Atmospheric Heterogeneous Chemistry of HO 2 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) Reaction Probabilities for HO 2 Loss on Aqueous Aerosols

Heterogeneous Reaction Rates  # of reactions # of collisions reaction probability uptake coefficient X X X X Z mass accommodation reaction in/on aerosol pH dependent rates and solubility non first-order chemistry major components of reaction probability

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

Questions to Address Is mass accommodation of HO 2 to both neutral and acidic aqueous surfaces efficient? Is loss of HO 2 on near neutral aqueous aerosol important in the lower troposphere? Is loss of HO 2 on aqueous acidic aerosol important in the lower troposphere? (Experimentally)

Aerosol Flow Tube—CIMS Apparatus Turbo Pump Rotary Pumps Quadrupole Mass Spectrometer submicron Aerosols/Humidified Bulk Flow NF 3 /N 2 Vertical Flow Tube 760 Torr Chemical Ionization Region 150 Torr Channeltron Multiplier Excess 210 Po H 2 /Ar O2O2 Beenakker Microwave Discharge Cavity H atoms To aerosol sizing instrumentation

Testing Mass Accommodation of HO 2 HO 2gas HO 2surf Cu(II) H 2 O 2 + O 2 k I > 1x10 9 s pH~5 Fast Cu-doped aerosols allow mass accommodation to be measured e.g. Mozurkewich, et al. JGR, 1987

Rapid HO 2 Loss in Cu(II) Doped Aerosols H 2 SO 4 RH=35% Loss on ~55 wt% H 2 SO 4 aerosol w/out Cu is > order of magnitude slower 8x10 -5 cm 2 /cm 3 2x10 -3 cm 2 /cm 3 Testing Mass Accommodation of HO 2

Cu(II) Drives  HO2 > 0.5 Mass Accommodation Is Efficient

Questions to Address Is mass accommodation of HO 2 to both neutral and acidic aqueous surfaces efficient? Is loss of HO 2 on near neutral aqueous aerosol important in the lower troposphere? Is loss of HO 2 on aqueous acidic aerosol important in the lower troposphere? YES NO  HO2 > 0.5  HO2 < 0.01

Heterogeneous Chemistry of HO 2 HO 2gas HO 2surf HO 2bulk + HO 2 H 2 O 2 + O 2 k II ~ 8x10 5 M -1 s -1 k II ~ 1x10 8 M -1 s -1 HO 2bulk H + + O 2 - HO 2 + O 2 - H 2 O 2 + O 2 + OH - H2OH2O (pH~5) In the absence of enough Cu (or Fe)

Heterogeneous Chemistry of HO 2 HO 2gas HO 2surf HO 2bulk H + + O 2 - HO 2 + O 2 - H 2 O 2 + O 2 + OH - k II ~ 1x10 8 M -1 s -1 what is the dependence of  on [HO 2 ] gas ? Depends on the competition between mass transport and chemistry

Loss of HO 2 to pH~5 Aerosol Aqueous (NH 4 ) 2 SO 4 RH=41% w/malonate-bimalonate buffer S a =1.5x10 -4 cm 2 /cm 3 implies  ~0.1

Loss of HO 2 to pH~5 Aerosol Aqueous (NH 4 ) 2 SO 4 RH=41% w/malonate-bimalonate buffer well described by 2 nd order kinetics

Loss of HO 2 to pH~5 Aerosol Our data yield k II ~4x10 7 M -1 s -1 for 2 nd order loss of HO 2 in super-saturated aqueous (NH 4 ) 2 SO 4 aerosol HO 2aq H aq + + O 2aq - HO 2aq + O 2aq - H 2 O 2aq + O 2aq + OH aq - H2OH2O literature: k II ~ 2x10 7 M -1 s -1 HO 2aq + HO 2aq H 2 O 2aq + O 2aq

Loss of Tropospheric HO 2 to Aerosol Our results: “  HO2 ” is function of [HO 2 ] gas, aerosol volume, pH and Temperature Lab:  HO2 ~ 0.1 pH 5 (NH 4 ) 2 SO 4 aerosol Troposphere:  HO2 < 0.01 pH 5 aqueous aerosol at room temperature (Due to [HO 2 ] lab >>[HO 2 ] trop )

Questions to Address Is accommodation of HO 2 to both neutral and acidic aqueous surfaces efficient? Is loss of HO 2 on near neutral aqueous aerosol important in the lower troposphere? Is loss of HO 2 on aqueous acidic aerosol important in the lower troposphere? YES NO Unlikely…but

Importance of Heterogeneous HO 2 Loss To Aqueous Aerosol (no metals) Gas phase Loss 10 4 cm cm -3 r p =100 nm pH=5,unless noted 10 2 cm -3, pH 3

Loss of HO 2 in Absence of Aerosol well described by HO 2gas +HO 2gas H 2 O 2

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

Adapted from Schwartz, JGR 1984 Transport Limitations Negligible Expect 2 nd Order Behavior