Steven Wagner

The effect of organic coating on the heterogeneous ice nucleation efficiency of mineral dust aerosols

The effect of organic coating on the heterogeneous ice nucleation (IN) efficiency of dust particles was investigated at simulated cirrus cloud conditions in the AIDA cloud chamber of Forschungszentrum Karlsruhe. Arizona test dust (ATD) and the clay mineral illite were used as surrogates for atmospheric dust aerosols. The dry dust samples were dispersed into a 3.7?m3 aerosol vessel and either directly transferred into the 84?m3 cloud simulation chamber or coated before with the semi-volatile products from the reaction of ?-pinene with ozone in order to mimic the coating of atmospheric dust particles with secondary organic aerosol (SOA) substances. The ice-active fraction was measured in AIDA expansion cooling experiments as a function of the relative humidity with respect to ice, RHi, in the temperature range from 205 to 210?K. Almost all uncoated dust particles with diameters between 0.1 and 1.0??m acted as efficient deposition mode ice nuclei at RHi between 105 and 120%. This high ice nucleation efficiency was markedly suppressed by coating with SOA. About 20% of the ATD particles coated with a SOA mass fraction of 17?wt% were ice-active at RHi between 115 and 130%, and only 10% of the illite particles coated with an SOA mass fraction of 41?wt% were ice-active at RHi between 160 and 170%. Only a minor fraction of pure SOA particles were ice-active at RHi between 150 and 190%. Strong IN activation of SOA particles was observed only at RHi above 200%, which is clearly above water saturation at the given temperature. The IN suppression and the shift of the heterogeneous IN onset to higher RHi seem to depend on the coating thickness or the fractional surface coverage of the mineral particles. The results indicate that the heterogeneous ice nucleation potential of atmospheric mineral particles may also be suppressed if they are coated with secondary organics.

VCSEL-based, high-speed, in situ TDLAS for in-cylinder water vapor measurements in IC engines

We report the first application of a vertical-cavity surfaceemitting laser (VCSEL) for calibration- and sampling-free, high-speed, in situ H2O concentration measurements in IC engines using direct TDLAS (tunable diode laser absorption spectroscopy). Measurements were performed in a single-cylinder research engine operated under motored conditions with a time resolution down to 100 μs (i.e., 1.2 crank angle degrees at 2000 rpm). Signal-to-noise ratios (1σ) up to 29 were achieved, corresponding to a H2O precision of 0.046 vol.% H2O or 39 ppm · m. The modulation frequency dependence of the performance was investigated at different engine operating points in order to quantify the advantages of VCSEL against DFB lasers.

Development and Metrological Characterization of a Tunable Diode Laser Absorption Spectroscopy (TDLAS) Spectrometer for Simultaneous Absolute Measurement of Carbon Dioxide and Water Vapor

Simultaneous detection of two analytes, carbon dioxide (CO2)and water vapor (H2O), has been realized using tunable diode laser absorption spectroscopy (TDLAS) with a single distributed feedback diode laser at 2.7 μm. The dynamic range of the spectrometer is extended from the low parts per million to the percentage range using two gas cells, a single-pass cell with 0.77 m, and a Herriott-type multipass cell with 76 m path length. Absolute measurements were carried out, i.e., amount fractions of the analytes were calculated based on previously determined spectral line parameters, without the need for an instrument calibration using gas standards. A thorough metrological characterization of the spectrometer is presented. We discuss traceability of all parameters used for amount fraction determination and provide a comprehensive uncertainty assessment. Relative expanded uncertainties (k = 2, 95% confidence level) of the measured amount fractions are shown to be in the 2-3% range for both analytes. Minimum detectable amount fractions are 0.16 μmol/mol for CO2 and 1.1 μmol/mol for H20 for 76 m path length and 5 s averaging time. This corresponds to normalized detection limits of 27 μmol/mol m Hz_1/2 for CO2 and 221 μmol/mol m Hz−1/2 for H2O. Precision of the spectrometer, determined using Allan variance analysis, is 3.3 nmol/mol for CO2 and 21 nmol/mol for H2O. The spectrometer has been validated using reference gas mixtures with known CO2 and H2O amount fractions. An application example of the absolute TDLAS spectrometer as a reference instrument to validate other sensors is also presented.

Distributed feedback diode laser spectrometer at 27 μm for sensitive, spatially resolved H_2O vapor detection

A new, compact, spatially scanning, open-path 2.7 μm tunable diode laser absorption spectrometer with short absorption path lengths below 10 cm was developed to analyze the spatiotemporal dynamics of one-dimensional (1D) spatial water vapor gradients. This spectrometer, which is based on a room-temperature distributed feedback diode laser, is capable of measuring absolute, calibration-free, line-of-sight averaged, but laterally resolved 1D H2O concentration profiles with a minimum fractional optical resolution of 2.1×10−3 optical density (OD) (2.5×10−4 OD after a background subtraction procedure), which permits a signal-to-noise-ratio of 407 (3400) at 10,000 parts in 106 (ppm)H2O, or normalized sensitivities of 2.6 ppm⋅m (0.32 ppm m) at 0.5 Hz duty cycle. The spectrometers lateral spatial resolution (governed by the 500 μm sampling beam diameter) was validated by analyzing a well-defined laminar jet of nitrogen gas in humidified air. This scanning setup was then used to (a) quantitatively investigate for what we believe to be the first time the H2O boundary layer from 0.7 to 11 mm beneath the stomatous side of a single, undetached plant leaf, and (b) to study the temporal boundary layer dynamics and its dependence on stepwise light stimulation of the photosynthetic system. In addition the 2.7 μm diode laser was carefully characterized in terms of spectral purity, beam profile, as well as quasi-static and dynamic wavelength tuning coefficients.

Broadband supercontinuum laser absorption spectrometer for multiparameter gas phase combustion diagnostics

We report on the development and application of a broadband absorption spectrometer utilizing a pulsed supercontinuum laser light source and dispersion compensating fiber with a single-pass absorption path to obtain absolute methane mole fractions in a laminar nonpremixed CH(4)/air flame supported on a Wolfhard-Parker burner. The basic principle of supercontinuum broadband absorption spectroscopy (SCLAS) provides advantageous means of combustion diagnostics since the broad spectral coverage allows for use in high-pressure high-temperature environments. Furthermore, a previously validated tunable diode laser absorption spectroscopy fitting algorithm was applied to the recorded spectra and found to be applicable to SCLAS measurements as well, by comparison of fitted methane gas concentrations to reference measurements on the Wolfhard-Parker burner. The spectrometer reached spectral resolutions of up to 0.152  cm(-1), while providing a spectral coverage of over 110  cm(-1), with an absorption path length of only 41 mm. First measurements of absolute CH(4) mole fractions showed the suitability of SCL-based spectroscopy for combustion diagnostics with short absorption path lengths in the nIR spectral region. Here, we achieved in-flame methane mole fraction resolutions of 3%(Vol.) (1210 ppm·m) and optical resolutions of up to 1.1×10(-2). Based on this first validation, this method can now be extended to other species and combustion parameters such as temperature and pressure.

Broadband fitting approach for the application of supercontinuum broadband laser absorption spectroscopy to combustion environments

In this work, a novel broadband fitting approach for quantitative in-flame measurements using supercontinuum broadband laser absorption spectroscopy (SCLAS) is presented. The application and verification of this approach in an atmospheric, laminar, non-premixed CH4/air flame (Wolfhard–Parker burner, WHP) is discussed. The developed fitting scheme allows for an automatic recognition and fitting of a B-spline curve reference intensity for SCLAS broadband measurements while automatically removing the influence of absorption peaks. This approach improves the fitting residual locally (in between absorption lines) and globally by 23% and 13% respectively, while improving the in-flame SNR by a factor of 2. Additionally, the approach inherently improves the time–wavelength-correlation based on recorded in-flame measurements itself in combination with a theoretical spectrum of the analyte. These improvements have allowed for the recording of complete spatially resolved methane concentration profiles in the WHP burner. Comparison of the measured absolute mole fraction profile for methane with previously measured reference data shows excellent agreement in position, shape and absolute values. These improvements are a prerequisite for the application of SCLAS in high-pressure combustion systems.

Direct single-mode fibre-coupled miniature White cell for laser absorption spectroscopy

We present the design, setup, and characterization of a new lens-free fibre-coupled miniature White cell for extractive gas analysis using direct tunable diode laser absorption spectroscopy (dTDLAS). The construction of this cell is based on a modified White cell design and allows for an easy variation of the absorption length in the range from 29 cm to 146 cm. The design avoids parasitic absorption paths outside the cell by using direct, lensless fibre coupling and allows small physical cell dimensions and cell volumes. To characterize the cell performance, different H2O and CH4 concentration levels were measured using dTDLAS. Detection limits of 2.5 ppm ⋅ m for CH4 (at 1.65 μm) and 1.3 ppm ⋅ m for H2O (at 1.37 μm) were achieved. In addition, the gas exchange time and its flow-rate dependence were determined for both species and found to be less than 15 s for CH4 and up to a factor of thirteen longer for H2O.

Optical Sensing of Turbine Inlet Temperature in a Pressurized Gas Turbine Combustor

A probe-based, fiber-coupled TDLAS sensor, operating at 1.4 µm for the measurement of gas turbine inlet temperatures using two line thermometry is developed and validated in a pressurized generic single-sector combustor.

Data analysis and uncertainty estimation in supercontinuum laser absorption spectroscopy

A set of algorithms is presented that facilitates the evaluation of super continuum laser absorption spectroscopy (SCLAS) measurements with respect to temperature, pressure and species concentration without the need for simultaneous background intensity measurements. For this purpose a non-linear model fitting approach is employed. A detailed discussion of the influences on the instrument function of the spectrometer and a method for the in-situ determination of the instrument function without additional hardware are given. The evaluation procedure is supplemented by a detailed measurement precision assessment by applying an error propagation through the non-linear model fitting approach. While the algorithms are tailored to SCLAS, they can be transferred to other spectroscopic methods, that similarly require an instrument function. The presented methods are validated using gas cell measurements of methane in the near infrared region at pressures up to 8.7 bar.

NO Mole Fraction Measurement in a Plasma-heated Auto Ignition Test Rig Using a 5.2 μm Interband Cascade Laser

In-situ concentration measurements of nitric oxide in the microwave-plasma-heated turbulent air-co-flow (800-1300 K) of an auto-ignition test rig have been performed, employing tunable diode laser absorption spectroscopy at a wavelength of 5.2 μm.