University of California, Davis - A new method for CH3O2 and ......and the direct and absolute Cavity Ring-Down Spectroscopy (CRDS) •FAGE has been used in kinetic studies of the

An intercomparison of methods for HO2 and CH3O2 detection and kinetic study of the HO2 + CH3O2 cross-reaction in the Highly

Instrumented Reactor for Atmospheric Chemistry (HIRAC)

Lavinia Onel, Alexander Brennan, Freja F. Østerstrøm, Michele Gianella, Lisa Whalley, Gus Hancock,

Paul Seakins, Grant Ritchie and Dwayne Heard

//upload.wikimedia.org/wikipedia/en/b/b8/Leeds_University_logo.svg


Overview

• Laser-Induced Fluorescence (LIF) spectroscopy at low-pressure, known asFluorescence Assay by Gas Expansion (FAGE) is most commonly used for theHO2 detection in the atmosphere

Overview


• A new method for CH3O2 radicals detection using FAGE has been developed

Overview



• As FAGE is an indirect method and requires calibration it has been validatedperforming intercomparisons of measurements of HO2 and CH3O2 by FAGEand the direct and absolute Cavity Ring-Down Spectroscopy (CRDS)

Overview



• As FAGE is an indirect method and requires calibration it has been validatedperforming intercomparisons of measurements of HO2 and CH3O2 by FAGEand the direct and absolute Cavity Ring-Down Spectroscopy (CRDS)

• FAGE has been used in kinetic studies of the HO2 + CH3O2 cross-reactionwithin the HIRAC chamber

HO2 and CH3O2 radicals in the troposphere• HO2 is generated directly by the oxidation of CO and indirectly by the oxidation

of VOCs

• CH3O2 is the most abundant RO2 in the atmosphere

• A dominant source of CH3O2 is the oxidation of CH4

O3 hn

OH

CH3O2

H2O

CH4

NO NO2

hn

HO2

NO2 NO

hn

O2

O2

CH3OCO

O2

HO2 and CH3O2 radicals in the troposphere• HO2 is generated directly by the oxidation of CO and indirectly by the oxidation

of VOCs

• CH3O2 is the most abundant RO2 in the atmosphere

• A dominant source of CH3O2 is the oxidation of CH4

O3 hn

OH

CH3O2

H2O

CH4

NO NO2

hn

HO2

NO2 NO

hn

O2

O2

CH3OCO

O2

cross-reactions otherand self-reactions RO2

chain terminationunder low NOx levels

HO2 and CH3O2 in the troposphere• HO2 is measured by FAGE by conversion to OH; typically the measured [HO2] peaks

at daytime at a few 108 cm-3.



• Currently there are no specific measurements of the concentration of CH3O2 orany other RO2 by any direct or indirect method.




• The sum of organic RO2 has been measured in the atmosphere using CIMS(Chemical Ionisation Mass Spectrometry), PERCA (Peroxy Radical ChemicalAmplifier) method and more recently ROxLIF method.




• The sum of organic RO2 has been measured in the atmosphere using CIMS(Chemical Ionisation Mass Spectrometry), PERCA (Peroxy Radical ChemicalAmplifier) method and more recently ROxLIF method.

• Box modelling using the Master Chemical Mechanism (MCM) has found that thedaytime [CH3O2] peaks in the range ~(0.5 - 6) × 108 cm-3.*

* Whalley et al. ACP, 2010; 2011; 2018

A new method for the selective and sensitive detection of CH3O2 radicals

• CH3O2 radicals are detected by titration with known amounts of NO togenerate CH3O radicals which are then detected using FAGE.

CH3O2 + NO CH3O + NO2

• The A X electronic excitation of CH3O radicals is used to detect theseradicals with high sensitivity and selectively by LIF.

LIF detection

A new method for the selective and sensitive detection of CH3O2 radicals

The method is similar to the FAGE method used for HO2 detection:

CH3O2 + NO CH3O + NO2

LIF detection LIF detection

HO2 + NO OH + NO2

FAGE instrument

CH3O2 CH3O HO2 OHDetectors

Gas flow, system at ~2.4 Torr

NO

Laser axis

Inlet pinhole

Fluorescence

NO

Laser excitation: ~308 nm ~298 nm



NO

Laser axis

Fluorescence

NOHg pen-ray lamp (184.9 nm)

40 slm humidified air0.3 % CH4

quartz window

FAGE calibration• Known concentrations of HO2 and CH3O2 are generated in a calibration flow tube

impinging outside the FAGE sample inlet



NO

Laser axis

Fluorescence

NOHg pen-ray lamp (184.9 nm)

40 slm humidified air0.3 % CH4

quartz window

FAGE calibration• Known concentrations of HO2 and CH3O2 are generated in a calibration flow tube

impinging outside the FAGE sample inlet

H2O OH + H

H + O2 HO2

OH + CH4 H2O + CH3

CH3 + O2 + M CH3O2 + M

184.9 nm

Methoxy (CH3O) excitation spectrum

297.78 297.79 297.80 297.810

10

20

FA

GE

sig

na

l / co

un

ts s

-1 m

W-1

/ nm

online

• online = 297.79 nm

• offline = online + 2.5 nm

online

Onel et al. AMT, 10, 3985–4000, 2017

FAGE calibration plotS/N = 2 • LOD(CH3O2) = (1.2 0.1) × 109 cm-3

(1.8 0.2) × 108 cm-3

30 s averaging time

20 min averaging time

slope = CCH3O2 = (4.1 ± 1.4) × 10-10 counts s-1 mW-1 cm3

Onel et al. AMT, 10, 3985–4000, 2017

FAGE calibration plotS/N = 2

slope = CCH3O2 = (4.1 ± 1.4) × 10-10 counts s-1 mW-1 cm3

Onel et al. AMT, 10, 3985–4000, 2017

LOD(HO2) = (1.6 0.2) × 106 cm-3• LOD(CH3O2) = (1.2 0.1) × 109 cm-3

(1.8 0.2) × 108 cm-3

30 s averaging time

20 min averaging time

Potential of the new method to be used in field measurements of CH3O2

LOD = (1.8 0.2) × 108 cm-3

~ 6 × 108 cm-3 CH3O2

tropical Atlantic ocean*

S/N = 2 and 20 min averaging time

* Whalley et al. ACP, 2010

Potential of the new method to be used in field measurements of CH3O2

LOD = (1.8 0.2) × 108 cm-3

S/N = 2 and 20 min averaging time

Further optimisations of FAGE sensitivity:

removal of the fibre optic cables to deliver the probe laser beam directly lower pressure (currently 2.4 Torr)

Validation of the FAGE method for detection of HO2 and CH3O2

FAGE–CRDS intercomparison


FAGE–CRDS intercomparisonHIRAC (Highly Instrumented Reactor for Atmospheric Chemistry)





First HO2 and CH3O2 CRDS measurementsin an atmospheric simulation chamber

NIR





Laser DetectorCirculating light~1.4 m cavity

0 1000 2000 30000

1

2

3

4

5

6

Lig

ht

inte

nsity / V

Data point

Absorbing species present

No absorbing species

NIR



~ 1.5 m

HO2

Cl + CH3OH CH2OH + HClCH2OH + O2 HO2 + CH2O

Cl2 2Cl360 nm



Cl + CH4 CH3 + HClCH3 + O2 + M CH3O2 + M

~ 1.3 m

CH3O2

Cl2 2Cl360 nm

Onel et al. AMT, 10, 4877–4894, 2017

Intercomparison of HO2 and CH3O2 by FAGE and CRDS

• CRDS probes HO2 using the excitation of the first O-H overtone at 1.5 m andCH3O2 using the AX electronic excitation at 1.3 m

lamps on

lamps off

FAGE–CRDS correlation plots

0.00 2.50x1010

5.00x1010

7.50x1010

0.0

2.0x1010

4.0x1010

6.0x1010

8.0x1010

1000 mbar

0.963 ± 0.005

[HO

2] F

AG

E /

cm

-3

[HO2]CRDS

/ cm-3

150 mbar

0.986 ± 0.002

HO2CH3O2

Onel et al. AMT, 10, 4877–4894, 2017

0.0 2.0x1011

4.0x1011

6.0x1011

0.0

2.0x1011

4.0x1011

6.0x1011

[CH

3O

2] F

AG

E /

cm

-3

[CH3O

2]CRDS

/ cm-3

1000 mbar

1.04 ± 0.01

100 mbar

1.019 ± 0.005

HO2 + CH3O2 cross-reaction

~ 6 × 108 cm-3 CH3O2

~ 2 × 108 cm-3 CH3O2

tropical Atlantic ocean tropical rainforest

• A main sink for CH3O2 radicals in remote areas (low NOx)

HO2 + CH3O2 O2 + CH3COOH 0.9O2 + HCHO + H2O 0.1

Kinetics of the HO2 + CH3O2 cross-reaction

• 23% 1 uncertainty in the value of the overall rate coefficient at 298 K5.2 × 10-12 cm3 molecule-1 s-1 [1]. The majority of previous studies usedUV-absorption spectroscopy, which is often an unselective technique.

• Clear need for kinetic studies of this reaction using a selective method.

[1] http://iupac.pole-ether.fr

HO2 + CH3O2 Productsk


HIRAC CH3O2 CH3O HO2 OHDetectors

Gas flow

NO

Laser axis

Fluorescence

NO


• HO2 and CH3O2 are measured in alternation


Gas flow

Laser axis

Fluorescence

NO

Laser excitation: ~308 nm




Gas flow

NO

Laser axis

Fluorescence

Laser excitation: ~298 nm

HO2 and CH3O2 second–order decays

0 100 2000.0

2.0x1011

4.0x1011

CH3O

2

co

nce

ntr

atio

n /

mo

lecu

le c

m-3

time / s2100 2200 2300 2400 2500

0.0

2.0x1011

4.0x1011

CH3O

2

time / s

lamps off

1000 1100 1200 13000.0

2.0x1011

4.0x1011

[HO2]

time / s

lamps on

• Reaction mixtures: CH4/CH3OH/Cl2/air

• Different [CH3O2]/[HO2] ratios


Reaction

CH3O2 + CH3O2 CH3OH + HCHO + O2

CH3O2 + CH3O2 CH3O + CH3O + O2

CH3O + O2 HCHO + HO2

CH3O2 + HO2 CH3OOH + O2

HCHO + H2O + O2

HO2 + HO2 + (M) H2O2 + O2 + (M)

CH3O2 self-reaction

HO2 self-reaction

cross-reaction

Kinetic analysis

Reaction

CH3O2 + CH3O2 CH3OH + HCHO + O2

CH3O2 + CH3O2 CH3O + CH3O + O2

CH3O + O2 HCHO + HO2

CH3O2 + HO2 CH3OOH + O2

HCHO + H2O + O2

HO2 + HO2 + (M) H2O2 + O2 + (M)

Kinetic analysis

0 20 400

1x1010

2x1010

3x1010

4x1010

[HO

2]

/ cm

-3

Fit

experimental data

time /s

0 10 20 30 40 50 600.0

4.0x1010

8.0x1010

1.2x1011

1.6x1011

time /s

[CH

3O

2]

/ cm

-3

Fit

Experimental

extracted: overall kCH3O2+HO2

HO2 + CH3O2 cross-reaction: Temperature dependence

• Experiments at 268, 284, 295, 323, and 344 K (-5 to 70 °C)

260 280 300 320 3400.0

4.0x10-12

8.0x10-12

1.2x10-11

kC

H3

O2

+H

O2 / c

m3 s

-1

T / K

This study

IUPAC

IUPAC: kCH3O2 + HO2 = 3.8 × 10-13 × exp(780/T) cm3 s-1 (205-580 K)E/R = 100 K




260 280 300 320 3400.0

4.0x10-12

8.0x10-12

1.2x10-11

kC

H3

O2

+H

O2 / c

m3 s

-1

T / K

This study

IUPAC

preliminaryresult



• Good agreement

• Smaller error margins

260 280 300 320 3400.0

4.0x10-12

8.0x10-12

1.2x10-11

kC

H3

O2

+H

O2 / c

m3 s

-1

T / K

This study

IUPAC

preliminaryresult


Conclusions and outlook Speciated detection of CH3O2 radicals by FAGE

Potential to use the method in field measurements of CH3O2

Speciated detection of C2H5O2 radicals by FAGE

New and selective method

Planned: development of the method to detect bigger RO2 radicals




FAGE–CRDS intercomparisons for HO2 and CH3O2 to validate the FAGEmethod.






FAGE–CRDS intercomparisons for HO2 and CH3O2 to validate the FAGEmethod.

The FAGE method has been used to detect HO2 and CH3O2 in HIRAC toreduce the uncertainty in the kHO2+CH3O2 reported by IUPAC.



Planned: New kinetic studies, e.g. OH + CH3COOH (CH3O2: intermediate)

• Alexander Brennan

• Freja F. Østerstrøm

• Joseph Parr

• Grace Ronnie

• James Hooper

• Dwayne Heard

• Paul Seakins

• Lisa Whalley

• Michele Gianella

• Ana Lawry Aguila

• Nicole Ng

• Gus Hancock

• Grant Ritchie 44



Documents

University of California, Davis - A new method for CH3O2 and ......and the direct and absolute Cavity Ring-Down Spectroscopy (CRDS) •FAGE has been used in kinetic studies of the