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  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 A missing sink for radicals Implications for radiative forcing from biomass burning Aerosol Fe speciation sustained by gas-phase HO2

  4. O2 hn O3 (Levy, Science, 1971) STRATOSPHERE 8-18 km TROPOSPHERE hn NO2 NO O3 hn hn, H2O OH HO2 H2O2 CH4,CO, VOCs Deposition H2O2 is a radical reservoir.

  5. 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). 20 S – 20 N The alternative explanation is that model OH is wrong, but how? 20 – 90 S (Shindell et al., JGR, 2006)

  6. Present Day OH Inter-hemispheric (N/S) ratio 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 Models Obs-derived (Naiket al., ACP, 2013)

  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). Aerosol uptake is only significant when gas-phase [HO2] is relatively low. Gas: L[HO2] ~ [HO2]∙ [HO2] Uptake: L[HO2] ~ [HO2] 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 HO2(g) H2O2(g) HO2(aq)+O2-(aq)→ H2O2 (aq) Conventional HO2 uptake: HO2 → H2O2(g) Cu(II) Cu(I) (Mao et al., ACP, 2010)

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

  11. Median vertical profiles in Arctic spring (observations vs. GEOS-Chemmodel) 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. 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: -e Fe(II) Fe(III) + e -e Cu(I) Cu(II) + 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. 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…

  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… But we missed one electron transfer reaction (very fast) Cu(I) + Fe(III) → Cu(II) + Fe(II)

  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) 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).

  18. Cu-Fe redox coupling in aqueous aerosols Cu only: HO2→ H2O2 Cu + Fe : HO2 → H2Oor H2O2 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., 2013, ACP)

  19. 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.

  20. 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.

  21. 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 Fe speciation)

  22. γ(HO2) dependence on aerosol pH and Cu concentrations Cu/Fe=0.1 Cu/Fe=0.01 typical rural site • γ(HO2) is high at typical rural conditions (0.4-1 at 298 K), even higher at low T. • Effective γ(HO2) can be higher than 1, due to the reactive uptake of H2O2. • γ(HO2) uptake is still higher than 0.1 when Cu is diluted by a factor of 10. (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(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

  24. 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., 2013, ACP)

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

  26. Conclusions • The product of HO2 uptake is likely to be H2O, not the radical reservoir H2O2. • γ(HO2) is somewhere between 0.1 and >1.0. This remains largely uncertain. • We find that the model results are largely improved when γ(HO2) set to 1 (both GEOS-Chem and AM3). • Further experimental work is needed, particularly at low T (< room temperature 298 K).

  27. Outline A missing sink for radicals Implications for radiative forcing from biomass burning Aerosol Fe speciation sustained by gas-phase HO2

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

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

  30. Model suggests a net warming effect when bb emissions increased by more than a factor of 2. (Mao et al., 2013, GRL)

  31. Outline A missing sink for radicals Implications for radiative forcing from biomass burning Aerosol Fe speciation sustained by HO2 uptake

  32. Why do we care about aerosol Fe speciation? We want to test our model for Cu-Fe-HOxchemistry. A dominant source of nutrient iron to open ocean, critical for plankton in surface waters. Oxidative stress and health impact of ambient aerosols Ocean Fe is mainly supplied by dust (95%) Phytoplankton blooms in the South Atlantic Ocean. (MODIS) “Give me a half a tanker of iron and I'll give you the next ice age”- John Martin

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

  34. Solubilization of dust Fe Fe(III)= Fe3+ + Fe(OH)2+ + Fe(OH)2+ + Fe(SO4)+ + … Fe(II)= Fe2+ + Fe(OH)++ Fe(SO4) + … Fe(II) is more bioavailable

  35. What do we know about Fe redox chemistry? Fe(II) + HO2 Fe(III) + H2O2 Fe(II) + H2O2 → Fe(III) + OH + OH− Fe(II) + OH → Fe(III) + OH− Fe(II) Fe(III) ??? Fe(III) + H2O2/HO2 are too slow to be important Fe(III)= Fe3+ + Fe(OH)2+ + Fe(OH)2+ + Fe(SO4)+ + Fe(C2O4) + … Fe(II)= Fe2+ + Fe(OH)+ + Fe(SO4) + Fe(C2O4) 0 …

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

  37. Current mechanisms for Fe(III)→Fe(II) (2) Enhanced photolysis by organic acids Fe2+ + CO2 (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).

  38. Current mechanisms cannot explain nighttime Fe(II) measurements!! Fe(II) + H2O2 → Fe(III) + OH + OH− Lifetime of Fe(II)< 1hr for 1ppb H2O2 N-nighttime D-daytime (Zhu et al., 1997) 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. Nighttime Fe(II) can be supplied by nighttime HO2 Diurnal cycle of HO2 over remote ocean HO2 >0 (Kanayaet al., 2000) Nighttime HO2 can be produced from O3/NO3 + 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) Solubility Aerosol mass (Baker et al., 2006)

  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 HO2 uptake • A major aqueous OH source (converted from gas-phase HO2 and H2O2), 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 Observations based on Jaffe et al. (2012) ΔO3/ΔCO is a measure of ozone production efficiency. (Mao et al., 2013, GRL)

  47. Fe(II)/Fe ratio modulated by gas-phase HO2 concentrations Field measurements of Fe(II)/Fe_total in MBL Fe(II)/Fe_total Higher Cu/Fe ratio leads to higher 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… Henry Taube Nobel Prize in 1983 Rudolph A. Marcus Nobel Prize in 1992 For contributions on electron transfer reactions between metal complexes.