The ICAD (iterative cavity-enhanced DOAS) method

Abstract

Abstract. Cavity-enhanced differential optical absorption spectroscopy (CE-DOAS or BB-CEAS DOAS) allows us to make in situ measurements while maintaining the kilometre-long light paths required by DOAS. This technique has been successfully used for several years to measure in situ atmospheric trace gases. A property of optical cavities is that in the presence of strong absorbers or scatterers the light path is reduced, in contrast to classical long-path DOAS measurements where the light path is fixed. Typical CE-DOAS or BB-CEAS evaluation schemes correct this effect using the measured total light intensity attenuation. This makes them sensitive to any variations in the light intensity not arising from the trace gas absorption. That means an important DOAS advantage, to be independent of total light intensity, is actually lost. In order to cope with this problem, the instrument setup would require a thorough stabilisation of the light source and a very rigid mechanical setup, which would make instrumentation more complex and error prone. We present a new approach to cavity-enhanced (CE) DOAS based on an iterative algorithm (ICAD) which actually models the light path reduction from the derived absorbers in the optical resonator. It allows a sensitive and robust data analysis that does not depend on the total light intensity, allowing a simpler and more compact instrument setup. The algorithm is discussed and simulated measurements demonstrate its sensitivity and robustness. Furthermore, a new ICAD NO2 instrument is presented. It takes advantage of the advanced data evaluation to build a compact (50 cm cavity) and lightweight instrument (<10 kg) with low power consumption (25 W) for sensitive measurements of NO2 with a detection limit of 0.02 ppbv at an averaging time of 7 min. The instrument is characterised with a NO2 calibration source and good long-term stability is demonstrated in a comparison with a commercial chemiluminescence detector. As a new application of ICAD we show measurements on an automobile platform to investigate the two-dimensional NO2 distribution in an urban area. The instrument is so robust that even strong vibrations do not lead to any measurement problems.

Intercomparison of MAX-DOAS vertical profile retrieval algorithms: studies on field data from the CINDI-2 campaign

“…In former publications (e.g. Irie et al, 2008;Clémer et al, 2010;Frieß et al, 2016;Bösch et al, 2018) and also during this comparison study, it was found that MAX-DOAS vertically integrated aerosol profiles systematically underestimate AOTs. It has already been pro-J.-L. Tirpitz et al: CINDI-2 profiling comparison Figure 10.…”

“…Multi-axis differential optical absorption spectroscopy (MAX-DOAS) (e.g. Hönninger and Platt, 2002;Hönninger et al, 2004;Wagner et al, 2004;Heckel et al, 2005;Frieß et al, 2006;Platt and Stutz, 2008;Irie et al, 2008;Clémer et al, 2010;Wagner et al, 2011;Vlemmix et al, 2015b) is a widely used ground-based measurement technique for the detection of aerosols and trace gases particularly in the lower troposphere: ultraviolet (UV) and visible (Vis) absorption spectra of skylight are analysed to obtain information on different atmospheric absorbers and scatterers, integrated over the light path (in fact, a superposition of a multitude of light paths). The amount of atmospheric trace gases along the light path is inferred by identifying and analysing their characteristic narrow spectral absorption features, applying DOAS (Platt and Stutz, 2008).…”

Intercomparison of MAX-DOAS vertical profile retrieval algorithms: studies on field data from the CINDI-2 campaign

“…5 minutes). Further, a CAPS (type AS32M, based on attenuated phase shift spectroscopy, Kebabian et al, 2005) and a CE-DOAS (cavity enhanced DOAS, Platt et al, 2009 andHorbanski et al, 2019) were continuously measuring at 27 m altitude. All the in situ measurements at the tower were combined to obtain another set of surface concentration measurements, more representative for concentrations close to the site.…”

An IBBCEAS system for atmospheric measurements of glyoxal and methylglyoxal in the presence of high NO<sub>2</sub> concentrations

“…Since the first application of IBBCEAS by Fiedler (Fiedler et al, 2003), it has been widely used in laboratory studies and field campaigns to measure atmospheric trace gases such as H 2 O, O 3 , O 4 , IO, I 2 , OIO, SO 2 , NO 2 , NO 3 , N 2 O 5 , HONO, HCHO, GLY, and MGLY (Vaughan et al, 2008;Washenfelder et al, 2008;Thalman and Volkamer, 2010;Axson et al, 2011;Kahan et al, 2012;Min et al, 2016;Duan et al, 2018). The core part of the instrument is a high-finesse cavity that holds of a pair of highreflectivity mirrors, which typically have reflectivity greater than 0.9999.…”