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| Hygroscopicity Tandem Differential Mobility Analyzer | |
|---|---|
| Name | Hygroscopicity Tandem Differential Mobility Analyzer |
| Type | Aerosol hygroscopicity analyzer |
| Related | Differential Mobility Analyzer |
Hygroscopicity Tandem Differential Mobility Analyzer A Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA) is an aerosol instrument used to determine the water uptake behavior of size-selected particles by measuring changes in mobility diameter as a function of relative humidity. The HTDMA couples mobility selection and controlled humidification to probe aerosol composition and mixing state, yielding parameters that are widely applied in atmospheric chemistry, climate science, and air quality studies. The technique has been deployed in field campaigns, laboratory experiments, and regulatory monitoring to relate hygroscopic growth to radiative forcing, cloud formation, and health impacts.
The HTDMA concept integrates elements from the Differential Mobility Analyzer lineage with humidification stages developed in laboratory aerosol science and field campaigns such as ACE-1, ACE-2, IMPACT, and Aerosol IOP. It provides a traceable route from physical quantities measured by instruments like the Condensation Particle Counter, Aerosol Mass Spectrometer, and Scanning Mobility Particle Sizer to derived hygroscopicity metrics used by modeling centers including the European Centre for Medium-Range Weather Forecasts, the National Center for Atmospheric Research, and the United States Environmental Protection Agency. HTDMA results inform parameterizations in global models operated by institutions such as NASA, NOAA, and the Met Office.
HTDMA operation relies on electrodynamic mobility selection followed by controlled humidification and remeasurement. A sheath-flow Differential Mobility Analyzer selects particles of a target electrical mobility corresponding to a dry mobility diameter; the monodisperse aerosol then enters a humidification flow where water uptake drives growth governed by thermodynamic models originally developed in the tradition of Köhler theory and refined with inputs from experiments by groups affiliated with MIT, Scripps Institution of Oceanography, University of Helsinki, and Max Planck Institute for Chemistry. After humidification, a second mobility selection determines the wet mobility diameter. The observed growth factor is compared to predictions from solute activity models and mixing rules used in studies by IPCC contributors and aerosol thermodynamics groups at Georgia Tech and ETH Zurich.
Typical HTDMA components combine hardware and control systems familiar to aerosol laboratories at institutions like CERN (for environmental control analogies), Brookhaven National Laboratory, and Los Alamos National Laboratory. Key elements include: - An inlet and aerosol neutralizer often employing principles from devices used at NIST and NPL. - A first-stage Differential Mobility Analyzer for size selection, based on designs standardized in workshops hosted by WMO and EUSAAR. - A humidifier or conditioning system using temperature control strategies similar to those at Purdue University and University of Colorado Boulder. - A second-stage Differential Mobility Analyzer and detection by a Condensation Particle Counter or equivalent detectors from manufacturers linked with CPC instrumentation used at NOAA ESRL. - Data acquisition and control software inspired by systems at Lawrence Berkeley National Laboratory and Argonne National Laboratory, with calibration components traceable to standards set by ISO committees.
Protocols for HTDMA measurements derive from intercomparison exercises coordinated by groups at EUCAARI, ACTRIS, and national networks such as AERONET partners. Typical protocols specify conditioning times, humidifier geometries, residence times, and relative humidity set points informed by studies from University of Manchester, University of Copenhagen, and Tohoku University. Data analysis workflows convert mobility spectra to growth factors using inversion techniques employed in SMPS analyses and adopt hygroscopicity parameterizations (e.g., κ-Köhler) developed by researchers at University of Hertfordshire, University of Reading, and University of Vienna. Uncertainty propagation methods draw on standards used by ISO/IEC and statistical approaches promoted in workshops at Oxford University and Harvard University.
HTDMA measurements have been central to case studies linking aerosol hygroscopicity to processes observed in field campaigns such as CalNex, MILAGRO, ACE-Asia, ARM Mobile Facility deployments, and urban studies in Beijing, Los Angeles, and London. Applications include partitioning of external versus internal mixtures in work by teams at Swiss Federal Institute of Technology Lausanne, source apportionment in studies by CSIC groups, and evaluation of cloud condensation nuclei parameterizations used by ECMWF and GFDL. Laboratory investigations at facilities like Aerosol Dynamics Inc. and university environmental chambers in Tokyo and Seoul have used HTDMA to study secondary organic aerosol formation linked to research from Caltech, Columbia University, and University of California, Berkeley.
Calibration approaches for HTDMA follow intercomparisons organized by ACTRIS and traceability recommendations from agencies including NIST and CEN. Uncertainties arise from sheath flow stability, humidity control accuracy, particle charging distributions, and mobility inversion algorithms—issues explored in interlaboratory studies involving MPI-C Mainz, University of Helsinki, and Potsdam Institute for Climate Impact Research. Limitations include reduced sensitivity for transient or highly inhomogeneous mixtures, challenges with semi-volatile components as highlighted in studies at Argonne National Laboratory and Lawrence Livermore National Laboratory, and assumptions in mixing rules critiqued by researchers at University of California, Irvine and Princeton University.
HTDMA is often compared with cloud chamber CCN counters developed at University of Manchester and University of Vienna, optical hygrometers used in campaigns by NOAA, and single-particle techniques such as aerosol optical tweezer experiments at University of Bristol and University of Georgia. While CCN instruments provide activation spectra tied to cloud formation relevant to IPCC assessments, HTDMA yields size-resolved equilibrium growth factors that complement chemical composition insights from Aerosol Mass Spectrometer deployments at Purdue University and Yale University. Each method has trade-offs in temporal resolution, size selectivity, and interpretive frameworks used by modelers at Princeton University and NCAR.
Category:Aerosol instrumentation