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Development of a mobile field instrument for the fast simultaneous measurement of OH- and HO2 radicals in the troposphere by high-repetition rate laser-induced fluorescence


This project aims to develop an operative mobile field instrument for the highly sensitive, fast in-situ measurement of OH and HO2 in the troposphere.

A highly sensitive measurement instrument for tropospheric hydroxyl (OH) radicals has been developed. It is based on laser induced flourescence (LIF) detection of OH. Oh is detected in a continuous free jet of ambient air expanding through a nozzle into a low pressure detection chamber. A tunable frequency double dye laser pumped by a high repetition rate pulsed copper vapour laser is used for fast signal accumulation rates. The OH flourescence emitted between 307 nm to 311 nm is measured by gated photon counting. The LIF instrument detects OH directly and with high specificity, a fact that was demonstrated by recording laser excitation spectra of ambient OH. In August 1994, the LIF instrument was tested and deployed in a field campaign to study the atmospheric photochemistry in a rural environment. Diurnal OH concentration profiles were measured with high time resolution showing maximum OH concentrations at noon. Minimum OH concentrations were measured in the morning and evening. The corectness of the concentration calibration was confirmed by the in situ intercomparison with the laser absorption spectrometer by Forchungszentrum Juelich.

The measure OH data will now be used to test troposperic chemistry models, in order to better understand the chemical role of OH radicals as the most important oxidant in the troposphere, being responsible for the removal of most atmospheric trace gases and pollutants.
The oxidizing capacity of the troposphere is mainly determined by the concentration of hydroxyl-radicals (OH), which control the removal of most naturally and anthropogenically produced atmospheric trace gases. Thus, the understanding of the tropospheric chemistry requires the direct measurement of OH-radicals in the atmosphere. There is an urgent need for highly sensitive instruments that can measure tropospheric OH-concentrations with good temporal and spatial resolution. Furthermore, simultaneous measurements of hydroperoxy-radicals (HO2) are needed to assist the interpretation of chemistry. The OH-radicals are sampled by fast gas-expansion of ambient air into a low pressure fluorescence chamber and are detected by laser-induced fluorescence (LIF) using the excitation wavelength 308 nm. A high-repetition rate laser system will be used to give adequate sensitivity (105 OH/cm3 in a few minutes). This technique is essentially free from interferences by laser generated OH. The nearly simultaneous measurement is achieved by chemical conversion of atmospheric HO2 into OH by titration with NO. The high-repetition rate laser system to be used in this project will be a combination of a optimized copper vapour laser (CVL) and a small-bandwidth tunable dye laser that will be modified and optimized for reliable field operation. The laser system will be integrated into an existing LIF experiment which will be developed into an operative mobile field instrument for ground based measurements. The instrument will be tested during a comprehensive photochemistry field campaign at Jülich. In a second part of this project, a novel concept for a tunable CVL-pumped Ti doped sapphire laser will be studied for use as a compact and lightweight laser radiation source, designed for an advanced OH/HO2 measurement system that could be operated on board of a small airplane.

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Forschungszentrum Jülich GmbH
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52425 Jülich

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