Development of the Horizontal Cloud Condensation Nuclei Counter (HCCNC) to Detect Particle Activation Down to 4 °C Temperature and 0.05% Supersaturation

Aerosol particles play a critical role as cloud condensation nuclei (CCN) in the atmosphere. The capacity of aerosol particles to activate into cloud droplets is measured experimentally using CCN counters (CCNCs). Recent findings suggest that the co-condensation effect of semi-volatiles can enhance aerosol particle growth and cloud droplet activation. Conventional CCNCs, such as the streamwise CCNC, heat particles (>30°C) as they transit the CCNC column and may inadvertently not capture the co-condensation effect, leading to an underestimate in CCN concentrations. Additionally, streamwise CCNCs struggle to achieve supersaturations (SS) below 0.13%. In pristine marine environments like the Southern Ocean, where particles are highly hygroscopic (κ≈0.9), getting reliable activation (i.e., critical supersaturation) of particles above 120 nm (i.e., the accumulation mode) could be challenging. This could result in 'activation blindness,' preventing precise CCN characterization of these climatically relevant particles.

To address these limitations, we developed the Horizontal CCNC (HCCNC), which can generate SS at temperatures down to 4 °C and SS level to 0.05%. This capability provides researchers with a unique platform to investigate the co-condensation effect, enabling studies that test the hypothesis that preserving semi-volatile fractions at atmospherically relevant temperatures may significantly enhance droplet activation. Furthermore, the ability to achieve stable SS down to 0.05% extends the observational window to include larger CCN (200 nm for pure ammonium sulfate) and highly hygroscopic particles characteristic of pristine marine environments like the Southern Ocean, as well as those used in weather modification and cloud seeding. The HCCNC also addresses operational inefficiencies inherent in current technology: streamwise CCNCs suffer from thermal inertia, requiring minutes to stabilize new SS setpoints, resulting in measurement dead time and data loss. In contrast, the HCCNC demonstrates rapid thermal response, enabling a “Flash Scan” capability that spans 0.05% to 0.8% SS in under one minute, combined with a modular, user-friendly design.

This study presents the development of the HCCNC, providing a detailed technical description of its 3D geometry, computational fluid dynamics simulations, and the key components that demonstrate its performance. Sampling and humidity generation followed the principle of the previously used continuous-flow thermal-gradient diffusion chambers. The instrument’s performance is validated by conducting laboratory tests using ammonium sulfate ((NH₄)₂SO₄) particles in the size range between 50 and 200 nm and for temperatures between 30 and 8 °C. To ensure these advancements are accessible to the wider scientific community, the HCCNC technology has been patent-filed, and commercialization efforts are currently underway to allow researchers to fully leverage its potential.