Particle formation through photosensitized reactions at the air-sea interface
Résumé
The sea surface microlayer (SML) represents more than 70% of the Earth’s surface and constitutes the boundary layer interface between the ocean and the atmosphere. The sea surface is characterized by an organic enriched microlayer originating from marine chemical and biological activities. Exposed to solar radiation, it has been demonstrated that the surface is a source of volatile organic compounds (VOC) as well as of highly functionalized VOCs.
Consequently, the question arises as to whether photo-induced chemical processes under relevant atmospheric conditions lead to the secondary organic aerosol (SOA) loading in the marine boundary layer. Previous works have shown that photochemical processing lead to the formation of highly functionalized VOCs which may be potential candidates to the SOA loading. In order to bring further comprehension, a multiphase atmospheric simulation chamber has been used in order to study the chemical processes occurring at the air-sea interface. Experiments have been performed in a 2 m3 chamber made of FEP film in which a glass container for liquids is inserted. Light processing was initiated using VUV lamps (centred at 365 nm) in order to irradiate the liquid mixture containing humic acid used as photosensitizer and nonanoic acid used as a surfactant. Particle formation was monitored using an ultrafine condensation particle counter (d50 > 2.5 nm) and particle growth was followed by a SMP
S. VOCs formed have been identified and analysed by Proton Transfer Reaction – Time of Flight Mass Spectrometer and GC/MS. The chamber is also equipped with trace gas analyzers continuously measuring NO-NOX and ozone.
The liquid mixture was exposed to VUV irradiation for 14 hours at least. When the light was turned off, ozone was added in the gas phase. Light-induced processes showed a low increase of particle concentration while secondary reaction implying ozone showed a significant contribution in SOA formation. These observations indicate the presence of SOA precursors among the VOCs formed during irradiation. Atmospheric implication of photosensitized reactions as a source of SOA loading in the marine boundary layer will be discussed.