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Why is the photochemistry in Arctic spring so unique?

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.

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Why is the photochemistry in Arctic spring so unique?

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

  2. 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.

  3. Troposphericphotochemistry uv The concentration of OH and HO2 determines the oxidizing power.

  4. Unique features in Arctic spring (1) • 2. thick ozone columns • Brewer-Dobson circulation • ozone is a strong absorber for UV radiation 1. high solar zenith angle Solar UV radiation is much weaker in Arctic!

  5. Unique features in Arctic spring (2) 3. Receptor of mid-latitude pollutants European influence Seasonal sulfate build-up the famous “Arctic haze” 1-2 2-5 5-8 km May Feb Scheuer et al., 2003 Air pollutants build up in Arctic spring.

  6. 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. (Shaw et al., 1995)

  7. 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

  8. Vertical Profile(Observation vs. GEOS-Chem) W. Brune(PSU), P. Wennberg(Caltech), R. Cohen(UCB), A. Weinheimer(NCAR), A. Fried(NCAR)

  9. HO2 uptake by aerosol ν is mean molecular speed A is surface area γ is reactive uptake coefficient 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-doped Aqueous Solid • To ensure effective HO2 uptake (γ>0.1): • aqueous • Cold or Cu-doped

  10. Arctic particles for HO2 uptake were likely aqueous Mass fraction • 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.

  11. Conventional HO2 uptake by aerosol with H2O2 formation

  12. Fate of HO2 in aerosol HO2 is weak acid (pKa ~ 4.7), not much O2-(aq) in acidic aerosols HO2(aq)+O2-(aq)→ H2O2 (aq) H2O2 (g) Pure HOy sink HO2 HSO4- HCOO-, HSO3- SO5- H SO5- OH(aq) ? HSO3- H2SO4 SO42- +2H+ HO2-H2SO4 complex

  13. Non-conventional HO2 uptake as a HOy sink This non-conventional HO2 uptake provides the best simulations for HOxand HOy.

  14. Relevant to ice core H2O2 data ??? The products from HO2 uptake is determined by the aerosol type and aerosol acidity. Greenland ice core data (Möller, 1999)

  15. Circumpolar HOybudget by GEOS-Chem (60-90N) • Transport from northern mid-latitudes accounts for 50% peroxides in upper troposphere. • H2O2+SO2(aq) is a minor HOysink in lower troposphere.

  16. 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. • 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.

  17. 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

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