Atmospheric Radiation Transfer And Sun Photometers 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 Equator at equinox sun overhead at noon L I0() Simple Idea of Sun Photometers: q 1. The sun’s output arriving at the top of the atmosphere doesn’t change from mid day to late afternoon. I0() 2. The path length of sunlight through the atmosphere changes during the day. 3. Equation with no clouds has two unknowns, so need at least 2 measurements. 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, q 1 = cos q 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 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. 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(1t) 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. Simple Model for Earth’s Atmosphere S0 = 1376 W/m2=Solar Irradiance at the TOA and =Stefan-Boltzmann constant Greenhouse Effect: from http://www.dnrec.delaware.gov/ClimateChange/Pages/Greenhouse%20Effect.aspx Greenhouse Effect: from http://www.dnrec.delaware.gov/ClimateChange/Pages/Greenhouse%20Effect.aspx Man’s Radiative Forcing From year 1750 (IPCC Report) Gas Concentrations and Projected Temperature Where we may be headed… 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 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 ( ) Scattering coefficient ( s ) = Extinction coefficient ( 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 2R 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 a p 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 1sr 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 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 !!!
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