Broadband Cavity Enhanced Differential Optical Absorption Spectroscopy (CE-DOAS) – applicability and corrections

Abstract

Abstract. Atmospheric trace gas measurements by cavity assisted long-path absorption spectroscopy are an emerging technology. An interesting approach is the combination of CEAS with broadband light sources, the broadband CEAS (BB-CEAS). BB-CEAS lends itself to the application of the DOAS technique to analyse the derived absorption spectra. While the DOAS approach has enormous advantages in terms of sensitivity and specificity of the measurement, an important implication is the reduction of the light path by the trace gas absorption, since cavity losses due to absorption by gases reduce the quality (Q) of the cavity. In fact, at wavelength, where the quality of the BB-CEAS cavity is dominated by the trace gas absorption (especially at very high mirror reflectivity), the average light path will vary nearly inversely with the trace gas concentration and the strength of the band will become only weakly dependent on the trace gas concentration c in the cavity, (the differential optical density being proportional to the logarithm of the trace gas concentration). Only in the limiting case where the mirror reflectivity determines Q at all wavelength, the strength of the band as seen by the CE-DOAS instrument becomes directly proportional to the concentration c. We investigate these relationships in detail and present methods to correct for the cases between the two above extremes, which are of course the important ones in practice.

Night-time chemistry above London: measurements of NO<sub>3</sub> and N<sub>2</sub>O<sub>5</sub> from the BT Tower

“…The BBCEAS technique was introduced in 2003 by Fiedler et al (2003) and has since been developed by a number of groups (Platt et al, 2009;Ruth et al, 2007;Washenfelder et al, 2008) including the current authors (Ball et al, 2004;Langridge et al, 2006Langridge et al, , 2008a. A detailed description of the BBCEAS technique and the instrument used in this study is provided in Langridge et al (2008a) and only a brief outline is provided here.…”

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

“…In this contribution we use the most direct way to define the light path L 0 by employing CRD parallel to recording broadband transmission spectra in order to achieve absolute absorber concentrations. The theoretical background of this procedure is discussed in great detail in a corresponding theory paper (Platt et al, 2009).…”

“…Note that the determination does not rely on the knowledge of the intensity I in coupled into the resonator, but this method is very sensitive to intensity fluctuations of the light source if they occur between the measurement of I 0 and I . Platt et al (2009) have recently shown corrections for this approximate formula derived by Fiedler et al (2003), which are necessary since there is a non-linear dependency between measured intensity and absorber concentration for high concentrations. For broadband techniques it is important to note that all parameters in Eq.…”

“…L 0 has to be interpreted as the average light path over all photons or partial rays propagating inside an evacuated resonator before they leave it by passing one of the mirrors (Fielder et al, 2003;Platt et al, 2009). This value can be estimated by:…”

Broadband Cavity-Enhanced Absorption Spectroscopy with Incoherent Light

“…Retrieval methods will not be discussed here -more 34 details can be found in refs. [5,33,55]. The species-specific LOD can be defined as the num-…”