Transcript Slide 1

Atmospheric RadiationTransfer:
And Sun Photometers
Madhu Gyawali
and
Pat Arnott
ATMS 360 Atmospheric Instrumentation
Univ NV Reno
Outline
 Solar and Terrestrial Spectrum
Modification of solar radiation reaching to the earth’s surface
 Radiation transfer in the Earth’s Atmosphere
Energy balance
Interaction with gas molecules and Aerosol
 Sun Photometer
Measurement of Aerosol Optical Depth
Infrared
From: http://www.sonoma.edu/users/f/freidel/global/. What’s missing?
Three Choices for Radiation
Emissivity is the same as absorptivity. Source can be visible or
infrared radiation, or other wavelengths as well.
Climate consequences of these choices….
(from www.ldeo.columbia.edu/.../solar_radiation)
Earth’s Surface Temperature


s

2
Te T Rs 2 (1 a)
Rse 2(1t)




1/4
t  0, Te  303K (Greenhouse Max)
t 1, Te  255 K (No Atmosphere)
t  0.2, Te  289 K (Just Right)
Te
Earth’s radiative temperature
Ts
Sun’s radiative temperature
Rs
Sun’s radius
Rse Sun to Earth distance
a
Earth’s surface solar reflectance
t
IR transmittance of Earth’s atmosphere.
Spectrum of Solar Radiation Flux
• The sun emits 41% of its radiation in the visible spectrum,
• 9% in the ultraviolet spectrum
• 50% in the near infrared spectrum
Spectral View of the Earth’s Radiation Balance from Space
SOLAR SPECTRUM:
TOP OF THE ATMOSPHERE
SOLAR SPECTRUM:
TOP OF THE ATMOSPHERE AND AT THE SURFACE
SOLAR SPECTRUM:
Effects of Rayleigh (gas) scattering, O2 and N2.
SOLAR SPECTRUM:
Effects of Rayleigh (gas) scattering, O2 and N2,
And effects of extinction by aerosol particles.
SOLAR SPECTRUM:
Effects of gaseous absorption.
From http://www.lib.utah.edu/services/prog/gould/1998/Figure_5.gif
Infrared Spectrum from the
Atmosphere to the Surface
CO2
H20
O3
CH4
Spectrum of Solar Radiation Flux
O3
O2
H2O
H2O ,CO2
.1 . 3
From Cunningham & Cunningham, 2004,
.5
1
1.5
2
 ( m) 
2.5
3
Global Energy Balance
Incoming = 45 +88 = 133
Outgoing = 104 + 24 + 5 = 133
From Cunningham & Cunningham, 2004,
Fig. 9.2
Major Atmospheric Windows
Composition of the atmosphere at ground
level
Gas
Concentration
% or ppm
Nitrogen(N2)
Residence Time
----
78.084%
Oxygen(O2)
20.94%
----
Argon(Ar)
.934%
---
Water(H2O)
.4 to 400
10 days
Carbon dioxide(CO2)
370 ppm
4 years
Ozone(O3)
10-100ppbv
Days-week
Methane(CH4)
1.75 ppm
10 years
Helium(He)
5.24 ppm
2. 10^6
Krypton(Kr)
1.14 ppm
---
Hydrogen(H2)
.4 to 1
---
Xenon(Xe)
.087 ppm
---
based on Junge, 1963; Andrews et al)
What are Aerosols?
 Definition;
Aerosols are tiny particles suspended in air, either in solid phase or liquid
phase or both .
Concentrations;
The highest concentrations are usually found in urban areas, reaching up
to 108 and 109 particles per cc (Seinfeld and Pandis, 1998).
 Size;
Aerosols range in size from around .001µm(molecular cluster) to 100
µm(small rain drop)
10µm
Human Hair(65 µm diameter)

Source; Thermo electron corporation
2.5µm
Aerosol Sources
 Primary and Secondary
Primary particles – introduced directly into the
atmosphere (e.g. smoke from combustion)
Secondary particles – formed by chemical
reactions in the atmosphere (e.g. gasto-particle conversion)
 Natural and Anthropogenic Aerosol
•
Sulfates, Soot
•
Biomass Burning
Natural – dominates in rural (remote)
areas
Anthropogenic – dominates in urban
areas
Sea Salt
Sources of Atmospheric Aerosol
Amount, Tg/yr [106 metric tons/yr]
NATURAL
Range
Best estimate
Soil dust
1000 - 3000
1500
Sea salt
1000 - 10000
1300
26 - 80
50
4 - 10000
30
3 - 150
20
100 - 260
180
40 - 200
60
2200 - 24000
3100
Botanical debris
Volcanic dust
Forest fires
Gas-to-particle conversion
Photochemical
Total for natural sources
ANTHROPOGENIC
Direct emissions
Gas-to-particle conversion
Photochemical
Total for anthropogenic
sources
50 - 160
120
260 - 460
330
5 - 25
10
320 - 640
460
(Data from: W.C. Hinds, Aerosol Technology, 2nd Edition, Wiley Interscience)
Effects Of Aerosol
 Direct effect —Scattering and absorption of radiation
 Indirect effect —Roles in cloud micro physics
Clean cloud
Large cloud droplets
Low albedo
Efficient precipitation
Polluted cloud
Small Cloud droplets
High albedo
Suppressed precipitation
Aerosol Optical Properties
 Optical thickness;τ(λ)
τ(λ)=

z2
z1
e
( z )dz
where e (z) is the extinction coefficient
and is the sum (s  a ) of scattering and absorption coefficient
It is the indirect measure of the size and number of particles present in a
given column of air.
 Phase function ;P(Θ,λ)
It describes the angular dependence of light scattering.
Aerosol Optical Properties
Single scattering albedo; 0 ( )
0 ( )
Scatteringcoefficient (  s )
=
Extinctioncoefficient (  s   a )
 The magnitude of single scattering albedo largely depends on the
complex part of refractive index, and particle size.
 It determines the sign(cooling/heating , depending on the planetary
albedo) of the aerosol radiative effect. Cooling when the value is larger
than about 0.85, and warming when it is below this value.
Optical Properties of Small Particles
µ= n + ik
µ = complex index of refraction
n = scattering (real part)
k = absorption (imaginary part)
The real part of the index of refraction is only a weak
function of wavelength, while the imaginary part, ik,
depends strongly on wavelength.

Seinfeld & Pandis, Atmospheric Chemistry and Physics,
Refractive indices of aerosol
particles at  = 589 nm
Water
1.333
10-8
Ice
1.309
10-8
NaCl
1.544
0
H2SO4
1.426
0
SiO2
1.55
0
Black Carbon
(soot)
1.96
0.66
Mineral dust
~1.53
~0.006
(seinfield,et al)
Scattering;
(Redirection of radiation out of the original direction)
 Rayleigh Scattering: Scattering from small
particles(comparison to the wavelength). Most effective for shorter
1
wavelengths,
s 
4
Scattering from atmospheric gases are well understood since major
gases (nitrogen and oxygen) that comprises 99% of the atmosphere are
well mixed
The effects due to aerosol scattering are quite variable due to wide range
of aerosol concentration and to the variety of aerosol found in the
atmosphere.
Particle scattering;
It occurs mostly in the lower portions of the
atmosphere where larger particles are more
abundant, and dominates when cloud conditions
are overcast
Nonselective scattering occurs when the particles
are much larger than the wavelength of the
radiation.
Rayleigh and Particle Scattering
Particle size parameter
x 
2R

Aerosol Radiative Effects
 Regional Haze, Air Quality and Visibility (COHA, FAQS)
 Photochemical Reaction (Atlanta Supersite)
 Photosynthesis and Crop Yields (ChinaMAP)
 Climate Change - Whitehouse Effect (ACE-Asia, ChinaMAP)
Directly - Scattering & Absorption of Solar
Radiation
Indirectly - Modifying Cloud Properties
Scattering and Absorption of Light by
Aerosols
Io=Light
Source (W/m2)
L=Path Length
I=Light
Detector (W/m2)
I
( sp   ap  eg ) L
 ext L
e
e
I0
  ( sp   ap ) * L;
   sp /( sp   ap );
 (b, g)
Scattering Model of an Aerosol Layer
F r a c tio n r e f le c te d u p w a r d
r  (1  e   ) 
F r a c tio n a b s o r b e d =
(1   ) (1  e   )
F r a c tio n s c a tte r e d d o w n w a r d
  (1   ) (1  e   )
F r a c tio n tr a n s m itte d = e  
T o ta l d o w n w a r d tr a n s m itte d f r a c tio n t= e   +  (1   ) (1  e   )
T o ta l r e f le c te d o f f s u r f a c e =  s t (  s = s u r f a c e a lb e d o )
F0= incident solar flux (wm-2)
t s
2
F   F0 (1  Ac )Ta [(r 
)   s ] Ac= fraction of the surface covered by clouds
1sr
Ta= fractional transmittance of the atmosphere
2
Aerosol Scattering and Absorption Coefficients
 sp ( ) 
D p ,max
  scat ( D p ,  , ri ) m( D p ) dD p
D p ,min
 ap ( ) 
Where:
D p ,max
  abs ( D p ,  , ri ) m( D p ) dD p
D p ,min
 = Wavelength (m)
Dp = Particle Diameter (m)
scat, abs = Mass Scattering and Absorption Efficiencies (m2/g)
ri = Refractive Index
m(Dp) = Aerosol Mass Size Distribution
Note: Aerosol Extinction Depends on Wavelength (Ångstrom Exponent, å = - d log ext / d log  ),
Chemical Composition, and Size
Major Aerosol Chemical Species that Contribute
to the Light Extinction
 Sulfate Aerosols
 SO2 from Fossil Fuel Combustion
 Carbonaceous Aerosols
 Organic Compounds (OC)
Biomass Burning, Automobile Emissions, Fossil Fuel Combustion,
Gas-to-particle Conversion of Hydrocarbons
 Elemental Carbon (EC) (Absorption, Warming Effect)
Incomplete Combustion of Fossil and Biomass Fuels
 Mineral Dust Aerosols
 Desert Dust, Construction, Road Dust
 Nitrate Aerosols
 Industrial and Automobile Emissions
Visibility Impairment of Aerosols Based on Aerosol
Chemical Speciation Data: IMPROVE Equation
 Bext = 3 * f(RH)* {[Ammonium Sulfate] + [Ammonium Nitrate]} +
4*1.4*[OC] + 10*[LAC] + 1*[Soil] + 0.6*[CM]+ 10 (Rayleigh Gas
Scattering)
[Sulfate] is the sulfate concentration, for example.
[OMC]=organic matter, [LAC]=light absorbing carbon [CM]=course
mass. f(RH)=hygroscopic growth factor.
 Visual Range (V.R.) = K/Bext
Where K is the Koschmieder Coefficient – the log of the contrast threshold of the human eye, K = 3 – 3.9
GOES View of the Dust Streak Across North America, April
17
GOES10 view of dust streak on the
morning of April 17
GOES8 view of dust streak on the
evening of April 17
29
Transport of the Asian dust to the United
States
The common weather conditions are usually associated
with the upper low pressure trough / cut-ff low and
surface low pressure system (low formed by a strong
cyclonic vortex) over northeast China and north Korea
[Kim et al., 2002]. Under this weather conditions, Asian
dust can move fast along the zonal wind distribution
due to the jet streak [Kim et al., 2002].
30
Origin of the Asian Dust
Strong low pressure system sitting in northeast Mongolia caused surface wind speeds to be as high as ~30 m/s
Given suitable weather conditions, dust can be lifted from the dry surface of the Asian Gobi desert region and
transported to the United States in about 7-10 days.
34
Optical Depth Measurement
Instrument: Sun Photometer, Technique: Beer’s Law
Light from the Sun causes the LED detector to generate a tiny electrical
current. This current goes to the operational amplifier , so that the LED current
is transformed into a voltage signal. This signal is then measured by an
attached digital voltmeter.
Source :www.http//patarnott.com
Beer’s Law
A connection between radiation at the top of
the atmosphere ( E0 ) and on the surface (E )
E( )  E0 ( ) exp[ ( )m]
is,

1
=
cos 
m
Langley plot method: calibration
Ln( E0 )
Top of the atmosphere
Ln( E)  Ln( E0 )  m ( )
Ln(E )
 ( )
0
...
1
m
Result: TOD from Langley plot method
8
y = -0.554x + 7.961
R² = 0.996
7.5
7
6.5
6
ln(V-Vd) 400nm
y = -0.4399x + 6.6943
R² = 0.9963
ln(V-Vd) 440nm
y = -0.3489x + 6.1819
R² = 0.9953
ln(V-Vd) 500nm
5.5
5
Ln(V-Vd)
4.5
4
3.5
air mass (m)
y = -0.1329x + 7.2993
R² = 0.9888
ln(V-Vd) 630nm
y = -0.0893x + 6.4771
R² = 0.986
ln(V-Vd) 840nm
y = -0.1153x + 5.1926
R² = 0.9845
ln(V-Vd) 920nm
3
1
1.5
2
2.5
3
3.5
4
Measurements from Sun Photometer and Spectrometer
AOD and Wavelength
4000
0.28
3500
0.23
0.6
Raw Data, 16:21,
12Sep07
Raw Data, 17:05
0.5
Optical Depth
Measured Spectrum (counts)
AOD
3000
2500
AOD(Sept2,2007)
AOD(sept3,2007)
0.18
0.13
2000
1500
0.4
0.3
0.2
1000
0.08
0.03
0.1
500
0
0
350
550
750
Wavelength
950
350
550
Wavelength (nm)
750
 ( AOD)  
Angstrom Coefficient:
 0
 4
very large particles
very small particles (Rayleigh regime)
  1.59
Ln( )
0
5.9
6
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
-0.2
Ln( ) -0.4
-0.6
Ln(AOD)
-0.8
Linear (Ln(AOD))
-1
-1.2
-1.4
-1.6
y = -1.598x + 9.394
R² = 0.967
Conclusion:
The interaction(scattering as well as absorption)of
solar radiation by atmospheric constituents is strongly
dependent on the nature of particles, size of particles,
as well as the wavelength of radiation.
The Sun Photometers offer an inexpensive as well as
convenient way of measuring aerosol optical depth.
By knowing the aerosol optical depth we can estimate
the size of suspended particles.
THANK YOU FOR YOUR ATTENTION !!!