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. Tropospheric photochemistry The concentration of OH and HO 2 determines the oxidizing power. uv

4. 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!

5. Scheuer et al., 2003 Feb May 1-2 2-5 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.

6. (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. Important for long range transport longer lifetime for air pollutants slow photochemical cycling dry air and low solar radiation

7. Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) Phase I: April 1 st ~ April 20 th 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. To ensure effective HO 2 uptake (γ>0.1): 1.aqueous 2.Cold or Cu-doped HO 2 uptake by aerosol Discrepancy cannot be solved by reasonable change of halogens or NO x. Temperature dependence of γ is expected by large enthalpy for HO 2 (g) ↔ HO 2 (aq). Cu-doped Aqueous Solid ν is mean molecular speed A is surface area γ is reactive uptake coefficient

10. The majority is OC and sulfate. Aqueous under Arctic condition from lab measurement. The main form of sulfate is bisulfate, so generally acidic (pK a (HSO 4 - )= 2.0). 95% surface area is contributed by submicron aerosols. Refractory aerosols contribute less than 10% of surface area. Mass fraction Arctic particles for HO 2 uptake were likely aqueous

11. Conventional HO 2 uptake by aerosol with H 2 O 2 formation

12. Fate of HO 2 in aerosol HO 2 HO 2 (aq)+O 2 - (aq)→ H 2 O 2 (aq) HSO 4 - SO 5 - H SO 5 - HCOO -, HSO 3 - OH(aq) HSO 3 - SO 4 2- +2H + H 2 SO 4 HO 2 -H 2 SO 4 complex ? H 2 O 2 (g) Pure HO y sink HO 2 is weak acid (pKa ~ 4.7), not much O 2 - (aq) in acidic aerosols

13. Non-conventional HO 2 uptake as a HO y sink This non-conventional HO 2 uptake provides the best simulations for HO x and HO y.

14. Relevant to ice core H 2 O 2 data ??? (Möller, 1999) The products from HO 2 uptake is determined by the aerosol type and aerosol acidity. Greenland ice core data

15. Circumpolar HO y budget by GEOS-Chem (60-90N) Transport from northern mid-latitudes accounts for 50% peroxides in upper troposphere. H 2 O 2 +SO 2 (aq) is a minor HO y sink in lower troposphere.

16. Main driver of this chemistry is by O ( 1 D)+H 2 O(70%) and transport (30%). Amplification by HCHO is comparable to primary source from O ( 1 D)+H 2 O. Aerosol uptake accounts for 35% of the HO y sink. Schematic diagram of HO x -HO y chemistry in Arctic spring Masses (in parentheses) are in units of Mmol. Rates are in units of Mmol d -1.

17. Conclusions Cold temperature, high aerosol loading and slow photochemical cycling suggest the important role of HO 2 uptake in HO x chemistry in Arctic spring. With HO 2 uptake as a HO y sink, we successfully reproduce HO x and their reservoirs in the model. HO 2 uptake accounts for 35% of HO y sink. Successful simulation of observed HO 2 and H 2 O 2 in ARCTAS implies HO 2 uptake that does not produce H 2 O 2 – possible mechanism coupled to HSO 4 - /H 2 SO 4 producing HSO 5 -. Changes in aerosol types (biomass burning vs. fossil fuel) or aerosol acidity (sulfuric acid vs. ammonium) may have large effects on H 2 O 2, and could be relevant to explaining the complex long-term trend of H 2 O 2 observed in Greenland ice cores