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

2. I took this picture

3. 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)

4. 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?

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 (Naik et al., ACP, 2013)

6. lifetime long enough for finding aerosols  ~ 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  electron donor, good for any redox reactions. Gas-phase loss: L[HO 2 ] ~ [HO 2 ]∙ [HO 2 ] Aerosol uptake: L[HO 2 ] ~ [HO 2 ] Aerosol uptake is only significant when gas-phase [HO 2 ] is relatively low. A missing sink: HO 2 uptake by aerosols Aerosol

7. 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

8. 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 by aerosol with H 2 O 2 formation Current models always assume HO 2 is converted to H 2 O 2 by aerosol uptake.

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

10. 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. 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 …

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: Fe(II)Fe(III) Cu(I)Cu(II) - e + e - e + e reduction(+e) + oxidation(-e) = redox

12. 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. Cu is mainly from combustion in submicron aerosols.

13. 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)

14. 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. 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)

15. 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)

16. 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) AM3 results

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

18. 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).