Risn Commane, Cdric Floquet,

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Aircraft Measurements of OH HO2 in the
Troposphere

• Róisín Commane, Cédric Floquet,
• Trevor Ingham, Dwayne Heard, Mat Evans
• School of Chemistry
• University of Leeds

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Hydroxyl Radical (OH)

• Primary oxidant in troposphere
• Vital to the breakdown of pollution
• Methane highest concentration trace gas in
  atmosphere

Estimate 80 global CH4 removed in tropics
(CH4 OH) Fast OH HO2 (HOx)
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Global OH Measurements

• Sparse tropical boundary layer and free
  tropospheric measurements of HOx
• Max HOx production expected in Tropical Summer

Horizontal Lines Fixed Sites Hatched areas
Airborne Measurements
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HOx Tropospheric Measurements Requirements

• OH Difficult to Measure
• Low concentration ( 106 molecule cm-3 0.04
  ppt)
• Very short lifetime in atmosphere (lt 1 s)
• Reacts quickly with surfaces
• Measurement Technique Requirements
• Highly Sensitive AND Selective
• Fast Time Resolution (ideally lt 1 min)
• In situ (aircraft ideal)

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FAGE Fluorescence Assay by Gas Expansion
A2?

?? 0

• On-resonance laser-induced fluorescence
• at low pressure
• High repetition laser (5 kHz).

Laser beam l308 nm
Fluorescence l308 nm
M
X2?i
?? 0

• Low pressure (1-2 Torr)
• reduces
• Conc. of interfering species
• Collisional quenching of OH
• OH Fluorescence longer
• than laser pulse/
• other scattering

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Leeds Air FAGE on the BAe-146during AMMA SOP-2
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Limitations during AMMA

• Laser Instability
• Temperature dependence (YAG unstable gt 36o C)
• Laser Wavelength Pressure dependence
• No profile measurements
• Stepper Motor (Failed sporadically)
• Lessons Learned
• Laser requires stable temperatures lower than 34o
  C
• High moisture conditions (with air conditioning)
  cause condensation on optics in the laser cavity

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Aircraft FAGE Data AMMA Flight Coverage
OH 7 flights over 6 days HO2 13 flights
over 11 days Typical OH 30 s integration Limit
of Detection (LOD) 7.2 x 105 molecule cm-3 at
1100 m Typical HO2 1 s integration LOD 3.1 x
106 molecule cm-3 at 1100 m.
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Measurement CycleReference Cell, Laser Power,
Cell Pressure
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OH and HO2 Online and Offline Raw Signal
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OH Time Integrated Datashowing associated limits
of detection
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HO2 Time Integrated Datashowing associated
limits of detection(LOD on separate scale)
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Interesting Features

• B228 Large variation observed in OH and HO2
  concentrations during the flight
• B229 OH and HO2 response to peak in isoprene
• B235A and B235B Day Vs Night
• OH below Limit of Detection
• HO2 still observed into the night
• B235A HO2 Uptake on water droplets?

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OH and HO2 concentrations observed during B228OH
ranges from 5 x 106 to lt 6 x 105 molecule cm-3
(LOD)HO2 2.9 x 107 9 x 108 molecule cm-3
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B229 HO2 Peak coincides with Isoprene Peak. OH
rise observed after
B229
Jim Hopkins, York
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Large Day-Night Variation in OH Night-time OH lt
LOD Large Day-Night Variation in HO2 Night-time
HO2 observed but greatly reduced
B235A B235B
Noon
Sunset
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Large Variation in HO2 when sampling in and out
of cloud
B235A B235B
In and Above Cloud
Noon
Sunset
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Reduction in HO2 when sampling in out of cloud
Claire Reeves
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Data interpretation strategy
What controls HOx and ROx ? Production - O3
? H2O ? J(O1D) vs J Oxygenates Isoprene
O3 Others? Loss - HO2 HO2, OH
NO2 HO2 Cloud drops OHIsoprene What
controls OH? Recycling from HO2? Loss
through VOCs? When does our understanding
fail? High isoprene loadings? Clouds?
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How does our understanding change? Conceptual
vision
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What should we expect?
Isoprene depletes OH
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What should we expect?
Isoprene depletes OH
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Future Treatment
What is the composition over West Africa? What
are the principal air masses over West
Africa? Use a cluster analysis approach Done
for ITOP Tie it together with the
meteorology Within each airmass type what
are the modes of variability? Principal
components analysis? New
Mat Evans
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Final Aircraft FAGE Characterisations

• Completion of Aircraft FAGE calibration
  characterisation (2 weeks)
• Application of calibration factor to all
  Aircraft FAGE AMMA data (lt1 week)
• Submission of 1 s Aircraft FAGE data to BADC
• (mid March)
• Addition of FAGE data to full Data Merge (?)

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Potential Papers ?

• Aircraft FAGE instrumental?
• OH Control Mechanism?
• HO2 Uptake on Clouds?
• Deviations from Model/Measurement agreement?