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

O3

O2

O3

OH HO2

hn, H2O

Deposition

NO

H2O2

CH4, CO, VOCs

NO2

STRATOSPHERE

TROPOSPHERE

8-18 km

hn

hn

hn

H2O2 is a radical reservoir.

(Levy, Science, 1971)

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?

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)

• 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/CH3O2/NO/NO2

• very reactive in aqueous phase electron donor, good for any redox reactions.

Gas-phase loss: L[HO2] ~ [HO2]∙ [HO2]Aerosol uptake: L[HO2] ~ [HO2]

Aerosol uptake is only significant when gas-phase [HO2] is relatively low.

A missing sink: HO2 uptake by aerosols

Aerosol

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

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

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 …

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-Fe redox coupling in aqueous aerosols

Cu only: HO2 → H2O2

Cu + Fe : HO2 → H2O or 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)

Dependence on aerosol pH and Cu concentrations

• 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.

Cu/Fe=0.1

Cu/Fe=0.01typical rural site

(Mao et al., 2013, ACP)

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., 2013, ACP)

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

Aerosol uptake has large impact on ozone production efficiency

ΔO3/ ΔCO is a measure of ozone production efficiency.

Observations

(Mao et al., 2013, GRL)

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).