Thursday, November 29, 2007

Inter-relationship between the chemistry of aerosols and precipitation and their implications

The large-scale anthropogenic emissions have been increasingly affecting the air quality as well as regional and global climate by altering Earth’s surface temperature and precipitation patterns. These pollutants are removed from the atmosphere via wet and dry deposition, which in turn may adversely affect the terrestrial and aquatic environments, ranging from acidification of soils and inland waters to the damage of buildings and monuments. The regional precipitation chemistry provides the simplest way to assess the influence of human activities on the composition of the atmosphere, and to improve knowledge of physico-chemical processes related to the atmospheric transport and deposition of pollutants. The chemical composition of an individual precipitation event is dictated by ‘in-cloud’ and ‘below-cloud’ scavenging of atmospheric aerosols and gaseous species derived from natural and anthropogenic sources. Wet and dry deposition also provides information on the exchange of chemical components between the atmosphere and the land/ocean and hence it is an important link in understanding the biogeochemical cycles of important chemical elements such as N, P and S.

Rastogi and Sarin (2007) have discussed the inter-relationship between aerosol and rain composition collected over a period of three years at Ahmedabad, an urban city located in a semi-arid region of India. They have proposed a simple way (comparison of ionic ratios in aerosol and rain) to understand the dominant scavenging processes (in-cloud/below-cloud) of chemical species and the phase (gaseous vis-à-vis particulate) from which it is scavenged by rain. By first direct measurement of alkalinity in rainwater over India, they have discussed the reason of alkaline rain over their study region, in spite of high concentrations of acidic pollutants like SO4 and NO3.

For more details, please read the following article and references therein:

Rastogi, N. and Sarin, M.M., 2007. Chemistry of precipitation events and inter-relationship with ambient aerosols over a semi-arid region in western India. Journal of Atmospheric Chemistry 56, 149-163. (DOI 10.1007/s10874-006-9047-5).

Sunday, November 4, 2007

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)


  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.

Conferences in the field of aerosol, cloud and climate

Recently I received quite a good number of conference related notifications. Many of reader of this blog may be participating it and they shouldn't miss the dead-line so I am listing them below. If you know conference that is not listed below but related to theme of this blog please let us know.

15th National Space Science Symposium, India (NSSS-2008)

National Space Science Symposium is one the largest gathering of space scientist in India. It covers wide variety of subjects including remote sensing of atmosphere and climate change study. The next NSSS is going to be held at Radio Astronomy Centre (NCRA-TIFR) in Ooty between 26 and 29 February 2008. Ooty is one of the famous hill-station in India.

Last date to submit abstract Dec 7, 2007.

Broad subject areas covered in this conference

  • Space- and ground-based astronomy and astrophysics, planetary science / exploration
  • Solar radiation and its interaction with earth's near and distant environment
  • Magnetosphere, ionosphere, thermosphere, and middle atmosphere phenomena
  • Space based oceanography, meteorology, and tropospheric studies
  • Climate changes and geosphere-biosphere interaction processes

EGU General Assembly 2008
European Geophysical Union's general assembly will be held in Vienna, Austria between 13 and 18 April 2008.
Last date to submit abstract: January 14, 2008
Last date to submit financial support application: December, 7, 2007
Last date for registration: March 31, 2008
Subject area covered are
It covers all disciplines of Earth, Planetary and Space Sciences

AOGS 2008
Asia Oceania Geosciences Society's (AOGS) 5th annual meeting will be convened between 16 and 20 June, 2008 in Busan, Korea.
Abstract submission dead-line: Jan 24, 2008
Author registration dead-line: Apr 22, 2008
This also covers all the subjects in Geoscience area.

International Conference on "Terrestrial Planets: Evolution through Time"

This conference will be held between 22 and 25 Jan 2008 in Physical Research Laboratory, Ahmedabad, India. Last date for abstract submission is November 15, 2007.

Following are the main theme of conference
  • Early solar system and Planetary processes
  • Evolution of the Indian Plate: Precambrian to Recent
  • Paleoclimate and Paleoenvironment
  • Tectonics-Erosion-Climate and Carbon cycle

Images used here are taken from the web-pages of those conferences.