Atmospheric Absorption Research Articles

A Rapid and Simplified Approach to Correct Atmospheric Absorptions in Infrared Spectra.

Infrared (IR) spectroscopy is a powerful analytical technique used to identify and quantify different components within a sample. However, spectral interference from fluctuating concentrations of water vapor and CO2 in the measurement chamber can significantly impede the extraction of quantitative information. These temporal fluctuations cause absorption variations that interfere with the sample's spectrum, making accurate analysis challenging. While several techniques to overcome this problem exist in the literature, many are time-consuming or ineffective. We present a simple method utilizing just two sample spectra taken sequentially. The difference of these spectra, multiplied by a scaling factor, determined by minimization of the point-to-point spectral length, provides a correction spectrum. Subtracting this from the spectrum to be corrected results in a fully corrected spectrum. We demonstrate the effectiveness of this method via the improved ability to determine analyte concentration from corrected spectra over uncorrected spectra using a partial least square regression (PLSR) model. This technique therefore offers rapid, effective, and automated spectral correction, which is ideal for a nonexpert user in a clinical or industrial setting.

A Three-Dimensional Time-Varying Channel Model for THz UAV-Based Dual-Mobility Channels

Unmanned aerial vehicle (UAV) as an aerial base station or relay device is a promising technology to rapidly provide wireless connectivity to ground device. Given UAV’s agility and mobility, ground user’s mobility, a key question is how to analyze and value the performance of UAV-based wireless channel in the terahertz (THz) band. In this paper, a three-dimensional (3D) time-varying channel model is proposed for UAV-based dual-mobility wireless channels based on geometric channel model theory in THz band. In this proposed channel model, the small-scale fading (e.g., scattering fading and reflection fading) on rough surfaces of communication environment and the atmospheric molecule absorption attenuations are considered in THz band. Moreover, the statistical properties of the proposed channel model, including path loss, time autocorrelation function (T-ACF) and Doppler power spectrum density (DPSD), have been derived and the impact of several important UAV-related and vehicle-related parameters have been investigated and compared to millimeter wave (mm-wave) band. Furthermore, the correctness of the proposed channel model has been verified via simulation, and some useful observations are provided for the system design of THz UAV-based dual-mobility wireless communication systems.

Compact Metasurface Terahertz Spectrometer

AbstractThe terahertz frequency region of the electromagnetic spectrum is crucial for understanding the formation and evolution of galaxies and stars throughout the universe's history, as well as the process of planet formation. Detecting the unique spectral signatures of molecules and atoms requires terahertz spectrometers, which must be operated in space observatories due to water vapor absorption in the Earth's atmosphere. However, current terahertz spectrometers face challenges such as low resolution, limited bandwidth, large volume, and complexity. In this paper, the issues of size and weight are addressed by demonstrating a concept for a centimeter‐sized, low‐weight terahertz spectrometer using a metasurface. The design of the metasurface spectrometer is first discussed for the 1.85 to 2.4 THz range, followed by its fabrication. Next, an array of quantum cascade lasers operating at slightly different frequencies around 2.1 THz is utilized to characterize the spectrometer. Finally, a spectrum inversion method is applied to analyze the measured data, confirming a resolution R (λ/Δλ) of at least 273. This concept can be extended to other application areas, such as planetary observations and various wavelengths in the far‐infrared (FIR) and near‐infrared (NIR) ranges.

Ozone Detector Based on Ultraviolet Observations on the Martian Surface

Ozone plays a key role in both atmospheric chemistry and UV absorption in planetary atmospheres. On Mars, upper-tropospheric ozone has been widely characterized by space-based instruments. However, surface ozone remains poorly characterized, hindered by the limited sensitivity of orbiters to the lowest scale height of the atmosphere and challenges in delivering payloads to the surface of Mars, which have prevented, to date, the measurement of ozone from the surface of Mars. Systematic measurements from the Martian surface could advance our knowledge of the atmospheric chemistry and habitability potential of this planet. NASA’s Mars 2020 mission includes the first ozone detector deployed on the Martian surface, which is based on discrete photometric observations in the ultraviolet band, a simple technology that could obtain the first insights into total ozone abundance in preparation for more sophisticated measurement techniques. This paper describes the Mars 2020 ozone detector and its retrieval algorithm, including its performance under different sources of uncertainty and the potential application of the retrieval algorithm on other missions, such as NASA’s Mars Science Laboratory. Pre-landing simulations using the UVISMART radiative transfer model suggest that the retrieval is robust and that it can deal with common issues affecting surface operations in Martian missions, although the expected low ozone abundance and instrument uncertainties could challenge its characterization in tropical latitudes of the planet. Other space missions will potentially include sensors of similar technology.

A deep convolutional neural network for blind element error correction of spatial heterodyne spectrometer using line selective convolutional blocks

The "GF Special Project" is a massive remote sensing technology initiative including a number of satellites and various observation platforms. GF-5 is the satellite with the most payloads, the highest spectral resolution, and the most difficulty in development, and it can monitor a variety of environmental elements using spatial heterodyne spectroscopy (SHS) technology, including atmospheric aerosols, carbon dioxide, methane, terrestrial vegetation, straw burning, and urban heat islands. In this study, a novel blind element error correction technique based on deep learning network is investigated and developed for spatial heterodyne interferograms, as well as the formation mechanism and distribution characteristics of the SHS interferometric data. LSConv-Net, a new CNN model, was created and trained to denoise in the presence of high-density and ultra-high-density blind element errors. We do this by introducing a new line-selective convolutional (LSConv) block. Simultaneously, experimental validation of blind element error correction utilizing laboratory water vapor interferometric data and atmospheric CO2 absorption interferometric data from GF-5, and the change in FWHM before and after the experiment was tested using potassium lamp interferograms. Experiments show that the Deep neural networks trained with this model may successfully suppress the effect of blind element noise on spectra, recover spectra that have been overwhelmed by high-density blind element noise without any effect on other non-blind pixels, and surpass all similar techniques in terms of spectral recovery.

Noise Study Auralization of an Open-Rotor Engine

Based on the performance and acoustic data files of reduced-size open-rotor engines in low-speed wind tunnels, the static sound pressure level was derived by converting the 1-foot lossless spectral density into sound-pressure-level data, the background noise was removed, and the results were corrected according to the environmental parameters of the low-speed wind tunnels. In accordance with the requirements of Annex 16 of the Convention on International Civil Aviation Organization and Part 36 of the Civil Aviation Regulations of China on noise measurement procedures, the takeoff trajectory was physically modeled; the static noise source was mapped onto the takeoff trajectory to simulate the propagation process of the noise during takeoff; and the 24 one-third-octave center frequencies that corresponded to the SPL data were corrected for geometrical dispersion, atmospheric absorption, and Doppler effects, so that the takeoff noise could be corrected to represent a real environment. In addition, the audible processing of noise data with a 110° source pointing angle was achieved, which can be useful for enabling practical observers to analyze the noise characteristics.

Indiscernible noise determination of a military system equipped with an internal combustion engine

The main objective of this paper is to explore the non-detectability criteria of military systems and to determine the indiscernible noise of military systems equipped with internal combustion engines. Non-detectability is one of the critical requirements evaluated for military defense systems and it is defined strictly in MIL-STD-1474E standard. Furthermore, this standard specifies the limits of sound pressure levels conservatively for 1/3 octave bands. Though, the standard does not define indiscernible or inaudible noise levels for events in the presence of background noise. It is also not explicitly well-defined in the existing literature. This study presents a comparison of non-detectability distance defined in MIL-STD-1474E standard and indiscernible noise of a military system extracted from the measured octave band sound levels, which are processed based on sound propagation theory. In this study, several parameters such as atmospheric absorption, geometric spreading, ground effect, terrain type, obstacle existence are included to determine the indiscernible noise level of the military system of interest. The results are compared with the non-detectability distance limits given in the MIL-STD-1474E standard, and it is observed that the proposed method of indiscernible noise, which is defined as extracted sound level should be lower than the background noise, is well-correlated.

In-flight pixel degradation of the Sentinel 5 Precursor TROPOMI-SWIR HgCdTe detector

Abstract The TROPOMI-SWIR HgCdTe detector on the Sentinel-5 Precursor mission has been performing in-orbit measurements of molecular absorption in Earth's atmosphere since its launch in October 2017. In its polar orbit the detector is continuously exposed to potentially harmful energetic particles. Calibration measurements taken during the eclipse are used to inspect the performance of this detector. This paper explores the in-orbit degradation of the HgCdTe detector.
After five years, the detector is still performing within specifications, even though pixels are continuously hit by cosmic radiation. The bulk of the impacts have no lasting effects, and most of the damaged pixels (95%) appear to recover on the order of a few days to several months, attributed to a slow spontaneous recovery of defects in the HgCdTe detector material. This is observed at the operational temperature of 140 K. The distribution of the observed recovery times has a mean around nine days with a significant tail towards several months. Pixels that have degraded have a significant probability to degrade again. The location of faulty pixels follows a Poissonian distribution across the detector. No new clusters have appeared, revealing that impacts are dominated by relatively low energetic protons and electrons. Due to the observed spontaneous recovery of pixels, the fraction of pixels meeting all quality requirements in the nominal operations phase has always been over 98.7%. 
The observed performance of the TROPOMI-SWIR detector in-flight impacts selection criteria of HgCdTe detectors for future space instrumentation.

Exploring sauna impulse responses

Sauna is an important element of Finnish tradition and culture, and its properties and features deserve preservation and archiving. Additionally, extreme temperature and humidity in a sauna comprise a unique environment for researching room acoustics in unusual atmospheric conditions. In the present study, we publish a dataset of room impulse response (RIR) measurements of a sauna during the process of warming up and throwing water on the stove. We explore several features of the dataset, such as the impact of the temperature and humidity distributions on the time-of-arrival of reflections, the atmospheric absorption of sound, and spectral coloration of measured RIRs. The results of this research show the influence of environmental factors on RIRs and allow an insight into the distinct room acoustics of a sauna while heating up.

Numerical modeling of infrasounds radiation and propagation in hurricanes

Infrasound can be useful for monitoring natural or human-induced energetic events in the atmosphere. For high-energy sources, the infrasonic signals can propagate over large distances around the globe, given the low rate of atmospheric absorption. This study focuses on the numerical simulation of infrasound propagation in realistic large-scale three-dimensional (3D) environments. More precisely, we are interested in the simulation of infrasound, called microbaroms, which are radiated from extreme weather conditions such as hurricanes. The simulation of hurricane-induced microbaroms relies on three main components: i) a hurricane model describing the atmospheric wind and pressure fields, ii) a microbarom source model, and iii) an infrasound propagation model. 3D simulations have been performed on a rectangular mesh. Azimuthal deviation caused by the hurricane and the stratified atmosphere can be seen in 3D. Further studies are needed to improve each model, and the correlation between them. The ultimate goal is to be able to simulate radiated infrasound propagation for any meteorological conditions.

Atmospheric CO2 absorption and counteraction by CH4 emission across contrasting habitats in a large eutrophic lake

Lakes are a crucial component of global carbon cycling and play a key role in the global CO2 and CH4 budgets. Intensified anthropogenic activity contributes to eutrophication, and reduces macrophytes while increasing phytoplankton in shallow lakes. However, the information on how the CO2 and CH4 fluxes at water–air interface temporally varies across habitats dominated by various primary producers in lakes and how CH4 emission counteracts CO2 adsorption is limited. In this study, in situ fluxes of CO2 and CH4 were measured monthly for one year at four sites in Lake Taihu, China. These sites had contrasting habitats dominated by high-density phytoplankton, low-density phytoplankton, floating-leaved macrophytes, and submerged macrophytes. We found that the CO2 fluxes ranged from − 439 ± 385 to − 106 ± 348μmol m−2h−1 and the CH4 fluxes ranged from 1.96 ± 3.79 to 10.7 ± 14.8μmol m−2h−1. The fluxes of CO2 and CH4 were mainly regulated by water pH, phosphorus levels, and sediment organic carbon. All four habitats acted as sinks of atmospheric CO2 and sources of atmospheric CH4, and the habitats with high-density phytoplankton showed the highest CO2 absorption and strongest CH4 emission. Generally, the CH4 emission counteracted 7–69 % of the atmospheric CO2 absorption in the four sites. These results collectively revealed that in a large, shallow eutrophic lake, habitats dominated by various primary producers consistently adsorb atmospheric CO2, even being counteracted by CH4 emission. These findings provide valuable insights for understanding carbon cycling and CO2 and CH4 dynamics across the water–air interface in shallow lakes.

Addressing atmospheric absorption in adaptive rectangular decomposition.

This paper focuses on the adaptive rectangular decomposition (ARD) scheme, a wave-based method employed for acoustic simulations. ARD holds promise for diverse applications. In architectural design, it can forecast acoustical parameters, facilitating the creation of spaces with superior sound quality. Moreover, in the domain of acoustic virtual reality, ARD can offer users a more immersive and lifelike acoustic environment. This enhancement proves advantageous for all of these applications, enabling the simulation of larger environments with heightened precision. Despite its notable efficiency, ARD faces a significant drawback: the absence of atmospheric absorption modeling. The principal aim of this study is to rectify this limitation, thereby augmenting the capabilities of the ARD algorithm.

JWST/NIRISS Reveals the Water-rich “Steam World” Atmosphere of GJ 9827 d

With sizable volatile envelopes but smaller radii than the solar system ice giants, sub-Neptunes have been revealed as one of the most common types of planet in the galaxy. While the spectroscopic characterization of larger sub-Neptunes (2.5–4 R ⊕) has revealed hydrogen-dominated atmospheres, smaller sub-Neptunes (1.6–2.5 R ⊕) could either host thin, rapidly evaporating, hydrogen-rich atmospheres or be stable, metal-rich “water worlds” with high mean molecular weight atmospheres and a fundamentally different formation and evolutionary history. Here, we present the 0.6–2.8 μm JWST/NIRISS/SOSS transmission spectrum of GJ 9827 d, the smallest (1.98 R ⊕) warm (T eq,A=0.3 ∼ 620 K) sub-Neptune where atmospheric absorbers have been detected to date. Our two transit observations with NIRISS/SOSS, combined with the existing HST/WFC3 spectrum, enable us to break the clouds–metallicity degeneracy. We detect water in a highly metal-enriched “steam world” atmosphere (O/H of ∼4 by mass and H2O found to be the background gas with a volume mixing ratio of >31%). We further show that these results are robust to stellar contamination through the transit light source effect. We do not detect escaping metastable He, which, combined with previous nondetections of escaping He and H, supports the steam atmosphere scenario. In water-rich atmospheres, hydrogen loss driven by water photolysis happens predominantly in the ionized form, which eludes observational constraints. We also detect several flares in the NIRISS/SOSS light curves with far-UV energies of the order of 1030 erg, highlighting the active nature of the star. Further atmospheric characterization of GJ 9827 d probing carbon or sulfur species could reveal the origin of its high metal enrichment.

Modeling and application of array light sources in ultraviolet communication systems

Reliable communication over long distances using ultraviolet (UV) light is challenging due to the strong scattering and atmospheric absorption of UV light. However, with advancements in UV LED technology, the performance of transmitters can be significantly enhanced by employing an array of light sources, which improves the overall quality of communication. Despite these advancements, the impact of various array configurations—such as shape, quantities, and other parameters—on light intensity distribution and UV optical communication remains largely unexplored. In this work, we first modeled the light intensity distribution of arrayed light sources with varying shapes, quantities, and other parameters. We improved the traditional Monte Carlo method to better accommodate these distributions, enabling more accurate simulations. Subsequently, we obtained simulation results for different array configurations. Building on these findings, we developed a UV LED array communication system that achieved a bit error rate (BER) of 10−6 for information transmission over a distance of 500 meters. This research provides valuable insights into the long-distance transmission capabilities of UV light using arrayed light sources.

Optical Characterization of Coastal Waters with Atmospheric Correction Errors: Insights from SGLI and AERONET-OC

This study identifies the characteristics of water regions with negative normalized water-leaving radiance (nLw(λ)) values in the satellite observations of the Second-generation Global Imager (SGLI) sensor aboard the Global Change Observation Mission–Climate (GCOM-C) satellite. SGLI Level-2 data, along with atmospheric and in-water optical properties measured by the sun photometers in the AErosol RObotic NETwork-Ocean Color (AERONET-OC) from 26 sites globally, are utilized in this study. The focus is particularly on Tokyo Bay and the Ariake Sea, semi-enclosed water regions in Japan where previous research has pointed out the occurrence of negative nLw(λ) values due to atmospheric correction with SGLI. The study examines the temporal changes in atmospheric and in-water optical properties in these two regions, and identifies the characteristics of regions prone to negative nLw(λ) values due to atmospheric correction by comparing the optical properties of these regions with those of 24 other AERONET-OC sites. The time series results of nLw(λ) and the single-scattering albedo (ω(λ)) obtained by the sun photometers at the two sites in Tokyo Bay and Ariake Sea, along with SGLI nLw(λ), indicate the occurrence of negative values in SGLI nLw(λ) in blue band regions, which are mainly attributed to the inflow of absorptive aerosols. However, these negative values are not entirely explained by ω(λ) at 443 nm alone. Additionally, a comparison of in situ nLw(λ) measurements in Tokyo Bay and the Ariake Sea with nLw(λ) values obtained from 24 other AERONET-OC sites, as well as the inherent optical properties (IOPs) estimated through the Quasi-Analytical Algorithm version 5 (QAA_v5), identified five sites—Gulf of Riga, Long Island Sound, Lake Vanern, the Tokyo Bay, and Ariake Sea—as regions where negative nLw(λ) values are more likely to occur. These regions also tend to have lower nLw(λ) values at shorter wavelengths. Furthermore, relatively high light absorption by phytoplankton and colored dissolved organic matter, plus non-algal particles, was confirmed in these regions. This occurs because atmospheric correction processing excessively subtracts aerosol light scattering due to the influence of aerosol absorption, increasing the probability of the occurrence of negative nLw(λ) values. Based on the analysis of atmospheric and in-water optical measurements derived from AERONET-OC in this study, it was found that negative nLw(λ) values due to atmospheric correction are more likely to occur in water regions characterized by both the presence of absorptive aerosols in the atmosphere and high light absorption by in-water substances.

Accurately Measuring the Infrared Spectral Emissivity of Inconel 601, Inconel 625, and Inconel 718 Alloys during the Oxidation Process.

Accurate measurement of the infrared spectral emissivity of nickel-based alloys is significant for applications in aerospace. The low thermal conductivity of these alloys limits the accuracy of direct emissivity measurement, especially during the oxidation process. To improve measurement accuracy, a surface temperature correction method based on two thermocouples was proposed to eliminate the effect of thermal conductivity changes on emissivity measurement. By using this method, the infrared spectral emissivity of Inconel 601, Inconel 625, and Inconel 718 alloys was accurately measured during the oxidation process, with a temperature range of 673-873 K, a wavelength range of 3-20 μm, and a zenith angle range of 0-80°. The results show that the emissivity of the three alloys is similar in value and variation law; the emissivity of Inconel 718 is slightly less than that of Inconel 601 and Inconel 625; and the spectral emissivity of the three alloys strongly increases in the first hour, whereafter it grows gradually with the increase in oxidation time. Finally, Inconel 601 has a lower emissivity growth rate, which illustrates that it possesses stronger oxidation resistance and thermal stability. The maximum relative uncertainty of the emissivity measurement of the three alloys does not exceed 2.6%, except for the atmospheric absorption wavebands.

Polarization purity and dispersion characteristics of nested antiresonant nodeless hollow-core optical fiber at near- and short-wave-IR wavelengths for quantum communications

Advancements in quantum communication and sensing require improved optical transmission that ensures excellent state purity and reduced losses. While free-space optical communication is often preferred, its use becomes challenging over long distances due to beam divergence, atmospheric absorption, scattering, and turbulence, among other factors. In the case of polarization encoding, traditional silica-core optical fibers, though commonly used, struggle with maintaining state purity due to stress-induced birefringence. Hollow core fibers, and in particular nested antiresonant nodeless fibers (NANF), have recently been shown to possess unparalleled polarization purity with minimal birefringence in the telecom wavelength range using continuous-wave (CW) laser light. Here, we investigate a 1-km NANF designed for wavelengths up to the 2-μm waveband. Our results show a polarization extinction ratio between ~−30 dB and ~−70 dB across the 1520 to 1620 nm range in CW operation, peaking at ~−60 dB at the 2-μm design wavelength. Our study also includes the pulsed regime, providing insights beyond previous CW studies, e.g., on the propagation of broadband quantum states of light in NANF at 2 μm, and corresponding extinction-ratio-limited quantum bit error rates (QBER) for prepare-measure and entanglement-based quantum key distribution (QKD) protocols. Our findings highlight the potential of these fibers in emerging applications such as QKD, pointing towards a new standard in optical quantum technologies.

An inter-satellite laser occultation method profiling atmospheric temperature and pressure from troposphere to lower mesosphere

We propose a laser occultation method for simultaneous profiling atmospheric temperature and pressure. Measurements can be performed on the optical link between two low-orbit satellites, where frequency-stepwise laser pulses are transmitted from one to the other. These pulses, covering several oxygen absorption lines in the wavelength domain, measure the broadened atmospheric absorption optical depth along the transmission path with a spectral resolution of tens of megahertz. In this way, atmospheric temperature and pressure are obtained by analysing the retrieved shape and intensity of the spectral lines. With the motion of the two satellites, the inter-satellite optical link penetrates different atmospheric layers at various altitudes, enabling the measurement of the vertical structure of atmospheric thermodynamic parameters from the troposphere to the lower thermosphere. This paper presents an end-to-end simulation of the proposed method, including models for laser occultation beam tracing, radiative transfer, and data inversion. The simulation results reveal that with minimal satellite payload resources, this method can accurately measure temperature and pressure at a vertical resolution of 100 m from 5 km to 90 km altitude with accuracies of ±1.5 K and 5 %, respectively. As the proposed differential absorption laser occultation method is independent of the hydrostatic equilibrium hypothesis for data inversion, it can eliminate errors associated with prior data at reference altitudes. It is believed that our method has provided a promising approach to laser satellite constellation for atmospheric observation.

Climate Regimes across the Habitable Zone: A Comparison of Synchronous Rocky M and K Dwarf Planets

M and K dwarf stars make up 86% of the stellar population and host many promising astronomical targets for detecting habitable climates in the near future. Of the two, M dwarfs currently offer greater observational advantages and are home to many of the most exciting observational discoveries in the last decade. But K dwarfs could offer even better prospects for detecting habitability by combining the advantages of a relatively dim stellar flux with a more stable stellar environment. Here we explore the climate regimes that are possible on Earth-like synchronous planets in M and K dwarf systems, and how they vary across the habitable zone. We focus on surface temperature patterns, water availability, and implications for habitability. We find that the risk of nightside cold trapping decreases with increased orbital radius and is overall lower for K dwarf planets. With reduced atmospheric shortwave absorption, K dwarf planets have higher dayside precipitation rates and less day-to-night moisture transport, resulting in lower nightside snow rates. These results imply a higher likelihood of detecting a planet with a moist dayside climate in a habitable “eyeball” climate regime orbiting a K dwarf star. We also show that “terminator habitability” can occur for both M and K dwarf land planets, but would likely be more prevalent in M dwarf systems. Planets in a terminator habitability regime tend to have slightly lower fractional habitability, but offer alternative advantages including instellation rates more comparable to Earth in regions that have temperatures amenable to life.

Transceiving telescope for a mobile TDLAS system for remote sounding of anthropogenic methane

In this paper, we report on stages of the development of a transceiving telescope for atmospheric gas analysis TDLAS systems. Design of transmitters and receivers for active remote sounding systems is an important problem when creating instruments for studying atmospheric parameters. Transmitter and receiver aperture size directly influences the minimum permissible and maximum practicable sounding ranges. The work describes the stages of designing the receiving and transmitting parts of an atmospheric gas analysis tunable diode laser absorption spectroscopy (TDLAS) system (with anthropogenic methane as a target gas). Calculations show that a backscattered signal is reliably detected over an optical path 0–1000 m long with the use of a transceiving telescope with aperture 50–80 mm diameter and magnification of no less than 5. This design enables minimizing the dead zone of the coaxial TDLAS system within the 1645–1655 nm spectral range, which is informative for anthropogenic methane sounding. The use of a transceiving telescope will ensure resistance to misalignment, namely, resistance to mismatch between the mutual overlap of the device field of view and the laser radiation spot on the topographic target. Results of this work can be useful when designing and manufacturing telescopes for TDLAS systems methane (CH4) and also other atmospheric gas analysis.