Dust Aerosol: Spherical Vs Non-Spherical

In most climate and radiative transfer models, optical properties of aerosols are modeled using spherical shape assumptions. This assumption is based on sound scientific reasoning. All liquid aerosols have spherical shape because of surface tension. Solid aerosols, which are water soluble also eventually absorbs water vapor from atmosphere and transform themselves into spherical shape.

However, dust aerosols are neither liquid nor water soluble. Soot aerosols also fall in this category. Hence, they may not necessarily have spherical shape. This requires that we should examine the validity of spherical shape assumption, particularly for these two types of aerosols. There are two aspects to look at for importance of spherical shape assumption. One is relative magnitude of non-spherical aerosol number concentration. If there are not quite large number of non-spherical particles in the atmosphere, then we need not worry about it. Li and Osada (2007) have shown using model study that dust particles are essentially spherical when away from source regions, this is due to preferential settling of non-spherical dust particles. (See our earlier blog). This kind of studies are relatively few and recent. We can expect to see in future their modeling results being compared with observations of Saharan dust transport over Atlantic ocean.

Second aspect is effect of non-sphericity on optical properties. It is believed that when non-spherical particle are randomly oriented, their overall impact can be modeled by assuming them spherical with some kind of equivalent effective radius. Though this assumption appears correct intuitively, not extensively validated. Recently, I come across an article by Yang et al. (2007), who have compared optical properties of aerosols for spherical and non-spherical (spheroid) shape assumptions. They have shown that the non-sphericity has negligible impact on optical properties in long-wave (terrestrial) spectrum. However quite a large effect can be seen in short wave (solar) spectrum.

Figure (12) of Yang et al. (2007) show the effect of spherical and spheroidal shape assumption on estimates of brightness temperature and top of the atmosphere reflectivity. The black curves represent clear sky condition, blue curves dust particle with spherical shape and red curves represent dust particle of spheroid shape (aspect ratio 1.7). When used spheroidal shape assumption to calculate top of the atmosphere reflectance, quite a large difference can be seen in shortwave (~30% difference at 500 nm)

References

1. Li, J. and K. Osada (2007, September). Preferential settling of elongated mineral dust particles in the atmosphere. Geophysical Research Letters 34, L17807+.
2. Yang, P., Q. Feng, G. Hong, G. W. Kattawar, W. J. Wiscombe, M. I. Mishchenko, O. Dubovik, I. Laszlo, and I. N. Sokolik (2007, October). Modeling of the scattering and radiative properties of nonspherical dust-like aerosols. Journal of Aerosol Science 38 (10), 995-1014.