That good, hearty smell of fresh …….cloud seed?!

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Have you ever stood at the seaside enjoying that good hearty smell of fresh sea air and wondered what causes that distinctive smell? Well, that smell is caused by a compound called dimethyl sulphide (DMS). It is derived from marine plants (phytoplankton) that live in the surface layer of the ocean where the sun’s rays penetrate the water. Phytoplankton produce a parent compound, dimethylsulfoniopropionate (DMSP), which marine bacteria convert to DMS.

Next, if you’re the contemplative type, you might also have wondered, ‘how do clouds form?’. Very simply put, clouds form when water vapour cools and condenses to form droplets which, unable to defy gravity, fall to the earth’s surface. Such condensation commonly happens around us too, when the bathroom mirror fogs up after a hot shower, or when the glass containing your iced 7up leaves a pool of water at its base for example. Around us, water vapour condenses onto surfaces, and it’s no different in the sky. To form rain droplets, water vapour must condense onto a surface. This is often provided by tiny particles like grains of dust, or sometimes by molecules such as volatile organic compounds (VOC) and volatile organic sulphur compounds (VOSCs). These surfaces are aptly called ‘cloud seeds’, or cloud condensation nuclei (CCN).

When DMS (derived from marine plants) reacts with molecules in the air (nitrate and hydroxyl), it forms cloud seed as VOSCs. The availability of cloud seed can affect both the location where cloud forms and the amount of cloud that forms.

Currently, humans are greatly altering the location and abundance of marine plants by adding nutrients to some marine environments. These nutrients derive mostly from fertiliser, animal and human waste, and waste water treatment plant discharges.

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In some parts of the world, humans are fertilising the ocean, particularly the coastal ocean, in much the same way you’d fertilise your garden to encourage lots of nice green grass to grow. In the Biogeochemistry Research Group in TCD Geography, we study such sources of nitrogen pollution, their pathways to the marine environment, and subsequent ecological effects.

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As humans alter the location and amount of marine plants, this may alter the location and amount of cloud seed available as VOSCs and thus the geography of rainfall.

When we hear the phrase ‘climate change’ we normally quickly jump to ‘the burning fossil fuels’, ‘release of carbon dioxide’ and ‘global warming’. However, like this example, there are other important though more subtle mechanisms of climate change which are highly sensitive to human perturbation. So, the next time you stand by the ocean and inhale that distinctive smell, think about plants, seeds, clouds, and the monumental impact we humans are having on the natural world.

Authored by Laura Foley, PhD student, TCD Geography

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Further reading:

Bentley, R. & Chasteen, T.G., 2004. Environmental VOSCs––formation and degradation of dimethyl sulphide, methanethiol and related materials. Chemosphere, 55(3), pp.291–317.

Vitousek, P.M. et al., 1997. Human alteration of the global nitrogen cycle: sources and consequences. Ecological applications, 7(3), pp.737–750.

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