U. Parchatka

Four-laser airborne infrared spectrometer for atmospheric trace gas measurements

We describe the four-laser airborne infrared (FLAIR) instrument, a tunable diode laser absorption spectrometer designed for simultaneous high-sensitivity in situ measurements of four atmospheric trace gases in the troposphere. The FLAIR spectrometer was employed during the large-scale airborne research campaign on tropospheric ozone (TROPOZ II) in 1991 and was used to measure CO, H(2) O(2), HCHO, and NO(2) in the free troposphere where detection limits below 100 parts in 10(12) by volume were achieved.

Ground‐based UV‐VIS spectroscopy: Diurnal OCIO‐profiles during January 1990 above Søndre Strømfjord, Greenland

Considerable amounts of chlorine dioxide, OClO, were observed from 5 January through 2 February, 1990 in the stratosphere above Soendre Stroemfjord showing a highly perturbed chlorine chemistry. Photolysis and simultaneous formation of the OClO leads to a typical concentration minimum at noon. Its changes in concentration indicate the release of the OClO precursors BrO and ClO from their respective reservoir substances in the morning. Two incidences of increased OClO production occur repeatedly at 92{degree} and 89{degree} solar zenith angle (SZA). Furthermore, in the beginning of January OClO morning values exceed those found at dusk for comparable SZA whereas towards the end of the month the morning values become depressed compared to the evening. The twilight vertical column densities of OClO often reach about 1.6 {times} 10{sup 13} molec/cm{sup 2} and a comparison shows an increase from 1988 to 1990.

Real-time measurements of atmospheric CO using a continuous-wave room temperature quantum cascade laser based spectrometer

A compact mid-infrared laser spectrometer based-on a thermoelectrically (TE) cooled continuous-wave room temperature (RT) quantum cascade laser working at a wavelength of 4.566 μm and TE-cooled detectors has been newly developed to demonstrate the applicability of high precision measurements of atmospheric CO. Wavelength modulation spectroscopy technique combined with a multiple pass absorption cell was employed to achieve high sensitivity. The Allan variance technique analysis indicates a 1-s CO measurement precision of 1.03 ppbv, improving to a minimum of 0.28 ppbv at the optimum integration time of 53 s. Long-term performance of the instrument was examined with alternate measurements of a sample gas with known concentration and ambient air at a typical 1-Hz sampling rate and a 1-hourly calibration cycle. This completely thermoelectrically cooled system shows the capability of long-term, unattended and continuous operation at RT without any complicated cryogenic cooling.

QUALITAS: A mid-infrared spectrometer for sensitive trace gas measurements based on quantum cascade lasers in CW operation

We describe QUALITAS, a mid-infrared spectrometer with multipass absorption cell, suitable to apply lead chalcogenide and quantum cascade lasers as light sources in CW operation at cryogenic temperatures. The instrument has been designed for the measurement of trace gases such as carbon monoxide, nitrous oxide or methane in the clean atmosphere from space-restricted and experimentally challenging mobile platforms. The design involves compact optics, which fit into the space limitations of a standard aircraft borne 19in. rack (W×H×D≈48×45×40cm3). The instrument allows high sensitivity (≈5×10−7m−1Hz−1∕2 during flight; ≈1×10−7m−1Hz−1∕2 in the laboratory) in combination with a low sample gas volume (0.3L) for high time resolution (<1s, using a moderately sized pump). We employ a combination of elements already applied in former spectrometers in our laboratory and a novel arrangement of spherical mirrors. The design criteria, the optical, electronic and mechanical set-up are described in detail. The applicatio...

The temporal evolution of the ratio HNO3/NOy in the Arctic lower stratosphere from January to March 1997

Aircraft-based measurements of HNO3, NOy, N2O, and O3 have been performed in the Arctic lower stratosphere in January (POLSTAR I) and March (STREAM III) of 1997. The two projects employed different aircraft platforms. In addition, NOy and O3 were measured using different instruments in the two campaigns. HNO3 and NOy were found strongly correlated with correlation coefficients of 0.84 (POLSTAR I) and 0.69 (STREAM III), respectively. The fraction of HNO3 within NOy decreased from 96% in January to 59% in March. The decrease is consistent with the lifetime of HNO3 due to photolysis after polar sunrise. The relationship of NOy and HNO3 with N2O shows that in January NOy and HNO3 values were markedly higher than expected, which may indicate nitrification by PSC-II particle sedimentation and evaporation. Contradictory, the ratios NOy/O3 observed in January are only slightly elevated. In March, NOy-N2O and NOy-O3 relations agree well with others reported in the literature. The difference between the NOy-O3 and NOy-N2O relationships is partly explained by an observed O3 decrease of about 30% between January and March.

A formaldehyde trace gas sensor based on a thermoelectrically cooled CW-DFB quantum cascade laser

We report the development of a trace gas sensor for the detection of atmospheric formaldehyde utilizing a thermoelectrically cooled distributed-feedback quantum cascade laser operating in continuous-wave mode at 5.68 μm. Wavelength modulation spectroscopy was combined with second harmonic detection and zero air based background subtraction techniques to enhance both detection sensitivity and precision to ∼2.5 ppbv for H2CO measurement with an integration time of less than 1 second and a 36 m optical path length. A novel analysis technique based on wavelet transform for noise reduction was successfully applied to improve the sensor performance, yielding sub-ppb measurement precision without reducing the fast temporal response.

Characterization and application of an externally mounted catalytic converter for aircraft measurements of NOy

A novel design for an airborne NOy converter was implemented, characterized in the laboratory, and used extensively for in situ tropospheric and stratospheric measurements of total reactive nitrogen (NOy). During field deployments, the converter is mounted outside the aircraft fuselage, avoiding the need for an inlet line. In flight, the converter can be calibrated by the addition of standard gases close to the sample inlet, compensating for any changes in the instrument sensitivity caused by changing operating conditions. The system has been used successfully during several Stratosphere Troposphere Experiments by Aircraft Measurements campaigns in the lowermost stratosphere and upper troposphere for the measurement of total reactive nitrogen. The detection limit of the system is approximately 100 pptv for 10 s integrated data (2σ). The precision, deduced from the reproducibility of the in-flight calibrations, is 7% and the accuracy is about 30%. Laboratory studies demonstrate that interference from HCN, ...

Real-Time Measurements of Atmospheric CO Using a Continuous-Wave Room Temperature Quantum Cascade Laser Spectrometer

A compact mid-infrared laser spectrometer based-on a thermoelectrically (TE) cooled continuous-wave room temperature (RT) quantum cascade laser working at a wavelength of 4.566 µm and TE-cooled detectors has been newly developed to demonstrate the applicability of high precision measurements of atmospheric CO. Wavelength modulation spectroscopy technique combined with a multiple pass absorption cell was employed to achieve high sensitivity. The Allan variance technique analysis indicates a 1-s CO measurement precision of 1.03 ppbv, improving to a minimum of 0.28 ppbv at the optimum integration time of 53 s. Long-term performance of the instrument was examined with alternate measurements of a sample gas with known concentration and ambient air at a typical 1-Hz sampling rate and a 1-hourly calibration cycle. This completely thermoelectrically cooled system shows the capability of long-term, unattended and continuous operation at RT without any complicated cryogenic cooling.