atmospheric heterogeneous chemistry of ho 2 joel thornton and jonathan abbatt department of...
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Atmospheric Heterogeneous Chemistry of HO2
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 HO2 Loss on Aqueous Aerosols
Heterogeneous Reaction Rates
# of reactions# of collisions
reactionprobability
uptakecoefficient
X
X X
XZ
RXN
111
mass accommodationreaction in/on aerosol•pH dependent rates and solubility•non first-order chemistry
major components of reaction probability
Questions to Address
Is mass accommodation of HO2 to both neutral and acidic aqueous surfaces efficient?
Is loss of HO2 on near neutral aqueous aerosol important in the lower troposphere?
Is loss of HO2 on aqueous acidic aerosol important in the lower troposphere?
(Experimentally)
Aerosol Flow Tube—CIMS Apparatus
Turbo Pump Turbo Pump
Rotary Pumps
Quadrupole Mass Spectrometer
submicron Aerosols/Humidified Bulk Flow
NF3/N2
Vertical Flow Tube760 Torr
Chemical Ionization Region150 Torr
ChanneltronMultiplier
Excess
210Po
H2/Ar
O2Beenakker Microwave
Discharge Cavity
H atoms
To aerosol sizing instrumentation
Testing Mass Accommodation of HO2
HO2gasHO2surf
Cu(II)H2O2 + O2
kI > 1x109 s-1
@ pH~5
RXN
111
Fast
Cu-doped aerosols allow mass accommodation to be measured
e.g. Mozurkewich, et al.JGR, 1987
0 5 10 15 20 25 30 35 40
1E-4
1E-3
0.01 O
2- Sig
na
l (a
rb u
nits
)
Injector Position (cm)
no aerosols H
2SO
4
Cu(II) doped
Rapid HO2 Loss in Cu(II) Doped AerosolsH2SO4
Aerosol @ RH=35%
•Loss on ~55 wt% H2SO4 aerosol w/out Cu is > order of magnitude slower
8x10-5 cm2/cm3
2x10-3 cm2/cm3
Testing Mass Accommodation of HO2
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.80.00
0.25
0.50
0.75
1.00
1.25
1.50
kI (
s-1)
Sa (10-4cm2/cm3)
(NH4)
2SO
4 w/Cu
H2SO
4 w/Cu
aI S
4k
Testing Mass Accommodation of HO2
Cu(II) Drives HO2 > 0.5 Mass Accommodation Is Efficient
Questions to Address
Is mass accommodation of HO2 to both neutral and acidic aqueous surfaces efficient?
Is loss of HO2 on near neutral aqueous aerosol important in the lower troposphere?
Is loss of HO2 on aqueous acidic aerosol important in the lower troposphere?
YES
NO
HO2 > 0.5
HO2 < 0.01
Heterogeneous Chemistry of HO2
HO2gasHO2surf
HO2bulk+ HO2 H2O2 + O2 kII ~ 8x105 M-1s-1
kII ~ 1x108 M-1s-1HO2bulk H+ + O2-
HO2 + O2- H2O2 + O2 + OH-
H2O(pH~5)
In the absence of enough Cu (or Fe)
RXN
111
Heterogeneous Chemistry of HO2
HO2gasHO2surf
HO2bulk H+ + O2-
HO2 + O2- H2O2 + O2 + OH- kII ~ 1x108 M-1s-1
what is the dependence of on [HO2]gas?
Depends on the competition between mass transport and chemistry
0 5 10 15 20 25 30 35 401E-4
1E-3
O
2- Sig
na
l (a
rb u
nits
)
Injector Distance (cm)
no aerosol (NH
4)
2SO
4 (pH 5)
Loss of HO2 to pH~5 AerosolAqueous (NH4)2SO4
Aerosol @ RH=41% w/malonate-bimalonate buffer
Sa=1.5x10-4 cm2/cm3
implies ~0.1
0 5 10 15 20 25 30 35 40
500
1000
1500
2000
2500
3000
3500
1
/Sig
na
l (a
rb)
Injector Position (cm)
no aerosol (NH
4)
2SO
4 (pH 5)
Loss of HO2 to pH~5 Aerosol
Aqueous (NH4)2SO4 Aerosol @ RH=41% w/malonate-bimalonate buffer
well described by 2nd order kinetics
0 5 10 15 20 25 30 35 400.000
0.005
0.010
0.015
0.020
no aerosol pH=5 aerosol
1/S
ign
al (
Hz-1
)
Injector Position (cm)
Loss of HO2 to pH~5 Aerosol
Our data yield kII~4x107 M-1s-1 for 2nd order loss of HO2 in super-saturated aqueous (NH4)2SO4 aerosol
HO2aq Haq+ + O2aq
-
HO2aq + O2aq- H2O2aq + O2aq + OHaq
-H2O
literature: kII ~ 2x107 M-1s-1
HO2aq + HO2aq H2O2aq + O2aq
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 aqueous aerosol at room temperature
(Due to [HO2]lab>>[HO2]trop)
Questions to Address
Is accommodation of HO2 to both neutral and acidic aqueous surfaces efficient?
Is loss of HO2 on near neutral aqueous aerosol important in the lower troposphere?
Is loss of HO2 on aqueous acidic aerosol important in the lower troposphere?
YES
NO
Unlikely…but
0
2
4
6
8
10
1E-15 1E-14 1E-13 1E-12 1E-11 1E-10 1E-9
280
260
240
220
200
Rate Constant (cm3 molec-1 s-1)
Alti
tud
e (
km)
Te
mp
era
ture
(K
)
Importance of Heterogeneous HO2 Loss To Aqueous Aerosol (no metals)
Gas phaseLoss
104 cm-3
102 cm-3
rp=100 nmpH=5,unless noted
102 cm-3, pH 3
5 10 15 20 25 30 35 40 450.01
0.1
1
Sig
na
l (a
rb. u
nits
)
Injector Distance (cm)
"wall" loss second order kinetics
Loss of HO2 in Absence of Aerosol
well described by HO2gas+HO2gas H2O2
Gas-Phase Loss Impacts Measurementsof Loss to Aerosol
0 5 10 15 20 25 30 35 401E10
1E11
=0.05=0.1
=0.2
=0.5
[HO
2](d
) (m
ole
c/cc
)
Injector Distance
222IIgas
2 HO4
AHOHOk
dt
HOd Model:
=0