The chemical composition of stratospheric aerosol particles

1. Daniel Murphy Chemical Sciences Division Earth System Research Laboratory National Oceanic and Atmospheric Administration The chemical composition of stratospheric aerosol particles Mossop, Nature, 1964

2. Outline • Known for many years: • Stratospheric aerosols are mostly sulfuric acid • from oxidation of OCS and SO2 • Newer results: • Almost all particles in the lower stratosphere are one of three types • Many solid inclusions in stratospheric particles • Black carbon data • Research needs:halogens, higher altitudes

3. PALMS: Particle Analysis by Laser Mass Spectrometry Particle enters vacuum. Trigger from light scattered from continuous laser. Excimer laser beam hits particle. Positive or negative ions analyzed with TOF mass spectrometer. - Size range about 0.25 (0.15 with low efficiency) to over 3 µm diameter Much of the mass and light scatteringMinority by numberOnly up to 19 km, all volcanic background

4. 3 types describe >95% of stratospheric particles Type 1: Sulfuric acid with metals Murphy et al., JGR, 2007

5. Source of stratospheric metals Meteorites Na, Mg, Ca, Mn, Ni also match meteoric ratios 2500 kg yr-1 global flux of meteoritic K Earth’s crust

6. Mechanism meteor 120 km Gas phase O, Fe, Si, etc. Transport, coagulation, add H2SO4 80 km nanoparticles Many nanoparticles per lower stratospheric particle 20 km Sidelight: Calibrations for Fe::S constrain the flux of meteors.Cziczo et al., Science, 2001

7. Another mode of sulfuric acid particles Clearly bimodal: There are at least two distinct source regions of stratospheric sulfuric acid particles.

8. Type 2: Sulfuric acid without metals C+ peak similar to SO+ => order 1% carbon No Fe! Also stratospheric in origin

9. Profile with and without metals • With metals: • High altitude, high latitude source. • Consistent with Curtius et al. nonvolatile CN in Arctic vortex. • ?? with new Campbell-Deshler CN data in Antarctic.

10. Type 3: Organic-sulfate mixtures C+ peak 100x larger than SO+=> more C than S Upper tropospheric particles CARIBIC found C/S elemental ratio about 0.5 in the lowermost stratosphere(Ngung et al. JGR 2008).

11. Organic content Particles formed in the troposphere have more organic content. They keep it: Tropospheric particles that spend months in the stratosphere don’t lose much organic content.-Little loss of semi-volatiles -Very slow loss of organic mass due to heterogeneous reactions. Compare to ~ 1 week against OH with = 0.1 Not shown: Biomass burning signature similar to upper troposphere-Pyrocumulus doesn’t dominate

12. Composition-resolved size distributions Costa Rica 18.8 to 19.5 km 17 to 18 km • Chuck Wilson FCAS size distributions apportioned by PALMS • Added sulfate just above tropical tropopause mostly an external mixture • Tropopause particles more organic than at 14 km • Meteoritic-sulfuric particles are larger (older).

13. Composition-resolved size distributions • PALMS measures precise aerodynamic diameters, convert to geometric • Fewer particles below 250 nm caused by PALMS transmission • Sulfate particles often larger than organic-sulfate(where PALMS has measured) • Sulfate sometimes monodisperse, but not always • Monodisperse distributions will occasionally wreak havoc with optical instruments

14. Pole Equator Pole summer winter Stratospheric particles Almost pure sulfuric Nucleation Nucleation Non-volatile CN Sulfuric w/ metals Troposphere Organic-sulfate

15. Solid or liquid? Sulfuric acid without metals: probably liquid sulfuric acid tetrahydrate stable but strong formation barriers many balloon impactor measurements show liquid particles

16. Organic-sulfate particles: Solid or liquid? Particles at tropical tropopause not very acidic, high organic content - from deep continental convection CR-AVE Pre-Ave Froyd et al., 2009

17. Solid or liquid? • Mixed organic-sulfate from troposphere: • high organic content: glassy or part-glassy (Murray, Zobrist, others) • often neutralized: will effloresce in dry stratosphere Also implications for scattering phase function vs 1960’s: liquid acid 1960’s: ammonium sulfate Now: liquid Now: ammon. sulfate or organic probably acid but satellite droplets can be organic Bigg et al., Tellus, 1970; also Junge, Mossop Nguyen et al. JGR 2008reinterpret Renard et al. papers on solid particles

18. Sulfuric acid with metals: Solid or liquid? When laser hits mixed particles: same mass spectrum regardless of location different laser hits give different ion ratios Mossop, 1964 after wash

19. Sulfuric acid with metals: Solid or liquid? stratosphere Extremely narrow Fe, Mg, Ni ratiossuggest they are dissolved (lab solubility data not consistent) Broader Al, Si ratios suggest inclusions (consistent with insoluble oxides) Stratospheric Mg/Fe is narrower than nominally identical sea salt particles! Mixing of elements from different micro-meteors

20. Black carbon is a minor constituent Compare: A few ppb by mass total aerosol

21. Halogens in PALMS • Commonly on organic-sulfate particles • Less common as perchlorate on sulfuric acid

22. Halogens in PALMS • Fluorine and tropospheric chlorine mostly on dust and soot • Br and Iodine peak just above tropopause • Rough calibrations indicate most of iodine and a few ppt of bromine are in aerosol – sedimentation • Perhaps 10 ppt of chlorine • Few other data

23. Summary • We can determine the source of individual particles: • Organic-sulfate mixtures: troposphere• Sulfate with meteoric metals: upper stratosphere, polar vortex• Sulfate without metals: lower stratosphere• Multiple sources imply properties are a sum over sources • Suggestions about phase: • Fe, Mg, Ni are probably dissolved in the sulfuric acid• Al, Si are probably inclusions• Organic-sulfate particles may be glassy and/or effloresced• Many particles are at least partially solid; will affect scattering phase functions • Trace constituents: • Recent data on black carbon• Halogens, mostly on tropospheric particles• Probably most of iodine, some bromine are in aerosol• Hints about perchlorate. On sulfuric acid so might respond to added sulfur.•Need better halogen data!

24. Thank you for your attention! Dan Cziczo, Storm Peak Dave Thomson, Costa Rica Paula Hudson, Houston Karl Froyd, on DC-8 Todd Sanford, Costa Rica Mike Schein, Boulder Murphy et al., Science, 282, 1664, 1998. initial resultsMurphy and Thomson, GRL, 3217, 2000. halogensFroyd et al., Atmos. Chem. Phys., 2009. TTL compositionMurphy et al., QJRMS, in press. synthesis paper Troy Thornberry, Costa Rica