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UP: Umweltphysik

UP 18: Atmosph
ärische Spurengase und Aerosole: Instrumentelles

UP 18.2: Fachvortrag

Dienstag, 14. März 2006, 16:45–17:00, E

Development of a measurement system for peroxy radicals using laser-induced fluorescence technique — •Hendrik Fuchs, Andreas Hofzumahaus, and Frank Holland — Institute for chemistry and dynamics of the geosphere, Forschungszentrum Jülich, Jülich, Germany

A new instrument for measuring the sum of atmospheric hydroperoxy and organic peroxy-radicals (HO₂+RO₂) was developed using a two-step chemical conversion and laser-induced fluorescence (LIF) technique. The detection is done by successive conversion of RO₂ to hydroxyl radicals (OH). The system consists of two differentially pumped chambers. About 7L/min of ambient air is sampled through a nozzle into the first chamber, in which pressure is reduced to 25hPa. An excess of NO and CO is added behind the nozzle leading to a conversion of RO₂ to HO₂. The pressure is further reduced in the second chamber. In this chamber HO₂ is converted to OH by adding an excess of NO. The detection of OH-radicals is done by time delayed gated photon counting after resonant excitation of OH-fluorescence at 308nm (A²Σ⁺ - X²Π). The sensitivity of the system is calibrated using a radical source. OH-radicals are produced by water photolysis. They react with methane resulting in methylperoxy-radicals. The typical detection limit is 2· 10⁶ cm⁻³ (0.08pptv) for 2-min averages and signal to noise ratio of 1. The estimated accuracy is 10%. Unlike chemical amplifying systems also used for measuring HO₂+RO₂-radicals, only a weak dependence of the sensitivity on water is found which can be explained by quenching of the fluorescence. Ambient air measurements were performed showing distinctive diurnal profiles.