1. Jingqiu Mao, Daniel Jacob Harvard 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)

2. 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

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

4. 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

5. 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 Please see Jenny Fisher’s (Wed 310PM) and Qiaoqiao Wang ‘s talk (Thursday 930AM) for aerosol simulations in GEOS- Chem. Arctic particles for HO 2 uptake were likely aqueous

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

7. 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

8. 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.

9. Sources and sinks for HO x and HO y (from observations) HO 2 uptake HO 2 uptake is needed to close HO x and HO y budgets above 5 km. Large observed HCHO below 3 km is inconsistent with independently computed HO x sinks. H 2 O 2 + hv is dominant HO x source above 4 km (unique in Arctic). OH+CH 3 OOH dominates HO y sink below 4km (unique in Arctic).

10. 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.

11. 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.

12. Possible aerosol effects of changing sources Neutralized aerosol Acidic aerosol Less SO 2 emission and more NH 3 emission ??

13. 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 -. The photolysis of H 2 O 2 becomes the dominant HO x source in middle and upper troposphere due to the long lifetime of HO y combined with the efficient cycling between HO x radicals and peroxide reservoirs. Future changes in aerosol acidity due to decreasing SO 2 and increasing NH 3 could lead to different chemistry and possibly increase OH.