Atmospheric ageing of sea spray aerosol

The Earths atmosphere contains a vast number of aerosols, which are dened as solid or liquid particles suspended in a gas. These aerosols have an important impact on climate by interacting with the incident solar radiation as well as their ability to form clouds. Depending on their size, particles can stay in the air for periods between seconds to weeks. During this time, several processes can occur, which lead to changes in the particles properties. For instance, particles can be oxidized or altered by changing conditions of relative humidity and temperature, or they can absorb condensable vapours present in the atmosphere. These alterations are defined as ageing processes and can have a substantial influence on the particles properties, hence changing their role for climate. The wide range of possible aerosol characteristics causes large uncertainties in the actual role of aerosols in climate. As the oceans cover 70% of the Earths surface, they are the largest single source of aerosol mass in the atmosphere. The particles directly emitted into the atmosphere as sea spray aerosols have already been investigated in previous studies, whereas changes due to ageing processes are still poorly characterized. Therefore, we propose the project entitled atmospheric ageing of sea spray aerosol . The main research question is: How do the properties of sea spray aerosol change during their lifetime in the atmosphere and how does this affect their role for climate? To this end we will conduct experiments at the University of Aarhus, which offers top-notch instrumentation together with outstanding expertise on sea spray aerosol. A so called environmental chamber will be used to simulate different atmospheric conditions as typically found above the ocean. Then different types of sea spray aerosols will be introduced into the chamber to investigate changes occurring as a result of ageing processes. The goal is to measure the particles chemical composition, ability to take up water and to form cloud droplets in different environments. As these properties can vary depending on the particles size, specific instrumentation is selected to provide information on a wide range of particle diameters. A set of well-established instruments will be complemented by a new and unique instrument, built at the Paul Scherrer Institute in Switzerland as part of the PhD project of the applicant, which allows to measure the water uptake by atmospheric particles in the diameter range from 300 to 700 nanometres. So far comparable techniques focused on smaller sizes, while significant amounts of sea spray particles are known to exist at larger diameters. Therefore, these studies will contribute to improved parameterizations of sea spray aerosol in climate models and ultimately improve our understanding of the Earths climate.

Within this project, we could show the major importance of the inorganic salt fraction of marine aerosols, which are tiny airborne particles originating from the oceans, for the interactions with solar radiation and their ability to form cloud droplets. We performed several experiments in a special laboratory setup equipped with unique techniques to produce marine particles that are comparable to real sea spray particles emitted from wave breaking over the oceans and a chamber where atmospheric processes can be simulated. Marine salt particles were chosen, as they constitute the largest source of aerosol particles in the atmosphere by mass, thus having a profound influence on Earth's climate. Our results uncovered that even solely the inorganic fraction of marine particles can undergo oxidation in the atmosphere thereby changing its composition. The chemical reactions occurring lead to the formation of hydrate forming salts within the marine particles. These hydrate forming salts contain water within their salt structure that hinders further water uptake. Thus, their ability to take up water and grow in size when the relative humidity is high is lowered compared to that of pure salts. At the same time, higher super saturations are needed to transform them into cloud droplets compared to pure salts. These results have a profound importance for their role in climate as they show that inorganic marine particles substantially change their properties during their lifetime in the atmosphere and such changes have not yet been included in climate models. In the marine atmosphere, these inorganic particles are typically present together with a large variety of organic material. Compounds including Sulphur are of special interest, as they are known to be crucial for cloud formation. Therefore, we investigated the compound "dimethyl-sulphide" emitted by microorganisms in the oceans and can be transferred into the atmosphere. This specific species was hypothesized to be particularly important, as it constitutes the largest natural source of Sulphur to the atmosphere. We investigated the efficiency of dimethyl-sulphide to form secondary particles by reactions occurring between dimethyl-sulphide and the highly reactive chemical species hydroxyl radicals (OH), which are ubiquitous in the atmosphere. Our results show that such a reaction indeed leads to the formation of new particles and that they are formed relatively slowly and do not grow to very large sizes. In our laboratory setup we also studied how changes in the relative humidity and temperature affect this new particle formation, showing that colder temperatures generally lead to less and smaller particles. The relative humidities and temperatures were selected in way to be representative of environmental conditions found in the marine atmosphere. Overall, this project was very successful, led to many new insights and became the basis for several future project ideas.