Two methods for the laboratory-scale formation of aerosols of nanosized particles of precipitated calcium carbonate (PCC), for potential use in Stratospheric Aerosol Injection (SAI), a Solar Radiation Management (SRM) technique, are described. The first uses the coarse fluidization of bulk PCC in a simple vessel, followed by dispersal using a commercially available two-fluid nozzle. The manufacturer’s measured particle mass distribution for the bulk material, and sprayed aerosol particle mass distributions are compared, indicating that the sprayed particles are well separated in spite of a notoriously problematic agglomeration tendency. The method is suitable for scale-up. A second dispersal method, useful for small laboratory experiments, using liquid carbon dioxide as a dispersant as well as spray propellant gave similar results. The mass mode diameters measured here (0.89 to 1.4 μm) differ from that stated by the manufacturer (0.7 μm), but the distributions are consistent in showing complete separation of the particles.

Marine Cloud Brightening (MCB) by effervescent spray atomization of mixed sea water brine with air is a candidate for solar radiation management to compensate for global warming. We discovered that the flow from mixing tee nozzle described earlier had occasional unstable slug flow. A new design that adding rotational swirl to the salt brine as it is mixed into the air stabilized the nozzle flow and no longer showed slug flow in spray pictures. Flow equations were developed for the relatively low speed of sound of a choked flow mixed brine and air nozzle. Experimental mixed flow measurements with 300b pressure and a 200 μm diameter nozzle and calculations using perfect gas, and isotropic processes equations compared well with the chocked flow equations. Analysis in EXCEL of particle sizers measurements from both a scanning mobility particle sizer (SMPS) and an aerodynamic particle sizer (APS) showed production of many nanometer sized particles estimated as usable for MCB. A small number of micron sized particles were also always present but with about 90% of the sprayed mass. This is a first report with good data over the complete size range. The micron sized particles measured were similar to the measurements of earlier reports which reported no nanometer sized particles. We hypothesize that many nano-particles are always produced by liquid-air effervescent sprays, but earlier, were not observed because SMPS instruments were not available. The presence of the large mass percentage of large particles in the spray may cause problems by evaporative cooling preventing the rise of the MCB particles. We suggest future systems design with an impactor filter to remove the large particles. Calculations combining increased brine concentration, lower pressure, and larger nozzle area showed that significant reductions in required power and number of nozzles could be realized. An EXCEL model is developed to calculate flow from experimental analysis equations and compare with mixed choked flow equations. Solving with the model predicted the power required and the number of nozzles required to produce 1015 particles/s. The model showed that increasing brine concentration strongly lowered total power. Lowering pressure decreased power and increased number of nozzles. Increasing nozzle area lowered the number of nozzles. This model predicted that, at 300b pressure and 200μm diameter nozzle as the experiment but using an increased brine concentration of 0.1 instead of 0.032 would require only 115 nozzles instead of 358 and power of 146 kw instead of 493 kw. Combining increased brine concentration, lower pressure, and larger nozzle area, the model predicted that with a 1 mm diameter nozzle at 30b pressure and salt concentration of 0.2, the nozzle count and power required would drop to only 24 nozzles and power of 28 kw. Whether extending the model to these conditions is valid is not known but suggests further development should be investigated. Filtering out and reusing the 90% or greater large particles mass sprayed combined with the lower power advantage of higher brine concentration is suggested for future systems.

The large-scale production of vast numbers of suitable salt nuclei and their upward launch is one of the main technological barriers to the experimental testing of marine cloud brightening (MCB). Very promising, though not definitive, results have been obtained using an adapted version of effervescent spray atomization. The process is simple, robust and inexpensive. This form of effervescent spraying uses only pressurized water and air sprayed from small nozzles to obtain very fine distributions. While it is far from optimized, and may not be the best method if full deployment is ever desired, we believe that even in its present form the process would lend itself well to preliminary field test investigations of MCB. Measurements obtained using standard aerosol instrumentation show approximately lognormal distributions of salt nuclei with median diameters of approximately 65 nm and geometric standard deviations slightly less than 2. However, these measurements are not in agreement with those based on scanning electron microscopy imaging of collected particles, an observation that has not yet been explained. Assuming the above distribution, 10 15 particles per second could be made with 21 kW of spray power, using approximately 200 nozzles. It is envisioned that existing snow making equipment can be adapted to launch the nuclei 60–100 m into the air, requiring approximately 20 kW of additional power.

Marine Cloud Brightening (MCB), should it ever need to be deployed, envisions the formation of 10 17 salt Cloud Condensation Nuclei (CCN) per second coming from each of several thousand vessels deployed worldwide.The creation of this many nuclei on such a vast scale, from micron-or submicron-sized seawater droplets, preferably mono-disperse, poses a considerable engineering challenge.Various existing or experimental spray methods were investigated for feasibility, resulting in the identification of a few with promising results.Electro-spraying from Taylor cone-jets, using either silicon micromachined long capillaries or short capillary polymer substrates attached to a porous substrate, appears to have the best potential for implementation of all the methods that have been investigated so far.

We review in some detail research into experimental spray methods for the production of salt nanoparticles from seawater, which would be needed for the implementation of Marine Cloud Brightening (MCB), a possible climate change mitigation method. We describe first an electrospray method employing Taylor cones spraying seawater, which fortuitously has the right conductivity to produce salt particles in the desired size range (40-100 nm equiv. sphere diameter). A second method involves spraying supercritical salt solutions to get particles in the same size range, as the absence of surface tension yields sprays that are extremely fine. Finally, we describe briefly our ongoing efforts to use effervescent sprays at very high pressure to obtain the desired particle size distribution (PSD).

The idea behind the marine cloud-brightening (MCB) geoengineering technique is that seeding marine stratocumulus clouds with copious quantities of roughly monodisperse sub-micrometre sea water particles might significantly enhance the cloud droplet number concentration, and thereby the cloud albedo and possibly longevity. This would produce a cooling, which general circulation model (GCM) computations suggest could-subject to satisfactory resolution of technical and scientific problems identified herein-have the capacity to balance global warming up to the carbon dioxide-doubling point. We describe herein an account of our recent research on a number of critical issues associated with MCB. This involves (i) GCM studies, which are our primary tools for evaluating globally the effectiveness of MCB, and assessing its climate impacts on rainfall amounts and distribution, and also polar sea-ice cover and thickness; (ii) high-resolution modelling of the effects of seeding on marine stratocumulus, which are required to understand the complex array of interacting processes involved in cloud brightening; (iii) microphysical modelling sensitivity studies, examining the influence of seeding amount, seed-particle salt-mass, air-mass characteristics, updraught speed and other parameters on cloud-albedo change; (iv) sea water spray-production techniques; (v) computational fluid dynamics studies of possible large-scale periodicities in Flettner rotors; and (vi) the planning of a three-stage limited-area field research experiment, with the primary objectives of technology testing and determining to what extent, if any, cloud albedo might be enhanced by seeding marine stratocumulus clouds on a spatial scale of around 100×100 km. We stress that there would be no justification for deployment of MCB unless it was clearly established that no significant adverse consequences would result. There would also need to be an international agreement firmly in favour of such action.