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

Martin, et al. JGR 2003

Predicted Effect of HO2 = 0.2 on HOx Loss

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

104 105 106 107 108 109 1010102

103

104

105

106

107

108

109

1010

1011

1012

1013

1014

aqueous phase

pH=6

pH=4.7pH=2

pHO2

(atm)

10-12

10-11

10-9

10-10

10-8

H o

r H

eff (

M a

tm-1)

kII or kII

eff (M-1 s-1)

Adapted from Schwartz, JGR 1984

Transport Limitations Negligible

Expect 2nd Order Behavior