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), Vaishali Naik (GFDL)

Outline 1.Tropospheric chemistry and potential issues 2.The role of aerosol uptake 3.Cu-Fe redox coupling in aerosols 4.Global implications for atmospheric oxidant chemistry 5.Other applications of aerosol TMI chemistry

O3O3 O2O2 O3O3 OHHO 2 h, H 2 O Deposition NO H2O2H2O2 CH 4, CO, VOC NO 2 STRATOSPHERE TROPOSPHERE 8-18 km Tropospheric radical chemistry Air Quality Climate h h h H 2 O 2 is a radical reservoir.

Models ONLY underestimate CO in Northern extratropics (Shindell et al., JGR, 2006) Cannot be explained by emissions: 1.Need to double current CO anthro emissions (Kopacz et al., ACP, 2010). 2.Why the discrepancy peak at spring? Should peak in winter if we underestimate heating or vehicle cold start. 3.Double CO emissions will lead to a higher ozone in northern extratropics (we already have too much ozone). MOPITT satellite (500 hPa) Multi-model mean (500 hPa) N 20 S – 20 N 20 – 90 S Annual cycle of CO

All models show that NH ≥ SH The alternative explanation is that model OH is wrong, but how? Observations show that SH ≥ NH (Prinn et al., Science, 2001) SH ≥ NH obs OH Conc OH ratio (NH/SH) N/S Interhemispheric OH Ratio Derived hemispheric OH concentrations from CH 3 CCl 3 measurements models

Outline 1.Tropospheric chemistry and potential issues 2.The role of aerosol uptake 3.Cu-Fe redox coupling in aerosols 4.Global implications for atmospheric oxidant chemistry 5.Other applications of aerosol TMI chemistry

O3O3 O2O2 O3O3 OHHO 2 h, H 2 O Deposition NO H2O2H2O2 CH 4, CO, VOC NO 2 STRATOSPHERE TROPOSPHERE 8-18 km Clouds/Aerosols h 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 ]

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

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 The role of copper has been ignored in HO 2 uptake because we thought it makes H 2 O 2.

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

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 …

I took this picture

Outline 1.Tropospheric chemistry and potential issues 2.The role of aerosol uptake 3.Cu-Fe redox coupling in aerosols 4.Global implications for atmospheric oxidant chemistry 5.Other applications of aerosol TMI chemistry

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

Cu and Fe are ubiquitous in crustal and combustion aerosols Cu/Fe ratio is between 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.

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

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

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

Cu-Fe redox coupling in aqueous aerosols driven by HO 2 uptake from the gas phase With Cu alone, HO 2 is converted to H 2 O 2. With both Cu and Fe, HO 2 is converted to either H 2 O 2 or H 2 O, 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., 2012, ACPD)

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

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)

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 ocean iron fertilization)

Dependence on aerosol pH and Cu concentrations (A)γ(HO 2 ) in the range at T = 298 K, should be close to 1 at lower T, due to higher solubility. (B)H 2 O 2 yield is more likely to be negative than positive. (C)HO 2 uptake is limited by aqueous diffusion until Cu = 5 x M. Cu/Fe=0.1 Cu/Fe=0.01 typical rural site (Mao et al., 2012, ACPD)

Outline 1.Tropospheric chemistry and potential issues 2.The role of aerosol uptake 3.Cu-Fe redox coupling in aerosols 4.Global implications for atmospheric oxidant chemistry 5.Other applications of aerosol TMI chemistry

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., 2012, ACPD)

All models show that NH ≥ SH Improvement on N/S Interhemispheric OH Ratio Observational constraints from CH 3 CCl 3 measurements (Prinn et al., Science, 2001) SH ≥ NH obs AM3 with aerosol uptake In AM3, methane lifetime increases from 8.5 year to 9.6 year ! OH ratio (NH/SH)

Aerosols CH 4 HFCs OH Implications for radiative forcing…warming effect from aerosols See poster on Thursday Mao et al., Sensitivity of tropospheric oxidants to wildfires: implications for radiative forcing (A43E-0205). trop ozone strat H 2 O

Other applications for aerosol TMI chemistry driven by HO 2 uptake (1) A major aqueous OH source (converted from gas-phase HO 2 and H 2 O 2 ), 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).

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

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

Extra slides

Test this mechanism in two 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

Impact on global OH (annual mean at surface) run with uptake – run with no uptake 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. (Liu et al., JGR, 2011) AM3 Obs

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…

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

Impact on global O 3 (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!!! BC (Lamarque et al., Climate Change, 2011) SO 2 BC Courtesy of V. Naik

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, NO x, halogen etc.).

Organic aerosols (insoluble organic) Organic-electrolyte mixtures tend to have liquid-liquid phase separation state. (Zuend et al., ACP, 2012) (Furukawa et al., ACP, 2010) Water soluble organic aerosols Fe(III)C 2 O 4 and Fe(II)C 2 O 4 complexes are very unstable. Cu complexes can also be a significant sink for aqueous HO 2 (Voelker et al., EST, 2000)

H 2 O 2 : Aircraft Observations Run with uptake Run with no uptake