Optical Properties of Aerosols
description
Transcript of Optical Properties of Aerosols
1LA on a smoggy day LA on a clear day
Optical Properties of Aerosols
ENVR 416Aerosol Technology
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Topics
•Definitions
• Extinction
• Scattering
• Visibility
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Introduction
• Aerosol scattering is responsible for many atmospheric events - sunsets - halos around the sun or moon - rainbows - white (extensive scattering from the surface) and black (complete scattering where light cannot penetrate) clouds - visibility degradation from pollution
• Aerosol light scattering is also a powerful method used by instruments that measure aerosol size and concentration - these instruments are sensitive and do not manipulate particles
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Light Scattering Regimes
Dp < 0.05 µm described by molecular scattering…aka “Rayleigh Scattering”
Dp > 100 µm described by geometric optics (diffracted, reflected, refracted rays)
0.05 µm < Dp < 100 µm Dp on the order of λ, described by “Mie Theory”
NOTE: All scattering can be derived via Mie Theory, developed by Gustav Mie in 1908 using Maxwell’s theory of Electromagnetic Radiation. Limiting cases such asDp << λ and Dp >> λ allow for simplifications to be made.
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Definitions
c = speed of light = 3x1010 cm/s = f*λ
For visible light, λ = 400-700 nm
m = refractive index relates the change in velocity that light experiences upon going from one medium to another (a material related property)
m = c/vp = speed of light in a vaccum/speed of light in a material, p
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Index of Refraction
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Index of Refraction
aimmaimm ''' 1
scattering absorption
Scattering portion measured with Snell’s Law:
1 2
'1m
'2m
1
2'2
'1
sinsin
mm
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Index of Refraction
Absorption often measured via spectrophotemtry
aA 4
Bulk absorption
aimmaimm ''' 1
0A0A
For electrically conductive material
For most aerosol particles
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Relative Index of Refraction (mr)
Used to describe a two phase system, i.e. a particle in air
p
m
m
pr V
Vmm
m
1m For air
1m For vacuum
mmr For aerosol particles in air
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Intensity of Light
areaunitpowerradiantI
__
2m
W
detector
Incident light
scattered light
Light arriving at a surface:
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Intensity of Light
Light from a point source:
anglesolidpowerradiantI
__
srW
A
4A
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Size Parameter (α)
d
- Added to simplify light scattering equations
- Makes α = ratio of particle size to wavelength of radiation
6 For dp on the order of mm
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Electromagnetic TheoryLight possesses wave/particle duality
we will treat it as the electric wave component of EM radiation
Light can be: 1) unpolarized (sunlight) 2) parallel polarized 3) perpendicular polarized
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Topics
•Definitions
• Extinction
• Scattering
• Visibility
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Extinction
Definition: the attenuation of light along an axis resulting from scattering and/or absorption
ParticlesExtinction is dependent upon the chemical composition of particles as well as particle size, shape, orientation and number.
LightExtinction is also dependent upon the polarization and frequency of theincident beam.
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Extinction
Mathematically, how do we quantify the results of extinction?
I0 I
eQdIN
dLdI
4
2
I
I
L
edLQdNIdI
0 0
2
4
LQdNII
e4ln
2
0
LQdN eeII 4
0
2
Lambert-Beer Law
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ExtinctionLQdN ee
II 4
0
2
Lambert-Beer Law
For a monodisperse aerosol:
ee QdN4
2
Extinction coefficient (L-1)Particle area
Extinction efficiency
# concentration
Represents fractional loss in intensity per unit length
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Extinction
particle aon incident lly geometricapower radiant particle aby absorbed and scatteredpower radiant
eQ
Extinction Efficiency eQ
• Represents the relative ability of a particle to remove light from a beam compared with blocking or interception by the projected area of the particle
• Does not have to approach 1… in fact:
50 eQ
SAe QQQ For polydisperse aerosols:
i
ieiie
QdN4
2
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Example Problem
If: 5.00
II md p m7.0 kmL 2 2eQ
What is: a) Number concentration in #/m3
b) Mass concentration in mg/m3 ?
LQdN eeII 4
0
2 Lambert-Beer Law
2m 3.85x10m 10466.3m 20005.0ln 213-1-4 Nxe
38 particles 105.4
mxN
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Example Problem
3
9
3
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mg 8.80
kgg 10
mkg 1000
6m 107.0particles 105.4 mm
xm
x
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Extinction
),,,,( is pe dshapeabsorptionscatteringfQ
Recall:
Therefore, there is no single equation to calculate eQ for all dp
mmmdQe m
05.0dfor
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p
2
2
24
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Extinction
For dp > 4mm 2eQ “Extinction Paradox”
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Extinction Paradox
Based on the condition that extinction must be observed at long relative distances
210d
For coffee cup 100 km(rarely observed condition)dobs >>
210d
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Beers Law (Mass Concentration)
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3dNC pm
p
m
dCN
6
3
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4
2
3
2 ddCQdN
p
mee
dQC
p
eme
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dLQC
L p
em
e eeII 2
3
0
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Topics
•Definitions
• Extinction
• Scattering
• Visibility
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Scattering• Responsible for optical effects caused by aerosols
• Basis for many aerosol measuring instruments
• Important for visibility and radiation balance
Think of an aerosol particle as a light source with its own angular distribution oflight intensity
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Scattering
Physical basis
• The scattering of EM radiation by any system is related to the heterogeneity of that system (the physics remains the same)
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Scattering
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ScatteringTwo cases
In this case, the whole particle “sees” the same E-fieldand scatters in phase
dp << (Rayleigh)
dp ~ (Mie)
In this case, the E-field is not the same for the entire particleand a complex interference pattern of scattered wavelets will result
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ScatteringRayleigh Region: dp<<
i2
2
2
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6p
4
s Icos12m1m
r8Nd
I
i2
2
2
24
6p
4
s,2 Icos2m1m
r8Nd
I
i2
2
24
6p
4
1s, I2m1m
r8Nd
I
Unpolarized light
Parallel to scattering plane
Perpendicular to scattering plane
31www.philiplaven.com
Inte
nsity
= 650 nm
dp = 0.02 mm
dp = 0.002 mm
Parallel Polarization
Perpendicular Polarization
Parallel Polarization
Perpendicular Polarization
i2
2
2
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6p
4
||s, Icos2m1m
r8Nd
I
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Mie Scattering
W.C. Hinds, Aerosol Technology: Properties, Behavior and Measurement of Airborne Particles, John Wiley and Sons, 1982
k = 10
k = 2
k = 0.8
k = 10
k = 2
k = 0.8
Rayleigh Regime
Mie Regime
Particle sizek =
pd
f||(ө,m,dp)
fL(ө,m,dp)
f ||, f
LSize Parameter
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Mie Equations• At a distance r in the direction Ө from a spherical particle the
intensity of scattered light is:
I(ө) =
IL(ө) =
I║(Ө) =
22
20
4),,(
rdmfI p
22
||2
0
8),,(),,(
rdmfdmfI pp
22||
20
4),,(
rdmfI p
Unpolarized light
Perpendicularly polarized
Parallel polarized
where f is a function of Ө, m and dp
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Mie Web Calculators
http://omlc.ogi.edu/calc/mie_calc.html
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http://omlc.ogi.edu/calc/mie_calc.html
Dp = 0.2mmm = 1.33
90º
Dp = 0.7mm
m = 1.33
Incident light = 532nm
Parallel
Unpolarized
Perpendicular
90º
0º180º
270º
0º180º
270º
k = 4.13 k = 1.18
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Mie Region dp ~ λ
Mars picture from Pathfinder
http://www.weatherstock.com/cloudcat.html
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Microchemistry: time dependence of and acid-base reaction in a single optically levitated microdroplet
M. Trunk, J. Popp, M. Lankers, W. Kiefer
Institut fur Physikalische Chemie Der Universitat WurzburgWurzburg, Germany
Chem. Phys. Lett. 264(1997) 233-237
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Experimental
• Optical levitation and Raman spectroscopy combined to study the following acid-base reaction:
NH3(g) + NH4C10H9O2(s)
• The appearance and position of MDRs in the Raman spectrum are monitored to determine change in droplet size due to processes such as evaporation and reaction.
Capric Acid
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Experimental Schematic
converter
mirror
mirror
mirror
mirrorspectrograph
Quartz plate
lenslens
lensargon-ion laser
lens
Interferencefilter
514.53 nm
Levitated dropletPhotodiode
t = 0
Spectrum Accumulation time ~ 1 sec
Droplet generation chamber
nebulizer
Observation chamber
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Optical Levitation
• The gravitational force exerted on a particle is balanced by photon pressure produced by a vertically directed laser beam
Fg
Flaser
cP Erad
where Prad is the radiative pressure, ΦE is theenergy flux, and c is the speed of light
Say for example, we have a particle with dp = 10 mm
Fg = mg = 5.14x10-12 N
Fg/A = 163.6 Pa = Prad
ΦE = 4.91x1010 Jm-2s-1
Given = 514.5nm, we need 3.88x1019 photons/sec to maintain levitation
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Morphology Dependent Resonance
• 355 nm light from Nd:YAG laser aligned with droplet edge optimizes coupling into a MDR
• Internally reflected light can circulate around circumference of the droplet on order of 10ns, provided an integral number of wavelengths circulate in the droplet
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Results
• Peaks that appear in the bulk case also appear after reaction between ammonia and the particle, indicating formation of the ammonium salt in or around particle
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C-H Stretching Region
Wavenumber (cm-1)
Ram
an In
tens
ity (a
rb. U
nits
)
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Laser Power Required for Levitation
Experimental
Theoretical
• Negative peaks correspond to MDRs
Post-Reaction time
• After the reaction, the particle size remains constant, and the required laser power for levitation will also remain constant
NH3(g) insertion
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Time (s)
Wav
e nu
mbe
r (cm
-1)
• This plot shows the movement of MDR #2 as a function of time
• From 0-200 s, evaporation occurs. When NH3 is introduced, the MDR moves to larger wavenumbers, indicating droplet growth via reaction
• After ~210 s, the MDR remains stationary, indicating droplet size change has ceased, and formation of ammonium salt has occurred at the surface
NH3(g) insertion
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Topics
•Definitions
• Extinction
• Scattering
• Visibility
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Visibility
http://www.dailymail.co.uk/news/worldnews/article-1215443/Australia-dust-storm-sweeps-eastern-coast.html
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Visibility
Visible range how far one can see in a given direction
Limited by:
1) Visual acuity
2) Contrast
Aerosol particles with 0.1 mm < dp < 1 mm reduce contrast by scattering light
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Contrast
''0
0 BBBC
Inherent contrast
Object luminance
Background luminance
Luminance luminous intensity per unit solid angle per unit area of surface
Units: lumens/m2•sr, cd/m2
r
r2
Total area = 24 r
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Typical Contrast Values
Sky near the horizon:
Clear day 104 cd/m2
Overcast night 10-4 cd/m2
White paper:
Sunlight 25,000 cd/m2
Overcast night 0.03 cd/m2
''0
0 BBBC
If '0 BB
00 C
If '0 BB 00 C
10 C For black object against white background
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What makes distant objects lighter (lower contrast)?
Aerosol particles!
Inherent contrast contrast that would exist w/o aerosol interference
CR = apparent contrast contrast that results when aerosol particles (scatterers) are present
''
R
RRR B
BBC 0CCR In the limit of no aerosol
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Koschmieder’s Law
Luminance Loss(scattering + absorption)
Luminance Gain (sunlight)
RB
B
L
ea
dxBB
dB
0 0
L
R
RR
eeBBBC
''0 L
ReeCC 0
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Perfect viewing
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Threshold of Brightness Contrast (ε)
LR
eeCC 0veLeC 0
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SIZE-RESOLVED MEASUREMENTS OF LIGHT SCATTERING BY AMBIENT PARTICLES IN THE
SOUTHWESTERN U.S.AWARREN H. WHITE and EDWARD S. MACIAS
Chemistry Department, Washington University, St Louis, MO 63130, U.S.A
ROBERT C. NININGERAerovironment Inc., Monrovia, CA 91016, U.S.A
andDAVID SCHORRAN
Desert Research Institute, Reno, NV 89506. USA
.4tmospheric Environment, Vol. 28. No 5, pp. 909 -921. 1994
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• Goal to look at extinction contribution from coarse particles
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