Oceanic emissions of gases of interest for atmospheric chemistry - Modeling oceanic dynamics of CO₂, isoprene and DMS

Phytoplankton activity is responsible for the oceanic production of volatile compounds which, once released into the atmosphere, have an impact on its oxidative capacity, on aerosol formation and on climate evolution. The oceanic emissions of most of these compounds are, however, poorly known because of a low number of in situ observations and a lack of knowledge in the oceanic processes involved. In a first step, I studied the oceanic cycles of carbon monoxide (CO) and isoprene (C5H8). In order to re-evaluate their emissions to the atmosphere, the sources and sinks of these gases in the water column have been embedded into the 3-D ocean circulation and marine biogeochemistry model NEMO-PISCES. In parallel, I compiled in situ measured oceanic concentrations and laboratory results from the literature to better constrain these oceanic processes. I produced the first spatialization of CO and isoprene emissions based on a 3-D ocean model and not relying on satellite data for the calculation of concentrations. For CO, global emissions are estimated to 4.0 Tg C yr^-1, with a spatial dynamic reflecting its photochemical source. This dynamic is very different from the one produced in the 80's, which was the only one available to the atmospheric chemistry community. For isoprene, I showed the importance of water temperature in regulating the phytoplankton production. Global emissions are estimated to 0.66 Tg C yr^-1, with a spatial dynamic reflecting this source. For both compounds, I also showed the importance of ocean circulation and mixing in determining surface concentrations and therefore emissions to the atmosphere. In a second step, I studied the responses of oceanic emissions of CO, isoprene and dimethyl sulfide (DMS) to climate change. This leads to an increase in the emissions of the 3 gases (+9.4, +4.2 and +6.5% for CO, isoprene and DMS in 2100 for a scenario with high emissions of greenhouse gases). Emissions tend to increase at high latitudes and decrease at lower latitudes. Global changes are mainly controlled by changes in water temperature, while changes in spatial distribution are controlled by a redistribution of phytoplankton production. Even if the uncertainties in modeling these cycles are still important, this work should allow integrating these cycles into an Earth System model coupling marine biogeochemistry and atmospheric chemistry, in order to better quantify the potential role of these interactions on the evolution of atmospheric chemistry and climate. (author)