The “ideal” spectrograph for atmospheric observations

Spectroscopy of scattered sunlight in the near-UV to near-IR spectral ranges has proven to be an extremely useful tool for the analysis of atmospheric trace gas distributions. A central parameter for the achievable sensitivity and spatial resolution of spectroscopic instruments is the étendue (product of aperture angle and entrance area) of the spectrograph, which is at the heart of the instrument. The étendue of an instrument can be enhanced by (1) upscaling all instrument dimensions or (2) by changing the instrument F number, (3) by increasing the entrance area, or (4) by operating many instruments (of identical design) in parallel. The étendue can be enhanced by (in principle) arbitrary factors by options (1) and (4); the effect of options (2) and (3) is limited

New methods for the calibration of optical resonators : integrated calibration by means of optical modulation (ICOM) and narrow-band cavity ring-down (NB-CRD)

Optical resonators are used in spectroscopic measurements of atmospheric trace gases to establish long optical path lengths L with enhanced absorption in compact in-struments. In cavity-enhanced broad-band methods, the ex-act knowledge of both the magnitude of L and its spectral dependency on the wavelength lambda is fundamental for the correct retrieval of trace gas concentrations. L(lambda) is connected to the spectral mirror reflectivity R (lambda), which is often referred to instead. L(lambda) is also influenced by other quantities like broad-band absorbers or alignment of the optical resonator. The established calibration techniques to determine L(lambda), e.g. introducing gases with known optical properties or measuring the ring-down time, all have limitations: limited spectral resolution, insufficient absolute accuracy and precision, inconvenience for field deployment, or high cost of implementation. Here, we present two new methods that aim to overcome these limitations: (1) the narrow-band cavity ring-down (NB-CRD) method uses cavity ring-down spectroscopy and a tunable filter to retrieve spectrally resolved path lengths L(lambda); (2) integrated calibration by means of op-tical modulation (ICOM) allows the determination of the op-tical path length at the spectrometer resolution with high ac-curacy in a relatively simple setup. In a prototype setup we demonstrate the high accuracy and precision of the new approaches. The methods facilitate and improve the determination of L(lambda), thereby simplifying the use of cavity-enhanced absorption spectroscopy.Peer reviewe

On the link between Earth tides and volcanic degassing

Long-term measurements of volcanic gas emissions conducted during the recent decade suggest that under certain conditions the magnitude or chemical composition of volcanic emissions exhibits periodic variations with a period of about two weeks. A possible cause of such a periodicity can be attributed to the Earth tidal potential. The phenomenology of such a link has been debated for long, but no quantitative model has yet been proposed. The aim of this paper is to elucidate whether a causal link from the tidal forcing to variation in the volcanic degassing can be traced analytically. We model the response of a simplified magmatic system to the local tidal gravity variations and derive a periodical vertical magma displacement in the conduit with an amplitude of 0.1–1 m, depending on geometry and physical state of the magmatic system. We find that while the tide-induced vertical magma displacement has presumably no significant direct effect on the volatile solubility, the differential magma flow across the radial conduit profile may result in a significant increase of the bubble coalescence rate in a depth of several kilometres by up to several ten percent. Because bubble coalescence facilitates separation of gas from magma and thus enhances volatile degassing, we argue that the derived tidal variation may propagate to a manifestation of varying volcanic degassing behaviour. The presented model provides a first basic framework which establishes an analytical understanding of the link between the Earth tides and volcanic degassing

First Results on the DOAS-Retrieval of OClO from SCIAMACHY Nadir Measurements

The Scanning Imaging Absorption Spectrometer for Atmospheric Chartography was launched successfully onboard ENVISAT on March 1, 2002. It observes solar radiation transmitted, backscattered from the atmosphere and reflected from the ground in nadir, limb and occultation viewing modes. Chlorinedioxide (OClO), an important indicator for stratospheric chlorine activation, can be measured in the UV spectral range by Differential Optical Absorption Spectroscopy (DOAS). First results of the DOAS retrieval of OClO slant column densities (SCDs) from the SCIAMACHY measurements are presented. The influence of several parameters like the wavelength range chosen as fitting window or the reference spectra included in the fit on the quality of the retrieval is examined. It is found that a proper correction of polarisation features in the spectra is essential for a good DOAS analysis of OClO. The OClO SCDs derived from SCIAMACHY are compared to measurements of the Global Ozone Monitoring Experiment (GOME) which has successfully measured OClO since 1995. SCIAMACHY flies in the same orbit, but measures approx. 30 minutes earlier than GOME. As OClO shows a strong diurnal variation, this leads to differences in the observed column densities, which may be useful to investigate the photochemistry of OClO and related compounds. Also, the spatial resolution of SCIAMACHY is higher (30*60 km^2 compared to 40*320 km^2 for GOME), which will allow a more detailed study of small scale effects like e.g. chlorine activation in mountain waves

Ground-Based Remote Sensing and Imaging of Volcanic Gases and Quantitative Determination of Multi-Species Emission Fluxes

The physical and chemical structure and the spatial evolution of volcanic plumes are of great interest since they influence the Earth's atmospheric composition and the climate. Equally important is the monitoring of the abundance and emission patterns of volcanic gases, which gives insight into processes in the Earth's interior that are difficult to access otherwise. Here, we review spectroscopic approaches (from ultra-violet to thermal infra-red) to determine multi-species emissions and to quantify gas fluxes. Particular attention is given to the emerging field of plume imaging and quantitative image interpretation. Here UV SO2 cameras paved the way but several other promising techniques are under study and development. We also give a brief summary of a series of initial applications of fast imaging techniques for volcanological research

The influence of nitrogen oxides on the activation of bromide and chloride in salt aerosol

Abstract. Experiments on salt aerosol with different salt contents were performed in a Teflon chamber under tropospheric light conditions with various initial contents of nitrogen oxides (NOx = NO + NO2). A strong activation of halogens was found at high NOx mixing ratios, even in samples with lower bromide contents such as road salts. The ozone depletion by reactive halogen species released from the aerosol, was found to be a function of the initial NOx mixing ratio. Besides bromine, large amounts of chlorine have been released in our smog chamber. Time profiles of the halogen species Cl2, Br2, ClNO2, BrNO2 and BrO, ClO, OClO and Cl atoms were simultaneously measured by various techniques (chemical ionization mass spectrometry, differential optical absorption spectrometry coupled with a multi-reflection cell and gas chromatography of hydrocarbon tracers for Cl and OH, employing cryogenic preconcentration and flame ionization detection). Measurements are compared to calculations by the CAABA/MECCA 0-D box model, which was adapted to the chamber conditions and took the aerosol liquid water content and composition into account. The model results agree reasonably with the observations and provide important information about the prerequisites for halogen release, such as the time profiles of the aerosol bromide and chloride contents as well as the aerosol pH.</jats:p

Non-dispersive UV Absorption Spectroscopy: A Promising New Approach for in-situ Detection of Sulfur Dioxide

A new type of instrument for in-situ detection of volcanic sulfur dioxide is presented on the basis of non-dispersive UV absorption spectroscopy. It is a promising alternative to presently used compact and low-cost SO2 monitoring techniques, over which it has a series of advantages, including an inherent calibration, fast response times (&lt; 2 s to reach 90 % of the applied concentration), a measurement range spanning about 5 orders of magnitude and small, well-known cross sensitivities to other gases. Compactness, cost-efficiency and detection limit (&lt; 1 ppm, few ppb under favorable conditions) are comparable to other presently used in-situ instruments. Our instrument prototype has been extensively tested in comparison studies with established methods. In autumn 2015, diverse volcanic applications were investigated such as fumarole sampling, proximal plume measurements and airborne measurements several kilometers downwind from the vent on Mt. Etna and White Island. General capabilities and limitations of the measurement principle are discussed, considering different instrument configurations and future applications

Remote measurement of high preeruptive water vapor emissions at Sabancaya volcano by passive differential optical absorption spectroscopy

Water (H2O) is by far the most abundant volcanic volatile species and plays a predominant role in driving volcanic eruptions. However, numerous difficulties associated with making accurate measurements of water vapor in volcanic plumes have limited their use as a diagnostic tool. Here we present the first detection of water vapor in a volcanic plume using passive visible-light differential optical absorption spectroscopy (DOAS). Ultraviolet and visible-light DOAS measurements were made on 21 May 2016 at Sabancaya Volcano, Peru. We find that Sabancaya's plume contained an exceptionally high relative water vapor abundance 6 months prior to its November 2016 eruption. Our measurements yielded average sulfur dioxide (SO2) emission rates of 800–900 t/d, H2O emission rates of around 250,000 t/d, and an H2O/SO2 molecular ratio of 1000 which is about an order of magnitude larger than typically found in high-temperature volcanic gases. We attribute the high water vapor emissions to a boiling-off of Sabancaya's hydrothermal system caused by intrusion of magma to shallow depths. This hypothesis is supported by a significant increase in the thermal output of the volcanic edifice detected in infrared satellite imagery leading up to and after our measurements. Though the measurement conditions encountered at Sabancaya were very favorable for our experiment, we show that visible-light DOAS systems could be used to measure water vapor emissions at numerous other high-elevation volcanoes. Such measurements would provide observatories with additional information particularly useful for forecasting eruptions at volcanoes harboring significant hydrothermal systems