Articles | Volume 9, issue 8
Atmos. Meas. Tech., 9, 3817–3836, 2016

Special issue: Results from the ice nucleation research unit (INUIT) (ACP/AMT...

Atmos. Meas. Tech., 9, 3817–3836, 2016

Research article 18 Aug 2016

Research article | 18 Aug 2016

Development and characterization of an ice-selecting pumped counterflow virtual impactor (IS-PCVI) to study ice crystal residuals

Naruki Hiranuma1, Ottmar Möhler1, Gourihar Kulkarni2, Martin Schnaiter1, Steffen Vogt1, Paul Vochezer1, Emma Järvinen1, Robert Wagner1, David M. Bell2, Jacqueline Wilson2, Alla Zelenyuk2, and Daniel J. Cziczo3 Naruki Hiranuma et al.
  • 1Institute of Meteorology and Climate Research – Atmospheric Aerosol Research, Karlsruhe Institute of Technology, Karlsruhe, Germany
  • 2Pacific Northwest National Laboratory, Richland, WA, USA
  • 3Earth, Atmospheric and Planetary Sciences, Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA

Abstract. Separation of particles that play a role in cloud activation and ice nucleation from interstitial aerosols has become necessary to further understand aerosol-cloud interactions. The pumped counterflow virtual impactor (PCVI), which uses a vacuum pump to accelerate the particles and increase their momentum, provides an accessible option for dynamic and inertial separation of cloud elements. However, the use of a traditional PCVI to extract large cloud hydrometeors is difficult mainly due to its small cut-size diameters (< 5 µm). Here, for the first time we describe a development of an ice-selecting PCVI (IS-PCVI) to separate ice in controlled mixed-phase cloud system based on the particle inertia with the cut-off diameter  ≥  10 µm. We also present its laboratory application demonstrating the use of the impactor under a wide range of temperature and humidity conditions. The computational fluid dynamics simulations were initially carried out to guide the design of the IS-PCVI. After fabrication, a series of validation laboratory experiments were performed coupled with the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) expansion cloud simulation chamber. In the AIDA chamber, test aerosol particles were exposed to the ice supersaturation conditions (i.e., RHice > 100 %), where a mixture of droplets and ice crystals was formed during the expansion experiment. In parallel, the flow conditions of the IS-PCVI were actively controlled, such that it separated ice crystals from a mixture of ice crystals and cloud droplets, which were of diameter  ≥  10 µm. These large ice crystals were passed through the heated evaporation section to remove the water content. Afterwards, the residuals were characterized with a suite of online and offline instruments downstream of the IS-PCVI. These results were used to assess the optimized operating parameters of the device in terms of (1) the critical cut-size diameter, (2) the transmission efficiency and (3) the counterflow-to-input flow ratio. Particle losses were characterized by comparing the residual number concentration to the rejected interstitial particle number concentration. Overall results suggest that the IS-PCVI enables inertial separation of particles with a volume-equivalent particle size in the range of  ~ 10–30 µm in diameter with small inadvertent intrusion (~  5 %) of unwanted particles.

Short summary
A new pumped counterflow virtual impactor (PCVI) called the ice-selecting PCVI (IS-PCVI) has been developed to collect ice crystal residuals for investigating physico-chemical properties of ice-nucleating particles. The results show that the ice crystals of volume-equivalent diameter ~ 10 to 30 µm can be efficiently separated from the supercooled droplets and interstitial particles. The IS-PCVI is efficient when the counterflow-to-input flow ratio is within 0.09 to 0.18.