A Study of Hygroscopic Seeding Agents: Characterization of Relevant CCN Ability and MRI Cloud Chamber Experiments

As a part of “advanced study on precipitation enhancement in arid and semi-arid regions” is supported by The United Arab Emirates Research Program for Rain Enhancement Science (UAEREP) , we have investigated two promising techniques for delivering efficient cloud condensation nuclei (CCN) that can be applied to warm-based clouds using airborne platform; salt micro-powder (MP) and pyrotechnic (hygroscopic flare) methods. The main composition of MP is pure NaCl, but including the insoluble particles to prevent the NaCl particles from clumping. The insoluble particles were composed of 5% SiO₂ and CaCO₃ by weight. The Ca₃(PO₄)₂ particles were also used instead of CaCO₃ in the in the previous research. Physico-chemical properties, such as size distribution, cloud condensation nuclei and/or ice nuclei (IN) abilities of MP particles with and without anti-caking agents and hygroscopic flare particles were measured by using CCN counter, IN counter and other aerosol instruments, especially focusing on hygroscopic parameter κ and temperature-deterministic ice nucleation active surface site (INAS) density for each particle size mode. For anti-caking agents, such as SiO₂, CaCO₃ and Ca₃(PO₄)₂, their κ-value were around 0.01, which are comparable to that of surrogates of mineral dust particles, and smaller than typical κ-value of atmospheric aerosols. Though, depending on their size distributions, anti-caking agents may not act as effective CCNs due to a large difference in κ-value compared to pure NaCl and/or (NH₄)₂SO₄, whose κ-value are 1.28 and 0.61, respectively (Petters and Kreidenweis, 2007). We also update κ-value of hygroscopic flare which consist of KCl, CaCl₂ and their internal mixed with metal-oxide.

The evolution of initial droplet size distribution in clouds seeded and unseeded by flare particles and MP particles were investigated in MRI cloud simulation chamber. The experiments showed that hygroscopic flare seeding tended to further increase droplet concentrations, which is consistent with numerical simulation results (Kuba and Murakami, 2012). On the other hand, MP seeding did not show any significant change in droplet number concentration, being not consistent with numerical simulation results. In those experiments, MP with anti-caking agents was used because it is much easier to handle than pure MP. To reexamine the effectiveness of MP seeding and hopefully optimize the relative amount of anti-caking agents, we have revisited MP seeding experiments with and without anti-caking agents. As a result of cloud formation experiment using without anti-caking agents (pure NaCl) , approximately 100s after the onset of cloud formation, the number concentration of cloud droplets larger than 2 μm measured with both the CAS and Welas-OPC attained a peak value, which was almost the same as CN concentration. The number of cloud droplets larger than 10 μm has kept increasing for a while, and reached nearly 1,000 cm⁻³ at 300sec of elapsed time. For experiment of MP with anti-caking agents, the number concentration of cloud droplets larger than 20μm was less than in the case of pure NaCl, in spite of the similarities of the concentration of droplet smaller than 10 μm. It was an evidence that MP without anti-caking agents causes relatively more intensive broadening of the cloud droplet spectrum as compared to MP with anti-caking agents. The differences in seeding effects of MP with and without anti-caking agent on cloud droplet size distribution activated on background CCN ((NH₄)₂SO₄) have been investigated to quantitatively understand. The size distribution of cloud droplets measured in the chamber experiments will be directly compared with those simulated by the detailed bin microphysics parcel model, which is based on the parcel model of Chen and Lamb (1994) and extended to implement the κ-Köhler theory (Yamashita et al, 2011).