ALBERT

Description: Over the last few decades, differential optical absorption spectroscopy (DOAS) has been used as a common technique to simultaneously measure abundances of a variety of atmospheric trace gases. Exploiting the unique differential absorption cross section of trace-gas molecules, mixing ratios can be derived by measuring the optical density along a defined light path and by applying the Beer–Lambert law. Active long-path (LP-DOAS) instruments can detect trace gases along a light path of a few hundred metres up to 20 km, with sensitivities for mixing ratios down to ppbv and pptv levels, depending on the trace-gas species. To achieve high measurement accuracy and low detection limits, it is crucial to reduce instrumental artefacts that lead to systematic structures in the residual spectra of the analysis. Spectral residual structures can be introduced by most components of a LP-DOAS measurement system, namely by the light source, in the transmission of the measurement signal between the system components or at the level of spectrometer and detector. This article focuses on recent improvements by the first application of a new type of light source and consequent changes to the optical setup to improve measurement accuracy. Most state-of-the-art LP-DOAS instruments are based on fibre optics and use xenon arc lamps or light-emitting diodes (LEDs) as light sources. Here we present the application of a laser-driven light source (LDLS), which significantly improves the measurement quality compared to conventional light sources. In addition, the lifetime of LDLS is about an order of magnitude higher than of typical Xe arc lamps. The small and very stable plasma discharge spot of the LDLS allows the application of a modified fibre configuration. This enables a better light coupling with higher light throughput, higher transmission homogeneity, and a better suppression of light from disturbing wavelength regions. Furthermore, the mode-mixing properties of the optical fibre are enhanced by an improved mechanical treatment. The combined effects lead to spectral residual structures in the range of 5-10×10-5 root mean square (rms; in units of optical density). This represents a reduction of detection limits of typical trace-gas species by a factor of 3–4 compared to previous setups. High temporal stability and reduced operational complexity of this new setup allow the operation of low-maintenance, automated LP-DOAS systems, as demonstrated here by more than 2 years of continuous observations in Antarctica.

Description: Over the last decades, Differential Optical Absorption Spectroscopy (DOAS) has been used as a common technique to simultaneously measure abundances of a variety of atmospheric trace gases. Exploiting the unique differential absorption cross section of trace gas molecules, mixing ratios can be derived by measuring the optical density along a defined light path and by applying the Beer-Lambert law. Active long-path (LP-DOAS) instruments can detect trace gases along a light path of a few hundred metres up to 20 km with sensitivities for mixing ratios down to ppbv and pptv levels, depending on the trace gas species. To achieve high measurement accuracy and low detection limits, it is crucial to reduce instrumental artefacts that lead to systematic structures in the residual spectra of the analysis. Spectral residual structures can be introduced by most components of a LP-DOAS measurement system, namely by the light source, in the transmission of the measurement signal between the system components or at the level of spectrometer and detector. This article focuses on recent improvements by the first application of a new type of light source and consequent changes to the optical setup to improve measurement accuracy. Most state-of-the-art LP-DOAS instruments are based on fibre optics and use xenon arc lamps or light emitting diodes (LEDs) as light sources. Here we present the application of a Laser Driven Light Source (LDLS), which significantly improves the measurement quality compared to conventional light sources. In addition the lifetime of LDLS is about an order of magnitude higher than of typical Xe-arc lamps. The small and very stable plasma discharge spot of the LDLS allows the application of a modified fibre configuration. This enables a better light coupling with higher light throughput, higher transmission homogeneity, and a better suppression of light from disturbing wavelength regions. Furthermore, the mode mixing properties of the optical fibre are enhanced by an improved mechanical treatment. The combined effects lead to spectral residual structures in the range of 5–10 · 10−5 RMS (in units of optical density). This represents a reduction of detection limits of typical trace gas species by a factor of 3–4 compared to previous setups. High temporal stability and reduced operational complexity of this new setup allow the operation of low-maintenance automated LP-DOAS systems as demonstrated here by more than two years of continuous observations in Antarctica.

Description: Multi-Axis Differential Optical Absorption Spectroscopy (MAXDOAS)is a widely used technique for the detection of atmospheric trace gases, e.g. NO2, SO2, but also for the oxygen collision complex O4, whose atmospheric distribution is well known. By comparing measured O4 differential slant column densities (dSCDs) with modelled ones, information on aerosol distributions and optical properties can be gained. In combination with a radiative transfer model, an inversion of measured dSCDs allows the retrieval of vertical aerosol extinction profiles and properties. Here the ability of MAX-DOAS observations to detect cloud altitude and cloud optical properties of different cloud covers will be discussed. An accurate retrieval of these parameters is crucial for an interpretation of trace gas dSCDs and a subsequent retrieval of vertical profiles from MAX-DOAS measurements under cloudy conditions. The ability of MAX-DOAS to retrieve cloud layer height and optical properties will be demonstrated with a comparison to co-located measurements of a commercial Ceilometer during several cruises of the German research vessel Polarstern. Advantages, limitations and possible applications of the technique will be discussed.

Description: Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) is a widely used technique for the detection of atmospheric trace gases, e.g. NO2, SO2, BrO, HCHO, but also for the oxygen collision complex O4. The atmospheric distribution of the latter is proportional to the square of the molecular oxygen concentration and thus well known. By comparing measured O4 differential slant column densities (dSCDs) from MAX-DOAS measurements with modeled ones, information on aerosol distributions and optical properties, as well as on clouds can be obtained using an algorithm based on optimal estimation. Here the ability of MAX-DOAS observations to detect cloud altitude and cloud optical properties of different cloud covers based on measurements of O4 will be discussed. The analysis uses measurements made by a shipborne instrument on two cruises of the German research vessel Polarstern to the Antarctic Weddell Sea from June to October 2013. During this time a broad range of cloud and aerosol conditions was encountered, in particular persistent low cloud cover with a high optical thickness. Aerosol and particle extinction profiles were retrieved with temporal resolutions of up to 15 minutes. For clouds at altitudes up to 2000 m the results show a very good agreement with co-located measurements of a commercial ceilometer and pictures from a cloud camera. Unless visibility was very poor due to fog, even rapid changes in cloud altitude or cover could be detected by MAX-DOAS. These results indicate that under homogeneous cloud cover an accurate retrieval of trace gas vertical profiles can be possible despite the strong influence of clouds on atmospheric light paths. We will discuss advantages and limitations of cloud detection with MAX-DOAS, implications for the subsequent retrieval of trace gas profiles and the possible use of external (ceilometer) data as a priori information for the profile retrieval algorithm.

Description: From January 2016 until August 2018, an automated Long-Path Differential Optical Absorption Spectroscopy (LP-DOAS) instrument was operated at the German research station Neumayer III (NMIII) in coastal Antarctica to measure trace gas mixing ratios in the boundary layer close to the ground (average altitude above the snow surface: 4m). Two different atmospheric light paths with total lengths of 3100 m and 5900m were available and used depending on the spectral window and visibility. Detectable species were bromine monoxide (BrO), bromine dioxide (OBrO), molecular bromine (Br2), chlorine monoxide (ClO), chlorine dioxide (OClO), iodine monoxide (IO), iodine dioxide (OIO), molecular iodine (I2), sulphur dioxide (SO2), nitrogen dioxide (NO2), nitrous acid (HONO), nitrate (NO3), and ozone (O3). The evaluated data was filtered based on RMS (root mean square) thresholds of optical density residuals of the DOAS fits (see individual data sets for respective values). Detection limits based on a 3-sigma criterion were used and are provided in the data sets for all detectable species. For BrO, ClO, OClO, IO, SO2, and NO2, observations above the respective detection limit were made. For all absorbers the retrieved mixing ratios/concentrations with respective total errors (corresponding to 1-sigma) are provided. The data sets include the observations below the respective detection limit scattered around 0, which due to the spectral analysis with the DOAS approach can include negative values. Significant data points can be selected using the separately provided detection limits (3-sigma criterion - recommended). Alternatively, the total errors can be used. Since total errors and hence detection limits are determined from spectral information, for absorbers not observed at significant levels (OBrO, Br2, OIO, I2, HONO, and NO3), the respective detection limits can serve as an upper limit for an atmospheric presence. The mixing ratios provided here were calculated using meteorological data from Neumayer III station already published on PANGAEA. A list of data sets is included in the "Related to" section. The spectral raw data of the measurements is stored on the measurement data server of the Institute of Environmental Physics, University of Heidelberg. -- Technical details and performance of employed LP-DOAS instrument: Nasse et al. (2019, doi:10.5194/amt-2019-69) Detailed description of measurements and evaluation: Nasse, Jan-Marcus (2019): Halogens in the coastal boundary layer of Antarctica (PhD Thesis). University of Heidelberg

Description: From January 2016 until August 2018, an automated Long-Path Differential Optical Absorption Spectroscopy (LP-DOAS) instrument was operated at the German research station Neumayer III (NMIII) in coastal Antarctica to measure trace gas mixing ratios in the boundary layer close to the ground (average altitude above the snow surface: 4m). Two different atmospheric light paths with total lengths of 3100 m and 5900m were used depending on visibility. The DOAS evaluation of I2 was performed between 532.5 and 549 nm. Fits with residual optical density RMS (root mean square) values larger than 4.0e-4 were discarded. The resulting mean residual RMS is 2.3e-4. The mean detection limit is 78 pmol/mol. The data sets include the observations below the respective detection limit scattered around 0, which due to the spectral analysis with the DOAS approach can include negative values. I2 was not detected above a 3-sigma detection limit. Since total errors and hence detection limits are determined from spectral information, the detection limit of I2 can serve as an upper limit for an atmospheric presence. The mixing ratios provided here were calculated using meteorological data from Neumayer III station already published on PANGAEA. A list of data sets is included in the "Related to" section of the bibliography this data set is part of. The spectral raw data of the measurements is stored on the measurement data server of the Institute of Environmental Physics, University of Heidelberg.

Description: From January 2016 until August 2018, an automated Long-Path Differential Optical Absorption Spectroscopy (LP-DOAS) instrument was operated at the German research station Neumayer III (NMIII) in coastal Antarctica to measure trace gas mixing ratios in the boundary layer close to the ground (average altitude above the snow surface: 4m). Two different atmospheric light paths with total lengths of 3100 m and 5900m were used depending on visibility. The DOAS evaluation of Br2 was performed between 532.5 and 549 nm. Fits with residual optical density RMS (root mean square) values larger than 4.0e-4 were discarded. The resulting mean residual RMS is 2.3e-4. The mean detection limit is 3.9 nmol/mol. The data sets include the observations below the respective detection limit scattered around 0, which due to the spectral analysis with the DOAS approach can include negative values. Br2 was not detected above a 3-sigma detection limit. Since total errors and hence detection limits are determined from spectral information, the detection limit of Br2 can serve as an upper limit for an atmospheric presence. The mixing ratios provided here were calculated using meteorological data from Neumayer III station already published on PANGAEA. A list of data sets is included in the "Related to" section of the bibliography this data set is part of. The spectral raw data of the measurements is stored on the measurement data server of the Institute of Environmental Physics, University of Heidelberg.

Description: From January 2016 until August 2018, an automated Long-Path Differential Optical Absorption Spectroscopy (LP-DOAS) instrument was operated at the German research station Neumayer III (NMIII) in coastal Antarctica to measure trace gas mixing ratios in the boundary layer close to the ground (average altitude above the snow surface: 4m). Two different atmospheric light paths with total lengths of 3100 m and 5900m were used depending on visibility. The DOAS evaluation of OBrO was performed between 532.5 and 549 nm. Fits with residual optical density RMS (root mean square) values larger than 4.0e-4 were discarded. The resulting mean residual RMS is 2.3e-4. The mean detection limit is 11 pmol/mol. The data sets include the observations below the respective detection limit scattered around 0, which due to the spectral analysis with the DOAS approach can include negative values. OBrO was not detected above a 3-sigma detection limit. Since total errors and hence detection limits are determined from spectral information, the detection limit of OBrO can serve as an upper limit for an atmospheric presence. The mixing ratios provided here were calculated using meteorological data from Neumayer III station already published on PANGAEA. A list of data sets is included in the "Related to" section of the bibliography this data set is part of. The spectral raw data of the measurements is stored on the measurement data server of the Institute of Environmental Physics, University of Heidelberg.

Description: From January 2016 until August 2018, an automated Long-Path Differential Optical Absorption Spectroscopy (LP-DOAS) instrument was operated at the German research station Neumayer III (NMIII) in coastal Antarctica to measure trace gas mixing ratios in the boundary layer close to the ground (average altitude above the snow surface: 4m). Two different atmospheric light paths with total lengths of 3100 m and 5900m were used depending on visibility. The DOAS evaluation of HONO was performed between 352.5 and 386.5 nm. Fits with residual optical density RMS (root mean square) values larger than 3e-4 were discarded. The resulting mean residual RMS is 1.5e-4. The mean detection limit is 174 pmol/mol. The data sets include the observations below the respective detection limit scattered around 0, which due to the spectral analysis with the DOAS approach can include negative values. HONO was not detected above a 3-sigma detection limit. Since total errors and hence detection limits are determined from spectral information, the detection limit of HONO can serve as an upper limit for an atmospheric presence. The mixing ratios provided here were calculated using meteorological data from Neumayer III station already published on PANGAEA. A list of data sets is included in the "Related to" section of the bibliography this data set is part of. The spectral raw data of the measurements is stored on the measurement data server of the Institute of Environmental Physics, University of Heidelberg.

Description: From January 2016 until August 2018, an automated Long-Path Differential Optical Absorption Spectroscopy (LP-DOAS) instrument was operated at the German research station Neumayer III (NMIII) in coastal Antarctica to measure trace gas mixing ratios in the boundary layer close to the ground (average altitude above the snow surface: 4m). Two different atmospheric light paths with total lengths of 3100 m and 5900m were used depending on visibility. The DOAS evaluation of BrO was performed between 301.7 and 346.4 nm as well as between 332.5 and 370.5 nm (for subsequent measurements in two different instrumental setups). Fits with residual optical density RMS (root mean square) values larger than 4.0e-4 and 3.5e-4 respectively were discarded. The resulting mean residual RMS are 2.4e-4 and 2.2e-4 respectively. The mean detection limits of the two evaluations are 2.2 and 3.8 pmol/mol. The data sets include the observations below the respective detection limit scattered around 0, which due to the spectral analysis with the DOAS approach can include negative values. Significant data points can be selected using the separately provided detection limits (3-sigma criterion - recommended). Alternatively, the total errors can be used. The mixing ratios provided here were calculated using meteorological data from Neumayer III station already published on PANGAEA. A list of data sets is included in the "Related to" section of the bibliography this data set is part of. The spectral raw data of the measurements is stored on the measurement data server of the Institute of Environmental Physics, University of Heidelberg.