why is the photochemistry in arctic spring so unique? jingqiu mao
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Why is the photochemistry in Arctic spring so unique?
Jingqiu Mao
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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.
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Tropospheric photochemistry
The concentration of OH and HO2 determines the oxidizing power.
uv
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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!
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Scheuer et al., 2003
Feb 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.
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(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.
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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
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Vertical Profile(Observation vs. GEOS-Chem)
W. Brune(PSU), P. Wennberg(Caltech), R. Cohen(UCB), A. Weinheimer(NCAR), A. Fried(NCAR)
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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 gHOvA
dt
gHOd
ν is mean molecular speedA is surface areaγ is reactive uptake coefficient
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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
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Conventional HO2 uptake by aerosol with H2O2 formation
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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
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Non-conventional HO2 uptake as a HOy sink
This non-conventional HO2 uptake provides the best simulations for HOx and HOy.
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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
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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.
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•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.
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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