SMPS. Inhalt. Atmosphärisches Aerosol Messgeräte : SMPS, DMA & CPC, DMPS, PM10 oh je oh je Datenauswertung Ausblick : FOX. Atmosphärisches Aerosol. Atmosphärisches Aerosol. Aerosole. Wie gross sind Aerosole ?. Typical Aerosol Particle Size Ranges.
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SMPS
DMPS, PM10 oh je oh je
Typical Aerosol Particle Size Ranges
Cloud droplets:
~ 4 - 100µm
Fog droplets:
~ 5 - 20µm
From Friedlander: Smoke, Dust and Haze
How do we measure particle number and particle size ?
In-situ: man misstwo die Partikelsind
Ex-situ: man misst von ausserhalb (z.B. Radar)
Important instruments in aerosol technology are Condensation Particle Counters (CPC).
They are used to measure the particle number concentration down to the nanometer size range.
The particles are enlarged due to supersaturation and a subsequent condensation of a condensable gas (normally Butanol, now also water!). The particles reach a size at which they can be optically detected.
The number concentration is measured for all particle larger than the lower detection diameter.
Modern CPCs operate with continuous aerosol flows and are able to count each single particle.
Model TSI 3010, 3760, 3762,…:
Principal: continuous flow, single particle counting
Lower detection Buthanol: 10 nm (Model 3025: 3 nm,)
diameter: Water CPC: 5nm
Upper detection
diameter: ca. 3000nm
Concentration
range: 0-10,000 cm-3 (Model 3025: 0-105 cm-3)
Accuracy: 10% compared to a reference instrument
Aerosol flow: 1.0 l/min
The aerosol flow is saturated with butanol in a slightly heated saturator.
The the temperature of the butanol-aerosol mixture is decreased by 17-27°C in the condenser of the CPC.
Here, the butanol become supersaturated and condenses onto the particles.
The particles grow to droplets of several µm in diameter.
The droplet flow is focused in a nozzle and introduced into a counting optic.
The droplets pass a laser beam, and each single particle creates a light pulse.
Pulses with an amplitude above a certain threshold are counted.
The particle number concentration can be calculated by knowing the aerosol flow rate
(critical orifice).
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Unser Massband:
Differential Mobility Analyser
Apfel: Durchmesser
Soot: ???
Partikel mit gleicher electricalmobility
APS
SMPS
Partikel mit gleicher electricalmobility
2. Particle Size - Differential Mobility Analyzer
2.1 Electric Mobility
Electrically charged particles move in an electric field according to their electrical mobility.
The electrical mobility ZP of a particle with a certain electric charge is defined to (given in [cm2/Vs]):
With ve derived analog to the sedimentation velocity
The electrical mobility depends mainly on the particle size and electricalcharge.
The smaller the particle the higher is the electrical mobility.
The higher the electrical charge the higher is the electrical mobility.
n = number of charges
e = elementary charge
B = particle mobility (“velocity per unit force”)
2. 2 Funktionsweise
Assumption:
All particles carry only one electrical charge.
The electrical mobility is than only a function of the particle size in case of constant temperature and pressure.
Example:
An electrically charged polydisperse aerosol is led through a plate capacitor.
Electrically charged particles are separated and deposited according their size.
2. Particle Size - Differential Mobility Analyzer
Plate mobility analyzer:
aerosol
d
sheath air
2.3 Theory of a plate mobility analyzer:
The most simple mobility analyzer is a plate capacitor.
A laminar particle-free sheath air flow Qsh is led through the capacitor (x-direction). An electric field is put between the plates (z-direction).
The aerosol flow QA (x-direction) is fed into the capacitor close to one plate.
The total volume flow is
The particle velocity in x-direction is given to:
The particle velocity in z-direction is defined to:
with
w = width of the capacitor
d = distance between plates
The electrical mobility for a certain deposition place is given to:
The voltage to select a certain mobility can be calculated by:
2.4 Theory of cylindrical mobility analyzer:
:
Folie neu drucken
2.4 Theory of cylindrical mobility analyzer:
:
The theory is analogous to the plate capacitor. The total flow is given to:
The particle velocity in x-direction is given to:
The radial velocity due to the electric field is described to:
with
The electrical mobility for a certain deposition place is given to:
The voltage to select a certain electrical mobility incl. the electrical mobility from Stokes‘ law can be calculated to:
The particle size for a certain deposition place and voltage cannot be analytically solved:
2. Differential Mobility Analyzer
The voltage to select a certain electrical mobility incl. the electrical mobility from Stokes‘ law can be calculated to:
e = elemental charge 1,602·10-19 As
n = number of charges
The particle size for a certain deposition place is:
∆DP= 1 nm or DP =10nm ± 0.5nm
2. Differential Mobility Analyzer
Aerosol particles can be classified due to their electrical mobility in a DMA.
A small volume flow Qs with particles of a defined mobility is taken out of the DMA through a slit at the end of the inner rod.
The mean mobility of these particles can be calculated to:
The ideal width of the mobility bin is described to:
for
QA=QS and QA =1/10 QSh
2. Differential Mobility Analyzer
Example:
DP= 10 nm
with ZP = 2.078.10-2 cm/Vs and ∆ ZP = 4.156·10-3 cm/Vs
∆DP= 1 nm or DP =10nm ± 0.5nm
the size resolution is excellent!
The size resolution depends mainly on the ratio of the volume flow rates QA/QSh.
The greater the ratio, the better becomes the size resolution.
2. 5 Transfer function
The transfer probability over the mobility bin is not unity.
The DMA-transfer function depends on the particle size and sample flow ratio.
Example:QA= Qs
The transfer function has the form of a symmetric triangle.
The transfer probability of the mean electrical mobility is unity.
The transfer probability of the upper and lower limit of the mobility bin is Zero.
For QA>QS und QA<QS, the transfer function becomes asymmetric. This cases are not discussed, because they are not the standard applications.
2. 5 Transfer function
Ideal transfer function:
2. 5 Transfer function
2.6 Generator for Monodisperse Particles
How do we measure particle number and particle size ?
Combination of DMA+CPC (most common application
for atmospheric measurements)
Electrical Mobility Spectrometer
Electrical Mobility Spectrometer
A electrical mobility spectrometer can measure a size distribution only for a certain size range.
This size range depends on the DMA-geometry and the sheath air flow rate.
Longer DMA larger particle diameter
Higher sheath air flow rate smaller particle diameter
Computer inversion routine:
Differential Mobility Particle Sizer (DMPS)
A pre-impactor removes all particles larger than the upper diameter of the size range to be measured
The particles are brought in the bipolar charge equilibrium in the bipolar diffusion charger.
A computer program sets stepwise the voltage for each selected mobility bin.
After a certain waiting time, the CPC measures the number concentration for each mobility bin.
The result is a mobility distribution.
The number size distribution must be calculated from the mobility distribution by a computer inversion routine.
Scanning Mobility Particle Sizer (SMPS)
The design of the system is identical to the DMPS.
The difference lies in the measurement principle.
The voltage is continuously increased.
There is no waiting time any longer.
The particle concentration is measured as function of time.
The relationship between electrical mobility and time (time between DMA entrance and CPC detection) must be determined for each SMPS system.
The results is again a mobility distribution.
The number size distribution must be calculated from the mobility distribution by a computer inversion routine.
Scanning Mobility Particle Sizer (SMPS)
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Scanning Mobility Particle Sizer (SMPS)
Particle Size Distributions
Particle Size Distributions
Particle Size Distributions
Assuming spherical geometry and dDp0
dS(Dp) = Dp2n(Dp)dDp
dV(Dp) = (/6)Dp3n(Dp)dDp
Area, Volume-Mass Distributions
Heterogeneous and multiphase reaction rates depend on surface area or volume, respectively.
Gravitational settling rates depend on mass and air quality standards are mass-based.
Typical Number Distribution for Urban Aerosols
Solid line: what would be observed, composed of 3 modes
Dotted/Dashed lines: Two common parameterizations
Indirecteffectofaerosols on climatepoorlyknown – especiallytheanthropogeniceffect
Zürich
From Jordi 2004
Image: courtesyofHanna Herich (adapted)
Droplets: 5-20mm (Wanner, 1979)
Total aerosol
Cloudresiduals Interstitial aerosols
Droplets
Droplets: 5-20mm (Wanner, 1979)
Droplets:
Diurnal / weeklycycle in thedropletconcentration?
Total aerosol:
Physicalproperties CCN activation
Cloudresiduals:
Anthropogenicmarkers
(metals & carbon)
?
?
Weekly & Daily cycle Industrial snow
Droplets: 5-20mm (Wanner, 1979)
Droplets:
Diurnal / weeklycycle in thedropletconcentration?
Fog Monitor
Total aerosol:
Physicalproperties CCN activation
SMPS & CCNC
Cloudresiduals:
Anthropogenicmarkers
(metals & carbon)
CVI /Massspectrometer
?
?
Weekly & Daily cycle Industrial snow
Additional: PM1, PM10 & gas phasemeasurements, anemometer (wind), ceilometer
Satellite observations: distribution & frequencymaps(Bachman & Bendix, 1993; Jordi, 2004)
garbage incineration plant
AMSL
CVI inlet: cloudresiduals
Mass Condensation
SpectrometerParticle
(chemicalCounter
Analysis)
Total
Inlet:
Scanning Cloud
Mobility Condensation
ParticleNuclei
SizerCounter
(Aerosol size
distribution)
CVI
Dryer
Dryer
Dropletmeasurements:
Fog Monitor
The aerosol flow is saturated with butanol in a slightly heated saturator.
The the temperature of the butanol-aerosol mixture is decreased by 17-27°C in the condenser of the CPC.
Here, the butanol become supersaturated and condenses onto the particles.
The particles grow to droplets of several µm in diameter.
The droplet flow is focused in a nozzle and introduced into a counting optic.
The droplets pass a laser beam, and each single particle creates a light pulse.
Pulses with an amplitude above a certain threshold are counted.
The particle number concentration can be calculated by knowing the aerosol flow rate
(critical orifice).
Modern CPCs operate with continuous aerosol flows and are able to count each single particle.
Model TSI 3010, 3760, 3762,…:
Principal: continuous flow, single particle counting
Lower detection Buthanol: 10 nm (Model 3025: 3 nm,)
diameter: Water CPC: 5nm
Upper detection
diameter: ca. 3000nm
Concentration
range: 0-10,000 cm-3 (Model 3025: 0-105 cm-3)
Accuracy: 10% compared to a reference instrument
Aerosol flow: 1.0 l/min
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8814
A.2 Aerodynamic Particle Sizer (APS)
The APS measures the aerodynamic particle diameter and can thus determine the aerodynamic particle size distribution.
Aerodynamic diameter:
The aerodynamic particle diameter is defined as:
„Diameter of a spherical particle with the density of One and the same sedimentation velocity of the measured particle".
= dynamic shape factor
0 = 1g/cm3, e.g. water
Specification of the APS:
The APS determines the aerodynamic number size distribution with a high time resolution.
The aerodynamic particle size range of the APS model TSI 3321 is between 0.5 and 20 µm.
Solid and non-volatile particles can be measured.
Schematic of the APS:
The main part of the APS are the acceleration nozzle and the laser anemometer.
Acceleration nozzle:
The acceleration nozzle consists of an inner and out nozzle.
The inner nozzle focuses the aerosol flow. The aerosol flow is then surrounded by the sheath air flow.
The entire flow is then accelerated through the outer nozzle.
The total flow rate of 5 l/min consists of 1 l/min aerosol flow and 4 l/min particle-free sheath air.
The velocity of the aerosol flow in the center is assumed to be constant.
Due to inertia, particles with a large aerodynamic diameter cannot follow the acceleration.
This means that particle with different aerodynamic diameters have different velocities directly behind the nozzle (calibrated instrument !).
Laser anemometer:
The laser anemometer measures the time-of-flight between two laser beams.
The laser beams are positioned directly behind the outer nozzle.
Particles passing the laser beams emit two light pulses.
The time difference between the two pulse maxima is the time-of-flight.
The time-of-flight is a measure for the aerodynamic particle diameter.
The relation between time-of-flight and aerodynamic particle size must be calibrated for each device.
Optical measurements:
Beside the determination of the aerodynamic particle size, the signal can also be taken to determine the optical diameter (see also Optical Particle Counter OPC).
Each particle is classified in relation to the refractive index of latex spheres.
B.3 Applications of DMPS-Systems
B.3.1 Twin Differential Mobility Particle Sizer (TDMPS)
A combination of two electrical mobility spectrometers allows to measure the size distribution of the entire submicrometer size range.
Hygroscopicity-Tandem-Differential-Mobility-Analyzer (HTDMA)
107
Sulfuric Acid
Sulfate
Particle
Number
Concentration
1/cm3
Organic
Sea Salt
105
Nitrate
Mineral
Carbonaceous
103
101
1
10
100
1000
10000
Particle Diameter (nm)
Elevated RH
Particle
Number
Concentration
Wet Dp
Dry Dp
Examples of HTDMA measurement