Synergistic Use of Remote Sensing and Modeling for Tracing Dust Storms in the Mediterranean

This study focuses on the detection of the dust source region and monitoring of the transport of the dust plume from its primary outflow to final deposition. The application area is the Sahara desert and the eastern Mediterranean, where two dust events occurred during the period 4–6 February 2009, an unusual event for a winter period. The Aqua-MODIS and OMI observations clearly define the spatial distribution of the dust plumes, while the CALIPSO observations of total attenuated backscatter (TAB) at 532 nm, depolarization ratio (DR), and attenuated color ratio (1064/532 nm) on 5 February 2009 provide a clear view and vertical structure of the dust-laden layer. The dust source region is defined to be near the Chad-Niger-Libyan borders, using satellite observations and model (DREAM) output. This dust plume is vertically extended up to 2.5 km and is observed as a mass plume of dust from surface level to that altitude, where the vertical variation of TAB (0.002 to 0.2) and DR (0.2–0.5) implies dust-laden layer with non-spherical particles. CALIPSO profiles show that after the dust plume reached at its highest level, the dust particles start to be deposited over the Mediterranean and the initial dust plume was strongly attenuated, while features of dust were limited below about 1–1.5 km for latitudes northern of ~36° (Greek territory).

Change of the Asian dust source region deduced from the composition of anthropogenic radionuclides in surface soil in Mongolia

Recent climate change, especially during the 2000s, may be the primary reason for the expansion of the Asian dust source region. The change in the dust source region was investigated by examining anthropogenic radionuclides contained in surface soil samples from Mongolia. Surface soil was globally labeled by radioactive fallout from nuclear testing during the late 1950s and early 1960s, but there are no current direct sources for anthropogenic radionuclides in the air (before the Fukushima nuclear power plant accident in 2011). Radionuclides in the atmosphere are therefore carried mainly by wind-blown dust from surface soil, that is, aeolian dust. Asian dust carries traces of 90Sr, 137Cs, and other anthropogenic radionuclides; the heaviest deposition occurs in spring and has been recorded in Japan since the early 1990s. The composition of anthropogenic radionuclides in atmospheric depositions would be affected by a change in the dust source. Previous studies of atmospheric deposition at long-term monitoring sites (e.g. in Tsukuba, Japan) have detected changes in the 137Cs/90Sr ratio and in the specific activity of the radionuclides. These changes in the composition of observed atmospheric depositions are supposed to reflect changes in the climatic conditions of the dust source region. To investigate this dust source change, we conducted a field survey of radionuclides (90Sr and 137Cs) in surface soil samples in September 2007 in the eastern and southern regions of Mongolia, where dust storms have occurred more frequently since 2000. The specific activities of both radionuclides as well as the 137Cs/90Sr ratio in the surface soil were well correlated with annual average precipitation in the Mongolian desert-steppe zone. Higher specific activities and a higher 137Cs/90Sr ratio were found in grassland regions that experienced greater precipitation. These findings suggest that the increased specific activities and the activity ratio detected in atmospheric depositions in Japan during years with frequent Asian dust transport events in the 2000s are a sign of grassland degradation.

Change of the Asian dust source region deduced from the relationship between anthropogenic radionuclides in surface soil and precipitation in Mongolia

The Asian dust source region may be expanding primarily as a result of recent climate change, especially during the 2000s. This change was investigated by examining anthropogenic radionuclides contained in surface soil samples from Mongolia. Surface soil was globally labeled by radioactive fallout from nuclear testing during the late 1950s and early 1960s. There are no current direct sources for anthropogenic radionuclides in the air, so the radionuclides in the atmosphere are mainly carried by dust from wind-blown surface soil, that is, aeolian dust. Asian dust carries 90Sr, 137Cs, and other anthropogenic radionuclides; the heaviest deposition occurs in spring and has been recorded in Japan since the early 1990s. The composition of anthropogenic radionuclides in atmospheric depositions would be affected by a change in the dust source. Previous studies of atmospheric depositions at long-term monitoring sites (e.g. in Tsukuba, Japan) have detected changes in the 137Cs/90Sr ratio and in the specific activity of the radionuclides. These changes in the composition of observed atmospheric depositions should be a reflection for a change in the climatic conditions of the dust source region. To investigate this dust source change, a field survey for radionuclides (90Sr and 137Cs) in surface soil samples was conducted in September 2007 in the eastern and southern regions of Mongolia, where dust storms have occurred more frequently since 2000. It was found that specific activities of both radionuclides as well as the 137Cs/90Sr ratio in the surface soil correlated well with annual average precipitation in the Mongolian desert-steppe zone. The higher specific activities and the higher 137Cs/90Sr ratio were found in the grassland region with the greater precipitation. This finding suggests that the increased specific activities and the activity ratio detected in the atmospheric depositions in Japan during years of the frequent Asian dust transport event since 2000 should be a sign of grassland degradation.