Atmos. Chem. Phys., 21, 3371–3393, 2021
https://doi.org/10.5194/acp-21-3371-2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.
Halogen activation in the plume of Masaya volcano: field
observations and box model investigations
Julian Rüdiger
1,2
, Alexandra Gutmann
1
, Nicole Bobrowski
3,4
, Marcello Liotta
5
, J. Maarten de Moor
6
, Rolf Sander
4
,
Florian Dinger
3,4
, Jan-Lukas Tirpitz
3
, Martha Ibarra
7
, Armando Saballos
7
, María Martínez
6
, Elvis Mendoza
7
,
Arnoldo Ferrufino
7
, John Stix
8
, Juan Valdés
9
, Jonathan M. Castro
10
, and Thorsten Hoffmann
1
1
Institute of Inorganic and Analytical Chemistry, Johannes Gutenberg University, Mainz, Germany
2
Environmental Chemistry and Air Research, Technical University Berlin, Berlin, Germany
3
Institute of Environmental Physics, University of Heidelberg, Heidelberg, Germany
4
Max Planck Institute for Chemistry, Mainz, Germany
5
Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Italy
6
Observatorio Vulcanológico y Sismológico de Costa Rica Universidad Nacional, Heredia, Costa Rica
7
Instituto Nicaragüense de Estudios Territoriales, Managua, Nicaragua
8
Department of Earth and Planetary Sciences, McGill University, Montreal, Canada
9
Laboratorio de Química de la Atmósfera, Universidad Nacional, Heredia, Costa Rica
10
Institute of Geosciences, Johannes Gutenberg University Mainz, Mainz, Germany
Correspondence: Thorsten Hoffmann (t.hoffmann@uni-mainz.de)
Received: 25 March 2020 – Discussion started: 22 June 2020
Revised: 9 January 2021 – Accepted: 26 January 2021 – Published: 4 March 2021
Abstract. Volcanic emissions are a source of halogens in the
atmosphere. Rapid reactions convert the initially emitted hy-
drogen halides (HCl, HBr, and HI) into reactive species such
as BrO, Br
2
, BrCl, ClO, OClO, and IO. The activation reac-
tion mechanisms in the plume consume ozone (O
3
), which
is entrained by ambient air that is mixed into the plume.
In this study, we present observations of the oxidation of
bromine, chlorine, and iodine during the first 11 min fol-
lowing emission, examining the plume from Santiago crater
of the Masaya volcano in Nicaragua. Two field campaigns
were conducted: one in July 2016 and one in September
2016. The sum of the reactive species of each halogen was
determined by gas diffusion denuder sampling followed by
gas chromatography–mass spectrometry (GC-MS) analysis,
whereas the total halogens and sulfur concentrations were
obtained by alkaline trap sampling with subsequent ion chro-
matography (IC) and inductively coupled plasma mass spec-
trometry (ICP-MS) measurements. Both ground and airborne
sampling with an unoccupied aerial vehicle (carrying a de-
nuder sampler in combination with an electrochemical SO
2
sensor) were conducted at varying distances from the crater
rim. The in situ measurements were accompanied by remote
sensing observations (differential optical absorption spec-
troscopy; DOAS). The reactive fraction of bromine increased
from 0.20 ± 0.13 at the crater rim to 0.76 ± 0.26 at 2.8 km
downwind, whereas chlorine showed an increase in the re-
active fraction from (2.7 ± 0.7) × 10
−4
to (11 ± 3) × 10
−4
in
the first 750 m. Additionally, a reactive iodine fraction of 0.3
at the crater rim and 0.9 at 2.8 km downwind was measured.
No significant change in BrO / SO
2
molar ratios was ob-
served with the estimated age of the observed plume ranging
from 1.4 to 11.1 min. This study presents a large complemen-
tary data set of different halogen compounds at Masaya vol-
cano that allowed for the quantification of reactive bromine
in the plume of Masaya volcano at different plume ages. With
the observed field data, a chemistry box model (Chemistry
As A Boxmodel Application Module Efficiently Calculat-
ing the Chemistry of the Atmosphere; CAABA/MECCA) al-
lowed us to reproduce the observed trend in the ratio of the
reactive bromine to total bromine ratio. An observed contri-
bution of BrO to the reactive bromine fraction of about 10 %
was reproduced in the first few minutes of the model run.
Published by Copernicus Publications on behalf of the European Geosciences Union.
3372 J. Rüdiger et al.: Halogen activation in the plume of Masaya volcano
1 Introduction
Volcanoes are known to be important emitters of atmo-
spheric trace gases and aerosols, both through explosive
eruptions and persistent quiescent degassing (von Glasow et
al., 2009). The most abundant gases in volcanic emissions
are water, carbon dioxide, sulfur compounds, and hydrogen
halides (Symonds et al., 1994). Typically, halogen emissions
are largely dominated by chlorine (HCl) and fluorine (HF),
whereas bromine (HBr) and iodine (HI) are 3 and 5 orders
of magnitude less abundant than chlorine and fluorine, re-
spectively (e.g., Aiuppa et al., 2005; Pyle and Mather, 2009).
Despite their low abundance, the heavy halogens (bromine
and iodine) can have a significant impact on the chemistry
of the atmosphere (e.g., von Glasow, 2010; Saiz-Lopez and
von Glasow, 2012; Platt and Bobrowski, 2015). The chemi-
cal composition of volcanic plumes is the subject of a large
number of studies, which are often aimed at gaining insights
into subsurface processes, such as the degassing of magma in
connection with changes in volcanic activity (e.g., Aiuppa et
al., 2007). In addition, the effects of volcanic gases on the at-
mosphere and biosphere at local, regional, and global scales
are also of interest, including acid deposition (wet and dry),
nutrient input (e.g., Delmelle, 2003), aerosol formation, and
the effects on the solar radiation balance (e.g., Mather et al.,
2013; Malavelle et al., 2017).
Volcanic halogen emissions have been studied for years
(e.g., Noguchi and Kamiya, 1963; Giggenbach, 1975), and
the determination of chlorine and sulfur is a common proce-
dure in such gas geochemical investigations. Bromine only
attracted more attention in later years, when the reactive
bromine species BrO was observed in volcanic plumes (e.g.,
Bobrowski et al., 2003; Oppenheimer et al., 2006). This also
proved that not only sulfur species (H
2
S, SO
2
) undergo ox-
idation by ambient reactants (such as OH and O
3
), and it
laid the foundation for various studies on oxidized halogen
species (such as BrO, ClO, OClO, and IO) (e.g., Lee et al.,
2005; Theys et al., 2014; General et al., 2015; Gliß et al.,
2015; Schönhardt et al., 2017). Despite the low abundance of
bromine in volcanic gas emissions, the relatively simple de-
tection of BrO by differential optical absorption spectroscopy
(DOAS) promoted research on the origin and fate of BrO in
volcanic plumes. Based on thermodynamic modeling, Ger-
lach (2004) hypothesized that BrO is not primarily emitted
by volcanoes and is instead formed only after the initial emis-
sions are mixed with entrained ambient air. As SO
2
can also
be easily measured by DOAS, and the oxidation of SO
2
plays
a minor role over a period of minutes to hours (McGonigle
et al., 2004), the ratio of BrO to SO
2
is used as a dilution-
compensated observation parameter.
An increase in the BrO / SO
2
ratio with increasing dis-
tance from the emitting vent has been observed at various
volcanoes (e.g., Bobrowski et al., 2007; Vogel, 2012; Gliß et
al., 2015) as have variations in BrO / SO
2
in a lateral plume
dimension with higher ratios at the edges of the plume (e.g.,
Table 1. Overview of halogen reactions in volcanic plumes (X = Cl,
Br).
Br
(g)
+ O
3(g)
→ BrO
(g)
+ O
2(g)
(R1)
BrO
(g)
+ HO
2(g)
→ HOBr
(g)
+ O
2(g)
(R2)
HBr
(g)
→ Br
−
(aq)
+ H
+
(aq)
(R3)
HOBr
(aq)
+ Br
−
(aq)
+ H
+
(aq)
→ Br
2(g)
+ H
2
O
(aq)
(R4a)
HOBr
(aq)
+ HCl
(aq)
→ BrCl
(g)
+ H
2
O
(aq)
(R4b)
Br
2(g)
hv
−→ 2Br
(g)
(R5a)
BrCl
(g)
hv
−→ Br
(g)
+ Cl
(g)
(R5b)
Cl
(g)
+ O
3(g)
→ ClO
(g)
+ O
2(g)
(R6)
BrO
(g)
+ BrO
(g)
→ 2Br
(g)
+ O
2(g)
(R7a)
BrO
(g)
+ BrO
(g)
→ Br
2(g)
+ O
2(g)
(R7b)
BrO
(g)
+ ClO
(g)
→ OClO
(g)
+ Br
(g)
(R8)
BrO
(g)
+ NO
2(g)
→ BrONO
2(g)
(R9)
BrONO
2(g)
+ H
2
O
(aq)
→ HOBr
(aq)
+ HNO
3(aq, g)
(R10)
Bobrowski et al., 2007; Louban et al., 2009; General et al.,
2015; Kern and Lyons, 2018). This has been explained by
a limited transfer of atmospheric O
3
to the center of the
plume, which is thought to promote the formation of BrO in
a chain reaction mechanism involving heterogeneous chem-
istry. Shortly after the discovery of the reactive bromine
species BrO, reactive chlorine species, ClO and OClO, were
also observed using the same DOAS techniques (e.g., Lee
et al., 2005; Bobrowski et al., 2007; Donovan et al., 2014;
Theys et al., 2014; General et al., 2015; Gliß et al., 2015;
Kern and Lyons, 2018). It was found that the abundance of
ClO and OClO is of the same order of magnitude as BrO, in
contrast to total chlorine, which is typically 3 orders of mag-
nitude more abundant than bromine. The formation of reac-
tive chlorine species is considered to be a secondary product
of the activation cycle of bromine (see Table 1). Recently,
reactive iodine species have also been detected by satellite
observations in the plume of Kasatochi (Schönhardt et al.,
2017), but they could not be confirmed by ground-based
measurements to date.
Both the transformation of halogen species in the plume
and their fate in the atmosphere are of interest. In particu-
lar, clarification regarding the amounts emitted into the at-
mosphere and the distribution of the halogens emitted by
quiescent (i.e., passive, non-eruptive) and eruptive degassing
is of interest. The global volcanic SO
2
flux has been esti-
mated as 23 Tg yr
−1
for the period from 2004 to 2016 (Carn
et al., 2017), resulting in estimated halogen fluxes of the
same order for chlorine and 3 orders of magnitude lower for
bromine, taking global mean sulfur / halogen ratios into ac-
count (Aiuppa et al., 2009).
Bromine from various sources (e.g., polar regions, salt
lakes, and volcanoes) is involved in tropospheric and strato-
spheric ozone depletion (e.g., Wennberg, 1999; Rose et al.,
Atmos. Chem. Phys., 21, 3371–3393, 2021 https://doi.org/10.5194/acp-21-3371-2021
J. Rüdiger et al.: Halogen activation in the plume of Masaya volcano 3373
2006; Simpson et al., 2007). Tropospheric ozone depletion
has also been observed in volcanic plumes (e.g., Hobbs et
al., 1982; Kelly et al., 2013; Surl et al., 2015; Roberts, 2018),
which supports the proposed reaction mechanisms for BrO
formation via autocatalytic chain reactions. Recent observa-
tions of halogen oxides by satellites (e.g., Theys et al., 2009;
Carn et al., 2016) and aircraft missions (Heue et al., 2011)
confirm that some large volcanic eruptions may inject a pro-
portion of their volcanic halogens into the free troposphere
or even to the stratosphere, thereby confirming their poten-
tial impact on stratospheric ozone. In addition to the effects
of volcanic degassing on atmospheric chemistry, measure-
ments of volcanic emission have become an important and
well-established tool in the assessment of volcanic hazard,
and gas monitoring is used at many volcanoes around the
world (e.g., Carroll and Holloway, 1994; Aiuppa et al., 2007;
de Moor et al., 2016).
It has been also observed that the BrO / SO
2
gas ra-
tio changes with the activity of volcanoes. Bobrowski and
Giuffrida (2012) observed lower BrO / SO
2
ratios in Etna’s
plume during eruptive phases. Moveover, long-term DOAS
observations by Lübcke et al. (2014), who used stationary
spectrometers within the Network for Observation of Vol-
canic and Atmospheric Change (NOVAC; Galle et al., 2010),
showed a decrease in the BrO / SO
2
ratio before and during
explosive activity at Nevado del Ruiz volcano. More recently,
a study by Dinger et al. (2018) at the Cotopaxi volcano
(Ecuador) showed low BrO / SO
2
ratios at the beginning of
eruptive activity compared with higher ratios present during
declining volcanic activity. Finally, Warnach et al. (2019)
found a low BrO / SO
2
ratio during high explosive periods
and an increased BrO / SO
2
ratio during less explosive peri-
ods at Tungurahua volcano.
However, it is not yet clear whether the BrO / SO
2
ratio
can be used as a robust diagnostic tool for forecasting vol-
canic activity. As BrO is a reactive secondary gas species,
its concentration in a volcanic plume potentially depends
on atmospheric variables such as humidity, oxidant abun-
dance, solar radiation, and aerosol surface. The BrO / SO
2
ratio might not always or may only partially be controlled
by the total bromine emission at a particular volcano under
study (Roberts et al., 2018). Therefore, further knowledge of
the chemistry that drives halogen activation is required.
Following the Introduction, an overview of the volcanic
plume halogen chemistry and related model studies is given
in Sect. 2. The comprehensive data set obtained during two
field campaigns at Masaya volcano using several measure-
ment techniques, including DOAS (e.g, Bobrowski et al.,
2003), Multi-GAS (Shinohara, 2005; Aiuppa et al., 2006), al-
kaline traps (e.g., Wittmer et al., 2014), and gas diffusion de-
nuder sampling (e.g., Rüdiger et al., 2017) is then presented
in Sect. 3. In Sect. 4, we outline the use of an unoccupied
aerial vehicle (UAV; e.g., Rüdiger et al., 2018; Stix et al.,
2018) that enabled the sampling of a downwind plume for
the investigation of halogen-induced plume-aging processes;
this process is then reproduced by the atmospheric modeling
of plume halogen chemistry. In Sect. 5, the outcome of the
modeling is compared with the field measurement results be-
fore Sect. 6 draws the conclusions and provides an outlook
for future studies.
2 Volcanic plume halogen chemistry
Besides numerous field surveys at various volcanoes, several
atmospheric modeling studies have been conducted that have
improved our understanding of the complex chemical reac-
tions in volcanic plumes marking the interface between vol-
canic trace gases (and aerosols) and ambient air. Two differ-
ent models have been developed by researchers to simulate
the in-plume chemistry, the microphysical marine boundary
layer model (MISTRA; Bobrowski et al., 2007; von Glasow,
2010) and PlumeChem (Roberts et al., 2009; Roberts et al.,
2014). While MISTRA is a 1-D box model including mul-
tiphase chemistry, PlumeChem additionally includes plume
dispersion and 3-D simulation by employing a multiple grid
box mode, and the multiphase chemistry is parameterized us-
ing uptake coefficients rather than being modeled explicitly
in the aqueous phase. The models used in this study (MIS-
TRA; PlumeChem; and Chemistry As A Boxmodel Appli-
cation Module Efficiently Calculating the Chemistry of the
Atmosphere, CAABA/MECCA) are initialized with the gas
composition of a so-called “effective source region”. This
gas composition is typically derived from a thermodynamic
equilibrium model (HSC, Outotec, Finland; e.g., Gerlach,
2004; Martin et al., 2006). Different mixtures of magmatic
gas and ambient air yield the hot gas mixture of the effec-
tive source region, which is quenched to ambient tempera-
ture and then mixed with ambient air including O
3
, OH, and
NO
x
. The suitability of the HSC model to represent high-
temperature chemistry in the plume has been debated, and it
has recently been shown that the chemistry should be better
represented by a kinetics model (Martin et al., 2012; Roberts
et al., 2019). The limitations of HSC with respect to repre-
senting the high-temperature chemistry include the assump-
tion of thermodynamic equilibria in the hot plume region,
which is quite improbable. The choices for temperature, a
mixing ratio of volcanic and magmatic gas, and a “quench-
ing factor” are rather arbitrary and do not necessarily reflect
reality. The kinetics model studies show that the timescale for
substantial NO
x
to be formed thermally appears longer than
a reasonable lifetime of a hot plume (Martin et al., 2012).
Moreover, Roberts et al. (2019) showed differences in the
predicted formation of H
x
O
y
in a kinetics model compared
with those assumed by thermodynamics, in terms of magni-
tudes and speciation. Therefore, conclusions drawn from at-
mospheric modeling that depend on the HSC initializations
are inherently limited by the uncertainties and limitation of
HSC. Nevertheless, we follow the former studies using HSC,
despite its limitations, because there is currently no kinetics
https://doi.org/10.5194/acp-21-3371-2021 Atmos. Chem. Phys., 21, 3371–3393, 2021
3374 J. Rüdiger et al.: Halogen activation in the plume of Masaya volcano
model with a more comprehensive chemistry available as an
alternative.
The initially emitted HBr is converted into reactive species
via an autocatalytic mechanism, involving multiphase reac-
tions, which constitute a so-called “bromine explosion” (von
Glasow et al., 2009). Under ozone consumption, Br radicals
– formed by high-temperature dissociation in the effective
source region – react to BrO (Table 1, Reaction R1), which
in turn reacts with HO
2
or NO
2
to form HOBr (Reaction R2)
or BrNO
3
, respectively. A subsequent uptake into aerosol en-
ables the conversion of HBr into Br
2
, which partitions into
the gas phase and is photolyzed to give two Br radicals and
start the cycle again (Reaction R5a). The self-reaction of
two BrO to give two Br (with Br
2
as a secondary product)
and O
2
is suggested to be the major ozone-depleting channel
at high bromine concentrations, as in a young plume (von
Glasow, 2009). Once HBr becomes depleted, the uptake of
HOBr / BrNO
3
may promote the formation of BrCl, which
also consumes O
3
and forms reactive chlorine species such
as ClO and OClO (Reactions R6, R8). The major reaction
pathways that involve the formation and degradation of BrO
in volcanic plumes are shown in Table 1.
An extensive review of the advances of bromine specia-
tion in volcanic plumes including a comparison of different
model approaches has recently been presented by Gutmann
et al. (2018). In this study, we present in situ measurements
along with remote sensing data on the activation of Br, Cl,
and I in the volcanic plume of Masaya and further investigate
the involved halogen species by atmospheric model simula-
tions using the CAABA/MECCA box model. Although flu-
orine has been measured as well, it is not discussed in detail
in this study due to the high water solubility and the non-
reactivity of fluoride towards oxidation.
3 Measurements
3.1 Site description and flight/sample strategy
Masaya volcano in Nicaragua is a shield volcano with a
caldera size of 6 km × 11 km. The caldera hosts a set of vents,
of which the Santiago pit crater, which formed in 1858–1859,
is currently active (McBirney, 1956). Since mid-November
2015, the Santiago crater has contained a persistent superfi-
cial lava lake (∼ 40 m × 40 m). The lava lake has been asso-
ciated with large volcanic gas emissions, making it one of the
largest contributors of SO
2
emissions in the Central Ameri-
can volcanic arc (Martin et al., 2010; de Moor et al., 2017;
Aiuppa et al., 2018). Due to the high emission rates and the
low-altitude ground-hugging plume, Masaya volcano has a
severe environmental impact on the downwind areas, affect-
ing human and animal health and vegetation (Delmelle et al.,
2002; van Manen, 2014). With its easy accessibility by car
and low altitude, the emissions of Masaya volcano have been
studied extensively throughout the last decades. Of particular
note is the establishment of molar halogen-to-sulfur ratios,
determined to be of the order of 0.3–0.7 for chlorine and
3 × 10
−4
for bromine (e.g., Witt et al., 2008; Martin et al.,
2010; de Moor et al., 2013). These halogen values are consid-
ered to be on the high end observed in magmas and plumes,
yet they are rather typical for arc volcanism (Aiuppa, 2009;
Gutmann et al., 2018). Measurements of reactive bromine
species (BrO) have been reported in the past (Bobrowski and
Platt, 2007; Kern et al., 2009). Continuous composition mon-
itoring (by Multi-GAS) has been realized since 2014, and gas
data for the onset of the superficial lava lake were presented
by Aiuppa et al. (2018).
In our field campaigns in 2016, UAV-based and ground-
based sampling approaches were undertaken to study the
plume of Masaya volcano with a focus on halogen emissions
and atmospheric reactions of the emitted halogens. Samples
were taken on the ground level at the edge of Santiago crater
(Fig. 1) at two locations (lookout south and pole site), at the
top of the rim of Nindirí crater (Nindirí rim) and at Cerro
Ventarrón. UAV-based sampling was conducted in the plume
hovering over the Nindirí crater and above the caldera bottom
and caldera rim (red points in Fig. 1d). Using ground-based
and UAV-based methods, the plume was sampled over a dis-
tance of about 2.8 km, covering an estimated age of 10 min,
depending on the wind velocity.
3.2 Alkaline traps
Total halogen amounts were obtained by ground-based sam-
pling using alkaline traps (Raschig tubes, RTs, and Drechsel
bottles, DBs) (Liotta et al., 2012; Wittmer et al., 2014) at
the locations marked in Fig. 1. The alkaline solution quanti-
tatively captures acidic gas species, due to an acid–base re-
action, and enables the determination of total halogens (F,
Cl, Br, and I) and sulfur (S) concentrations. The sampled
solutions were measured by ion chromatography (IC) and
inductively coupled plasma mass spectrometry (ICP-MS) at
the Geochemistry Laboratories of the Istituto Nazionale di
Geofisica e Vulcanologia, Palermo (Italy). A 1 M NaOH so-
lution was used in the RTs and a 4 M NaOH solution was
used in the DBs. Both solutions were made from 99 % pu-
rity NaOH (Merck, Germany) in 18.2 M cm
−1
water. The
plume samples were pumped through the RTs using a GilAir
Plus pump (Sensidyne, USA) for about 1 h at 2.8–4 L min
−1
.
Total volume data logging enabled mixing ratio calculation
of the RT samples. A custom-built pump (without data log-
ging) was used to pump approx. 1 L min
−1
of gas through the
DBs for between 18 and 30 h each. These samples were used
for gas ratio comparisons over a longer time period.
3.3 Gas diffusion denuder sampling
Reactive halogen species (RHS) were sampled by gas dif-
fusion denuder samplers using 1,3,5-trimethoxybenzene as a
reactive coating (Rüdiger et al., 2017) on borosilicate brown-
Atmos. Chem. Phys., 21, 3371–3393, 2021 https://doi.org/10.5194/acp-21-3371-2021
J. Rüdiger et al.: Halogen activation in the plume of Masaya volcano 3375
Figure 1. (a) Location of the Masaya volcano in Central America (© Google Maps). (b) The Masaya pit crater system in the Masaya
caldera. The flight area (green patch and red points) and sampling locations marked by colored circles in a sketched map (c) and 3-D plot
(d).
glass tubes with a diameter of 0.9 cm. An electrophilic sub-
stitution reaction occurs within this coating, effectively trap-
ping halogen species with an oxidation state (OS) of +1 or 0
(HalX, e.g., Br
2
(OS 0) or BrCl, BrNO3, or BrO (OS + 1)),
which are considered as reactive species in contrast to the -1
OS species Br
−
(aq)
or HBr
(g)
. However, it is not yet experi-
mentally evidenced that the denuder technique is completely
naïve to halogen radicals, although following the chemical
reaction mechanism, the reaction of the denuder coating with
halogen radicals is unlikely. Ground-based denuder measure-
ments employed a serial setup of two denuders (2 × 50 cm)
at a flow rate of 250 mL min
−1
using a GilAir Plus pump
and were conducted simultaneously to the RT sampling for
60 min to give the ratios of reactive species to total halo-
gens (e.g., HalX / Br) or total sulfur (e.g., HalX / S). For the
UAV-based sampling, a remotely controlled sampler (called
Black Box) was used and is described in detail in Rüdi-
ger et al. (2018). The typical sampling flow rate was about
180 mL min
−1
for 5 to 15 min. The Black Box enabled log-
ging of the sampling duration and SO
2
mixing ratios via
the built-in SO
2
electrochemical sensor (CiTiceL 3MST/F,
City Technology, Portsmouth, United Kingdom). Further-
more, the SO
2
sensor signal was transmitted to the remote
control, which helped to identify regions of high SO
2
con-
centrations in real time. The SO
2
signal of the sensor was
time integrated over the sampling period of the denuders to
derive the HalX / S ratios at the location where the UAV hov-
ered during sampling.
3.4 Unoccupied aerial vehicle sampler
The UAV used for this study is a small four-rotor multicopter
with foldable arms (Black Snapper, Globe Flight, Germany)
called RAVEN (Rüdiger et al., 2018). We achieved flight
times of up to 15 min with a payload of approximately 1 kg,
depending on the sampling setup. Global Positioning System
(GPS) data of the flights were recorded on board using the
micro-SD data logger (Core 2, Flytrex, Aviation, Tel Aviv,
Israel) with a 2 Hz time resolution. The four batteries of the
UAV were charged in the field with a car battery, enabling up
to eight flights per day.
3.5 DOAS
DOAS measurements of SO
2
and BrO were performed by a
scanning-DOAS station from NOVAC (Galle et al., 2010),
which is located approximately 1.5 km west-southwest of
Santiago crater at an altitude of 387 m a.s.l. (above sea level,
Aiuppa et al., 2018). This UV spectrometer records the inten-
sity spectra of the diffuse solar radiation over a wavelength
range from 280 to 450 nm for different viewing angles by
scanning the sky from horizon to horizon at steps of 3.6
◦
. For
most of the time, the volcanic plume transects the scan plane
nearly orthogonally. The slant column densities (SCDs) are
retrieved from these spectra via the DOAS method (Platt and
Stutz, 2008). Due to the rather high BrO detection limit,
spectral and arithmetical averaging is required for a reli-
able retrieval of the BrO SCDs and, ultimately, the calcu-
lated BrO / SO
2
molar ratios. As a drawback, the temporal
resolution of the BrO and BrO / SO
2
data is reduced to a
data point roughly every 30 min. For a detailed methodolog-
ical description, see Lübcke et al. (2014) and Dinger (2019).
Due to a data gap caused by an instrument outage, DOAS
data for July 2016 were not available; therefore, the times
series only covers the later part of the field study. The ob-
tained BrO / SO
2
ratios were investigated for a period be-
tween 6 August and 30 September 2016. The plume age was
estimated by employing wind speed data obtained at the air-
https://doi.org/10.5194/acp-21-3371-2021 Atmos. Chem. Phys., 21, 3371–3393, 2021