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Methane hydrates exist in solid form within the pores of sand and have a significant impact on its mechanical and deformation properties. Therefore, it is essential to simulate the mechanical behavior of methane hydrate-bearing sand (MHBS) and evaluate its stability prior to commercial methane gas extraction. In this article, an elastoplastic constitutive model for MHBS is proposed. The yield surface function, which accounts for the bonding strength due to the hydrate cementation effect, is discussed, and the critical state line equation as a function of hydrate saturation is presented. Additionally, a simple debonding law is incorporated as a function of the plastic damage strain. Furthermore, an expression for the dilatancy is established based on the nonassociated flow rule. The hardening rule of the proposed model aligns with that of the Cam-Clay model. Model parameters are calibrated based on drained triaxial compression test data on MHBS. The simulation results closely match the test data, demonstrating the model’s effectiveness in predicting the stress–strain and volume change behavior of MHBS across a broad spectrum of initial densities, hydrate saturation, and effective confining pressures.

The vertical profile of atmospheric temperature is a principal state variable to study atmospheric stability. A lidar system, constructed using a 355 nm Nd:YAG laser transmitter, measures the temperature profile using the rotational Raman technique. In comparison with traditional Raman lidar, the major innovations are the use of a low peak power and high repetition rate laser to achieve eye-safe operation in a compact reliable instrument and the use of an angle tuning filter to select operating wavelengths. We demonstrate the capability of both nighttime and daytime measurements as a step toward a future stand-alone capability for routine measurements of important meteorological properties in the lower atmosphere.

Water vapor is the most important atmospheric greenhouse gas, but its variability and distribution, particularly the vertical profile, are not well known due to a lack of reliable long-term observations in the upper troposphere and stratosphere. Additional design and testing is necessary to extend Water Vapor Sensor System (WVSS) sensitivity to water vapor from a threshold of 100 ppmv to 2.8 ppmv to support operational and climate applications. Laser photoacoustic spectroscopy (LPAS) technique can extend the sensitivity to this level without extending absorption chamber path or using expensive laser emitting at stronger absorption line. A laser photoacoustic spectroscopy sensor based on inexpensive telecommunication style packaged, fiber-coupled near IR distributed feedback (DFB) laser diodes was developed to quantify concentrations of water vapor (H2O), CO2, and methane in ambient air. The LPAS sensor assembled in a compact package was designed for airborne, real-time measurements of atmospheric components. A resonant photoacoustic cell is used to increase the photoacoustic signal, electrical modulation is applied to replace mechanical chopper, and wavelength modulation spectroscopy is used to minimize the interfering background signal from window absorption in the sample cell. The minimum detection sensitivities (1) of 5 ppm at 1.39 m (5 mW) for water vapor, 6 ppm at 1.6 m (15 mW) for CO2, and 3 ppm at 1.6 m (15 mW) for methane, are reported.

Formaldehyde is a trace species that plays a key role in atmospheric chemistry. It is an important indicator of nonmethane volatile organic compound emissions. Also, it is a key reactive intermediate formed during the photochemical oxidation in the troposphere. Because the lifetime of formaldehyde in the atmosphere is fairly short (several hours), its presence signals hydrocarbon emission areas. The importance of measuring formaldehyde concentrations has been recognized by the National Academy's Decadal Survey and two of NASA's forthcoming missions the GEO-CAPE and GACM target its measurement. There are several techniques some of which are highly sensitive (detection limit ~ 50 parts-per-trillion) for in-situ measurement of formaldehyde and many reported atmospheric measurements. However there appear to be no reported standoff lidar techniques for range resolved measurements of atmospheric formaldehyde profiles. In this paper, we describe a formaldehyde lidar profiler based on differential laser induced fluorescence technique. The UV absorption band in the 352 - 357nm is well suited for laser excitation with frequency tripled Neodymium lasers and measuring the strong fluorescence in the 390 - 500nm region. Preliminary nighttime measurements of formaldehyde were demonstrated with a lidar using a commercial Nd:YAG laser (354.7 nm) with a rather large linewidth (~.02 nm). The measured sensitivity was ~1 ppb at 1 km with 100 meters range resolution even with this non-optimized system. In this paper we describe our approach for increasing the sensitivity by many orders and for daytime operation by improving the laser parameters (power and linewidth) and optimizing the receiver.

US EPA's Clean Air Act lists 187 hazardous air pollutants (HAP) or airborne toxics that are considered especially harmful to health, and hence the measurement of their concentration is of great importance. Numerous sensor systems have been reported for measuring these toxic gases and vapors. However, most of these sensors are specific to a single gas or able to measure only a few of them. Thus a sensor capable of measuring many of the toxic gases simultaneously is desirable. Laser photoacoustic spectroscopy (LPAS) sensors have the potential for true broadband measurement when used in conjunction with one or more widely tunable laser sources. An LPAS gas analyzer equipped with a continuous wave, room temperature IR Quantum Cascade Laser tunable over the wavelength range of 9.4 m to 9.7 m was used for continuous real-time measurements of multiple gases/chemical components. An external cavity grating tuner was used to generate several (75) narrow line output wavelengths to conduct photoacoustic absorption measurements of gas mixtures. We have measured various HAPs such as Benzene, Formaldehyde, and Acetaldehyde in the presence of atmospheric interferents water vapor, and carbon dioxide. Using the preliminary spectral pattern recognition algorithm, we have shown our ability to measure all these chemical compounds simultaneously in under 3 minutes. Sensitivity levels of a few part-per-billion (ppb) were achieved with several of the measured compounds with the preliminary laboratory system.