Fundamental of Aerosol: Formation and Dynamics

1. Fundamental of Aerosol: Formation and Dynamics

2. Objective • What is aerosol? • Life time and transport of aerosol compared to gases. • How do aerosol look like? • Why do we bother about these tiny particles? • Aerosol formation? • Aerosol size and shape • Forces on Aerosol

3. COURSE OUTLINE • Review of Viscous Flows • Introduction to Aerosols • Properties of Gases • Uniform Particle Motion • Particle size Statistics • Straight- Line Acceleration and Curvilinear Particle Motion • Adhesion of Particles • Brownian motion and Diffusion • Coagulation • Condensation and Evaporation • Electrical Properties

4. Suspended particles in medium

5. Sources of AtmosphericAerosol

6. Size and Shape of Aerosol • Size range: 0.001 mm (molecular cluster) to 100 mm (small raindrop) Iron oxide particles Granite cutting particle Fly ash particle from coal burning

7. Aerosol Size Distribution • - «nucleation: radius isbetween 0.002 and 0.05 mm. Theyresultfrom combustion processes, photo-chemicalreactions, etc. • - « accumulation:radius isbetween 0.05 mm and 0.5 mm. Coagulation processes. • -« fine:particles (nucleation and accumulation) resultfromanthropogenicactivities, • - «coarse: largerthan 1 mm. Frommechanicalprocesseslikeaeolianerosion. 0.01 0.1 1.0 10.0

8. What is radiative forcing by aerosols? DFlTOA DFlSUR

9. Visibility Degradation from Aerosols Glacier National Park, Montana 7.6 µgm-3 12.0 µgm-3 65.3 µgm-3 21.7 µgm-3

10. Aerosol and climate change Top of the Atmosphere (+ve forcing) Knowledge gap: Large uncertainty in quantification of impact of aerosol on climate [IPCC, 2007]. Surface (-Ve forcing) Radiative Forcing (Wm-2) due to aerosol Cloud with aerosol Numerous cloud nuclei Small droplets, Brighter cloud, less prone to rain Drought Annual mean precipitation (1976-2003) minus (1948-1975): Green/blue (red/Yellow) decrease (increase)

11. Ship Track Formation – the First Evidence of Aerosol Indirect Effect N ~ 40 cm-3 W ~ 0.30 g m-3 re ~ 11.2 µm N ~ 100 cm-3 W ~ 0.75 g m-3 re ~ 10.5 µm “Borrowed” from Michael King

12. Aerosol-indirect climate effect Ship tracks off the Washington coast • Adding CCN makes clouds with more, smaller droplets. • These clouds are whiter, reflect more sunlight  net cooling

13. Formation of aerosol • Aerosol formation at source: • Primary aerosol formation: Product of incomplete combustion • Elemental carbon • Organic carbon Elemental cabon Organic cabon EC+OC

14. Secondary aerosol formation in the Atmospher Soil dust Sea salt Environmental importance: health (respiration), visibility, climate, cloud formation, heterogeneous reactions, long-range transport of nutrients…

15. Can you name the laws of motion for a moving object? • Newton’s law • stokes law

16. Do aerosol follow Newton’s law? • Newton’s law • Inertial force dominates the viscous force • What is Reynolds number? • Re>1000 • Large body such as cannonballs (not for particles) • http://csep10.phys.utk.edu/astr161/lect/history/newton3laws.html

17. Stokes’s Law • Viscosity dominates over Inertia • http://members.shaw.ca/gp.lagasse/Centrifuge%20Training/basic2.html • Solution of Navier-Stokes equations (differential eqs describing the fluid motion) Navier-Stokes equations are derived from application of Newton’s second law to a fluid element on which the forces include body forces, pressure, and viscous forces.

18. Stoke’s Law: Assumptions for solving Navier-Stoke equtions • Resulting equations are very difficult to solve because they are nonlinear partial differential equations. • Therefore, Stoke’s solution involved the assumptions • Inertial force is negligible compared to viscous force: this eliminates the higher order terms in Navier-Stokes equation and yield linear equation that can be solved. • Fluid is incompressible. • There are no wall or other particles nearby • The motion of the particle is constant • The particle is rigid sphere • The fluid velocity at the particle surface is zero. The net force acting on the particle is obtained by integrating the normal and tangential forces over the surface of the particle. Re<1.0 (error in the drag force will be 12% error at Re=1.0 and 5% error at Re=0.3)

19. Stokes’s law: Assumptions valid or invalid • Fluid is incompressible • Air around the particle can not be compressed significantly when particle moves through it. Valid • Presence of the wall within 10 diameters of particle will modify the drag coefficient . Aerosols are of small size therefore only a tiny fraction of aerosol will be within 10 particle diameters in any real container or tube. Valid

20. Stokes Law: Non-rigid particle • What if it is water droplet (non rigid sphere)? • Settle 0.6% faster than predicted • Reason circulation develop within the droplet caused by resisting force at drop let surface

21. What about Particles 0.1 mm to 1.0 mm diameter? Particle dp<1.0 mm Particle dp>1.0 mm Include slip correction factor or Cunningham correction factor (Cc) = Mean free path (For air at 1 atm 0.066 mm) Slip correction factors for particles 1.0 mm size is 1.15 that means the particle settles 15% faster than predicted by stokes equ

22. Nonspherical Aerosol • Liquid droplets less than 1 mm and some solid particle are spherical. Most other type of particles are non spherical. • Some have regular geometric shapes, such as cubic (sea salt particles), cylindrical (bacteria and fibers). • Agglomerated particles, crushed material have irregular shape. • Dynamic shape factor (α) is applied to Stoke’s Law to account for effect of shape on particle motion. α is the ratio of actual resistance force of the nonspherical particle to the resistance force of sphere having the same volume and velocity as nonspherical particle. de = equivalent volume diameter

23. Aerodynamic diameter Aerodynamic diameter always greater than stokes’s equivalent dia.

24. When particle is travelling in accelerated field • This is important for understanding the collection mechanism of aerosols. Such as cascade impactor. • Relaxation time and stopping distance are important • Relaxation time characterizes the time required by the particle to adjust or relax its velocity to anew condition of force. • Relaxation time (τ) = mass X mobility=mB

25. References • Hinds, W. C. (1999) Aerosol Technology: Properties, Behavior, and measurement of air born particles. John Willey & Sons Inc. • Friedlander S. K. (2000) Smoke, dust, and haze: fundamentals of aerosol dynamics. Oxford University Press.