Chemistry of hydrogen oxide radicals (HOx) in Arctic spring

Jingqiu Mao, Daniel JacobHarvard University

Jennifer Olson(NASA Langley), Xinrong Ren(U Miami), Bill Brune(Penn State), Paul Wennberg(Caltech), Mike

Cubison(U Colorado), Jose L. Jimenez(U Colorado), Ron Cohen(UC Berkeley), Andy Weinheimer(NCAR), Alan

Fried(NCAR), Greg Huey (Gatech)

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

Vertical Profile(Observation vs. GEOS-Chem)

W. Brune(PSU), P. Wennberg(Caltech), R. Cohen(UCB), A. Weinheimer(NCAR), A. Fried(NCAR)

To ensure effective HO2 uptake (γ>0.1):1. aqueous2. Cold or Cu-doped

HO2 uptake by aerosol

Discrepancy cannot be solved by reasonable change of halogens or NOx.

Temperature dependence of γ is expected by large enthalpy for HO2 (g) ↔ HO2(aq).

Cu-dopedAqueousSolid

)]([4

)]([2

2 gHOvAdtgHOd

ν is mean molecular speedA is surface areaγ is reactive uptake coefficient

Sul-fate58%

OC32%

NH46%

Nitrate3% Chloride

1%

•The majority is OC and sulfate. • Aqueous under Arctic condition from lab measurement.•The main form of sulfate is bisulfate, so generally acidic (pKa (HSO4

-)= 2.0).•95% surface area is contributed by submicron aerosols.•Refractory aerosols contribute less than 10% of surface area.

Mass fraction

Please see Jenny Fisher’s (Wed 310PM) and Qiaoqiao Wang ‘s talk (Thursday 930AM) for aerosol simulations in GEOS-Chem.

Arctic particles for HO2 uptake were likely aqueous

Conventional HO2 uptake by aerosol with H2O2 formation

Fate of HO2 in aerosol

HO2

HO2(aq)+O2-(aq)→ H2O2 (aq)

HSO4-

SO5- H SO5

- HCOO-, HSO3

-

OH(aq)HSO3

-

SO42- +2H+H2SO4

HO2-H2SO4 complex

?

H2O2 (g)

Pure HOy sink

HO2 is weak acid (pKa ~ 4.7), not much O2

-(aq) in acidic aerosols

Non-conventional HO2 uptake as a HOy sink

This non-conventional HO2 uptake provides the best simulations for HOx and HOy.

Sources and sinks for HOx and HOy (from observations)

HO2 uptake

• HO2 uptake is needed to close HOx and HOy budgets above 5 km.

• Large observed HCHO below 3 km is inconsistent with independently computed HOx sinks.

• H2O2 + hv is dominant HOx source above 4 km (unique in Arctic).

• OH+CH3OOH dominates HOy sink below 4km (unique in Arctic).

Circumpolar HOy budget by GEOS-Chem (60-90N)

•Transport from northern mid-latitudes accounts for 50% peroxides in upper troposphere.

•H2O2+SO2(aq) is a minor HOy sink in lower troposphere.

•Main driver of this chemistry is by O (1D)+H2O(70%) and transport (30%).•Amplification by HCHO is comparable to primary source from O (1D)+H2O.•Aerosol uptake accounts for 35% of the HOy sink.

Schematic diagram of HOx-HOy chemistry in Arctic spring

Masses (in parentheses) are in units of Mmol .

Rates are in units of Mmol d-1.

Possible aerosol effects of changing sources

Neutralized aerosol

Acidic aerosol

Less SO2 emission and more NH3 emission

??

ConclusionsCold temperature, high aerosol loading and slow photochemical cycling suggest the important role of HO2 uptake in HOx chemistry in Arctic spring.

With HO2 uptake as a HOy sink, we successfully reproduce HOx and their reservoirs in the model. HO2 uptake accounts for 35% of HOy sink.

Successful simulation of observed HO2 and H2O2 in ARCTAS implies HO2 uptake that does not produce H2O2 – possible mechanism coupled to HSO4

-/H2SO4 producing HSO5- .

The photolysis of H2O2 becomes the dominant HOx source in middle and upper troposphere due to the long lifetime of HOy combined with the efficient cycling between HOx radicals and peroxide reservoirs.

Future changes in aerosol acidity due to decreasing SO2 and increasing NH3 could lead to different chemistry and possibly increase OH.