Lecture 3: Aerosol processes

1. Lecture 3: Aerosol processes Ken Carslaw

2. The aerosol lifecycle transport < months Nucleation Low vapor pressure Free troposphere Growth oxidation coagulation Vertical transport hours/days Dissolution Wet scavenging Precursor gases CCN insoluble Ageing/growth Cloud processing Primary particles Boundary layer soluble Dry deposition Emission Surface hours/days EmissionTransformationRemoval

3. Nucleation • Formation of new particles through gas-to-particle conversion • Occurs in almost every part of the atmosphere • Nuclei detectable using condensation particle counters (CPCs) > 3nm diameter (10s to 100s of molecules) Formation and growth rates of ultrafine atmospheric particles: a review of observations, Kulmala, et al., Journal of Aerosol Science, 35, 2004.

4. Nucleation mechanisms Remote Pacific • Free and upper troposphere probably Binary Homogeneous Nucleation of H2SO4-H2O particles • Other possibilities: • Ion-induced • Ternary (with NH3) • Anything fast… Clarke et al. Europe Schroder et al. Kulmala, et al., Parameterizations for sulfuric acid/water nucleation rates, J. Geophys. Res. - Atmos., 103, 8301–8307, 1998.

5. Mean profile and single measurement over tropical Pacific Binary homogeneous H2SO4-H2O nucleation 109 cm-3 hour-1 105 cm-3 hour-1 Courtesy V.-M. Kerminen Particle distribution becomes independent of sources gases: Clement, et al., Journal of Aerosol Science, 37, 2006.

6. Nucleation in the boundary layer Hyytiälä, Finland • Occurs at most continental sites • Organics may play a role • Iodine in coastal environments • Nucleation rates (particles cm-3 s-1) are available, but uncertainties are large 100 nm 10 nm 1 – 108 cm-3 hour-1 Formation and growth rates of ultrafine atmospheric particles: a review of observations, Kulmala, et al., Journal of Aerosol Science, 35, 2004.

7. Condensation and growth • Condensation is a kinetic gas uptake process • Solution of radial diffusion equation • E.g., H2SO4, oxidised organic compounds ~ zero vapor pressure Vapor concentration Particle radius Calculated atmospheric growth rates For vapor concentration = 107 molec cm-3 10 nm particle: dr/dt ~ 8 nm/h 100 nm dr/dt ~ 0.8 nm/h = 19 nm/day 1 mm dr/dt ~ 0.08 nm/h = 1.9 nm/day mv=molecule mass, rp=particle density, Diffusion

8. Observed particle growth At a Boreal Forest site 100 nm ~2 nm/h 10 nm ~16 nm/h Courtesy M Kulmala Days Observed atmospheric nuclei growth rates ~ 0.003 – 20 nm/h Implies condensing vapor concentrations ~ 104 – few 107 cm-3

9. Dissolution of soluble gases • Dissolution is the dissolving of gases into solution • A combination of gas diffusion to the particle and equilibration (not reaction) Soluble volatiles H2O SOLUBILITY Henry’s law const = f(T) NH3 Partial pressure HNO3 Activity coefficient = f(composition, RH, T) Soluble involatiles

10. Dissolution timescales • How long does it take for particles to reach equilibrium with soluble gases? 1 day Time to reach gas-liquid equilibrium Days for coarse particles – i.e., never in equilibrium Hours for fine particles Meng and Seinfeld, Atmos. Env. 1995

11. Hygroscopicity • Water uptake by soluble particles • Reversible process • f(RH, composition) • Timescale for water equilibration • ~ seconds Petters and Kreidenweis, A single parameter representation of hygroscopic growth and cloud condensation nucleus activity, Atmos. Chem. Phys., 7, 2007

12. Production Loss Coagulation • Smoluchowski coagulation equation (1916) number radius 1 2 3 4 i • Coagulation is a volume preserving, growth and number-reducing process • Loss of small particles, creation of large ones

13. Coagulation timescales • How quickly are particle concentrations changed by coagulation? Brownian Coagulation with 0.1 mm particle Brownian diffusion kernel Radius of large particle and diffusion coeff of small particle Typical coagulation loss rates For coagulation with 1000 cm-3 0.1 mm particles 1 nm particle: t ~ 300 s 3 nm t ~ 1.5 h 10 nm t ~ 12 h cf dr/dt ~ 8 nm/h 50 nm t ~ 10 days Remember, coagulation also creates particles at each size!

14. Growth and coagulation together Fast diffusion Rapid coagulation Slow diffusion Slow coagulation • Coagulation and condensation have greatest effect on the smallest particles (<200 nm). The larger particles are determined mostly by primary emissions Main sink 37000 3 nm particles = 1 100 nm particle! Number concentration Very little change Rapid growth and loss 10 nm 50 nm 100 nm size

15. Nucleation and CCN Slow growth results in coagulation loss • Coagulation rate of 10 nm particle is ~50 times less than 1 nm particle • Rapid initial growth maximises nuclei “survival” to climate-relevant sizes Rapid growth increases survival Pierce and Adams, Efficiency of cloud condensation nuclei formation from ultrafine particles, ACP, 2007 Spracklen et al., Contribution of particle formation to global cloud condensation nuclei concentrations, GRL, 2008

16. Particle activation in clouds 1000-10000 cm-3 100-1000 cm-3 • Cloud drops are: • Larger (volume 105 > than aerosol) • More dilute (lower vp of dissolving gases) growth maximum supersaturation in cloud equates to minimum radius of activation See lecture by Graham Feingold

17. Cloud “processing” • SO2 + H2O2 SO4 dominates in summer • SO2 + O3 SO4 dominates in winter and high latitudes Rate is fast: limited by replenishment of H2O2 and SO2 in clouds SO2 Involatile H2SO4 remains in particles evaporation cloud Easter and Hobbs, The Formation of Sulfates and the Enhancement of Cloud Condensation Nuclei in Clouds, Journal of the Atmospheric Sciences, 1974

18. Particle ageing • Some primary particles are initially insoluble • They become partly soluble through • Coagulation • Chemical transformation • Condensation • Results in greater CCN activity Condensation and chemical reaction Insoluble Partially soluble More soluble

19. Ageing timescales Rate of ageing Time to form 1 monolayer (~1019 molec m-2) of H2SO4 on a 100 nm particle ~104s (3 h) in polluted conditions (H2SO4 = 107 cm-3) ~105 – 106 s (1-10 days) in clean conditions Factor >10 longer for coarse particles

20. BC/OC condensation aged Example of ageing BC/OC not condensation aged Results from the Leeds GLOMAP model

21. Summary of timescales mins hours weeks days Formation Growth Coagulation Ageing Equilibration Deposition Nucleation Formation Growth Coagulation Ageing Equilibration Deposition Aitken Formation Growth Coagulation Ageing Equilibration Deposition Accum. Formation Growth Coagulation Ageing Equilibration Deposition Coarse

22. Explaining the size distribution Accumulation Produced by cloud processing (from Aitken) Slow loss through deposition Lost in raindrops Aitken Produced by emissions and coagulation Coarse Produced by emissions. Rapidly lost through deposition and scavenging Number concentration Nucleation Produced by nucleation. Rapidly lost through growth, coagulation, deposition 10 nm 50 nm 100 nm size

23. The aerosol lifecycle minutes Nucleation Free troposphere days/weeks hours/days oxidation Growth/coagulation hours/days transport Precursor gases Rapid scavenging CCN hours hours/weeks Cloud processing insoluble Ageing Primary particles Boundary layer soluble Emission hours/days Dry deposition Surface

24. Global sulfate production • Sulfur budget in Tg(S) per year 0.01 deposition H2SO4 (gas) condensation OH 13.4 13.0 nucleation 0.07 SO2 36.0 H2SO4 (particles) 31.7 deposition