Rowland-Molina Hypothesis

Have you ever had an older person tell you that hairspray or deodorant damages the ozone layer? That's because aerosol propellants used to contain chemicals called chlorofluorocarbons (called CFCs for short). These chemicals affect the ozone layer, the invisible shield over Earth.

You may have heard of CFCs, but have you ever heard of the scientists who first suggested that they impacted the ozone? Perhaps not – their paper was met with heavy criticism, affecting the reputation of the scientists.

However, evidence gradually appeared to support their hypothesis. As other scientists grew concerned, the work of Rowland and Molina helped to shape the Montreal Protocol.

The Rowland-Molina Hypothesis: Overview

In 1974, the chemist Frank Rowland and his post-doctoral student Mario Molina published a controversial paper. It suggested that long-lived halogen compounds, such as chlorofluorocarbons (CFCs), could deplete the ozone layer.

For a couple of years, it was difficult to find evidence for the hypothesis. But by 1976, the United States National Academy of Sciences published a report concluding that ozone depletion was strongly supported by scientific evidence. The world's governments responded by signing and ratifying the Montreal Protocol – a UN treaty that set out plans to regulate the production and consumption of ODS.

In 1995, Rowland and Molina shared a Nobel Prize with Paul Crutzen for their work in atmospheric chemistry.

The Rowland-Molina Hypothesis: Main Idea

Molina discovered that unlike many other chemicals, CFCs were persistent. They didn't break down in the lower atmosphere. Instead, they travelled all the way to the top of the stratosphere, where they were broken down by harsh ultraviolet (UV) radiation.

The carbon-chlorine bonds were broken, and the chlorine atoms became free radicals.

He then learned that chlorine free radicals react catalytically with ozone (O₃), breaking down ozone molecules into chlorine monoxide (ClO) and oxygen. When the chlorine monoxide meets another molecule of ozone, it breaks up and releases the chlorine free radical.

Fig. 1 – Symbol equations showing the two-step process of ozone depletion. The dots indicate free radicals. Source: Vaia Originals

Molina compared the effects of CFCs to natural ozone depletion mechanisms, and discovered that CFCs could lead to even more ozone destruction! Just one chlorine atoms could destroy up to 100,000 ozone molecules!

He and Rowland worked together to produce mathematical models and publish a paper, warning that CFC production could damage the ozone layer.

Importance of the Rowland-Molina Hypothesis

The Rowland-Molina hypothesis aimed to protect the ozone layer and therefore it was important. Why was this? Read on to find out!

The Importance of the Ozone Layer

High up in the stratosphere, ozone molecules act as a shield over Earth. They absorb 98% of incoming UV radiation.

Without the ozone layer, Earth's surface would receive high levels of UV radiation, especially harmful UV-B.

In humans, exposure to high levels of UV-B radiation leads to the following conditions:

• Eye damage (such as cataracts)

• Premature ageing

• Immune suppression

• Skin cancers

With mass ozone depletion, humans may also suffer from famines due to the effects on plants. UV-B radiation damages the cell membranes and organelles of plants. This impacts metabolic processes, including photosynthesis and growth. As a result, UV-B radiation has adverse effects on crop yields.

The Effect of UV-B on Marine Organisms

Due to its unique atmospheric conditions, Antarctica experiences a large ozone 'hole' during spring. The Southern Ocean experiences enhanced levels of UV-B radiation penetrating the surface of the ocean, killing phytoplankton.

Phytoplankton are the base of the Antarctic marine food chain. A vast decline in numbers could have knock-on effects on the entire continent.

It's not just microscopic organisms that are affected. The Southern Ocean is home to the unique icefish – the only vertebrate lacking haemoglobin.

Fig. 2 – A captive icefish in Japan. Adapted for cold temperatures, this fish has clear blood due to its lack of haemoglobin and red blood cells. Source: Wikimedia Commons

In normal conditions, a lack of haemoglobin would benefit the icefish. It allows its blood to flow easily at low temperatures. However, it leaves their DNA unprotected. As ozone depletion has led to increased UV-B radiation, icefish have experienced DNA lesions. Damage to icefish eggs and larvae is closely associated with the amount of UV radiation penetrating the ocean.

Weakness of the Rowland-Molina Hypothesis

Unlike many other academic papers, Rowland and Molina’s paper was not backed up with experimental evidence. Instead, it focused on an over-arching hypothesis, (that CFCs caused ozone depletion) based on many supporting hypotheses. These supporting hypotheses had their own experimental evidence, but also their own uncertainties. If just one supporting hypothesis was found to be false, it could ruin Rowland and Molina’s overarching hypothesis.

Many scientists were sceptical of the Rowland-Molina hypothesis – not due to their personal work, but due to some of the supporting hypotheses.

The Industry Reaction

The paper was also met with heavy criticism by the aerosol and halocarbon industries.

CFCs had been discovered in 1928, and were considered the 'perfect compound' for decades due to their non-toxic and seemingly inert properties. In 1976, the US alone was producing over 340 million kilograms of CFCs every year. The industry was worth over $8 billion, and employed 200,000 people.

Fig. 3 – The main uses of CFCs during the twentieth century, as stated in the Los Angeles Times.

Manufacturers regarded the hypothesis as 'purely speculative' and Rowland's reputation was damaged.

Early Tests of the Rowland-Molina Hypothesis

As we learned earlier, there was no experimental evidence to support the hypothesis when Rowland and Molina's paper was published in 1974. They knew that there would be delay between the release of CFCs and subsequent ozone depletion, so the pair had predicted a change in ozone levels by 1975. Unfortunately, this was not to be. Ozone levels fluctuate by up to 10%. It wasn't possible to differentiate any change from natural fluctuations.

However, alternative evidence came about in 1975. Two independent groups of scientists used aircraft and balloon instruments to measure concentrations of CFCs at different altitudes. Their results confirmed Rowland and Molina's theories that:

• CFCs persisted through the lower atmosphere, eventually reaching the stratosphere

• CFCs were the primary source of stratospheric chlorine

In 1976, the atmospheric scientist James Anderson proved the existence of chlorine monoxide in the atmosphere. His results showed that the Cl : ClO ratio matched the expected value of the Rowland-Molina hypothesis.

That same year, the United States National Academy of Sciences concluded that there was scientific evidence to support Rowland and Molina's theory. They calculated that if CFC production continued to increase until 1990, half of the world's ozone would have been lost by 2050.

The Rowland-Molina Hypothesis: A Post-Evidence Timeline

After the National Academy publication, the attitude towards CFCs began to change. Key events regarding the Rowland-Molina hypothesis are summarised in this table.

I hope that this article has clarified the Rowland-Molina hypothesis for you. To summarise, the theory suggested that CFCs led to ozone depletion. Initially there was no evidence to support the claim, but the work of other scientists supported the theory, and led to the Montreal Protocol.

^{1. Candace Gilet, Ozone Depletion, Understanding Science, 20222. Jeremy Norman, Rowland & Molina Suggest that CFCs Deplete the Ozone Layer, History Information, 2022}

^{3. John Durkee, Management of Industrial Cleaning Technology and Processes, 2006}

^{4. Lanie Jones, Ozone Warning : He Sounded Alarm, Paid Heavy Price, Los Angeles Times, 1988}