Saturday, October 4, 2008

Long Range Transport of Dust Aerosols

Continuing from our last blog post on the issue of aerosol radiative forcing (ARF) and the importance of its vertical distribution, l would like to bring to attention another important aspect associated with aerosols that affect global ARF. This aspect is the transport of aerosols which is very closely related to its vertical distribution. One of the largest sources of aerosols is desert. These desert dust aerosols have gained much attention due to the long range transport associated with them. The dust from these deserts is lifted higher in the atmosphere and ‘because of the fast large-scale transport in the upper troposphere, aerosols such as dust and black carbon, once they enter the upper troposphere (above ~8 km), can be transported around the earth in a latitudinal belt in a week or two’. ‘The impact of long range transport of dust and air pollution from their continental sources over oceanic regions is one of the outstanding problems in understanding regional and global climate change’ and in also determining the air quality of regions to which dust is advected to. Using Calipso and surface micropulse lidar, Huang et al., [2008] observe the long-range transport and vertical distribution of Asian dust aerosols during the PACific Dust EXperiment (PACDEX). ‘The MPL measurements were made at the Loess Plateau (35.95°N, 104.1°E) near the major dust source regions of the Taklamakan and Gobi Deserts’ in China during March-May 2007. They find that ‘Dust events were more frequent in the Taklamakan, where floating dust dominates, while more intensive, less frequent dust storms were more common in the Gobi region. The vertical distribution of the CALIPSO backscattering/depolarization ratios indicate that non-spherically shaped dust aerosols floated from near the ground to an altitude of approximately 9 km around the source regions. This suggests the possible long-range transport of entrained dust aerosols via upper tropospheric westerly jets. A very distinct large depolarization layer was also identified between 8 and 10 km over eastern China and the western Pacific Ocean corresponding to dust aerosols transported from the Taklamakan and Gobi areas, as confirmed by back trajectory analyses. The combination of these dust sources results in a two-layer or multilayered dust structure over eastern China and the western Pacific Ocean.’ We have seen in our last blog spot that ‘the vertical distribution of dust aerosols is another critical factor impacting the effects of dust on radiative forcing and climate [Claquin et al., 1998; Zhu et al., 2007; Forster et al., 2007].


Huang, J., P. Minnis, B. Chen, Z. Huang, Z. Liu, Q. Zhao, Y. Yi, and J. Ayers (2008), Long-range Transport and Vertical Structure of Asian Dust from CALIPSO and Surface Measurements during PACDEX, J. Geophys. Res., doi:10.1029/2008JD010620, in press.

Claquin, T., M. Schulz, Y. J. Balkanski, and O. Boucher (1998), Uncertainties in assessing radiative forcing by mineral dust, Tellus, Ser. B, 50, 491–505.

Forster, P., V. Ramaswamy, P. Artaxo, T. Berntsen, R. Betts, D.W. Fahey, J. Haywood, J. Lean, D.C. Lowe, G. Myhre, J. Nganga, R. Prinn,G. Raga, M. Schulz and R. Van Dorland (2007), Changes in Atmospheric Constituents and in Radiative Forcing. In: Climate Change 2007: The Physical Science Basis. contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

Zhu, A., V. Ramanathan, F. Li, and D. Kim (2007), Dust plumes over the Pacific, Indian, and Atlantic oceans: Climatology and radiative impact, J. Geophys. Res., 112, D16208, doi:10.1029/2007JD008427

No comments: