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

O3

O2

O3

OH HO2

hn, H2O

Deposition

NO

H2O2

CH4, CO, VOC

NO2

STRATOSPHERE

TROPOSPHERE

8-18 km

Tropospheric radical chemistry

Air Quality

Climate

hn

hn

hn

H2O2 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)20-90 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

Co

nc

OH ratio (NH/SH)

N/S Interhemispheric OH RatioDerived hemispheric OH concentrationsfrom CH3CCl3 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

O3

O2

O3

OH HO2

hn, H2O

Deposition

NO

H2O2

CH4, CO, VOC

NO2

STRATOSPHERE

TROPOSPHERE

8-18 km

Clouds/Aerosolshn

hn

Uniqueness of HO2 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/CH3O2/NO/NO2).• very reactive in aqueous phase (superoxide, a major reason for DNA

damage and cancer).Gas: L[HO2] ~ [HO2]∙ [HO2]Uptake: L[HO2] ~ [HO2]

Gas phase HO2 uptake by particles

HO2

aerosol

HO2(aq)

NH4+

NH4+

NH4+

NH4+

SO42-

SO42-

SO42-

SO42-

HSO4-

HSO4-

HSO4-

Aqueous reactions

NH4+

HSO4-

④① ② ③

γ(HO2) defined as the fraction of HO2 collisions with aerosol surfaces resulting in reaction.

① ② ③ ④

Laboratory measured γ(HO2) on sulfate aerosols are generally low…

Except when they add copper in aerosols…

Cu-dopedAqueousSolid

(Mao et al., ACP, 2010)

HO2(aq)+O2-(aq)→ H2O2 (aq)

Cu(II) Cu(I)

HO2(g) H2O2(g)

Conventional HO2 uptake by aerosol with H2O2 formation

The role of copper has been ignored in HO2 uptake because we thought it makes H2O2.

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

Conventional HO2 uptake does not work over Arctic!

(Mao et al., ACP, 2010)

Joint measurement of HO2 and H2O2 suggest that HO2 uptake by aerosols may in fact not produce H2O2 !

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 HO2 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 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.

Cu(II) + HO2 → Cu(I) + O2 + H+

Cu(I) + HO2 Cu(II) + H2O2

What we thought was happening in aerosols…

As Fe(III) + HO2 is 300 times slower than Cu(II) + HO2, so we thought Fe was unimportant…

Net: HO2 +HO2 → H2O2 + O2

Cu(II) + HO2 → Cu(I) + O2 + H+

Cu(I) + HO2 Cu(II) + H2O2

What we thought was happening in aerosols…

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)

Net: HO2 +HO2 → H2O2 + O2

Cu(II) + HO2 → Cu(I) + O2 + H+

Cu(I) + HO2 Cu(II) + H2O2

What we thought was happening in aerosols…

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)

Fe(II) + HO2 Fe(III) + H2O2

With three reactions to close the cycle…

Fe(II) + H2O2 → Fe(III) + OH + OH−

Fe(II) + OH → Fe(III) + OH−

The product from HO2 uptake depends on the fate of Fe(II).

Net: HO2 +HO2 → H2O2 + O2

Net: HO2 + H2O2 → OH + O2 + H2O

Net: HO2 +HO2 → H2O2 + O2

Net: HO2 + OH → O2 + H2O

Cu-Fe redox coupling in aqueous aerosols driven by HO2 uptake from the gas phase

With Cu alone, HO2 is converted to H2O2.

With both Cu and Fe, HO2 is converted to either H2O2 or H2O,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., 2012, ACPD)

Modeling framework for HO2 aerosol uptake

HO2

aerosol

[HO2]surf

2

1)4

( HOg

in AnvD

aR

*

21][

)4

(H

HOA

vD

aR surf

gout

Rin

[HO2]surf

[HO2]bulk

outinbulk RR

dt

HOd

][ 2

2HOn Rout

[HO2]surf is higher than [HO2]bulk because of its short lifetime.

0][)][

(1

2222

2 HO

Iaq PHOk

dt

HOdr

dr

d

rD

provides a relationship between [HO2]surf

and [HO2]bulk.

The diffusion equation with chemical loss (kI[HO2]) and production (PHO2)

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. NH4

+, H+, HSO4-, SO4

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.

iii cAa Ai 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 NH4HSO4 aerosols at RH=85%, T=298 KCu/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 ocean iron fertilization)

Dependence on aerosol pH and Cu concentrations

(A) γ(HO2) in the range 0.4-1 at T = 298 K, should be close to 1 at lower T, due to higher solubility.

(B) H2O2 yield is more likely to be negative than positive.

(C) HO2 uptake is limited by aqueous diffusion until Cu = 5 x 10-4 M.

Cu/Fe=0.1

Cu/Fe=0.01typical 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 extratropicsBlack: NOAA GMD Observations at remote surface sites Green: GEOS-Chem with (γ(HO2) = 1 producing H2O) Red: GEOS-Chem with (γ(HO2) = 0)

(Mao et al., 2012, ACPD)

All models show that NH ≥ SH

Improvement on N/S Interhemispheric OH Ratio

Observational constraints from CH3CCl3 measurements

(Prinn et al., Science, 2001)

SH ≥ NH

obsAM3 with aerosol uptake

In AM3, methane lifetime increases from 8.5 year to 9.6 year !

OH ratio (NH/SH)

Aerosols

CH4

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 H2O

Other applications for aerosol TMI chemistry driven by HO2 uptake (1)

• A major aqueous OH source (converted from gas-phase HO2 and H2O2), 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 HO2 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. MarcusNobel Prize in 1992

Henry TaubeNobel 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(HO2) = 1 producing H2O 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)

20-90 N

20 S – 20 N

20 – 90 S

AM3 simulations

Impact on global O3 (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)

SO2 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, NOx, 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)C2O4 and Fe(II)C2O4 complexes are very unstable.

Cu complexes can also be a significant sink for aqueous HO2 (Voelker et al., EST, 2000)

H2O2: Aircraft Observations Run with uptake Run with no uptake