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

Abstract

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.

Perspectives on the Future of Ice Nucleation Research: Research Needs and Unanswered Questions Identified from Two International Workshops

“…Sampling in-cloud with selective inlets, such as counterflow virtual impactors (CVIs), are helpful in reducing sampling error by separating cloud particle residuals from interstitial aerosol [204]. Additionally, recent development of an ice-selective CVI (i.e., that separates interstitial aerosol, supercooled liquid droplets, and ice crystals) can enable more detailed characterization of ice residuals by segregating cloud liquid from ice [205].…”

Activation of Intact Bacteria and Bacterial Fragments Mixed with Agar as Cloud Droplets and Ice Crystals in Cloud Chamber Experiments

“…The detection size ranges of the two OPCs, referred to as welas1 and welas2, are ~0.7 to 46 µm and ~5 to 240 µm, respectively (Wagner and Möhler, 2013). The simultaneous use of these two OPCs effectively spans the size range of droplets and ice 20 crystals formed in the chamber.…”

“…A pumped counterflow virtual impactor (PCVI) (Boulter et al, 2006) and an ice-selecting PCVI (IS-PCVI) (Hiranuma et al, 2016) were used to selectively sample cloud droplet and ice crystals, the residuals of which were counted with the CPC and characterized by miniSPLAT and other instruments. PCVIs use a counterflow to reject smaller, interstitial particles, while particles that have enough inertia to pass through the counterflow are transmitted through the inlet.…”

“…Most importantly, it was used to characterize the residuals of liquid droplets and ice crystals formed during the expansions. Cloud particles were separated from interstitial particles with a pumped counterflow virtual impactor (PCVI) or an ice-selecting PCVI (IS-PCVI) (Hiranuma et al, 2016). Comparison 20 between the properties of cloud residuals with the overall particle population in the chamber before and after the expansion allows for the determination of cloud and ice active versus interstitial particles.…”

Immersion Freezing of Total Ambient Aerosols and Ice Residuals

“…Previously, many ground-and airborne-based studies have collected and characterized the nature of IRs using real-time and offline analysis techniques [24][25][26][27][28][29][30][31][32][33][34][35][36]. For example, DeMott et al (2003) [35] utilized a ice nucleation chamber to induce ice nucleation at cirrus cloud temperature conditions on ambient aerosols, and these ice crystals were separated to obtain IRs.…”