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Modeling the lava heat flux during severe effusive
volcanic eruption an important impact on surface air
quality

Jonathan Durand, Pierre Tulet, Maud Leriche, Soline Bielli, Nicolas

Villeneuve, Andrea Di Muro, Jean-Baptiste Fillipi

► To cite this version:

Jonathan Durand, Pierre Tulet, Maud Leriche, Soline Bielli, Nicolas Villeneuve, et al. Modeling the lava heat flux during severe effusive volcanic eruption an important impact on surface air quality. Journal of Geophysical Research: Atmospheres, 2014, pp.119. 10.1002/2014JD022034. hal-01164000

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https://hal.science/hal-01164000

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Distributed under a Creative Commons Attribution 4.0 International License
Modeling the lava heat flux during severe effusive volcanic
eruption: an important impact on surface air quality

²

Jonathan Durand,¹ Pierre Tulet,¹ Maud Leriche,² Soline Bielli,¹ Nicolas

Villeneuve,³ Andrea Di Muro,³ and Jean-Baptiste Fillipi⁴

¹LACy, UMR 8105, CNRS - Université de
La Réunion, Météo-France

²LA, UMR 5560, CNRS - Université de
Toulouse III

³IPGP, UMR 7154, CNRS - Université Paris
Diderot

⁴SPE, UMR 6134, CNRS - Université de
Corse
**Abstract.** The Reunion Island experienced its biggest eruption of Piton de la Fournaise volcano during April 2007. Known as “the eruption of the century”, this event degassed more than 230 KT of SO₂. Theses emissions led to important health issues, accompanied by environmental and infrastructure degradations. This modeling study uses the mesoscale chemical model MesoNH-C to simulate the transport of gazeous SO₂ between April 2nd and 7th, with a focus on the influence of heat fluxes from lava. This study required the implementation of a reduced chemical scheme, a basic surface model and an estimation of lava heat fluxes in the atmospheric model. The model was able to reproduce general trends of this eruption, in particular the crossing of trade wind inversion, the SO₂ surface concentration (with highest peak of SO₂ of 600 µg m⁻³ observed April 4th for western Reunion locations), and the wet deposition associated to rainfall. A sensitivity study shows that without heat fluxes over the vent and the lava flow, simulated SO₂ surface concentration are up to 45 times higher than observed.
# 1. Introduction

## 1.1. Generalities

Volcanoes are one of the most important natural sources of air pollution, both during and between eruptions [Oppenheimer, 2003]. It is essential for different areas of atmospheric science to have a good knowledge of volcanic volatile emissions in time and space, their atmospheric chemistry, physical and radiative effects. There are two different types of volcanoes: "reds" volcanoes characterized by relatively quiet effusive eruptions and transmitting any fluid lava in the form of castings, and "gray" volcanoes characterized by explosive eruptions and emitting pasty lava and ash in the form of pyroclastic flows. Each type of volcano is impacting the atmosphere in very different ways, particularly in terms of injection depth and nature of the products ejected. Explosive volcanic eruptions such as those of El Chichon (Mexico) in 1982 [Pollack et al., 1983; Hoffman, 1987] and Mount Pinatubo in 1991 [McCornick et al., 1995; Fiocco et al., 1996; Robock, 2002], mainly affected climate because of radiative and chemical impact of the plumes formed by aerosols injected into the stratosphere [Solomon, 1999; Robock, 2000, 2002]. For effusive volcanic eruptions such as those of Piton de la Fournaise (Reunion Island, Indian Ocean), the problem is different. Knowledge of their atmospheric and environmental impacts in the troposphere and degassing processes has some shortcomings. By chemical oxidation reactions, volcanic gases such as SO₂, which is predominant during the degassing of the lava, become acidic and can interact with the aerosol phase as precursors of particles through nucleation and/or condensation. These tropospheric volcanic aerosols play an important role in atmospheric radiation, directly by scattering and absorbing of short wave radiation, and indirectly by changing cloud cover and cloud properties [Hobbs et al., 1982; Al-
brecht, 1989; Kaufman et al., 2000; Yuan et al., 2011a, b]. Tropospheric volcanic aerosols and gaseous compounds, especially sulfur dioxide, can also be source of risks to terrestrial ecosystems and health at local or regional scales [Baxter et al., 1982; Mannino et al., 1996; Allen et al., 2000; Delmelle et al., 2001]. Piton de la Fournaise is a typical basaltic shield volcano located on the Indian Ocean Island of Réunion and, as Etna and Kilauea, it is one of the world's most active effusive volcanoes, with an eruption occurring every 10 months in average [Roult et al., 2012]. Reunion island is born 3 million years ago in the emergence of a gigantic volcano, in the southwest of the Indian Ocean at 21.06°S and 55.32°E. It presents on its 2512 km² a unique variety of landforms and landscapes with Piton des Neiges (3071m) being its highest point. The orographic influence on local dynamics of Reunion Island expected to be major. The interaction of the high mountainous terrain with the synoptic flow induces a large variability of wind field at local scale. The maritime and tropical location of the island, as well as the complexity of the terrain and wind exposure, imply a multitude of local circulations and weather, marked by large variations in temperature and precipitation. Modeling of local circulations is very complex to achieve in an environment like Réunion Island because it is the result of a complex interaction between topographic circulations, thermal breezes and local formation of clouds and precipitation. During an eruption, other parameters make modeling more difficult, including the effects of the dynamics of volcanic flows and thermal secondary effects associated with lava flow. Few studies, as the Vog Measurement and Prediction Project (VMAP) on island of Hawaii (http://mkwc.ifa.hawaii.edu/vmap/hysplit), have been able to accurately represent the distribution of volcanic pollution at characteristic scales of a volcanic island. The Piton de la Fournaise eruption of April 2007 presented all the characteristics of complex flow of sulfur dioxide with a temporal discontinuity between the highest concentrations of surface SO₂
observed and the paroxysmal period of emission from the vent. The sulfur dioxide SO₂ is the second gas emitted at the Piton de la Fournaise volcano after water H₂O, followed by carbon dioxide CO₂ and hydrochloric acid HCl. The objectives in the framework of this case study are two-folds. First the paper aims to investigate the complex transport and distribution of the sulfur dioxide influenced by steep topography and three-dimensional atmospheric circulation. The second objective is to highlight the influence of the cloud scavenging and the heat fluxes on the SO₂ surface concentration. For the latter, high resolution numerical simulations have been used to analyze the sensitivity of the heat fluxes on the volcanic pollutants. This paper starts with a brief description of April 2007 eruption as well as numerical methods (section 2). Section 3 is devoted to analyzing the estimation of the heat flux release during the eruption. The section emphasizes the influence of lava flow on the convection and its consequences on the SO₂ distribution.

## 1.2. Description of the April 2007 eruption of the Piton de la Fournaise

In April 2007, the Reunion's Island has known its biggest eruption of Piton de la Fournaise volcano at least three centuries [Michon et al., 2013]. Within a month, 210 Mm³ of lava flowed out with 90 Mm³ reaching the sea. Above all, the collapse of the summit caldera caused significant morphological change [Michon et al., 2007]. Due to this nearby events and large environmental and civil protection impacts, this eruption is very well described in literature [Staudacher et al., 2009; Vlastélic et al., 2012; Barde-Cabusson et al., 2011; Tulet and Villeneuve, 2010; Di Muro et al., 2014]. After two short eruptive events (18 February and 30 March), a critical phase of the eruption started at 06 UTC on 2 April, located on the lower south-eastern part of the volcano (55°46" 25.5' E; 21°16" 54.6' S, WGS84) at only 590m above the sea level and only 3km from the coast (Figure 1). In less than eleven hours, two main lava streams reached the sea,
producing significant water vapor plumes with a very low pH due to strong presence of sulfuric and chlorohydric compound (pH<2, Staudacher et al. [2009]). During the first two days, lava fountains up between 50m and 150m high were observed. From 4 April, MODIS sensor shows a significant increase of the lava flow rate until the 6th of April, where the peak of emissions of lava was observed (greater than 200 m³/s at the vent, Coppola et al. [2009]; Staudacher et al. [2009]). SO₂ emissions, proportional to lava emissions, were estimated at 80 kg s⁻¹ the 4 April 12 UTC, 320 kg s⁻¹ the 5 April 12 UTC and 1600 kg s⁻¹ the 6 April 12 UTC before a strong decrease until 8 April 12 UTC at 55 kg s⁻¹, and finally a constant emission of 55-70 kg s⁻¹ until 11 April [Tulet and Villeneuve, 2010]. The peak of degassing was simulated at 1800 kg s⁻¹ on April 6, with the total budget estimated at 230kT, which is in agreement with the petrologic estimation of 311kT [Di Muro et al., 2014]. The Observatoire Volcanologique du Piton de la Fournaise (OVPF) recorded the 5 April at 20:48 UTC an earthquake of 4.8 magnitude synchronous with the caldeira collapse [Michon et al., 2007; Staudacher et al., 2009]. As described in Tulet and Villeneuve [2010] the location of this ash plume is well separate from the SO₂ one, as well as the vapor plume. The 6 April, lava fountains reached more than 200m high, and there were several tens of individual lava flows from 2 to 20m wide [Staudacher et al., 2009]. At this moment, the lava flow reaches its maximum lateral and longitudinal extents. In late 6 April, the intensity dramatically decreases, and the eruption became more “typical” compared to usual eruptions of Piton de la Fournaise. The 12th of April, the shallow seismicity came back at its highest, causing a new Dolomieu crater collapse. In the next days, the lava eruption intensity became steady with effusion measured at 15-20 m³.s⁻¹. This last event continued until May 1st 2007, the last eruption day.
1.3. ORA observations

ORA (Observatoire Réunionnais de l'air) provides daily monitoring of air pollution levels.

It is equipped with several fixed and mobile stations along the Reunion coastline, measuring
continuously primary and secondary pollutants. During the April 2007 eruption, 8 stations
measured the SO₂ surface concentration from Saint Louis to Saint Denis passing through Cam-
baie in the west (Figure 1). Data from these three stations are compared to simulations. The
first day, surface concentration of SO₂ is very low with value under 20 µg m⁻³ for all stations
(Figure 2). The next day from 02 UTC, Saint Louis and Cambaie stations measured signifi-
cant increases of SO₂ concentrations with a peak of more than 60 µg m⁻³ for Cambaie and 200
µg m⁻³ for Saint Louis at 06 UTC. The 4th of April, new strong increases of surface concentra-
tion have been measured by all stations located on the southwest and northwest coast. A peak
of 600 µg m⁻³ was observed at Cambaie and 587 µg m⁻³ in Saint Louis at 13 UTC. A significant
decrease followed this peak of SO₂ surface concentration, with SO₂ concentration falling below
the 100 µg m⁻³ threshold at the end of the day. The 5th, a slight increase appears at dawn (from
03 UTC) with 200 µg m⁻³ for Cambaie and 269 µg m⁻³ for Saint Louis. Subsequently, from the
5th (12 UTC) to the 10th of April (06 UTC), the concentration varied between 20 µg m⁻³ to 120
µg m⁻³ for all west coast station except Saint Louis station, where brief peaks appeared for few
hours on the 10th (345 µg m⁻³) and the 24th (390 µg m⁻³). The highest values measured by
ORA are not in phases in time with the maximum emitted from the vent. This paradox needs a
detailed study of sulfur dioxide transport.
## 2. Model description

### 2.1. Atmospheric model

The mesoscale non hydrostatic atmospheric model (MesoNH) developed by the Centre Na-
tional de la Recherche Météorologique and the Laboratoire d'Aérologie [Lafore et al., 1998]
has been used for the study. MesoNH can be used at all scales ranging from synoptic to large
eddy scales (http://mesonh.aeroobs-mip.fr/). It can be run in a two way nested mode involving
up to eight nesting stages. Different sets of parameterizations have been introduced for convec-
tion [Bechtold et al., 2001; Pergaud et al., 2009], cloud microphysics [Cohard and Pinty, 2000],
turbulence [Bougeault and Lacarrere, 1989], lightning [Barthe et al., 2007], gaseous chemistry
[Suhre et al., 1998; Tulet et al., 2003], cloud chemistry [Leriche et al., 2000] and aerosols [Tulet
et al., 2005; Grini et al., 2006].

### 2.2. Surface model

The SURFEx surface scheme is coupled with MesoNH to simulate surface processes, ther-
modynamic and chemical exchanges with the atmosphere. (http://www.cnrm.meteo.fr/surfex/;
Masson et al, 2013.). SURFEx is composed by various parameterizations for natural land sur-
face [Noilhan and Mahfouf, 1996; Bougeault and Lacarrere, 1989], urbanized area [Masson,
2000], lakes and oceans [Salgado and Le Moigne, 2010] and chemistry and aerosols surface
processes [Tulet et al., 2003; Mokhtari et al., 2012]. The coupling with the atmospheric model
is performed by averaging the surface fluxes over a model grid box. Within SURFEx, the lava
flow is represented by a line of potential heat flux emission, starting from the vent of the volcano
at 21.28°S and 55.77°E to the coastline at 21.28°S and 55.80°E. This representation implies two
major approximations for the lava flow. The first one is the misrepresentation of lava flow shape,
as the observed lava flow has a triangular shape. The second is relative to the static represen-
tation of the lava under SURFEx, when the lava propagation is not integrated in time. The
increasing surface of lava flow and its heat flux is modeled by multiplying the line of potential
emission with a coefficient proportional to the increase of the lava flow surface.

**2.3. Model configuration**

The simulation starts at 00 UTC on April 2nd 2007, and ends at 00 UTC on April 7th 2007.

The simulation has two nested domains with Kessler microphysics scheme and TKE turbulence scheme (prognostic turbulent kinetic energy, one and a half order closure). The largest domain with high model grid spacing (2km) is centered over the Reunion island. The domain extends over 330km from north to south and 450km from east to west. The second domain covers only the Reunion Island and its coastline with a horizontal model grid spacing of 500m. The vertical grid is composed of 72 levels for both models stretching up to 31km altitude with a first level 5m above ground level. Initial and lateral boundary conditions are extracted from ECMWF analysis for the meteorological fields and from MOCAGE (http://www.cnrm.meteo.fr/gmgec/spip.php?article87) for gaseous chemistry fields. The gas phase chemistry is resolved on both domains using the ReLACS chemical mechanism [Crassier et al., 2009], which is a reduced version of RACM including the oxidation of sulfur dioxide by OH radical. In SURFEX, the entire SO₂ emission is released at the vent. This assumption is well correlated with the fact that the magma begin to degas when it reaches the vent and its surroundings. In consequence, with a 500m MesoNH horizontal model grid spacing, the location of SO₂ emission is well represented. A simulation protocol was implemented to limit the model drift by reinitializing the model dynamic (wind, humidity and temperature filed) in the middle of the simulation while the chemical fields have been preserved along the whole period (Figure 3). A second simulation starts the 3rd at 18 UTC until the 4th of April at 00 UTC. This latter
provides at its end the dynamic fields refresh, while suppressing the need for model spin-up
(time taken by the model to reach equilibrium state). Finally, a new simulation starts the 4 April
at 00 UTC with the dynamic of simulation 2 and chemistry of simulation 1.

Sensitivity tests are made in this study to highlight the influence of heat fluxes in the transport
of volcanic pollutants and the influence of cloud chemistry in scavenging sulfur dioxide. The 3
simulations configurations are sum up in the table 1.

3. Estimations of thermodynamic emissions

**3.1. Heat flow estimation**

Lava heat is released in the atmosphere from the core of an active flow by conduction through
the basal, lateral and surface crusts [Oppenheimer, 1991; Klingelhofer et al., 1999; Quareni
et al., 2004]. At the surface, heat losses are dominated by radiation (5×10⁴ W m⁻²) and con-
vection (10⁴ W m⁻²), whereas conduction from the base to the ground is predominant (10³ W
m⁻², Harris et al. [2005]). For this study, only convective heat fluxes are implemented, with the
assumption that heat losses by conduction and by rain falling on the flow (250 W m⁻², Harris
et al. [2005]) are negligible. As the influence of radiant heat fluxes is inversely proportional
to the square of the distance; we have also neglected it for our simulation. The heat flow by
convection is calculated from:

$Q_{conv} = hc(T_{surf} - T_{air})$

With *hc* the heat transfert coefficient estimated at 50 W m⁻² by Keszthelyi *et al.* [2003], *T*<sub>surf</sub>
the lava surface temperature and *T*<sub>air</sub> the air temperature (290K). Estimation of the sensible heat
fluxes, and hence the lava cooling, is mainly controlled by the surface winds. The heat flux
relation to the wind from Keszthelyi observations are taken into account in our model (Figure
4). The surface covered by hot (1100°C) liquid lava flow and the crusted (400°C) lava is taken from the day by day observation given by *Bachèlery et al.* [2014] between April 2nd and 8th, allowing the estimation of the heat flow (in m²) from the ground.

# 4. Sulfur transport during the April 2007 eruption of Piton de la Fournaise volcano using MesoNH atmospheric model

## 4.1. SO₂ mass burden

Figure 5 represents the evolution of SO₂ mass burden simulated by MesoNH between April 3rd and April 6th at 18 UTC. The first period until 4 April shows that the plume is oriented to the west with a maximum of mass burden of 210 DU. From April 5 and 6, the plume at the vent, above 5km ASL is oriented to the north with a large value (330 DU) over Reunion island. During this period, the strong presence of SO₂ in the north of Reunion Island indicates a plume separation. The change in direction between the two periods is due to the SO₂ plume crossing the inversion of trade winds around 3500m-4500m ASL. Below the inversion, a lower branch of the SO₂ plume is transported westward by the trade winds, while above, an upper plume is advected eastward. This analysis of SO₂ plume evolution in the first 4 days of the eruption is consistent with the study based on satellite data OMI and CALIOP by Tulet and Villeneuve (2010).

## 4.2. Simulated SO₂ surface concentrations

In the morning of April 2, the SO₂ plume at the surface is oriented southwestwards, contourn-ing by the south the Piton de la Fournaise area. The plume is then transported along the coastline influenced by the trade winds circumventing the island, where a strong gradient of SO₂ appears. During this day, the SO₂ plume reaches Cambaie, at the northwest of the island with low surface concentration of SO₂ in order of some tens of µg m⁻³. The strongest simulated concentrations
204 are located at low altitude, near Saint Joseph in the south, with 350 µg m⁻³. For other stations, 205 SO₂ surface concentration is 125 µg m⁻³ at Saint Louis in the southwest or a comparatively low 206 25 µg m⁻³ is observed in Sainte Thérèse in the northwest, while concentrations in Saint Denis 207 to the north is even lower with 1 µg m⁻³. Not much changes occur the 3th of April, when the 208 overall atmospheric dynamics confine the volcanic pollutants to the west of the island (Figure 6, 209 dots represent ORA stations, Saint Denis at the north, Cambaie at the northwest and Saint-Louis 210 at the southwest). Concentrations for the majority of the stations are stronger than the eve, with 211 200 µg m⁻³ at Saint Louis and 75 µg m⁻³ at Cambaie and Sainte Thérèse (Figure 6). However, 212 the strong SO₂ gradient noted earlier is no longer present on April 3rd. SO₂ distribution appears 213 larger on the south and west side of La Reunion and to the west above the ocean.

214 Higher concentrations are simulated the 4th of April, with a peak of 680 µg m⁻³ obtained at 215 St. Joseph and 350 µg m⁻³ obtained at St. Louis. For stations in the north of the island, the 216 concentrations are close to those April 3rd. The maximum SO₂ concentration at the surface 217 is thus located in the heights of the island, where high concentrations are simulated notably 218 with more 1000 µg m⁻³ in the heights above the city of Saint Joseph (south of Réunion Island). 219 On the 5th of April, an increase of heat flow and a decrease in the stability of the atmospheric 220 boundary layer allows the plume to reach higher altitude and in consequence be oriented directly 221 to the northwest.

222 Finally, the 6 April, high concentrations are only simulated over the entire southern half of 223 the island, with concentrations over 5000 µg m⁻³, 10 times the threshold recommended by 224 European standards. Unfortunately, no observations are available in the south of the island to 225 validate these very high-simulated concentrations the 6 April.
4.3. Comparison between MesoNH simulation and ORA data

From 2 to 5 April, the simulation succeeds to correctly reproduce general trends for all sim-
ulated stations on the island (Figure 7). On the 2nd, the SO₂ surface concentration given by
MesoNH corresponds to ORA observations for these three stations with values below 15 µg m⁻³
for Cambaie and Saint Denis and a peak of 200 µg m⁻³ in the middle of the day for Saint Louis.
On the 3rd, Saint Denis station did not record any presence of SO₂ while Cambaie station had
a gradual increase with a peak of 85 µg m⁻³ for ORA observation and 135 µg m⁻³ in MesoNH
simulation. Saint Louis has experienced a significant increase with a SO₂ surface concentration
of 480 µg m⁻³ measured by ORA and of 605 µg m⁻³ simulated by MesoNH, once again in the
middle of the day. The 4th of April, no changes occured for Saint Denis, while a strong SO₂ in-
crease appears at Cambaie with 500 µg m⁻³ in MesoNH and 601 µg m⁻³ for ORA observations.
The same behaviour is also seen for Saint Louis station, with 585 µg m⁻³ observed against a
strong over concentration simulated value of 1135 µg m⁻³. It is important to note that this lat-
ter station is positioned on a very strong gradient of SO₂ (1135 µg m⁻³ to 220 µg m⁻³ at 5km
away). This strong increase is immediately followed by a sharp decrease at the end of the day,
with value below 150 µg m⁻³ for Cambaie and 200 µg m⁻³ for Saint Louis. On the 5th SO₂, con-
centration varies between 30 to 200 µg m⁻³ for Cambaie, and 10 to 300 µg m⁻³ for Saint Louis
except in the evening (18 UTC) where the simulation does not succeed to keep low concentra-
tion values (550 µg m⁻³ simulated instead of 35 µg m⁻³ observed). For the simulation's last day,
the SO₂ surface concentration given by the model is stronger than observations, with highest
values of more than 450 µg m⁻³ (against 45 µg m⁻³ observed) at Cambaie and a peak of 590
µg m⁻³ at Saint Louis instead of 55 µg m⁻³. The same anomaly appears for Saint Denis, when
shortly after 00 UTC, the simulation gives a peak of 450 µg m⁻³ instead of a total absence of
volcanic SO₂ highlighted by ORA measurements. Despite these orders of magnitude anomalies
from the April 6th, and the global over exposition of SO₂ surface concentration for Saint Louis,
SO₂ concentrations between 2 and April 7 are generally consistent with ORA measurements.
The simulation succeed to recreate the paradoxical situation between the highest surface SO₂
concentration measured by ORA the 4 April whereas the paroxysmal intensity of the eruption
is in the night (from 20 UTC) of the 5 April and on April 6th.

**4.4. Vertical transport above the eruption**

The increased lava flow and its greater surface coverage between April 2 and 7 consequently
involves an increase in heat flux over the lava flow. Heat flow for the first three days are moderate
with an average of 12800 W m⁻². Local circulation is still dominated by trade winds with
surface winds around 5 m s⁻¹. The plume reached 2.5km ASL. (under 100 µg m⁻³) and the
highest SO₂ concentration value are close to the surface (23000 µg m⁻³). However, from April
5th, the general trend is the increasing of trade winds (11 m s⁻¹ around the eruption zone). This
increase induces more heat flux (22500 W m⁻² April 5) and a local breeze in the eruption area
which creates a more efficient vertical transport of volcanic sulfur.

The surface warming and the heat flow associated with the lava flow generate atmospheric
instability in the low layers of the troposphere. On Figure 8, the strong convection above the lava
flow creates a large mixing area with a maximal negative vertical gradient of equivalent potential
temperature of ∂θe/∂z = -1.5K/km between the lava and 7km ASL. Under the influence of the
trade winds, the vertical structure of the plume in altitude is moving slightly westward. The
vertical wind above the eruption reached 14 m s⁻¹ from 3 to 5km ASL. and transports SO₂ up
to 8 km high (under 100 µg m⁻³), ie above the inversion zone trade winds situated between 2.5
and 3.5 km of altitude. At this altitude the plume is in thermodynamic equilibrium with the
environment and oriented according to the wind direction, ie west/southwest. The plume is no longer transported to the west, but in the direction of Mauritius and Australia. High values of SO₂ concentration are modeled up to 6km (above 30000 µg m⁻³) the 6 April (Figure 9, cross section in left panel corresponds to blue line (2 April) in Figure 1 and cross section in right panel corresponds to red line (6 April)).

## 4.5. Rainfall and aqueous chemistry

The 5 and the 6 April, strong clouds formations appear in the Piton de la Fournaise area. The associated accumulated rainfall simulated between 2 and 7 April 2007 by MesoNH is consistent with Meteo-France observations (Figure 10). Only the southeast weather station gives high rainfall value (67mm cumulated), whereas the western weather station recorded lower value (15mm and 6mm). One possible causes of concentrations overprediction for April 6th is that the scavenging of SO₂ by rain and cloud water leading to sulfuric acid formation is not taken into account in the simulation. However, Meteo-France measurements have shown that between April 2 and 7, the largest quantity of rain were observed only for the 6 April in the volcanic region. As MesoNH includes a cloud chemistry module [Leriche et al., 2013] a sensitivity test was realized from April 5 18 UTC to April 7 00 UTC (limited period due to high computational cost) by activating this module. A simplified mechanism in aqueous phase was used including the oxidation of SO₂ into sulfuric acid by hydrogen peroxide, ozone and pernitric acid [Leriche et al., 2003]. The module includes also the mass transfer kinetic for the exchange between the gas phase and liquid phases of soluble gases and their redistribution between cloud water and rainwater by microphysical processes (collision/coalescence leading to precipitation and sedimentation of raindrops leading to wet deposition).
One of the main consequence of cloud chemistry activation is a global decrease of SO₂ surface concentration due to SO₂ scavenging by rainfall and aqueous phase SO₂ chemistry transformation inside clouds over Reunion Island. The 6 April at 13 UTC, the difference of SO₂ surface concentration between the simulation with cloud chemistry activated (AQ simulation) and reference simulation (REF) reach up to -700 µg m⁻³ over the high terrain in the center of the island (Figure 11). The SO₂ surface concentration for the western coastline is 200 µg m⁻³ lower for the AQ simulation than the REF simulation and for the Piton de la Fournaise area, a strong decrease appears due to the proximity of the vent with the presence of high rainfall this day over the volcano. Generally, a 30% to 60% decrease is simulated by MesoNH, giving SO₂ surface concentrations close to ORA observations (Figure 12).

## 5. Influence of sensible heat fluxes in the transport of SO₂

A sensitivity study was performed to characterize the influence of heat flux forcings over the vent and lava on the vertical transport of SO₂ (Figure 9). To do so, an additional simulation has been made without thermodynamic flux (NO-FLX) to highlight the contribution of these fluxes in the transport and the dispersion of sulfur dioxide. This simulation rapidly presents large discrepancy from the reference simulation (REF) as shown by Figure 13 in the differences of SO₂ concentration (in µg m⁻³) between the NO-FLX simulation and the REF simulation on the 3,4,5 and 6 of April at 13 UTC. A strong positive difference in concentrations appears for the whole southern part of the island with a maximum of 32000 µg m⁻³. The northwestern part of the island is also overexposed to higher concentrations of the order of 500 µg m⁻³ for April 3 and 1000 µg m⁻³ from April 3 to 5. Conversely, negative anomalies are simulated the 4th of April for the northwest with less than 340 µg m⁻³ compared to concentrations in the REF simulation. As a main consequence, concentrations obtained with the simulation NO-FLX are
also far from ORA measurements. In general, unrealistic peaks are simulated (Figure 14) with
a factor of 5 to 35 in the south, and 5 to 10 in the northwest compared to ORA observations.

Taking into account sensible heat flux from lava is therefore of prime importance as the NO-
FLX simulation did not recreate correctly the spatial and temporal distribution of sulfur dioxide
for the 2007 eruption of Piton de la Fournaise. Here, the lack of heat flux injection did not allow
adequate vertical transport, essential to get an overall good representation of SO₂ distribution.

As a general consequence we estimate that numerical modeling of the April 2007 eruption
cannot be represented without heat flux correctly estimated and injected at the eruptive vent.

**6. Conclusions**

The objective of this study was to model fine scale spatial distribution of SO₂ degassed during
the eruption of the Piton de la Fournaise in April 2007. It was necessary to adequately modeled
the heat flux injection over the vent and lava flow to simulate the atmosphere dynamics that
drives this SO₂ distribution. The simulation has been found to be in relatively good agreement
with observations, and highlighted two phases. With moderate value of heat fluxes from lava
flow (12800 W.m⁻²), the first phase, between April 2 and 4, shows a SO₂ plume still contained
under the trade wind inversion at 3km ASL. The main consequence is a high SO₂ surface con-
centration for western stations (600 µg m⁻³ for ORA observations, 500 µg m⁻³ for simulation at
Cambaie). The second phase, between April 5 and 7, corresponds to the eruption maximum in-
tensity. This high intensity is accompanied by a strong increase in lava heat flux (22500 W m⁻²)
that allows the SO₂ plume to cross the trade wind inversion, and reach an altitude of 8km ASL
on the 6th of April. This deep convection reduces surface SO₂ concentration (600 µg m⁻³ to 100
µg m⁻³ in few hours at Cambaie), but the model fails to keep low SO₂ surface concentration on
the last simulation day (peaks at 400 µg m⁻³ instead of 55 µg m⁻³ at the end of 6 April). These
over predictions were addressed by taking into account the cloud chemistry in a sensitivity study realized from 12 UTC on April 5 to April 7. During this period, the scavenging of SO₂ by rain water and cloud water significantly reduces SO₂ surface concentration, producing sulfuric acid and as a consequence acid rain. Overall, the reference simulation was largely in good agreement and within the same order of magnitude, with the observation values from ORA. To highlight heat flux influence, a second sensitivity study was performed, in which the heat fluxes from the vent and lava flow were totally suppressed. Without these additional contributions of heat flux, the simulated surface concentrations are up to 45 times higher than the observations. One of the main conclusions of the study is that heat flux above lava is a crucial parameter to take into account in order to reproduce correctly SO₂ distribution. This additional energy allows the development of strong convection that injects volcanic discharges over the atmospheric boundary layer. The heat flux model, although still imperfect by its surface representation, significantly improve the SO₂ spatial distribution, as shown in this study by respecting orders of magnitude compared to observations and by displaying correct temporal evolution of the simulated surface concentrations.

A perspective of improvement is the implementation of a new deep convection scheme to improve the representation of sub-grid convective transport in MesoNH model. The initial deep convection scheme from MESO-NH basic package is not adapted for an extreme event such as volcanic eruption. Indeed, some important processes are not taken into account or are not representative of a phenomenology of an eruption, such as the speed of ejection of gas and heat flow, or the absence of the mixing vertical processes. A strategy could be a coupling system between a more detailed lava surface model and MesoNH atmospheric model to better reproduce the distribution and evolution of the lava during the period.
**Acknowledgments.** We greatly acknowledge the MesoNH assistance, especially C.Lac and J.Escobar. Computer resources were provided by CINES (Centre Informatique National de l'Enseignement Supérieur http://www.cines.fr/, project no. lac6309) and CCUR (Centre de Calcul de l'Université de la Réunion) for the access to super-computer. We also thank ORA (http://www.atmo-reunion.net/) for free access to their data. Finally, we would like to thank OSU-R, OMNCG federation, Reunion University and LEFE/INSU program for funding this work.

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<table><thead><tr><td>Simulation</td><td>Period</td><td>Lava heat flux</td><td>Cloud chemistry</td></tr></thead><tbody><tr><td>REF</td><td>04/02 00 UTC - 04/07 00 UTC</td><td>Yes</td><td>No</td></tr><tr><td>NO-FLX</td><td>04/02 00 UTC - 04/07 00 UTC</td><td>No</td><td>No</td></tr><tr><td>AQ</td><td>04/05 18 UTC - 04/07 00 UTC</td><td>Yes</td><td>Yes</td></tr></tbody></table>

**Table 1.** The 3 simulations configurations. REF is the reference simulation, NO-FLX is the simulation without heat fluxes from lava flows and AQ is the simulation with cloud chemistry activated
<figure><img src="image_3.png" /></figure>

**Figure 1.** Orography and geographic situation of Reunion Island. The blue line and the red line in the Piton de la Fournaise area correspond respectively to the cross section of 2 April 2007 and 6 April 2007 describes in section 4-3.
<figure><img src="image_4.png" /><figcaption>**Figure 2.** ORA measurements between April 1st and April 28th for Cambaie in the northwest (red), Saint Louis in the southwest (blue) and Saint Denis in the north (Green). Thin dashed line is the public information threshold and the large dashed line is the health threshold.</figcaption></figure>
<figure><img src="image_5.png" /></figure>

**Figure 3.** Updating the model dynamic: As a first step, the reference simulation begins the 2 April until 4 April 00 UTC (1). Then a new simulation begins April 3 18 UTC until 4 April 00 UTC (2). This latter will give the new model dynamics while avoiding the early simulation spin up. Finally, the REF simulation resumes the 4 April 00 UTC into the end, with chemical fields of (1) and model dynamic of (2).
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Heat flux evolution with lava surface temperature (Kezsthelyi et al, 2003). The green, red, blue and black lines represent heat fluxes respectively for 10 m s⁻¹, 5 m s⁻¹, 1 m s⁻¹ and without surface wind.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Integrated column of SO₂ (DU) between April 3 and April 6 at 13 UTC above the Réunion Island from first model domain (2km horizontal model grid spacing). April 2 and 3, the SO₂ plume, is influenced by the trade winds below the thermic inversion, is oriented to the west. The 4 and 5 April, a large part of the SO₂ plume are crossing the trade winds inversion and is transported to the northeast.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Figure 6.** Surface concentration of SO₂ between April 3 and 6 at 13 UTC from MesoNH mesoscale atmospheric model smallest domain (500m horizontal model grid spacing)</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Figure 7.** Comparison between ORA observation (blue points) and MesoNH simulation (red points) from April 2 to 7 April. The large dashed line is the health threshold while the thin dashed line is the information threshold.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure 8.** Cross section of equivalent potential temperature (K) for the 6th April at 13 UTC along red line in Figure 1. The strong convection above the lava flow creates a large mixing area with a maximal negative vertical gradient of equivalent potential temperature of $\partial\theta_e/\partial z = -1.5\text{K/km}$ between the lava and 7km ASL. Under the influence of the trade winds, the vertical structure of the plume in altitude is moving slightly westward.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Figure 9.** Cross section along SO₂ plumes for April 3 (left) and April 6 (right) at 13 UTC. The cross sections are not in the same direction due to change of plume orientation. The left panel corresponds to blue line in Figure 1, the right panel to red line. Color filling corresponds to SO₂ concentrations (µg m⁻³), isocontours values represent the upward velocity intensity (m s⁻¹) generated by lava heat flow.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Figure 10.** Accumulated rainfall given by MesoNH model between 2 and 7 April 2007. The numbers correspond to Meteo-France observations.</figcaption></figure>
<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

**Figure 11.** Difference of SO₂ concentration between AQ and REF simulation the 6 April at 13 UTC at the surface (left panel) and in the plume (right panel).
<figure><img src="image_15.png" /><figcaption>**Figure 12.** Comparison of surface SO₂ concentration between REF simulation and AQ simulation with observation providing by ORA.</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>**Figure 13.** SO₂ concentration difference at the surface between NO-FLX and REF simulations, for April 3,4,5 and 6 2007 at 13 UTC. The arrows represent the difference of the wind field between NO-FLX simulation and REF simulation.</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>**Figure 14.** Comparison of surface SO₂ concentration between NO-FLX simulation (green), REF simulation (red) and ORA measurements (blue). Thin dashed line is the public information threshold.
Large dashed line is the health threshold.</figcaption></figure>