Particle formation and growth
Gas phase reactions
Formation of low volatility
products; nucleation or
condensation; (coagulation)
E.g., SO2 oxidation → H2SO4
Reactions of gases on particle surfaces
Formation of condensed phase products
E.g., HNO3 (g) + sea salt → NaNO3
Formation of secondary particles
Chemical reactions in
aqueous phase in clouds
E.g., SO2 oxidation to
sulphate
1.
Gas phase reactions:
Homogeneous nucleation:
Direct condensation of low volatility compounds to form a new
particle
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 collisions
When vapour is supersaturated, get higher
concentration of molecules and clusters
Clusters grow by sequential attachment of
molecules up to a critical diameter, D*
D > D* → clusters stable and grow
D < D* → clusters evaporate
4
D* 
kT ln s
γ = surface tension
ν = molecular volume
s = saturation ratio
= actual VP
equilibrium VP
Binary homogeneous nucleation
Formation 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 compounds
Most 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 predicted
Possibly have condensation onto pre-existing molecular clusters
(“prenucleation embryos”)
E.g., H3O+(H2O)n + HSO4-(H2SO4)m(H2O)q
→ large, stable cluster embryos
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
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
Field data suggests both homogeneous and heterogeneous nucleation
Similar results from smog chamber studies of DMS oxidation:
growth of particles in initially particle-free system
with seed particles (34 μm mean diameter), we observe:
• oscillation in number of
fine particles produced
• periodic bursts of nucleation
Coagulation:
Collision and sticking together of two smaller particles to form a
larger particle
Coagulation of small particle and large particle
depends on diameter of large particle
reduces number of smaller particles
adds little to mass of larger particle
rate depends on diameter of larger particle, diffusion rate
of smaller particle, concentrations of small and large
particles
Self-coagulation:
can change size distribution significantly
depends a lot on particle size and concentration
2.
Reactions of gases at particle surfaces
Examples:
O3 oxidation of PAHs
Reaction 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
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
3.
Reactions in the aqueous phase
E.g., Reaction of SO2 in clouds and fogs plays a key role in
formation of H2SO4
Later evaporation of H2O → suspended particles
May 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