FAGE

OH, HO2, RO2

OH, HO₂ and RO2₂ measurements are made using the fluorescence assay by gas expansion technique (FAGE). A single FAGE fluorescence cell is used for sequential measurements of OH and HO₂. This is operated from the roof of a customised shipping container but can be operated remotely from the container (at a distance of up to 30 m) for example on a tower to collocate with other measurements. The cell is held at 1 Torr using a roots blower backed rotary pump system which is housed in the air-conditioned shipping container and connected to the cell by flexible hosing. 308 nm tunable, pulsed laser light is used to electronically excite OH radicals, this is delivered to the cell via a fibre optic cable with the laser system housed in the shipping container. Fluorescence is detected by a channel photomultiplier and gated photon counting. Data are acquired every second (photon counts from 5000 laser shots), consisting of up to five minutes tuned to the OH transition (NO can be injected after a time to rapidly convert HO₂ to OH, to facilitate detection of HO₂) and up to 2 minutes tuned away from the OH transition to determine background laser scatter.

A second fluorescence cell is used to measure the sum of RO2 radicals. This is achieved by coupling a low-pressure FAGE cell (as described above) to a differentially pumped reaction tube (held at approximately 30 Torr) to allow for the conversion of RO₂ radicals to OH. Ambient air is drawn into the reaction tube and in HOx mode, CO flows into the centre of the reaction tube just beneath the inlet. Hydroxyl radicals are converted to HO₂ by reaction with CO as they pass through the reaction tube:

CO + OH (+O₂) → HO₂ + CO₂

Air from the reaction tube is sampled by the second FAGE cell coupled to the reaction tube (held at approximately 1.5 Torr). Ambient HO₂ (and ambient OH which was converted to HO₂ in the reaction tube) is titrated to OH by NO injected into the cell and detected as described above. In ROx mode, NO is added to the CO flow to promote the conversion of RO₂ to OH in the reaction tube:

RO₂ + NO → RO + NO₂

RO + O₂ → HO₂ + R-HO

HO₂ + NO → OH + NO₂

the excess CO present rapidly converts OH to HO₂. Ambient RO₂, HO₂ and OH radicals (converted to HO₂ in the reaction tube) enter the FAGE detection cell, are reconverted to OH by NO and detected.

By varying, the amount of NO injected it is possible to switch from conditions where certain RO₂ types – alkene and aromatic-derived RO₂ (RO₂i) – are efficiently converted to OH to conditions where the conversion is poor and with knowledge of the conversion efficiency of RO₂ and HO₂ at different NO concentrations, changing the NO flow in the FAGE cells during ambient measurements can selectively provide a measurement of the concentration of RO₂i and HO2.

OH reactivity

OH reactivity measurements are made using a laser-induced pump and probe technique. Ambient air is drawn through a flow-tube and a beam of 266 nm light, pulsed along the flow-tube, photolyses ambient ozone (or lab generated ozone if ambient levels are low) generating O(1D) which reacts with ambient H2O (v) present to form OH:

O₃ + hv → O(¹D) + O₂

O(¹D) + H₂O → 2OH

The [OH] generated in the flow tube is kept sufficiently low that pseudo-first-order conditions are maintained, thus the absolute OH concentration does not need to be known. The OH undergoes an exponential decay, monitored by a FAGE cell coupled to the end of the flow-tube:

[OH]_t=[OH]₀ exp(−(k’ + k’ wall)t)

where k’ represents the reactive decay rate, the inverse of the chemical lifetime, τOH, given by:

k’=Σki [X]i

where [X]i represents the concentration of atmospheric trace gas and ki the rate coefficient for its reaction with OH at the temperature of the flow-tube, and k’wall is the non-reactive loss of OH mainly due to wall losses or diffusion out of the 266 nm beam.

The instrument is calibrated at regular intervals (twice weekly) during deployment. Known concentrations of OH and HO₂ are generated in a turbulent flow reactor by flowing humidified air past a Hg pen ray lamp which photolyses H₂O at 185 nm to form OH (and HO₂ in the presence of O₂):

H₂O + 185 nm → OH + H

H + O₂ → HO₂

With knowledge of the H₂O(v) concentration (measured using a calibrated GE dewpoint hygrometer), the absorption cross-section of H₂O at 185 nm, flow velocities and lamp flux the concentration of the radicals can be accurately calculated.

The lamp flux is determined before and after each deployment using N₂O actinometry: Briefly, a known concentration of N₂O flows through the turbulent flow reactor, N₂O is photolysed to NO by the Hg lamp and detected using a trace level NOx analyser which has been calibrated using a certified NO standard.

Consumables

• The user will need to supply a 3-phase power cable (32 A, 400 V, 5P connector).
• OH, HO₂ measurements:
  • 1 x nitric oxide (purity 2.5 – 99.5%) cylinder size AK
  • 3 x BTCA Air cylinder size AK
  • 1 x Nitrogen premier grade cylinder size AK
  • 1 x 450 ppb nitric oxide certified standard, cylinder size AV
  • 2 x 22.7 kg Sofnofil (http://www.molecularproducts.com/pdf/Sofnofil%20UK%20DS.pdf)
  • 1 x 50 kg Chemsorb1000 (http://www.molecularproducts.com/pdf/Chemsorb%201000%20UK%20DS.pdf)
  • 20 L HPLC grade water
• If RO₂ measurements are required:
  • 3 x 5 % carbon monoxide in nitrogen, cylinder size AV
  • 2 x 500 ppmv nitric oxide in nitrogen, cylinder size AV
  • 1 x methane CP grade, cylinder size F
• If OH reactivity measurements are required:
  • 3 x BTCA Air cylinder size AK

Costs

• Instrument Insurance
  • This system must be insured by the user for £500K and covers loss, theft or damage to the instrument: damage is that over and above general wear and tear. The system has been designed to be rugged and autonomous. Even so, the end-user must respect the fact that the system is a precision optical instrument that must be treated with great care.
  • The user is responsible for the instrument from the time it leaves the AMOF to the time it is returned and signed off as in an acceptable operating condition by the IS: this will be done as soon as is possible on its return.
• Public Liability Insurance
  • NCAS is not liable for any damage or injury arising from the deployment or operation of this instrument when unattended by the IS.
• Shipping Expenses
  • The user is liable for all costs arising from the shipping of the instrument both to and from deployment.
• IS T&S
  • The user is responsible for coving the travel and subsistence expenses of the IS while attending the instrument.

Shipping

• The instrument is packed in a 20 ft shipping container.
• Shipping dimensions: 5.9 m (L) x 2.35 m (W) x 2.35 m (H)
• Shipping weight: 10 Tonnes

3 phase power is required to operate the instrument.

Adequate access and hard-standing for container lorry for drop-off and pick-up.

Solid, reasonably level ground for positioning the container.

Minimal shading.

Eye safety

• This instrument contains a class 4 laser system. Only laser-trained individuals should have access to the container when the instrument is in operation. There are no beams external to container / Roof-top box. All beams within the container are enclosed and delivered by optic fibers. Laser goggles are worn for all alignments. Door sign indicating lasers on; restricted access.

Manual handling

• Two people are required for set-up and pack-up of the instrument and good lifting practices should be employed. No access is permitted on the roof of the container before safety barriers have been erected by trained individuals. A side-lift is fitted to the container and is used for moving items on and off the roof. Safety hats required when walking close to the container and on the roof. Wires/tubing should be kept away from pathways, held overhead height in the container. Walkways kept free of clutter and access to exit kept clear.

Electric safety

• Single point insulation fitted. Waterproof container, armoured cable, RCD protection. All equipment electrical safety tested.

Chemical hazards

• Toxic gases are used during the operation of the instrument (CO, NO): Cylinders are secured and the container is kept well ventilated. Vacuum lines are leak tested and gas usage is monitored. Gases should be locked away when unattended and a CO alarm is housed in the container.

Footprint:

• 5.9 m (L) x 2.35 m (W) x 2.35 m (H)

Weight :

• 10 Tonnes

Power:

• 10 kW(at 400 V, 32 amps)

Operation temperature:

• -10 min°C to +40 max°C

The FAGE instrument provides measurements of OH, HO2, sum of RO2 and OH reactivity (kOH) as standard.

It is possible to begin to partially speciate between alkene, aromatic derived RO2 species and those derived from short-chain (<C4) alkanes.

Field Data

• The instrument produces a range of out files and all are text format.
• The user can download (but not delete) this data from the instrument but it should be noted that this data will not have been quality controlled.

Archive data

• Data is provided in NetCDF files following the AMOF data standard
• Files contain no more than 24hr of data.
• Instrument name is
  • ncas-fage-1
• The data product(s) associated with this instrument:
  • oh-concentration
• Example data file
  • How to use example data
  • ncas-fage-1_ral_29001225_oh-concentration_v0.1.nc