Diapositiva 1

1
SAHARAN DUST OVER ITALY SIMULATIONS WITH THE
REGIONAL AIR QUALITY MODEL BOLCHEM
Mihaela Mircea, Massimo D'Isidoro, Alberto
Maurizi, Francesco Tampieri, Maria Cristina
Facchini, Stefano Decesari, Sandro Fuzzi Istituto
di Scienze dellAtmosfera e del Clima, Consiglio
Nazionale delle Ricerche, Bologna, Italy
PREAMBLE The dust aerosols, besides of changing
climate through the scattering and absorption of
solar and thermal radiation, also affect the
environment by fertilizing marine and terrestrial
ecosystems, which in turn influence the carbon
cycle. Moreover, the dust particles contribute
substantially to the total aerosol mass usually
employed in the developing of the environmental
policy regulations, therefore, a reliable
forecast of dust events is mandatory. Italy is
often reached by dust produced in the Saharan
regions. To the scope of predicting the advection
of dust and its physical and chemical properties
over Italy, a dust emission scheme has been
implemented in the air quality model BOLCHEM
(Mircea et al., 2007 P 4.5). The flux of dust at
surface is parameterized as a function of
friction velocity, which depends on winds and on
threshold friction velocity, and on particle
size. Here, we show the sensitivity of the dust
model to threshold velocity values and to number
of dust size bins. And a preliminary analysis of
model ability to predict a dust storm over Italy.
BOLCHEM-DUST BOLCHEM (Mircea et al., 2007)
couples on-line, one-way, meteorology with
chemistry and dust production. The dust model
implemented in BOLCHEM was developed by Tegen et
al. (2002), based on the results from Marticorena
and Bergametti (1995). The horizontal and
vertical dust fluxes are calculated based on the
location of the preferential dust sources, soil
texture, surface roughness, vegetation cover,
soil moisture content and surface wind velocity.
The ratio between the vertical and the horizontal
dust fluxes varies with the type of soil and the
size of the particle mobilized. The size
distribution of the mobilized dust depends on
both soil texture and surface wind speed. The
threshold friction velocities used to initiate
dust emissions are computed as a function of
particle size (Marticorena and Bergametti, 1995),
assuming constant roughness (0.001 cm) within
model grid cells.

• The July 2003 dust storm over Italy
• A strong Saharan dust outbreak occurred from 15
  to 19 July 2003 over Italy. The figures below
  show the SeaWIFS image of the 16 July at 1235
  UTC and the dust loading in g/m2 for two
  simulations performed with BOLCHEM model V1 and
  V2. By comparing BOLCHEM simulations V1 and V2 to
  the AQUA/MODIS satellite/sensor image, the model
  seems to predict relatively well both the extent
  and the timing of the dust event over Italy. In
  all images, it can be noted that the plume of
  dust over the Mediterranean sea comes from
  north-west and north of Africa and goes
  straightforward to the center and north of Italy
  with only a little veil over Sicily and Messina
  Strait. However, the dust loading given by the
  simulation V1 is much higher than that given by
  simulation V2 since more dust particles are
  mobilized when the threshold friction velocity
  is lowered. The comparison of model results with
  the observations (surface concentrations from
  EMEP stations and aerosol optical depth (AOD)
  from AERONET stations) shows better agreement in
  the case V1 than in the case V2.

Dust model sensitivity
BOLCHEM-DUST-V1
Sensitivity to threshold friction velocity The
graphs above show the ratio of dust
concentrations at surface for the simulations V1
(threshold friction velocities lowered by a
factor of 0.50) and V2 (threshold velocities
lowered by a factor of 0.75) with respect to the
values calculated according with Marticorena and
Bergametti (1995). The different increase of dust
concentration in the coarse mode with respect to
accumulation mode is a consequence of their
different production mechanisms coarse particle
are produced by deflation and accumulation
particles by saltation.
B1 Accumulation mode
B1 Coarse mode
BOLCHEM-DUST-V2.1 (same as V2, with changed
sedimentation)
BOLCHEM-DUST-V2 (same as V1, with lowered
friction velocity)?
The figures below show the vertical distribution
in function of forecast time of dust
concentrations (?g/m3) at Etna, from 14 to 19
July, for the cases V1 and V2. The results are in
agreement with Tafuro et al. (2006), the dust
layers are located below 6 km (ca. level 19).
B1 Coarse mode
B2 Accumulation mode
Sensitivity to number of dust size bins The
graphs above show the ratio of dust loading for
two cases 12 bins (B1) and 48 bins (B2) with
respect to the reference case with 24 bins. The
results show that the dust loading increase with
increasing the number of bins and decrease
substantially by reducing the number of bins in
both accumulation and coarse mode.

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• Gery, W., Witten, G. Z., Killus, J. P., Dodge.
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• Marticorena, B., Bergametti, G., 1995, J.
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• Mircea, M., D'Isidoro, M., Maurizi, A., Vitali,
  L., Monforti, F., Zanini, G., Tampieri, F., 2007,
  submitted to Atmos.Environ.
• Tafuro,A.M., Barnaba, F., De Tomasi, F., Perrone,
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• Tegen, I., Harrison, S.P., Kohfeld, K, Colin
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  3.

BOLCHEM-DUST-V1
BOLCHEM-DUST-V2
The simulations presented here were carried out
on a domain which extends from -15.91 W to 33.58
E and from 11.37 N to 53.37 N with a resolution
of 0.5 degrees. The ECMWF data were used as
initial and boundary conditions for meteorology.
The sedimentation velocity of the dust particles
has been considered constant in the simulations
V1,V2 and variable in the simulations V2.1, B1
and B2.
ACKNOWLEDGEMENTS This work was conducted in the
frame of ACCENT and GEMS EC projects, Italian
MIUR project AEROCLOUDS, and was also supported
by the Italian Ministry of Environment through
the Program Italy-USA Cooperation on Science and
Technology of Climate Change.