Journal: Atmospheric Measurement Techniques Discussions

Abbreviation

Atmos. meas. tech., Pap. open discuss.

Publisher

Copernicus

Journal Volumes

ISSN

1867-8610

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2009 - 200912010 - 20195
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Publications1 - 6 of 6
  • A Convolutional Neural Network for Classifying Cloud Particles Recorded by Imaging Probes
    Item type: Working Paper
    Touloupas, Georgios; Lauber, Annika; Henneberger, Jan; et al. (2019)
    Atmospheric Measurement Techniques Discussions
    During typical field campaigns, millions of cloud particle images are captured with imaging probes. Our interest lies in classifying these particles in order to compute the statistics needed for understanding clouds. Given the large volume of collected data, this raises the need for an automated classification approach. Traditional classification methods that require extracting features manually (e.g. decision trees and support vector machines) show reasonable performance when trained and tested on data coming from a unique dataset. However, they often have difficulties to generalize to test sets coming from other datasets where the distribution of the features might be significantly different. In practice, we found that each new dataset requires labeling a huge amount of data by hand using those methods. Convolutional neural networks have the potential to overcome this problem due to their ability to learn complex non-linear models directly from the images instead of pre-engineered features, as well as by relying on powerful regularization techniques. We show empirically that a convolutional neural network trained on cloud particles from holographic imagers generalizes well to unseen datasets. Moreover, fine-tuning the same network with a small number (256) of training images improves the classification accuracy. Thus, the automated classification with a convolutional neural network not only reduces the hand-labeling effort for new datasets but is also no longer the main error source for the classification of small particles.
  • Flux correction for closed-path laser spectrometers without internal water vapor measaurements
    Item type: Working Paper
    Hiller, Rebecca V.; Zellweger, Christoph; Knohl, Alexander; et al. (2012)
    Atmospheric Measurement Techniques Discussions
    Recently, instruments became available on the market that provide the possibility toperform eddy covariance flux measurements of CH4and many other trace gases, in-cluding the traditional CO2and H2O. Most of these instruments employ laser spec-troscopy, where a cross-sensitivity to H2O is frequently observed leading to an increased dilution effect. Additionally, sorption processes at the intake tube walls modifyand delay the observed H2O signal in closed-path systems more strongly than the sig-nal of the sampled trace gas. Thereby, a phase shift between the trace gas and H2Ofluctuations is introduced that dampens the H2O flux observed in the sampling cell. Forinstruments that do not provide direct H2O measurement in the sampling cell, transfer functions from externally measured H2O fluxes are needed to estimate the effect ofH2O on trace gas flux measurements. The effects of cross-sensitivity and the damp-ing are shown for an eddy covariance setup with the Fast Greenhouse Gas Analyzer(FGGA, Los Gatos Research Inc.) that measures CO2, CH4, and H2O fluxes. Thisinstrument is technically identical with the Fast Methane Analyzer (FMA, Los Gatos Research Inc.) that does not measure H2O concentrations. Hence, we used mea-surements from a FGGA to derive a modified correction for the FMA accounting fordilution as well as phase shift effects in our instrumental setup. With our specific setupfor eddy covariance flux measurements, the cross-sensitivity counteracts the dampingeffects, which compensate each other. Hence, the new correction only deviates very slightly from the traditional Webb, Pearman, and Leuning density correction, which iscalculated from separate measurements of the atmospheric water vapor flux
  • Effective density of Aquadag and fullerene soot black carbon reference materials used for SP2 calibration
    Item type: Working Paper
    Gysel, Martin; Laborde, Marie J.G.; Olfert, Jason S.; et al. (2011)
    Atmospheric Measurement Techniques Discussions
    The mass and effective density of black carbon (BC) particles generated from aque-ous suspensions of Aquadag and fullerene soot was measured and parametrized asa function of their mobility diameter. The measurements were made by two indepen-dent research groups by operating a differential mobility analyser (DMA) in series with an aerosol particle mass analyser (APM) or a Couette centrifugal particle mass anal-yser (CPMA). Consistent and reproducible results were found in this study for differentproduction lots of Aquadag, indicating that the effective density of these particles isa stable quantity and largely unaffected by differences in aerosol generation proce-dures and suspension treatments. The effective density of fullerene soot particles from one production lot was also found to be stable and independent of suspension treat-ments. Some difference to previous literature data was observed for both Aquadagand fullerene soot at larger particle diameters. Knowledge of the exact relationshipbetween mobility diameter and particle mass is of great importance, as DMAs arecommonly used to size-select particles from BC reference materials for calibration of single particle soot photometers (SP2), which quantitatively detect the BC mass insingle particles.
  • Pore structure 3-D imaging by synchrotron micro-tomography of graupel grains
    Item type: Working Paper
    Enzmann, Frieder; Miedaner, M.M.; Kersten, Michael; et al. (2010)
    Atmospheric Measurement Techniques Discussions
    Three dimensional air bubble structure including size distribution, concentration andspatial distribution are important clues in identifying the growth regime of graupel andhailstone. For imaging of the bubble structure, a cryo-stage was developed to adapt tothe standard setup of the SLS X04SA tomography beamline (actually replaced by the TOMCAT beamline) at the Swiss Light Source synchrotron facility to the requirementsof ice particle micro-tomography. The cryo-stage setup provides for the first time 3-D-data on the individual inner pore shape delineation down to μm spatial (voxel) resolutionof sub-mm small naturally as well as wind tunnel rimed graupel particles. Special caremust be taken for maintaining a cooling chain between sampling and measurement. It must be kept at liquid nitrogen temperature (77 K) until measurement of the originalstructure at the μm spatial scale. However, even at that temperature there is no chanceto preserve any ice bubble structure at sub-μm spatial resolution due to the Kelvineffect. In natural graupel grains, Y-shaped morphology of air-filled pores was found.This morphology transformed into smaller and rounded voids well-known from literature when the ice particle was annealed for as short as half an hour at 265K and must,therefore, be regarded as artificial rather than representing the in situ pore structure.With the new synchrotron tomography approach, quantitative information on the in situpore structure statistics within individual samples representative for a known or, thus,deduced growth mode or history can be derived, in particular if combined with airplane sampling in the troposphere at in situ growth conditions.
  • A High Speed Particle Phase Discriminator (PPD-HS) for the classification of airborne particles, as tested in a continuous flow diffusion chamber
    Item type: Working Paper
    Mahrt, Fabian; Wieder, Jörg; Dietlicher, Remo; et al. (2019)
    Atmospheric Measurement Techniques Discussions
    A new instrument, the High Speed Particle Phase Discriminator (PPD-HS) developed at the University of Hertfordshire, for sizing individual cloud hydrometeors and determining their phase is described herein. PPD-HS performs an in-situ analysis of the spatial intensity distribution of near forward scattered light for individual hydrometeors yielding shape properties. Discrimination of spherical and aspherical particles is based on an analysis of the symmetry of the recorded scattering patterns. Scattering patterns are collected onto two linear detector arrays, reducing the complete 2D scattering pattern to scattered light intensities captured onto two linear, one dimensional strips of light sensitive pixels. Using this reduced scattering information, we calculate symmetry indicators that are used for particle shape and ultimately phase analysis. This reduction of information allows for detection rates of a few hundred particles per second. Here, we present a comprehensive analysis of instrument performance using both spherical and aspherical particles, generated in a well-controlled laboratory setting using a Vibrating Orifice Aerosol Generator (VOAG) and covering a size range of approximately 3–32 micron. We use supervised machine learning to train a random forest model on the VOAG data sets that can be used to classify any particles detected by PPD-HS. Classification results show that the PPD-HS can successfully discriminate between spherical and aspherical particles, with misclassification below 5 % for diameters > 3 micro meter. This phase discrimination method is subsequently applied to classify simulated cloud particles produced in a continuous flow diffusion chamber setup. We report observations of small, near-spherical ice crystals at early stages of the ice nucleation experiments, where shape analysis fails to correctly determine the particle phase. Nevertheless, in case of simultaneous presence of cloud droplets and ice crystals, the introduced particle shape indicators allow for a clear distinction between these two classes independent of optical particle size. We conclude that PPD-HS constitutes a powerful new instrument to size and discriminate phase of cloud hydrometeors and thus study microphysical properties of mixed-phase clouds, that represent a major source of uncertainty in aerosol indirect effect for future climate projections.
  • Water vapor δ2H and δ18O measurements using off-axis integrated cavity output spectroscopy
    Item type: Working Paper
    Sturm, Patrick; Knohl, Alexander (2009)
    Atmospheric Measurement Techniques Discussions
    We present a detailed assessment of a commercially available water vapor isotopeanalyzer (WVIA, Los Gatos Research, Inc.) for simultaneous in-situ measurements ofδ2H andδ18O in water vapor. This method, based on off-axis integrated cavity outputspectroscopy, is an alternative to the conventional water trap/isotope ratio mass spectrometry (IRMS) techniques. We evaluate the analyzer in terms of precision, memoryeffects, concentration dependence, temperature sensitivity and long-term stability. Acalibration system based on ink jet technology is used to characterize the performanceand to calibrate the analyzer. Our results show that the precision at an averagingtime of 15 s is 0.16‰ forδ2H and 0.08‰ forδ18O. The isotope ratios are strongly dependent on the water mixing ratio of the air. Taking into account this concentrationdependence as well as the temperature sensitivity of the instrument we obtained along-term stability of the water isotope measurements of 0.38‰ forδ2H and 0.25‰forδ18O. The accuracy of the WVIA was further assessed by comparative measure-ments using IRMS and a dew point generator indicating a linear response in isotopic composition and H2O concentrations. The WVIA combined with a calibration systemprovides accurate high resolution water vapor isotope measurements and opens newpossibilities for hydrological and ecological applications
Publications1 - 6 of 6