Radical loss in the atmosphere from Cu-Fe redox coupling in aerosols

1. Radical loss in the atmosphere from Cu-Fe redox coupling in aerosols Jingqiu Mao(Princeton/GFDL), Songmiao Fan (GFDL), Daniel Jacob (Harvard), Katherine Travis (Harvard), Larry Horowitz (GFDL), VaishaliNaik (GFDL)

2. Outline Tropospheric chemistry and potential issues The role of aerosol uptake Cu-Fe redox coupling in aerosols Global implications for atmospheric oxidant chemistry Other applications of aerosol TMI chemistry

3. Tropospheric radical chemistry Air Quality O2 hn O3 STRATOSPHERE Climate 8-18 km TROPOSPHERE hn NO2 NO O3 hn hn, H2O OH HO2 H2O2 CH4,CO, VOC Deposition H2O2 is a radical reservoir.

4. Models ONLY underestimate CO in Northern extratropics Annual cycle of CO MOPITT satellite (500 hPa) Multi-model mean (500 hPa) 20-90 N • Cannot be explained by emissions: • Need to double current CO anthro emissions (Kopacz et al., ACP, 2010). • Why the discrepancy peak at spring? Should peak in winter if we underestimate heating or vehicle cold start. • Double CO emissions will lead to a higher ozone in northern extratropics (we already have too much ozone). 20 S – 20 N 20 – 90 S (Shindell et al., JGR, 2006)

5. The alternative explanation is that model OH is wrong, but how? N/S Interhemispheric OH Ratio Derived hemispheric OH concentrations from CH3CCl3 measurements OH ratio (NH/SH) SH ≥ NH models OH Conc obs (Prinn et al., Science, 2001) Observations show that SH ≥ NH All models show that NH ≥SH

6. Outline Tropospheric chemistry and potential issues The role of aerosol uptake Cu-Fe redox coupling in aerosols Global implications for atmospheric oxidant chemistry Other applications of aerosol TMI chemistry

7. Uniqueness of HO2 in heterogeneous chemistry: • lifetime long enough for het chem(~ 1-10 minvs ~1 s for OH). • high polarity in its molecular structure (very soluble compared to OH/CH3O2/NO/NO2). • very reactive in aqueous phase(superoxide, a major reason for DNA damage and cancer). O2 Gas: L[HO2] ~ [HO2]∙ [HO2] Uptake: L[HO2] ~ [HO2] hn O3 STRATOSPHERE 8-18 km TROPOSPHERE Clouds/Aerosols hn NO2 NO O3 hn, H2O OH HO2 H2O2 CH4,CO, VOC Deposition

8. Gas phase HO2 uptake by particles aerosol HSO4- NH4+ NH4+ HO2 HO2(aq) SO42- ① ② ③ ④ NH4+ Aqueous reactions HSO4- SO42- HSO4- NH4+ γ(HO2) defined as the fraction of HO2 collisions with aerosol surfaces resulting in reaction. HSO4- SO42- NH4+ SO42- ① ③ ④ ②

9. Laboratory measured γ(HO2) on sulfate aerosols are generally low… Except when they add copper in aerosols… Cu-doped Aqueous Solid Conventional HO2 uptake by aerosol with H2O2 formation HO2(g) H2O2(g) HO2(aq)+O2-(aq)→ H2O2 (aq) (Mao et al., ACP, 2010) Cu(II) Cu(I) The role of copper has been ignored in HO2 uptake because we thought it makes H2O2.

10. Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) Phase I: April 1st ~ April 20th ARCTAS-A DC-8 flight track

11. Median vertical profiles in Arctic spring (observations vs. model) Joint measurement of HO2 and H2O2 suggest that HO2 uptake by aerosols may in fact not produce H2O2 ! Conventional HO2 uptake does not work over Arctic! We hypothesized a bisulfate reaction to explain this: But it is not catalytic and thereby inefficient to convert HO2 radical to water. There must be something else … (Mao et al., ACP, 2010)

12. I took this picture

13. Outline Tropospheric chemistry and potential issues The role of aerosol uptake Cu-Fe redox coupling in aerosols Global implications for atmospheric oxidant chemistry Other applications of aerosol TMI chemistry

14. Cu is one of 47 transitional metals in periodic table… Trace metals in urban aerosols (Heal et al., AE, 2005) Transitional metals have two or more oxidation states: -e Fe(II) Fe(III) + e -e Cu(I) Cu(II) + e reduction(+e) + oxidation(-e) = redox

15. Cu and Fe are ubiquitous in crustal and combustion aerosols Cu is mainly from combustion in submicron 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.

16. What we thought was happening in aerosols… Cu(II) +HO2 → Cu(I) + O2 + H+ Cu(I) +HO2 Cu(II) + H2O2 Net: HO2 +HO2 →H2O2 + O2 As Fe(III) + HO2 is 300 times slower than Cu(II) + HO2, so we thought Fe was unimportant…

17. What we thought was happening in aerosols… Cu(II) +HO2 → Cu(I) + O2 + H+ Cu(I) +HO2 Cu(II) + H2O2 Net: HO2 +HO2 →H2O2 + O2 As Fe(III) + HO2 is 300 times slower than Cu(II) + HO2, so we thought Fe was unimportant… But we missed one electron transfer reaction (very fast) Cu(I) + Fe(III) → Cu(II) + Fe(II)

18. What we thought was happening in aerosols… Cu(II) +HO2 → Cu(I) + O2 + H+ Cu(I) +HO2 Cu(II) + H2O2 Net: HO2 +HO2 →H2O2 + O2 As Fe(III) + HO2 is 300 times slower than Cu(II) + HO2, so we thought Fe was unimportant… But we missed one electron transfer reaction (very fast) Cu(I) + Fe(III) → Cu(II) + Fe(II) With three reactions to close the cycle… Fe(II) + HO2 Fe(III) + H2O2 Net: HO2 +HO2 →H2O2 + O2 Net: HO2 + H2O2→ OH + O2 + H2O Fe(II) + H2O2 → Fe(III) + OH + OH− Net: HO2 + OH → O2 + H2O Fe(II) + OH → Fe(III) + OH− The product from HO2 uptake depends on the fate of Fe(II).

19. Cu-Fe redox coupling in aqueous aerosols driven by HO2 uptake from the gas phase With Cu alone, HO2 is converted to H2O2. With both Cu and Fe, HO2is converted to either H2O2 or H2O, and may also catalytically consume H2O2. Conversion of HO2 to H2O is much more efficient as a radical loss. In gas phase, H2O2 can photolyze to regenerate OH and HO2. (Mao et al., 2012, ACPD)

20. Modeling framework for HO2 aerosol uptake aerosol Aqueous chemistry include Cu, Fe, Cu-Fe coupling, odd hydrogen and photolysis. Rout HO2 [HO2]surf Rin [HO2]bulk [HO2]surf Uptake rate [HO2]surf is higher than [HO2]bulk because of its short lifetime. Volatilization rate The diffusion equation with chemical loss (kI[HO2]) and production (PHO2) Chemical loss rate provides a relationship between [HO2]surf and [HO2]bulk.

21. Ionic strength correction for aerosol aqueous chemistry Ideal solution (cloud droplets) Non-ideal solution (aqueous aerosol) Ai is activity coefficient for any species and also a function of ionic strength. - - - - - + + + - - - - - - - + - - - - - - Non-ideal behavior due to the electrostatic interactions between the ions. + - + + - - - - - - - + - - - Use Aerosol Inorganic Model (AIM) to calculate the ionic strength and activity coefficients for major ions (i.e. NH4+, H+, HSO4-, SO42-). Calculate activity coefficients for trace metal ions and neutral species based on specific ion interaction theory. Account for salting-out effect on Henry’s law constant.

22. Chemical budget for NH4HSO4 aerosols at RH=85%, T=298 K Cu/Fe = 0.05, HO2(g) = 10 pptv, H2O2(g) = 1 ppb • 70% of HO2 gas uptake is lost in aerosols (γ(HO2) = 0.7) • no H2O2 is net produced. • Fe(III) reduction is dominated by Fe(III) + Cu(I), instead of photoreduction(implications for ocean iron fertilization)

23. Dependence on aerosol pH and Cu concentrations Cu/Fe=0.1 Cu/Fe=0.01 typical rural site γ(HO2) in the range 0.4-1 at T = 298 K, should be close to 1 at lower T, due to higher solubility. H2O2 yield is more likely to be negative than positive. HO2 uptake is limited by aqueous diffusion until Cu = 5 x 10-4 M. (Mao et al., 2012, ACPD)

24. Outline Tropospheric chemistry and potential issues The role of aerosol uptake Cu-Fe redox coupling in aerosols Global implications for atmospheric oxidant chemistry Other applications of aerosol TMI chemistry

25. Improvement on modeled CO in Northern extratropics Black: NOAA GMD Observations at remote surface sites Green: GEOS-Chemwith (γ(HO2) = 1 producing H2O) Red: GEOS-Chemwith (γ(HO2) = 0) (Mao et al., 2012, ACPD)

26. Improvement on N/S Interhemispheric OH Ratio Observational constraints from CH3CCl3 measurements All models show that NH ≥SH OH ratio (NH/SH) SH ≥ NH AM3 with aerosol uptake obs (Prinn et al., Science, 2001) In AM3, methane lifetime increases from 8.5 year to 9.6 year !

27. Implications for radiativeforcing…warming effect from aerosols Aerosols OH CH4 HFCs trop ozone strat H2O See poster on Thursday Mao et al., Sensitivity of tropospheric oxidants to wildfires: implications for radiativeforcing (A43E-0205).

28. Other applications for aerosol TMI chemistry driven by HO2 uptake (1) • A major aqueous OH source (converted from gas-phase HO2 and H2O2), critical for SOA formation. • Dust iron solubilization (dust provides 95% of ocean iron) • Oxidative stress and health (sustain soluble form of transitional metals in aerosols).

29. Other applications for aerosol TMI chemistry driven by HO2 uptake (2) • Aerosol optical properties.

30. We only explored two transitional metals here… Manganese (Mn) Chromium (Cr) ? Cobalt (Co) ? Vanadium (V) ? Zinc (Zn)? Titanium (Ti)?? They may be all redox-coupled ! The theory is well established… Henry Taube Nobel Prize in 1983 Rudolph A. Marcus Nobel Prize in 1992 For contributions on electron transfer reactions between metal complexes.

31. Extra slides

32. Test this mechanism in two models GFDL AM3 chemistry-climate model (nudge) GEOS-Chem chemical transport model In both models, we assume γ(HO2) = 1 producing H2O for all aerosol surfaces (based on effective radius and hygroscopic growth). Typical aerosol distribution number area Aerosol surface area is mainly contributed by submicron aerosols (sulfate, organic carbon, black carbon) volume

33. Impact on global OH (annual mean at surface) run with uptake – run with no uptake Obs Both model confirms significant decrease of northern hemisphere OH by aerosol uptake. GEOS-Chem show a larger decrease over Arctic due to a larger aerosol surface area. AM3 (Liu et al., JGR, 2011)

34. Impact on global CO (annual mean at surface) run with uptake – run with no uptake Story is consistent with CO… We saw a large increase of CO in spring in GEOS-Chem, but not much so in AM3, maybe due to aerosol surface area…

35. MOPITT (500 hPa) Multi-model mean (500 hPa) AM3 simulations 20-90 N 20 S – 20 N 20 – 90 S (Shindell et al., JGR, 2006)

36. Impact on global O3 (annual mean at surface) run with uptake – run with no uptake We see a large decrease of ozone over East Asia in both models. This means that ozone can be a lot higher without man-made aerosols!!! SO2 BC (Lamarqueet al., Climate Change, 2011) BC Courtesy of V. Naik

37. Conclusions We propose a new catalytic mechanism (Cu-Fe redox coupling) in aerosol aqueous chemistry and largely improve model-to- observation comparisons. This mechanism has a major and previously unrecognized impact on atmospheric oxidant chemistry, and has important implications for air quality and radiative forcing. This mechanism may also help to understand the supply of dust iron to the ocean. There are many trace metals in aerosols. We only explored two here…heterogeneous process may be responsible for other unresolved issues in atmospheric chemistry (ozone, SOA, NOx, halogen etc.).

38. Organic aerosols (insoluble organic) Organic-electrolyte mixtures tend to have liquid-liquid phase separation state. (Zuend et al., ACP, 2012) Water soluble organic aerosols Fe(III)C2O4 and Fe(II)C2O4 complexes are very unstable. Cu complexes can also be a significant sink for aqueous HO2 (Voelker et al., EST, 2000) (Furukawa et al., ACP, 2010)

39. H2O2: Aircraft Observations Run with uptake Run with no uptake