Volker Ebert

Simultaneous in situ measurement of CO, H2O, and gas temperatures in a full-sized coal-fired power plant by near-infrared diode lasers

We present what is to our knowledge the first near-infrared diode-laser-based absorption spectrometer that is suitable for simultaneous in situ measurement of carbon monoxide, water vapor, and temperature in the combustion chamber (20-m diameter, 13-m path length) of a 600-MW lignite-fired power plant. A fiber-coupled distributed-feedback diode-laser module at 1.56 microm served for CO detection, and a Fabry-Perot diode laser at 813 nm was used to determine H2O concentrations and temperature from multiline water spectra. Despite severe light losses (transmission, <10(-8)) and strong background radiation we achieved a resolution of 1.9 x 10(-4) (1sigma) fractional absorption, equivalent to 200 parts in 10(6) by volume of CO (at 1450 K, 10(5) Pa) with 30-s averaging time.

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.

Effect of sulfuric acid coating on heterogeneous ice nucleation by soot aerosol particles

[1]xa0The low-temperature aerosol and cloud chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere) of Forschungszentrum Karlsruhe was used to investigate the effect of sulfuric acid coating on the ice nucleation efficiency of soot aerosol particles from a spark discharge generator. The uncoated (sulfuric acid–coated) soot aerosol showed a nearly lognormal size distribution with number concentrations of 300–5000 cm−3 (2500–56,000 cm−3), count median diameters of 70–140 nm (90–200 nm), and geometric standard deviation of 1.3–1.4 (1.5–1.6). The volume fraction of the sulfuric acid coating to the total aerosol volume concentration ranged from 21 to 81%. Ice activation was investigated in dynamic expansion experiments simulating cloud cooling rates between about −0.6 and −3.5 K min−1. At temperatures between 186 and ∼235 K, uncoated soot particles acted as deposition nuclei at very low ice saturation ratios between 1.1 and 1.3. Above 235 K, ice nucleation only occurred after approaching liquid saturation. Coating with sulfuric acid significantly increased the ice nucleation thresholds of soot aerosol to saturation ratios increasing from ∼1.3 at 230 K to ∼1.5 at 185 K. This immersion mode of freezing nucleates ice well below the thresholds for homogeneous freezing of pure sulfuric acid solution droplets measured in previous AIDA experiments. A case study indicated that in contrast to the homogeneous freezing the nucleation rate of the immersion freezing mechanism depends only weakly on relative humidity and thereby the solute concentration. These results show that it is important to know the mixing state of soot and sulfuric acid aerosol particles in order to properly assess their role in cirrus formation.

SIMULTANEOUS DIODE-LASER-BASED IN SITU DETECTION OF MULTIPLE SPECIES AND TEMPERATURE IN A GAS-FIRED POWER PLANT

We have developed a diode-laser (DL)-based spectrometer and demonstrated, to our knowledge, the first simultaneous in situ detection of all major combustion species and the temperature in the same measurement volume for active combustion control purposes and to ensure a safe ignition procedure of large-scale multi-burner gas-fired combustion systems. Two distributed-feedback DLs at 760 nm and 1.65 μ m were used to detect O 2 , CH 4 , and CO 2 , while a Fabry-Perot DL at 812 nm served to extract absolute H 2 O concentrations and the temperature from multiline water spectra. Permanent alignment of the laser beams could be ensured, despite strong wall deformation, with a new active alignment control loop. We analyzed the instationary ignition procedure of a full-scale gas-fired power plant with a 10 m furnace diameter using the spectrometer. A time resolution of 1.6 s and a minimum detectable absorption better than 10 −3 OD could be achieved. CH 4 could be detected with a dynamic range of more than two orders of magnitude and a detectivity in the 100 ppm range. A strong dependence of the CH 4 signal on the burner height was found. This spectrometer is well suited to enable an on-line control of the furnace atmosphere and a rapid detection of ignition delays by unburned CH 4 .

In situ detection of potassium atoms in high-temperature coal-combustion systems using near-infrared-diode lasers

Direct tunable diode laser absorption spectroscopy at 769.9 and 767.5 nm was used to measure potassium (K) atom concentrations in situ in the high temperature (up to 1650 K) flue gas of two different pulverized coal dust combustion systems (atmospheric or pressurized (12 bar)). Two laser types (Fabry-Pérot (FP) and vertical-cavity surface-emitting lasers (VCSEL)) were used for the spectrometer and characterized with respect to the magnitude and linearity of their static and dynamic wavelength tuning properties. The wide continuous current-induced tuning range of the VCSEL of 20 cm(-1) (compared to 1 cm(-1) for the FP) make this laser ideal for species monitoring in high pressure processes. Two VCSELs were time-multiplexed to realize the simultaneous detection of the potassium D1 and D2 lines. Several oxygen absorption lines in the A-band, which are in close spectral vicinity of the K lines, were detected simultaneously, showing the possibility of multi-species detection with one laser. Using the FP-DL for the atmospheric process and the VCSEL for the high pressure process, the pressure-dependent coefficients for spectral broadening as well as a shift of the K line in the flue gas were determined to be (0.18 +/- 0.01) and (-0.060 +/- 0.003) cm(-1) per atm (at 1540 K and 11.2 bar). The total width and shift of the D1 line (11.2 bar/1540 K) were 60 and -20 GHz, respectively. The K atom concentration was determined continuously for several days in both plants under various operation conditions. Typical concentrations in the atmospheric plant were around 2 microg m(-3) with a range of 50 ng m(-3)-30 microg m(-3). Averaging 100 scans for each concentration value, we achieved a time resolution of 1.7 s and a detection limit of 10 ng m(-3), which corresponds to a fractional absorption in the 10(-3)-10(-4) range. A strong anti-correlation with the oxygen concentration could be verified. At the 12 bar plant, the concentration was again typically around 2 microg m(-3) but K levels up to 60 microg m(-3) were observed. Here, a strong dependence of the K-signal on the type of fuel could be verified.

ACRIDICON–CHUVA Campaign: Studying Tropical Deep Convective Clouds and Precipitation over Amazonia Using the New German Research Aircraft HALO

AbstractBetween 1 September and 4 October 2014, a combined airborne and ground-based measurement campaign was conducted to study tropical deep convective clouds over the Brazilian Amazon rain forest. The new German research aircraft, High Altitude and Long Range Research Aircraft (HALO), a modified Gulfstream G550, and extensive ground-based instrumentation were deployed in and near Manaus (State of Amazonas). The campaign was part of the German–Brazilian Aerosol, Cloud, Precipitation, and Radiation Interactions and Dynamics of Convective Cloud Systems–Cloud Processes of the Main Precipitation Systems in Brazil: A Contribution to Cloud Resolving Modeling and to the GPM (Global Precipitation Measurement) (ACRIDICON– CHUVA) venture to quantify aerosol–cloud–precipitation interactions and their thermodynamic, dynamic, and radiative effects by in situ and remote sensing measurements over Amazonia. The ACRIDICON–CHUVA field observations were carried out in cooperation with the second intensive operating period...

Experimental investigation of ice nucleation by different types of aerosols in the aerosol chamber AIDA : implications to microphysics of cirrus clouds

The aerosol chamber AIDA was used as a moderate expansion cloud chamber with cooling rates at the onset of ice nucleation between -1.3 and -3.0 K min -1 to investigate the nucleation and growth of ice crystals in sulphuric acid, ammonium sulphate, and mineral dust aerosols at temperatures between 196 and 224 K. Supercooled sulphuric acid droplets with mean diameters of about 0.2 to 0.3 μm nucleated ice by homogeneous freezing at RH ice increasing from 144 to 166 % with temperatures from 220 and 196 K. This is in good agreement both with previous results of AIDA experiments and literature data. In contrast, ammonium sulphate particles of similar size nucleated ice at the significantly lower RH ice of 120 to 127 % in the same temperature range. Fourier-Transform infrared (FTIR) extinction spectra of the aerosol revealed that the ammonium sulphate particles, mainly consisted of the liquid phase. The number concentration of ice crystals formed during the homogeneous freezing experiments agree well with model results from the literature. Higher ice crystal number concentrations formed during the ammonium sulphate, compared to the sulphuric acid experiments, can be explained by the also somewhat higher cooling rates at ice nucleation. Deposition ice nucleation on mineral dust particles turned out to be the most efficient ice nucleation mechanism both with respect to RH ice at the onset of ice nucleation (102 to 105 % in the temperature range 209 to 224 K) and the ice crystal number concentration. Almost all mineral dust particles nucleated ice at the lower temperatures.

ML-CIRRUS - The airborne experiment on natural cirrus and contrail cirrus with the high-altitude long-range research aircraft HALO

AbstractThe Midlatitude Cirrus experiment (ML-CIRRUS) deployed the High Altitude and Long Range Research Aircraft (HALO) to obtain new insights into nucleation, life cycle, and climate impact of natural cirrus and aircraft-induced contrail cirrus. Direct observations of cirrus properties and their variability are still incomplete, currently limiting our understanding of the clouds’ impact on climate. Also, dynamical effects on clouds and feedbacks are not adequately represented in today’s weather prediction models.Here, we present the rationale, objectives, and selected scientific highlights of ML-CIRRUS using the G-550 aircraft of the German atmospheric science community. The first combined in situ–remote sensing cloud mission with HALO united state-of-the-art cloud probes, a lidar and novel ice residual, aerosol, trace gas, and radiation instrumentation. The aircraft observations were accompanied by remote sensing from satellite and ground and by numerical simulations.In spring 2014, HALO performed 16 f...

Lightweight diode laser spectrometer CHILD (Compact High-altitude In-situ Laser Diode) for balloonborne measurements of water vapor and methane

A new lightweight near-infrared tunable diode laser spectrometer CHILD (Compact High-altitude In-situ Laser Diode spectrometer) was developed for flights to the stratosphere as an additional in situ sensor on existing balloonborne payloads. Free-air absorption measurements in the near infrared are made with an open-path Herriott cell with new design features. It offers two individual absorption path lengths optimized for CH4 with 74 m (136 pass) and H2O with 36 m (66 pass). New electronic features include a real-time gain control loop that provides an autocalibration function. In flight-ready configuration the instrument mass is approximately 20 kg, including batteries. It successfully measured stratospheric CH4 and H2O profiles on high-altitude balloons on four balloon campaigns (Environmental Satellite validation) between October 2001 and June 2003. On these first flights, in situ spectra were recorded from ground level to 32,000-m altitude with a sensitivity of 0.1 ppm [(parts per million), ground] to 0.4 ppm (32,000 m) for methane and 0.15-0.5 ppm for water.

In situ determination of molecular oxygen concentrations in full-scale fire-suppression tests using tunable diode laser absorption spectroscopy

The fast and accurate determination of oxygen in air is important for fire research. Available O2 sensors (paramagnetic, electrochemical, ZrO2) are only of limited use because of significant errors caused by the specific sampling or measurement process, so that a purely optical, in situ detection is of great interest. Optical methods can account for the dilution of O2 by water (both vapor and drops) and are thus valuable tools for studying the effectiveness of water for replacing halogenated fire suppressants. To fulfill this need, a tunable diode laser based absorption spectrometer (TDLAS) has been developed for the in situ detection of molecular oxygen at 760 nm (A band, b1Σ+g←X3Σ−g). The device was successfully tested during fullscale fire-suppression tests carried out at the Naval Research Laboratory Chesapeake Bay Detachment facility in a 28-m3 rest compartment. A specially protected open-path Herriott multipass setup with an absorption length of 1.8 m was developed to restrict the probe volume to a base length of 30 cm. Various scenarios including water mist only, unsumppressed fires, and water-suppressed fires (methanol and n-heptane, both pool and cascading fires up to 400 kW) were investigated. O2 concentrations were measured at a 2.5 Hz repetition rate with a resolution of 0.01 to 1 vol % O2 depending on the transmission conditions. This demonstrated for the first time the capability for in situ oxygen measurements under fire-suppression conditions with large and rapid obscuration changes (transmission of as little as 0.8% of the emitted laser power). It also showed that the TDLAS results account for dilution of O2 by water vapor without any interference of other species.