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The Two Signatures of Chinese Dust Storms in Beijing. Three major dust sources + Loess Plateau. Northern low- dust deserts. Northern high- dust deserts. Northwesterndeserts. Loess area. Main transport pathways. Takla Makan. Gobi. Rocky desert with less salts.
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Three major dust sources + Loess Plateau Northern low- dust deserts Northern high- dust deserts Northwesterndeserts Loess area
Main transport pathways Takla Makan Gobi Rocky desert with less salts. Sandy desert with abundant salts (chlorides, sulfates, carbonates). (Makra L. et al, 2002) (Sun, J. 2002)
The 6 dust storms and their 3 types Site: BNU I II II II III III 950 µg m-3 Dust episodes defined by meteorological reports, air pollution indices, and our TSP data.
DS3 DS4 I II Back trajectories for the two types of episodes
Elements that don’t work Fe, Ti, V, Mg, Na, Zn, Cu
Why these six elements? Maybe just ionic substitution for Ca++ in the gypsum matrix
Principles of ionic substitution • Free substitution when: • ionic radii are within about 15% • charges differ by zero or one unit. • The next slide shows the ions that can freely substitute for Ca in gypsum. • They are Na (which will go to halite instead), Sr, the REE (not measured here), and Sc. • Except for Co, these cations match the observed cationic enrichments in the high-Ca signal.
Ionic radii main 1.8 1.6 1.4 1.2 1 Ionic radius, A 0.8 0.6 0.4 0.2 0 0 1 2 3 4 5 6 7 Charge Cs Rb Ba K Pb La Sr Ca U Na REE Mn Sc Zn ZrHf In Co Cu NbTa Ti Li Fe Ni Mg Mn Ga V GeSe Al Si Be P S B C
Previous attempts to find chemical tracers • Yang D. et al. (1997) In Chinese • Concentrations of 12 elements from five sites between Beijing and source areas, but no explicit characterization of sources. • Zhang X. et al. (1996, 1997) • Four-element tracer system (Al, Fe, Mg, Sc) that reportedly differentiated three desert sources in China (NW, N, NE). S not measured, and Ca eliminated because of “postdepositional effects.”
Previous attempts, cont’d • Zhang X. et al. (2003) • NW deserts have 50% higher Ca, Fe, K, and Mg, and 20% lower Si, Al, Mn, and Ti than N deserts. • Nakano et al. (2004) • Sr/Nd isotopic data from soils used to claim that contemporary dustfall on Beijing comes more from nearby soils (<200 km) than from deserts or Loess Plateau. • Direct dustfall not measured. • Xuan J. (2005) • Emissions of Fe, Al, K, Mg, Mn, Na, Ca, and Ti in dust calculated from surface soils of six sources (three in Mongolia and three in China). X/Al ratios 14%–36% higher from NW deserts than from N deserts, with Mn and Ca the highest and Fe and Ti the lowest.
Summary • The main tracer system previously suggested (Fe/Al, Mg/Al, Sc/Al) does not work after transport to Beijing. • Instead, a five-ratio system (Ca/Al, S/Al, Sr/Al, Co/Al, Sc/Al) distinguishes the Takla Makan from the Gobi. • The enriched Ca and S of the upper line agree with the enrichment of gypsum and other salts in the Takla Makan (Zhang et al 2003; Makra et al., 2002; Okada 2004; Chinese Soil Atlas, 1994). • Rivers from surrounding mountains drain into the Tarim Basin and dry up there, depositing their salts (NaCl, CaCO3, CaSO4, etc.). • The co-enrichments of Sr, Co, and Sc are consistent with ionic substitution for Ca in gypsum of the Takla Makan.
These results also provide a way to deal with the high Ca from Beijing. • The two sources can be differentiated by S/Al or Ca/S. • Dust storms from Xinjiang replace the high Ca of Beijing (construction activities?) with high Ca from the Takla Makan Desert. • Thus the aerosol really does change when a dust storm arrives.
The straightness of the 1:1 lines over an episode • The straightness of the 1:1 lines over two orders of magnitude of concentration means that the dust storms remain a single material throughout the episode. This in turn implies at least two things: • (1) No significant SO2 is converted to sulfate on the surface of the dust as it enters Beijing. This agrees with the findings of Zhang D. et al. for Qingdao (AE, 2003) and Song et al. for ACE-Asia (AE, 2005).
(2) The signal from the dust overwhelms the signal of local Beijing dust, even down to very low concentrations. • This is consistent with the “clear-out” phase in dust storms (Guo et al, 2004), which can be stronger and last longer than first thought. • It further means that the wind speeds during dust storms in Beijing, much lower than those at the source, are too low to resuspend much dust in Beijing. • This is particularly so when the first stage of a dust storm is falling dust, which usually has very low wind speeds because it arrives ahead of the cold front.
Possible areas of research • Verifying the two signatures with samples from other times and places. • Searching for other signatures, say from the northern low-dust area or from Kazakhstan (northwest of the Takla Makan). • Explaining the two signatures. • Tracking dust clouds by means of their signatures. • Using the signatures to interpret existing sets of data.
Explaining the two signatures • Dust storms may come from small “hot spots” in deserts rather than from evenly over the entire surface. • In Africa (the Sahara), these hot spots are “wadis,” or ephemeral dried river channels and lake bottoms. • Does the same thing hold for Chinese deserts? • Prospero reports that the early stages of dust storms from the Gobi are composed of narrow plumes from hot spots that later diffuse into a broad dust cloud. (As seen from satellite photos).
Identifying the Chinese hot spots • Examine satellite photos from early stages of dust storms in the Gobi and the Takla Makan. • See whether hot spots can be verified. • If so, see whether they occupy consistent locations. • If so, see whether those locations can be identified on maps. • If so, are they wadis or something else?
Determining properties of hot spots • Go to the hot spots and sample their soil. • Compare the hot spots with each other and with the surrounding soil (and with the average desert soil). • Average desert soil may be available from the literature. • Data on hot spots may also be available somewhere, but I am guessing not. • May have to take multiple samples along a dried river bed—don’t know how many yet. • Also don’t know how many hot spots have to be sampled.
Determining properties of aerosol blown out of hot spots • Don’t yet know how best to do it. • Sample at the very beginning of dust storm? • Create aerosol from the soil in a wind tunnel? • Create the aerosol right there in the desert? (Jinghua’s idea)
The possible stages of fractionation • Hot spots vs. deserts. • One hot spot vs. another. • Salt in soils promotes lifting. • Aerosol vs. soil in hot spots. • Aerosol vs. aerosol-sized soil in hot spots. • Some minerals may be lifted more effectively than other minerals. • Depletion of coarse aerosol during transport.
