Activités de recherche

CloUd MULtiphase chemistry of organic compoUndS in the troposphere

Context

Global Change issues cannot be solved within the restricted frame of traditional disciplines, but require a multidisciplinary approach that gathers specialists from different fields. In order to improve our capability of predicting future air quality and climate change, there is an urgent need to obtain a detailed understanding of the linkages between aerosols, cloud formation, and the specific role that organic material plays in these processes. The atmospheric photooxidation of Volatile Organic Compounds (VOC) leads to the formation of less volatile oxidized species. These products can nucleate or condensate on pre-existing particles, thus explaining the formation of Secondary Organic Aerosols (SOA) in the atmosphere. Nevertheless, these oxidation products are also water soluble, and they have been observed in clouds’ water droplets. In this phase, the atmospheric reactivity is modified compared to the gas phase one, leading to the formation of more oxidized and less volatile products. Clouds are present on a large part of the lower atmosphere (60% of the earth surface on the first 4-6 km in altitude), and they continuously appear and disappear through evapo-condensation cycles. They can be assimilated to authentic photoreactors. While only 10% of clouds precipitate, the remaining 90% dissipate, inducing volatile species’ evaporation, and the condensation of the low volatile organic compounds. This is a new route for SOA formation which is potentially very important, but has barely been experimentally investigated before. Indeed, the atmospheric in situ formation of SOA has often, if not always been explained by the gas phase initiated photooxidation of VOC, thus neglecting the presence of water.

Objectives of CUMULUS

Our aim is to investigate the contribution of in-cloud processes to the formation of SOA, because aqueous phase photooxidation of soluble organic compounds can potentially induce completely new and relevant pathways, leading to the formation of SOA with different physical and chemical properties. Isoprene is the major VOC emitted on a global scale. Its gas phase ozonolysis and photooxidation are relatively well-known, and do not lead to the formation of important amounts of SOA (SOAgas), but produce water soluble VOCs (e.g. methacrolein, methylvinylketone, hydroxyacetone, glycolaldehyde, …). The latter compounds can lead to the formation of important amounts of SOA, after in-cloud photochemical processes (SOAcloud). Thus, isoprene’s large emission fluxes can induce the formation of important amounts of SOA through in-cloud processes and thereby deeply modify the current yield estimation of SOA on the global scale, and their impact on air quality and on the climate. Isoprene thus appears to be extremely relevant for our study. An estimation of the quantity of SOAcloud formed through these processes in the atmosphere will be achieved thanks to an integrated laboratory experimental approach (i.e. the coupling between photochemistry and cloud evapo-condensation cycles), complemented by an air parcel model, and a mesoscale model simulating real case cloud events. The resulting mesoscale model will be used for a quantification of SOA relatively to the global SOA budget: in particular, the influence of in-cloud chemistry, and the influence of isoprene multiphase photochemistry will be tested. The model will also provide a solid basis for field data analysis and interpretations, in particular to seek tracers of biogenic SOA derived from isoprene multiphase photochemistry. Finally, the model will provide a solid basis for further global modelling of atmospheric chemistry, climate and air quality. The understanding of cloud processing of organic compounds is still in its infancy, and would benefit from a careful examination.

The scientific objectives of the project are:

1. To determine the impact of aqueous phase reactivity of biogenic organic compounds, in particular isoprene, on the transformation of SOA, and the formation of new SOA during evaporation of clouds;
2. To quantify the importance of these new SOA particles relatively to the global SOA budget, using models initialized on real case studies;
3. To determine the physical and chemical properties of these new particles. In particular, the modification of their hygroscopic properties as a function of their chemical composition will indicate their potential climatic impacts. CUMULUS aims at developing a scientifically sound knowledge on such issues, and represents an innovative step toward yet unexplored prospects of the SOA formation issue.