Optical Properties of Aerosols

1. LA on a smoggy day LA on a clear day Optical Properties of Aerosols ENVR 416 Aerosol Technology

2. Topics • Definitions • Extinction • Scattering • Visibility

3. 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

4. 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 as Dp << λ and Dp >> λ allow for simplifications to be made.

5. 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

6. Index of Refraction

7. Index of Refraction scattering absorption Scattering portion measured with Snell’s Law:

8. Index of Refraction Absorption often measured via spectrophotemtry Bulk absorption For electrically conductive material For most aerosol particles

9. Relative Index of Refraction (mr) Used to describe a two phase system, i.e. a particle in air For air For vacuum For aerosol particles in air

10. Intensity of Light Light arriving at a surface: scattered light detector Incident light

11. Intensity of Light Light from a point source: A

12. Size Parameter (α) • - Added to simplify light scattering equations • Makes α = ratio of particle size to wavelength of radiation For dp on the order of mm

13. Electromagnetic Theory • Light 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

14. Topics • Definitions • Extinction • Scattering • Visibility

15. Extinction Definition: the attenuation of light along an axis resulting from scattering and/or absorption Particles Extinction is dependent upon the chemical composition of particles as well as particle size, shape, orientation and number. Light Extinction is also dependent upon the polarization and frequency of the incident beam.

16. Extinction Mathematically, how do we quantify the results of extinction? I I0 Lambert-Beer Law

17. Extinction For a monodisperse aerosol: # concentration Represents fractional loss in intensity per unit length Extinction efficiency Extinction coefficient (L-1) Particle area Lambert-Beer Law

18. Extinction Extinction Efficiency • 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: For polydisperse aerosols:

19. Example Problem If: What is: a) Number concentration in #/m3 b) Mass concentration in mg/m3 ? Lambert-Beer Law

20. Example Problem

21. Extinction Recall: Therefore, there is no single equation to calculate for all dp

22. Extinction “Extinction Paradox” For dp > 4mm

23. Extinction Paradox Based on the condition that extinction must be observed at long relative distances For coffee cup  100 km (rarely observed condition) dobs >>

24. Beers Law (Mass Concentration)

25. Topics • Definitions • Extinction • Scattering • Visibility

26. 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 of light intensity

27. Scattering Physical basis • The scattering of EM radiation by any system is related to the heterogeneity of that system (the physics remains the same)

28. Scattering

29. Scattering Two cases In this case, the whole particle “sees” the same E-field and scatters in phase dp << l (Rayleigh) In this case, the E-field is not the same for the entire particle and a complex interference pattern of scattered wavelets will result dp ~ l (Mie)

30. Scattering Rayleigh Region: dp<< l Unpolarized light Parallel to scattering plane Perpendicular to scattering plane

31. Perpendicular Polarization dp = 0.02 mm l= 650 nm Parallel Polarization Intensity Perpendicular Polarization dp = 0.002 mm Parallel Polarization www.philiplaven.com

32. Mie Scattering Size Parameter k = k = 10 k = 10 Mie Regime f||, fL Particle size k = 2 k = 2 fL(ө,m,dp) f||(ө,m,dp) k = 0.8 k = 0.8 Rayleigh Regime W.C. Hinds, Aerosol Technology: Properties, Behavior and Measurement of Airborne Particles, John Wiley and Sons, 1982

33. 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) = Unpolarized light Perpendicularly polarized Parallel polarized where f is a function of Ө,m and dp

34. Mie Web Calculators http://omlc.ogi.edu/calc/mie_calc.html

35. Incident light = 532nm Dp = 0.7mm Dp = 0.2mm k = 4.13 m = 1.33 m = 1.33 k = 1.18 90º Unpolarized 90º Parallel Perpendicular 180º 0º 180º 0º 270º 270º http://omlc.ogi.edu/calc/mie_calc.html

36. Mie Region dp ~ λ Mars picture from Pathfinder http://www.weatherstock.com/cloudcat.html

37. 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 Wurzburg Wurzburg, Germany Chem. Phys. Lett. 264(1997) 233-237

38. 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

39. Experimental Schematic t = 0 Droplet generation chamber nebulizer Levitated droplet Photodiode lens lens mirror Observation chamber lens argon-ion laser l = 514.53 nm lens Quartz plate mirror spectrograph mirror Interference filter mirror converter Spectrum Accumulation time ~ 1 sec

40. Fg Flaser Optical Levitation • The gravitational force exerted on a particle is balanced by photon pressure produced by a vertically directed laser beam where Prad is the radiative pressure, ΦE is the energy 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 l = 514.5nm, we need 3.88x1019 photons/sec to maintain levitation

41. 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

42. 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

43. C-H Stretching Region Raman Intensity (arb. Units) Wavenumber (cm-1)

44. Laser Power Required for Levitation • Negative peaks correspond • to MDRs Experimental Theoretical 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

45. Wave number (cm-1) Time (s) NH3(g) insertion • 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

46. Topics • Definitions • Extinction • Scattering • Visibility

47. Visibility http://www.dailymail.co.uk/news/worldnews/article-1215443/Australia-dust-storm-sweeps-eastern-coast.html

48. Visibility Visible range  how far one can see in a given direction Limited by: Visual acuity 2) Contrast Aerosol particles with 0.1 mm < dp < 1 mm reduce contrast by scattering light

49. Contrast Object luminance Background luminance Inherent contrast Luminance  luminous intensity per unit solid angle per unit area of surface Units: lumens/m2•sr, cd/m2 r2 r Total area =

50. 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 If If For black object against white background