2005 SOLAS Summer School Introduction to Marine Aerosols

1. 2005 SOLAS Summer SchoolIntroduction to Marine Aerosols Eric S. SaltzmanEarth System ScienceUniv. of CA, Irvine

2. What driving marine aerosol research? • geochemical cycles • metals, nutrients, organics • acidification (sulfur, nitrogen) • climate change • direct/indirect effects • aerosol optical properties, aerosol/cloud interactions • nutrients • N deposition • coastal, HNLC • desert dust and iron deposition • human health • air quality, airborne pathogen transport

3. Earth’s energy balance...

4. Aerosol effects on radiation budget radiation Global mean radiative forcing of climatefor year 2000 relative to 1750 (IPCC) • Direct effects (cloud-free): • scatter  cooling • absorption  heating • Indirect effects (clouds): • more (but smaller) droplets  scatter (Twomey) • more droplets  longer cloud lifetime (Albrecht) • absorption  heating  evaporates clouds

5. The life cycle of marine aerosols... • What are marine aerosols made of ? • Where do they come from? • How long do they stay in the atmosphere? How are they removed? • How do they evolve while in the atmosphere? • How do they interact with the climate system? • forcing • feedback Note on observational constraints: intensive – many param., poor coverage network – good coverage, few param. satellite – excellent coverage, limited interpretability...

6. “...there are known knowns. These are things we know that we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns. These are things we don't know we don't know.” Donald Rumsfeld Things we “know we know”... (or don’t)... Unknown knowns...? • “nucleation is rare” • “sulfate is the source of CCN (cloud condensation nucleii) over the oceans” • “sea spray doesn’t make submicron aerosols"

7. Terminology... • aerosol - a dispersion of solid and liquid particles suspended in gas (air). • note: in common practice, “aerosol” is used to refer to the particles only! • primary aerosol - emitted directly into the atmosphere. • Saharan dust, sea spray, pollen, plant waxes, soot • secondary aerosol - created by nucleation of new particles, aggregation of existing particles, or growth of preexisting particles from gas phase molecules (gas to particle conversion). • either type → natural, anthropogenic, or both • internal vs. external mixtures – in an internally mixed aerosol all particles have the same composition • CN – condensation nucleii - aerosols that scatter light at very high supersaturation levels • CCN – cloud condensation nucleii - aerosols that scatter light at very low superaturation levels • How much aerosol is there? • typically ~10’s 100’s of ug/m3 (air density ~1kg/m3)

8. Aerosol size distributions... number distribution surface area distribution volume distribution (Seinfeld and Pandis)

9. the log-normal aerosol size distribution... Aitken mode accumulation mode number distribution surface area distribution volume distribution coarse mode (Seinfeld and Pandis)

10. The aerosol modes... • Aitken mode – 0.01-0.1 um • accumulation mode – 0.1-1 um • coarse mode - >1 um and sometimes, the elusive • nucleation mode <0.01 um (C. Leck)

11. Humidity and aerosol size... deliquescence • hygroscopic aerosols grow/shrink with RH (with hysteresis!) efflorescence • aerosol size strongly affects light scattering cross-section

12. a process-oriented view of the size distribution ... ... reflecting competition between production/transformation/removal no ultrafines here... at the time, there was no instrumentation to detect them

13. why “accumulation” mode? impaction, settling diffusion, coagulation

14. removal mechanisms... gravitational settling • 10 um particle  1000 cm hr-1 • 1 um particle  10 cm hr-1 coarse particles

15. fine particles Diffusion ... You can estimate the distance a particle will diffuse in a given time from the equation: where D is the diffusion coefficient

16. The chemical perspective ... a chemical size distribution 1. chemical size distributions resemble mass, not number 2. sulfate and organics dominate the accumulation mode, but there’s a surprising amount of seasalt 3. there are a lot of unidentified organics 4. the coarse mode has the expected mechanically generated aerosols, but also nitrate and sometimes sulfate Mass (C. Leck)

17. Marine aerosol system... cloud processing nucleation

18. Mineral Dust • Dust (mineral aerosols) • diameter size: 2-300 µm • main material: sand, silt, clay • includes essential trace metals such as Fe • consists of insoluble and soluble fractions

19. Seasalt aerosols... wind bubbles spray whitecap coverage W α U3+! seasalt production via bubble bursting... • film drops (many, small, drags along surface organics) • jet drops (fewer, larger, bulk) • spume drops (larger still)

20. Seasalt aerosols... Film Drops Jet Drops Number 20 nm 80 nm 1 m 10 m Particle Diameter (Dry) (from C. Leck)

21. Seasalt – number and mass as a function of surface wind speed seasalt particle number seasalt mass (compiled by Lewis and Schwartz, 2004)

22. Seasalt – mass fraction as a function of size (Lewis and Schwartz, 2004)

23. Marine aerosol mass fractions...

24. The sulfur story (in brief) ... • emissions: fossil fuel SO2, volcanic SO2, oceanic DMS • DMS oxidation ... gas phase ... complex! (mod. from Yin et al., 1990)

25. Happily, SO2 oxidation in the gas phase is simple... but most SO2 oxidation occurs in the aqueous phase... some basics...

26. heterogeneous oxidation of SO2 • in-cloud oxidation • weakly buffered, pH ~4 • oxidation by H2O2 • growth of CN, split Aitken mode • seasalt aerosols • strongly buffered by carbonate system • rapid oxidation by O3 • slower oxidation by H2O2 (also OH, halogen radicals...) • growth of existing particles, inhibits nucleation of new particles (Chameides and Stelson, 1992)

27. the single-particle view... Murphy et al., 1998; Buseck and Posfai, 1999

28. Organic aerosols - burning soot – “elemental carbon” formed in flames little spectral abs. dependence carbon-only “brown carbon”: sugars alcohols aromatics di/tri acids ketoacids hydroxyacids

29. Marine-derived organic aerosols... (O’Dowd et al., 2004)

30. Organic cluster Diatom Virus like Organic film Bacterium A surprise from the Arctic sea ice... Bacterium C. Leck

31. More organics in marine aerosols ... SEM micrograph of an Arctic marine aerosol sample. Single particles (diameter 1 μm) and groups of particles are lying between the fibers of the filter. About half of the particles are coated by an organic layer. Negative TOF-SIMS spectra of marine aerosol. mass 255  C15H31COOH (palmitic acid) C15 C14 C15 C17 C18 Tervahattu et al., 2002

32. Iodine and nucleation... • coastal I emissions  I2O5 • macroalgae • CH2I2, I2 ? • broader importance? O’Dowd et al., 2002a,b

33. Multi-year time series of bulk aerosol chemistry at island stations... eg. Prospero et al., 2003

34. Modeled aerosol sources... annual average sources (kg km-2 hr-1) IPCC, 2001

35. Aerosol properties from space... • mineral dust • urban pollution • biomass burning • aerosol size info Data from POLDER-1 (IPCC, Deuze et al., 1999)

36. Understanding forcing/feedback requires realistic aerosols in climate models...

37. What’s left to do...? • in situ aerosol/cloud radiation experiments! • shape – non-sphericity • mixing state • “life cycle” of marine aerosol organics, marine microlayer... • gas phase  aerosol chemistry (sulfur, iodine, organics) • aerosol  gas phase chemistry (organics, halogens) • depositional fluxes • coupled aerosol/chemistry/climate models • marine aerosols are woefully undersampled!

38. The end.