This paper investigates a multi-elevation Fast Synchronous observation MAX-DOAS system(FS MAX-DOAS) that can quickly obtain trace gas profiles. Compared to the traditional sequential scanning of elevation angles by motors, the system employs a grating spectrometer with a two-dimensional array CCD, we also designed telescopes of small field(<1⁰), a high-speed shutter switching module, and a multi-mode multi-core fiber which is divided into twelve beams to achieve multitrack spectroscopy. It greatly improves the time resolution of spectra collection(a elevation cycle within two minutes). The influence of spectral resolution on FS MAX-DOAS detection of trace gases was analyzed, and the optimal resolution range (0.277-0.569nm) was determined to select the grating used in the spectrometer. The selection of actual binning rows takes into account the SNR of each row of pixels to improve the quality of spectral data, and two-step acquisition is used to overcome the influence of difference in light intensity for low elevation angles. The stability of the system was analyzed using Allan variance. The outfield comparison experiment with differential optical absorption spectroscopy was conducted, and the comparison test was conducted with the ground-based MAX-DOAS system for NO₂ and HCHO in the actual atmosphere. The Pearson correlation coefficient of NO₂ reached 0.9, HCHO had a good correlation(Pearson’s R was mostly between 0.65-0.78). In the experiment, it was found that the RMS of FS MAX-DOAS spectral inversion can be stably lower than that of MAX-DOAS system for a long time, and the gas profile obtained by the former can show more details due to the improved time resolution. Compared to the near surface concentration of NO₂ using active Long Path DOAS instrument, the Pearson’s R of FS MAX-DOAS data is higher. New system can quickly and simultaneously obtain vertical distribution profiles of NO₂ and HCHO with high accuracy, which provides a possibility for mobile MAX-DOAS to achieve gas profile inversion.

How to cite: Xu, J., Li, A., Hu, Z., and Zhang, H.: Study of NO2 and HCHO vertical profile measurement based on Fast Synchronous MAX-DOAS, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3706, https://doi.org/10.5194/egusphere-egu24-3706, 2024.

In polluted cities with large sources of NO_x, rapid photolysis of nitrous acid (HONO) may be a major daytime source of the main atmospheric oxidant, OH. Current understanding of urban HONO is problematic. Its abundance is generally underestimated by models, and urban and rural networks. Campaigns with in situ instruments routinely identify a mystery midday HONO source. These measurements do not directly measure HONO and offer no insight into its vertical distribution. We use remotely sensed daytime differential slant column density (dSCD) measurements of HONO from a Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) instrument which was installed on a 60 m rooftop during summer 2022 on the Bloomsbury University College London (UCL) campus. Modelled (GEOS-Chem) and surface network measurements of air quality and meteorology are used to characterise conditions conducive to HONO detection. To be detected, dSCDs must exceed the detection limit, which we define as 2 times the root mean square of the fit of residuals divided by the maximum absorption cross section of HONO. We find that MAX-DOAS HONO dSCDs at elevation angles measuring the lowest layers of the atmosphere are only ever above the instrument detection limit in winter mornings (8:30 am–12:00 pm local time), suggesting substantial nighttime accumulation of HONO. This early morning HONO decreases rapidly (within 3-4 hours) to below detection after sunrise. Consistent characteristics of these mornings include cloud-free, cold (< 0°C) and calm conditions (wind speeds < 2.5 m s^-1), a shallow boundary layer (< 100 m), substantial surface ozone depletion (< 10 µg m^-3), and relatively large contribution of NO to total NO_x (NO/NO_x mass ratio ³ 0.3). Work is underway to further characterise urban HONO using 190 m measurements of NO_x from the Central London BT Tower observatory and assess the reaction kinetics balancing HONO loss and formation. New knowledge of HONO gained from MAX-DOAS measurements will then be used to evaluate best understanding of urban HONO as simulated with the GEOS-Chem model.

How to cite: Gershenson-Smith, E., Marais, E. A., Ryan, R. G., and Lu, G.: Assessment of variability in urban HONO using MAX-DOAS measurements in Central London, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8237, https://doi.org/10.5194/egusphere-egu24-8237, 2024.

Correspondence: Pinhua Xie(phxie@aiofm.ac.cn)

The understanding of the causes of photochemical pollution in Changjiang-Huaihe (Jianghuai) region, China, is limited due to the lack of investigation on the spatial-temporal distribution of secondary pollutants and its precursors, meteorological data and atmospheric oxidation. Aiming at this need, an integrated three-dimensional detection system for the key atmospheric components has been established by combining the in-situ and ground based telemetry. To meet the needs of the three-dimensional distribution detection of key atmospheric trace components (precursors - intermediates - secondary pollutants), a trace gas profile inversion algorithm of 50 m vertical resolution based on Monte Carlo method, named McPrA (Monte carlo Profile retrieval algorithm by AIOFM), and a machine learning algorithm based on MAX-DOAS observation were established, respectively. And the Open-path BroadBand Cavity Enhanced Absorption Spectroscopy (OP-BBCEAS) based on ball platform was established to carry out comparative verification and prior optimization for MAX-DOAS inversion algorithm. Long-term spatial-temporal distribution observations of key atmospheric trace components were carried out over the Jianghuai region, China. Aerosol profiles mainly showed the Gaussian shape with the high value concentrated in the range of 0.5-1.5 km. And the key gases profiles showed the exponential shape and concentrated within 1 km of the surface. The potential sources of trace gases at different heights of Hefei City were studied. It was found that aerosol transport from northern Anhui and northern Jiangsu was significant, and its transport layer was concentrated at the altitude of 500 m. For the polluted gases (NO₂, SO₂ and HCHO), the junction of eastern Anhui and Jiangsu was the main source area for the altitude below 500 m, especially the transport of NO₂ was the most significant. With the increase of the altitude, the influence of northern Anhui on Hefei City was gradually enhanced. The result of an integrated analysis of the meteorological factors affecting ozone pollution in Hefei was shown that the highest ozone concentration was mainly under the control of peripheral subsidence of typhoon. Additionally, a typical site for Jianghuai region, named Science Island in Hefei, was set up as a super observatory for photochemical pollution. The characteristics of the ozone control regime in the Jianghuai region were analyzed, and the quantitative response relationship between ozone generation and precursors were revealed.

This work was supported by the National Natural Science Foundation of China (Nos. U19A2044, 42105132, 42030609) and the National Key Research and Development Program of China (No. 2022YFC3700303). 

How to cite: Tian, X., Xie, P., Qin, M., Hu, R., Xu, J., Zheng, J., Hu, F., and Duan, J.: DOAS related techniques and application in photochemical measurement in Changjiang-Huaihe region, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20373, https://doi.org/10.5194/egusphere-egu24-20373, 2024.

Wind and weather in Taiwan are strongly influenced by the monsoons. Taiwan is on the lee side of the Asian winter monsoon, originating on the Asian continent. It receives continental air masses transported by the monsoon, thus air pollutants originating in the eastern and northern parts of China. While polluted air reaches Taiwan during winter monsoon, clean air masses from the remote western North Pacific are predominant in summer, making Taiwan an ideal location to investigate variations in atmospheric composition due to the monsoons. In addition to that, the monsoons themselves are also subject to regional climate changes in the future.

Since June 2023, a Multi-AXis-DOAS (MAX-DOAS) instrument has been installed at the Cape Fuguei Research Station (CAFE) at the northernmost point of Taiwan, measuring vertical and horizontal distributions of trace gases, including NO₂, SO₂, HCHO, and aerosols. The measurements aim to investigate local air pollution and study the impact of pollution export from mainland China on tropospheric composition and local air quality. The MAX-DOAS and the other measurements at the site will also be very valuable in the validation of the data products from the GEMS satellite.

The measurements are part of the project “Investigation of Pollution Transport to Taiwan” (IPToT), which has been established as a cooperation between the Insitute of Environmental Physics (IUP) of the University of Bremen (Germany) and the Research Center for Environmental Changes of the Academia Sinica (Taiwan), funded by the DFG (Deutsche Forschungsgemeinschaft).

Here, we present first MAX-DOAS observations and retrieved profiles of NO₂, SO₂, HCHO, and aerosols at this new station and compare them to collocated in-situ and (aerosol) LIDAR measurements. The data are evaluated for diurnal and weekly pollution signals and the dependency on the prevailing wind direction. A first comparison to satellite measurements is also shown.

How to cite: Seyler, A., Lange, K., Richter, A., Bösch, T., Behrens, L., Chou, C. C. K., Chen, W.-N., Ho, S., Hsu, K.-S., Kuo, C., Raman, M. R., Burrows, J. P., and Bösch, H.: MAX-DOAS measurements of air pollution on the northern tip of Taiwan, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11921, https://doi.org/10.5194/egusphere-egu24-11921, 2024.

The presence of formaldehyde (HCHO) in the atmosphere is an indicative of the existence of oxidation processes of volatile organic compounds (VOCs), and plays a big role in a number of tropospheric chemical processes, most importantly in the formation of tropospheric ozone. It can also be used as a proxy for local air quality and air pollution studies. In the framework of the FRM4DOAS project, a series of MAX-DOAS measurements has been carried out in an unpolluted environment situated at the north of the city of Torrejón de Ardoz (Madrid, Spain, 41° N), to determine the distribution of NO2 and HCHO in both urban and green areas, with special emphasis on the detection of the spatial heterogeneity of both species. The instrumentation’s operation site was situated in TOTEM, an observation station inside the premises of INTA, and on the top of a tower which height ensures that there are no immediate obstacles in the observations’ lines of sight.
This work is focused on HCHO measurements. The series of measurements spans from 2019 to 2022, with some gaps in between due to instrumental problems, and 4 different azimuth angles that enable the study of airmasses around nearby urban areas, highways and airport landing strips, and also around nearby fields and mountains. Some measurements have also been filtered out with a HCHO detection limit criterion to ensure that the studied spectra properly detect this trace gas.
The instrument involved in these measurements was a MAX-DOAS spectrometer observing in the UV region (320-415 nm), with a resolution of FHWM 0.55 nm, and equipped with a calibrated inclinometer to precisely adjust to the selected elevation angles. The instrument was configured to measure in the following elevation angles: 0°, 1°, 2°, 3°, 5°, 10°, 30°, 60° and zenith, and in 4 different azimuth angles (50°, 100°, 180° and 220° N, clockwise).
The spectral analysis for the HCHO retrieval was made by a software developed at INTA. O4 retrieval was also performed, at a different spectral range than that used for HCHO but still in the UV region, to obtain information of the optical paths that the light followed before being measured by our instrument.
A clear seasonality can be observed throughout the years, with a maximum peak around July- August, and a minimum in the months of February-March. This seasonal behaviour can be observed in all azimuthal directions, but it’s more prevalent in the 100° N azimuth direction, pointing to the city of Alcalá a few kilometers away.

How to cite: Blanco, M., Puentedura, O., Navarro-Comas, M., Gomez-Martin, L., Iglesias, J., Prados-Roman, C., and Yela, M.: Spatially resolved MAX-DOAS measurements of Formaldehyde at Torrejon de Ardoz (41 N)., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9344, https://doi.org/10.5194/egusphere-egu24-9344, 2024.

Volcanic eruptions in recent years, including the long-lasting eruption of Cumbre Vieja, La Palma (September-December 2021) and the fissure eruption at Reykjanes Peninsula, Iceland (December 2023, currently ongoing), have put a spotlight on the severe impacts volcanic gas emissions can have on human activities and the environment. Observations of such volcanic degassing were the target of the Volcanic Emissions Observation and Modeling (VOLCOM) field campaign, carried out on Sakurajima volcano, Japan, during November 2023, chosen due to the volcano’s long-lasting activity (currently ongoing since 1955) and surrounding population (>1 million residents in the surrounding 20 km).

VOLCOM provided an excellent opportunity to employ and test a well-known remote sensing methodology (Car-DOAS). To reduce the noise level in Car-DOAS measurements, we incorporated a temperature stabilization device. The temperature stabilized-Avantes spectrometer was mounted on a vehicle equipped with a UV bandpass filter. The planning of the measurements was supported by a dedicated high-resolution meteorological forecasting approach using the Weather Research and Forecasting (WRF) and FALL3D models. With the presented setup, SO₂ emission rates from the vent were monitored, and strong signals (SCD >10¹⁸ molecules cm^-2) were observed.

High concentrations of SO₂ in the plume, however, can make proper retrieval of the signal challenging as the strong absorption of SO₂ pushes the DOAS method to its limits. It was seen that shifting the fit window depending on the SO₂ signals ensures that the weak absorber assumption remains valid, which is used to create a composite product featuring multiple fit windows. Based on the composite product, the passive emissions during that period were estimated using a mass-balance approach with the forecasted wind field over the volcano. Proper retrieval of the SO₂ signal will help us establish a new dataset for the volcano and allow us to gain insight into the chemistry and environmental impacts of volcanic gas emissions.

How to cite: Bittner, S., Poulidis, A. P., Richter, A., Iguchi, M., and Vrekoussis, M.: Observations and retrieval of volcanic SO2 emissions from the Sakurajima volcano, Japan, using temperature-stabilised Car-DOAS, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9155, https://doi.org/10.5194/egusphere-egu24-9155, 2024.

Most of our knowledge about the global distribution of atmospheric iodine is based on measurements of iodine monoxide (IO) radicals in the marine boundary layer and at high latitudes. Recent evidence of IO in the free troposphere is limited to few available aircraft measurements over oceans. We report the first ground-based Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) observations of tropospheric IO radicals over the central continental United States. IO vertical columns measured at Storm Peak Laboratory, CO (40.455°N, 106.744°W, 3220 m.a.s.l.) are similar to those over oceans and vary significantly. A priori sensitivity studies indicate that IO mixing ratios increase with altitude. Back trajectory modeling indicates that IO is consistently observed in air masses quickly transported from over the Pacific Ocean. We compare the IO columns and their variability with predictions by a global model and use the observations to constrain a chemical box model to assess the atmospheric implications for ozone and mercury oxidation. Iodine-induced mercury oxidation is currently missing in atmospheric models, understudied, and helps to partially explain the elevated concentrations of oxidized mercury measured at SPL.

How to cite: Lee, C., Elgiar, T., David, L., Wilmot, K., Reza, M., Hirshorn, N., McCubbin, I., Lyman, S., Hallar, G., Gratz, L., and Volkamer, R.: Detection of iodine over the continental United States: Implications for ozone and mercury oxidation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14103, https://doi.org/10.5194/egusphere-egu24-14103, 2024.

To date, efforts to map ground-level NO₂ distribution have primarily utilized visible-light scanning grating spectrometers, such as MAX-DOAS and Pandora. Although these instruments boast high retrieval accuracy, their spatiotemporal resolution remains relatively low, limiting the detection of dynamic features and strong spatial gradients.

In recent years, a novel passive remote sensing instrument called the NO₂ camera was developed at the Royal Belgian Institute for Space Aeronomy (BIRA-IASB). This instrument aims to measure 2D distributions of NO₂ slant column densities (SCDs) with enhanced spatiotemporal resolution over both extensive areas and in point-source plumes. Central to this instrument is the acousto-optical tunable filter (AOTF), providing both sufficient spectral resolution (~0.7nm) for resolving NO₂ absorption cross-section structures and tunability across a broad spectral range.

The measurement approach involves sequential acquisitions of monochromatic images of a scene. A hypercube is constructed by tuning the AOTF to specific wavelengths within the 440-450nm range. This hypercube is a 3D array with two spatial dimensions and one spectral axis.

Conventional methods, such as DOAS, process the acquired light spectrum for each pixel's field of view to retrieve NO₂ SCD. The hypercube resulting from a measurement with this AOTF-based NO₂ camera is a 512x512 array of NO₂ SCD values, taken at approximately 50 different wavelengths collected in around 1 minute, achieving a spatial sampling of less than 1 m, from distance of 1 km. Due to this high spatial resolution of the NO₂ camera, the pixels that will be processed can be selected very carefully.

Under the QA4EO IDEAS+ framework, the NO₂ camera has been selected for further development. At EGU, we will present the results from the associated measurement campaign, performed in spring 2024 on the rooftop of the Sapienza University in Rome.

This work is partially supported by the QA4EO contract QA4EO/SER/SUB/33.

How to cite: Busschots, C., Gramme, P., Baker, N. C., Dekemper, E., Casadio, S., Iannarelli, A. M., Di Bernardino, A., and Vanhamel, J.: Urban application of the AOTF-based NO2 camera , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15754, https://doi.org/10.5194/egusphere-egu24-15754, 2024.

Accurately monitoring atmospheric temperature and water vapor profiles with high spatial and temporal resolutions is crucial for weather forecasting and climate research. Hyperspectral radiance measurements offer a promising opportunity to retrieve these profiles due to the distinct absorption features of various atmospheric compositions.

Three clear-sky field campaigns were conducted in Ottawa to collect collocated hyperspectral measurements from two sophisticated instruments: the High Spectral Resolution Airborne Microwave Sounder (HiSRAMS) and the Atmospheric Emitted Radiance Interferometer (AERI). HiSRAMS operates within the microwave oxygen band (49.6-58.3 GHz) and microwave water vapor band (175.9-184.6 GHz), while AERI operates in the infrared spectral range (520-1800 cm^-1). Both instruments possess high spectral resolution, enabling the detection of subtle changes in temperature and water vapor profiles. Radiosonde measurements were simultaneously taken during each field campaign to serve as the truth for this study.

Initially, we performed a simultaneous assessment of the radiometric accuracy of HiSRAMS and AERI through radiative closure tests. A persistent warm radiance bias was detected in AERI observations in the window band. Correcting this bias improved radiative closure within the same band. HiSRAMS observations, when directed towards the nadir, displayed a smaller brightness temperature bias compared to zenith observations. We diagnosed and compared the radiometric accuracy of both instruments based on the relationship between radiometric bias and optical depth. HiSRAMS exhibited similar radiometric accuracy to AERI in nadir-pointing measurements but demonstrated comparatively poorer accuracy in zenith-pointing measurements, necessitating further characterization.

Subsequently, clear-sky temperature and water vapor concentration profiles were successfully retrieved from collocated HiSRAMS flight measurements and AERI ground measurements. These retrieved profiles were validated against radiosonde measurements, demonstrating good agreement. When both instruments were positioned on the ground for zenith-pointing measurements, infrared hyperspectral measurements provided higher information content and better vertical resolution for temperature and water vapor retrievals compared to microwave hyperspectral measurements. Combining airborne nadir-pointing microwave measurements and ground-based zenith-pointing infrared measurements, termed the “sandwich” sounding approach, exhibited increased information content and reduced retrieval uncertainty for temperature and water vapor concentrations across all retrieval levels.

How to cite: Liu, L., Bliankinshtein, N., Huang, Y., Gyakum, J., Gabriel, P., Xu, S., and Wolde, M.: Clear-sky temperature and water vapor retrievals utilizing collocated microwave and infrared hyperspectrometers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6920, https://doi.org/10.5194/egusphere-egu24-6920, 2024.

Absorption in the deep-UV region often excites strong electronic bands. In cases where vibronic structure is evident, these strong absorptions could be exploited for sensitive and selective quantification of trace gases, as is done in long-path DOAS systems to monitor ammonia emissions. Accurate and appropriate resolution absorption cross sections are essential for this purpose, but literature cross sections of adequate quality are not always available. In this study, we report new cross section for major inorganic species (NO and SO₂), as well as important biogenic and anthropogenic volatile organic compounds (BVOC and AVOC). 
A spectrometer using a xenon arc flashlamp was used to measure absorption spectra in the wavelength range of 197 to 225 nm. Absorption cross section measurements using a flow cell were validated against SO₂ absorption cross-sections, for which there is excellent agreement in the literature. We report a new absorption cross section of nitric oxide (NO) using the flow cell. Despite being a key nitrogen oxide species in the atmosphere, there are few NO absorption cross-sections between 190 and 230 nm and little agreement among the spectra. VOCs measurements were made in a static cell and validated against the isoprene absorption cross-section. New absorption cross sections are reported for important AVOCS (benzene, toluene, ethylbenzene, xylene) and key BVOCS (α-pinene, β-pinene, limonene, 3-carene, and myrcene). The measured absorption cross-sections are compared and discussed.
We discuss the potential and challenges of using deep-UV measurements and the sensitivities that would be needed to be useful for trace gas detection in a range of contexts.

How to cite: Wang, M., C. Connolly, S., and S. Venables, D.: Measurement of deep-ultraviolet cross-sections of strongly absorbing atmospheric species and their potential for trace gas detection, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7919, https://doi.org/10.5194/egusphere-egu24-7919, 2024.

Ultra-broadband spectroscopy in the mid-infrared (MIR) wavelength range, where most molecular species have strong, distinct absorption features, has a great potential for gas sensing applications. Novel MIR supercontinuum (SC) sources excel in their ability to provide broadband light together with a high spatial coherence. Using this unique combination of properties, we have recently demonstrated the potential of MIR SC sources in combination with a tailor-made Fourier Transform Spectrometer (FTS) and a multipass absorption cell for multispecies trace gas detection [1]. Moreover, a novel application is to utilize the spatial coherence of the source to monitor outdoor (greenhouse) gas concentrations through free space. In open-path absorption spectroscopy, the light beam is guided over an (outdoor) path, instead of sampling the gas in a cell. The concentrations of the gases of interest are integrated over this path, which is useful for detecting greenhouse gases in an area or around an emission source. Wastewater treatment plants are a known source of greenhouse gas emissions of CO₂, CH₄, and N₂O. However, little is known about the fluctuation in the emission rates of these gases over time and their relation to internal and external factors.

We have developed a new, transportable instrument for open-path absorption spectroscopy, which comprises of a unique, novel SC source with an ultra-broad spectrum from 2 – 11.5 µm with ~3 W output power and a custom-built Fourier transform spectrometer [2]. The beam of the SC source is sent over an open path to a cubic retroreflector, where it is reflected back to the FTS. Using the spatial coherence of the beam, extensive, outdoor optical paths can be achieved, while the broad spectrum enables simultaneous detection of many different gas species.

We present the results of field measurements at a wastewater treatment plant where we monitored the concentration of greenhouse gases (CH₄, N₂O, and CO₂) and other trace gases (e.g., NH₃ and CO) simultaneously in the atmosphere surrounding the aerobic tank of the plant (Figure 1).  We will shed light on the perspective of this novel instrumentation for greenhouse gas monitoring around emitting sources, as it is providing reliable data for modelling studies on the dynamics of the emissions of wastewater treatment plants and other sources.

Figure 1: Satellite image of the aerobic tank of the wastewater treatment plant with the beam path over the tank (in red). Left insert: Retrieved concentrations of methane using the open-path instrument (in black) and using a validation instrument (in red, measured at point marked “x” in the photo). Right insert: Close-up of the open-path instrument.

[1] M. A. Abbas, K. E. Jahromi, M. Nematollahi, R. Krebbers, N. Liu, G. Woyessa, O. Bang, L. Huot, F. J. M. Harren, and A. Khodabakhsh, "Fourier transform spectrometer based on high-repetition-rate mid-infrared supercontinuum sources for trace gas detection," Opt. Express 29, 22315-22330 (2021).

[2] R. Krebbers, K. van Kempen, F. J. M. Harren, S. Vasilyev, I. Peterse, S. Lücker, A. Khodabakhsh, and S. M. Cristescu, “Ultra-broadband spectroscopy using a 2–11.5 µm IDFG-based supercontinuum source”, Optica Open. Preprint. DOI:10.1364/opticaopen.24967692.v1.

How to cite: Krebbers, R., van Kempen, K., Lin, Y., Khodabakhsh, A., and Cristescu, S.: Ultra-broadband mid-infrared supercontinuum-based spectroscopy for greenhouse gas monitoring at wastewater treatment plants, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8526, https://doi.org/10.5194/egusphere-egu24-8526, 2024.

The Oxygen Spectrometer for Atmospheric Science on a Balloon (OSAS-B) is a heterodyne receiver for the thermally excited ground state transition of neutral atomic oxygen at 4.75 THz [1]. It has been shown that this transition is favorable for the determination of atomic oxygen in the mesosphere and lower thermosphere (MLT) region of Earth [2]. Due to water absorption, it cannot be observed from ground. Atomic oxygen is the dominant species in the MLT, and thus plays an important role for the chemistry and energy balance in the MLT region as well as for the deceleration of low-earth orbit satellites. OSAS-B uses a combined helium/nitrogen cryostat for the detector of the instrument, a superconducting hot-electron bolometer mixer, as well as for cooling the quantum-cascade laser, which serves as the local oscillator for heterodyne detection. A turning mirror allows for measurements at different vertical inclinations and for radiometric calibration against two blackbody sources. The first flight took place as a one-day flight in September 2022 from Esrange, Sweden in the framework of the EU-funded Hemera 2020 program. During the course of the flight, several hundred spectra for different elevation angles and azimuth directions were recorded. Preliminary results show a reasonable agreement with predictions from the current MSIS model (NRL MSIS 2.0/2.1). Besides, we are able to observe the spectral signatures of shear winds in the MLT as they are predicted by the horizontal-wind model (HWM 2014).

[1] Wienold, M. et al. 48th IRMMW-THz, Montreal, Canada (2023), doi: 10.1109/IRMMW-THz57677.2023.10299165
[2] Richter, H. et al. Commun Earth Environ 2,19 (2021), doi: 10.1038/s43247-020-00084-5

How to cite: Wienold, M., Semenov, A., Hansen, P., and Hübers, H.-W.: Observations of atomic oxygen in the MLT from a stratospheric balloon with the OSAS-B terahertz heterodyne spectrometer, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11512, https://doi.org/10.5194/egusphere-egu24-11512, 2024.

Jupiter’s atmosphere is primarily composed of molecular hydrogen and helium with a mixing ratio almost identical to our Sun.

Since the atmosphere of this gaseous giant represents a high-density environment, spanning many orders of magnitude along its radius, the H₂-H₂ and H₂-He Collision Induced Absorption (CIA) bands represent its main source of opacity in the infrared part of the spectrum, particularly between 1 and 5 μm, a spectral range widely used by remote sensing instruments.

Thus, it is of primary importance to have experimental data on the CIA absorption and to compare them with the theoretical models present in the literature, to have the most accurate estimation of Jupiter’s atmospheric opacity, at least for the CIA.

Consequently, measurements of the hydrogen CIA fundamental band have been performed at three different temperatures and pressure conditions typical of the Jovian upper-tropospheric profile [1], using an H₂-He mixture with 13.8 % helium, a typical value of Jupiter’s atmosphere.

For this scope, we used an experimental setup called PASSxS, which allows us to record absorption spectra in the spectral range from 1 to 6 μm.

It comprises two stainless steel concentric vessels, as shown in Figure 1. The inner one contains the gas or mixture of gases under investigation, while the external one can be evacuated to ensure thermal insulation of the sample chamber from the external environment.

The inner vessel contains a Multi-Pass cell, characterized by an optical path of about 3.2 m, coupled with an FT-IR spectrometer. The spectral resolution achievable with the present FT-IR spans from 0.06 to 10 cm^-1. For a more detailed description of the experimental setup refer to [2].

Figure 2 shows the experimental absorption coefficients (blue curve) acquired at 402 K and 19.2 bar, compared with Abel’s theoretical model [3] (red curve). The band shows a maximum absorption around 4200 cm^-1 where the absorption coefficients reach a value of  5.8 10^-4 cm^-1.

Some discrepancies between the data and the model are evident and have to be further investigated. This work presents the first experimental study of the CIA fundamental band of H₂-H_e at this high temperature.

Since our experimental setup can reach temperatures up to 550 K, one of the main objectives will be to perform measurements at still higher temperatures, to further investigate a not-yet explored temperature range (work in progress at the time of this abstract).

Figure 2 also shows the so-called interference dips [4] around 4150 cm^-1, which represent a lack of absorption at specific wavelengths. The spectral resolution of our setup allowed, for the first time, to resolve four interference dips.

A further investigation of these features might be important for future modeling.

Acknowledgments: This work has been developed under the ASI-INAF agreement n. 2023-6-HH.0.

References:

[1] A. Seiff (1997), Science Vol 276, pp.102-104.

[2] M. Snels et al. (2021), AMT 14, 7187–7197.

[3] M. Abel et al. (2012), The Journal of Chemical Physics, 136.

[4] J. Van Kranendonk (1968), Canadian Journal of Physics Vol. 46, N. 10.

How to cite: Vitali, F., Stefani, S., Piccioni, G., Grassi, D., and Snels, M.: Experimental results on the H2-H2 and H2 -He collisional-induced absorption coefficients at typical Jupiter’s upper tropospheric conditions. , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12107, https://doi.org/10.5194/egusphere-egu24-12107, 2024.

How to cite: Jágerská, J., Seton, R., Zakoldaev, R., Salaj, J., and Vlk, M.: Trace gas detection on a photonic chip with sub-ppb detection levels and isotope discrimination , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16415, https://doi.org/10.5194/egusphere-egu24-16415, 2024.

Hydroxyl radical (OH) is a pivotal oxidant in the atmosphere, exerting significant influence on atmospheric chemistry and participating in diverse environmental processes. However, accurately measuring OH in the atmosphere is challenging due to its short half-life and low ambient concentrations. Various methods, such as laser-induced fluorescence coupled with fluorescence assay by gas expansion (LIF-FAGE), differential optical absorption spectroscopy (DOAS), and ion-chemical ionization mass spectrometry (CIMS), have been employed for OH quantification, each with its associated complexities and limitations.

This study introduces a novel measurement approach utilizing broadband cavity-enhanced absorption spectroscopy (BBCEAS) for detecting OH under ambient atmospheric conditions. The BBCEAS instrument, known for its portability and resilience to interferants owing to its spectroscopic nature, emerges as a practical solution for field measurements. The instrument's characterization involved detecting OH in an open flame, and subsequent enhancements were implemented to render it field-ready, enabling it to compete with established methods like LIF-FAGE, CIMS, and DOAS.

The application of BBCEAS in OH detection represents a valuable tool for atmospheric researchers, offering a balance between portability and sensitivity. This study highlights the potential of BBCEAS as a reliable method for field measurements of OH concentrations, contributing to a more comprehensive understanding of atmospheric processes and chemical reactions.

How to cite: Flowerday, C., Thalman, R., Asplund, M., and Hansen, J.: OH Radical Detection Using Broadband Cavity Enhanced Absorption Spectroscopy (BBCEAS) in an Open-Path Configuration, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6822, https://doi.org/10.5194/egusphere-egu24-6822, 2024.

Cavity-Enhanced Absorption Spectroscopy (CEAS) [1-3], stands out as a cavity-based approach for sensitive measurement of sample absorption, without the need of complex optical setup and fast electronic devices for the used optical cavity compared to Cavity Ring-Down Spectroscopy (CDRS). CEAS offers continuous data acquisition within rapid scanning of laser frequency. The absorption spectrum is derived from the measured intensity outputs from the cavity with and without sample inside. While in tunable diode laser spectroscopy with wavelength modulation, the baseline is constructed of a high background signal, resulted from direct modulation of the laser power, superimposed with a small gas absorption signal. Achieving precise absorption assessments demands thus overcoming challenges in baseline structure, due to the laser intensity variation during laser frequency scan. Therefore, stabilization of laser intensity to maintain the baseline constant could improve the instrument's stability as well as measurement accuracy and precision.

This work introduces the development and application of a laser Intensity-Stabilized Cavity-Enhanced Spectroscopy (IS-CEAS) operating near 1506 nm. The key innovation lies in employing an acousto-optic modulator (AOM) as an external power actuator for laser intensity stabilization. This strategy resulted in a remarkable 5-fold reduction in the laser intensity noise at lower frequency ranges where the integrated signal is detected. The fluctuations of cavity output intensity during laser scans were effectively eliminated, offering a baseline-free measurement of the absorption. This advancement notably bolstered system stability, enabling nearly 10 times longer integration than the conventional CEAS approach. The developed IS-CEAS system was employed to quantify the concentration of HO₂ radicals produced in laboratory, yielding a bandwidth-normalized (1σ) limit of detection for HO₂ at 1.6×10⁸ molecule.cm^-3.Hz^-1/2. This value is comparable to the detection limits achieved by CRDS systems operating at the same wavelength.

The experimental detail and the preliminary results will be presented and discussed.

Acknowledgments The authors thank the financial support from the French ANR Foundation : ICAR-HO₂ project (ANR-20-CE04-0003). This work has been partly supported by the EU H2020-ATMOS project, the ANR LABEX CaPPA (ANR-10-LABX-005) project and the regional CPER ECRIN program.

References

[1] M. Mazurenka, A. J. Orr-Ewing, R. Peverall, G. A. D. Ritchie. Annu. Reports Prog. Chem. - Sect. C 2005, 101, 100–142.

[2] W. Chen, A. A. Kosterev, F. K. Tittel, X. Gao, W. Zhao. Appl. Phys. B 2008, 90, 311–315.

[3] M. Ngo, T. Nguyen-Ba, D. Dewaele, F. Cazier, W. Zhao, L. Nähle, W. Chen. Sensors & Actuators: A. Physical 2023, 362, 114654

How to cite: Ngo, M. N., Nguyen-Ba, T., Ghysels-Dubois, M., Fittschen, C., Schoemaecker, C., and Chen, W.: Laser-driven stabilized cavity-enhanced absorption spectroscopy for HO2 detection near 1506 nm , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21306, https://doi.org/10.5194/egusphere-egu24-21306, 2024.