1. Heterogeneous chemistry and its potential impact on climate Jingqiu Mao (Princeton/GFDL) North Carolina State University, 11/25/2013

2. Acknowledgement Songmiao Fan (GFDL) Daniel Jacob (Harvard) Larry Horowitz (GFDL) Vaishali Naik (GFDL)

3. Outline 1.A missing sink for radicals 2.Implications for radiative forcing from biomass burning 3.Aerosol Fe speciation sustained by gas-phase HO 2

4. O3O3 O2O2 O3O3 OHHO 2 h, H 2 O Deposition NO H2O2H2O2 CH 4, CO, VOCs NO 2 STRATOSPHERE TROPOSPHERE 8-18 km h h h H 2 O 2 is a radical reservoir. (Levy, Science, 1971)

5. Models ONLY underestimate CO in Northern extratropics (Shindell et al., JGR, 2006) Cannot be explained by emissions: Need to double current CO anthro emissions (Kopacz et al., ACP, 2010). MOPITT satellite (500 hPa) Multi-model mean (500 hPa) 20-90 N 20 S – 20 N 20 – 90 S Annual cycle of CO The alternative explanation is that model OH is wrong, but how?

6. All models have more OH in NH than SH (N/S > 1) Obs-derived estimates show the opposite (N/S < 1), with 15-30% uncertainties (Naik et al., ACP, 2013) Obs-derivedModels Present Day OH Inter-hemispheric (N/S) ratio

7. O3O3 OHHO 2 h, H 2 O Deposition NO H2O2H2O2 CH 4, CO, VOC NO 2 Clouds/Aerosols h Uniqueness of HO 2 in heterogeneous chemistry: lifetime long enough for het chem (~ 1-10 min vs ~1 s for OH). high polarity in its molecular structure (very soluble compared to OH/CH 3 O 2 /NO/NO 2 ). very reactive in aqueous phase (superoxide, a major reason for DNA damage and cancer). Gas: L[HO 2 ] ~ [HO 2 ]∙ [HO 2 ] Uptake: L[HO 2 ] ~ [HO 2 ] Aerosol uptake is only significant when gas-phase [HO 2 ] is relatively low.

8. 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

9. Laboratory measured γ(HO 2 ) on sulfate aerosols are generally low… Except when they add copper in aerosols… Cu-doped Aqueous Solid (Mao et al., ACP, 2010) HO 2 (aq)+O 2 - (aq)→ H 2 O 2 (aq) Cu(II) Cu(I) HO 2 (g)H 2 O 2 (g) Conventional HO 2 uptake: HO 2 → H 2 O 2 (g)

10. Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) Phase I: April 1 st ~ April 20 th,2008 ARCTAS-A DC-8 flight track

11. Conventional HO 2 uptake does not work over Arctic! (Mao et al., ACP, 2010) Joint measurement of HO2 and H 2 O 2 suggest that HO 2 uptake by aerosols may in fact not produce H 2 O 2 ! Median vertical profiles in Arctic spring (observations vs. GEOS-Chem model) We hypothesized a bisulfate reaction to explain this: But it is not catalytic and thereby inefficient to convert HO 2 radical to water. There must be something else …

12. I took this picture

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

14. 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.

15. Cu(II) + HO 2 → Cu(I) + O 2 + H + What we thought was happening in aerosols… As Fe(III) + HO 2 is 300 times slower than Cu(II) + HO 2, so we thought Fe was unimportant… Net: HO 2 +HO 2 → H 2 O 2 + O 2

16. Cu(II) + HO 2 → Cu(I) + O 2 + H + What we thought was happening in aerosols… As Fe(III) + HO 2 is 300 times slower than Cu(II) + HO 2, so we thought Fe was unimportant… But we missed one electron transfer reaction (very fast) Cu(I) + Fe(III) → Cu(II) + Fe(II) Net: HO 2 +HO 2 → H 2 O 2 + O 2

17. Cu(II) + HO 2 → Cu(I) + O 2 + H + What we thought was happening in aerosols… As Fe(III) + HO 2 is 300 times slower than Cu(II) + HO 2, 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) + H 2 O 2 → Fe(III) + OH + OH − Fe(II) + OH → Fe(III) + OH − The product from HO2 uptake depends on the fate of Fe(II). Net: HO 2 +HO 2 → H 2 O 2 + O 2 Net: HO 2 + H 2 O 2 → OH + O 2 + H 2 O Net: HO 2 +HO 2 → H 2 O 2 + O 2 Net: HO 2 + OH → O 2 + H 2 O

18. Cu-Fe redox coupling in aqueous aerosols Cu only: HO 2 → H 2 O 2 Cu + Fe : HO 2 → H 2 O or H 2 O 2 and may also catalytically consume H 2 O 2. Conversion of HO 2 to H 2 O is much more efficient as a radical loss. In gas phase, H 2 O 2 can photolyze to regenerate OH and HO 2. (Mao et al., 2013, ACP)

19. 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

20. 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) + + ++ + + - - - - - - - - - - - - - - - - - - - - - - -- - - -

21. 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  70% of HO 2 gas uptake is lost in aerosols ( γ(HO 2 ) = 0.7)  no H 2 O 2 is net produced.  Fe(III) reduction is dominated by Fe(III) + Cu(I), instead of photoreduction (implications for Fe speciation)

22. γ(HO 2 ) dependence on aerosol pH and Cu concentrations γ(HO 2 ) is high at typical rural conditions (0.4-1 at 298 K), even higher at low T. Effective γ(HO 2 ) can be higher than 1, due to the reactive uptake of H 2 O 2. γ(HO 2 ) uptake is still higher than 0.1 when Cu is diluted by a factor of 10. Cu/Fe=0.1 Cu/Fe=0.01 typical rural site (Mao et al., 2013, ACP)

23. Test this mechanism in two global models GFDL AM3 chemistry-climate model (nudge) GEOS-Chem chemical transport model In both models, we assume γ(HO 2 ) = 1 producing H 2 O for all aerosol surfaces (based on effective radius and hygroscopic growth). number area volume Aerosol surface area is mainly contributed by submicron aerosols (sulfate, organic carbon, black carbon) Typical aerosol distribution

24. Improvement on modeled CO in Northern extratropics Black: NOAA GMD Observations at remote surface sites Green: GEOS-Chem with (γ(HO 2 ) = 1 producing H 2 O) Red: GEOS-Chem with (γ(HO 2 ) = 0) (Mao et al., 2013, ACP)

25. CO at 500 hPa AM3 with het chem off MOPITT AM3 with het chem on MOPITT (2000-2004) AM3(2001-2005) OH ratio (NH/SH) (Mao et al., 2013, GRL) Improvement in AM3 model

26. Conclusions The product of HO 2 uptake is likely to be H 2 O, not the radical reservoir H 2 O 2. γ(HO 2 ) is somewhere between 0.1 and >1.0. This remains largely uncertain. We find that the model results are largely improved when γ(HO 2 ) set to 1 (both GEOS-Chem and AM3). Further experimental work is needed, particularly at low T (< room temperature 298 K).

27. Outline 1.A missing sink for radicals 2.Implications for radiative forcing from biomass burning 3.Aerosol Fe speciation sustained by gas-phase HO 2

28. The impact of biomass burning emissions on oxidants and radiative forcing IPCC AR4 only estimates the direct forcing from biomass burning aerosols (+0.03 ±0.12 W m -2 ). Cooling or warming?

29. Perturbation tests of biomass burning emissions on global OH and ozone Computed change of global mean OH is 6.3% for doubling 2000 bb emissions. (Prinn et al., 2005). 1997 Indonesian fires, 6% AM3 model with different magnitude of biomass burning emissions (for year 2000). Estimated global OH from CH3CCl3 (Mao et al., 2013, GRL)

30. Model suggests a net warming effect when bb emissions increased by more than a factor of 2.

31. Outline 1.A missing sink for radicals 2.Implications for radiative forcing from biomass burning 3.Aerosol Fe speciation sustained by HO 2 uptake

32. We want to test our model for Cu-Fe-HO x chemistry. A dominant source of nutrient iron to open ocean, critical for plankton in surface waters. Oxidative stress and health impact of ambient aerosols Why do we care about aerosol Fe speciation? “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%)

33. Crystal structure of hematite Fe(II) solubilities ~ 0.1% Solubilization of dust Fe by atmospheric processing (Shi et al., 2012)

34. Solubilization of dust Fe Fe(III)= Fe 3+ + Fe(OH) 2 + + Fe(OH) 2+ + Fe(SO 4 ) + + … Fe(II)= Fe 2+ + Fe(OH) + + Fe(SO 4 ) + … Fe(II) is more bioavailable

35. What do we know about Fe redox 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 Fe(III)= Fe 3+ + Fe(OH) 2 + + Fe(OH) 2+ + Fe(SO 4 ) + + Fe(C 2 O 4 ) + … Fe(II)= Fe 2+ + Fe(OH) + + Fe(SO 4 ) + Fe(C 2 O 4 ) 0 …

36. 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. Most time they are still in aerosol form. cloud aerosol ? Fe(OH) 2+ + hv

37. (2) Enhanced photolysis by organic acids (photolysis rate ~ 10 -2 s -1 ) (Zuo and Hoigné, 1992, Johnson et al., 2013) Limitation: need continuous supply of oxalic acid in aerosols (still unidentified yet). Current mechanisms for Fe(III)→Fe(II) Fe 2+ + CO 2

38. 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!! Significant amount of nighttime Fe(II) found in marine boundary layer!

39. A new driver for aqueous Fe(II) production Fe(II) is sustained by gas-phase HO2!!!!

40. Diurnal cycle of HO 2 over remote ocean (Kanaya et al., 2000) HO 2 >0 Nighttime Fe(II) can be supplied by nighttime HO2 Nighttime HO2 can be produced from O 3 /NO 3 + VOCs.

41. Future measurements to test such mechanism This mechanism can be easily tested by concurrent measurements of Cu and Fe in aerosols.

42. Extra slides

43. 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

44. Sensitivity of tropospheric oxidants to biomass burning emissions Global OH decreases with larger bb emissions. Global ozone increases with larger bb emissions.

45. Other applications for aerosol TMI chemistry driven by HO 2 uptake A major aqueous OH source (converted from gas-phase HO 2 and H 2 O 2 ), critical for SOA formation aerosol aging (O/C ratio). Oxidative stress and health (sustain soluble form of transitional metals in aerosols).

46. Aerosol uptake has large impact on ozone production efficiency ΔO 3 / ΔCO is a measure of ozone production efficiency. Observations based on Jaffe et al. (2012) (Mao et al., 2013, GRL)

47. 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

48. 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)

49. 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… For contributions on electron transfer reactions between metal complexes. Rudolph A. Marcus Nobel Prize in 1992 Henry Taube Nobel Prize in 1983