Low-altitude ozone: a double-edged sword

Sensitivity of human health to global surface temperature increases under three adaptation scenarios: limited adaptation, incomplete adaptation, and proactive adaptation^[1]. Excerpt from the Summary for Policymakers of the 2022 IPCC Working Group II report.

In its latest report, the The IPCC highlights ozone as one of the four major human health issues related to climate change. Thus, alongside deaths caused by heat stress, malaria, and mosquito-borne diseases, ozone-related deaths appear to be just as significant. This is a good reason for our expert, Cyril Caram, to revisit this gas, its origin, and its impacts in a question-and-answer format.

Where is ozone found?

In the upper layers of the atmosphere, the ozone layer blocks some of the harmful UV rays from reaching the Earth's surface, thereby protecting living organisms. We're talking about stratospheric ozone here.

The stratosphere lies above the troposphere, which ranges in altitude from 8 km to 18 km from the poles to the equator, up to an altitude of about 50 km. It may seem counterintuitive, but the higher you go in this layer, the warmer it gets! This is due to the presence of the stratospheric “ozone layer.” This layer is of fundamental importance because it filters out mutagenic ultraviolet rays.

Chemical mechanisms govern the natural balance of ozone in the stratosphere. These mechanisms are disrupted by human activities, such as emissions of CFCs (chlorofluorocarbons), which have a sufficiently long lifespan to be transported into the stratosphere.

It is at these altitudes that CFCs break down under the influence of UV radiation into highly reactive radicals (Cl and ClO), effectively destroying the ozone layer on a global scale, and contributing significantly to the emergence of the famous “ hole in the ozone layer "above the poles."

Following the discovery of the role of CFCs in ozone depletion in the 1970s, the international community signed the Montreal Protocol in 1987 to limit their use. Since then, the stratospheric ozone layer has been recovering, and it is estimated that it will fully recover by 2060^[2].

Diagram explaining the difference between the stratospheric ozone layer, which protects the Earth from the sun’s harmful rays, and tropospheric ozone, which is an air pollutant and a greenhouse gas. Source: Carbone 4

In contrast, in the lower layers of the atmosphere, ozone is a air pollutant which degrades air quality and is a greenhouse gas that has an impact on the climate. We are referring here to tropospheric ozone.

What is the relationship between the two ozone layers?

Generally, a slow exchange of air occurs across the troposphere-stratosphere boundary. Due to higher ozone levels in the upper atmosphere, there is a net transfer of ozone from the upper layers to the lower layers of the atmosphere. This exchange, however, plays only a minor role in determining ozone abundances in the stratosphere and troposphere. Therefore, there is no strong connection between the two layers.

Where does tropospheric ozone come from?

Tropospheric ozone, also known as low-altitude ozone, is not directly emitted by natural processes or human activities. It is the product of chemical reactions between carbon compounds, including methane (CH4)^[3] and nitrogen oxides (NOx), both of which come from anthropogenic sources (vehicles, industry, agriculture, etc.) and natural sources. That is why it is referred to as air pollutantsecondary.

As emissions of its precursors have increased since the preindustrial era, Tropospheric ozone has increased, especially in recent decades.

Its formation is promoted by summer season, under the influence of the sun's UV rays, and its concentration varies significantly from region to region. The suburban and rural areas are generally more affected than city centers.

Number of days with levels exceeding 120 μg/m³ over an 8-hour period in the Île-de-France region in 2019.That year, the quality objective related to health protection^[4] is exceeded, especially in suburban and rural areas. Map taken from the Airparif website^[5].

Why is there such a disparity in tropospheric ozone concentrations?

Simply put, the burning of fossil fuels in cities leads to high concentrations of NOx. When it is hot and solar radiation is strong, NOx participates in specific chemical cycles that increase the amount of ozone produced. Paradoxically, however, once a certain threshold is reached, ozone can be broken down by the very NOx compounds that also produce it.

This destruction does not occur in suburban and more remote areas, where primary NOx emissions are lower and concentrations of carbon compounds are higher. It is a fairly complex chemical cycle, but one that is very well understood by air quality specialists.

Nevertheless, we should not conclude that the air in cities is less polluted than in rural areas. Overall, the air in cities remains more polluted due to the presence of other air pollutants such as fine particulate matter, nitrogen oxides, carbon monoxide, etc.

What are the effects of tropospheric ozone on living organisms?

From a health perspective, ozone is a pollutant that irritates the eyes and respiratory system of humans and animals. Even small increases in atmospheric ozone concentrations can affect health. In many regions of the world, the Ozone levels often exceed the established threshold by air quality agencies, which poses a serious problem for public health and is associated with a increased risk of premature death^[6].

The effects of tropospheric ozone on agricultural production are also felt, especially during the growing season. This oxidizing agent penetrates the leaves through the stomata and attacks plant tissues. Many cereals and legumes are vulnerable to ozone, including wheat, soybeans, corn, and rice. Yield losses for these crops are substantial and are estimated to range from 2 to 16 percent globally.^[7], thereby affecting CO2 uptake by plants.

How is this related to the climate?

Ozone is a significant greenhouse gas In terms of radiative forcing, it ranks third after carbon dioxide and methane^[8]. Compared to these, its lifetime is relatively short—on the order of about twenty days. Consequently, tropospheric ozone can be transported from one continent to another but is not well mixed on a hemispheric scale. If ozone production declines, the radiative effect caused by ozone will decrease in the same proportion.

On the other hand, in urban areas during the summer, rising concentrations of tropospheric ozone are the main cause of the formation of photochemical smog that can last for several days. Global warming is intensifying these events and exacerbates urban heat islands^[9] which are characterized by an increase in average temperatures in cities compared to surrounding rural areas, both day and night. Ozone, being a greenhouse gas, fuels this feedback loop by trapping thermal energy near the ground.

The effect of the urban heat island on late-afternoon temperatures in urban, suburban, and rural areas. Figure adapted from Fuladlu et al., 2018.^[10]

What about recent and future trends in tropospheric ozone concentrations?

According to Airparif, an organization that monitors air quality in the Île-de-France region, Ozone is the only pollutant for which recent annual trends show no improvement in the Northern Hemisphere^[11].

As for future concentrations of ground-level ozone, they are are highly uncertain and depend on future socioeconomic trajectories :

• By 2050, ozone levels are likely to rise under so-called “pessimistic” climate scenarios^[12] particularly in East Asia, South Asia, the Middle East, Africa, and Southeast Asia.
• In the “optimistic” scenarios^[13], ozone levels are decreasing in all regions in response to a sharp reduction in emissions starting in 2030 in North America, Europe, Eurasia, East Asia, the Middle East, and South Asia.
• In trend scenarios described as “middle-of-the-road”^[14], by the middle of the century, and in most regions, tropospheric ozone will be slightly reduced or will remain close to current levels.

Are there any mitigation strategies?

It is important to establish and strengthen the regulations on tropospheric ozone concentrations to develop mitigation strategies science-based and support the various sectors in the decrease in emissions fromprecursors tropospheric ozone.

The sharp decline in these, and including methane will result in a decrease in low-altitude ozone of nearly 30% on average, globally, by 2050^[15].

Regional, national, and continental policies can very effectively reduce the impact of the air pollution, and contribute to the short- and long-term mitigation of the climate change.

How about we talk about adaptation?

To limit the impact on public health, investment in health, information, and air quality monitoring systems can be very effective.

In order to improve the future food security, governments and local authorities should identify agricultural management options to address multiple stress factors (water, temperature, chemicals, etc.). One option would be to select and develop varieties adapted to specific geographic regions,ozone-resistant to prevent the worst effects of ozone pollution episodes^[16].