Measurement of Engine Exhaust
Particle Size
David B. Kittelson
Center for Diesel Research
University of Minnesota
presented at
University of California, Davis
17 February 2000
Background
• Current emission standards are mass based. Recently interest
in other measures, i.e, size, number, surface, has increased.
• Concerns about particle size
– New ambient standards on fine particles
– Special concerns about ultrafine and nanoparticles
– Indications that reductions in mass emissions may increase number
emissions
• Difficulties associated with measurement of ultrafine and
nanoparticles
– Often more than 90% of particle number are formed during exhaust
dilution
– Particle dynamics during sampling and dilution are highly nonlinear large changes in of particle number may result from small changes
dilution and sampling conditions
Typical Diesel Particle Size Distribution
0.18
PM10
Dp < 10 µm
0.14
0.12
0.9
µ
Ultrafine Particles
Dp < 100 nm
0.16
1
Fine
Particles
Nanoparticles
Dp < 50 nm
0.8
0.7
0.6
Fractional deposition of particle
with density of 1 g/um
0.1
0.5
0.08
0.4
0.06
0.3
0.04
Nuclei
Mode
0.02
0.2
Accumulation
Mode
Coarse
Mode
0.1
0
0.001
0
0.010
0.100
1.000
10.000
Diameter (µ
µm)
Mass Weighting
Number Weighting
Alveolar Deposition Fraction
Alveolar Deposition Fraction
Normalized Concentration, dC/Ctotal/dlogDp
0.2
Typical Diesel Particle Size Distribution - Log Scale
Typical Engine Exhaust Size Distribution
Both Mass and Number Weightings are Shown
Normalized Concentration,dC/Ctotal/dlogDp
1
Fine Particles
Dp < 2.5 µm
Nanoparticles
Dp < 50 nm
0.1
PM10
Dp < 10 µ m
0.01
0.001
0.0001
0.00001
0.001
Nuclei
Mode
0.010
Ultrafine Particles
Dp < 100 nm
Coarse
Mode
Accumulation
Mode
0.100
1.000
Diameter (µ m)
Mass Weighting
Number Weighting
10.000
Health
• Correlations between fine particles and excess deaths
• Increased asthma in children living near roadways
• Special concerns about ultrafine and nanoparticles
Special Concerns about Nanoparticles and
Health
• Increased deep lung deposition
• Increased number and surface area at same mass exposure
• Particles which are non-toxic in µm size range may be
toxic in nm range
– Rats exposed to the same mass of 0.25 and 0.02µm diameter ΤiΟ2
retain more nanoparticles in the interstitial tissue of the lung and
develop marked inflammatory response. (Seaton et al., 1995).
– Comparison of surface free radical activity of ambient PM10
particles, and 0.5 and 0.025 µm diameter TiO2 showed significant
activity for PM10 and much more activity for 0.025 than for 0.5µm
TiO2 (Donaldson et al., 1996).
– Modest concentrations of 0.03µm Teflon fume particles caused
acute pulmonary toxicity in rats (Ferin et al., 1992).
A Recent HEI Study Gave a Surprising Result for a
Low Emission Engine
• Particle concentrations and size distributions were measured
for a 1988 and a 1991 engine
– When run on very low sulfur fuel particle number emissions were much
higher, 30 -100 times, from the new engine although mass emissions
were about 3 times lower
– The 1988 engine produced high number emissions, but not as high as
the 1991 engine when run on a 1988 type fuel (higher sulfur)
– This raised concerns that new engines might be producing large
numbers of nanoparticles while still meeting mass emission standards
and focused attention on nanoparticle emissions
• However reviews of measurements made on and near
roadways in the 70’s and 80’s show high nanoparticle
emissions. High nanoparticle emissions may be a problem
but are not a new development!
Number Size Distribution Data from HEI
Report and 1979 CRC Roadway Study
1.0E+10
3
dN/d(log(Dp)) (part./cm )
1.0E+09
1979 Roadway measurments
1.0E+08
Old engine, low S fuel
1.0E+07
New engine, low S fuel
1.0E+06
0.0010
0.0100
0.1000
Particle Diameter (µ
µ m)
1.0000
Particle Composition and Structure
Particles consist mainly of highly agglomerated solid carbonaceous
material and ash and volatile organic and sulfur compounds.
Solid Carbonaceous / Ash Particles with
Adsorbed Hydrocarbon / Sulfate Layer
Sulfuric Acid Particles
0.3 µm
Hydrocarbon / Sulfate Particles
Typical Composition of Diesel Particulate
Matter: Lube oil contributes to SOF, ash, sulfate
13%
7%
Carbon
Ash
Sulfate and Water
10%
60%
10%
Lube Oil SOF
Fuel SOF
Particle Formation History:
From the Start of Combustion to the Nose
Carbon formation/oxidation
t = 2 ms, p = 150 atm.,
T = 2500 K
Formation
Ash Condensation
t = 10 ms, p = 20 atm.,
T = 1500 K
Exit Tailpipe
t = 0.5 s, p = 1 atm.,
T = 600 K
Increasing
Time
Atmospheric Aging
Exposure
Sulfate/SOF
Nucleation and Growth
t = 0.6 s, p = 1 atm.,
Fresh Aerosol over
D = 10, T = 330 K Roadway–Inhalation/Aging
t = 2 s, p = 1 atm.,
D = 1000 T = 300 K
University of Minnesota
Significant Gas to Particle Conversion Takes
Place as the Exhaust Dilutes and Cools
• More than 90% of the particle number may form
through homogeneous nucleation of nanoparticles
• From 5 to more than 50% of the particle mass may
form through adsorption, absorption, and nucleation
• These processes are extremely sensitive to dilution
conditions
• Thermodynamics and fluid mechanics of mixing
during dilution are very complicated and vary widely
Atmospheric Dilution Leads to Nucleation,
Absorption, and Adsorption
A dilution ratio of 1000 may be reached in 1 - 2 s
Studies of Diesel Nanoparticle Formation Using a
Variable Residence Time Dilution System
Heated or Cooled
Compressed Air
Compressed
Air
Exhaust
Pipe
Pressure
Gauge
Primary
Orifice
Emission
Rack, CO2,
NOx
Air Inlet
Thermocouple
Pressure
Gauge
Retractable
tubing
Overflow
Bypass
Diluted
NOx
SMPS
Three Way
Valve
Primary
Ejector
Pump Humidity and
temperature
Sensor
Sampling
Probe
Engine
Thermocouple
Insulated Variable
Residence Time
section
Secondary
Orifice
Secondary
Ejector
Pump
Three Way
Valve
Tygon
Flexible
Tube
Overflow
Bypass
CPC
Sensitivity of Diesel Particle Size Distribution to
Dilution Conditions - Residence Time Effects
1.00E+09
900 ms
Heavy-Duty Diesel
DN/DLog Dp (Part./cm³)
Medium Load and Speed
1.00E+08
400 ms
1.00E+07
90 ms
Primary dilution ratio 12
Temperature 48 °C
1.00E+06
1
10
100
Dp (nm)
1000
Sensitivity of Diesel Particle Number Emissions to Dilution
Conditions - Residence Time and Temperature Effects
7.00E+08
Heavy-Duty Diesel
Medium Load and Speed
Primary Dilution Ratio = 12
6.00E+08
5.00E+08
4.00E+08
Number
Concentrations
(Part./cm³)
3.00E+08
2.00E+08
900
1.00E+08
400
0.00E+00
90
33
50
Dilution Temperature (°C)
65
Residence Time
(ms)
Nanoparticle formation downstream of an exhaust
filter may be even more sensitive to dilution conditions
DN/DLog Dp (Part./cm³)
1.00E+09
Upstream
1.00E+08
1.00E+07
Mode 6
1.00E+06
Downstream
Peak torque
1.00E+05
speed, full load
1.00E+04
Figure 11
1.00E+03
1
10
100
1000
Dp (nm)
Upstream, Res. 40 ms
Upstream, Res. 6000 ms
Dow nstream, Res. 40 ms
Dow nstream, Res. 6000 ms
Our results suggest nanoparticles are mainly volatile,
we have demonstrated this with a catalytic stripper
Typically more than 99% of the nanoparticles
disappear by 300 C (except with metal additives)
DN/DLog Dp (Part./cm³)
1.000E+10
Engine
Out
(Res.1000
1000ms)
ms)
Engine
Out
(Res.
1.000E+09
Engine Out (40 ms)
Stripper– 160 °C
1.000E+08
1.000E+07
Stripper - 295 °C
1.000E+06
1
10
100
1000
Dp (nm)
Engine Out-1000 ms
Engine Out-40 ms
Stripper-160 °C-1000 ms
Stripper-295 °C-1000 ms
How do nanoparticles form during dilution? Driving
force for gas-to-particle conversion is saturation ratio
The saturation ratio, S, is defined as:
p
S=
pv
where p is the partial pressure and pv is the vapor pressure of the volatile component. The dilution ratio,
D, is defined as:
Q
D = mix
Qexh
where Qmix and Qexh refer to the standard volumetric flows of diluted and raw exhaust, respectively.
Also:
1
p∝
D
and
pv ∝ e
− ∆H
RTmix
so that
S∝
1
− ∆H
RTmix
De
and for adiabatic dilution.
For typical diesel exhaust conditions, saturation
ratio may not be high enough for nucleation, S > ~ 3
Influence of Dilution Upon Extractables
0.45
Assumes extractable mass of
8 mg/m3 is nonodecane
Tex = 240 C, Ta = 27 C
0.4
Saturation ratio
0.35
0.3
0.25
These levels are too low for nucleation
but would drive absorption and adsorption
0.2
0.15
0.1
0.05
0
1
10
Dilution Ratio
Sat. Ratio
Adiab. Sat. Ratio
100
What does nucleate? Consider the threshold for
Binary H2SO4-H2O nucleation
• An extreme dependence of the nucleation rate upon temperature, H2SO4,
and H2O concentrations makes theoretical predictions of nucleation
difficult.
• Seinfeld and Pandis give an empirical expression to predict the onset of
nucleation in this system:
Ccrit = 0.16exp(0.1T - 3.5RH - 27.7)
where Ccrit is the threshold H2SO4 concentration in µg/m3, T is temperature,
RH is relative humidity (0 to 1).
• I have used this expression and mass and energy balances applied to the
dilution process to predict the ratio of the actual H2SO4 concentration to the
critical one, C/Ccrit. When this critical concentration ratio exceeds one,
nucleation is likely.
Influence of Fuel and Exhaust Conditions
on Sulfuric Acid Nucleation
100
Tex = 150 C
Fuel sulfur conversion to sulfate = 3%,
240 C
Critical Concentration Ratio C/C crit
330 C
Air-fuel ratio = 30, Tair = 27 C, RH = 40%
10
500 ppm Fuel Sulfur
Nucleation likely for
C/C crit > 1
Tex = 150 C
1
240 C
330 C
0.1
5 ppm Fuel Sulfur
0.01
1
10
100
Dilution ratio
1000
10000
H2SO4 Nucleation Is Suppressed by
Adsorption on Carbon Particles
1.00E+02
Increasing dimensionless adsorption rate
Concentration Ratio C/Ccrit
1.00E+01
H2SO4 / H2O nucleation
1.00E+00
500 ppm S fuel, 3% conversion
Texh =330 C, Tair = 27 C, RH =40%
Nucleation suppressed by increasing
carbon concentration or slower dilution
1.00E-01
1.00E-02
1
10
100
1000
10000
Dilution Ratio
Rads = 0
0.01
0.02
0.08
0.27
0.92
3.20
Nanoparticle Nucleation and Growth
• It appears that with current engines binary sulfuric
acid - water nucleation triggers the process.
• The initial size of these nuclei is about 1 nm.
• In most cases there is not enough sulfuric acid present
in the exhaust to explain the observed rates of particle
growth.
• Hydrocarbons normally associated with the soluble
organic fraction apparently are absorbed by the
concentrated sulfuric acid nuclei leading to the
observed growth rates.
Summary Diesel - 1
• A significant amount of particulate matter (e.g. 90 % of the
number and 30% of the mass) is formed during exhaust
dilution from material present in the vapor phase in the tailpipe
(e.g., sulfuric acid, fuel and oil residues).
– New particles are formed by nucleation. This is likely to be the source
of most of the ultrafine and nanoparticles (and particle number)
associated with engine exhaust.
– Preexisting particles grow by adsorption or condensation.
– Nucleation and adsorption are competing processes. Soot agglomerates
provide a large surface area for adsorption that suppresses nucleation.
Thus, engines with low soot mass emissions may have high number
emissions.
Summary Diesel - 2
• Nanoparticle measurements are very strongly influenced by
the sampling and dilution techniques employed.
– Nucleation, adsorption, absorption, and coagulation during sampling
and dilution depend upon many variables, including dilution rate, (or
residence time at intermediate dilution ratio), humidity, temperature,
and relative concentrations of carbon and volatile matter.
– Changes of more than two orders of magnitude in nanoparticle
concentration may occur as dilution conditions are varied over the
range that might be expected for normal ambient dilution, e.g., 0.1 to 2
s dilution time scales.
• Sampling systems should mimic atmospheric dilution to obtain
samples representative of the tailpipe to nose process.
The most difficult problem associated with exhaust particle measurement
is understanding the dilution process from tailpipe to nose -- this is being
examined in the CRC E-43 program
This illustration shows a typical application of the University
of Minnesota Mobile Aerosol laboratory developed for the
CRC E-43 Program
Nanoparticles are produced by new engines, but
concentrations may be much lower than feared
1.0E+06
Hard Acceleration
dN/d(log(Dp)) (part./cm3)
1.0E+05
Cruise
1.0E+04
1.0E+03
1.0E+02
1
10
100
Particle Diameter (nm)
1000
Summary Diesel - 3
• Currently most of the particles in the nanoparticle size range
are volatile. However, as engines become cleaner, metallic
ash particles from the lubricating oil (or fuel if metallic
additives are present) may become more important.
– Solid particles raise new issues…
– But they are easy to control with exhaust filters
• Spark ignition engines typically emit smaller particles than
diesel engines and are an important source of fine particles and
nanoparticles.
– A recent study in Colorado concluded that up to 2/3 of the fine particle
mass emitted by vehicles was from spark ignition engines
– New gasoline direct injection engines emit much higher particle
concentrations than conventional engines and may approach diesel
levels under some conditions
Particles from Spark Ignition Engines - Approximate
Composition of Exhaust Particulate Matter
Well Maintained Port Fuel Injection Engines
Unresolved complex
mixture (UCM)*
Ash
5% 5%
10%
Sulfates, carbon, etc.
Oxygenated and PAC
80%
* Includes branched and cyclic compounds
Nanoparticle Emissions from Spark Ignition Engines
Also Depend upon Dilution Conditions
Number Concentration, dN/dlog(Dp) [particles/cm3]
1.0E+09
DR = Dilution Ratio
DR = 10.9
DR = 11.7
DR = 15.3
DR = 20.5
DR = 25.4
1.0E+08
1.0E+07
1.0E+06
1.0E+05
Port Fuel Injection, 4 Valve per Cylinder
1.0E+04
1
10
100
Particle Diameter, D p [nm]
1000
Port Fuel Injected Spark Ignition Engine Influence of Load and Additives
7.0E+13
Brake Specific Particle Number Emissions
[part./kW-hr]
non-additized
6.0E+13
OX13391C
OX13003L
5.0E+13
4.0E+13
3.0E+13
2.0E+13
1.0E+13
95% Confidence Interval: +/- ~10%
0.0E+00
0
5
10
15
20
25
30
35
40
Power [kW]
Figure 2. Brake Specific Particle Number Emissions as a Function of
Power with 95% Confidence Intervals
45
Port Fuel Injected Spark Ignition Engine - Influence
of Additives on Size Distributions
5.0E+06
non-additized
OX13391C
OX13003L
p)
4.5E+06
Exhaust Number Concentration, dN/dlog(d
[particles/cm 3 ]
4.0E+06
3.5E+06
3.0E+06
2.5E+06
2.0E+06
1.5E+06
1.0E+06
5.0E+05
0.0E+00
0.001
0.01
0.1
1
Particle Diameter, dp [µ
µ m]
Figure 3. Representative Baseline Size Distributions for the OX13391 and
OX13003 Additives [2500 RPM, 90 kPa]
Summary - Particle Emissions from Port Fuel
Injection (PFI) Spark Ignition Engines
• PFI engine exhaust particles are quite different from diesel
particles
– They usually smaller
– They are composed primarily of volatile materials
– Lube oil may play an important role
• PFI emissions are strongly influenced dilution conditions
– Thus they are formed from volatile precursors during dilution
– Sulfuric acid-water nucleation and hydrocarbon absorption likely play a
role although direct nucleation of heavy hydrocarbon derivatives may
play a role
– Formation likely to be associated by local inhomogeneous conditions big droplets, crevices
Penetration Through Catalytic
Converter
Particle Number Penetration through the Catalytic
Converter on a Port Fuel injection Engine
1.0
0.8
0.6
0.4
0.2
Variable Speed and Load
0.0
0
10
20
30
Brake Power [kW]
40
50
Time-Resolved Polydisperse Number Concentration in
an SI engine Under Steady State Operating Conditions
2.0E+07
2.3L PFI SI Engine
2000 RPM, 55 kPa MAP
Exhaust Particle Concentration
[particles/cm3]
1.8E+07
1.6E+07
1.4E+07
1.2E+07
Navg = 1.11 x 10 6 particles/cm3
1.0E+07
8.0E+06
6.0E+06
4.0E+06
~ 5 x 105 part/cm3
2.0E+06
0.0E+00
0
500
1000
Time [seconds]
1500
2000
Stable and Unstable (Spiking) Particle Size
Distributions from a Spark Ignition Engine
Exhaust Particle Number Concentration,
dN/d(log dp) [particles/cm 3]
1.00E+09
1.00E+08
1.00E+07
Baseline Aerosol
Spiking Aerosol
DGNSPIKE = 11.4 nm
1.00E+06
1.00E+05
DGNBASELINE = 35.3 nm
1.00E+04
1.00E+03
0.001
0.01
0.1
Particle Diameter [ µ m]
1
Size Distributions for a GDI Engine
1.00E+09
dN/dlog(Dp) [particles/cm3]
1.00E+08
1.00E+07
1.00E+06
13 km/hr
32 km/hr
50 km/hr
70 km/hr
90 km/hr
1.00E+05
10
100
Particle Diameter, D p [nm]
1000
Summary - Particle Emissions from GDI
Spark Ignition Engines
• These engines are likely to come into widespread use
in Europe and Asia
• Particles structure and emission rates are much
more like those from diesels than PFI engines
• Only limited studies of fuel and additive effects have
been done