Stromboli volcano, the northernmost island of the Aeolian volcanic arc (Italy), rises 924 m above sea level (a.s.l.). The island represents the top part of a large 2500 m high stratovolcano emerging from the Tyrrhenian Sea floor. Stromboli is well known for its regular (∼ every 10–20 min) explosive activity (Strombolian activity). Intermittently, continuous passive degassing occurs from the active vents, which are located in the crater terrace at about 750 m a.s.l. Ejected lava material is dominantly deposited in a northwestern direction, forming a horseshoe-shaped area (Sciara del Fuoco) that is not safely accessible on foot. The summit above the craters is easily accessible, and numerous monitoring stations have been installed here for continuous observation of the ongoing volcanic activity. Thus, Stromboli has been a laboratory volcano for studying magma degassing processes (Allard et al., 2008) and field testing new instrumentations for many years.

While most gas-monitoring station positions benefit from a northwesterly wind direction, a southeasterly wind only allows gas plume detection by spectroscopic methods. In this study, due to the dominance of southeasterly winds in early April 2016 an approach from an easterly direction was chosen, using a multicopter as a carrier for gas sampling and sensing instruments. The UAV was mostly launched at the northern shelter (see Fig. 1).

Masaya volcano (elevation ∼ 600 m a.s.l.), Nicaragua, is a basaltic andesite shield volcano caldera (6 × 11 km in size) with a set of vents. The currently active vent is situated in the Santiago pit crater, formed in 1858–1859 (McBirney, 1956). Masaya persistently emits voluminous quantities of SO2, with fluxes typically ranging from 500 to 2500 T d-1 (Mather et al., 2006; de Moor et al., 2013; Carn et al., 2017), currently making this volcano one of the largest contributors of volcanic gas emissions in the Central American Volcanic Arc (de Moor et al., 2017). The reappearance of a superficial lava lake (∼ 40 × 40 m) at Masaya in January 2016 marked an increase in degassing activity. Due to high emission rates and the low-altitude plume, Masaya volcano has a detrimental environmental impact on the downwind areas, diminishing vegetation and potentially affecting human health (Delmelle et al., 2002). Continuous monitoring of the gas emissions is realized by a stationary MG system (through the Deep Carbon Observatory – Deep Earth Carbon Degassing initiative, DCO-DECADE) at the crater rim and two scanning DOAS instruments (Network for Observation of Volcanic and Atmospheric Change (NOVAC); Galle et al., 2010) in the downwind direction. Besides the presence of a strong plume, Masaya volcano provides perfect conditions for field testing new methods and studying plume chemistry using UAVs: it is easily accessible by car, at a low altitude, and has a relatively stable dominant wind direction (northeast). In July 2016 flights were launched from the caldera bottom, marked “flight area” in Fig. 2c.

Turrialba is a stratovolcano with a peak elevation of 3340 m a.s.l. and is located about 35 km east and directly upwind of San José, the capital of Costa Rica. It is the southernmost active volcano of the Central American Volcanic Arc. In the 2000s, an almost 150-year-long period of quiescence ended and since 2010 several vent opening phreatic eruptions have occurred, marking an ongoing but erratic phase of unrest (Martini et al., 2010) characterized by variable ash and gas emission intensities (up to 5000 tons day-1 of SO2; de Moor et al., 2016a). This made Turrialba volcano the second most substantial emitter in the arc, besides Masaya volcano (de Moor et al. 2017). The proximity of Turrialba volcano to the densely populated central valley, which has Costa Rica's major international airport, and the dominant western wind direction is responsible for ash depositions, causing health and air traffic problems. Therefore, the activity of Turrialba is continuously monitored by various systems including permanent Multi-GAS stations to observe short-term precursory changes in the gas composition prior to eruptive events (de Moor et al., 2016a). However, stations located near the active vent suffer from ash deposition and ballistic impacts during more frequent episodes, making maintenance demanding and risky or impossible. Furthermore, the accessibility of the summit and surrounding areas is decreasing due to intense erosion following vegetation destruction by acid rain, heavy rainfall, ash deposition and remobilization, and the lack of infrastructural maintenance following community evacuation. Thus, the use of UAV-based systems might represent the only viable approach for in situ measurements of the open-vent plume during periods of high activity. In 2016 flights were conducted starting at the La Silvia site (Fig. 2d) to investigate the feasibility of a UAV-based gas sensing system in this challenging environment (high altitude and thick ash plumes).

The UAV (called RAVEN, remote-controlled aircraft for volcanic emission analysis; see Fig. 3a–b) used during the field campaigns was a four-rotor multicopter with foldable arms (Black Snapper, Globe Flight, Germany) and an E800 motor set using propellers with a diameter of 13 inch (0.33 m) and a pitch of 4.5 inch (0.11 m) (DJI Innovations, Shenzhen, China). It was flown manually in line-of-sight conditions. The multicopter had a weight of 2.3 kg including a 22.2 V (6S) 4.5 Ah battery. A maximum payload of 1.3 kg was achieved with various mounted instruments. The foldable frame of the UAV was beneficial with regards to the usual requirement of personally carrying equipment into the field, especially on volcanoes like Stromboli. Another advantage of this system was that the battery capacity is within the guidelines of air travel restrictions, allowing the system to be transported on commercial airplanes. The main controller (NAZA M-2, DJI Innovations, Shenzhen, China) of the multicopter was connected to a combined denuder sampling and SO2 sensing instrument (hereafter named black box; see Sect. 3.3) to transmit the measured SO2 data as an 0–5 V signal to the remote control, where it is displayed. This allowed the operator to find areas with dense plumes in which to hover the system for stationary sampling and to react to changes in the plume direction. This is challenging if relying only on visual observation of the plume. A data logger (Core 2, Flytrex Aviation, Tel Aviv, Israel) with a micro SD card was used to log the flight data from the main controller consisting of GPS coordinates, pressure and temperature data at 2 Hz. The payloads were attached below the main body of the multicopter with an inlet for the in situ CO2 and SO2 sensing instrument (hereafter called Sunkist; see Sect. 3.2), close to the center of the copter. The sampled air volume can be assumed to originate from within a radius of a few meters around the inlet (see e.g., Roldan et al., 2015; Alvarado et al., 2017; Palomaki et al., 2017), which represents homogeneous plume gas for a widely spread out plume. This assumption is confirmed by a self-developed method for the estimation of the origin of sample air, which is described in the Supplement (Sect. 4).

A gas sensor system was developed for use in volcanic environments with a focus on robustness and has a compact and lightweight design for application on UAVs. The system, called Sunkist (SK), contains two gas sensors:

A CO2 sensor (K30 FR, SenseAir, Delsbo, Sweden) uses the principle of nondispersive infrared absorption spectroscopy: the sample gas gets sucked into a multi-reflection cell, where it is exposed to the radiation of a small infrared light source. The CO2 concentration is determined by measuring the light attenuation on distinct CO2 absorption bands in the near infrared.

The sensors are read out by a custom-built Arduino Uno Rev 3 computer with a micro SD card logger at a sampling rate of 2 Hz and powered by a rechargeable 3.7 V lithium polymer (LiPo) battery (Fig. 4b). The SO2 sensor was powered by a separate 9 V alkaline battery. The whole system is sheltered in a polystyrene foam case and has a total weight of 500 g (Fig. 3c). A 45 µm pore size PTFE filter was attached to the inlet to prevent particles from entering the sensors. Gas was pumped through to the sensors in series (1. SO2, 2. CO2) by a small pump using a flow rate of 500 mL min-1. The system was calibrated before and after field deployments with CO2 (0–1500 ppm) and SO2 (0–30 ppm) test gases. Detailed information on the specifications of the sensors is shown in Table 1 and in the Supplement (Fig. S1).

An in situ gas sampling system was constructed to enable gas diffusion denuder sampling in the plume at various distances from the vent using the UAV. To compensate for dilution, a CiTiceL 3MST/F SO2 sensor (City Technology, Portsmouth, United Kingdom) was implemented to obtain halogen/sulfur ratios combining denuder samples and sensor data. The sampler (called black box – BB) consisted of the following components, which are introduced in the order the gas passes through:

inlet system for three denuders,

Gas diffusion denuder sampling, which enriches gaseous compounds while being insensitive to the particle phase, was applied by using two types of coating materials as derivatization agents for the gas diffusion sampling. Total gaseous reactive molecular bromine species, Brx (Br2, BrCl, (H)OBr), were determined by denuders coated with 15 µmol of 1,3,5-trimethoxybenzene (TMB) – which reacts to 1-bromo-2,4,6-trimethoxybenzene – and subsequent gas chromatography-mass spectrometry (GC-MS) analysis (Rüdiger et al., 2017). Hydrogen bromide (HBr) was sampled with denuders coated with 7.2 µmol of 5,6-epoxy-5,6-dihydro-1,10-phenanthroline (EP) – which is selectively reactive towards halogen acids through its epoxy function, forming 5-halogeno-6-hyrdroxy-5,6-dihydro-1,10-phenanthroline – and analyzed by liquid chromatography mass spectrometry (LC-MS). For the UAV-based application 15 cm long denuders were used at a sampling flow rate of 208 mL min-1 to ensure quantitative sampling. Denuders with both coating types were sampled at the same flow rate simultaneously with the recording of Multi-GAS (Sunkist) data.

A miniature UV spectrometer system (Galle et al., 2003) was employed to fly mobile DOAS traverse measurements which could conduct estimations of the SO2 flux at Turrialba volcano. This system consisted of an UV spectrometer (Fig. 8a; USB2000+, Ocean Optics, USA), a miniature telescope (Ocean Optics 74-DA collimating lens, diameter: 5 mm, focal length 10 mm), a GPS antenna (BU-353-S4, GlobalSat, Taipei, Taiwan) and a miniature on-board computer (VivoStick TS10, ASUS, Taipei, Taiwan). The system was powered by a 11.1 V LiPo battery (1000 mAh), which was connected via a switching regulator (CC BEC 10 A, Castle Creations, USA) to give 9 V at 2 A. The spectrometer and the GPS antenna were connected (and powered) at the computer via USB ports. The NOVAC mobile DOAS software developed at the Chalmers University (Sweden) was run on the computer for data acquisition and later for evaluation. The miniature PC in the spectrometer system was accessed via a remote desktop connection on a different computer to initialize the data acquisition by the mobile DOAS” software. Validation of the SO2 fluxes obtained by DROAS traverses was achieved by comparison with the SO2 fluxes derived by two NOVAC stationary DOAS instruments (La Silvia and La Central; see Fig. 2d) located in proximity to the flight area.

During the deployment at three volcanoes, the RAVEN multicopter conducted more than 50 flights under moderate wind conditions, in most cases below 10 m s-1. The multicopter achieved a maximum operation altitude of 3320 m at Turrialba volcano and records showed a maximum speed of 85 km h-1. With a takeoff weight of 2.45 kg and a payload of maximum 1.3 kg flights at Turrialba volcano were still possible, although the flight time was reduced to about 5 to 8 min. At the Masaya volcano sites (takeoff altitude ∼ 500 m), a maximum ascent above ground level of 1080 m was recorded. A typical flight time at this site was between 10 and 15 min. The telemetrically transmitted SO2 mixing ratios from the black box apparatus allowed the localization of high plume densities and therefore adjustments on the optimal hover location. While flying in the line of sight the system could be reliably controlled within a distance of 1–2 km to the operator. However, entering the dense plume proved to challenge the connection between the remote control and the receiver, resulting in multiple connection losses during flights, in which the UAV also left the line of sight of the operator. Nevertheless, a GPS connection was still present and an automated return mechanism allowed control to be regained after the UAV left areas of high plume densities. At Masaya this phenomenon was observed mostly in a plume with condensed water and SO2 mixing ratios in low one-digit ppmv numbers, while in a plume without condensation close to the crater, control was maintained even with SO2 levels up to 40 ppmv. This is probably due to the attenuation effect of fog and cloud droplets on millimeter waves, as they are used with the 2.4 Ghz transmitter of the remote control (Zhao and Wu, 2000).

During two field deployments, at the Stromboli (Table 2) and Masaya (Table 3) volcanoes, the lightweight gas-monitoring system SK determined CO2 / SO2 gas ratios at various airborne and ground-based locations. A comparison with a stationary MG instrument at Masaya volcano for the same site and time (lookout point south, 14 July 2016, 11:19) and with inlets of both systems in proximity to each other gave results on CO2 / SO2 ratios that were within each other's 2σ intervals (CO2 / SO2 of 2.9 ± 0.3 for MG and 3.6 ± 0.4 for SK). The time series of the SO2 mixing ratios of SK and MG also showed good agreement (R2 = 0.89) (Fig. 6). An application of SK further downwind (0.5 km from the rim) gave a CO2 / SO2 ratio of 3.3 ± 1.2, while the MG measured a CO2 / SO2 ratio of 3.1 ±  0.1 during the same time at the rim. This shows SK's ability to be deployed in a more diluted plume, but with the disadvantage of higher errors, due to the higher relative background in CO2 at more distant locations. Furthermore, UAV-based application (nine flights, 17–20 July 2017) between 1.5 km and 2 km downwind of the Masaya plume resulted in 42 % higher CO2 / SO2 ratios on average with a larger standard deviation compared to the crater rim (14–16 July 2017), but still within each other's errors (CO2 / SO2: 5.4 ± 2.3 at 1.5–2 km; 3.8 ± 0.3 at the rim). Due to the limitations of the CO2 sensing, acquiring useful CO2 / SO2 data with SK is more feasible in dense plumes close to the crater rim but not limited to it, as the airborne application has shown. Additionally, SO2 mixing ratios were measured as a plume dilution proxy by the black box (BB) system. Both the BB and SK systems use an identical SO2 sensor and showed a good agreement of their SO2 time series and time-integrated SO2 mixing ratio (SK: 1.75 ± 0.08 ppmv, BB: 1,84 ± 0.08 ppmv) (Supplement S6).

At Stromboli volcano, the Sunkist and black box systems were deployed on 2 days (5 and 6 April 2016), resulting in seven flights into the plume. These flights covered distances of between 11 and 419 m from the vent in a downwind direction (northeast), above the Sciara del Fuoco. As shown in Fig. 1, discrete gas clouds with different CO2 / SO2 compositions were measured. The retrieved CO2 / SO2 ratios ranged between 7 and 64, with the higher values typically detected directly above the vent (11 to 26 m) (see Table 3 and Fig. 1). During the multicopter operations close to the vent regular strombolian ash explosions occurred (see Fig. 3d and e), which are likely accompanied by CO2-rich gas clouds (La Spina et al., 2013) and therefore act as a possible explanation for the detected high CO2 / SO2 ratios. On both flight days the predominant wind direction was southwest (207∘ ± 15∘ and 210∘ ± 18∘, data from weather station at 77 m a.s.l., commercially available from http://www.windfinder.com, Kiel, Germany), which resulted in the plume mostly being present across the Sciara del Fuoco and therefore only accessible by UAV. Nevertheless, a local Multi-GAS station (placed on the SE rim of the crater terrace) discontinuously measured the plume during the period of the UAV survey, showing CO2 / SO2 ratios between 2.2 and 13.6, which is in agreement with some of the multicopter-based measurements. Similar CO2 / SO2 ratios have been observed in the past and are exemplary for ordinary Strombolian activity (Aiuppa et al., 2009). It has to be taken into account that both instruments did not measure simultaneously or in proximity to each other. The MG instrument only measures four times a day for 30 min and averages over this time, while with the SK instrument we identified discrete peaks of gas clouds with different CO2 / SO2 ratios. Furthermore, a comparison of SK and MG CO2 / SO2 data might be more accurate with the high SK CO2 / SO2 values (associated with eruptive degassing) left aside, as we lack the observations on passive or eruptive degassing behavior for the MG data records.

Bromine species were detected by gas diffusion denuder sampling on three of the seven flights at Stromboli volcano. Reactive bromine species were measured between 0.14 and 0.65 ppb (see Table 4), while HBr was determined qualitatively in all three samples.

The obtained ratios of reactive bromine (Brx) to SO2 (1.9 × 10-4–9.8 × 10-4) are within the range of bromine to sulfur ratios produced by other methods at Stromboli volcano, e.g., alkaline trap sampling by Wittmer et al. (2014) (4.3 × 10-4–2.36 × 10-2). Figure 7b shows the Brx / SO2 ratios for different plume ages, which was calculated by taking wind speeds into account. Although the general feasibility of the used methods for the investigation of reactive halogen species in an aging plume is demonstrated in this proof-of-principle approach, a trend in the Brx / SO2 ratio over age is not recognizable for this limited data set. Without information on abundances of other halogen species such as BrO(g), HBr(g) and aqueous particulate bromine (HBr(aq)), interpretation of the Brx / SO2 ratio is difficult, as the emitted gas composition may also change on shorter timescales (La Spina et al., 2013) compared to the campaign duration. However, an increase in the reactive bromine species BrO with distance from the crater rim has previously been observed by DOAS measurements at various volcanoes and is well described in the literature (Oppenheimer et al., 2006; Bobrowski et al., 2007). Although the bromine speciation in volcanic plumes has been the subject of several ground-based (e.g., Gliß et al., 2015) airplane (e.g., General et al., 2015), satellite (e.g., Theys et al., 2009; Hörmann et al., 2013) and model studies (e.g., Bobrowski et al., 2007; Roberts et al., 2009, 2014; von Glasow, 2010; Jourdain et al., 2016) in recent years, in situ measurements are scarce. The data presented here for the first minute after emission highlight the potential for UAV-based measurements to improve the sample acquisition and thus obtain a better understanding of plume aging.

As shown in Fig. 7c, the Brx to SO2 ratio seems to change not only with the plume age but also with CO2 / SO2 mixing ratios, which were simultaneously obtained. However, with only three data points a further interpretation is inadequate due to the lack of a statistical basis. Nevertheless, these first results show the principal practicality of the used denuder sampling and gas sensing methods for simultaneous investigation of halogen, carbon and sulfur emissions.

On 27 September, two DROAS plume transects were performed at Turrialba volcano during mild ash emission at around 2800 m a.s.l. with a total flight time of approximately 10 min. The flight was conducted in the downwind direction west of the active vent and crossed underneath the plume on a south–north axis (Fig. 8b), exiting the plume on both ends of the flight path, which is indicated by the low SO2 column amounts close to the baseline in the northern- and southernmost section of the flight path (Fig. 8c). SO2 fluxes were calculated for that flight and resulted in 1776 ± 1108 T d-1 SO2 for traverse A and 1616 ± 1007 T d-1 SO2 for traverse B (see Table 5). Calculation of the SO2 fluxes obtained by the two scanning DOAS instruments from the NOVAC network, located on the southern edge of the plume, gave average results of 1533 ± 986  T d-1 SO2 for La Silvia and 1094 ± 704 T d-1 SO2 from La Central for a 2 h period, in which the flight took place. Therefore, the results of the DROAS traverses are in good agreement with the scanning DOAS stations. De Moor et al. (2016a) conducted car-based mobile DOAS transects, which typically take ∼ 45 min for a round trip due to rough terrain. Dynamic gas plumes can significantly change the plume's travel direction on scales of minutes, introducing a significant source of error to car traverses. A major advantage of the UAV method is that it is much quicker, about 10 min for a round trip, thus providing a more accurate snapshot of the plume.