Particle formation and growth

1. 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

2. 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* γ = surface tension ν = molecular volume s = saturation ratio = actual VP equilibrium VP D > D* → clusters stable and grow D < D* → clusters evaporate

3. 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

4. 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

5. 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

6. 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

7. 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

9. 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

10. 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

11. 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