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.

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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.

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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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