Lavinia Onel, Alexander Brennan, Freja F. Østerstrøm, Michele Gianella, Lisa Whalley, Gus Hancock, Paul Seakins, Grant Ritchie and Dwayne Heard
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1 An intercomparison of methods for HO 2 and CH 3 O 2 detection and kinetic study of the HO 2 + CH 3 O 2 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
2 Overview Laser-Induced Fluorescence (LIF) spectroscopy at low-pressure, known as Fluorescence Assay by Gas Expansion (FAGE) is most commonly used for the HO 2 detection in the atmosphere
3 Overview Laser-Induced Fluorescence (LIF) spectroscopy at low-pressure, known as Fluorescence Assay by Gas Expansion (FAGE) is most commonly used for the HO 2 detection in the atmosphere A new method for CH 3 O 2 radicals detection using FAGE has been developed
4 Overview Laser-Induced Fluorescence (LIF) spectroscopy at low-pressure, known as Fluorescence Assay by Gas Expansion (FAGE) is most commonly used for the HO 2 detection in the atmosphere A new method for CH 3 O 2 radicals detection using FAGE has been developed As FAGE is an indirect method and requires calibration it has been validated performing intercomparisons of measurements of HO 2 and CH 3 O 2 by FAGE and the direct and absolute Cavity Ring-Down Spectroscopy (CRDS)
5 Overview Laser-Induced Fluorescence (LIF) spectroscopy at low-pressure, known as Fluorescence Assay by Gas Expansion (FAGE) is most commonly used for the HO 2 detection in the atmosphere A new method for CH 3 O 2 radicals detection using FAGE has been developed As FAGE is an indirect method and requires calibration it has been validated performing intercomparisons of measurements of HO 2 and CH 3 O 2 by FAGE and the direct and absolute Cavity Ring-Down Spectroscopy (CRDS) FAGE has been used in kinetic studies of the HO 2 + CH 3 O 2 cross-reaction within the HIRAC chamber
6 HO 2 and CH 3 O 2 radicals in the troposphere HO 2 is generated directly by the oxidation of CO and indirectly by the oxidation of VOCs CH 3 O 2 is the most abundant RO 2 in the atmosphere A dominant source of CH 3 O 2 is the oxidation of CH 4 hn O 3 H 2 O hn NO 2 OH NO CO O 2 HO 2 O 2 CH 3 O O 2 CH 4 CH 3 O 2 NO NO 2 hn
7 HO 2 and CH 3 O 2 radicals in the troposphere HO 2 is generated directly by the oxidation of CO and indirectly by the oxidation of VOCs CH 3 O 2 is the most abundant RO 2 in the atmosphere A dominant source of CH 3 O 2 is the oxidation of CH 4 hn O 3 H 2 O hn NO 2 OH NO CO O 2 HO 2 O 2 CH 3 O O 2 CH 4 CH 3 O 2 NO NO 2 cross-reactions other and self-reactions RO 2 hn under low NO x levels chain termination
8 HO 2 and CH 3 O 2 in the troposphere HO 2 is measured by FAGE by conversion to OH; typically the measured [HO 2 ] peaks at daytime at a few 10 8 cm -3.
9 HO 2 and CH 3 O 2 in the troposphere HO 2 is measured by FAGE by conversion to OH; typically the measured [HO 2 ] peaks at daytime at a few 10 8 cm -3. Currently there are no specific measurements of the concentration of CH 3 O 2 or any other RO 2 by any direct or indirect method.
10 HO 2 and CH 3 O 2 in the troposphere HO 2 is measured by FAGE by conversion to OH; typically the measured [HO 2 ] peaks at daytime at a few 10 8 cm -3. Currently there are no specific measurements of the concentration of CH 3 O 2 or any other RO 2 by any direct or indirect method. The sum of organic RO 2 has been measured in the atmosphere using CIMS (Chemical Ionisation Mass Spectrometry), PERCA (Peroxy Radical Chemical Amplifier) method and more recently RO x LIF method.
11 HO 2 and CH 3 O 2 in the troposphere HO 2 is measured by FAGE by conversion to OH; typically the measured [HO 2 ] peaks at daytime at a few 10 8 cm -3. Currently there are no specific measurements of the concentration of CH 3 O 2 or any other RO 2 by any direct or indirect method. The sum of organic RO 2 has been measured in the atmosphere using CIMS (Chemical Ionisation Mass Spectrometry), PERCA (Peroxy Radical Chemical Amplifier) method and more recently RO x LIF method. Box modelling using the Master Chemical Mechanism (MCM) has found that the daytime [CH 3 O 2 ] peaks in the range ~(0.5-6) 10 8 cm -3. * * Whalley et al. ACP, 2010; 2011; 2018
12 A new method for the selective and sensitive detection of CH 3 O 2 radicals CH 3 O 2 radicals are detected by titration with known amounts of NO to generate CH 3 O radicals which are then detected using FAGE. CH 3 O 2 + NO CH 3 O + NO 2 LIF detection The A X electronic excitation of CH 3 O radicals is used to detect these radicals with high sensitivity and selectively by LIF.
13 A new method for the selective and sensitive detection of CH 3 O 2 radicals The method is similar to the FAGE method used for HO 2 detection: CH 3 O 2 + NO CH 3 O + NO 2 LIF detection HO 2 + NO OH + NO 2 LIF detection
14 FAGE instrument Laser excitation: ~308 nm ~298 nm HO 2 OH Detectors CH 3 O 2 CH 3 O Fluorescence Laser axis Inlet pinhole NO NO Gas flow, system at ~2.4 Torr
15 FAGE calibration Known concentrations of HO 2 and CH 3 O 2 are generated in a calibration flow tube impinging outside the FAGE sample inlet 40 slm humidified air 0.3 % CH 4 HO 2 OH Detectors Fluorescence Laser axis CH 3 O 2 CH 3 O Hg pen-ray lamp (184.9 nm) quartz window NO Gas flow, system at ~2.4 Torr NO
16 FAGE calibration Known concentrations of HO 2 and CH 3 O 2 are generated in a calibration flow tube impinging outside the FAGE sample inlet H 2 O nm OH + H H + O 2 HO 2 OH + CH 4 H 2 O + CH 3 CH 3 + O 2 + M CH 3 O 2 + M 40 slm humidified air 0.3 % CH 4 HO 2 OH Detectors Fluorescence Laser axis CH 3 O 2 CH 3 O Hg pen-ray lamp (184.9 nm) quartz window NO Gas flow, system at ~2.4 Torr NO
17 Methoxy (CH 3 O) excitation spectrum online FAGE signal / counts s -1 mw online / nm Onel et al. AMT, 10, , 2017 online = nm offline = online nm
18 FAGE calibration plot S/N = 2 LOD(CH 3 O 2 ) = ( ) 10 9 cm -3 ( ) 10 8 cm s averaging time 20 min averaging time slope = C CH3O2 = (4.1 ± 1.4) counts s -1 mw -1 cm 3 Onel et al. AMT, 10, , 2017
19 FAGE calibration plot S/N = 2 LOD(CH 3 O 2 ) = ( ) 10 9 cm s averaging time LOD(HO 2 ) = ( ) 10 6 cm -3 ( ) 10 8 cm min averaging time slope = C CH3O2 = (4.1 ± 1.4) counts s -1 mw -1 cm 3 Onel et al. AMT, 10, , 2017
20 Potential of the new method to be used in field measurements of CH 3 O 2 LOD = ( ) 10 8 cm -3 S/N = 2 and 20 min averaging time ~ cm -3 CH 3 O 2 tropical Atlantic ocean * * Whalley et al. ACP, 2010
21 Potential of the new method to be used in field measurements of CH 3 O 2 LOD = ( ) 10 8 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)
22 Validation of the FAGE method for detection of HO 2 and CH 3 O 2 FAGE CRDS intercomparison
23 Validation of the FAGE method for detection of HO 2 and CH 3 O 2 FAGE CRDS intercomparison HIRAC (Highly Instrumented Reactor for Atmospheric Chemistry)
24 Validation of the FAGE method for detection of HO 2 and CH 3 O 2 FAGE CRDS intercomparison HIRAC (Highly Instrumented Reactor for Atmospheric Chemistry) NIR First HO 2 and CH 3 O 2 CRDS measurements in an atmospheric simulation chamber
25 Validation of the FAGE method for detection of HO 2 and CH 3 O 2 FAGE CRDS intercomparison HIRAC (Highly Instrumented Reactor for Atmospheric Chemistry) 6 Light intensity / V Absorbing species present No absorbing species NIR Data point Laser Circulating light ~1.4 m cavity Detector
26 Validation of the FAGE method for detection of HO 2 and CH 3 O 2 FAGE CRDS intercomparison HIRAC (Highly Instrumented Reactor for Atmospheric Chemistry) 360 nm Cl 2 2Cl ~ 1.5 m HO 2 Cl + CH 3 OH CH 2 OH + HCl CH 2 OH + O 2 HO 2 + CH 2 O
27 Validation of the FAGE method for detection of HO 2 and CH 3 O 2 FAGE CRDS intercomparison HIRAC (Highly Instrumented Reactor for Atmospheric Chemistry) 360 nm Cl 2 2Cl ~ 1.3 m CH 3 O 2 Cl + CH 4 CH 3 + HCl CH 3 + O 2 + M CH 3 O 2 + M
28 Intercomparison of HO 2 and CH 3 O 2 by FAGE and CRDS CRDS probes HO 2 using the excitation of the first O-H overtone at 1.5 m and CH 3 O 2 using the A X electronic excitation at 1.3 m lamps off lamps on Onel et al. AMT, 10, , 2017
29 FAGE CRDS correlation plots [HO 2 ] FAGE / cm x x x x mbar ± HO 2 CH 3 O mbar ± x x x10 10 [HO 2 ] CRDS / cm -3 [CH 3 O 2 ] FAGE / cm x x x mbar ± x x x10 11 [CH 3 O 2 ] CRDS / cm mbar 1.04 ± 0.01 Onel et al. AMT, 10, , 2017
30 HO 2 + CH 3 O 2 cross-reaction HO 2 + CH 3 O 2 O 2 + CH 3 COOH 0.9 O 2 + HCHO + H 2 O 0.1 A main sink for CH 3 O 2 radicals in remote areas (low NO x ) ~ cm -3 CH 3 O 2 ~ cm -3 CH 3 O 2 tropical Atlantic ocean tropical rainforest
31 Kinetics of the HO 2 + CH 3 O 2 cross-reaction HO 2 + CH 3 O 2 k Products 23% 1 uncertainty in the value of the overall rate coefficient at 298 K cm 3 molecule -1 s -1 [1]. The majority of previous studies used UV-absorption spectroscopy, which is often an unselective technique. Clear need for kinetic studies of this reaction using a selective method. [1]
32 Kinetics of the HO 2 + CH 3 O 2 cross-reaction HIRAC HO 2 OH Detectors Fluorescence CH 3 O 2 CH 3 O Laser axis NO Gas flow NO
33 Kinetics of the HO 2 + CH 3 O 2 cross-reaction Laser excitation: ~308 nm HIRAC HO 2 OH Detectors Fluorescence CH 3 O 2 CH 3 O Laser axis NO Gas flow HO 2 and CH 3 O 2 are measured in alternation
34 Kinetics of the HO 2 + CH 3 O 2 cross-reaction Laser excitation: ~298 nm HIRAC HO 2 OH Detectors Fluorescence CH 3 O 2 CH 3 O Laser axis Gas flow NO HO 2 and CH 3 O 2 are measured in alternation
35 HO 2 and CH 3 O 2 second order decays Reaction mixtures: CH 4 /CH 3 OH/Cl 2 /air Different [CH 3 O 2 ]/[HO 2 ] ratios concentration / molecule cm x10 11 CH 3 O 2 2.0x time / s lamps off lamps on 4.0x10 11 [HO 2 ] 2.0x time / s 4.0x x10 11 CH 3 O time / s HO 2 and CH 3 O 2 are measured in alternation
36 Kinetic analysis Reaction CH 3 O 2 + CH 3 O 2 CH 3 OH + HCHO + O 2 CH 3 O 2 + CH 3 O 2 CH 3 O + CH 3 O + O 2 CH 3 O + O 2 HCHO + HO 2 CH 3 O 2 + HO 2 CH 3 OOH + O 2 HCHO + H 2 O + O 2 HO 2 + HO 2 + (M) H 2 O 2 + O 2 + (M) CH 3 O 2 self-reaction cross-reaction HO 2 self-reaction
37 Kinetic analysis Reaction CH 3 O 2 + CH 3 O 2 CH 3 OH + HCHO + O 2 CH 3 O 2 + CH 3 O 2 CH 3 O + CH 3 O + O 2 CH 3 O + O 2 HCHO + HO 2 CH 3 O 2 + HO 2 CH 3 OOH + O 2 HCHO + H 2 O + O 2 HO 2 + HO 2 + (M) H 2 O 2 + O 2 + (M) extracted: overall k CH 3O2+HO2 [HO 2 ] / cm -3 [CH 3 O 2 ] / cm -3 4x x x x x x x x10 10 Fit experimental data time /s time /s Fit Experimental
38 HO 2 + CH 3 O 2 cross-reaction: Temperature dependence Experiments at 268, 284, 295, 323, and 344 K (-5 to 70 C) k CH3O2+HO2 / cm 3 s x x x10-12 This study IUPAC T / K IUPAC: k CH3O2 + HO2 = exp(780/t) cm 3 s -1 ( K) E/R = 100 K
39 HO 2 + CH 3 O 2 cross-reaction: Temperature dependence Experiments at 268, 284, 295, 323, and 344 K (-5 to 70 C) k CH3O2+HO2 / cm 3 s x x x10-12 preliminary result This study IUPAC T / K IUPAC: k CH3O2 + HO2 = exp(780/t) cm 3 s -1 ( K) E/R = 100 K
40 HO 2 + CH 3 O 2 cross-reaction: Temperature dependence Experiments at 268, 284, 295, 323, and 344 K (-5 to 70 C) k CH3O2+HO2 / cm 3 s x x x10-12 preliminary result This study IUPAC Good agreement Smaller error margins T / K IUPAC: k CH3O2 + HO2 = exp(780/t) cm 3 s -1 ( K) E/R = 100 K
41 Conclusions and outlook Speciated detection of CH 3 O 2 radicals by FAGE New and selective method Potential to use the method in field measurements of CH 3 O 2 Speciated detection of C 2 H 5 O 2 radicals by FAGE Planned: development of the method to detect bigger RO 2 radicals
42 Conclusions and outlook Speciated detection of CH 3 O 2 radicals by FAGE New and selective method Potential to use the method in field measurements of CH 3 O 2 Speciated detection of C 2 H 5 O 2 radicals by FAGE Planned: development of the method to detect bigger RO 2 radicals FAGE CRDS intercomparisons for HO 2 and CH 3 O 2 to validate the FAGE method.
43 Conclusions and outlook Speciated detection of CH 3 O 2 radicals by FAGE New and selective method Potential to use the method in field measurements of CH 3 O 2 Speciated detection of C 2 H 5 O 2 radicals by FAGE Planned: development of the method to detect bigger RO 2 radicals FAGE CRDS intercomparisons for HO 2 and CH 3 O 2 to validate the FAGE method. The FAGE method has been used to detect HO 2 and CH 3 O 2 in HIRAC to reduce the uncertainty in the k HO2+CH3O2 reported by IUPAC. Planned: New kinetic studies, e.g. OH + CH 3 COOH (CH 3 O 2 : intermediate)
44 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
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