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

Abstract

<p><strong>Abstract.</strong> 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.</p>

Analysis of Volatile Organic Compounds during the OCTAVE Campaign: Sources and Distributions of Formaldehyde on Reunion Island

“…The diurnal variations in O3, NOx, and relevant meteorological parameters at Bélouve and Maïdo are presented in Figure 6A. Similar diurnal variations in NOx and meteorological parameters were previously measured during the 2015 Forests-gAses-aeRosols-Clouds Exploratory (FARCE) campaign, which was also conducted at the Maïdo observatory and along the Maïdo mountain slopes [29]. At Maïdo during the night, O3 concentrations remain almost constant due to the chemical conditions (no photolysis and low NOx conditions) and stable dynamical conditions.…”

“…The Maïdo observatory is then located in the free troposphere under Figure 6A. Similar diurnal variations in NO x and meteorological parameters were previously measured during the 2015 Forests-gAses-aeRosols-Clouds Exploratory (FARCE) campaign, which was also conducted at the Maïdo observatory and along the Maïdo mountain slopes [29]. At Bélouve, O3 concentrations during the night were lower than at Maïdo due to the lower altitude of the site [28], higher concentrations of NO, ozonolysis of terpenes, and surface deposition [55].…”

“…The average concentration of the main biogenic and anthropogenic VOCs is reported for the three sites in Figure 3. A slight reduction in VOCs concentrations was observed for the Maïdo site, except for isoprene, which is emitted close to the Maïdo observatory by neighboring tamarind forests [29]. The concentrations between the Maïdo observatory and Bélouve were comparable.…”

“…The climate is tropical with wet and hot summers with a cyclonic season from November to March and, depending on the altitude, mild to cool winters from May to September. VOCs, NO x , O 3 , and SO 2 concentrations were measured at the end of the cyclonic season, and the temperatures observed during the campaign varied between 12 and 16 • C at the Maïdo observatory, 23 On the synoptic scale, under the influence of trade winds, air masses are mainly advected toward the northwest over the mountains of Reunion Island, known as the over-flow phenomenon [28][29][30]. The observatory is also influenced by local air circulation patterns with thermodynamic processes linked to upslope and downslope winds such as the catabatic-anabatic slope winds and sea-land breeze.…”

“…These air masses are mostly within the marine boundary layer (MBL), which contains biogenically-or anthropogenically-emitted compounds [28]. In contrast, during the night, the observatory is mostly in the free troposphere, the surface cools, and air masses move downward [28][29][30][31]. Figure 2 presents the spatial distribution of back-trajectories points during the measurement campaign period.…”