CSIRO scientists have developed a new way to account for ozone in computer simulations of the climate. This latest modeling shows that the oceans take much less ozone out of the atmosphere than previously thought. This has implications not only for the understanding of future global warming, but also on human health, plant productivity and the economy.

Ozone (O₃) is formed by reactions of chemicals such as nitrogen oxides and volatile organic compounds—i.e., precursor species.

Ozone is the third most important greenhouse gas after carbon dioxide and methane, in terms of potential for global warming. Ozone concentrations have increased by about 40% since preindustrial times due to the increased emission of precursor species, resulting in about 15% 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.

—Dr Ashok Luhar, an atmospheric scientist who leads the aerosol and chemistry modelling team at CSIRO

Dr Luhar 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. 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.

—Dr Luhar

Until now, one number represented this complicated deposition process in advanced global climate and chemistry models. Researchers previously used a constant waterside deposition velocity—an indication of the intensity or rate at which ozone is deposited into the ocean, Dr Luhar explained. 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, he said.

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.

—Dr Luhar

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 the Australian Community Climate and Earth-System Simulator (ACCESS), the researchers 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; i.e., 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% 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.Dr Luhar

The difference between the previous estimate and the new results is most pronounced in the Southern Hemisphere.T he results better explain observations of ozone recorded at the Cape Grim Baseline Air Pollution Station in northwest Tasmania.

The improved ozone deposition understanding may have implications on other chemical species as well.

While it has been known for some time that the carbon cycle is important for modeling 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.

Resources

• 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. doi: 10.5194/acp-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. doi: 10.5194/acp-17-3749-2017