Preliminary results from the FARCE 2015 campaign: multidisciplinary study of the forests–gases–aerosols–clouds system on the tropical island of La Réunion

The Forests gAses aeRosols Clouds Exploratory (FARCE) campaign was conducted in March–April 2015 on the tropical island of La Réunion. For the first time, several scientific teams from different disciplines collaborated to provide reference measurements and characterization of La Réunion vegetation, (biogenic) volatile organic compounds (BVOCs), (bio)aerosols and composition of clouds, with a strong focus on the Maïdo mount slope area. The main observations obtained during this two-month intensive field campaign are summarized. They include characterizations of forest structure, concentrations of VOCs and precursors emitted by forests, aerosol loading and optical properties in the planetary boundary layer (PBL), formation of new particles by nucleation of gas-phase precursors, ice nucleating particles concentrations, and biological loading in both cloud-free and cloudy conditions. Simulations and measurements confirm that the Maïdo Observatory lies within the PBL from late morning to late evening and that, when in the PBL, the main primary sources impacting the Maïdo Observatory are from marine origin via the Indian Ocean and from biogenic origin through the dense forest cover. They also show that (i) the marine source prevails less and less while reaching the Observatory, (ii) when in the PBL, depending on the localization of a horizontal windshear, the Maïdo Observatory can be affected by air masses coming directly from the ocean and passing over the Maïdo mount slope, or coming from inland, (iii) bioaerosols can be observed in both cloud-free and cloudy conditions at the Maïdo Observatory, (iv) BVOCs emissions by the forest covering the Maïdo mount slope can be transported upslope within clouds and are a potential way of secondary organic aerosols formation in aqueous phase at the Maïdo Observatory, (v) the simulation of dynamics parameters, emitted BVOCs and clouds life cycle in the Meso-NH model are realistic, and more advanced Meso-NH simulations should use an increased horizontal resolution (100 m) to better take into account the orography and improve the simulation of the windshear front zone within which lies the Maïdo Observatory. The FARCE campaign provides a unique set of multi-disciplinary data and results that can be used to better understand the forest–gases–aerosols–clouds system in an insular tropical environment.