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
TROPOSPHERE8-18 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
Con
c
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 aerosolsCu/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
vDaR surf
gout
Rin
[HO2]surf
[HO2]bulk
outinbulk RR
dtHOd
][ 2
2HOn Rout
[HO2]surf is higher than [HO2]bulk because of its short lifetime.
0][)][(122
222 HO
Iaq PHOk
dtHOdr
drd
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
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