| Photo-oxidation in the troposphere is highly complex, being initiated by short lived free
radical species, in the daytime dominated by the hydroxyl radical, OH. Chemical oxidation
cycles, which also involve peroxy radicals (HO2 and RO2), remove natural or anthropogenic
emissions (for example methane) and generate a range of secondary products, for example
ozone, nitrogen dioxide, acidic and multifunctional organic species, and secondary
organic aerosol, which impact on human health and climate. Owing to their short
lifetime in the atmosphere, the abundance of radicals is determined solely by their
rate of chemical production and loss, and not by transport. Field measurements of
the concentrations of radicals and comparison with calculations using a numerical
model therefore constitutes one of the very best ways to test whether the chemistry
in each of these locations is understood and accurately represented in the model.
Validation of the chemistry is important, as the predictions of climate and air quality
models containing this chemistry are used to drive the formulation of policy and
legislation.
However, in situ measurements of radical species, owing to their very low abundance
(often sub part per trillion) and short lifetimes (< 1 second for OH), remain extremely
challenging. Laser-induced fluorescence spectroscopy (LIF) has enjoyed considerable success
worldwide for the quantitative detection of radicals in a range of environments. The radicals
are either excited directly by the laser (e.g. OH, IO) or are first chemically converted to OH
prior to detection (e.g. HO2, RO2). Recent developments in the LIF technique for radical
detection, which uses a supersonic expansion with detection at low pressure and multi kHz
pulse repetition rate tunable laser systems, will be discussed, together with calibration
methods to make signals absolute, and identification of potential interferences. LIF
instruments have been operated on ground, ship and aircraft platforms at a number of
locations worldwide, and examples from recent fieldwork involving the Leeds instruments
will be presented. |