Andreas Held

The canopy horizontal array turbulence study

The Canopy Horizontal Array Turbulence Study (CHATS) took place in spring 2007 and is the third in the series of Horizontal Array Turbulence Study (HATS) experiments. The HATS experiments have been instrumental in testing and developing subfilterscale (SFS) models for large-eddy simulation (LES) of planetary boundary layer (PBL) turbulence. The CHATS campaign took place in a deciduous walnut orchard near Dixon, California, and was designed to examine the impacts of vegetation on SFS turbulence. Measurements were collected both prior to and following leafout to capture the impact of leaves on the turbulence, stratification, and scalar source/sink distribution. CHATS utilized crosswind arrays of fast-response instrumentation to investigate the impact of the canopy-imposed distribution of momentum extraction and scalar sources on SFS transport of momentum, energy, and three scalars. To directly test and link with PBL parameterizations of canopy-modified turbulent exchange, CHATS also included a 30-m profile ...

Chemical classes of atmospheric aerosol particles at a rural site in Central Europe during winter

Ambient atmospheric particles were studied at an ecosystem research site in the Fichtelgebirge mountains in Central Europe by single-particle analysis and bulk impactor measurements. Fuzzy clustering analysis of mass spectra ofindividual aerosol particles allowed chemical classi7cation ofthe atmospheric aerosol. During the campaign, inorganic salts, mineral particles, and carbonaceous particles, with varying degrees ofsecondary components, were identi7ed. These chemical classes exhibited preferential size ranges leading to a characteristic pattern ofrelative abundances with respect to particle size. A more detailed analysis revealed that 65 –80% of all particles were assigned almost exclusively to one chemical class. These particle populations are assumed to be externally mixed with respect to the identi7ed chemical classes. The temporal variations ofthe ratio ofnitrate to ammonium (ranging between 0.37 and 0.81) determined by both impactor measurements and single-particle analyses were in good agreement. Through Monte-Carlo-type simulations, con7dence intervals ofthe mean NO − =NH + ratio were calculated for sub-samples of the total particle population. ? 2002 Elsevier

Towards direct measurement of turbulent vertical fluxes of compounds in atmospheric aerosol particles

] Vertical fluxes of gaseous and particulate compoundsin the planetary boundary layer are mainly establishedthrough turbulence. To date, it is a challenge to measurevertical fluxes of particles and, even more, the fluxes ofcompounds in particulates. The combination of disjuncteddy sampling and time-of-flight MS single particleanalysis bears the potential to directly measure theturbulent particle flux together with the chemical particlecomposition. Several obstacles must be overcome beforethis goal may be achieved. In this paper, we present astatistical procedure, using Monte-Carlo-type simulations,to obtain the flux direction of particulate compounds suchas nitrate. A first experimental application of this methodyielded emission of particulate NO

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.

A Computational Approach for the Identification of Small GTPases Based on Preprocessed Amino Acid Sequences

The prediction of essential biological features based on a given protein sequence is a challenging task in computational biology. To limit the amount of in vitro verification, the prediction of essential biological activities gives the opportunity to detect so far unknown sequences with similar properties. Besides the application within the identification of proteins being involved in tumorigenesis, other functional classes of proteins can be predicted. The prediction accuracy depends on the selected machine learning approach and even more on the composition of the descriptor set used. A computational approach based on feedforward neural networks was applied for the prediction of small GTPases. Consequently, this was realized by taking secondary structure and hydrophobicity information as a preprocessing architecture and thus, as descriptors for the neural networks. We developed a neural network cluster, which consists of a filter network and four subfamily networks. The filter network was trained to identify small GTPases and the subfamily networks were trained to assign a small GTPase to one of the subfamilies. The accuracy of the prediction, whether a given sequence represents a small GTPase is very high (98.25%). The classifications of the subfamily networks yield comparable accuracy. The high prediction accuracy of the neural network cluster developed, gives the opportunity to suggest the use of hydrophobicity and secondary structure prediction in combination with a neural network cluster, as a promising method for the prediction of essential biological activities.

A Thermal Desorption Chemical Ionization Ion Trap Mass Spectrometer for the Chemical Characterization of Ultrafine Aerosol Particles

The development of a thermal desorption chemical ionization ion trap mass spectrometer for the chemical characterization of ultrafine aerosol particles is reported and first experimental results are presented. Atmospheric particles are size-classified and collected using a unipolar charger, a radial differential mobility analyzer and an electrostatic precipitator, and analyzed after thermal desorption and chemical ionization using an ion trap mass spectrometer. Integration of an ion trap mass spectrometer allows for fast scans of the entire mass spectrum every 0.5 s and bears the potential to identify unknown particulate compounds by tandem mass spectrometry. Particle collection efficiencies range from 90–100% for 25 nm particles to about 50% for 40 nm particles. In the current configuration, the absolute sensitivity of the instrument with regard to ammonium is in the range of 10–100 pg NH + 4 . In ambient samples collected in the Colorado Front Range, NH + 4 was the major signal peak in the positive ion spectrum, and additional minor signals and peak patterns of organic compounds including methylamine were found.

Evaluation and Application of an Electrical Low Pressure Impactor in Disjunct Eddy Covariance Aerosol Flux Measurements

A micrometeorological application of an electrical low pressure impactor (ELPI) for the measurement of size-resolved particle fluxes between the surface and the atmosphere is proposed. Particles are introduced into the system for a very short time period through an inlet valve and analyzed in distinct size bins with the ELPI instrument for several seconds. Together with sonic anemometer measurements, size-resolved deposition velocities can be derived employing disjunct eddy covariance (DEC). Laboratory experiments studying ELPI instrument response indicate that a sampling interval of 0.3 s and a measurement interval of at least 5 s yield particle concentration data comparable to continuous sampling, and suitable for DEC. Calculations and Monte-Carlo simulations show that the finite length of the average period as well as Poisson and instrumental noise frequently introduce uncertainties in deposition velocities which may be an order of magnitude larger than the measurement value. In first field measurements with a prototype system, the measured fluxes were often below the detection limit as defined by the measurement uncertainties. At high particle concentrations, the DEC measurements could potentially extend current measurement capabilities to cover submicron particles fluxes in several size classes simultaneously.

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.

The Emissions of Biogenic Volatile Organic Compounds (BVOC) and Their Relevance to Atmospheric Particle Dynamics

The exchange of carbon compounds between the earth’s surface and the atmosphere is a key process in global change. The sources and sinks of CO2 and CH4 are major triggers for the greenhouse gas budget of the atmosphere. The importance of carbon sequestration in terrestrial ecosystems is still one of the open questions in efforts to quantify key processes within the global biogeochemical carbon cycle. Vice versa, the exchange of C between land surface and the atmosphere is highly sensitive to global change. Changes of solar insulation, temperature, humidity and precipitation will affect the C fluxes in ecosystems. Secondary effects such as changed species interaction through land-use change and the deposition of toxic substances into ecosystems are also important. In the short term, major feedback loops include effects of stomatal control on the uptake of CO2 and the loss of water by the vegetation as a result of the direct coupling of gas exchange to the energy input into ecosystems. On longer time scales (years to decades and centuries), additional feedbacks on biogeochemical cycles play an important role due to the time lag between, e.g., vegetation growth (C storage) and C loss via organic matter degradation, or vegetation extinction through extreme events (fire, her-bivory), resulting in changed dynamics of the vegetation cover including its management.

Ion – particle interactions during particle formation and growth at a coniferous forest site in central Europe

13 In this work, we examined the interaction of ions and neutral particles during atmospheric 14 new particle formation (NPF) events. The analysis is based on simultaneous field 15 measurements of atmospheric ions and total particles using a neutral cluster and air ion 16 spectrometer (NAIS) across the diameter range 2 25 nm. The “Waldstein” research site is 17 located in a spruce forest in NE Bavaria, Southern Germany, known for enhanced radon 18 concentrations, presumably leading to elevated ionization rates. Our observations show that 19 the occurrence of the ion nucleation mode preceded that of the total particle nucleation mode 20 during all analysed NPF events. The time difference between the appearance of 2 nm ions and 21 2 nm total particles was typically about 20 to 30 minutes. A cross correlation analysis showed 22 a rapid decrease of the time difference between the ion and total modes during the growth 23 process. Eventually, this time delay vanished when both ions and total particles did grow to 24 larger diameters. Considering the growth rates of ions and total particles separately, total 25 particles exhibited enhanced growth rates at diameters below 15 nm. This observation cannot 26 be explained by condensation or coagulation, because these processes would act more 27 efficiently on charged particles compared to neutral particles. To explain our observations, we 28