Research looks into whether sea spray is losing its sparkle

Sea spray, which is produced in abundance across all the world’s oceans, is one of the greatest sources of atmospheric aerosols - tiny particles that not only scatter and absorb sunlight but also influence climate indirectly through their role in cloud formation.

Understanding how these particles take up water from the atmosphere, a process known as hygroscopicity, is important because it determines how much sunlight they reflect and how well they can form clouds.

Sea spray is a complex mixture of inorganic salts, organic material present in the ocean and living organisms such as bacteria, viruses and fungi.

Now, an international team of researchers, including Dr Daniel Partridge from the University of Exeter, has looked at the ability of one of the inorganic components to take up water, as part of the process of cloud formation.

Using a large suite of well-controlled laboratory experiments the research has shown, for the first time, that the hygroscopicity of these components is significantly lower than pure sodium chloride, a substance routinely used to describe their hygroscopicity in climate models.

The research is published in leading scientific journal Nature Communications.

Dr Partridge, a Mathematics lecturer at the University of Exeter said: “ Ocean-derived aerosol particles play an important role for the modulation of the Earth’s radiation budget and climate; however, an accurate description of their chemical, physical and radiative properties are still subject to large uncertainties. Climate models are tasked with representing a myriad of complex atmospheric processes that act over a wide range of temporal and spatial scales. Accordingly, the treatment of aerosol properties and processes in these models is often simplified to facilitate the computer simulation of future climate change scenarios within a reasonable time frame.

“One such simplification is the representation of the chemical composition of the inorganic component of sea spray aerosol which is typically represented to have the same properties as pure sodium chloride - salt. Through detailed laboratory studies we have shown that the affinity of inorganic sea spray aerosol to take up water vapour is significantly lower than sodium chloride, as assumed in climate models.

“This has implications for the simulated historical radiative forcing which has been shown to be sensitive to the characterisation of natural aerosols in the clean, pre-industrial atmosphere, of which sea salt is one of the most widely distributed components.”

Matt Salter, researcher at the Department of Environmental Science and Analytical Chemistry (ACES), Stockholm University, and co-author of the study added: “It is widely understood that aerosols have a net cooling effect on climate, counteracting the warming caused by greenhouse gases. However, the magnitude of this cooling is highly uncertain, in part due to knowledge gaps in how natural aerosol particles interact with solar radiation and clouds.

We suspect that water bound within sea salt, known as hydrates, play a significant role in defining the hygroscopicity of inorganic sea spray aerosol, If true, it means that the particles would take up less water because of the water already present as hydrates and, as a result, they would grow less. Overall, improving our knowledge of one of the largest natural aerosol sources is critical if we are to understand the effects of man-made aerosols on climate.”

Using models, the authors were able to show that the reduced hygroscopicity of sea spray means that these particles will grow less and reflect less sunlight than previously thought. However, the picture may be more complicated.

“All numerical models are simplified reflections of reality which require approximations. It was previously thought that sodium chloride was a good approximation for the hygroscopicity of the inorganic fraction of sea spray. We have now shown that the hygroscopicity of pure inorganic sea spray particles is significantly lower than sodium chloride. This finding has implications for the role of sea spray aerosols in climate, especially on how they interact with solar radiation, ” says Paul Zieger, assistant professor at ACES and co-author of the study.