Bio-physicochemistry of tropical clouds at Maïdo (La Réunion Island): processes and impacts on secondary organic aerosols formation – BIO-MAIDO

The BIO-MAÏDO measurement campaign took place from March 13 to April 4, 2019 on Reunion Island with the objective of documenting the life cycle of clouds on the slope of the Maïdo, the development of the boundary layer and the evolution of atmospheric chemical composition (primary and secondary aerosols and precursor gases) along the slope to the receptor site, the Maïdo Observatory. The campaign took place at five sites on the slope of the Maïdo up to the Observatory at 2165m, which is located in the receptor area where numerous measurements of physico-chemistry for aerosols and chemistry for gases make it possible to characterize transformed and oxidized air masses.
The objective of the data analysis is to assess the multi-phase pathways of formation and oxidation of SOA by including the effect of bacteria in cloud water. Retro-trajectory and trajectory calculations make it possible to estimate the major origins of air masses and the contributions of different local sources (marine, urban from the coast).
The data obtained will be exploited in an explicit cloud chemistry model (CLEPS) that allows the effect of bacteria in cloud water to be integrated. CLEPS results will optimize the multiphase reduced chemical mechanism of the 3D transport/chemistry model (Meso-NH). A parameterization of the effect of bacteria will be taken into account in Meso-NH. Meso-NH will be applied to several case studies of the campaign to determine its ability to simulate SOA formation, the contribution of multiphase pathways in this formation and the potential role of bacteria.

The measurement campaign was successful with largely favorable weather conditions with the formation of cloud on slope almost every day of the campaign.
Chemical analyses for gases, aerosols and cloud water, and biological analyses for aerosols and cloud water are almost complete. Retro-trajectory calculations with FLEXPART/ECMWF or/AROME were performed. Chemical analyses of aerosols show that the anthropogenic contribution is very low, that the contribution of isoprene, compound of biogenic origin, in the formation of SOA is significant at the Maïdo Observatory and that the presence of sulfate is strong whether in submicron or super-micron mode. The main origin of this sulfate does not appear to be marine and remains to be identified. Chemical analyses of cloud water also show the presence of sulfate of unknown origin. Concentrations of inorganic ions are generally high with a dominant marine contribution (Na, Cl). Levels of trace metals, particularly iron, are low. Finally, the composition of dissolved organic compounds is extremely complex showing the significant presence of biogenic compounds such as sugars and amino acids. Oxygenated compounds are also present in the aqueous phase such as carbonylated compounds (dominant formaldehyde) and carboxylic acids, which are also measured in particulate phase lower on the slope. However, the levels of organic matter identified in cloud water by these much more comprehensive chemical analyses than are generally done, at best, only account for 30% of the total organic matter present in cloudy water.
Biological analyses for aerosols and cloud water show that 4 groups of bacteria dominate what is representative of the atmospheric microbial core.

In order to determine for which days there is a connection between i.e. where the sampled air mass at the observatory passed through the sites on the slope, additional trajectory calculations are in progress with another Lagrangian model, CAT. CAT uses data from Meso-NH dynamical simulations, which are available for the whole campaign at horizontal resolutions of 2km, 500m and 100m. The trajectories obtained are therefore able to determine finely where the air masses have passed. The complete dataset of gases, aerosols and cloud water will be analyzed using these new trajectories. This work will result in a general publication on the results of the campaign.
The analysis of the chemical composition of cloud water is currently being finalized and will result in a publication that is expected to be submitted in early 2021. Other publications are pending on precursor gases and aerosols.
Work on 3D chemical modeling with Meso-NH will begin in 2021. It will first examine the study of precursor gases and aerosols before the introduction of cloud chemistry. The Meso-NH chemical mechanism for the aqueous phase will benefit from the simulations carried out with CLEPS, which is expected to start in 2021. Particular attention will be paid to the introduction of the effect of bacteria on the organic chemical composition of cloud water and its simplification for consideration in the 3D model.

Conferences :
Leriche, M., C. Jambert, A. Colomb, A. Borbon, L. Deguillaume, P. Tulet, V. Duflot, S. Houdier, J.-L. Jaffrezo, M. Vaitilingom, F. Burnet et al. : Bio-physicochemistry of tropical clouds at Maïdo (La Réunion Island): processes and impacts on SOA formation, workshop des acteurs et utilisateurs ACTRIS-FR, 21-24 mai 2019, Saint-Pierre D’Oléron, France, May 2019.
Rocco, M., A. Colomb, J.-L. Baray, A. Borbon, L. Deguillaume, M. Leriche, P. Tulet, C. Amelynck, N. Schoon, B. Verreyken, V. Gros, C. Jambert, L. Bouvier, R. Sarda-Esteve, G. Péris, C. Guadagno, F. Burnet, T. Bourrianne: Characterization of reactive gases during BIO-MAIDO campaign along the Maïdo slope on Indian Ocean tropical island (Reunion Island), Atmospheric Chemical Mechanisms Conference, 9-20 novembre 2020, conférence virtuelle organisée par UC Davis Air Quality Research Center, November 2020.
Dissemination action:
Press release at the start of the measurement campaign in March 2020.
TV report on the campaign broadcasted during a news of Antenne Réunion March 2020, www.linfo.re/la-reunion/societe/changement-climatiques-les-scientifiques-s-installent-au-maido
Press conference with tour of the measurement sites organized by the University of La Réunion in March 2020.
Leriche, M. : BIO-MAÏDO : une campagne de mesures tropicale pour l’étude de la formation d’aérosols organiques dans les nuages, La Météorologie, 8, 106, 17-19, August 2019.

The BIO-MAIDO project aims at better understanding the chemical and biological multiphasic mechanisms that control the Secondary Organic Aerosol (SOA) formation. The tropical environment of the Reunion Island represents an ideal area to study SOA formation by the multiphasic atmosphere since: (1) numerous biogenic volatile organic compounds, precursors of SOA are emitted in huge amount and the high solar intensity flux and the temperature favours their chemical transformations; (2) the high occurrence of fogs and clouds over the area can lead to the formation of SOA by aqueous phase processes (chemistry and biology). The strategy adopted here is based on an intensive field campaign with the objective to characterize primary sources of gases and aerosols and to evaluate multiphasic pathways controlling the formation and oxidation of SOA. This work is done in synergy with modelling investigations using a 0D process cloud model (CLEPS) together with a 3D chemistry/transport model (Meso-NH).
The field campaign will be deployed on two sites: one in source area with biogenic emissions and submitted to fog (mid-slope site in forest); one in receiver zone with transformed and oxidized air masses (Maïdo observatory). Previous results from numerical simulations and observations show the presence at the Maïdo observatory of air masses coming along the slope and mixing anthropogenic, marine and biogenic influences. Simulations of trajectory origins along the slope of the Maïdo will be performed during the campaign to help the interpretation of in-situ observations. Marine and anthropogenic sources will be assessed thanks to results from the OCTAVE project (Oxygenated Compounds in the Tropical Atmosphere: Variability and Exchange), chemical markers identification and back-trajectories. A particular attention will be paid of to the boundary layer features thanks to the MARLEY lidar deployed on a mobile pick-up truck. State-of-art instrumentations will be deployed for the characterization of SOA precursors gases including primary compounds, the physico-chemical properties of aerosol particles and the bio-physico-chemical properties of clouds forming along the slope of the Maïdo. The campaign will be synchronize with the ANR NEPHELAE campaign, which will document the 4D structure of clouds thanks to a fleet of autonomous aerial vehicles that coordinate themselves in real-time as an intelligent network. These sets of observation will help understand the formation of SOA in a humid tropical atmosphere by improvement of the CLEPS and Meso-NH models. The French INSU labelled 3D Meso-NH model will benefit from the observations and the simulations performed with the 0D CLEPS model to improve its parameterizations of SOA formation from gas phase and cloud droplets processes including bacterial activities. This last point is very innovative as never studied until now.