The detection and measurement of gas concentrations using the characteristic optical absorption of the gas species is important for both understanding and monitoring a variety of phenomena from industrial processes to environmental change. This study reviews the field, covering several individual gas detection techniques including non-dispersive infrared, spectrophotometry, tunable diode laser spectroscopy and photoacoustic spectroscopy. We present the basis for each technique, recent developments in methods and performance limitations. The technology available to support this field, in terms of key components such as light sources and gas cells, has advanced rapidly in recent years and we discuss these new developments. Finally, we present a performance comparison of different techniques, taking data reported over the preceding decade, and draw conclusions from this benchmarking.

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Optical gas sensing: a review

Reactive species in non-equilibrium atmospheric-pressure plasmas: Generation, transport, and biological effects

General formalism of absorption and emission spectra, and of radiative and nonradiative decay rates are derived using a thermal vibration correlation function formalism for the transition between two adiabatic electronic states in polyatomic molecules. Displacements, distortions, and Duschinsky rotation of potential energy surfaces are included within the framework of a multidimensional harmonic oscillator model. The Herzberg−Teller (HT) effect is also taken into account. This formalism gives a reliable description of the Qx spectral band of free-base porphyrin with weakly electric dipole-allowed transitions. For the strongly dipole-allowed transitions, e.g., S1 → S0 and S0 → S1 of linear polyacenes, anthracene, tetracene, and pentacene, the HT effect is found to enhance the radiative decay rates by ∼10% compared to those without the HT effect. For nonradiative transition processes, a general formalism is presented to extend the application scope of the internal conversion theory by going beyond the promo...

Theory of excited state decays and optical spectra: application to polyatomic molecules.

. Oxidation of biogenic volatile organic compounds (BVOC) by the nitrate radical (NO3) represents one of the important interactions between anthropogenic emissions related to combustion and natural emissions from the biosphere. This interaction has been recognized for more than 3 decades, during which time a large body of research has emerged from laboratory, field, and modeling studies. NO3-BVOC reactions influence air quality, climate and visibility through regional and global budgets for reactive nitrogen (particularly organic nitrates), ozone, and organic aerosol. Despite its long history of research and the significance of this topic in atmospheric chemistry, a number of important uncertainties remain. These include an incomplete understanding of the rates, mechanisms, and organic aerosol yields for NO3-BVOC reactions, lack of constraints on the role of heterogeneous oxidative processes associated with the NO3 radical, the difficulty of characterizing the spatial distributions of BVOC and NO3 within the poorly mixed nocturnal atmosphere, and the challenge of constructing appropriate boundary layer schemes and non-photochemical mechanisms for use in state-of-the-art chemical transport and chemistry–climate models. This review is the result of a workshop of the same title held at the Georgia Institute of Technology in June 2015. The first half of the review summarizes the current literature on NO3-BVOC chemistry, with a particular focus on recent advances in instrumentation and models, and in organic nitrate and secondary organic aerosol (SOA) formation chemistry. Building on this current understanding, the second half of the review outlines impacts of NO3-BVOC chemistry on air quality and climate, and suggests critical research needs to better constrain this interaction to improve the predictive capabilities of atmospheric models.

/pdf/nitrate-radicals-and-biogenic-volatile-organic-compounds-48l9l9gpi9.pdf

Nitrate radicals and biogenic volatile organic compounds: oxidation, mechanisms, and organic aerosol

Optical properties of secondary organic aerosols and their changes by chemical processes.

https://depositonce.tu-berlin.de/bitstream/11303/1384/1/Dokument_44.pdf

Incoherent broad-band cavity-enhanced absorption spectroscopy

A new application of incoherent broad-band cavity enhanced absorption spectroscopy (IBBCEAS) to weak transitions in solution through a very straightforward modification of commercially available double-beam UV/VIS absorption spectrometers is reported. The improved sensitivity of the new approach is demonstrated on basis of the weak Franck–Condon inhibited absorption of the fifth C–H stretch overtone in liquid benzene. The theoretical limits of the enhancement of the signal-to-noise ratio of IBBCEAS in comparison with single pass absorption experiments are discussed for a set of given experimental cavity parameters. The optical loss properties of a typical transparent cuvette window in the cavity are also discussed.

Incoherent broad-band cavity-enhanced absorption spectroscopy of liquids

A cavity-enhanced absorption setup employing an incoherent broadband light source was used in combination with a Fourier-transform spectrometer to measure the spin-forbidden B-band of gaseous oxygen at approximately 688 nm and several weak absorption transitions of water vapor in the same spectral region at room temperature in ambient air. The experiments demonstrate that the sensitivity of a Fourier-transform spectrometer can be significantly improved by increasing the effective path length, while retaining a rather small sample volume. In comparison with a single-pass absorption measurement, we report a path-length enhancement factor of 200 and an improvement of the signal-to-noise ratio of approximately 6 in the present cavity-enhanced absorption experiment. The practical advantages and limitations of this novel approach are outlined and potential applications are briefly discussed.

Fourier-transform cavity-enhanced absorption spectroscopy using an incoherent broadband light source

Incoherent broad-band cavity-enhanced absorption spectroscopy of azulene in a supersonic jet

The incoherent broadband cavity-enhanced absorption spectroscopy is a technique in measuring small absorptions over a broad wavelength range. The setup consists of a conventional absorption spectrometer using an incoherent lamp and a charge coupled device detector, as well as a linear optical cavity placed around the absorbing sample, which enhances the effective path length through the sample. In this work the consequences of cavity length, mirror curvature, reflectivity, different light injection geometries, and spot size of the light source on the output intensity are studied and the implications to the signal-to-noise ratio of the absorption measurement are discussed. The symmetric confocal resonator configuration is identified as a special case with optimum imaging characteristics but with higher requirements for mechanical stability. Larger spot sizes of the light source were found to be favorable in order to reduce the negative effects of aberrations on the intensity.

Sven E. Fiedler

Papers

Incoherent broad-band cavity-enhanced absorption spectroscopy

Incoherent broad-band cavity-enhanced absorption spectroscopy of liquids

Fourier-transform cavity-enhanced absorption spectroscopy using an incoherent broadband light source

Incoherent broad-band cavity-enhanced absorption spectroscopy of azulene in a supersonic jet

Influence of the cavity parameters on the output intensity in incoherent broadband cavity-enhanced absorption spectroscopy