1. Modelling Base Cations in Europe - A first approach … and what wind blown dust has to do with this.
Maarten van Loon and Leonor Tarrasón (met.no/EMEP)
Max Posch (RIVM/CCE)
6th TFMM meeting, Zagreb, Croatia, 4-7 April 2005

2. Outline Why model base cations? Approach Sources of base cations
Wind blown dust
Results & Discussion

3. Why model base cations?
Base cations play an important role in neutralizing acidity.
Need to identify sources and their contributions.
Present procedure based on measurements:
of wet deposition – unsatisfactorily
different assumptions followed in the countries.
==>Request for model estimates from WGE

4. Use of the EMEP Unified Model
Approach
Use of the EMEP Unified Model
Extention with Ca, Mg, K and Na in the fine and coarse mode
Using the same physical parametrisations
Dry and wet deposition (as for fine/coarse particles)
Advection
No chemistry taken into account
Routines for handling emissions developped
Boundary conditions from the global Oslo-CTM.
Inventorying emissions
Natural
Anthropogenic

5. Sources of base cations
Wind Blown Dust (WBD) seems to be the
dominant source in large parts of Europe.
WBD of various origin:
Saharan dust (partly within the model area, partly through boundary conditions)
From agricultural soils
Possibly also from other ”bare” soils
Other sources:
Sea salt
Anthropogenic sources

6. dust storm in the Sahara Desert along the Algeria/Niger
Source: NASA
dust storm in the Sahara Desert along the Algeria/Niger
border (25.0N, 10.0E)

9. WBD – illustration of the process
Picture loant from WERU

10. WBD parametrisation (I)
Horizontal flux (”saltation”):
Fh = const·Klim · u*3 ·(1-R)(1+R)2
with
R = u*,threshold/u*
u*,threshold the threshold friction velocity, which is
dependent on soil, soil moisture and Z0
Klim represents the supply of soil aggregates
(taken 1 for desert, 0.02 for agricultural soils and 0.1 otherwise)

11. WBD parametrisation (II)
Vertical flux (”sandblasting”): α·Fh
α dependent on soil characteristics.
Values in literature range from 10-5 to 10-7.
In this study taken constant:
α = 5 · 10-5
Size distribution:
function of u* and soil characteristics.
In this study ”average” profile taken
Percent of emitted mass that consists of
particles < 10μm for different values of u*

12. WBD parametrisation (III)
Right: % in fine mode
Bottom: % in coarse mode

13. Base cation content of the soil
Many soil types, with different percentages of b.c.
For Ca quite complete, but not for Mg, K and Na
Therefore emphasis on Ca in presenting the results

14. Results Simulation for the year 2000
Measurements of concentra-tions in precipitation from:
EMEP network
ICP-forest network
Seasalt correction applied (next slide)
Computations done for different sources separately. (linear process)
Observed Ca concentrations in precip. in 2000.
From work by NILU/CCC

15. Sea salt - correction
Wet deposition of Na seems to be underestimated by a factor of ~8
Air concentrations of Na are generally underestimated by a factor of ~2

16. Ca – comparison with measurements annual wet deposition
104 EMEP stations, 334 ICP-Forest locations

17. Results - statistics Spatial correlation Mean flux all EMEP ICP-F Obs.
model
bias Ca
0.73
0.78
427
362
-15% Mg
0.72
152
145
-5% K
0.53
n.a.
124 72
-42% Na
1005
984
-2%
Taking out WBD leads to low(er) correlations!

18. Modelled origin of Ca deposition

19. % contributions Ca
UL: WBD-EUR, UR: WBD-rest, LL: bound, LR: ant.

20. PM10 – from a global scale model
Average computed PM10 concentration (Oslo-CTM) for the year 2000 (source: A. Grini)

21. Conclusions Good performance for in particular Ca
Evidences that WBD is important indeed
Effort needed to obtain better information on soil content
Refine input to WBD parametrisation
Consistent u* and z0
More information on soil characteristics and content
Proper assignment to fine and coarse mode
Need for addition of ”verycoarse” mode
Sea salt
Other mechanism than wash/rain out???
Air concentrations
Seem too high (factor 2-6 for Ca seen), but … based on very few measurements
Potentially large contributions to PM concentrations, al least episodically