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Nucleation: Formation of Stable Condensed Phase

Nucleation: Formation of Stable Condensed Phase. Homogeneous – Homomolecular H 2 O (g) H 2 O (l). Not relevant to atmosphere. Homogeneous – Heteromolecular nH 2 O (g) + mH 2 SO 4(g)  (H 2 O) n (H 2 SO 4 ) m. Appears to be important in atmosphere. Heterogeneous Homomolecular

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Nucleation: Formation of Stable Condensed Phase

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  1. Nucleation: Formation of Stable Condensed Phase Homogeneous – Homomolecular H2O(g)H2O(l) Not relevant to atmosphere Homogeneous – Heteromolecular nH2O(g) + mH2SO4(g) (H2O)n(H2SO4)m Appears to be important in atmosphere Heterogeneous Homomolecular nH2O(g) + Xs/l (H2O)nXs/l Certainly happens (clouds)

  2. New Particle Formation in Atlanta

  3. Formation Rate of Cluster i: ki-1N1Ni-1 + kri+1Ni+1 Loss Rate of Cluster i: kiN1Ni + kriNi dNi/dt = ki-1N1Ni-1 + kri+1Ni+1 - kiN1Ni - kriNi dNi/dt = kfi-1Ni-1 + kri+1Ni+1 - kfiNi - kriNi Kinetics of Cluster Formation kri+1 i-3 i-2 i-1 i i+1 i+2 Because N1 >>Ni Describes time rate of change of cluster i as system adjusts to some initial perturbation and approaches steady state

  4. dNi/dt = kfi-1Ni-1 + kri+1Ni+1 - kfiNi - kriNi Steady State Cluster Flux = 0 At steady state, the concentration of cluster i no longer changes with time. But, there is a steady state flux of molecules from one cluster to another as the system approaches equilibrium kfi-1Ni-1 - kriNi = kfiNi - kri+1Ni+1 = J J describes the net rate of formation of any cluster size and hence, for S > 1, it is the nucleation rate

  5. Question What is J, once equilibrium has been achieved? Does it make sense to calculate a nucleation rate by assuming the cluster distribution is at equilibrium? Which is larger, kfi or kfi+1?

  6. Forward and Reverse Rate Constants Forward Rate Constant: “reaction” of monomer with cluster i A + Ai Ai+1 From kinetic theory: Rate proportional to collision frequency Reverse Rate Constant: evaporation from cluster i More challenging, but should only depend on T and ri Connect to Kelvin Equation

  7. Thermodynamics of Cluster Formation • Recall Kelvin Equation Derivation • Obtained from examining free energy change associated with increasing size of arbitrary particle S<1 S>1 i*A Ai* G Ri*

  8. Critical Radii and Numbers

  9. Binary Nucleation is now a surface is now a saddle point where na* and nb* are such that H2SO4-H2O

  10. New Particle Formation in the Atmosphere Wide-spread phenomenon Observed in continental and marine boundary layers, forested regions, polluted urban areas, and cloud outflow Regional and frequent Tend to occur over 100’s of km, with a frequency of 5 –40% of days. Photochemical in nature Events tend to be in morning to midday suggesting a photochemical process with possible influence from boundary layer dynamics

  11. Impacts of New Particle Formation Formation events tend to increase aerosol number concentrations by factors of 2-10. Newly formed particles (<10nm) tend to grow into accumulation mode particles (100 nm) at a rate of 1-20 nm/hr (fast). Accumulation mode particles act as cloud condensation nuclei such that new particle formation may impact cloud cover and direct scattering of solar radiation.

  12. Nuclei vs Measured New Particles A typical stable nuclei will have a radius <~ 1nm Size measurements are limited to particles with r > 3nm Thus significant post-nucleation growth will have occurred before measurement What formed the nuclei? What contributed to growth?

  13. When Will New Particle Formation Be Observed? Formation of stable clusters Rapid growth of nuclei to observable size with slow loss of nucleated particles i* monomer condensational growth to observable sizes condensation sink coagulation loss

  14. Existing Aerosol Limits Observation of New Particle Formation Nucleation Rate will scale with N1 khetAvailable surface area Net production of observable new particles Pobs = Condensational Growth Rate – Coagulation Sink Area of nuclei relative to area of preexisting aerosol important for Pobs>0

  15. nuclei j coagulation loss rate condensational growth rate (jj+1) The “McMurry” Number When coagulation of nuclei with preexisting aerosol dominates their condensational growth, new particle formation will not be observed even though nucleation may be occurring. L: McMurry number L = 1: equal # condensing vapor molecules lost to preexisting aerosols as contribute to nuclei growth

  16. L>1 not observed; L<1 observed Questions When should new particle formation be observed, when L>1 or L<1? How do we interpret the <10nm particles that appear when L>1?

  17. What are the key players to nucleation and growth? H2SO4-NH3-H2O: Binary or Ternary nucleation H2SO4-Organic acid complexes: Zhang, et al. 2004 Location dependent?

  18. Mass Spectrometer to Measure New Particle Composition

  19. What’s in those new particles? Mass spectrometry of new particles suggests H2SO4 and NH3 are most important constituents. No organics were observed.

  20. Hygroscopicity and Volatility Apparatus size select If hygroscopic: will grow with humidification humidify or volatilize If volatile: will shrink with heat resize

  21. Hygroscopicity of New Particles These new particles should take up water like AS Volatility measurements also suggest no significant organic component. OC volatile @~ 100oC Sulfates involatile Consistent with GF for small ammoniated sulfates measured in lab

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