Nucleation: Formation of Stable Condensed Phase Homogeneous – Homomolecular H 2 O (g) H 2 O (l) Homogeneous – Heteromolecular nH 2 O (g) + mH 2 SO 4(g) (H 2 O) n (H 2 SO 4 ) m Heterogeneous Homomolecular nH 2 O (g) + X s/l (H 2 O) n X s/l Not relevant to atmosphere Appears to be important in atmosphere Certainly happens (clouds)
New Particle Formation in Atlanta
Kinetics of Cluster Formation ii+1i+2i-2i-1i-3 Formation Rate of Cluster i:k i-1 N 1 N i-1 + k r i+1 N i+1 k r i+1 Loss Rate of Cluster i:k i N 1 N i + k r i N i dN i /dt =k i-1 N 1 N i-1 + k r i+1 N i+1 -k i N 1 N i - k r i N i Because N 1 >>N i dN i /dt =k f i-1 N i-1 + k r i+1 N i+1 -k f i N i - k r i N i Describes time rate of change of cluster i as system adjusts to some initial perturbation and approaches steady state
Steady State Cluster Flux dN i /dt =k f i-1 N i-1 + k r i+1 N i+1 -k f i N i - k r i N i = 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 k f i-1 N i-1 - k r i N i = k f i N i - k r i+1 N i+1 = J J describes the net rate of formation of any cluster size and hence, for S > 1, it is the nucleation rate
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, k f i or k f i+1 ?
Forward and Reverse Rate Constants Forward Rate Constant: “reaction” of monomer with cluster i A + A i A i+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 r i Connect to Kelvin Equation
Thermodynamics of Cluster Formation Recall Kelvin Equation Derivation Obtained from examining free energy change associated with increasing size of arbitrary particle R i* S<1 S>1 GG i * A A i*
Critical Radii and Numbers S=2 dynes cm -1 v l x10 23 cm 3 molec -1 R i* Ang i* H2OH2O Acetone Ethanol
Binary Nucleation is now a surface is now a saddle point where n a * and n b * are such that H 2 SO 4 -H 2 O
New Particle Formation in the Atmosphere Observed in continental and marine boundary layers, forested regions, polluted urban areas, and cloud outflow Tend to occur over 100’s of km, with a frequency of 5 –40% of days. Events tend to be in morning to midday suggesting a photochemical process with possible influence from boundary layer dynamics Wide-spread phenomenon Regional and frequent Photochemical in nature
Impacts of New Particle Formation Formation events tend to increase aerosol number concentrations by factors of 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.
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?
When Will New Particle Formation Be Observed? Formation of stable clusters Rapid growth of nuclei to observable size with slow loss of nucleated particles condensational growth to observable sizes coagulation loss monomer condensation sink i*
Existing Aerosol Limits Observation of New Particle Formation Nucleation Rate will scale with N 1 k het Available surface area Net production of observable new particles P obs = Condensational Growth Rate – Coagulation Sink Area of nuclei relative to area of preexisting aerosol important for P obs >0
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. nuclei j coagulation loss rate condensational growth rate (j j+1) L: McMurry number L = 1: equal # condensing vapor molecules lost to preexisting aerosols as contribute to nuclei growth
Questions When should new particle formation be observed, when L>1 or L<1? L>1 not observed; L<1 observed How do we interpret the 1?
What are the key players to nucleation and growth? H 2 SO 4 -NH 3 -H 2 O: Binary or Ternary nucleation H 2 SO 4 -Organic acid complexes: Location dependent? Zhang, et al. 2004
Mass Spectrometer to Measure New Particle Composition
What’s in those new particles? Mass spectrometry of new particles suggests H 2 SO 4 and NH 3 are most important constituents. No organics were observed.
Hygroscopicity and Volatility Apparatus size select humidify or volatilize resize If hygroscopic: will grow with humidification If volatile: will shrink with heat
These new particles should take up water like AS Hygroscopicity of New Particles Consistent with GF for small ammoniated sulfates measured in lab Volatility measurements also suggest no significant organic component. OC 100 o C Sulfates involatile