A Continuous-Flow Streamwise Thermal-Gradient CCN Chamber for Atmospheric Measurements

By Gregory Charles Roberts, A. Nenes. 2005

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We have addressed the need for improved measurements of cloud condensation nuclei (CCN) by developing a continuous-flow instrument that provides in situ measurements of CCN. The design presented in this article can operate between 0.1 and 3% supersaturation, at sampling rates sufficient for airborne operation. The design constitutes a cylindrical continuous-flow thermal-gradient diffusion chamber employing a novel technique of generating a supersaturation: by establishing a constant streamwise temperature gradient so that the difference in water vapor and thermal diffusivity yield a quasi-uniform centerline supersaturation. Our design maximizes the growth rate of activated droplets, thereby enhancing the performance of the instrument. The temperature gradient and the flow through the column control the supersaturation and may be modified to retrieve CCN spectra. The principle of the CCN instrument was validated in controlled laboratory experiments at different operating conditions using a monodisperse aerosols with known composition and size. These experiments yield sharp activation curves, even for those kinetically limited particles that have not exceeded their critical diameter. The performance of the CCN instrument was also assessed using polydisperse laboratory-generated aerosol of known composition and size distributions similar to ambient particulate matter. In all tests, the measured CCN concentrations compared well with predicted values and highlight the instrument's ability to measure CCN at various size distributions. The full potential of the new design has yet to be explored; however, model simulations suggest that direct measurements in the climatically important range of supersaturations of less than 0.1% (certainly down to 0.07%) are possible. The new instrument clearly offers a unique level of design simplicity, robustness, and flexilibity (temperature control, large range of supersaturations without flow reversal, and multiple configurations for same supersaturation) necessary for atmospheric studies.

Subfields: Atmospheric Science, Global and Planetary Change, Health, Toxicology and Mutagenesis

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Citations in the corpus

Calibration and measurement uncertainties of a continuous-flow cloud condensation nuclei counter (DMT-CCNC): CCN activation of ammonium sulfate and sodium chloride aerosol particles in theory and experiment (2007, methodology): “The corresponding value reported by Roberts and Nenes (2005) was1Seff/1Q=0.06 % per 0.1 L min−1 for 1000 hPa and 8.3 K m−1, which is the same as we obtained with the CCN flow model, but significantly higher than what we determined experimentally.”
Cloud condensation nuclei in pristine tropical rainforest air of Amazonia: size-resolved measurements and modeling of atmospheric aerosol composition and CCN activity (2009, neutral): “Size resolved CCN efficiency spectra (activation curves) were measured with a Droplet Measurement Technologies continuous flow CCN counter (DMT-CCNC, Roberts and Nenes, 2005; Lance et al., 2006) coupled to a Differential Mobility Analyzer (DMA; TSI 3071; sheath flow 10 L min −1 ) and a condensation…”
Towards closing the gap between hygroscopic growth and activation for secondary organic aerosol - Part 2: Theoretical approaches (2008, methodology): “…measured at up to 99.6% relative humidity with the Leipzig Aerosol Cloud Interaction Simulator (LACIS) (Stratmann et al., 2004) and CCN activity measured with a Droplet Measurement Technologies (DMT) continuous flow CCN instrument (Roberts and Nenes, 2005; Lance et al., 2006; Rose et al., 2008).”
Particulate emissions from commercial shipping: Chemical, physical, and optical properties (2008, methodology): “The uncertainty associated with the CCN number concentrations is estimated to be less than ±10% [ Roberts and Nenes, 2005 ].”
Microbiology and atmospheric processes: the role of biological particles in cloud physics (2007, neutral): “The activation of aerosol particles as CCN can be determined by so-called cloud condensation nucleus counters Biogeosciences, 4, 1059–1071, 2007 www.biogeosciences.net/4/1059/2007/ (Delene et al., 1998; Roberts and Nenes, 2005).”
Cloud condensation nucleation activity of biomass burning aerosol (2009, methodology): “…larger than what is obtained for typical laboratory studies, likely due to larger than usual fluctuations of temperature in the Fire Science Laboratory (the supersaturation depends on the inlet temperature [Roberts and Nenes, 2005] but this dependence is not accounted for in the data processing).”
Scanning Mobility CCN Analysis—A Method for Fast Measurements of Size-Resolved CCN Distributions and Activation Kinetics (2010, methodology): “The classified aerosol is then split between a condensation particle counter (CPC, TSI 3010 or 3022a) for measurement of total aerosol (condensation nuclei, CN) concentration, and a Droplet Measurement Technologies Continuous-Flow, Streamwise Thermal-Gradient Chamber (CFSTGC) (Roberts and Nenes 2005”
Microbiology and atmospheric processes: research challenges concerning the impact of airborne micro-organisms on the atmosphere and climate (2011, neutral): “The evaluation of some of these properties, such as cloud condensation nucleation are being facilitated by the emergence of new techniques (Roberts and Nenes, 2005).”
The deposition ice nucleation and immersion freezing potential of amorphous secondary organic aerosol: Pathways for ice and mixed‐phase cloud formation (2012, methodology): “The CCN activation of size-selected SOA particles was measured with a Continuous Flow CCN Counter (CCNC, Droplet Measurement Technologies) [Roberts and Nenes, 2005; Lance et al., 2006] to derive the particle hygroscopicity parameter.”
The Arctic Summer Cloud Ocean Study (ASCOS): Overview and experimental design (2013, methodology): “The instrument used to measure the CCN number concentration was a continuousflow stream-wise thermal gradient CCN counter (CCNC) described in full detail by Roberts and Nenes (2005).”
A Method for Particle Size Amplification by Water Condensation in a Laminar, Thermally Diffusive Flow (2005, background): “Recently, Roberts and Nenes (2005) introduced a CCN counter that also utilizes a wet-walled tube with a positive axial temperature gradient, and that addresses issues of buoyancy and flow stability.”
Aerosol Mixing State: Measurements, Modeling, and Impacts (2019, background): “A critical advance in our ability to understand CCN activity was the development of the CCN counter (Hudson, 1989; Roberts & Nenes, 2005).”