Displays

Open-path measurements of atmospheric gas species over km-scale path lengths are well suited to quantify emissions from sources like oil and gas, forest fires, and industry. is a relatively new technique that combines high-resolution and broad spectral coverage with no instrument lineshape and near perfect frequency calibration. These features have enabled open-path DCS to provide accurate measurements of multiple trace gas species simultaneously in the near-infrared across path lengths ranging from 100 m to several km. However, in order to reach the sensitivity necessary to detect many atmospheric trace constituents, including volatile organic compounds (VOCs), operation in the mid-infrared (or UV/Vis) is required.

Here, we show a mid-infrared open-path dual comb spectrometer operating in the 3-4 and 4.5-5 μm spectral regions. We have used this spectrometer to measure methane, ethane, and propane (arising primarily from oil and gas activity) across a 1-km-long path in Boulder, CO for 1 week with an ethane sensitivity of ∼0.1 ppb for a 2-minute time resolution. In addition, we show quantitative measurements of intentionally released acetone and isopropanol with a 1-σ sensitivity of 5.7 ppm·m and 2.4 ppm·m, respectively. In the 4.5-5 μm region, we have used this system to detect N₂O, CO, and O₃. Finally, we have developed a second-generation instrument in the 3-4 μm region that is more compact and has improved stability. This system was recently deployed in a van at an active oil and gas drilling operation. We present preliminary measurements of methane, ethane, and higher hydrocarbons from this deployment as well as initial efforts at emissions quantification.

How to cite: Cossel, K., Waxman, E., Giorgetta, F., Baumann, E., Friedlein, J., Herman, D., Ycas, G., Coddington, I., and Newbury, N.: Measurement of VOCs using open-path mid-infrared dual comb spectroscopy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12396, https://doi.org/10.5194/egusphere-egu2020-12396, 2020.

In contrast to land-based sources of air pollution, which have been regulated and reduced since several decades, NO_x and SO_x emissions from ships were only recently identified as significant sources of air pollution. As one consequence the sulphur content of ship fuel used within the so-called Sulphur Emission Control Areas (SECA) was recently regulated to a maximum of 0.1% (m/m) (MARPOL Annex VI). Therefore, especially monitoring the emission of sulphur compounds is of particular interest.

Within a 6-week measurement campaign in July and August of 2016, ship emissions were measured at the river Elbe in Germany, near Hamburg using the Long Path (LP)-DOAS technique. The measurements were carried out within the framework of the project MeSMarT (MEasurements of Shipping emissions in the MARine Troposphere), which investigates the influence of ship emissions on chemical processes in the atmosphere. Currently, monitoring of ship emission plumes is typically achieved by a combination of in situ trace gas monitors and meteorological sensors. In contrast to that the LP-DOAS technique is capable of simultaneously measuring signatures of multiple trace gases along an absorption path across a well-frequented waterway close to the ship exhaust-pipes and thus directly in the emission plume at a time resolution of a few seconds.

For our study, a LP-DOAS instrument was set up side by side to an in situ MeSMarT measurement station at the river Elbe at Wedel (15 km downriver of Hamburg harbour) where NO₂ and SO₂ emission signatures of a total of 5037 ship passes (of 1044 individual ships) were monitored. While the in situ method detected 16% of the ships, the LP-DOAS was able to assign emission plumes to 41% of all passing ships. With meteorology mainly limiting the in situ detection yield, the major limitation for the LP-DOAS was found to be due to the high traffic density and thus the difficulty to unambiguously assign recorded plumes to particular vessels, rather than to the sensitivity to the emission plume itself.

Based on the results of this feasibility study, we present a newly designed LP-DOAS system fulfilling the requirements for operational ship emission monitoring (robust mechanical setup, broad-band long-lifetime light source, compact sealed housing, automized alignment and data acquisition). This new system is now operated continuously to measure the ship emissions on the river Elbe.

How to cite: Schmitt, S., Pöhler, D., Weigelt, A., Wittrock, F., Seyler, A., Krause, K., Kattner, L., Mathieu-Üffing, B., Lampel, J., and Platt, U.: Towards operational monitoring of ship emissions using Long Path Differential Optical Absorption Spectroscopy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13775, https://doi.org/10.5194/egusphere-egu2020-13775, 2020.

The NO₃ radical is the main atmospheric oxidant at night. The night period is favorable to the formation and accumulation of NO₃ radicals in the atmosphere. On the one hand, it is formed by the reaction of nitrogen dioxide with ozone while, on the other hand, NO₃ being highly photosensitive, it cannot accumulate significantly during the day (S. S. Brown and J. Stutz, Chem. Soc. Rev. 2012). In addition, the reaction between NO and NO₃ is very fast and so, urban environment is considered so far, being not favorable to the occurrence of NO₃ radicals. However, atmospheric nitrogen chemistry near the earth surface is strongly linked to the dynamics of the boundary layer and in summer NO is rapidly depleted by ozone. A large variability of the mixing ratios for NO₃ as a function of height above the ground is thus expected with non-negligible concentrations in altitude (Brown et al., Atmos. Chem. Phys., 2007). The contribution of NO₃ radical to the atmospheric evolution of VOCs in urban and sub-urban areas may therefore also be influenced by this vertical distribution.

To demonstrate the potential importance of NO₃ radical even in urban environment, a field campaign was carried out at night during July 2018 inside Paris. A newly developed field instrument dedicated to the measurement of NO₃ radical was deployed on a high payload touristic tethered balloon located in Paris 15^th district that was used as vertical vector. The NO₃ instrument is a compact, robust and easily deployable on field instrument based on the IBB-CEAS (Incoherent Broad band Cavity Enhanced Absorption Spectroscopy) technique. NO₃ measurements were completed by ground and airborne measurements of NO (chemiluminescence analyzer), NO₂ (CAPS cavity) and O₃ (absorption analyzer) concentrations as well as particle number concentrations (OPC GrimmTM) and 355 nm lidar (Leosphere ALS300) measurement for mixing layer probing.

Vertical profiles from 0 to up to 300 m were obtained at night characterized by high concentrations of ozone and moderate humidity. In this presentation, vertical profiles of the species measured and implications for VOC oxidation in urban environment will be discussed.

How to cite: Cirtog, M., Michoud, V., Fouqueau, A., Cazaunau, M., Bergé, A., Maisonneuve, F., Zapf, P., Pangui, E., Landsheere, X., Giacomoni, J., Gobbi, M., Hanottel, L., Paris, A., Roulier, N., Formenti, P., Mellouki, A., Cantrell, C., Doussin, J.-F., and Picquet-Varrault, B.: Night-time vertical profiles of nitrate radical concentrations in urban environment (Paris, France), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15843, https://doi.org/10.5194/egusphere-egu2020-15843, 2020.

Free tropospheric ammonia plays critical roles in aerosol nucleation and ammonium nitrate formation with significant impacts on the Earth’s radiative forcing and tropospheric photochemistry. Remote sensing measurements on aircraft and satellite report large values (> 1 ppbv) in the upper troposphere in the outflow of deep convection over source regions. Accurate, in-situ “point” ammonia measurements from aircraft in the free troposphere are non-existent because of surface adsorption effects on existing instrument surfaces and inlets. Such higher spatiotemporal resolution measurements are needed to better deduce the processes that impact the transport of ammonia into the free troposphere from biomass burning and deep convection and its subsequent transformation into particulate ammoniated aerosols. To this end, we are developing an open-path, airborne-based ammonia instrument for the NASA DC-8 aircraft in order to measure ammonia without sampling biases throughout the troposphere. Development of such an instrument requires characteristics of fast response (10 Hz) and low detection limits (10 pptv), requiring instrument attributes of high-stability and high sensitivity. Complicating matters, these measurement attributes have to occur under a wide range of temperatures (210-310 K), pressures (150-1013 hPa), absolute humidities (ppmv to %), and environmental sampling challenges (high airspeed, vibrations, aerodynamic stresses) over the flight envelope (e.g. for vertical profiles for satellite validation). To avoid thermal management issues with the laser under the extreme temperatures experienced by the sensor, a 9.06 micron, distributed feedback quantum cascade laser (c-mount) is mounted inside a custom housing and located inside the aircraft cabin. The laser light is coupled into a 200 micron hollow core fiber for single mode operation and passed through the fuselage of the aircraft to a Herriott cell mounted 35 cm above the fuselage. The fiber and laser housing are continuously purged with dry nitrogen filtered by an ammonia scrubber to avoid interstitial absorption of ammonia in the optical path internal to the Herriott cell. Light emanating from the back facet of the laser is passed through a reference cell of ethylene and ammonia at 50 hPa to ensure appropriate linelocking and laser tuning characterization. The Herriott cell (61 m) consists of polished, aluminum mirrors held together by invar rods to minimize thermal effects on the mirror spacing (55 cm).  The mirrors are heated slightly above ambient by 50 W heaters to avoid water and ice condensation. Allan deviation experiments of the instrument show a precision of 80 pptv (1 Hz) and 13 pptv (60 s), and drift of the calibration is much less than these values up to 3000 s. Wavelength modulation spectra are fit to reference conditions over the range of the flight envelope with accuracies of fit to better than 10%. Field tests of the instrument will be shown, particularly at cold temperatures representative of the upper troposphere. The instrument was test fit onto the NASA DC-8 in summer 2019 and test flights are planned for 2020. The design attributes needed for such measurements – particularly in an aircraft platform - and laboratory and field data supporting the instrument performance will be demonstrated.

How to cite: Zondlo, M., Yi, H., Tao, L., Pan, D., McSpiritt, J., and Guo, X.: Airborne ammonia measurements with a fiber-coupled quantum cascade laser, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11798, https://doi.org/10.5194/egusphere-egu2020-11798, 2020.

Atmospheric gas monitoring is of major importance for climate change and air quality. Indeed, emissions regulations and control rely on the detection and quantification of the concentration of gases such as CO₂, CH₄, NO₂, O₃, etc. Good control on emissions is key to reduce those gases impacts on climate change and people’s health.

The accuracy and relevance of such measurements depend on higher spatial, spectral and temporal resolutions. To this end, conventional dispersive hyperspectral imaging systems are typically used. However, these sensors are submitted to compromises in terms of price, spectral and spatial resolutions and temporal acquisition frequency. To overcome these compromises, a new ground-breaking device is currently developed under the name Imaging Spectrometer on Chip (ImSPOC). It is based on an interferometric imaging system that allows real time acquisition with significant spatial and spectral resolutions. The device, which takes the volume of a matches’ box, could in the future be a building block for Nano-satellites, drone, or ground based measurements platforms. The particularity of this device is the snapshot acquisition of an interferometer by pixel of the imaged scene instead of a spectrum. This is obtained by using a matrix of Fabry-Perot interferometers with different thicknesses placed in front of a photodetector. ImSPOC is then of great interest for real-time acquisitions. However, the acquisition of interferometers requires signal processing developments to reconstruct the corresponding spectra. This reconstruction relies typically on the resolution of an inverse problem. Some models of the device have been proposed to this end.

To validate the efficiency of this new device and to test the developed algorithms, acquisitions were conducted tracking the sun during a whole day using our device and a conventional diffraction grating based spectrometer. In this way, the reconstructed spectra from our device can be compared to the classical spectrometer ones. Particularly, the absorption peaks are compared (their central wavelengths, amplitude, etc.). To go further, the gas characterization from both devices will be compared (gas detection, evolution over time of the vertical concentration profiles, etc.). These results allow the validation of our device to this application and highlight the signal processing improvements that could be done in the future to have more accurate measurements.

How to cite: Dolet, A., Picone, D., Gousset, S., Dalla Mura, M., Le Coarer, E., and Voisin, D.: A new snapshot interferometric imaging spectrometer: a first comparison with a classical grating spectrometer., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8353, https://doi.org/10.5194/egusphere-egu2020-8353, 2020.

The NDMC (Network for the Detection of Mesopause Change) is a global network of ground based observatories with the objective of monitoring key parameters of the mesopause region. For temperature monitoring GRound-based Infrared P-branch Spectrometers (GRIPS) are widely deployed. These spectrometers allow for the retrieval of the mesopause temperature from the OH* P-band emission lines around 1530 nm. A common technology for GRIPS instruments are spectrometers based on diffraction gratings. To overcome the limitations of conventional grating spectrometers, a new type of spectrometer is being developed within the project Metrology for Earth Observation and Climate - 3 (MetEOC-3) which is coordinated by the European Metrology Project for Innovation and Research (EMPIR). The new spectrometer shall improve the quality and traceability of the atmospheric data obtained by the NDMC. It is intended to serve as a reference instrument with significantly smaller measurement uncertainties. It is also designed to identify temperature trends of 1K/decade. A Spatial Heterodyne Interferometer (SHI) was chosen as the most promising technology, offering several advantages. Compared to conventional grating spectrometers, the throughput and resolution of the interferometer is one order of magnitude larger. The use of a two-dimensional detector array in combination with an imaging optics enables the detection of spatial temperature distributions in the mesopause region, as caused by dynamical processes like gravity waves. The talk gives an introduction to the technology of spatial heterodyne interferometry, and the new instrument design and calibration results are presented.

How to cite: Stehr, J., Knieling, P., Olschewski, F., Kaufmann, M., Mantel, K., and Koppmann, R.: GRIPS-HI - a Novel Interferometer for Measuring Two-Dimensional Temperature Distributions at the Mesopause, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19075, https://doi.org/10.5194/egusphere-egu2020-19075, 2020.

We present an exhaustive study of the gamma activity increase measured at ground level for the atmospheric radon daughter ²¹⁴Pb. We demonstrate the effectiveness of proximal gamma-ray spectroscopy in continuously gathering reliable measurements of rain-induced ²¹⁴Pb gamma signal related to the rain intensity and amount. Since every impulse of rain produces a sudden increase of gamma signal, we study such transient activity to obtain information on precipitations and rain formation.

A novel spectroscopic instrument specifically tailored for gathering reliable and unbiased estimates of atmospheric and terrestrial gamma emitters has been developed. After seven months of continuous acquisition, we analyze the temporal evolution of the ²¹⁴Pb net count rate with an innovative and reproducible mathematical model for extracting information on this radon daughter’s content in the rain water. The effectiveness of the model is proved by an excellent coefficient of determination (r² = 0.91) between measured and reconstructed ²¹⁴Pb count rates. We observe that the impulsive increase of ²¹⁴Pb count rates ΔC is clearly related to the rain rate R by the power law dependence ΔC = A·R^{0.50 ± 0.03}, where the parameter A is equipment dependent. This means that the expected increase of atmospheric ²¹⁴Pb activity measured at ground level during a rain event is proportional to the square root of the rain rate √R.

We observe that the ²¹⁴Pb abundance (G) of the rain water is inversely related to the rain rate G ∝ 1/R^{0.48 ± 0.03} and to the rain median volume diameter λ_m with G ∝ 1/ λ_m^2.2.  We proved that, for a fixed rainfall amount, the longer is the rain duration (i.e. the lower is the rainfall intensity and the smaller is the mean raindrop volume), the higher is the ²¹⁴Pb content of the rain water.

Since the developed algorithm is detector independent, it can be used for analysing the data collected by the networks of thousands of gamma sensors distributed around the Earth, typically utilised for monitoring the air radioactivity in case of a nuclear fallout. From this spectroscopic technique we shall learn a lot more about the rain formation and scavenging mechanisms which are responsible for the attachment of ²¹⁴Pb to rain droplets in-cloud. Finally, our research provides a comprehensive characterization of the background radiation assessments relevant for radioprotection, earthquake predictions, cosmic rays research and anthropic radiation monitoring.

How to cite: Strati, V., Albéri, M., Bottardi, C., Chiarelli, E., Montuschi, M., Raptis, K. G. C., Serafini, A., and Mantovani, F.: Monitoring rain rate with proximal gamma-ray spectroscopy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15888, https://doi.org/10.5194/egusphere-egu2020-15888, 2020.

The HITRAN database is an integral component of numerous atmospheric radiative transfer models and it is therefore essential that the database contains the most appropriate up-to-date spectroscopic parameters. To this end, the HITRAN2020 database is scheduled to be released at the end of this year.  The compilation of this edition (as is the tradition for the HITRAN database) exemplifies the efficiency and necessity of worldwide scientific collaborations. It is a titanic effort of experimentalists, theoreticians and atmospheric scientists, who measure, calculate and validate the HITRAN data.

The HITRAN line-by-line lists for almost all 49 molecules have been updated in comparison to HITRAN2016 (Gordon et al., 2017), the previous compilation. The extent of these updates depend on the molecule, but range from small adjustments for a few lines of an individual molecule to complete replacements of line lists and the introduction of new isotopologues. Many new vibrational bands have been added to the database, thereby extending the spectral coverage and completeness of the datasets. In addition the accuracy of the parameters for major atmospheric absorbers has been substantially increased, often featuring sub-percent uncertainties.

Furthermore, the amount of parameters has also been significantly increased. For example, HITRAN2020 will now incorporate non-Voigt line profiles for many gases, broadening by water vapour (Tan et al., 2019), as well as updated collision induced absorption sets (Karman et al., 2019). The HITRAN2020 edition will continue taking advantage of the new structure and interface available at www.hitran.org (Hill et al., 2016) and the HITRAN Application Programming Interface (Kochanov et al., 2016).

This talk will provide a summary of these updates, emphasizing details of some of the most important or drastic improvements.

References:

Gordon, I.E., .et al., (2017), JQSRT 203, 3–69.  (doi:10.1016/j.jqsrt.2017.06.038)

Hill, C., et al., (2016), JQSRT 177, 4–14.  (doi:10.1016/j.jqsrt.2015.12.012)

Karman, T., et al. (2019), Icarus 328, 160–175.  (doi:10.1016/j.icarus.2019.02.034)

Kochanov, R.V., et al.,( 2016), JQSRT 177, 15–30.  (doi:10.1016/j.jqsrt.2016.03.005)

Tan, Y., et al., (2019), J. Geophys. Res. Atmos. 124, 11580-11594. (doi:10.1029/2019JD030929)

How to cite: Hargreaves, R., Gordon, I., Rothman, L., Hashemi, R., Karlovets, E., Skinner, F., Conway, E., Tan, Y., Hill, C., and Kochanov, R.: HITRAN2020: An overview of what to expect, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20533, https://doi.org/10.5194/egusphere-egu2020-20533, 2020.

An ultraviolet (UV) spectroradiometer is refurbished and upgraded with a fore-optical module. In addition to measuring total UV irradiance, the UV spectroradiometer can measure solar direct beam and sky radiance at any preset azimuth and elevation angle. This double Czerny-Turner spectroradiometer, with an ion-etched holographic grating operating in the first order with 3600 lines per mm, enables wavelength scanning range from 290 nm to 410 nm, with a nominal bandwidth of 0.1 nm.  It can operate with a step-size of 0.0005 nm and a full width at half maximum of 0.1 nm. It has an out-of-band rejection ratio of approximately 10^–10. This high resolution spectroradiometer can be used as a reference instrument for UV radiation measurements and to monitoring atmospheric gases (O3, SO2, NO2). Recently laboratory work suggests that water vapor displays structured absorption features over 290-350 nm region with maximum and minimum cross-sections of 8.4×10-25 and 1.4×10-25 cm2/molecule. To investigate the water vapor absorption features in UV region in real atmosphere, we did a series of field observations by using this high resolution spectroradiometer. A residual analysis method is developed to analyze the absorption of atmospheric components and to retrieve the atmospheric optical depth. The residual spectra of multiple cases have spectral features similar to that of in-lab measured water vapor absorption in some wavelength regions, and the inferred ozone amount from residual analysis agrees with OMI retrievals. Through multiple case studies, the magnitude of residual optical depth from observed UV spectra is sensitive to the atmospheric water vapor amount. The greater the water vapor path, the larger the difference between observational spectra and calculated spectra without considering water vapor absorption.

How to cite: Min, Q., Yin, B., Berdnt, J., Lee, H., and Zhu, L.: A high resolution ultraviolet spectroradiometer and its application in solar radiation measurement, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3775, https://doi.org/10.5194/egusphere-egu2020-3775, 2020.

Measurements of volatile organic compounds (VOCs) are important in a wide variety of scientific disciplines, including air quality, biogeochemistry, hydrology, plant and animal physiology, human health, and petrochemistry. Generally, these measurements are performed with expensive laboratory-based mass spectrometers, slow gas chromatographs, non-speciated flame- or photo-ionization detectors, or insensitive Fourier transform infrared spectrometers. Laser-based spectrometers based upon cavity enhanced techniques like cavity ring-down spectroscopy (CRDS) would in principle provide significant advantages over these methods in sensitivity, speed of response, simplicity, stability, and portability, as has been demonstrated in the last decade for the quantification of simple molecules like carbon dioxide, methane, or ammonia.  However, the majority of these instruments are based upon narrowly tunable lasers.  These lasers cannot tune across the broad spectral features, characteristic of VOCs.  In this paper we present a novel CRDS instrument based upon a broadband laser source that can in principle span more than hundred nanometers.  We show early measurements of ethylene oxide and BTEX (benzene, toluene, ethylbenzene, and xylene) at parts per billion and even parts per trillion levels.  Though our initial work has focused on measurements of hazardous air pollutants, there is future potential for speciated detection of other VOCs as well as greenhouse gases. These systems can be deployed on a bench-top and/or vehicle, and with per-second measurement intervals, are ideal for correlation with weather data for accurate source attribution. The analyzers are being developed to meet indoor and outdoor air quality requirements and may be deployed near sources (stack or fenceline monitoring) or far from sources (community monitoring). 

How to cite: Lucic, G., Rella, C., Hoffnagle, J., Skog, K., and McHale, L.: Novel, real-time measurements of VOCs using a Cavity Ring-Down Spectrometer (CRDS), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20831, https://doi.org/10.5194/egusphere-egu2020-20831, 2020.

We have developed a thermal dissociation cavity enhanced absorption spectroscopy (TD-CEAS) for in situ measurement of NO₂, total peroxy nitrates (PNs) and total alkyl nitrates (ANs) in the atmosphere. The instrument uses one optical cavity for measuring NO₂ absorption at 435 - 455 nm. Three channels with heating modules are set before the detecting cell in parallel, for measuring ANs, PNs and NO₂ by stabilizing the temperature of 653 K, 453 K and normal, respectively. Three-channel cycle measurement is realized by dynamic switching design with the cycle time of 3 min, by assuming the air mass change is negligible when measuring on adjacent channels in each cycle. Therefore, the instrument is feasible in relative stable air masses, such as the chamber studies or field campaigns away from the emission source regions. The limit of detection (LOD) is estimated to be 97 pptv (1σ) at 6 s intervals for NO₂. The measurement uncertainty of NO₂ is estimated to be 8%, which mainly originates from effective cavity length, mirror reflectivity, and NO₂ absorption cross section. The uncertainty of PNs and ANs measurement should be enlarged due to the sampling loss and the thermal dissociation efficiency. This instrument had been successfully applied in a field observation in China. Up to 3 ppbv PNs and ANs were observed during ozone pollution episodes. The inter-comparison of NO₂ with that measured by PL-CLD, as well as PNs with that measured by GC-ECD will be presented. 

How to cite: Li, C., Lu, K., Wang, H., Chen, X., Zhai, T., Chen, S., Li, X., and Zeng, L.: Thermal dissociation cavity enhanced absorption spectrometer for detecting ANs and PNs in the atmosphere, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12284, https://doi.org/10.5194/egusphere-egu2020-12284, 2020.

Formaldehyde (HCHO) is the most abundant atmospheric carbonyl compound and plays an important role in the troposphere. However, HCHO detection via traditional incoherent broadband cavity enhanced absorption spectroscopy (IBBCEAS) is limited by short optical path lengths and weak light intensity. Thus, a new light-emitting diode (LED)-based IBBCEAS was developed herein to measure HCHO in ambient air. Two LEDs (325 and 340 nm) coupled by a Y-type fiber bundle were used as an IBBCEAS light source, which provided both high light intensity and a wide spectral fitting range. The reflectivity of the two cavity mirrors used herein was 0.99965 (1 – reflectivity = 350 ppm loss) at 350 nm, which corresponded with an effective optical path length of 2.15 km within a 0.84 m cavity. At an integration time of 30 s, the measurement precision (1σ) for HCHO was 380 parts per trillion volume (pptv) and the corresponding uncertainty was 8.3%. The instrument was successfully deployed for the first time in a field campaign and delivered results that correlated well with those of a commercial wet-chemical instrument based on Hantzsch fluorimetry (R² = 0.769). The combined light source based on Y-type fiber bundle overcomes the difficulty of measuring ambient HCHO via IBBCEAS in near-ultraviolet range, which may extend IBBCEAS technology to measure other atmospheric trace gases with high precision.

How to cite: Liu, J., Li, X., Yang, Y., Wang, H., Kuang, C., Zhu, Y., Chen, M., Hu, J., Zeng, L., and Zhang, Y.: Sensitive Detection of Ambient Formaldehyde by Incoherent Broadband Cavity Enhanced Absorption Spectroscopy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6561, https://doi.org/10.5194/egusphere-egu2020-6561, 2020.

Hydroxyl (OH) radicals play a vital role in the degradation of trace gases and pollutants in the troposphere and in controlling the atmospheric oxidation capacity. Due to its short lifetime and low concentration, interference-free high sensitivity in situ OH monitoring by laser spectroscopy represents a challenge. In this presentation, we will report the development of Faraday rotation spectroscopy (FRS) instruments operating at 2.8 µm for quantitative measurement of OH concentrations in an atmospheric simulation chamber and the total atmospheric OH reactivity (k’_OH). The Q (1.5) double lines (²Π_3/2 (ν=1<-0)) at 3568 cm^-1 were selected for the detection. Different detection methods have been studied. The FRS technology relies on the particular magneto-optic effect observed for paramagnetic species (including most radicals and some compounds with unpaired electrons), which can significantly reduce excess laser noise and makes it capable of enhancing the detection sensitivity and mitigation of spectral interferences from diamagnetic species in the atmosphere. With the use of a multipass enhanced FRS, a detection limit of 3.2 × 10⁶ OH/cm³ (2σ, 4s) was achieved with an absorption path length of 108 m. We demonstrated that FRS method provides a unique method for atmospheric chemistry research.

How to cite: Zhao, W., Fang, B., Wei, N., Yang, N., Zhang, W., and Chen, W.: Quantitative measurement of OH radical using Faraday rotation spectroscopy at 2.8 µm, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3857, https://doi.org/10.5194/egusphere-egu2020-3857, 2020.

The contribution of light absorption by brown carbon aerosols to the Earth’s energy balance still poses a significant uncertainty in our understanding of climate forcing. As a result, one of the main open questions regarding organic aerosols in atmospheric chemistry is related to the formation and degradation of light-absorbing compounds during aging processes. Towards this goal, we explore the use of photophoresis for high sensitivity measurements of light absorption by a single levitated particle in an experimental setup that facilitates realistic atmospheric gas concentrations and aging time.

Photophoresis occurs when the surface of an illuminated, light-absorbing particle is unevenly heated relative to its surroundings. The temperature difference between the illuminated and the ‘dark’ side of the particle results in an uneven momentum transfer from colliding gas-phase molecules. This leads to a net photophoretic force, acting on the particle in the direction of the momentum transfer gradient. The photophoretic force is related to the complex refractive index of the particle and to its size parameter through the distribution of internal electric fields. As such, it may lead to a net force away from (positive) or towards (negative) the light source.

Using this phenomenon, we were able to retrieve the imaginary part of the complex refractive index (k) of a single particle levitated in an electrodynamic balance (EDB) at 473 nm wavelength. Extremely low values of k from 10^-4 to 10^-5 were successfully retrieved with uncertainty of less than 35% during a photo-bleaching experiment of a slightly absorbing organic particle used as a model for atmospheric brown carbon.    

An advantage of the EDB is that heterogeneous chemistry and photochemistry experiments are performed on a single particle that is levitated for days and weeks allowing for realistic atmospheric gas concentrations and aging times. In such experiments, measurements of the particle’s light absorption properties using photophoresis would add valuable information on the evolution of light absorption by the aging of organic aerosols.

A future study will implement this approach in an EDB-MS system where the EDB will be coupled with a soft ionization mass spectrometer. This will allow the identification of the molecular species responsible for light absorption as it evolves.  

How to cite: Bluvshtein, N. and Krieger, U.: Photophoresis used for measurements of light absorption by a single particle , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10574, https://doi.org/10.5194/egusphere-egu2020-10574, 2020.

We describe the continued development of a new laser heterodyne radiometry (LHR) technique:  Precision Heterodyne Oxygen-Calibration Spectrometry, or PHOCS. The prototype instrument is equipped with two active laser channels for oxygen and water (measured near 1.28 µm) and carbon dioxide (near 1.57 µm) determination. The latter may be substituted by a heterodyne receiver module equipped with a laser to monitor atmospheric methane near 1.65 µm). Oxygen measurements provide dry gas corrections and – more importantly – determine accurate temperature and pressure profiles that, in turn, improve the precision of the CO₂ and H₂O column retrievals. Vertical profiling is enabled by interrogating the very low-noise, absorption lines shapes collected by the O(10^-3 cm^-1) instrument. The presentation will describe (1) the continued development of column concertation retrieval protocols and (2) the results of initial tests performed at the Smithsonian Environmental Research Center in Edgewater, Maryland during the summer/fall of 2019 and spring of 2020.

How to cite: Miller, J. H., Flores, M., and Bomse, D.: Precision Heterodyne Oxygen-Calibration Spectrometry: Vertical Profiling of Water and Carbon Dioxide in the Troposphere and Lower Stratosphere, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18961, https://doi.org/10.5194/egusphere-egu2020-18961, 2020.

A significant portion of the production and consumption of trace gases (e.g. CO₂, CH₄, N₂O, NH₃, etc.) occurs in areas without sufficient infrastructure or easily available grid power to run traditional closed-path flux stations. Open-path analyzer design allows such measurements with power consumption 10-150 times below present closed-path technologies, helping to considerably expand the global coverage and improve the estimates of gas emissions and budgets, informing the remote sensing and modeling communities and policy decisions, all the way to IPCC reports. Broad-band NDIR devices have been used for open-path CO₂ and H₂O measurements since the late 1970s, but since recently, a growing number of new narrow-band laser-based instruments are being rapidly developed.

The new design comes with its own challenges, specifically: (i) mirror contamination, and (ii) uncontrolled air temperature, pressure and humidity, affecting both the gas density and the laser spectroscopy of the measurements. While the contamination can be addressed via automated cleaning, and density effects can be addressed via the Webb-Pearman-Leuning approach, the spectroscopic effects of the in-situ temperature, pressure and humidity fluctuations on laser-measured densities remain a standing methodological question.

Here we propose a concept accounting for such effects in the same manner as Webb et al. (1980) proposed to account for respective density effects. Derivations are provided for a general case of flux of any gas, examined using a specific example of CH₄ fluxes from a commercially available analyzer, and then tested using "zero-flux" experiment.

The proposed approach helps reduce errors in open-path, enclosed, and temperature- or pressure-uncontrolled closed-path laser-based flux measurements due to the spectroscopic effects from few percent to multiple folds, leading to methodological advancement and geographical expansion of the use of such systems providing reliable and consistent results for process-level studies, remote sensing and Earth modeling applications, and GHG policy decisions.

How to cite: Burba, G., Anderson, T., and Komissarov, A.: Importance of Spectroscopic Effects in Laser-based Flux Measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1254, https://doi.org/10.5194/egusphere-egu2020-1254, 2020.

Measurements of vertical concentration profiles of greenhouse gases (GHGs) is extremely important for our understanding of regional air quality and global climate change trends. In this context, laser heterodyne radiometer (LHR) technique has been developed ^[1-5] for ground-based remote measurements of GHGs in the atmospheric column.

Solar radiation undergoing absorption by multi-species in the atmosphere is coupled into a LHR instrument where the sunlight is mixed with a local oscillator (LO), being usually a tunable laser source, in a fast photodetector. Beating note at radio frequency (RF) resulted from this photomixing contains absorption information of the LO-targeted molecules. Scanning the LO frequency across the target molecular absorption lines allows one to extract the corresponding absorption features from the total absorption of the solar radiation by all molecules in the atmospheric column. Near-IR (~1.5 µm) and mid-IR (~8 µm) ^[6] LHRs have been recently developed in the present work. Field campaigns have been performed on the roof of the platform of IRENE in Dunkerque (51.05°N/2.34°E).

The developed LHR instruments as well as the preliminary results of their applications to the measurements of CH₄, N₂O, CO₂ (including ¹³CO₂/¹²CO₂), H₂O vapor (and its isotopologue HDO) in the atmospheric column will be presented and discussed.

Acknowledgments The authors thank the financial supports from the LABEX CaPPA project (ANR-10-LABX005), the MABCaM (ANR-16-CE04-0009) and the MULTIPAS (ANR-16-CE04-0012) contracts, as well as the CPER CLIMIBIO program. S. F. thanks the program Labex CaPPA and the "Pôle Métropolitain de la Côte d’Opale" (PMCO) for the PhD fellowship support.

References

[1] R. T. Menzies, and R. K. Seals, Science 197 (1977) 1275-1277

[2] D. Weidmann, T. Tsai, N. A. Macleod, and G. Wysocki, Opt. Lett. 36 (2011) 1951-1953

[3] E. L. Wilson, M. L. McLinden, and J. H. Miller, Appl. Phys. B 114 (2014) 385-393

[4] A. Rodin, A. Klimchuk, A. Nadezhdinskiy, D. Churbanov, and M. Spiridonov, Opt. Express 22 (2014) 13825-13834

[5] J. Wang, G. Wang, T. Tan, G. Zhu, C. Sun, Z. CAO, W. Chen, and X. Gao, Opt. Express 27 (2019) 9600-9619

[6] F. Shen, P. Jeseck, Y. Te, T. Tan, X. Gao, E. Fertein, and W. Chen, Geophys. Res. Abstracts, 20 (2018) EGU2018-79

How to cite: Wang, J., Shen, F., Tan, T., Cao, Z., Gao, X., Jeseck, P., Te, Y., and Chen, W.: Laser heterodyne radiometers (LHR) for in situ ground-based remote sensing of greenhouse gases in the atmospheric column, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22368, https://doi.org/10.5194/egusphere-egu2020-22368, 2020.

Tibet Plateau is known as the third pole of the world, the environmental changing in this area profoundly impacts on east Asian or even global climate. HDO is the stable isotope of water vapor and is the ideal tracer of water cycle, which has been applied to atmospheric circulation and climatic studies. For monitoring the water vapor isotopic abundance in Tibetan Plateau and providing reliable information for environmental and climatic studies, a portable laser heterodyne radiometer was operated at Golmud (Qinghai Province) in summer 2019. The radiometer adopted a narrow linewidth 3.66 μm DFB laser as the local oscillator and performed high resolution(~0.009 cm^-1) and high signal-to-noise ratio(~160). Furthermore, the absorption spectra of atmospheric HDO and H₂O were obtained and the retrieval algorithm of water vapor isotopic abundance was discussed. The optimal estimation method based on LBLRTM was chosen for retrieving, the ratio of HDO/H₂O at Golmud is 185±7×10^-6 during the observation, the value is less than the Vienna Standard Mean Ocean Water (VSMO, 311.5×10^-6) but larger than Standard Light Antarctic Precipitation (SLAP, 178.2×10^-6).

How to cite: Lu, X., Huang, J., Cao, Z., Huang, Y., Liu, D., Tan, T., and Rao, R.: Water vapor isotopic abundance measurement in Tibetan Plateau based on portable laser heterodyne radiometer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6294, https://doi.org/10.5194/egusphere-egu2020-6294, 2020.

The use of high reflectivity dielectric mirrors to form a high finesse optical cavity allows one to achieve long optical path lengths of up to several kilometres for high-sensitivity spectroscopy applications [1]. However, the high reflectivity of a dielectric mirror is achieved via constructive interference of the Fresnel reflection at the interfaces produced by multilayer coatings of alternate high and low refractive index materials. This wavelength-dependent coating limits the bandwidth of the mirror's high reflectivity to only a few percent of the designed central wavelength [2].

In this paper, we report on the development of a novel optical cavity based on prism used as cavity reflector through total internal reflection combined with Brewster angle incidence [3], which offers a high-finesse optical cavity operating in a broadband wavelength region from 400 to longer than 1600 nm. Cavity Enhanced Absorption Spectroscopy (CEAS) of NO₂, NO₃, and H₂O vapor was applied to determine the achieved prism reflectivity over a broad spectral range from 400 nm to 1600 nm.

Experimental details and preliminary results will be presented. The developed prism-based cavity is specifically adapted for the needs of broadband measurement of multi-molecular absorber or/and wavelength-dependent extinction coefficient of aerosols over a broad spectral region.

Acknowledgments. This work is supported by the French national research agency (ANR) under the CaPPA (ANR-10-LABX-005), the MABCaM (ANR-16-CE04-0009) and the MULTIPAS (ANR-16-CE04-0012) contracts. The authors thank the financial support from the CPER CLIMIBIO program.

REFERENCES

[1] S. S. Brown, "Absorption spectroscopy in high-finesse cavities for atmospheric studies", Chem. Rev. 103 (2003) 5219-5238.

[2] G.R. Fowles, Introduction to Modern Optics, 2nd ed. (Rinehart and Winston, 1975), p. 328.

[3] B. Lee, K. Lehmann, J. Taylor and A. Yalin, "A high-finesse broadband optical cavity using calcium fluoride prism retroreflectors", Opt. Express 22 (2014) 11583-11591.

How to cite: Chen, W., Wang, G., Meng, L., Gou, Q., Yalin, A., Nguyen Ba, T., Coeur, C., and Tomas, A.: Prism-based Broadband Optical Cavity (400 – 1600 nm) for High-Sensitivity Trace Gas Sensing by Cavity Enhanced Absorption Spectroscopy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4213, https://doi.org/10.5194/egusphere-egu2020-4213, 2020.

Water vapor is the dominant greenhouse gas, and its abundance in the upper tropospheric/lower stratospheric region (UTLS, 8-25 km altitude) is of great importance to the Earth's radiative balance. Reliable predictions of the climate evolution as well as the understanding of cloud-microphysical processes require the accurate and frequent measurement of water vapor concentrations at these altitudes. The only established method for high-accuracy UTLS water vapor measurements aboard of meteorological balloons is cryogenic frost-point hygrometry (CFH). However, the cooling agent required for its operation (CHF₃) is to be phased out due to its strong global warming potential. It is, therefore, a major, worldwide challenge to ensure the continuation of the observation of this key Environmental Climate Variable (ECV) of the World Meteorological Organization (WMO). As an alternative method, we present a compact and lightweight instrument based on quantum cascade laser absorption spectroscopy (QCLAS) that reduces systematic errors by contactless and contamination-minimized measurements. Its construction addresses the stringent constraints posed by the harsh environmental conditions found in the UTLS. This is achieved by a fundamental reconsideration of main components of the spectrometer. We developed a highly versatile segmented circular multipass cell (SC-MPC) which supports compact and well-controlled beam folding [1]. The SC-MPC consists of a monolithic aluminum ring with 10.8 cm inner radius, containing 57 quadratic, spherically curved segments, seamlessly shaped into the internal ring surface. The collimated mid-IR beam (λ = 6 µm) from the distributed feedback quantum cascade laser (DFB-QCL) is directly coupled to the MPC without the need for additional beam-shaping optics. This leads to a resilient optical setup suitable for mobile applications and rough environmental conditions. Water vapor amount fractions of <10 ppmv can be measured with a precision better than 1% at 1 Hz. Measuring in open-path mode ensures quick response and minimal interference by water desorbing from surfaces. The instrument weighs less than 4 kg (including battery) and has an average power consumption of 15 W. An elaborate thermal management system that comprises phase change materials and thermoelectric cooling ensures excellent internal temperature stability despite an outside temperature difference of up to 80 K. Specifically developed hard- and software guarantee autonomous operation for the duration of flight [2]. Extensive stability assessments in climate chambers as well as validation experiments using dynamically generated, SI-traceable water vapor mixtures were performed in collaboration with the Swiss Federal Institute of Metrology (METAS). In cooperation with the German Weather Service (DWD) in Lindenberg, the instrument was successfully tested and compared to CFH in two consecutive balloon-ascents in December 2019 up to 28 km altitude, experiencing temperatures and pressures as low as –65°C and 16 hPa, respectively. The drastic reduction in mass and size of a laser absorption-spectrometer and its successful deployment under harshest conditions represents a paradigm change in portable laser spectroscopy and opens the door to previously inaccessible applications.

[1] Graf, M.; Emmenegger, L.; Tuzson, B. Opt. Lett. 2018, 43, 2434-2437

[2] Liu, C. et al., L. Rev. Sci. Instrum. 2018, 89 (6), 065107 (9 pp.)

How to cite: Graf, M., Scheidegger, P., Looser, H., Kupferschmid, A., Peter, T., Emmenegger, L., and Tuzson, B.: Mid-IR Laser Spectrometer for Balloon-borne Lower Stratospheric Water Vapor Measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10923, https://doi.org/10.5194/egusphere-egu2020-10923, 2020.

We described an open-path cavity enhanced absorption spectroscopy (OP-CEAS) technique for ambient measurement of nitrate radical (NO₃) near 662 nm. Compared with the close type CEAS system with a sampling line, the OP-CEAS is featured with high accuracy due to free of quantifying NO₃ loss in the sampling line and cavity. Based on a 0.84 m long open path cavity, the effective absorption length of ~5 kilometers is achieved by a coupled high reflectivity mirrors with the reflectivity of 0.99985 at 662 nm. The detection limit of OP-CEAS for NO₃ measurement is 3.0 pptv (2σ) in 30 seconds. The uncertainty is 11.2% and dominated by the cross section of NO₃. The instrument was successfully applied in a field measurement at low particulate matter (PM) loading condition. As the sensitive would be decreased due to the strong PM extinctions under heavy PM pollution condition, we highlight the feasibility of this OP-CEAS configuration in the field application under the low PM condition, such as the forest region affected by anthropogenic emissions. This technique also appropriates to be expended in the field detection of other reactive trace gases in future studies.

How to cite: Wang, H. and Lu, K.: Monitoring Ambient Nitrate Radical by Open Path Cavity Enhanced Absorption Spectroscopy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18725, https://doi.org/10.5194/egusphere-egu2020-18725, 2020.

The hydroxyl radical (OH) is considered as a primary agent responsible to remove a majority of trace gas in the atmosphere [1]. It is also responsible to initiate the reactions leading to the formation of a wide range of secondary species such as ozone (O₃) and secondary organic aerosols (SOAs) [2]. Reliable and real-time assessment of the OH radical concentration change and related chemical process in the atmosphere is a key factor to understand and determinate the oxidation capacity of the atmosphere. Because of its very high reactivity, very short lifetime (≤ 1 s) associated with very low atmospheric concentration (~10⁶ OH/cm³), the development of optical instrument allowing accurate, interference-free and ultra-high sensitivity in-situ direct measurement of OH concentration presents a great challenge for atmospheric science and climate change research.We report in this paper our recent development of an OH sensor based on Faraday Rotation Spectroscopy (FRS) [3]. FRS exploits magnetic circular birefringence (MCB) observed in the vicinity of Zeeman split absorption line of paramagnetic species such as O₂, NO, NO₂, OH. The Q(1,5e) and Q(1,5f) double lines of OH at 3568,52 cm^-1 and 3568,41 cm^-1 were chosen for quantification of OH radicals [4,5]. In order to enhance the detection sensitivity, multi-pass absorption approach was coupled to FRS. A 1σ (SNR=1) detection limit of about 5×10⁷ OH/cm³ was achieved.

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

Acknowledgments

The authors thank the financial supports from the CPER CLIMIBIO program and the Labex CaPPA project (ANR-10-LABX005).

 References

[1] D.E. Heard, M.J. Pilling, Chem. Rev. 103 (2003) 5163-5198.

[2] D. Stone, L.K. Whalley, and D.E. Heard, Chem. Soc. Rev. 41 (2012) 6348-6404.

[3] G. Litfin, C.R. Pollock, R.F. Curl, F.K. Tittel, J. Chem. Phys. 72 (1980) 6602-6605.

[4] W. Zhao, G. Wysocki, W. Chen, et al., Opt. Express 19, (2011) 2493-2501.

[5] W. Zhao, G. Wysocki, W. Chen, W. Zhang, Appl. Phys. B 109 (2012) 511-519.

How to cite: Nguyen Ba, T., Zhao, W., Chen, J., Liu, K., Gao, X., Fertein, E., and Chen, W.: High-sensitivity measurement of OH radicals using multi-pass enhanced Faraday Rotation Spectroscopy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22371, https://doi.org/10.5194/egusphere-egu2020-22371, 2020.

Atmospheric Brown Carbon (BrC) is an important component of aerosol particles that Influences the climate through interactions with incoming solar and emitted terrestrial radiation. BrC can be generated from a variety of primary emissions (such as traffic, coal combustion, biomass burning) and secondary formation. Nitrophenols are classified as Brown Carbon due to their strong absorption in near-ultraviolet and visible regions.

A heated single path absorption spectroscopy system is been built to measure the cross section of nitrophenols. Due to its semi-volatility, the nitrophenols were introduced into the cell by N₂. The cross section of nitrophenols is obtained by calculating the integrated absorption.

A Thermal Decomposition - Incoherent Broadband Cavity-Enhanced Absorption Spectroscopy (TD-IBBCEAS) system was setup for atmospheric measurement. This instrument covered the spectral region from 320 to 440nm which could contain the interested absorption of nitrophenols. A thermal decomposition device was used to heating the sample.  The system was characterized based in laboratory experiment.

How to cite: Wang, M., Chen, J., Lou, S., and Venables, D.: The Measurement of Nitrophenols by Integrated Spectrum and TD-IBBCEAS, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6306, https://doi.org/10.5194/egusphere-egu2020-6306, 2020.

Light-absorbing carbonaceous aerosols mainly generated from the combustion of biomass and fossil fuels, play an important role in the global environment ^[1]. Multi-wavelength in-situ measurement of carbonaceous aerosol optical absorption is important both for reduce errors in assessing radiative forcing and component identification or source appointment of aerosols (such as biomass burning and diesel soot) with absorption Ångström exponent (AAE) ^[2]. A differential photoacoustic spectrometer (PAS) using a 438 nm laser diode was developed for simultaneously measure the aerosol optical absorption coefficient and the concentration of NO₂. In order to evaluate the reliability of the differential photoacoustic spectrometer, we compared the NO₂ concentration measured by PAS with the data from environmental monitoring station and showed good consistency. In the actual atmospheric measurement process, we observed a good correlation between the light absorption characteristics of aerosols and the concentration of NO₂ within a certain time range. In addition, a novel multi-wavelength photoacoustic spectrometer (MW-PAS) was developed to measure the aerosol optical absorption coefficients and its wavelength-dependent characteristics in the UV-VIS-NIR bands (405, 638, 808 nm). The performance of MW-PAS was evaluated by measuring the light absorption characteristics of kerosene soot aerosol. The measurement results are agreed with the results reported in literatures ^[3].

Reference

[1] J.G. Radney, R. You, M.R. Zachariah, C.D. Zangmeister, Direct in-situ mass specific absorption spectra of biomass burning particles generated from smoldering hard and softwoods. Environ. Sci. Technol. 51, 5622-5629 (2017)

[2] T. Ajtai, N. Utry, M. Pintér, B. Major, G. Szabó, A method for segregating the optical absorption properties and the mass concentration of winter time urban aerosol. Atmos. Environ.122, 313-320 (2015)

[3] M. Gyawali, W.P. Arnott, R.A. Zaveri, C. Song, H. Moosmüller, L. Liu, M.I. Mishchenko, L.-W.A. Chen, M.C. Green, J.G. Watson, and J.C. Chow, Photoacoustic optical properties at UV, VIS, and near IR wavelengths for laboratory generated and winter time ambient urban aerosols. Atmos. Chem. Phys. 12, 2587-2601 (2012)

How to cite: Cao, Y., Liu, K., Chen, W., and Gao, X.: Aerosol optical absorption and spectral dependence measurement with photoacoustic spectroscopy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1861, https://doi.org/10.5194/egusphere-egu2020-1861, 2020.

The scattering and absorption of light by atmospheric aerosols are constrained poorly in climate models. In particular, there is large uncertainty in aerosol light absorption arising from a lack of accurate measurements for absorbing aerosols. Photoacoustic spectroscopy (PAS) is the technique of choice for contact-free light absorption measurements by aerosol particles. In PAS instruments, the light intensity of a laser source is modulated periodically at typical frequencies in the range 1 – 2 kHz and the light absorbing species of interest absorbs energy from this modulated light. The absorbed energy is subsequently transferred to translational degrees-of-freedom of the surrounding bath gas through collisional relaxation and generates an acoustic pressure wave that is detected by a sensitive microphone. The recorded amplitude of the microphone response is related directly to the sample absorption coefficient, while the phase shift of the microphone response with respect to the laser power modulation provides information on the timescale for energy transfer to the bath gas.

Recent years have seen PAS instruments deployed in the field on aircraft measurement platforms. These airborne studies facilitate spatially-resolved measurements of aerosol light absorption, including with variation in altitude. The accuracy of the resulting aerosol absorption measurements depends chiefly on the calibration of the PAS microphone response. Moreover, this calibration for microphone response varies with pressure, with an increased sample pressure dampening the microphone membrane motion to a greater extent. This pressure-dependent microphone sensitivity is particularly pertinent to measurements from aircraft platforms that sample at varying pressures typically over the range 400 – 1000 mbar. Largely, field instruments have used ozone-laden gas to calibrate PAS instruments operating at visible wavelengths, and repeated this calibration for several values of absolute pressure.

In this contribution, we report photoacoustic amplitude and phase shift measurements which demonstrate ozone-laden gas is a poor calibrant of PAS instruments operating at visible wavelengths and at pressures reduced from those at ambient conditions (~1000 mbar). The nascent photodissociation products following photoexcitation of O₃ do not liberate their energy to the surrounding bath gas on a fast timescale compared to the photoacoustic modulation frequency regardless of the bath gas composition. Instead, we show that the PAS instrument can be calibrated at ambient pressure and then a miniature speaker can be used to excite an acoustic response for calibrating the pressure sensitivity in the microphone response. In this way, we show that we accurately measure aerosol absorption at reduced pressure for sub-micrometre diameter aerosols consisting of dyed polystyrene latex spheres or nigrosin dye. These results will be of utmost interest to those measuring aerosol absorption using PAS from airborne platforms or those calibrating PAS instruments for ground based or laboratory measurements.

How to cite: Cotterell, M., Szpek, K., Tiddeman, D., Haywood, J., and Langridge, J.: Calibration of Photoacoustic Spectrometers at Reduced Pressure Using Aerosols or Ozone-Laden Gas, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2932, https://doi.org/10.5194/egusphere-egu2020-2932, 2020.

Atmospheric aerosols influence radiative forcing through interaction with solar radiation and indirectly by acting as cloud condensation nuclei and have a negative impact on air quality especially in urban scenarios. With socio-economic models suggesting that in a growing global population, 70% of the humans will live in urban areas by 2050, the adverse impact on urban air quality is a prominent societal and health issue, expected to become more and more severe in the future. In order to introduce effective mitigation strategies and monitor their effect, the state and characteristics of pollution need to be characterized and main sources identified. Offline-analysis of particulate matter (PM) collected on filter samples offers such insight. However, PM chemical composition is highly complex, and its comprehensive characterization and quantification requires advanced instrumentation and data analysis techniques and strategies.

Here, we present the development and application of a novel analytical nondestructive method. We acquired Fourier-transform infrared spectroscopy (FTIR) spectra of ambient PM collected on Teflon filters at various locations in Italy. FTIR allows to obtain high-resolution spectral data non-destructively and therefore to detect and quantify functional groups of organic and inorganic species present in the aerosol PM. The spectral dataset was analyzed by applying partial least squares regression (PLS regression) methods in order to allow quantification of ammonium, sulphate and nitrate ionic PM components. This statistical method allowed to disentangle the inner complexity of the PM sample and to train a statistical model for each of the three ionic species. In our conference contribution, the so developed models are discussed and compared with the more traditional analytical method, ionic chromatography (IC).

References:

Cuccia, et al. (2011). Atmospheric Environment, 45(35), 6481–6487. https://doi.org/10.1016/j.atmosenv.2011.08.004

Piazzalunga, A., et al. (2013). Analytical and Bioanalytical Chemistry, 405(2–3), 1123–1132. https://doi.org/10.1007/s00216-012-6433-5

Russell, L. M., et al. (2009). Atmospheric Environment, 43(38), 6100–6105. https://doi.org/10.1016/j.atmosenv.2009.09.036

How to cite: Molteni, U., Piazzalunga, A., and Fermo, P.: Non-destructive method based on infrared spectroscopy and partial least square regression for the quantification of the ionic component of atmospheric particulate matter, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16412, https://doi.org/10.5194/egusphere-egu2020-16412, 2020.

Accounting for the wavelength- and source-dependent optical absorption properties of the abundant light-absorbing organic (brown) carbon (BrC) and the mixing state of atmospheric black carbon (BC) are essential to reduce the large uncertainty in aerosol radiative forcing. Estimation of BrC absorption online by subtraction is highly uncertain and may be biased if not decoupled from the potential BC absorption enhancement (lensing) due to non-refractory (organic and inorganic) coating acquisition.

Here, the reported total particulate absorption is based on long-term, filter-based seven-wavelength Aethalometer (AE33 model) data, corrected for multiple scattering effects with Multi-Wavelength Absorbance Analyzer (5λ MWAA) measurements. Using ultraviolet-visible spectroscopy absorbance measurements along with particle size distributions obtained by a scanning mobility particle sizer, we have conducted Mie calculations to assess the importance of source-specific extractable particulate BrC (Moschos et al., 2018) versus BC absorption.

For the species-specific optical closure, the wavelength dependence of bare BC absorption is estimated using MWAA measurements upon successive filter extractions to remove the influence of BrC/coatings. The lensing contribution, supported by observations from field-emission scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, is estimated at longer wavelengths using a refined proxy for the BC coating thickness. The approach is validated independently by applying a novel positive matrix factorization-based approach on the calibrated total AE33 absorption data.

Based on the observational constraints established in this study, we demonstrate for various distinct case studies that the interplay between lensing and BrC absorption results in lower than expected BC absorption at shorter wavelengths. This indicates that the volume additivity assumption is not valid for particulate absorption by internally mixed heterogeneous atmospheric aerosol populations. These comprehensive experimental analyses verify the BC lensing suppression predicted for simplified core-shell structures containing moderately absorbing BrC (Lack & Cappa, 2010). The implications discussed in this work are relevant for co-emitted species from biomass burning or aged plumes with high BrC to BC mass/absorption ratio.

References

Moschos, V., Kumar, N. K., Daellenbach, K. R., Baltensperger, U., Prévôt, A. S. H., and El Haddad, I.: Source apportionment of brown carbon absorption by coupling ultraviolet-visible spectroscopy with aerosol mass spectrometry, Environ. Sci. Tech. Lett., 5, 302-308, https://doi.org/10.1021/acs.estlett.8b00118, 2018.

Lack, D. A. and Cappa, C. D.: Impact of brown and clear carbon on light absorption enhancement, single scatter albedo and absorption wavelength dependence of black carbon, Atmos. Chem. Phys., 10, 4207–4220, https://doi.org/10.5194/acp-10-4207-2010, 2010.

How to cite: Moschos, V., Gysel-Beer, M., Modini, R. L., Corbin, J. C., Massabò, D., Costa, C., Danelli, S. G., Vlachou, A., Daellenbach, K. R., Prati, P., Prévôt, A. S. H., Baltensperger, U., and El Haddad, I.: Experimental evidence of the lensing effect suppression for atmospheric black carbon containing brown coatings, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7278, https://doi.org/10.5194/egusphere-egu2020-7278, 2020.

Climate models which predict the Earth’s temperature and which are a basis for estimating climate change have large uncertainties induced by the lack of understanding of aerosol effects. Polarimetry is the most promising technique to gain information about aerosols and to understand their effect on the climate.

The airborne multispectral sunphotometer and polarimeter (AMSSP) can measure complete polarization information, including circular polarization for the visible spectral range. The transformation of intensity measurements measured by the AMSSP to polarization information is only possible with a sufficient calibration of the instrument. Laboratory calibration measurements resulted in calibration parameters that convert the intensity measurements to accurate polarization information with only small deviations.

On the basis of recent experiences during a previous field campaign, an improved ground-based polarimeter is going to be developed. A Pan-Tilt tracking system allows direct measurements of the sun which enables the optimization of the relative adjustments of the optical paths. The ground-based system allows a flexible measurement geometry within the upper hemisphere without additional mirrors. With this, the impact and uncertainty of the previously used mirror sytsem are eliminated. In addition, it is planned to optimize the optical components of the polarimeter.

The new setup will be tested during the measurement campaign ASKOS in Cape Verde in summer 2020.

How to cite: Jänicke, L. and Ruhtz, T.: Advancement of the AMSSP for Ground-Based Measurements of the Complete Stokes Vector , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4955, https://doi.org/10.5194/egusphere-egu2020-4955, 2020.

Anusanth Anantharajah^a, Fridolin Kwabia Tchana^a, Jean-Marie Flaud^a , Pascale Roy^b and Laurent Manceron^b,c

• a- Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), UMR CNRS 7583,
  Université de Paris et Université Paris-Est Créteil, Institut Pierre Simon Laplace,
  61 Avenue du Général de Gaulle, 94010 Créteil Cedex, France.
• b- Synchrotron SOLEIL, AILES Beamline, L’Orme des Merisiers, Saint-Aubin F-91192, France.
• c-  Sorbonne Université, CNRS, MONARIS, UMR 8233, 4 place Jussieu, F-75005 Paris, France. 

Nitryl chloride (ClNO₂) and Chlorine Nitrate are molecules of great interest for atmospheric chemistry since these are produced by heterogeneous reactions, in the marine troposphere, between NaCl sea-salt aerosols or ClO and gaseous N₂O₅ [1,2], and on polar stratospheric clouds, between N₂O₅ and solid HCl [3,4].

Many high-resolution spectroscopic studies in the microwave and mid-infrared regions are available. However, these molecules present low-lying vibrational levels and thus numerous hot bands in the regions of the NOx stretching and bending mode absorptions in the 8-12 µm atmospheric transparency window which could serve for remote sensing and quantification of these species.

Fourier Transform Spectrometry is a useful technique to observe broad band high resolution spectra (0.001 cm^-1) of these molecules and a significant advantage is gained by combining interferometry with the high brightness of a synchrotron source [5]. At SOLEIL we have developed specific instrumentation to study such reactive molecules and a few results concerning chlorine-containing compounds will be presented.

1. B. J. Finlayson-Pitts, M. J. Ezell, and J. N. Pitts Jr, Nature 337, 241-244 (1989).
2. W. Behnke, V. Scheer, and C. Zetzsch, J. Aerosol Sci. 24, 115-116 (1993).
3. . M. A. Tolbert, M. J. Rossi, and D. M. Golden, Science 240, 1018-1021 (1988).
4. M. T. Leu, Geophys. Res. Lett. 15, 851-854 (1988).
5.  J-M. Flaud, A. Anantharajah, F. Kwabia Tchana, L. Manceron, J. Orphal, G. Wagner, and M. Birk, J Quant Spectrosc Radiat Transf 224, 217-221 (2019).

How to cite: Manceron, L.: Synchrotron radiation and long path cryogenic cells: New tools and results for modelling chlorinated compounds absorption in the 8-12µm atmospheric window, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8639, https://doi.org/10.5194/egusphere-egu2020-8639, 2020.