Why measure HO_x?

OH is highly reactive, with the balance between production and destruction rates giving an atmospheric lifetime under clean conditions of ~1 s (less under polluted conditions), and consequently tropospheric concentrations are extremely low. Daytime maxima are in the range of (1-10) x 10⁶ molecule cm^-3 (0.04 - 0.4 pptV). HO₂ has a longer lifetime of ~100 s with a typical daytime maximum concentration of ~10⁸ molecule cm^-3 (~4 pptV). Both OH and HO₂ concentrations react swiftly to changes in local ambient conditions, such as solar flux and levels of NO_x or hydrocarbons, but are largely unaffected by transport phenomena.

If the values of all the local controlling parameters are used as input, a photochemical model can predict the OH and HO₂ concentrations. Measurements of ambient OH and HO₂ are thus highly desirable, as they enable comparison with the model output, providing a stringent test of our understanding of the mechanisms of fast photochemical oxidation processes.

The FAGE Technique

OH concentrations are extremely low in the troposphere, making detection of this radical a great experimental challenge. The FAGE technique is utilised by the University of Leeds for the task.

FAGE, Fluorecence Assay by Gas Expansion is essentially laser-induced fluoresence at low pressure. It is a highly selective and sensitive technique, ideal for the measurment of OH in the atmosphere. Ambient air is drawn into a detection cell at low pressures (~1 Torr) through a pinhole nozzle (~1mm diameter). A laser beam tuned to 308 nm is passed perpendicularly across the gas beam, causing on-resonance fluoresence of the OH radicals. The fluoresence is filtered and focussed onto a photomultiplier tube and analysed by photon counting. The resultant signal is converted into an OH concentration by calibration. The FAGE technique can also be used to make HO₂ measurements by means of chemical conversion to OH with NO, followed by LIF detection. Current detection limits are 6 x 10⁵ molecule cm^-3 for OH and 1 x 10⁶ molecule cm^-3 for HO₂. Data is taken with 30 second integration time.

Why measure IO?

Iodine species are present in the marine boundary layer (MBL) due to the release I₂ and photo-labile iodocarbons from macro- and micro-algae. The photolysis of these species yield iodine atoms, which react with ozone to generate IO.

This radical plays an important role in the chemistry occurring in the MBL. It has been implicated in catalytic ozone destruction cycles, it influences the oxidising capacity of the region through the perturbation of the HO_x cycle, and is thought to be involved in the formation and growth of new particles.

LIF Detection of IO

Laser induced fluorescence, LIF, has been used to detect ambient IO radicals.

Air was drawn into a detection cell, held at 150 torr, via a 0.8 mm aperture cone shaped nozzle. A laser beam, tuned to 445 nm, passed through the cell and was used to excite the (2,0) band of the IO A²Π _3/2 electronic transition, with off-resonance fluorescence in the (2,5) band detected at 521 nm using a CPM and photon counting. The fluorescence signal was converted into an IO concentration via calibration in which a known concentration of IO was generated by photolysis of N₂O in the presence of CF₃I.

The ground-based instrument made the first ever LIF measurements of IO in Roscoff in 2006 as part of the RHaMBLe project, whilst the aircraft instrument made the first ever ship-borne measurements of IO in 2007.