Sub-micrometer salt aerosol production intended for marine cloud brightening
Introduction
Marine Cloud Brightening (MCB) is one of several Solar Radiation Management (SRM) ideas designed to produce a global cooling to roughly balance the warming resulting from fossil-fuel burning.
A significant number of papers (e.g., Latham, 1990, Latham, 2002, Bower et al., 2006, Salter et al., 2008, Latham et al., 2008, Latham et al., 2012a, Latham et al., 2012b, Rasch et al., 2009, Jones et al., 2009, Jones et al., 2011, Korhonen et al., 2010, Bala et al., 2010) have been published on MCB, which is based on the idea that the albedo of maritime stratocumulus clouds, and possibly their longevity, can be substantially enhanced (Twomey, 1977, Albrecht, 1989) by seeding the clouds with salt or seawater aerosol particles, which are activated as Cloud Condensation Nuclei (CCN), so as to increase the cloud droplet number concentration, N. Seawater aerosol or salt particles would be produced at or near the ocean surface beneath selected clouds and turbulence would cause a high fraction of them to rise into the clouds and become activated to cloud droplets. GCM (General Circulation Model) computations indicate that this geo-engineering technique could produce a cloud albedo enhancement and negative forcing sufficient to maintain the Earth's average surface temperature and polar sea-ice coverage at approximately current values, at least up to the CO2-doubling point.
One concern that has rightfully been raised regarding deployment of any of the SRM geo-engineering techniques that is capable of creating significant global negative forcing, is that if the technology breaks down, or has to be terminated because it produces unresolvable negative consequences, a major global-scale warming will rapidly be produced. It would therefore be sensible, if SRM deployment is ever deemed to be necessary, to have at least two such techniques acting in concert, so that (hopefully) the remaining technique(s) could be ratcheted up in order to eliminate the possibility of sudden temperature rise. Computations indicate that both the stratospheric sulfur seeding geo-engineering technique (Crutzen, 2006) and MCB–if they are found to function as predicted in GCM modeling studies–could maintain the global average surface temperature at roughly current values until at least the CO2-doubling point, which may not be reached for several decades. So the two techniques acting in parallel could perhaps buy sufficient time for a clean form of energy to take over, globally, from fossil-fuel burning. An additional possible advantage of having the two techniques deployed at the same time is that fine-tuning (using the sub-global flexibility associated with MCB) should be possible for a significant time.
In our view MCB, and any other SRM technique, should never be deployed unless its usage is approved and deemed to be necessary by a yet-to-be-formed fully international panel representing all countries. It would be vital for a comprehensive examination of all techniques under consideration to be conducted and to accept only those that would not create unacceptable consequences that could not be fully resolved in advance of deployment.
There exist several unresolved questions regarding MCB. For example: (1) GCM modeling assumes a more simplistic picture of these clouds than is warranted, and more complex models need to be developed; (2) It is vital to determine whether undesirable consequences of MCB seeding exist. If they do, and if they cannot be eliminated, then work on this idea should be abandoned; (3) there still exist some technical problems that need to be definitively resolved, particularly in relation to the development of a spray device for producing, in copious quantities, sea-salt or sea-water aerosol particles.
We present herein 1) a discussion of the theory behind a novel technique, 2) a description of an experimental apparatus based on that theory, and 3) results of experiments using the apparatus for the production of salt aerosol of the size-range and particle flux required for effective utilization of MCB, should that ever be warranted. The technique could also be of significant value in aerosol studies, both in the laboratory and in the field.
Section snippets
Particle size
In simplified terms, in order for seawater aerosol particles to function as CCN and convert into cloud droplets, they must contain a minimum salt mass, which is a function of the supersaturation observed in the cloud, as calculated by the Köhler equation. The average supersaturation in marine stratocumulus clouds is a matter of discussion and various observations, but is estimated to be on the order of 0.5% (Hudson, 2009).
Based on this simplified consideration, one would estimate the minimum
Summary description
A schematic of the conceptually quite simple apparatus built to heat and spray saltwater through a small orifice is shown in Fig. 3. It comprises water reservoirs, a pump, a pressure gauge, a serpentine heating tube enclosed in a block heater, and a nozzle enclosed in a separate block heater. The apparatus is designed such that at the desired orifice flow rate, the fluid exit temperature can reach the desired value by controlling the temperature of the heating block. As it flows through the
Experimental results
During the development of the process 53 experiments resulted in the collection of 103 salt-on-wafer samples. Thirty-nine of these salt samples were judged to have meaningfully analyzable particle distributions (enough particles to be representative and clear enough images to be analyzed). Many of the 53 experiments, especially the earlier ones, were to some degree compromised by such exigencies as leaks, inability to accurately determine fluid temperature, collection of wet droplets, and
Discussion of results and conclusions
The efficiency of the supercritical saltwater spray method is bounded on the one side by the ineffectiveness of the smallest particles at the lower end of the distribution to act as CCN, and on the other side by the increasing number of nozzles and amount of energy required if the diameter is increased. For a CCN effectiveness cut-off diameter of 40 nm, 29% of the particles in the distribution of Fig. 5 are ineffective (Fig. 9), while 42% of those in the distribution of Fig. 6 are ineffective.
Acknowledgments
The authors acknowledge the financial support of FICER for part of this work and in particular thank David Keith and Ken Caldera for their encouragement. We thank J. Bischoff and B. Thomas from the USGS for very informative discussions on supercritical seawater, and Qin Wang and David Johnston of Aqua Metrology Systems for their expert help and suggestions with some of the experimental setup. We acknowledge our debt to several suppliers who provided us with free samples, services or expertise:
References (27)
- et al.
A note on the chemistry of seawater in the range 350°–500 °C
Geochim. Cosmochim. Acta
(1983) - et al.
Computational assessment of a proposed technique for global warming mitigation via albedo enhancement of marine stratocumulus clouds
Atmos. Res.
(2006) - et al.
Statistical description of the size properties of nonuniform particulate substances
J. Franklin Inst.
(1929) Amelioration of global warming by controlled enhancement of the albedo and longevity of low-level maritime clouds
Atmos. Sci. Lett.
(2002)- Abraham, J.P., 2009. Purdue University, private...
Aerosols, cloud microphysics, and fractional cloudiness
Science
(1989)- et al.
Albedo enhancement of marine cloud to counteract global warming: impacts on the hydrological cycle
Clim. Dyn.
(2010) Densities of liquids and vapors in boiling NaCl solution. A PVTX summary from 300° to 500 °C
Am. J. Sci.
(1991)- et al.
Liquid–vapor relations for the system NaCl–H2O: summary of the P-T-x surface from 300° to 500 °C
Am. J. Sci.
(1989) - et al.
An empirical equation of state for hydrothermal seawater (3.2 percent NaCl)
Am. J. Sci.
(1985)
A review of some experimental spray methods for marine cloud brightening
Int. J. Geosci.
Albedo enhancement by stratospheric sulfur injections: a contribution to resolve a policy dilemma
Clim. Chang.
Cited by (6)
Precipitation enhancement by cloud seeding using the shell structured TiO<inf>2</inf>/NaCl aerosol as revealed by new model for cloud seeding experiments
2018, Atmospheric ResearchCitation Excerpt :The seeding height is 200 m above the ground and the seeding material is injected in the updraft below the modelled cloud. It can be seen that the pure NaCl also contributes to precipitation enhancement, as has previously been demonstrated in the number of studies (Kristensen et al., 2014; Neukermans et al., 2014). However, the novel seeding material significantly increases surface precipitation when compared to pure NaCl case.
Review of geoengineering approaches to mitigating climate change
2015, Journal of Cleaner ProductionCitation Excerpt :They indicated that with the correct drop size, the amounts of spray needed to give a useful reduction of incoming power are surprisingly small. In order to carry it out, Neukermans et al. (2014) designed a simple apparatus built to heat and spray saltwater through a small orifice. It comprises water reservoirs, a pump, a pressure gage, a serpentine heating tube enclosed in a block heater, and a nozzle enclosed in a separate block heater.
SYSTEMATIC REVIEW AND CLASSIFICATION OF THE ENGINEERING FOR GLOBAL DEVELOPMENT LITERATURE BASED ON DESIGN TOOLS AND METHODS FOR SOCIAL IMPACT CONSIDERATION
2022, Proceedings of the ASME Design Engineering Technical ConferenceClimate intervention: Reflecting sunlight to cool earth
2015, Climate Intervention: Reflecting Sunlight to Cool EarthPreliminary results for salt aerosol production intended for marine cloud brightening, using effervescent spray atomization
2014, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences