Chemical reactions of ozone depletion

2O3—→3O2 

Ozone becoming oxygen is the basic reaction of ozone depletion, during which OH·,NO·,Cl· and Br·  act as catalysts for ozone destruction cycles in the stratosphere. Among these catalysts, OH· and NO· mainly come from the nature, however, Cl· and Br· are mostly released by human. Furthermore, CFC and Halons are the major sources of Cl· and Br· in the stratosphere. 

An example of reaction is shown below (CF2Cl2 is involved in CFC) :

CF2Cl2 → CF2Cl· + ·Cl

Cl· + O3 —→ ClO· + O2

ClO· + O —→ Cl· + O2

This process is able to  produce ClO and Cl again and again; thus, a small amount of CFC will consume  of ozone until it is transported to out of the stratosphere.

The reason why CFC and Halon are the major sources of catalysts is that CFC and Halon are stable in the troposphere which can reach to stratosphere and break down, thus producing Cl and Br which is able to consume ozone. However, recently, scientists found that short-lived compounds like CH2Cl2 could also release Cl· and lead to ozone depletion. What's more, it rose by 8% from 2004 to 2014 and may result fromf fast-rising emissions of a chemical used in paint stripper. 

For latest ozone consuming compounds, I will continue to focus on news and keep posting.

The impacts of the ozone hole on environment

In the first post, we mentioned that the ozone layer is able to absorb most of the sun's harmful ultraviolet rays. This kind of harmful ultraviolet rays is called UV-B, as shown in figure 1. It also presents how the ozone layer protects the earth by absorbing UV-B.

Figure 1. The fundamental of how the ozone layer protects the earth (Source: NASA).

This section will talk about how the ozone hole threats environment from different aspects. Firstly, the depletion of ozone will reduce the ability of the ozone layer to absorb UV-B, thus damaging the plants, such as reducing crop production and plant leaves as well as photosynthesis, making crops more susceptible to weeds, etc. Furthermore, there are many indirect effects, including changing plant form, altering growth cycle of plant, modifying plant metabolism. These can have more profound effects on the environment because it may break the balance of biogeochemical cycles by influential herbivores.

Secondly, the ozone layer is closely linked to human health. UV can promote the synthesis of vitamin D on the skin, which plays a beneficial role in the formation of bone. However, excessive irradiation with ultraviolet B (λ = 280 to 320 nm) in ultraviolet light (λ = 200 to 400 nm) can cause diseases such as skin cancer and eye diseases. It is estimated that a 1% decrease in stratospheric ozone (an increase in UVB of 2%) will increase the incidence of skin cancer by 4% to 6%. By now, about 100,000 people die of skin cancer each year around the world, and those who die of skin cancer increase by about 5,000 every year. In light-skinned people exposed to the sun for a long time, more than 50% of the skin diseases are sun-induced, that means light-skinned people are more susceptible to various sun-induced skin cancers than other ethnic groups. In addition, ultraviolet light can cause premature aging of the skin.

The ozone hole can also change the atmosphere and ocean circulations. It may significantly impact the changes in the climate system and weather within the Southern Hemisphere, especially in the summer (December to February). Darryn W. Waugh and his team show that subtropical intermediate waters in the southern oceans have become fresher, because surface winds have strengthened as the ozone layer has been thin. Antarctic ozone hole has changed the mix in the southern oceans, a situation that could change the amount of carbon dioxide in the atmosphere and could eventually have an impact on global climate change.