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Atmospheric Heterogeneous Chemistry of HO 2

Atmospheric Heterogeneous Chemistry of HO 2. Reaction Probabilities for HO 2 Loss on Aqueous Aerosols. Joel Thornton and Jonathan Abbatt Department of Chemistry University of Toronto. $$ Natural Sciences and Engineering Research $$ Council of Canada (NSERC) PREA Ontario (J. P.D. Abbatt).

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Atmospheric Heterogeneous Chemistry of HO 2

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  1. Atmospheric Heterogeneous Chemistry of HO2 Reaction Probabilities for HO2 Loss on Aqueous Aerosols Joel Thornton and Jonathan Abbatt Department of Chemistry University of Toronto $$ Natural Sciences and Engineering Research $$ Council of Canada (NSERC) PREA Ontario (J. P.D. Abbatt)

  2. reaction probability uptake coefficient # of reactions g = # of collisions Heterogeneous Reaction Rates X X Z X X major components of reaction probability reaction in/on aerosol mass accommodation • pH dependent rates and solubility • non first-order chemistry

  3. Predicted Effect of HO2 = 0.2 on HOx Loss Martin, et al. JGR 2003

  4. Questions to Address (Experimentally) Is mass accommodation of HO2 to both neutral and acidic aqueous surfaces efficient? Is loss of HO2 on aqueous acidic aerosol important in the lower troposphere? Is loss of HO2 on near neutral aqueous aerosol important in the lower troposphere?

  5. Aerosol Flow Tube—CIMS Apparatus H atoms H2/Ar submicron Aerosols/Humidified Bulk Flow Beenakker Microwave Discharge Cavity O2 Excess Vertical Flow Tube 760 Torr Quadrupole Mass Spectrometer To aerosol sizing instrumentation 210Po Channeltron Multiplier NF3/N2 Chemical Ionization Region 150 Torr Turbo Pump Turbo Pump Rotary Pumps

  6. Testing Mass Accommodation of HO2 Cu(II) kI > 1x109 s-1 @ pH~5 HO2gas HO2surf H2O2 + O2 Fast e.g. Mozurkewich, et al. JGR, 1987 Cu-doped aerosols allow mass accommodation to be measured

  7. Testing Mass Accommodation of HO2 Rapid HO2 Loss in Cu(II) Doped Aerosols H2SO4Aerosol @ RH=35% 2x10-3 cm2/cm3 8x10-5 cm2/cm3 • Loss on ~55 wt% H2SO4aerosol w/out Cu is > order of magnitude slower

  8. Cu(II) Drives HO2 > 0.5 Mass Accommodation Is Efficient Testing Mass Accommodation of HO2

  9. Questions to Address Is mass accommodation of HO2 to both neutral and acidic aqueous surfaces efficient? YES HO2 > 0.5 Is loss of HO2 on aqueous acidic aerosol important in the lower troposphere? NO HO2 < 0.01 Is loss of HO2 on near neutral aqueous aerosol important in the lower troposphere?

  10. HO2bulk H+ + O2- H2O HO2 + O2- H2O2 + O2 + OH- Heterogeneous Chemistry of HO2 In the absence of enough Cu (or Fe) HO2bulk+ HO2 H2O2 + O2 kII ~ 8x105 M-1s-1 kII ~ 1x108 M-1s-1 HO2gas HO2surf (pH~5)

  11. Heterogeneous Chemistry of HO2 what is the dependence of  on [HO2]gas? HO2bulk H+ + O2- HO2gas HO2surf HO2 + O2- H2O2 + O2 + OH- kII ~ 1x108 M-1s-1 Depends on the competition between mass transport and chemistry

  12. Loss of HO2 to pH~5 Aerosol Aqueous (NH4)2SO4Aerosol @ RH=41% w/malonate-bimalonate buffer Sa=1.5x10-4 cm2/cm3 implies ~0.1

  13. Loss of HO2 to pH~5 Aerosol well described by 2nd order kinetics Aqueous (NH4)2SO4Aerosol @ RH=41% w/malonate-bimalonate buffer

  14. Loss of HO2 to pH~5 Aerosol HO2aq Haq+ + O2aq- H2O HO2aq + O2aq- H2O2aq + O2aq + OHaq- HO2aq + HO2aq H2O2aq + O2aq literature: kII ~ 2x107 M-1s-1 Our data yieldkII~4x107 M-1s-1for 2nd order loss of HO2 in super-saturated aqueous (NH4)2SO4 aerosol

  15. Loss of Tropospheric HO2 to Aerosol Our results: “HO2” is function of [HO2]gas, aerosol volume, pH and Temperature Lab: HO2 ~ 0.1 pH 5 (NH4)2SO4 aerosol Troposphere: HO2 < 0.01 pH 5 aqueousaerosol at room temperature (Due to [HO2]lab>>[HO2]trop)

  16. Questions to Address Is accommodation of HO2 to both neutral and acidic aqueous surfaces efficient? YES Is loss of HO2 on aqueous acidic aerosol important in the lower troposphere? NO Is loss of HO2 on near neutral aqueous aerosol important in the lower troposphere? Unlikely…but

  17. Importance of Heterogeneous HO2 Loss To Aqueous Aerosol (no metals) rp=100 nm pH=5,unless noted 102cm-3, pH 3 102cm-3 104cm-3 Gas phase Loss

  18. Loss of HO2 in Absence of Aerosol well described by HO2gas+HO2gas H2O2

  19. Gas-Phase Loss Impacts Measurements of Loss to Aerosol Model: =0

  20. Transport Limitations Negligible Expect 2nd Order Behavior Adapted from Schwartz, JGR1984

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