Particle formation and growth

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Particle formation and growth Formation of secondary particles Gas phase reactions Formation of low volatility products; nucleation or condensation; (coagulation) E.g., SO 2 oxidation → H 2 SO 4 Chemical reactions in aqueous phase in clouds E.g., SO 2 oxidation Reactions of gases on particle surfaces Formation of condensed phase products E.g., HNO 3 (g) + sea salt → NaNO 3

description

Particle formation and growth. Gas phase reactions Formation of low volatility products; nucleation or condensation; (coagulation) E.g. , SO 2 oxidation → H 2 SO 4. Reactions of gases on particle surfaces Formation of condensed phase products E.g. , HNO 3 (g) + sea salt → NaNO 3. - PowerPoint PPT Presentation

Transcript of Particle formation and growth

Page 1: Particle formation and growth

Particle formation and growth

Formation of secondary particles

Gas phase reactionsFormation of low volatility products; nucleation or condensation; (coagulation)E.g., SO2 oxidation → H2SO4

Chemical reactions in aqueous phase in cloudsE.g., SO2 oxidation to sulphate

Reactions of gases on particle surfacesFormation of condensed phase productsE.g., HNO3 (g) + sea salt → NaNO3

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1. Gas phase reactions:

NB: VP over a curved surface > VP over flat surface Small size → higher VP & greater tendency to evaporate

How does particle formation occur?Molecular clusters formed by gas phase collisionsWhen vapour is supersaturated, get higher concentration of molecules and clustersClusters grow by sequential attachment of molecules up to a critical diameter, D*

Homogeneous nucleation:Direct condensation of low volatility compounds to form a new particle

skTD

ln4*

D > D* → clusters stable and growD < D* → clusters evaporate

γ = surface tensionν = molecular volume s = saturation ratio = actual VP  

equilibrium VP

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Binary homogeneous nucleationFormation of particle from two different gas phase compounds

E.g., H2O and H2SO4

Can occur when concentrations of individual species are too low for nucleation of pure compoundsMost secondary particles are probably formed by formation and growth of clusters of several species

H2SO4 nucleation

Can describe nucleation processes theoretically.Observed nucleation rate of H2SO4 is much higher than predictedPossibly have condensation onto pre-existing molecular clusters (“prenucleation embryos”)E.g., H3O+(H2O)n + HSO4

-(H2SO4)m(H2O)q

→ large, stable cluster embryos

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Formation of ultrafine particles occurs at lower concentrations of gaseous H2SO4 than predicted

Dependence of cluster formation smaller than predicted – other species may be involved

E.g., NH3 may assist in nucleation process; VP reduced by 2-3 orders of magnitude with NH3:H2SO4 in 1:1 ratio

Rate of growth of ultrafine particles larger than expected from H2SO4 condensation – other species (like organics) probably also taken up

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Heterogeneous condensation: The scavenging of low volatility gas phase products by preexisting particles

If particle concentration is high, heterogeneous condensation dominates over formation of new nuclei via homogeneous nucleation

Factors affecting heterogeneous condensation:• rate of gas collisions with particle surface• mass uptake coefficient (probability of uptake per collision)• size of existing particles• difference in partial pressure of condensing species between

the air mass and particle surface

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Field data suggests both homogeneous and heterogeneous nucleation

Similar results from smog chamber studies of DMS oxidation:growth of particles in initially particle-free systemwith seed particles (34 μm mean diameter), we observe:

• oscillation in number of fine particles produced

• periodic bursts of nucleation

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Coagulation:Collision and sticking together of two smaller particles to form a larger particle

Coagulation of small particle and large particledepends on diameter of large particlereduces number of smaller particlesadds little to mass of larger particlerate depends on diameter of larger particle, diffusion rate of smaller particle, concentrations of small and large particles

Self-coagulation:can change size distribution significantlydepends a lot on particle size and concentration

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Examples:O3 oxidation of PAHsReaction of NaCl and NaBr in sea salt particles with nitrogen oxides

NaCl(s) + HNO3 (g) → HCl(g) + NaNO3 (s)

Unclear how reactions affect growth and transformations of particles

2. Reactions of gases at particle surfaces

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E.g., HNO3 (or NO2) + NaCl → (dry) no change in particle morphology→ (humid) formation of microcrystals of NaNO3 on salt particle

surface(Possible route for formation of free particles in MBL free of Cl-)

Water adsorption plays a critical role in interaction of gases with surfaces usually thought of as solids

Reaction of SO2 and NO2 at liquid interfaces• may have unique reaction mechanisms compared to bulk or gas

phase• difficult processes to study

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E.g., Reaction of SO2 in clouds and fogs plays a key role in formation of H2SO4

Later evaporation of H2O → suspended particlesMay explain bimodal distribution within the accumulation mode

different modes are observed within clouds vs just below clouds

- Small particles taken up in droplets; subsequent evaporation of droplet → agglomeration of particles together → larger particles

- SO2 absorbed and oxidised to H2SO4 → evaporation → sulphate

- bimodal distribution

3. Reactions in the aqueous phase