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Characterisation of Differential
Mobility Analysers for the Droplet
Aerosol Analyser instrument
Michael Gradmann
Supervisor: Maria Berghof, Staffan Sjörgen
and Göran Frank
Examiner: Carl Erik Magnusson
Lund University
Department of Physics
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Understand mechanisms of climate and
weather
Provide a reliable weather forecast
Understand how and how much weather is
affected by humans
The DAA instrument
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Measures size of cloud droplets
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Dries the droplets
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Measures number and size of the particles
(CCN)
→
unique data set
The DAA instrument
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Sketch by Maria Berghof
Differential Mobility Analyser
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Cylindrical capacitor
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Radial electrical field
Sheath air flow
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→
Uncurled, smooth
precise manufacturing
required
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Picture courtesy of [3]
Differential Mobility Analyser
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Radial electrical field
Radial velocity
Differential notation
E=
U
r∗ ln
ra
ri
er
U∗ Z
vr r =
ra
r∗ ln
ri
dt=
ra
r∗ ln
ri
U∗ Z
dr
Differential Mobility Analyser
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Different differential notation dz=
Integration, transposition
Electrical mobility
Z=
ra
r∗ ln
ri
U∗ Z
ra
Q∗ ln
ri
2 ∗ U∗ l
neC C
Z=
3
dp
v z dr
DMA preparation
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Cleaning
Check for scratches or
damages
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Mesh size checked
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Mesh replaced
Leak test
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Picture courtesy of [3]
Set-up
Transfer function
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Ideal triangular
function
Losses and
broadening
because of
imperfections
inside the DMA
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Picture courtesy of [2]
Transfer function
f
f
, , Z0 , Z =
, , Z0 , Z =
f
∗ 1
Qs Z
Qa
Z
− 1 ;1−
≤ ≤1
Qa Z 0
Qs Z 0
∗ 1
Qs − Z
Qa Z 0
Z
1 ;1≤ ≤ 1
Z0
Qa
Z
Z
, , Z0 , Z = 0;
1−
∨
1
Z0
Qs Z 0
Qa
Qs
Qa
Qs
Experimental method
Experimental method
Experimental method
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Theoretical data can be calculated by
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compared with measurement
n2 Z i ∫ f 1 1, 1, Z ' 0 , Z ∗ f 2 2, 2, Z i , Z dZ
=
N1
∫ f 1 1, 1, Z ' 0 , Z dZ
X = ∑ exi − thi
2
2
i
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λ, μ changed (iteration), until X² has its smallest
value
Results
DMA name
DMA1a
DMA2a
DMA2b
DMA2c
DMA1b
DMA2d
DMA2e
DMA2f
UDMA
λa
0.599
0.917
0.934
0.838
0.953
0.686
0.821
0.858
0.896
μa
0.903
NA
0.942
0.783
0.910
0.710
0.942
NA
0.781
λb
0.660
0.811
0.980
0.904
0.969
0.786
0.842
0.958
0.962
μb
1.022
1.056
0.756
0.885
0.832
0.714
0.881
0.921
0.873
Discussion
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Too clean DMAs
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Turbulances caused by changed flow ratio
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Modifications added to DMAs (plastic
mount/mesh)
Unstable particle number concentration
Discussion
Discussion
Reliable results
DMA name
DMA2b
DMA1b
DMA2e
DMA2f
λa
0.953
0.821
0.858
μa
0.910
0.942
NA
λb
0.980
0.969
μb
0.756
0.832
Conclusion
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Values for λ and μ depend on the stability of
particle number concentration
Broad distribution of particle number has a
larger effect than slow changes
The time a particle takes to reach a CPC is
important
Perspective
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Check of well known flow ratio to reproduce
results published in [1] and [2]
Small changes made on the set-up
Particle number concentration distribution could
be reduced to less than 2%
Significantly better results
References
[1] Martin N.A. Karlsson, Bengt G. Martinsson, Methods to measure and predict the
transfer function size dependence
of individual DMAs, J. Aerosol Sci., 34, 603625, 2003
[2] Bengt G. Martinsson, Martin N.A. Karlsson, and Göran Frank, Methodology to
estimate the transfer function of individual Differential Mobility Analyzers, Aerosol Sci.
Techn., 35, 815-823, 2001
[3] Anna Persson, Design av mätmetodik för droppaerosolanalysatorn, Examensarbete
för Kandidatsexamen, Lunds Universitet, 2008
[4] William C. Hinds, Aerosol Technology, 2nd edition, Wiliey Interscience, 1999
[5] Göran Frank and Bengt G. Martinsson, An instrument for studies of the relation
between cloud droplet size and dry residual particle size - The Droplet Aerosol
Analyser, Proceedings of the International Conference on Cloud and Precipitation
(Abstract), 2008
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Thank you for your attention
You are welcome to ask questions