Aerosol Characterisation and Modelling in the Marine Environment (ACMME)

Aerosol particles play an important role in climate forcing but the processes governing their behaviour still remain uncertain. Oceans, which cover the majority of the earth's surface, are known to generate particulate matter of different chemical natures (sea salt, organic material and sulphate) but the extents and climactic implications of the different aerosol types are very poorly understood. More recently there has been considerable interest in the organic component of marine particles. It has been suggested that that seaspray derived particles contain large quantities of organic matter generated mechanically from the sea surface microlayer which can influence cloud forming potential significantly. The ability of an aerosol particle to take up water, its hygroscopicity, is a highly important property, as this will increase particle size (and therefore scattering potential) in sub-saturated environments and also dictates whether they will act as an effective Cloud Condensation Nucleus (CCN). This property is determined by particle size and composition. Whilst the hygroscopic behaviour of the inorganic fraction is relatively well understood, our knowledge of the behaviour of the organic fraction is far less well developed. Organic matter in general is not particularly hygroscopic. However many classes of organic compounds are known to suppress surface tension. In multicomponent aerosol, this property leads to a reduction in the supersaturation required to activate in a cloud, modifying the number of droplets formed. Because of this potential significance of the marine organic fraction, it is vital that this component is better characterised and its spatial distribution evaluated so allowing quantification of its role in climatic processes. In addition, particulate matter from marine environments is known to significantly affect the chemistry of the atmosphere. There is significant current research interest in the roles of aerosols in halogen cycling through heterogeneous processes, which can significantly alter the concentrations of ozone and other oxidants in the Earth's atmosphere. The rates of these processes depend on the available particulate surface area and the acidity of the particles, both poorly characterised. To address the uncertainty in the roles of marine aerosol, ACMME will accurately quantify the chemical and the physical properties of particulate matter in a poorly characterised region of the globe, using these results in state of the art models to predict properties of these aerosol and their impacts on cloud droplet formation. These measurements will be made onboard the NERC research vessel Discovery, on the 2007 cruise between Lisbon and the Cape Verde Islands funded by UK SOLAS within the Reactive Halogens in the Marine Boundary Layer (RHaMBLe). ACMME will make direct measurements and model predictions of the ability of an aerosol to form a CCN, the cloud-activation potential, and hygroscopic growth of the sampled aerosol. The former plays a major role in determining the indirect radiative effect and the latter, the direct effect. Predictions of the effect of multicomponent marine aerosol on cloud droplet number will be made using a new model developed under the UK-SOLAS funded project APPRAISE. Similarly, additional compositional data will be used to provide constraint on the heterogeneous reaction rates which have significance in the tropospheric oxidant budget. The measurements of compositions, mixing states and ambient water contents will be used in conjunction with chemical modelling activities in associated proposals to assess their importance in the heterogeneous chemical processes that control halogen cycling in the marine atmosphere. Finally, the data will also be compared with those from similar atmospheric studies at sea, both by the investigators and others, in order to assess the spatial variability of marine aerosol properties and their effects.