Abstract. We describe the first high precision real-time analysis of the N 2 O site-specific isotopic composition at ambient mixing ratios. Our technique is based on mid-infrared quantum cascade laser absorption spectroscopy (QCLAS) combined with an automated preconcentration unit. The QCLAS allows for simultaneous and specific analysis of the three main stable N 2 O isotopic species, 14 N 15 N 16 O, 15 N 14 N 16 O, 14 N 14 N 16 O, and the respective site-specific relative isotope ratio differences δ 15 N α and δ 15 N β . Continuous, stand-alone operation is achieved by using liquid nitrogen free N 2 O preconcentration, a quasi-room-temperature quantum cascade laser (QCL), quantitative sample transfer to the QCLAS and an optimized calibration algorithm. The N 2 O site-specific isotopic composition (δ 15 N α and δ 15 N β ) can be analysed with a long-term precision of 0.2 ‰. The potential of this analytical tool is illustrated by continuous N 2 O isotopomer measurements above a grassland plot over a three week period, which allowed identification of microbial source and sink processes.

We describe the first high precision real-time analysis of the N₂O site-specific isotopic composition at ambient mixing ratios. Our technique is based on mid-infrared quantum cascade laser absorption spectroscopy (QCLAS) combined with an automated preconcentration unit. The QCLAS allows for simultaneous and specific analysis of the three main stable N₂O isotopic species, ¹⁴N¹⁵N¹⁶O, ¹⁵N¹⁴N¹⁶O, ¹⁴N¹⁴N¹⁶O, and the respective site-specific relative isotope ratio differences δ¹⁵N^α and δ¹⁵N^β. Continuous, stand-alone operation is achieved by using liquid nitrogen free N₂O preconcentration, a quasi-room-temperature quantum cascade laser (QCL), quantitative sample transfer to the QCLAS, and an optimized calibration algorithm. The N₂O site-specific isotopic composition (δ¹⁵N^α and δ¹⁵N^β) can be analysed with a long term precision of 0.2‰. The potential of this analytical tool is illustrated by continuous N₂O isotopomer measurements above a grassland plot over three weeks period, which allowed identification of microbial source and sink processes

These findings validate QCLAS as a viable alternative technique with even higher precision than state-of-the-art IRMS. Thus, laser spectroscopy has the potential to contribute significantly to a better understanding of N turnover in soils, which is crucial for advancing strategies to mitigate emissions of this efficient greenhouse gas.

Abstract. Small-, medium-, and large-scale measurements of sea ice surface roughness were carried out in the Baltic Sea over a 3-year period. A clear relationship was found between the rms height and correlation length of Baltic sea ice. In most cases the surface correlation function was close to exponential. The logarithm of rms height showed a linear dependence on that of measured distance. Similarly, the correlation length was in many cases linearly dependent on the measured distance. This dependence on length suggests that ice surface roughness could be characterized as a fractal. Surface roughness is usually described with rms height •r and correlation length L. Actually, these are pseudoparameters, as they depend on the measured length [Church, 1988]. For conventional surfaces the dependence is sometimes so weak that these parameters can be taken as constants, but this is not so with fractal-like surfaces. Rms height and correlation length can be determined also for fractal-like surfaces but will not then represent intrinsic surface properties and will depend on the measurement process used. Fractal surfaces will typically have measured correlation lengths, which are of the same order as the measured length no matter what length is used. In nature the fractal characteristics of a surface will not extend to infinity; there is an upper limit to the measurement length, the surpassing of which leads to saturation of the rms height and correlation length values. The surface then behaves like a random Gaussian surface with a very high maximum surface height difference. The adequate surface roughness description to be chosen depends on the application in question. If the surface area to be characterized is smaller than required for the saturation of the conventional roughness parameters, the effect of measurement length cannot be neglected. Field ExperimentsThe years 1992 and 1993 were very mild, making ice measurements possible only in the Bay of Bothnia (Figure 1). In 1994 the ice extent was mostly very close to average for the Baltic Sea, and measurements were accordingly carried out in the Bay of Bothnia, the Sea of Bothnia, and the Gulf of Finland. The ice types were also much more varied in 1994 than during the previous 2 years.In 1992, measurements were carried out mostly in ridged old ice areas, with ice blocks in the ridges some 40 cm thick. Because the whole ice field was deformed repeatedly, no large level ice areas were available for comparison. In 1993 the study area was the edge of an old, many times deformed ice field (Figure 2). Here the ice block thickness was typically about 20 cm.In 1994, surface roughness measurements were carried out on old level ice and deformed areas in the Bay of Bothnia and the Gulf of Finland. The ice field of the Sea of Bothnia was newer and had deformed radically about 1 week prior to the field experiment. The deformation appeared in the Bay of Bothnia and the Gulf of Finland either as long ridge sails, rubble fields (Figure 3), or consolidated pancake ice. The ice in ...

A multipass cell (MPC) design for laser absorption spectroscopy is presented. The development of this new type of optical cell was driven by stringent criteria for compactness, robustness, low volume, and ease of use in optical systems. A single piece of reflective toroidal surface forms a near-concentric cavity with a volume of merely 40 cm(3). Contrary to traditional MPCs, this design allows for flexible path-length adjustments by simply changing the aiming angle of the laser beam at the entrance window. Two effective optical path lengths of 2.2 and 4.1 m were chosen to demonstrate the cell's suitability for high-precision isotope ratio measurements of CO(2) at 1% and ambient mixing ratio levels.

The design, construction and characterization of a novel circular multipass cell for sensitive trace gas analysis are presented. This cell allows for easy modification of the optical path length without any changes of its physical parameters. Furthermore, it is suited for three different detection techniques: direct absorption, wavelength modulation and photoacoustics. To demonstrate its performance, mixing ratios of 13 CO 2 and N 2 O were measured from ambient air, using a quantum cascade laser. With the direct absorption method, noise equivalent 1-s precisions of 2.7 ppb and 0.2 ppb are achieved for 13 CO 2 and N 2 O, respectively. The wavelength modulation technique resulted in 4.3 ppb precision with 1-s averaging for the 13 CO 2 measurements. A Q-factor of 190 and a normalized noise equivalent minimum absorption of 1.3 × 10 −8 cm −1 W Hz −1/2 are achieved using the photoacoustic technique.A. Manninen ( ) · B. Tuzson · L. Emmenegger

A hyperspectral remote sensing instrument employing a novel near-infrared supercontinuum light source has been developed for active illumination and identification of targets. The supercontinuum is generated in a standard normal dispersion multi-mode fiber and has 16 W total optical output power covering 1000 nm to 2300 nm spectral range. A commercial 256-channel infrared spectrometer was used for broadband infrared detection. The feasibility of the presented hyperspectral measurement approach was investigated both indoors and in the field. Reflection spectra from several diffusive targets were successfully measured and a measurement range of 1.5 km was demonstrated.

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