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Gas-To-Particle Conversion

Gas-To-Particle Conversion. Reading: Chap 13.5. Q: What’s the interaction between gas molecules and particles?. Activated Carbon. VOC. Water droplet. Water vapor. Q: Examples?. ==> Condensation. Q: How do we have the first particle for vapor condensation?. ==> Nucleation. Q: Examples?.

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Gas-To-Particle Conversion

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  1. Gas-To-Particle Conversion Reading: Chap 13.5 Q: What’s the interaction between gas molecules and particles? Activated Carbon VOC Water droplet Water vapor Q: Examples? ==>Condensation Q: How do we have the first particle for vapor condensation? ==>Nucleation Q: Examples? Aerosol & Particulate Research Laboratory

  2. (Saturation) Vapor Pressure Time to reach equilibrium How does vapor pressure change if the temperature increases? Vapor Pressure • The pressure required to maintain a vapor in equilibrium with the condensed vapor (liquid or solid) with a flat surface at a specified temperature Pv in mmHg and T in oC(if Table 9.2 is used) Aerosol & Particulate Research Laboratory

  3. Aerosol & Particulate Research Laboratory

  4. What is the vapor pressure of water at 20 oC? If the measurement is conducted on Mars (the atmospheric pressure is about 0.006 atm), what will be the value? Aerosol & Particulate Research Laboratory

  5. Aerosol & Particulate Research Laboratory

  6. Partial Pressure: the pressure that a gas (or vapor) in a mixture of gases would exert if it were to occupy the entire volume occupied by the mixture Saturation Ratio (or relative humidity for water) Supersaturation: S > 1 (RH > 100%) 4 moles of N2 1 mole of O2 @ 1 atm Q: What would happen if S > 1? Q: When does it happen for S > 1? Q: How much is PO2? Q: After a rain at dusk, the temperature starts to drop. How do PV and PA change correspondingly? How about S? Can you predict the weather at dawn? Adiabatic Expansion, Mixing, Cooling, Speciation change by Gas Phase Reaction Aerosol & Particulate Research Laboratory

  7. Adiabatic Expansion An expansion allowing no heat input from the surroundings Q: Saturated water vapor @ 20 oC (p = 17.6 mmHg) expands 20%. Calculate the new saturation ratio. Ps @ 0 oC = 4.7 mmHg, k = 1.38. Example: Cloud formation, smoke generated in the neck of a wine bottle Aerosol & Particulate Research Laboratory

  8. Kelvin Effect Change in Gibbs free energy accompanying the formation of a single droplet of pure material A of radius Rp (initial) (final) NT: # of molecules Nu: # of vapor molecules Nl: # of liquid molecules gu: Gibbs free energy of a molecule in the vapor phase gl: Gibbs free energy of a molecule in the liquid phase ul: volume of one liquid molecule s: surface tension Rp: particle radius A system moves toward a lower energy state whenever possible. Aerosol & Particulate Research Laboratory

  9. = 0 at constant T Q: How does DG change as Rp increases? Q: How do we determine the critical size? Aerosol & Particulate Research Laboratory

  10. Maximizing the G Q: Calculate Rp* and Nl* for water at 273 K, S = 2 (s = 75.6 dynes/cm) (Critical radius) (Critical Number) (Max G) (Kelvin Equation) (Kevin Ratio) The partial pressure of vapor at the surface of a small droplet is greater than the saturation vapor pressure defined for a flat surface Aerosol & Particulate Research Laboratory

  11. Critical number and Radius for water droplets Equilibrium vapor pressure over a pure water droplets for various dp at T = 298 K Aerosol & Particulate Research Laboratory

  12. t = 0 dp* ?t = t1? Saturation ratio vs dp dp* Aerosol & Particulate Research Laboratory

  13. Homogeneous Nucleation • Nucleation of vapor on embryos comprised of vapor molecules in the absence of foreign substances Equilibrium Cluster Distribution For a fluid at equilibrium, the concentration of clusters obeys a Boltzmann distribution Nl: number of molecules in the cluster N1: number of gas molecules Aerosol & Particulate Research Laboratory

  14. Classical Nucleation Rate Surface area Equilibrium # of critical size Flux of monomer m1: mass of one molecule p1: gas pressure Aerosol & Particulate Research Laboratory

  15. Critical cluster size and droplet current for homogeneous nucleation of water at 293 K log J Critical saturation ratio set at J = 1 #/cm3.s Aerosol & Particulate Research Laboratory

  16. Self-Consistent Kinetic Theory Q: What if Nl = 1? Classical Kinetic Toluene Q: What is the physical meaning of s for a very small cluster? Aerosol & Particulate Research Laboratory

  17. Reflection Aerosol & Particulate Research Laboratory

  18. Adiabatic Expansion Aerosol & Particulate Research Laboratory

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