1. Modeling of aerosol processes in the atmosphere Peter Tunved Department of Environmental Science and Analytical Chemistry Stockholm University Sweden

2. GOALS: -Develop a fundamental understanding of processes affecting aerosols in the atmosphere -Apply knowledge gained to atmospheric data collected during the GoAmazon campaign. Atmospheric measurements Process models

3. Modeling the atmospheric aerosols Nucleation Chemical reactions Coagulation Dry deposition Gas-to-particle conversion (“condensation”) Primary emissions Cloud processing Wet removal

4. The aerosol is continously changed via a number of dynamical (both physical and chemical) processes, altering the properties of the size distribution Knowledge of these processes is necessary in order to accurately assess both smaller scale interactions (e.g. aerosol cloud interactions) as well as large scale transport and processing Thus, aerosol processes are relevant on both micro and synoptic scale

6. Dry deposition Transport through air to a surface: Turbulent transport, transport over the laminar surface layer, surface properties Wet deposition Up-take in cloud droplets (”in-cloud scavenging”), up-take in falling rain droplets (”below-cloud scavenging”) Chemical reactions in the atmosphere Reactions of different compounds with OH, ozone och NO 3 C 2 H 6  CO 2 +H 2 O

7. Inorganic SO 2 (DMS,COS, CS 2 )  H 2 SO 4 NO x, NH 3  HNO 3, NH 4 NO 3 Organic Biogenic VOC (BVOC’s; eg Monoterpenes) Anthropogenic VOC’s (generally high Mw)

8. Monoterpenes Sesquiterpenes Isoprene Kesselmeier and Staudt, 1999.

9. Very complex chemistry! Most well studied for α- pinene and β-pinene For most compounds, secondary oxidation steps are largely unknow α-pinene most commonly abundant monoterpene The ozone reaction believed to dominate SOA production Kanakidou et al., 2005

10. Few but heavy! (contains much mass, but lifetime is short) Small but many! (visibility, climate, longer lifetime) Recall: The mass of ONE 1µm particle is equal to the mass of ONE THOUSAND 100nm particles, or ONE MILLION 10nm particles

11. SO 2 CONDENSATION/ COAGULATION OXIDATION SO 2 +H 2 SO 4(g) NUCLEATION SO 2 +H 2 SO 4(g) +H 2 SO 4(l) FURTHER AGEING; TRANSPORT; DEPOSITION

12. Basic processes acting on single aerosol particles Gravitational settling Drag force Brownian motion

13. Gravitational settling n.b. that effects of Drag need to be corrected for if Re>0.1, i.e Dp>20um

14. Single particle dynamics and Knudsen number Knudsen number Continuum regime Free molecular regime transition regime

15. Brownian diffusion Continuum regime Free molecular regime transition regime

16. Only removal path in the dry atmosphere Depends on: Atmospheric turbulence Phase of species (gas or particle) Physio-chemical properties of depositing species Particles: size, density Gases: water solubility, reactivity Surface properties (Reactive? Sticky? Irregular?(eg vegetation)

17. Dry deposition Diffusion Impaction Turbulence Sedimentation

19. Ageing due to dry deposition Brownian Diffusion Sedimentation, impaction

20. F=-v d *C where: F=flux to surface m -2 s -1 v d =deposition velocity (m/s) C=concentration of particles

21. C3C3 rara rbrb rsrs C1C1 C2C2 C 0 =0 Laminar resistance; laminar surface layer: ~f(molecular or brownian diffusivity) Aerodynamic resistance; surface layer: ~f(temperature; windspeed; surface roughness) Surface resistance; surface properties: ~f(reactivitiy, solubility, pH etc.)

23. Coagulation Mainly the result of Brownian motion, although other forces may come into play (electrical, gravitational etc) Key concepts:

24. Coagulation Free molecular regime

25. Coagulation Continuum regime  the kernel in this regime is found by solving the time-dependent diffusion equation around a stationary spherical absorber in an infinite medium with suspended particles. Transport and collision through “random walk” Transition regime (~1<Kn<50)  Semi-empirical solution to the collision kernels (Fuchs 1964, “Flux matching”)

26. Coagulation; Fuchs form of the coagulation coefficient K12=COAG_COEFF(Dp 1, Dp 2, T, P)

28. Condensation on existing particles SIZE NUMBER Precursorgases (e.g. SO 2 ) High saturation vapor pressure Nucleation Oxidation products (e.g. H 2 SO 4 ) Low saturation vapor pressure ([H 2 SO 4 ]>[H 2 SO 4 ] sat )

29. S=saturation ratio p a= partial pressure of a p a s =saturation vapor pressure of a at temperature T Clausius-Clapeyron relation

31. (hot) gas [x]<[x] sat cooling [x]>[x] sat [x]=[x] sat [x]<[x] sat [x 2 ]>[X 2 ] sat Transformation: X  X 2 [x]<[x] sat [x 2 ]=[X 2 ] sat

32. Amount of pre-existing aerosol surface crucial Generation of supersaturated conditions + low surface area of pre-existing particles favours formation of particle number via nucleation Generation of supersaturated conditions + High concentration of pre-existing particles favours formation of particle mass via condensation This is often referred to as ”condensation sink”

33. Condensation β=Fuchs and Sutugin non-contnuum correction factor

35. In-cloud scavenging SO 2 (g) SO 2 (aq) Below-cloud scavenging Cloud droplet Formation (”nucleation”) Nucleation scavenging Aerosol Particles

37. Scavenging of aerosols: Impaction, diffusion and interception

38. Below cloud scavengning of particles

40. Scavengning Thus, we need a droplet distribution Marshal palmer droplet distribution

41. Scavengning coefficient, Λ

42. Simulating below cloud scavengning

44. “At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid” [A(aq)]pA(g)

45. [A(aq)]pA(g) A(eq) Z Where Kc is a mass transfer coefficient: However, both C aq and C g is function of altitude and time

46. Aitken mode Accumulation mode Coarse Particles 0.0010.010.1110100 Particle Diameter (  m) Sedimentation Rainout and Washout Wind blown dust + Emissions + Sea spray + Volcanoes + Plant Particles Coagulation Condensation nuclei Agglomerates Droplets Coagu- lation Primary particles Homogenous nucleation and condensation Hot vapours Low volatility gases Gas phase chemical reactions Coagu- lation Nucleation mode Activation Condensation Diffusion

47. All processes

49. Cloud formation Aerosolpartiklar Precipitation SO 2 Liquid phase chemistry OZONE + SO 2 H 2 O 2 H 2 SO 4 O3O3 H2O2H2O2 SO 2 H 2 SO 4 Gas phase chemistry

50. Nucleation Coagulation Condensation Deposition (Brownian Diffusion) Deposition (sedimentation) Clouds! Wet deposition

51. Creating your own Lagrangian model Processes included Nucleation Coagulation Condensation Dry deposition Meteorological input from trajectory Temperature (T) Pressure (Pa) Relative humidity (RH)