OH Lifetime

Lifetime Instrument

The highly reactive hydroxyl radical is the major oxidising compound in the troposphere, and governs the atmospheric lifetimes of many trace species of biogenic and anthropogenic origin. It initiates oxidative radical chain reactions which remove primary pollutants and greenhouse gases such as CO, methane, non-methane volatile organic compounds, NOx, and SO₂. These processes convert these compounds into secondary pollutants such as O₃, peroxy acetyl nitrate, and sulphuric acid, which are potentially harmful e.g. for the oxidation of a hydrocarbon and generation of O₃ via the formation and subsequent photolysis of NO₂.

An experimental method to measure the atmospheric lifetime of OH will allow the resolution of the [OH] measured to model discrepancies as the direct measurement of the total OH loss rate, the inverse of its lifetime, is a parameter that encompasses all the sinks of OH. This measurement determines the situations when the full range of OH co-reactants has not been fully characterised.

OH Decay

The technique employed for the measurement of total OH loss rate is based on measuring the decay of artificially produced OH in an atmospheric pressure flow tube reactor. OH radicals are generated in the tip of a movable injector tube which is co-axial with the much larger diameter main flow tube. Downstream of the injector gas from the main tube is sampled into a FAGE detection cell where OH radicals are detected by laser induced fluorescence. The decay of [OH] is measured as a series of individual OH signal measurement points by stepping the injector back in fixed increments to increase the distance from injection to sampling, thus increasing the time available for OH to react with ambient trace gases. The flow velocity in the main flow tube is measured so that the distance between the injector and the sampling point can be converted into a reaction time.

A plot of ln[OH] against reaction time as shown can be used to calculate a crude loss rate which includes a physical loss and the removal of the OH by reaction with the ambient air. The physical loss rate is a constant that can be obtained by measuring a loss rate in very clean air or nitrogen.

Instrument development

A new lifetime instrument is currently being built and tested by the FAGE group and uses a flash photolysis system to generate OH radicals. Further details will be added once the instrument has been fully characterised.