Title: Hydrogen peroxide serves as pivotal fountainhead for aerosol aqueous sulfate formation from a global perspective.
Authors: Jie Gao, Haoqi Wang, Wenqi Liu, Han Xu, Yuting Wei, Xiao Tian, Yinchang Feng, Shaojie Song & Guoliang Shi
Journal: Nature Communications
Year: 2024

Many fossil fuels contain small amounts of sulfur which enters the atmosphere as sulfur dioxide (SO₂) when these fuels are burned. Once in the air, this SO₂ can oxidise to form aqueous-phase sulfate, either dissolved in cloud droplets or small particles suspended in the air (known as aerosols). Sulfate increases the acidity of rain particles, which has important environmental impacts, such as harming aquatic life and trees. Sulfate aerosol also reflects incoming solar radiation, which can result in a cooling of the Earth’s climate.

Legislation passed by governments around the world to limit emissions of SO₂ has meant that recent decades have seen declines in atmospheric concentrations of both SO₂ and sulfate. However, the declines in sulfate have not been as rapid as the declines in gas-phase SO₂ indicating that increases in the rate of SO₂ oxidation have counteracted the reductions in SO₂ concentrations. There are many factors that must be accounted for as the atmospheric oxidation of SO₂ is a complex process with many different potential pathways, many of which involve heterogeneous chemistry between gas and particle-phase species. 

In their recent publication in Nature Communications, Gao et al. aim to probe the importance of different sulfate aerosol formation pathways using data from a global atmospheric chemistry model. The work investigates the relative importance of four different sulfate formation pathways involving the oxidation of SO₂ by: hydrogen peroxide (H₂O₂), nitrogen dioxide (NO₂), ozone (O₃), or transition metal ions (TMI). The rates of each of these processes are sensitive to a wide range of physical properties, however one of the key parameters is aerosol acidity which can change as a result of the constituent compounds. For example, increased sulfate concentrations result in more acidic particles. The H₂O₂ pathway is relatively insensitive to changes in pH, whereas each of the other pathways show strong responses to changes in pH. The NO₂ and O₃ pathways are most prevalent in higher-pH (more alkaline) aerosol, whereas the TMI pathway is enhanced in acidic aerosol.

Through calculations based on results from a global atmospheric chemistry model, the researchers demonstrated that oxidation by H₂O₂ was the dominant aerosol sulfate formation pathway in most global regions. However, the TMI pathway was important in areas with high aerosol acidity. While the NO₂ pathway only played a small role in sulfate formation in most environments, the O₃ pathway was most important over desert regions such as the Sahara due to the high aerosol alkalinity over these regions. 

Given the importance of aerosol pH for the formation of sulfate, the researchers note that the contribution of each of these pathways could shift as changes to human pollutant emissions impact aerosol acidity. For example, reductions in acidic aerosol components such as nitrogen oxides (NO_x) could increase aerosol alkalinity and increase the importance of the O₃ pathway outside of desert regions. Projected increases in ammonia (NH₃) emissions could also increase the alkalinity of atmospheric aerosol.

A consideration of these oxidation processes is vital to properly avoid the negative impacts of atmospheric sulfate, beyond simply reducing SO₂ emissions. For example, Gao et al. note that increases in H₂O₂ concentrations in Europe and North America have counteracted the recent decreases in SO₂, slowing the decrease in sulfate concentrations. Finally, given the importance of H₂O₂ in sulfate formation, the researchers note that there is still lots of uncertainty around the formation processes of H₂O₂ in the atmosphere warranting further research.

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