Development of the Horizontal Cloud Condensation Nuclei Counter (HCCNC) to detect particle activation down to 4 °C temperature and 0.05 % supersaturation

Abstract. 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 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 below 0.13 %, limiting their applicability for studying hydrophilic particles like (NH4)2SO4 larger than 111 nm. To address these limitations, we developed the Horizontal Cloud Condensation Nuclei Counter (HCCNC), which can generate supersaturation (SS) down to 0.05 % for temperatures down to 4 °C. 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, showing accurate performance at low temperatures and SS, which the widely used, commercially available Droplet Measurement Technologies Inc. (DMT) CCNC cannot achieve. The main chamber parts were 3D-printed from an aluminium alloy. Sampling and humidity generation followed the principle of the previously used continuous-flow thermal-gradient diffusion chambers. Particles were detected using a commercially available optical particle counter (OPC; MetOne Instruments, Inc., Model 804). 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. Future work will focus on exploring the co-condensation effect on cloud droplet activation of levoglucosan and ammonium sulfate particles.