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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.

Ice nucleation abilities of surface collected mineral dust particles from the Sahara (SD) and Asia (AD) are investigated for the temperature range 253–233 K and for supersaturated humidity conditions in the immersion-freezing regime. The dust particles were also coated with a proxy of secondary organic aerosol (SOA) from the dark ozonolysis of α-pinene to better understand the influence of atmospheric coatings on the immersion freezing ability of mineral dust particles. The measurements are conducted on poly-disperse particles in the size range 0.01–3 µm with three different ice nucleation chambers. Two of the chambers follow the continuous flow diffusion chamber (CFDC) principle (Portable Ice Nucleation Chamber, PINC) and the Colorado State University CFDC (CSU-CFDC), whereas the third was the Aerosol Interactions and Dynamics in the Atmosphere (AIDA) cloud expansion chamber. From observed activated fractions (AF) and ice nucleation active site (INAS) densities, it is concluded within experimental uncertainties that there is no significant difference between the ice nucleation ability of the particular SD and AD samples examined. A small bias towards higher INAS densities for uncoated versus SOA coated dusts is found but this is well within the 1σ (66 % prediction bands) region of the average fit to the data, which captures 75 % of the INAS densities observed in this study. Furthermore, no systematic differences are observed between SOA coated and uncoated dusts in both SD and AD cases, regardless of coating thickness (3–60 nm). The results suggest that differences observed are mostly within the uncertainty of the measurements or differences in cloud chamber parameters such as size fraction of particles sampled, and residence time, as well as assumptions in using INAS densities to compare poly-disperse aerosol measurements which may show variable composition with particle size. In the atmosphere, coatings with similar properties to that of the SOA tested here for a thickness up to 60 nm, the maximum thickness tested here, is not expected to impede or enhance the ice nucleation ability by immersion mode of mineral dust in the mixed-phase cloud regime.

A number of new measurement methods for ice nucleating particles (INPs) have been introduced in recent years, and it is important to address how these methods compare. Laboratory comparisons of instruments sampling major INP types are common, but few comparisons have occurred for ambient aerosol measurements exploring the utility, consistency and complementarity of different methods to cover the large dynamic range of INP concentrations that exists in the atmosphere. In this study, we assess the comparability of four offline immersion freezing measurement methods (Colorado State University Ice Spectrometer, IS; North Carolina State University Cold Stage, CS; National Institute for Polar Research Cryogenic Refrigerator Applied to Freezing Test, CRAFT; University of British Columbia Micro-Orifice Uniform Deposit Impactor – Droplet Freezing Technique, MOUDI-DFT) and an online method (continuous flow diffusion chamber, CFDC) used in a manner deemed to promote/maximize immersion freezing, for the detection of INP in ambient aerosols at different locations and in different sampling scenarios. We also investigated the comparability of different aerosol collection methods used with offline immersion freezing instruments. Excellent agreement between all methods could be obtained for several cases of co-sampling with perfect temporal overlap. Even for sampling periods that were not fully equivalent, the deviations between atmospheric INP number concentrations measured with different methods were mostly less than one order of magnitude. In some cases, however, the deviations were larger and not explicable without sampling and measurement artifacts. Overall, the immersion freezing methods seem to effectively capture INP that activate as single particles in the modestly supercooled temperature regime (> −20 °C), although more comparisons are needed in this temperature regime that is difficult to capture with online methods. Relative to the CFDC method, three immersion freezing methods that disperse particles into a bulk liquid (IS, CS, CRAFT) exhibit a positive bias in measured INP number concentrations at below −20 °C, increasing with decreasing temperature. This bias was present, but much less pronounced for a method that condenses separate water droplets onto limited numbers of particles prior to cooling and freezing (MOUDI-DFT). Potential reasons for the observed differences are discussed, and further investigations are required to elucidate the role of all factors involved.