Axis 1: The evolution of organic carbon and its impact on the production and evolution of secondary organic aerosol
Organic matter represents a significant fraction of tropospheric aerosol and in this organic fraction, a significant share is of secondary origin i.e. outcome of chemical conversion process. These processes transform volatile chemical compounds in products with low saturated vapor pressure that, if any, can condensate and/or nucleate and thus form secondary organic aerosol (SOA).
Despite a more detailed production of SOA, current models underestimate systematically the masses of aerosols from one to two orders of magnitude. In addition, it has been shown that this gap grows with the aging of the air mass. These disagreements are the result of the lack of understanding of the processes involved that are extremely complex and varied (reactivity in gas phase, heterogeneous reactivity, phase transfers, polymerization (Figure 1) so as the very large number of gaseous precursors.
Figure 1: Interactions between the gas phase and the condensed phase in the atmosphere
To improve the understanding of the processes of formation and evolution of the secondary organic aerosol, LISA has implemented field experiments, laboratory studies in simulation chambers and modeling. Specifically, it is :
- Measure on the field the VOC precursors of SOA. The objective of these measures is to allow, in synergy with the models and simulations in atmospheric chambers, to describe the evolution of the organic carbon in order to determine organic products mostly involved or always not considered in the formation of SOA (part anthropic/biogenic, primary/secondary) to, in term, assess the impact.
- Develop reliable chemical schemes to simulate the production of SOA in 3D models. The implementation approach is to (i) develop a detailed mechanism in which the formation processes of the SOA are treated explicitly, (ii) assess this mechanism by comparing the simulation chamber experiments and (iii) use this detailed mechanism as reference mechanism to develop optimized chemical patterns for 3D models. The explicit chemical schemes generator developed at LISA (GECKO-A) is used for this study.
- Study the properties of the SOA according to its sources and its evolution by laboratory experiments. It relates the basic physicochemical properties whose knowledge is indispensable for the quantification of direct and indirect climatic impacts (properties of diffusion, of absorption, of hygroscopicity) to the "history" of the aerosol. For this, aerosols models are generated and aged in a simulation chamber by exposure to solar radiation, oxidants, hydrometeors, (Figure 2).
Figure 2: Formation of secondary organic aerosols observed in an experiment of ozonolysis of a-pinned in the CESAM simulation chamber