Atmospheric Heterogeneous Chemistry of HO2

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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)
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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

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Predicted Effect of ?HO2 0.2 on HOx Loss
Martin, et al. JGR 2003
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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?
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Aerosol Flow TubeCIMS 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
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Testing Mass Accommodation of HO2
Cu(II)
kI gt 1x109 s-1 _at_ pH5
HO2gas
HO2surf
H2O2 O2
Fast
e.g. Mozurkewich, et al. JGR, 1987
Cu-doped aerosols allow mass accommodation to be
measured
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Testing Mass Accommodation of HO2
Rapid HO2 Loss in Cu(II) Doped Aerosols
H2SO4 Aerosol _at_ RH35
2x10-3 cm2/cm3
8x10-5 cm2/cm3

• Loss on 55 wt H2SO4 aerosol w/out Cu is gt order
  of magnitude slower

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Testing Mass Accommodation of HO2
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Questions to Address
Is mass accommodation of HO2 to both neutral and
acidic aqueous surfaces efficient?
YES
?HO2 gt 0.5
Is loss of HO2 on aqueous acidic aerosol
important in the lower troposphere?
NO
?HO2 lt 0.01
Is loss of HO2 on near neutral aqueous aerosol
important in the lower troposphere?
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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
(pH5)
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Heterogeneous Chemistry of HO2
what is the dependence of ? on HO2gas?
HO2bulk
H O2-
HO2gas
HO2surf
HO2 O2-
H2O2 O2 OH-
kII 1x108 M-1s-1
Depends on the competition between mass
transport and chemistry
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Loss of HO2 to pH5 Aerosol
Aqueous (NH4)2SO4 Aerosol _at_ RH41
w/malonate-bimalonate buffer
Sa1.5x10-4 cm2/cm3
implies ?0.1
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Loss of HO2 to pH5 Aerosol
well described by 2nd order kinetics
Aqueous (NH4)2SO4 Aerosol _at_ RH41
w/malonate-bimalonate buffer
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Loss of HO2 to pH5 Aerosol
HO2aq
Haq O2aq-
H2O
HO2aq O2aq-
H2O2aq O2aq OHaq-
HO2aq HO2aq
H2O2aq O2aq
literature kII 2x107 M-1s-1
Our data yield kII4x107 M-1s-1 for 2nd order
loss of HO2 in super-saturated aqueous (NH4)2SO4
aerosol
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Loss of Tropospheric HO2 to Aerosol
Our results ?HO2 is function of HO2gas,
aerosol volume, pH and Temperature
Lab ?HO2 0.1 pH 5 (NH4)2SO4 aerosol
Troposphere ?HO2 lt 0.01 pH 5 aqueous aerosol at
room temperature
(Due to HO2labgtgtHO2trop)
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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?
Unlikelybut
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Importance of Heterogeneous HO2 Loss To Aqueous
Aerosol (no metals)
rp100 nm pH5,unless noted
102 cm-3, pH 3
102 cm-3
104 cm-3
Gas phase Loss
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Loss of HO2 in Absence of Aerosol
well described by HO2gasHO2gas H2O2
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Gas-Phase Loss Impacts Measurements of Loss to
Aerosol
Model
?0
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Transport Limitations Negligible Expect 2nd
Order Behavior
Adapted from Schwartz, JGR 1984