Eddy covariance (EC) flux measurements of nitrous oxide (N2O) obtained by using a 3-D sonic anemometer and a tunable diode laser gas analyzer for N2O were investigated. Two datasets (Sorø, Denmark and Kalevansuo, Finland) from different measurement campaigns including sub-canopy flux measurements of energy and carbon dioxide are discussed with a focus on selected quality control aspects and flux error analysis. Although fast response trace gas analyzers based on spectroscopic techniques are increasingly used in ecosystem research, their suitability for reliable estimates of EC fluxes is still limited, and some assumptions have to be made for filtering and processing data. The N2O concentration signal was frequently dominated by offset drifts (fringe effect), which can give an artificial extra contribution to the fluxes when the resulting concentration fluctuations are correlated with the fluctuations of the vertical wind velocity. Based on Allan variance analysis of the N2O signal, we found that a recursive running mean filter with a time constant equal to 50 s was suitable to damp the influence of the periodic drift. Although the net N2O fluxes over the whole campaign periods were quite small at both sites (~5 μg N m−2 h−1 for Kalevansuo and ~10 μg N m−2 h−1 for Sorø), the calculated sub-canopy EC fluxes were in good agreement with those estimated by automatic soil chambers. However, EC N2O flux measurements show larger random uncertainty than the sensible heat fluxes, and classification according to statistical significance of single flux values indicates that downward N2O fluxes have larger random error.

A cryogenically operated laser diode spectrometer (COLD) for the airborne measurement of carbon monoxide is described. The instrument is designed, and fully qualified, for operation on a high-altitude aircraft and the scientific mission is the in situ measurement of trace gases in the upper troposphere and lower stratosphere. Sensitivities achieved so far during in-flight operation are a few ppbV with a time resolution of 4 s, coupled with a good reliability. Airborne data, obtained by COLD during research campaigns in Australia and Brazil in the frame of international projects, are also presented to demonstrate COLD in-flight performance.

The combined use of a novel multipass cell and a sample modulation scheme based on the Stark effect in molecular spectra is used to suppress time dependent background signals, which in general limit spectrometer performance during measurements. A rapid background subtraction scheme, in which the external electric field was turned off on alternate scans, as well as a double modulation experiment show drift free, white noise limited characteristics up to integration times of more than 1000 s. This exceeds the generally obtained spectrometer stability by about one order of magnitude.

A multipass cell for absorption measurements with an additionally applied homogeneous electric field for Stark effect measurements is described. The configuration is based on two ring mirrors, where the laser beam propagates between two nested cylindrical-wall electrodes. The total optical path length achieved is 40 m. The beam pointing stability of this setup is investigated and compared to a confocal-type Herriott cell of the same base length, employing numerical simulations. The exit beam pointing stability is found to be very good. The response measurements show fast exchange times, which agree well with theoretical values.

Diode-lasers find increasingly applications in industry, medicine and environmental monitoring, but still measurements challenges have to be solved in the fields of stable isotopes, trace gas emissions from ecosystems and on in-situ airborne stratospheric platforms. 2006 Optical Society of America OCIS Codes: (140.2020) Diode-Laser; (300.6260) Spectroscopy; (010.1120) Air Pollution Monitoring

Diode-laser absorption spectroscopy finds increasing applications in the emerging field of stable isotope research. To meet the requirements of the water isotopes measurement challenge in environmental research, ways have to be found to cope with the present limitations of spectroscopic systems. In this article, we discuss an approach based on the Stark effect in molecular spectra to reduce the influence of time-dependent, unwanted background structures generally superimposed on the desired signal from the spectral feature under investigation. A road map to high-sensitivity isotopic ratio measurements of water isotopes is presented. On the basis of an Allan Variance analysis of measured data, the detection limits have been calculated as a function of the integration time. To achieve the required optical density of about 6 x 10(-7) for H(2)(17)O measurements, the duty cycle has to be optimized and the implementation of a sample modulation within an optical multipass cell is a promising approach to increase the stability of spectroscopic instrumentation required for ecosystem research and airborne atmospheric platforms.

Optical sensors based on semiconductor lasers are at the threshold of routine applications in gas analysis and increasingly these sensors are used for industrial and environmental monitoring applications whenever sensitive, selective and fast insitu analysis in the nearand mid-infrared spectral region is required. With the increasing complexity of processes, online gas analysis is becoming an issue in automated control of various industrial applications such as combustion and plasma diagnostics, investigations of engines and automobile exhaust measurements. Other challenges are online analysis of high purity process gases, medical diagnostics and monitoring of agricultural and industrial emissions (VDI 2002). The need to meet increasingly stringent environmental and legislative requirements has led to the development of analyzers to measure concentrations of a variety of gases based on nearand mid-infrared absorption spectroscopy.

Laser-optical sensors are applied whenever sensitive, selective and fast in-situ gas analysis is required. This contribution illustrates the performance of selected near- and mid-infrared spectrometers based on tunable diode-lasers. Applications of lead-salt diode-lasers for NO2 and CH4 sensing, of antimonide lasers for CH4 and HCHO sensing in the 3-4 μm range and a near-infrared gas sensor for CO2 based on a room temperature 2 μm Indium-Phosphide laser will be presented and spectrometer performance will be discussed.

Currently available semiconductor lasers for spectroscopy in the near- and mid-infrared region based on direct band-to-band transitions as gallium-arsenide, indium-phosphide, antimonides and lead-salt containing compounds will be discussed together with the main features of different tunable diode-laser absorption spectrometers for trace gas analysis. Measurements of atmospheric carbon dioxide with a room-temperature 2 microm indium-phosphide laser, applications of antimonide lasers for methane and formaldehyde sensing in the 3-4 microm range and a fast chemical sensor for methane flux measurements based on lead-salt diode-lasers operating near 7.8 microm will be presented.

We present a detailed analysis of the tuning characteristics of InAsSb continuous-wave (cw) single-frequency lasers emitting at 3.3 μm (3050 cm−1). The lasers demonstrate a tuning range of −7.5 cm−1 by changing the current and −3.5 cm−1 by changing the heat sink temperature without mode hopping. The tuning rates are of −0.015 to −0.089 cm−1/mA and −0.11 to 0.27 cm−1/K. The laser tunes to the blue side both with increasing injection current and heat sink temperature. The extended tuning is attributed to the carrier heating effect in the cw operation and the band-filling effect in the presence of strong losses.

Semiconductor diode lasers were first developed in the mid-1960s and found immediate application as much needed tunable sources for high-resolution laser spectroscopy commonly referred to as tunable diode laser absorption spectroscopy (TDLAS). In this paper, currently available semiconductor lasers for spectroscopy in the near- and mid-infrared spectral region based upon gallium arsenide, indium phosphite, antimonides and lead-salt containing compounds will be reviewed together with the main features of TDLAS. Room-temperature measurements of atmospheric carbon dioxide near 2μm will be discussed and recent results obtained with a fast chemical sensor for methane flux measurements based on lead-salt diode lasers operating near 7.8μm will be presented.

Eddy covariance measurements of methane were carried out over the fenMurnauer Moosa in the south of Germany in order to evaluate the performance of a newly developed eddy covariance measurement system, based on a frequency- modulated tunable diode laser spectrometer as a fast chemical sensor. During a six-day period, an average day- time methane emission of (5.4$1.8) mg CH m h was measured. We "nd this value moderate, considering the favorable meteorological and soil conditions for methane emission. Diurnal cycles of the #uxes of methane and carbon dioxide as well as of sensible and latent heat are presented. Results are discussed in terms of relevant micrometeorological quantities, and quality control procedures based on Allan variance and spectral analysis are discussed. 2001 Elsevier Science Ltd. All rights reserved.

Eddy covariance measurements of methane were carried out over the fen “Murnauer Moos” in the south of Germany in order to evaluate the performance of a newly developed eddy covariance measurement system, based on a frequency-modulated tunable diode laser spectrometer as a fast chemical sensor. During a six-day period, an average daytime methane emission of (5.4±1.8) mg CH4 m−2 h−1 was measured. We find this value moderate, considering the favorable meteorological and soil conditions for methane emission. Diurnal cycles of the fluxes of methane and carbon dioxide as well as of sensible and latent heat are presented. Results are discussed in terms of relevant micrometeorological quantities, and quality control procedures based on Allan variance and spectral analysis are discussed.

A fast diode laser sensor has been applied for micrometeorological flux measurement of methane emissions from rice paddy fields in order to assess the quality of data on methane fluxes and allow a comparison with simultaneously recorded data provided by the closed chamber method. Systematic differences between chamber and the eddy correlation technique have been found as closed chamber measurements report about 70% higher emissions than eddy correlation measurements. This demonstrates that diode laser spectroscopy is a valuable tool for quality assurance in atmospheric research.

A fast tunable diode laser sensor has been applied for micrometeorological flux measurement of methane emissions from rice paddy fields in order to assess the quality of data on methane fluxes and allow a comparison with simultaneously recorded data provided by the closed chamber method. Systematic differences between chamber and the eddy correlation technique have been found and it seems that chamber measurements overestimate the actual emission up to 70%. This finding demonstrates once more that TDLAS is a valuable tool for atmospheric research and quality assurance.