Marine Primary Aerosol in the Mediterranean atmosphere: physical and chemical properties from a mesocosm study
Résumé
The Mediterranean Sea is a special marine environment characterized by low biological activity and high anthropogenic pressure. It is often difficult to discriminate the contribution of Primary Sea Salt Aerosol (SSA) formed at the sea-air interface from background level of the aerosol. An alternative tool to study the sea-air exchanges in a controlled environment is provided by the mesocosms, which represent an important link between field studies and laboratory experiments. A mesocosms experiment was performed in May 2013 at the Oceanographic and Marine Station STARESO in Western Corsica. Three mesocosms were simultaneously filled with pooled and screened (<1000 ľm) subsurface (1 m) seawater from the Bay. Each mesocosm had a maximum water column depth of 2 m and contained 2260 L of Bay water and covered with transparent (teflon film) dome to prevent atmospheric contamination. The three mesocosms were equipped with a pack of optical and physicochemical sensors and received different treatements: one was left unchanged as control and two were enriched by addition of nitrates and phosphates respecting Redfield ratio (N:P = 16). The evolution of the three systems was followed for 20 days. The set of sensors in each mesocosm was allowed to monitor, at high frequency (every 10 min), the water temperature, conductivity, pH, incident light, fluorescence of chlorophyll a and dissolved oxygen concentration. The mesocosms waters were daily sampled for chemical (colored dissolved organic matter, dicarboxylic acids and related polar compounds, transparent polysaccharides and nutrients concentration) and biological (chlorophyll a, virus, phytoplankton and zooplankton concentration) analyses. Finally, few liters of sea-water from each mesocosm were sampled daily and immediately transferred to a bubble-bursting apparatus to simulate SSA. Size distribution and particle number were followed by SMPS and APS in the range of 10nm to 10mm. The hygroscopic properties were investigated by a CCN device. On-line chemical analysis of the sub-micrometer fraction was performed by a c-TOF-AMS. Off-line analysis of the SSA generated included TEM-EDX , LC-MS and IC, Thermo-optical analysis of EC-OC. The objective of the present study is to investigate the influence of water chemical and biological composition and biological activity on physical and chemical properties of the primary generated aerosol.