Linking Seawater Biogeochemistry to the Chemical and Biological Signatures of Nascent Marine Aerosol

Abstract. Interactions between the ocean and atmosphere are a key component in the Earth's climate system. Oceans represent an important source of aerosol particles in the atmosphere. However, uncertainties are still associated with our understanding of the complex processes involved in these interactions. This work presents a unique set of in-situ measurements focusing on the relationship between marine biogeochemistry and nascent marine aerosols over the Southern Ocean. These measurements were performed on nascent seawater aerosols generated, in an enclosed tank, from a continuous flow of seawater, during the Sea2Cloud cruise. This experimental set up provided the opportunity to study the interactions between the ocean and the atmosphere without the influence of background transported aerosols. These nascent seawater aerosols were characterised for their physical, biological, and chemical properties, using a combination of aerosol mass spectrometry and fluorescence spectroscopy. In parallel, detailed measurements of seawater biogeochemistry provided concentrations of different bacterial species, amino acids, and sugars in the seawater, enabling us to make links between organic aerosol types and seawater biochemistry.

Organic aerosol contributed 34–46 % of the nascent sea spray. We observed that the contribution of oxidized organic aerosol dominated the nascent organic sea spray, indicating a large contribution of chemically or biologically processed organic matter already in the seawater. The contribution of organic material was highest at the start of the campaign in biologically active waters (>40 %) and decreased to <40 % later in the campaign. The contributions of the different type of OA changed across the seawater types showing that a marine OA evolves with the biogeochemical composition of seawater. The POA signature in the nascent sea spray was suspected to be less of a signature of biological debris and more representative of the biologically refractory and aged background organic matter of the ocean. The fluorescent aerosol particles (FAP) were logically proportional to most microorganism cell numbers but represented only 3 % of the total aerosol nascent sea spray, and less than 10 % of the total organic matter. Among these fluorescing particles, type A dominated with correlations with MOA, bacteria, but also with nanophytoplankton, diatoms and in general to total Chl-a. Classes B and C were uncorrelated to any microorganism but instead were sensitive to the presence of OOA and to a lesser extend MOA.