Johannes Ofner

Chemometric Analysis of Multisensor Hyperspectral Images of Precipitated Atmospheric Particulate Matter

The chemometric analysis of multisensor hyperspectral data allows a comprehensive image-based analysis of precipitated atmospheric particles. Atmospheric particulate matter was precipitated on aluminum foils and analyzed by Raman microspectroscopy and subsequently by electron microscopy and energy dispersive X-ray spectroscopy. All obtained images were of the same spot of an area of 100 × 100 μm(2). The two hyperspectral data sets and the high-resolution scanning electron microscope images were fused into a combined multisensor hyperspectral data set. This multisensor data cube was analyzed using principal component analysis, hierarchical cluster analysis, k-means clustering, and vertex component analysis. The detailed chemometric analysis of the multisensor data allowed an extensive chemical interpretation of the precipitated particles, and their structure and composition led to a comprehensive understanding of atmospheric particulate matter.

Time resolved Infrared Spectroscopy of Formation and Processing of Secondary Organic Aerosols

Abstract An aerosol flow reactor was coupled to an infrared absorption cell to study aerosol formation processes with high temporal resolution. The recorded infrared spectra were referenced using aerosol smog chamber experiments. Evaluation was done by studying the formation of secondary organic aerosol from α-pinene and catechol as precursors and ozone as oxidant. Three main infrared absorptions: ν(O-H), ν(C-H) and ν(C=O) were considered, and humic like properties of the secondary organic aerosol are mainly interpreted according to the formation and variations of carbonyl bands in the region between 1850 and 1600 cm−1, especially the ν(C=O) of aryl carbonyls from catechol oxidation products below 1700 cm−1. The relative intensities of two major ν(C=O) stretching vibrations at 1690 cm−1 and 1755 cm−1 were observed to depend strongly on the available ozone concentration. At high precursor/ozone ratios (2:1 or 1:1) the vibration at 1690 cm−1 predominates, indicating aryl carbonyl vibrations. With increasing ozone concentrations this vibration is replaced by the higher carbonyl vibration at 1755 cm−1 indicating unsaturated carbonyl-containing compounds. This is a strong hint at ring opening processes leading to unsaturated aliphatic compounds in the resulting particle. Aryl carbonyls and aromatic or olefinic ν(C=C) at 1620 cm−1 in aged particles remain visible, as aerosol smog chamber studies exhibit – thus a strong hint at humic like properties of the SOA from the spectroscopic point of view.

Halogen-induced organic aerosol (XOA): a study on ultra-fine particle formation and time-resolved chemical characterization

The concurrent presence of high values of organic SOA precursors and reactive halogen species (RHS) at very low ozone concentrations allows the formation of halogen-induced organic aerosol, so-called XOA, in maritime areas where high concentrations of RHS are present, especially at sunrise. The present study combines aerosol smog-chamber and aerosol flow-reactor experiments for the characterization of XOA. XOA formation yields from alpha-pinene at low and high concentrations of chlorine as reactive halogen species (RHS) were determined using a 700 L aerosol smog-chamber with a solar simulator. The chemical transformation of the organic precursor during the aerosol formation process and chemical aging was studied using an aerosol flow-reactor coupled to an FTIR spectrometer. The FTIR dataset was analysed using 2D correlation spectroscopy. Chlorine induced homogeneous XOA formation takes place at even 2.5 ppb of molecular chlorine, which was photolysed by the solar simulator. The chemical pathway of XOA formation is characterized by the addition of chlorine and abstraction of hydrogen atoms, causing simultaneous carbon-chlorine bond formation. During further steps of the formation process, carboxylic acids are formed, which cause a SOA-like appearance of XOA. During the ozone-free formation of secondary organic aerosol with RHS a special kind of particulate matter (XOA) is formed, which is afterwards transformed to SOA by atmospheric aging or degradation pathways.

An unusually water-poor 5,5′-azobistetrazolate of dysprosium: stabilization of a nitrogen-rich heterocycle by a minimum of hydrogen bonds

In inorganic 5,5′-azobistetrazolate (C2N102−; ZT) compounds, the energetic ZT moiety is usually stabilized by H-bonds to lattice H2O and/or coordination of an N-atom of the tetrazole ring to the metal ion. Here we present the synthesis and characterization of a novel, salt-like compound, [Dy(H2O)8]2ZT3, which constitutes a unique exception to the above rule. We used supercritical CO2 as an unconventional desiccant in a straightforward metathesis reaction without abstaining from the benefits aqueous chemistry offers. It caused the crystallization of the title compound with a complex system of H-bonds between [Dy(H2O)8]3+ and ZT2− that involves almost every N-atom of the ZT2− anions as H-bond acceptors, which sufficiently stabilizes the energetic ZT despite the lack of lattice H2O or a coordinative bond to the metal ion.

Direct Deposition of Aerosol Particles on an ATR Crystal for FTIR Spectroscopy Using an Electrostatic Precipitator

An electrostatic precipitator (ESP) has been developed for collecting aerosol samples for attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) from an aerosol chamber. The ESP deposits the aerosol particles directly onto the ATR crystal with high efficiency. The ESP-ATR spectra are observed to agree well with the spectrum of a filter sample, transferred by impression. ZnSe ATR crystals have been found most suitable for electrostatic precipitation due to surface hardness, transmission characteristics and deposition behavior. ESP-ATR-FTIR spectroscopy might be a powerful tool not only for in situ but also for online measurements of aerosols.

Quasi-Simultaneous In-Line Flue Gas Monitoring of NO and NO2 Emissions at a Caloric Power Plant Employing Mid-IR Laser Spectroscopy

Two pulsed thermoelectrically cooled mid-infrared distributed feedback quantum cascade lasers (QCLs) were used for the quasi-simultaneous in-line determination of NO and NO2 at the caloric power plant Dürnrohr (Austria). The QCL beams were combined using a bifurcated hollow fiber, sent through the flue tube (inside diameter: 5.5 m), reflected by a retro-reflector and recorded using a fast thermoelectrically cooled mercury-cadmium-telluride detector. The thermal chirp during 300 ns pulses was about 1.2 cm(-1) and allowed scanning of rotational vibrational doublets of the analytes. On the basis of the thermal chirp and the temporal resolution of data acquisition, a spectral resolution of approximately 0.02 cm(-1) was achieved. The recorded rotational vibrational absorption lines were centered at 1900 cm(-1) for NO and 1630 cm(-1) for NO2. Despite water content in the range of 152-235 g/m(3) and an average particle load of 15.8 mg/m(3) in the flue gas, in-line measurements were possible achieving limits of detection of 73 ppb for NO and 91 ppb for NO2 while optimizing for a single analyte. Quasi-simultaneous measurements resulted in limits of detection of 219 ppb for NO and 164 ppb for NO2, respectively. Influences of temperature and pressure on the data evaluation are discussed, and results are compared to an established reference method based on the extractive measurements presented.

Application of a ring cavity surface emitting quantum cascade laser (RCSE-QCL) on the measurement of H 2 S in a CH 4 matrix for process analytics.

The present work reports on the first application of a ring-cavity-surface-emitting quantum-cascade laser (RCSE-QCL) for sensitive gas measurements. RCSE-QCLs are promising candidates for optical gas-sensing due to their single-mode, mode-hop-free and narrow-band emission characteristics along with their broad spectral coverage. The time resolved down-chirp of the RCSE-QCL in the 1227-1236 cm-1 (8.15-8.09 µm) spectral range was investigated using a step-scan FT-IR spectrometer (Bruker Vertex 80v) with 2 ns time and 0.1 cm-1 spectral resolution. The pulse repetition rate was set between 20 and 200 kHz and the laser device was cooled to 15-17°C. Employing 300 ns pulses a spectrum of ~1.5 cm-1 could be recorded. Under these laser operation conditions and a gas pressure of 1000 mbar a limit of detection (3σ) of 1.5 ppmv for hydrogen sulfide (H2S) in nitrogen was achieved using a 100 m Herriott cell and a thermoelectric cooled MCT detector for absorption measurements. Using 3 µs long pulses enabled to further extend the spectral bandwidth to 8.5 cm-1. Based on this increased spectral coverage and employing reduced pressure conditions (50 mbar) multiple peaks of the target analyte H2S as well as methane (CH4) could be examined within one single pulse.

New particle formation above a simulated salt lake in aerosol chamber experiments

Environmental context Deforestation in Western Australia beginning in the mid-19th century led to a considerable change of the land surface, and Western Australia is now suffering more often from droughts. Particle formation induced by salt lakes has been identified as a potential control factor for changed precipitation patterns. This study aims to determine key factors involved in the particle formation process by simulating a simplified salt lake in an aerosol chamber in the laboratory. Abstract In recent field experiments, particle formation has been observed above salt lakes in Western Australia and related to changes in regional precipitation patterns. This work investigates the particle formation potential above a simulated salt lake in aerosol chamber experiments under various conditions. The salt lake mixture comprised fixed concentrations of NaBr, NaCl and Na2SO4, and varying concentrations of FeSO4 and FeCl3. Further, an organic mixture of 1,8-cineol and limonene was added under dark and light conditions. Both the presence of organic compounds and of light were found to be essential for new particle formation in our experiments. There were clear indications for conversion of FeII to FeIII, which suggests a Fenton-like reaction mechanism in the system. Contrary to the idea that a Fenton-like reaction mechanism might intensify the oxidation of organic matter, thus facilitating secondary organic aerosol formation, the observed particle formation started later and with lower intensity under elevated FeII concentrations. The highest particle number concentrations were observed when excluding FeII from the experiments. Chemical analysis of the formed aerosol confirmed the important role of the Fenton-like reaction for particle formation in this study. Ultrahigh-resolution mass spectrometry and Raman spectroscopy provide analytical proof for the formation of organosulfates and halogenated organic compounds in the experiments presented. Even though halogens and organic precursors are abundant in these experimental simulations, halogen-induced organic aerosol formation exists but seems to play a minor overall role in particle formation.

Circular multireflection cell for optical spectroscopy.

We constructed a circular multireflection (CMR) cell, allowing multireflection around the center of the cell. This is caused by a skewed adjustment of the entering beam (equivalent to a simple parallel shift/offset), avoiding the center of the cell, thus leading to multiple reflections. The experimental setup with a cell with an inner diameter of 6 cm showed up to 17.5 beam passes on polished aluminum and attained path lengths up to 105 cm, demonstrated by Fourier transform infrared measurements of CO(2) gas between 2283 and 2400 cm(-1). The circular concept, i.e., the centering of the reflections, is useful for absorption spectroscopy on trace gases and aerosols. The optical alignment of the cell can completely be performed from outside the experimental setup, e.g., an aerosol flow reactor or a vacuum system. The variation of the path length is easily possible by adjusting the position of the cell with respect to the entering light beam.

In situ formation of reduced graphene oxide structures in ceria by combined sol–gel and solvothermal processing

Raman and IR investigations indicated the presence of reduced graphene oxide (rGO)-like residues on ceria nanoparticles after solvothermal treatment in ethanol. The appearance of such structures is closely related to cerium tert-butoxide as precursor and ethanol as solvothermal solvent. The rGO-like residues improve the catalytic CO oxidation activity. This was also confirmed by introduction of “external” graphene oxide during sol–gel processing, by which the rGO structures and the catalytic activity were enhanced.