Catalysis in the clouds

Science 340, 727–730 (2013)

Credit: © STEPHAN MERTES/TROPOS

Clouds have a big impact on climate. Within them, aerosol particles scatter heat and light through 'aerosol radiative forcing'. The size and properties of these aerosol particles control a cloud's reflectivity — so, small droplets make brighter clouds that are more reflective and block sunlight from reaching the Earth's surface. Thus, aerosol forcing typically works to oppose greenhouse gases and causes cooling. Sulfate aerosols are an important constituent of clouds, so understanding the in-cloud sulfur cycle is critical for modelling the impact of aerosol radiative forcing. Sulfates occur naturally from, for example, volcanic dust and sea salt, and anthropogenically through the combustion of fossil fuels. They are added to aerosol particles in clouds through both in situ oxidation of SO₂ and the direct uptake of H₂SO₄ gas and ultrafine particulates by cloud droplets.

SO₂ can be oxidized by H₂O₂, O₃ and O₂, but its reaction with O₂ can also be catalysed by transition metal ions (TMIs). The contribution of TMI catalysis is not fully understood, making it difficult to accurately predict the effect of sulfate aerosol radiative forcing in climate models. Now, a group of scientists, led by Eliza Harris and Bärbel Sinha from the Max Planck Institute in Mainz, have found that the pathway catalysed by naturally occurring TMIs is the main contributor to the oxidation of sulfur in clouds. The team used instruments located in Mount Schmucke in Thuringia, Germany to measure upwind, downwind and within-hill cap clouds. By measuring sulfur isotope abundances in gas-phase SO₂ and sulfate particles they were able to discriminate between sulfate particles created through TMIs and those created through oxidation by H₂O₂ and O₃.

As the dominant oxidation pathway involving natural TMIs takes place on large dust particles, which drop out of the clouds relatively quickly, the sulfate particles will actually have shorter lifetimes than previously estimated. The team maintain that this result will lead to significant changes in the contribution of aerosol forcing in major global climate models.