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CPMA mass and mobility distribution measurements with direct measurement of the aerosol charge state
David Walker, Kingsley Reavell
Cambustion, Cambridge, UK
Cambridge Particle Meeting 19/06/2020
Motivation
• Mass distribution (area under the curve represents total aerosol mass).
• Mass and mobility spectrum without charging model.
• Accurate measurement of large soot agglomerates.
2
An aerosol instrument for measuring mass
• The Cambustion Centrifugal Particle Mass Analyser (CPMA) uses a spinning drum with a variable voltage supply to
select particles based on the balance of centrifugal and electrostatic forces, i.e. their mass to charge ratio.
3
w
Electric
attraction+
V
Centrifugal
force
These would all be classified as the same
The 1st problem: particle charging uncertainty
• The SMPS and other traditional aerosol size distribution techniques rely on models for the charge distribution because
classification depends on the ratio of particle size to particle charge.
• Different models must be used for spherical and agglomerate particles, and the actual charge state depends on the
details of the morphology of agglomerate particles, and the details of the charging process.
• For the CPMA the density must be known to infer the particle charge.
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Bipolar Diffusion Charging of Soot Aggregates, M. Matti Maricq
DMS500 Mean Particle Charge
0
5
10
15
20
25
30
35
0 50 100 150 200 250 300 350 400 450 500
mobility diameter (nm)
me
an
ch
arg
e
NaCl / ejector pump
DEHS / peristaltic pump
Diesel, 4th Gear 70kph
Diesel, 5th Gear 70kph
Unipolar Diffusion Charging, Cambustion
The 1st solution: measure the average charge per particle
• Using a CPC (condensation particle counter) and electrometer the average charge per particle is directly measured
without any charging model.
• The CPMA selects particles of a known mass to charge ratio therefore by measuring the charge per particle this gives
the particle mass.
• For this to be true the mass selected particles must have a tight charge distribution.
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The 2nd problem: uncharged particle penetration
• At low speeds the centrifugal force generated in the CPMA is low which means small uncharged particles can penetrate
through the classifier. These are counted by the CPC resulting in the incorrect concentration but not by the
electrometer resulting in the wrong charge per particle and therefore the wrong particle mass.
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Transmission (%)
Uncharged
Single charge
Double charge
The 2nd solution: unipolar diffusion charging
Using the Cambustion unipolar diffusion aerosol charger (UDAC) extremely high ion concentrations are possible, so that
above 20nm the zero charge state is negligible. This has the following benefits:
• Dramatically reduced quantity of uncharged particles limited small sizes.
• Improved electrometer sensitivity because particles carry many charges.
• Any uncharged particles can be removed by toggling an electrostatic precipitator (ESP) or by using a CPC with a large d50.
• Improved CPMA working range, maximum size extended to several microns.
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Bigger particles attract
large numbers of
charges in the UDAC
Massively reduced
number of uncharged
particles
Tight charge
distribution
The 3rd problem: variable CPMA transfer function width
• The transfer function of the CPMA depends on the mobility of the particle because whether a particle outside the
setpoint mass emerges depends on the time taken for it to drift across the gap, which depends on its drag or mobility.
• Therefore the mobility is required to calculate mass spectral density. For aerosol of known density and shape factor a
model can be used to calculate the mobility from the mass however for agglomerates or aerosol of unknown material
this introduces uncertainty into the concentration measurement.
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m* is the setpoint mass
m’ is the upper mass transmitted
Particle trajectory through CPMA annular gap
The 3rd solution: mobility separator
• Split the electrometer flow into two electrometers depending on
electrical mobility.
• The device is similar to a DMA but with a low resolution and
dual aerosol outlets.
• Operating conditions of the separator are set to ensure all the
aerosol particles pass to one or other electrometer. This means
the ratio of combined current from the two electrometers to a
CPC concentration measurement in parallel gives the average
charge per particle.
• Used with the CPMA, mass and mobility are thus measured
simultaneously from which the effective density of the particle is
calculated.
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Mobility separator patent pending.
Full schematic: Mass & Mobility Aerosol Spectrometer (M2AS)
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Aerosol highly
charged
Monodisperse
with known
mass to charge
Particle number
measured
Electrical mobility
measured
Particle charge
measured
Raw results: putting it all together
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Aerosol: DOS from
Collison nebuliser
CPMA voltage and speed stepped scan
CPC concentration & electrometer currents
Mobility separator voltage & electrical mobility
Average charge per particle
Particle mass
Particle mobility
diameter
CPMA transfer
function
Particle mass to
charge ratio
Mass spectral
density
Processed results: Diameter distribution
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Left hand tail
limited by CPMA
maximum speed
Processed results: Mass distribution
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Right hand tail here
limited by CPMA
minimum speed
Right hand tail here
limited by signal to
noise ratio
Processed results: Density plot
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Gradient indicates
differences in
shape factor
Processed results: Charge plot
Another interesting
morphology comparison,
critically showing
differences between
aerosols
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Validation: Integrated mass
Test Filter papermg/m3
MMASmg/m3
1 135 148
2 (with catalytic stripper)
98 97
Catalytic stripper added here to
remove any gaseous hydrocarbons
• Particle losses in the UDAC ~10%.
• Particles losses in the CPMA are ~20%.
• Integrated number to be evaluated in
future.
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Validation: Effective density of known spherical particles (DOS)
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Majority of the error is
caused by a non linear
power supply in the
prototype.
Summary
Novel technique for simultaneous measurement of aerosol mass and mobility, dubbed the “Mass & Mobility Aerosol
Spectrometer” (M2AS).
• No charge model – dramatically improves the accuracy for non spherical aerosols.
• Measurement of mass and mobility in a single scan.
- However some bipolar distributions will not be resolved.
• Wide measuring range.
• Further work would enable a faster continuous scan instead of step scan.
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Questions and acknowledgements
Acknowledgements:
• Matt Pengelly, Summer Student 2019, Cambustion
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