Why is the photochemistry in Arctic spring so unique?Jingqiu Mao

Why do we care about the photochemistry in Arctic?

• Arctic is a beacon of global climate change.• Arctic is a receptor of mid-latitude pollution and also

influenced by boreal forest fires.• To understand the evolution of aerosols, ozone, mercury

and other pollutants in the Arctic.– Impacts on radiative forcing and global warming

• To understand the lifetime of greenhouse gases.• To understand the ice core data.

Tropospheric photochemistry

The concentration of OH and HO2 determines the oxidizing power.

uv

Unique features in Arctic spring (1)

1. high solar zenith angle

2. thick ozone columns• Brewer-Dobson circulation• ozone is a strong absorber for

UV radiation

Solar UV radiation is much weaker in Arctic!

Scheuer et al., 2003Feb May

1-22-5

5-8 km

Unique features in Arctic spring (2)

3. Receptor of mid-latitude pollutants

European influence Seasonal sulfate build-up the famous “Arctic haze”

Air pollutants build up in Arctic spring.

(Shaw et al., 1995)

Unique features in Arctic spring (3)4. Cold temperature and dry air • shallow boundary layer (~100 m) and capped by a strong thermal inversion layer.

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

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.

Relevant to ice core H2O2 data ???

(Möller, 1999)

The products from HO2 uptake is determined by the aerosol type and aerosol acidity.

Greenland ice core data

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.

Conclusions

Cold 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- .

Changes in aerosol types (biomass burning vs. fossil fuel) or aerosol acidity(sulfuric acid vs. ammonium) may have large effects on H2O2, and could be relevant to explaining the complex long-term trend of H2O2 observed in Greenland ice cores