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A missing sink for radicals

A missing sink for radicals. Jingqiu Mao (Princeton/GFDL). With Songmiao Fan (GFDL), Daniel Jacob (Harvard), Larry Horowitz (GFDL) and Vaishali Naik (GFDL). I took this picture . O 2. h n. O 3. (Levy, Science, 1971). STRATOSPHERE. 8-18 km. TROPOSPHERE. h n. NO 2. NO. O 3.

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A missing sink for radicals

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  1. A missing sink for radicals Jingqiu Mao (Princeton/GFDL) With Songmiao Fan (GFDL), Daniel Jacob (Harvard), Larry Horowitz (GFDL) and VaishaliNaik (GFDL)

  2. I took this picture

  3. 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.

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

  5. Present Day OH Inter-hemispheric (N/S) ratio: All models have more OH in NH than SH (N/S > 1) Obs-based estimates indicate N/S < 1 with 15-30% uncertainties (Naiket al., ACP, 2013)

  6. A missing sink: HO2 uptake by aerosols • lifetime long enough for finding aerosols • ~ 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 • electron donor, good for any redox reactions. Aerosol Gas-phase loss: L[HO2] ~ [HO2]∙ [HO2] Aerosol uptake: L[HO2] ~ [HO2] Aerosol uptake is only significant when gas-phase [HO2] is relatively low.

  7. 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

  8. 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) Current models always assume HO2 is converted to H2O2 by aerosol uptake.

  9. 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

  10. 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)

  11. 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

  12. 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.

  13. 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)

  14. 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. • HO2 uptake is still higher than 0.1 when Cu is diluted by a factor of 10. (Mao et al., 2013, ACP)

  15. 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)

  16. AM3 results 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)

  17. Aerosol uptake has large impact on ozone production efficiency Observations ΔO3/ΔCO is a measure of ozone production efficiency. (Mao et al., 2013, GRL)

  18. 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).

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