1. Measurement and modeling of aerosol Fe speciation Jingqiu Mao (GFDL/NOAA), Songmiao Fan (GFDL/NOAA), Ying Chen (Fudan U, China)

2. A dominant source of nutrient iron to open ocean, critical for plankton in surface waters: -Biological pump of CO2 -DMS production->sulfate->marine clouds->climate Why do we care about aerosol Fe? “Give me a half a tanker of iron and I'll give you the next ice age”- John Martin Phytoplankton blooms in the South Atlantic Ocean. (MODIS) Ocean Fe is mainly supplied by dust (95%)

3. Fe(II) is the bioavailable form of aerosol Fe Crystal structure of hematite Fe(II) solubilities ~ 0.1% Solubilization of dust Fe

4. Solubilization of dust Fe by atmospheric processing Soil has low Fe solubilities ~ 0.1% Solubilities of aerosol Fe in remote regions: up to 80% Fine aerosols (<2.5 µm) tend to yield larger iron solubilities than coarse aerosols (Siefert et al., 1999; Baker et al., 2006) (Baker et al., 2006) Aerosol mass Solubility

5. Solubilization of dust Fe Dust Fe(III)= Fe 3+ + Fe(OH) 2 + + Fe(OH) 2+ + Fe(SO 4 ) + + … Fe(II)= Fe 2+ + Fe(OH) + + Fe(SO 4 ) + …

6. Aqueous Fe chemistry Fe(II) + H 2 O 2 → Fe(III) + OH + OH − Fe(II) + OH → Fe(III) + OH − Fe(II)Fe(III) ??? Fe(III) + H 2 O 2 /HO 2 are too slow to be important

7. Current mechanisms for Fe(III)→Fe(II) (Zhuang et al., 1992, Nature) (1) Enhanced photolysis of Fe(III) by cloud processing Cloud: pH~4 Aerosols: pH<3 Cannot maintain the steady state of Fe(II)/Fe(III) after clouds evaporate. cloud aerosol ? Fe(OH) 2+ + hv

8. (2) Enhanced photolysis by organic acids (photolysis rate ~ 10 -2 s -1 ) (Zuo and Hoigné, 1992) Limitation: need continuous supply of oxalic acid in aerosols. We need something to sustain the steady state of Fe(II)/Fe(III)! Current mechanisms for Fe(III)→Fe(II) Fe 2+ + CO 2

9. Fe(II) + H 2 O 2 → Fe(III) + OH + OH − Lifetime of Fe(II)< 1hr for 1ppb H2O2 (Zhu et al., 1997) N-nighttime D-daytime Current mechanisms cannot explain nighttime Fe(II) measurements!! aerosol Fe(II) measurements in marine boundary layer O 2 - is the only electron donor we can think of at night, but Fe(III) + O 2 − is too slow. However, Cu(II) + O 2 − is faster by a factor of 300 Cu(I) + Fe(III) is very fast Fe(III) → Fe(II) ???????

10. A new driver for aqueous Fe(II) production-HO 2 uptake Cu(I) + Fe(III) → Cu(II) + Fe(II) electron transfer reaction (very fast) Cu(II) + HO 2 → Cu(I) + O 2 + H + Fe(II) + H 2 O 2 → Fe(III) + OH + OH − Fe(II) + OH → Fe(III) + OH − Cu-Fe redox coupling Fe(II) is sustained by gas-phase HO2!!!!

11. HO 2 measurement over remote ocean (Kanaya et al., 2000) HO 2 >0 Unique role of HO2 in heterogeneous chemistry: (1) its lifetime (~ 1-10 min), long enough for het chem (OH lifetime only ~1 s). (2) high polarity in its molecular structure. (very soluble compared to OH/CH 3 O 2 /NO/NO 2 ). (3) very reactive in aqueous phase (a major reason for DNA damage and cancer, superoxide). Nighttime Fe(II) can be supplied by nighttime HO2

12. Gas phase HO 2 uptake by particles HO 2 aerosol HO 2 (aq) NH 4 + SO 4 2- HSO 4 - Aqueous reactions NH 4 + HSO 4 - ④①②③ γ(HO 2 ) defined as the fraction of HO 2 collisions with aerosol surfaces resulting in reaction. ① ② ③④

13. Modeling framework for HO 2 aerosol uptake HO 2 aerosol [HO 2 ] surf R in [HO 2 ] surf [HO 2 ] bulk R out [HO 2 ] surf is higher than [HO 2 ] bulk because of its short lifetime. provides a relationship between [HO 2 ] surf and [HO 2 ] bulk. The diffusion equation with chemical loss (k I [HO 2 ]) and production (P HO2 ) Aqueous chemistry include Cu, Fe, Cu- Fe coupling, odd hydrogen and photolysis. Uptake rate Volatilization rate Chemical loss rate

14. Chemical budget for NH 4 HSO 4 aerosols at RH=85%, T=298 K Cu/Fe = 0.05, HO 2 (g) = 10 pptv, H 2 O 2 (g) = 1 ppb  Fe(III) reduction is dominated by Fe(III) + Cu(I), instead of photoreduction (implications for dust iron to ocean…)  This process is entirely driven by HO 2 ( γ(HO 2 ) = 0.7)  OH budget is controlled by TMI.

15. Fe(II)/Fe ratio modulated by gas-phase HO 2 concentrations Field measurements of Fe(II)/Fe_total in MBL Higher Cu/Fe ratio leads to higher Fe(II)/Fe_total Fe(II)/Fe_total

16. Future measurements to test such mechanism

17. Transitional metal is abundant in crustal and combustion aerosols Transitional metals have two or more oxidation states: Fe(II)Fe(III) Cu(I)Cu(II) - e + e - e + e reduction(+e) + oxidation(-e) = redox + + ++ + + - - - - - - - - - - - - - - - - - - - - - - -- - - - Redox coupling driven by gas-phase HO2 uptake

18. What else in dust aerosols? Measurements from dust aerosols There are tens of transitional metals in dust aerosols. We don’t know chemical kinetics for most of them. (Sun et al., 2005)

19. HO 2 Ocean Fe supply is driven by atmospheric HO 2 aerosol uptake!

21. Cu and Fe are ubiquitous in crustal and combustion aerosols Cu/Fe ratio is between 0.01-0.1 IMPROVE Cu is fully dissolved in aerosols. Fe solubility is 80% in combustion aerosols, but much less in dust.

22. Ionic strength correction for aerosol aqueous chemistry Non-ideal behavior due to the electrostatic interactions between the ions. 1.Use Aerosol Inorganic Model (AIM) to calculate the ionic strength and activity coefficients for major ions (i.e. NH 4 +, H +, HSO 4 -, SO 4 2- ). 2.Calculate activity coefficients for trace metal ions and neutral species based on specific ion interaction theory. 3.Account for salting-out effect on Henry’s law constant. A i is activity coefficient for any species and also a function of ionic strength. + - + - Ideal solution (cloud droplets) Non-ideal solution (aqueous aerosol) + + ++ + + - - - - - - - - - - - - - - - - - - - - - - -- - - -

23. Dependence on aerosol pH and Cu concentrations (A)γ(HO 2 ) in the range 0.4-1 at T = 298 K, should be close to 1 at lower T, due to higher solubility. (B)H 2 O 2 yield is more likely to be negative than positive. (C)HO 2 uptake is limited by aqueous diffusion until Cu = 5 x 10 -4 M. Cu/Fe=0.1 Cu/Fe=0.01 typical rural site

24. Organic aerosols (insoluble organic) Organic-electrolyte mixtures tend to have liquid-liquid phase separation state. (Zuend et al., ACP, 2012) (Furukawa et al., ACP, 2010) Water soluble organic aerosols Fe(III)C 2 O 4 and Fe(II)C 2 O 4 complexes are very unstable. Cu complexes can also be a significant sink for aqueous HO 2 (Voelker et al., EST, 2000)