Technical Note: Using a high finesse optical resonator to provide a long light path for differential optical absorption spectroscopy: CE-DOAS

摘要

Cavity enhanced methods in absorption spectroscopy have seen a considerableincrease in popularity during the past decade. Especially Cavity EnhancedAbsorption Spectroscopy (CEAS) established itself in atmospheric trace gasdetection by providing tens of kilometers of effective light path lengthusing a cavity as short as 1 m. In this paper we report on the constructionand testing of a compact and power efficient light emitting diode basedbroadband Cavity Enhanced Differential Optical Absorption Spectrometer(CE-DOAS) for in situ observation of atmospheric NO₃. This devicecombines the small size of the cavity with the advantages of the DOASapproach in terms of sensitivity, specificity and insensivity to intensityfluctuations of the light source. In particular, no selective removal of theanalyte (here NO₃) is necessary for calibration of the instrument ifappropriate corrections are applied to the CEAS theory. Therefore theCE-DOAS technique can – in principle – measure any gas detectable by DOAS.We will discuss the advantages of using a light emitting diode (LED) aslight source particularly the precautions which have to be considered forthe use of LEDs with a broad wavelength range. The instrument was tested inthe lab by detecting NO₃ formed by mixing of NO₂ and O₃ inair. It was then compared to other trace gas detection techniques in anintercomparison campaign in the atmosphere simulation chamber SAPHIR atForschungszentrum Jülich at NO₃ concentrations as low as 6.3 ppt.