By Katie Reed
The ozone layer acts as the earth’s natural sunscreen by blocking harmful, high energy UV light. Without it, UV light would be able to reach earth’s surface and damage plants, animals, and humans. Unfortunately, the ozone layer is depleted when human-made substances called CFCs are released into the atmosphere. One ingredient of CFCs is chlorine which can react with ozone, breaking up the ozone molecules, and depleting the layer. This process has created an ozone hole in Antarctica, which is credited with causing shifts in spring ice coverage.
In 1987, the Montreal Protocol on Substances that Deplete the Ozone Layer was held. Since then, these chlorine containing substances have slowly been phased out of use, but are still present in the atmosphere. The Antarctic ozone hole is expected to be observed for at least the next 35 – 50 years.
Dr. H. Nagase and his colleagues in Japan, Germany, and the United States have a radical idea to remove some of the chlorine from the stratosphere: inject ice particles into the atmosphere.
“The ozone destruction in Antarctica could be significantly reduced if a systematic method was established that would reduce HCl.” said the study, published in May. They propose a geo-engineering approach which involves injecting ice at an altitude of about 25 km. This process would work, they say, because HCl dissolves in ice particles. These particles can then fall to earth, effectively removing the chlorine before it can react with ozone.
The study found that five conditions had to be adjusted in order to optimize chlorine removal. Size and concentration of the particles, along with altitude, latitude, and seasonality of injection, all affect the efficacy of ice injection.
The most important parameter was size of the ice particle. For this process to be effective, ice particles have to be small to provide adequate surface area, but large enough for gravity to quickly remove it from the stratosphere, away from the ozone layer.
Altitude, latitude, and timing of injection all affect how long the particles can remain ice, and also how much ozone is present. The researchers limited the injection to a 10-day period, and found injection to be most effective in Antarctic fall because of seasonality conditions.
Finally, concentration is important. High concentrations may be more effective, but also more expensive.
Overall, the models determined that 40µm ice particles injected at about 25 km, between 70-80 degrees south, at a concentration of 3x1014 molecules/cm3, in May-July would remove the most HCl.
The researchers stress this was an initial presentation of a possible solution, and that more information needs to be collected before this can be considered a feasible process. Factors like evaporation, other particle interactions and, of course, cost need to be further investigated.
The biggest challenge, Nagase and his colleagues say, will be designing an effective and successful method to inject the ice.
 Nagase, H., Nagase, H., Kinnison, D., Petersen, A., & Vitt, F. (2015). Earth's future: Effects of injected ice particles in the lower stratosphere on the Antarctic ozone hole. American Geophysical Union Publications. 3(5):143-158.