- Auxiliary Material -
The material presented below describes the geochemical methods associated with the
measurement of dust deposition rates and sources in the Hongyuan peat core and the
parameters and equations used to simulate dust emission fluxes in the regional
chemistry/aerosol climate model REMOTE.
The authors associated with this work are:
Dr. Marion Ferrat,
Dept. of Earth Science and Engineering, Imperial College London, UK.
College of Global Change and Earth System Science, Beijing Normal University, China.
Dr. Baerbel Langmann,
Institute of Geophysics, University of Hamburg, Germany.
Dr. Xuefeng Cui,
College of Global Change and Earth System Science, Beijing Normal University, China.
Dr. Jefferson Gomes,
Dept. of Earth Science and Engineering, Imperial College London, UK.
Dr. Dominik Weiss,
Dept. of Earth Science and Engineering, Imperial College London, UK.
Dept. of Mineralogy, the Natural History Museum, London, UK.
1. Geochemistry
The 6.2 m-long peat core was drilled using a 5 cm inner diameter hand-operated
corer and sub-sampled at 1, 2 and 5 cm intervals. Sixteen samples were selected for
radiocarbon dating at the AMS-14C laboratory in the Key Laboratory of Isotope
Geochronology and Geochemistry, Chinese Academy of Sciences in Guangzhou (China)
and at the Angstrom Laboratory, Uppsala University (Sweden). Results show a near
linear age-depth relationship, with the oldest peat samples dating back to 9.5 kyr BP
[Large et al., 2009]. The mineral dust was recovered from the plant matter by dry ashing
the peat samples at 450°C for 12 hours and collecting the residual ash in acid-cleaned
vials [Ferrat et al., 2012a, b, c].
The REE and trace elements Y, Sc and Th were measured in the peat ash and source
samples by inductively-coupled plasma mass spectrometry at the Natural History
Museum, London, after dissolution following an HF-HNO3 digestion method optimized
for these samples [Ferrat et al., 2011; 2012a, c].
Dust fluxes Φ to the core (g m-2 y-1) were calculated as follows:
Φ = (ρhci)/(Δt)*100
(1)
where ρ is the peat density (g cm-3), h the thickness of the sample (cm), ci the inorganic
concentration of the sample, taken as the ash content (cg g-1), Δt the time interval
represented by each sample (yr) and 100 is a unit conversion factor (Fig. A1). In order to
provide constraints for our numerical simulations, it is of interest to consider only that
fraction of the deposited dust which was transported from the non-local sources (i.e. the
Taklamakan desert and Chinese loess plateau), and to assess the relative contributions
from these two sources. At any given depth, the fraction of dust from each source was
estimated by solving three-simultaneous equations based on the La/Th vs. Y/Tb signature
of the peat samples and of the three end-members (Ferrat et al., 2012a).
Figure A1. (A) Total dust flux Φ (g m-2 y-1) to the peat core as a function of age (cal. yrs
BP); (B) Relative contributions of dust from the Chinese loess plateau and Taklamakan
desert to the total dust deposited at each depth; (C) Resulting dust fluxes from the
Taklamakan desert; and (D) Resulting dust fluxes from the Chinese loess plateau. The
grey bar indicates the position of the mineral-rich layer. Two columns on the right hand
side indicate wet (green), dry (yellow) and cold (blue) periods identified in the core based
on carbon density and δ13C measurements (Large et al., 2009).
2. Numerical simulations
2.1. Smooth roughness length z0s
The smooth roughness length as defined by Marticorena and Bergametti [1995] is the
roughness induced by soil grains on a bare erodible surface, which is source dependent.
Marticorena et al. [1997] suggested that z0s can be calculated as one thirtieth of the coarse
diameter of the soil population. However, according to Marticorena and Bergametti
[1995], the sensitivity of the results to the smooth roughness length over a range of one
order of magnitude (z0s = 10-4 to 3.10-3 cm) can be represented by a unique value of 10-3
cm. This value has been used in previous model simulations over Asia [Gong et al.,
2003]. Given that the soil size distribution in the present study is assigned estimated
values only, z0s is assigned a value of 10-3 cm as well.
2.2. Wind friction velocity
The wind friction velocity U*, or shear velocity, is computed from the model output
wind velocity U in the surface layer:
U* = (Uk)/ln(z/z0s)
(A1)
where k is the van Karman constant (k = 0.4), z is the height above the surface and z0s is
the smooth roughness length of the surface.
2.3. Dust emission fluxes
Dust particles may only be mobilized when the calculated wind friction velocity
exceeds a threshold value which depends on the soil surface characteristics. For a smooth
surface, Marticorena and Bergametti [1995] defined the threshold friction velocity as a
function of particle size dp, U*t1(dp), as:
U*t1 (dp) = (0.129K)/(1.928Re0.092-1)0.5
for 0.01 < Re ≤ 10
(A2)
with
Re = a(dp)X + b
(A3)
where a = 1331 cm-X, b = 0.38 and X = 1.56 are constants determined semi-empirically
and
K = ((ρpgdp)/ρa)0.5 * (1 + 0.006/(ρpgdp2.5))0.5
(A4)
The parameters ρp = 2.65 g cm-3 and ρa = 0.00123 g cm-3 are the density of soil dust and
air respectively, g = 981 cm s-2 is the acceleration due to gravity and dp is the soil particle
radius.
The expression for the threshold friction velocity U*t(dp) defined by Marticorena and
Bergametti [1995] is also corrected for the effects of non-erodible elements:
U*t (dp) = U*t1 (dp) * (1 – (ln[Zm – z0s]/ln[0.35(10/z0s)0.8]))
(A5)
where U*t1(dp) is the expression of the friction velocity as a function of particle radius
alone, Zm is the roughness length of the surface (cm) and z0s that of the erodible elements.
It is assumed that all prescribed surfaces are erodible as a first order estimate and Zm is set
to z0s. U*t(dp) thus becomes equal to U*t1(dp).
When the threshold value U*t(dp) is exceeded by the wind friction velocity U*,
particles of size dp enter a horizontal movement called saltation. This horizontal flux G is
defined in their scheme as:
G = E Σ[(ρa/g)U*3(1 + R)(1 – R2)]Srel(dp)
(A6)
where
R = (U*t(dp))/U*
(A7)
and E and Srel(dp) are the fraction of erodible to total surface and the relative proportion
of the surface covered by particles of size dp respectively. It is assumed as a first order
estimate that all areas prescribed in the model as dust sources are erodible and E is
assigned a value of 1.
Finally, the vertical flux F represents the flux of smaller particles escaping the
saltation layer and mobilized into higher levels in the model atmosphere and is
proportional to the horizontal flux G by a factor α such that:
α = F / G = 10 (0.134(%clay))
(A8)
Laurent et al. [2006] calculated α for various deserts regions in northern China and found
values of 1.85×10-6 and 2.23×10-6 for the Taklamakan and Tengger deserts respectively.
Given that a similar size distribution was assigned to all potential sources in this study, an
average value of 2.0×10-6 was chosen to represent the ratio of vertical to horizontal flux
in the model.