosphere particle size can roughly be categorized with origin. Particles about 1 µm Dp or larger originate from mechanical processes, such as wind-blown dust or sea spray. Particles smaller than 1 µm, often called "fine" particulate matter, originate from gas-to-liquid/solid conversion either due to phase change or chemical reaction.

After nitrogen and oxygen, the most abundant gas in our atmosphere is water vapor. Given typical temperatures in our atmosphere, The thermodynamically stable state of water is liquid or solid, yet water vapor persists. This is because energy is required to make a phase change: in this case, for water to "nucleate," reduce several gas molecules into one cluster binding those molecules together (reducing entropy). Classical nucleation theory hypothesizes that pure water will not, in reality, homogeneously nucleate, i.e. condense without a seed/surface that assists the phase change, in our atmosphere but could if combined with sulfuric acid as the mixture will have a lower vapor pressure, and observations support this theory. Moreover, other observations lead scientists to believe that a third component in the gas phase, such as ammonia or ions, will induce nucleation.

Then, to understand origins of sub-micron (< 1 µm Dp) atmospheric particles, we investigate the smallest particles detectable in an attempt to know condensing compounds as a function of size, knowing that at this moment particles < 3 nm Dp are just too small to chemically characterize. As you can see in the figure below, nanometer-sized particles do contribute to the overall mass distribution because they grow over time.

Above image : Nanoparticle (3-50 nm Dp) size distributions recorded by Matt Dunn et al. with a custom scanning mobility particle sizer (from the UMN Particle Technology Lab) shown with gas phase SO2 concentrations and solar irradiance measurements (from Oscar Fentanes and Ben de Foy in participation of the Mexico City Metropolitan Area 2003 campaign). The diagram likely depicts a new particle formation event at Santa Ana Tlacotenco, a rural location 30km southeast of Mexico City in April 2003, while elevated concentrations of SO2 (H2SO4 when the sun is shining) occur simultaneously suggests at least binary nucleation. Over time the greatest concentration of particles evolves to larger sizes, likely due to coagulation and condensational growth.

Bibliography

Dunn, M.J.; Jimenez, J.-L., Baumgardner, D.; Castro, T.; P.H.; Smith, J.N. Measurement of Mexico City nanoparticle size distributions: Observations of new particle formation and growth. Geophys. Res. Lett. Vol. 31, L10102, doi:10.1029/2004GL019483, 2004.

Eisele, F.L.; McMurry, P.H. Recent progress in understanding particle formation and growth. Phil. Trans. R. Soc. Lond. B, 352, 191-201, 1997.

Seinfeld, John H.; Pandis, Spyros N. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change; John Wiley & Sons : New York, 1998.

return to home page
site last revised 21 May 2006