Ozone uptake in oceans much lower than previously thought
THE many personalities of ozone can be confusing. Located high up in the stratosphere it has an important role in reducing the amount of cancer-causing ultraviolet light reaching the Earth’s surface. Closer to Earth, in the troposphere, it is a greenhouse gas that has an important influence on global climate. Near the surface, ozone is an air pollutant.
Made up of three oxygen atoms, ozone (O3) is not emitted into the atmosphere in the way we emit other greenhouse gases. Instead, it is formed by reactions of chemicals such as nitrogen oxides and volatile organic compounds, known as precursor species.
“Ozone is the third most important greenhouse gas after carbon dioxide and methane, in terms of potential for global warming,” says Dr Ashok Luhar, an atmospheric scientist who leads the aerosol and chemistry modelling team at CSIRO.
“Ozone concentrations have increased by about 40 per cent since preindustrial times due to the increased emission of precursor species, resulting in about 15 per cent of global warming caused by human activity.
“It is also an air pollutant that is harmful to human health. And it has implications for plant ecosystems and the economy, as ozone damages plant stomata, damaging the leaves, and hence reduces productivity.”
He says the amount of ozone in the troposphere is determined by how much is produced and destroyed through photochemical reactions, how much is transported from the stratosphere above, and how much is deposited at the Earth’s surface.
“Understanding how much ozone the ocean and land remove is important because it tells us how much is left in the atmosphere,” says Luhar.
“Dry deposition is a process where aerosols and gases such as ozone are removed from the atmosphere when they come into contact with the Earth’s surface. Ozone deposits on all surfaces, but the amount deposited depends on the surface type; whether it is vegetation, bare soil, ice or water. In the case of loss to the ocean, ozone dissolves into water, reacts with the naturally occurring iodide present in water and undergoes molecular and turbulent mixing, all at the same time.”
Gaining a better understanding of oceanic processes
Until now, one number represented this complicated deposition process in advanced global climate and chemistry models.
“Researchers previously used a constant waterside deposition velocity, which is an indication of the intensity or rate at which ozone is deposited into the ocean. This was an old figure suggested in the late 1980s before there were any open-ocean observations to support an understanding of the relevant processes. It incorrectly yielded too high an uptake of ozone by the ocean surface in global models.”
Although dry deposition of ozone to the ocean is generally less intense than to land surfaces, the large area of the Earth covered by oceans means the total amount is significant.
“The understanding has been that total global ozone deposition was approximately 1000 teragrams per year, with around 300 teragrams of that being deposited to the ocean.”
A teragram is one trillion (1012) grams.
Luhar and colleagues at CSIRO developed a new mathematical mechanism that better describes the main oceanic processes and thus deposition velocity. This in turn enabled a better description of observations of ozone in the troposphere, and means better simulations of tropospheric ozone in computer models of the Earth system and climate.
“With the new mechanism in our Australian Community Climate and Earth-System Simulator (ACCESS), we found that the total deposition to the ocean is about a third of what was previously estimated by global climate models. So, the number drops from 300 to 100 teragrams per year, and thus 200 teragrams of ozone that was supposed to be deposited each year is now left in the atmosphere.”
In other words, the total global dry deposition has dropped from 1000 to 800 teragrams a year, a 20 per cent decrease.
“That, on average, leads to about a five to eight per cent increase in the simulated concentration of ozone in the troposphere. This amount may appear small, but an increased concentration of ozone in the troposphere means you might estimate increased damage to vegetation, implications for human health, and greater radiative forcing and hence greater climatic impact,” says Luhar.
Result is a better alignment with observations
He says the difference between the previous estimate and the new results is most pronounced in the Southern Hemisphere.
“You see most of the difference from the new deposition mechanism in the Southern Hemisphere, from mid- to high-latitudes, with the modelled ozone concentrations larger by as much as 15 per cent. So it is very relevant to us here in Australia.”
The results better explain observations of ozone recorded at the Cape Grim Baseline Air Pollution Station in northwest Tasmania.
“Our mechanism gives a better simulation of tropospheric ozone observations including those from Cape Grim. It’s describing what we’re seeing significantly better, which means we have improved the understanding of oceanic deposition.”
Atmospheric models taking up the new mechanism
The improved ozone deposition understanding may have implications on other chemical species as well.
“Ozone is an oxidant that is linked with the hydroxyl radical, a scavenger or cleansing agent in the atmosphere. It also is involved in reactions with carbon monoxide and methane in the atmosphere. With all these chemical species, if you change one, it changes others. But we’re yet to explore what happens to other species.”
While it has been known for some time that the carbon cycle is important for modelling future climate, computer models are increasingly including reactions relating to chemically active climate forcing agents (e.g. ozone) and thus better accounting for feedbacks from processes such as emissions due to changes in land use. This means reduced uncertainties in climate change projections.
“Eventually, climate models will become Earth system models that also include interactions between chemistry and the carbon cycle, with improved simulations of future climate.
“Already, our ozone mechanism has been included in ACCESS and in the U.K. Met Office’s base atmospheric model. Other global models are starting to use this mechanism, as a result of what our papers have shown, with improved predictions of ozone and other chemical species in the troposphere,” says Luhar.
For more reading:
Luhar, A.K., Woodhouse, M.T. and Galbally, I.E., 2018. A revised global ozone dry deposition estimate based on a new two-layer parameterisation for air-sea exchange and the multi-year MACC composition reanalysis. Atmospheric Chemistry & Physics, 18(6), p. 4329-4348. https://www.atmos-chem-phys.net/18/4329/2018/
Luhar, A.K., Galbally, I.E., Woodhouse, M.T. and Thatcher, M., 2017. An improved parameterisation of ozone dry deposition to the ocean and its impact in a global climate-chemistry model. Atmospheric Chemistry and Physics, 17(5), p. 3749-3767. https://www.atmos-chem-phys.net/17/3749/2017/