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

Abstract

Abstract. Cavity enhanced methods in absorption spectroscopy have seen a considerable increase in popularity during the past decade. Especially Cavity Enhanced Absorption Spectroscopy (CEAS) established itself in atmospheric trace gas detection by providing tens of kilometers of effective light path length using a cavity as short as 1 m. In this paper we report on the construction and testing of a compact and power efficient light emitting diode based broadband Cavity Enhanced Differential Optical Absorption Spectrometer (CE-DOAS) for in situ observation of atmospheric NO 3 . This device combines the small size of the cavity with the advantages of the DOAS approach in terms of sensitivity, specificity and insensivity to intensity fluctuations of the light source. In particular, no selective removal of the analyte (here NO 3 ) is necessary for calibration of the instrument if appropriate corrections are applied to the CEAS theory. Therefore the CE-DOAS technique can -in principlemeasure any gas detectable by DOAS. We will discuss the advantages of using a light emitting diode (LED) as light source particularly the precautions which have to be considered for the use of LEDs with a broad wavelength range. The instrument was tested in the lab by detecting NO 3 formed by mixing of NO 2 and O 3 in air. It was then compared to other trace gas detection techniques in an intercomparison

An instrument for measurements of BrO with LED-based Cavity-Enhanced Differential Optical Absorption Spectroscopy

“…Ball et al (2004) combined BBCEAS for the data evaluation with the established DOAS technique (Platt and Stutz, 2008), creating the CE-DOAS. With this technique, different trace gases can be detected simultaneously and at high spatial resolution (Ball et al, 2004;Gherman et al, 2008;Platt et al, 2009;Meinen et al, 2010;Thalman and Volkamer, 2010). We note that the BBCEAS retrieval algorithm published by Fiedler et al (2003) relies -like DOAS -on the characteristic narrow band absorption features of the molecules for their quantification and is thus in principle equivalent to DOAS.…”

“…Both effects are discussed in this paper. To estimate the optical path length in the cavity, the instrument applies the cavity ringdown technique (Meinen et al, 2010) …”

“…In practice, this average path length L 0 (λ) can be determined from the known Rayleigh scattering of two different gases, e.g., helium and dry zero air (Washenfelder et al, 2008), from a CRD decay time measurement (Meinen et al, 2010) or from the differential absorption of a trace gas of known concentration. In this work L 0 (λ) was determined from the absorption bands of O 4 at 343.5 and 360.4 nm, making use of the known O 2 concentration (Ball et al, 2004).…”

“…Also, A denotes the initial signal at t = 0 and b denotes any offset signal at t = ∞. From this fit and known τ 1 , the CRD decay time can be determined and converted to an optical light path or mirrors reflectivity according to Meinen et al (2010). However, in our case it is not possible to derive with this method the accurate absorption light path for the data evaluation, as the mirror reflectivity varies too much over the LED spectral range (see Sect.…”

Light extinction by secondary organic aerosol: an intercomparison of three broadband cavity spectrometers

“…The CE-DOAS instrument comprised an optical cavity formed by two highly reflective mirrors (R = 99.9985 %) separated by 0.62 m (Platt et al, 2009;Meinen et al, 2010). Light from a red LED, stabilized to 27 ± 2 • C, was focussed into the optical cavity.…”