The major volcanic eruption of Mount Pinatubo in 1991 has been shown to have
significant effects on stratospheric chemistry and ozone depletion even at
midlatitudes. Since then, only “moderate” but recurrent volcanic eruptions
have modulated the stratospheric aerosol loading and are assumed to be one
cause for the reported increase in the global aerosol content over the past
15 years. This particularly enhanced aerosol context raises questions about the
effects on stratospheric chemistry which depend on the latitude, altitude and
season of injection. In this study, we focus on the midlatitude Sarychev
volcano eruption in June 2009, which injected 0.9 Tg of sulfur dioxide (about
20 times less than Pinatubo) into a lower stratosphere mainly governed by high-stratospheric temperatures. Together with in situ measurements of aerosol
amounts, we analyse high-resolution in situ and/or remote-sensing
observations of NO
In the stratosphere, the photo-oxidation of N
The hydrolysis of ClONO
Some works also suggest that the hydrolysis of BrONO
After large volcanic eruptions, the aerosol loading in the stratosphere and
the surface area densities (hereafter SADs) available for Reaction (R1) to
occur are dramatically enhanced (e.g. Deshler et al., 2003). As a result,
the amount of ozone-depleting NO
However, the NO
In periods following major eruptions, the year-to-year variability in
stratospheric ozone at northern midlatitudes appears closely linked to
dynamical changes induced by the volcanic aerosol radiative perturbation
(e.g. Telford et al., 2009; Aquila et al., 2013) and to changes in chlorine
partitioning (e.g. Solomon, 1999; Chipperfield, 1999). Effects on
stratospheric chemistry are expected in periods of elevated chlorine levels
from anthropogenic activities (Tie and Brasseur, 1995; Solomon et al.,
1996). In the past decade no event comparable to the 1991 Pinatubo or 1982
El Chichón eruptions was observed. However, several volcanic eruptions,
though of much lesser amplitude, impacted the aerosol burden in the lower
stratosphere over periods of months (Vernier et al., 2011). These
“moderate” eruptions have occurred in a period of still high chlorine loading with a potential impact on stratospheric ozone chemistry. Their effects depend on
the amount of released SO
The goal of this paper is to show how such moderate eruptions are likely to modify the chemical balance of the Northern Hemisphere lower stratosphere at periods excluding wintertime or springtime halogen-activating photochemistry. We specifically focus on the eruption of the Sarychev volcano on 15 and 16 June 2009, which injected 0.9 Tg of sulfur dioxide into the lower stratosphere (Clarisse et al., 2012), resulting in enhanced sulfate aerosol loading up to 19 km, for a period of about 8 months ending before winter (Haywood et al., 2010; Kravitz et al., 2011; O'Neill et al., 2012; Jégou et al., 2013).
The approach consists in analysing some key aspects of
lower-stratospheric chemistry and ozone loss in a context of high aerosol
surface area densities and high-stratospheric temperatures using
balloon-borne observations conducted in August–September 2009 from
Kiruna/Esrange in Sweden (67.5
Our study is based on in situ and remote-sensing balloon-borne observations obtained during summer 2009 in northern Sweden. More details about the instrument descriptions and retrieval techniques are given in the Appendix and in the references. Data can be found in ESPRI data Centre (2016).
Aerosol in situ measurements were performed by the STAC (Stratospheric and Tropospheric Aerosol Counter) instrument which is an optical particle counter providing aerosol size distributions (Ovarlez and Ovarlez, 1995; Renard et al., 2008). This instrument has been used in a number of studies dedicated to the quantification of the aerosol content in the stratosphere at various locations and seasons (e.g. Renard et al., 2002, 2005, 2010). Eight vertical aerosol concentration profiles were obtained between August and September 2009 as reported by Jégou et al. (2013).
Our study presents in situ vertical profiles of the N
Since 1996 stratospheric NO
The SALOMON (Spectroscopie d'Absorption Lunaire pour
l'Observation des Minoritaires Ozone et NO
Variations in solar zenith angle (SZA) along solar occultation lines of
sight and associated concentration variations are likely to impact the
retrieved vertical profiles near sunrise and sunset especially below 20 km
(Newchurch et al., 1996; Ferlemann et al., 1998). Some works propose using a photochemical model to correct for this effect (e.g. Harder et al., 2000;
Butz et al., 2006) depending on the considered chemical compound, the
observation geometry (i.e. balloon ascent or occultation) and daytime (SZA
variation). Typically, concentrations are converted to values expected at
90
In our study, the NO
Photochemical effects on the BrO profile obtained by the SALOMON instrument
from solar occultation measurements are estimated to be 10 % and are
taken into account in the error estimation in accordance with the study of
Ferlemann et al. (1998). Photochemical changes in the BrO slant column
densities (SCDs) recorded during the balloon ascent are small, and the DOAS BrO
profile has not been corrected to 90
The REPROBUS (Reactive Processes Ruling the Ozone Budget in the Stratosphere) 3-D CTM has been used in a number of studies of stratospheric chemistry involving nitrogen and halogen compounds in particular through comparisons with space-borne and balloon-borne observations (e.g. Krecl et al., 2006; Berthet et al., 2005; Brohede et al., 2007). It is designed to perform annual simulations as well as detailed process studies. A description of the model is given in Lefèvre et al. (1994) and Lefèvre et al. (1998), as well as in the Appendix.
In this study, REPROBUS was integrated from 1 October 2008 to 1 October 2009
with a horizontal resolution of 2
As sulfur chemistry is not included in REPROBUS, we have conducted a
simulation (hereafter called Ref-sim) constrained with typical background
aerosol levels inferred from H
We have conducted another type of simulation (hereafter called Bal-sim)
consisting in adjusting the input H
Range of aerosol SAD values (black lines) as derived from several
balloon-borne observations in the lower stratosphere in summer 2009
(1
N
As expected, increasing SAD values in the model to reproduce the volcanic
aerosol levels has no effect on N
This situation implies that the balloon flights performed from 7 August 2009
in the Kiruna region match the photochemical conditions for which
volcanic aerosols likely have an impact on NO
Seasonal variation in N
For the Sarychev situation, minima in NO
Vertical profiles of NO
Figures 5 and 6 present the measured profiles of NO
Vertical profile of NO
Top: vertical profile of NO
It may be noted that the REPROBUS calculations do not reproduce some of the vertical structures detected by the SPIRALE instrument, i.e. between 17.5 and 19.5 km for SPIRALE-07082009 and at 17 and 20.5 km for SPIRALE-24082009. This is likely due to the vertical resolution of the model or inaccurate simulation of mixing effects in the CTM as already mentioned in previous studies showing this kind of comparison (e.g. Berthet et al., 2006).
Calculated differences between the reference and the volcanic-aerosol-constrained simulations provide an estimation of the chemical perturbation
induced by the Sarychev aerosols. Reductions in NO
Some small model–measurement discrepancies in the 20–35 km altitude range as
shown in the embedded plots in Figs. 5 and 6 suggest that the
model–measurement differences in the lower stratosphere may be only partly
attributed to remaining uncertainties in calculations of transport. A way to
discard a possible remaining effect of transport and improve the modelling
of total NO
Total NO
N
The 1-D-REPROBUS reference simulation is computed with background aerosol
levels, whereas the Sarychev aerosol-affected simulation is constrained with
the mean observed aerosol profile presented in Fig. 2. As a result of the
NO
Overall the 1-D NO
The reduction of NO
We consider here total HNO
The NO
For the Pinatubo-aerosol-loaded stratosphere, maximum HNO
Several studies have revealed the impact of the Pinatubo eruption on the stratospheric halogen chemistry. This has been shown to be of particular importance regarding ozone destruction processes through the partitioning of chlorine reservoir species and the activation of chlorine radicals on volcanic aerosols (e.g. Solomon, 1999, and references therein).
Some volcanic eruptions are likely to inject halogenated compounds within
the stratosphere, therefore impacting directly the halogen content and
bypassing (or adding to) in situ heterogeneous processes. For the Sarychev
volcano eruption, an injection of several ppbv of HCl into the stratosphere
has been reported by Carn et al. (2016) using Microwave Limb Sounder (MLS)
data, mainly below the 140 hPa level (see their Fig. 4). However, because of
the low vertical resolution of MLS data, i.e.
We therefore examine the direct impact of the Sarychev sulfate aerosols on
the chlorine partitioning in connection with NO
Simulated changes on various stratospheric key species due to the
Sarychev volcanic aerosols over the August–September 2009 period at 16.5 km.
Numbers are taken from the Sat-sim simulation. Effects for daytime and
night-time conditions are provided depending on statistically significant
amounts in the diurnal cycle of a given compound. Also, the contribution of
BrONO
The impact of the volcanic aerosols on the chlorine partitioning appears
somewhat small since it is primarily the consequence of the increasing
losses of HCl by enhanced OH through reaction HCl
NO
Same as Fig. 5 but for the NO
Coupling between chlorine and bromine compounds is of particular importance
in the lower stratosphere (e.g. Lary et al., 1996; Erle et al., 1998;
Salawitch et al., 2005). Heterogeneous bromine reactions are expected to
increase the coupled gas-phase ClO
Since direct injection of bromine into the stratosphere was insignificant
after the Sarychev eruption (Hörmann et al., 2013), we expect that
stratospheric bromine chemistry was only modified by the enhanced aerosol
loading. BrO was the only key halogenated radical detected during the summer
2009 balloon campaign. Vertical profiles were provided by the SALOMON and
DOAS instruments on 25 August 2009 and 7 September 2009 respectively (Fig. 10).
They were simultaneously measured with the NO
Same as Fig. 6 but for BrO. The SALOMON data in the lower
stratosphere were obtained between 19:15 UT (SZA
Simulated results related to the bromine chemistry at 16.5 km are presented
in Table 1 for the August–September 2009 period. During the daytime, part of the BrO
enhancement is linked to the decreased loss by the three-body reaction with
decreased NO
However, it is not clear if BrONO
As shown in Table 1 for an altitude of 16.5 km, at night BrONO
This additional release of OH radicals has significant consequences in the
chemistry of the lower stratosphere. In our study the reduction in NO
It is interesting to estimate the stratospheric ozone depletion induced by the Sarychev eruption. As said above, the model does not directly calculate possible effects of aerosols on stratospheric temperature and circulation. All our simulations use the same transport calculations, whereas ozone loss from Pinatubo in the northern midlatitudes can be both attributed to chemical and transport (such as increased tropical upwelling) effects (e.g. Telford et al., 2009; Dhomse et al., 2015). In the following, we therefore solely calculate the change in ozone due to photochemistry.
We then compare model simulations with enhanced and background aerosol
levels (Fig. 11). Results indicate chemical reductions in ozone of a few
percent following the eruption when aerosol levels are computed from the
OSIRIS space-borne data. Accumulated ozone depletion reaches its maximum
above Kiruna near 16 km from around mid-September with changes of
Changes in ozone over Kiruna (67.5
We note that for the post-Pinatubo eruption period, ozone reductions as
large as
In the lower stratosphere, ozone removal rates are mainly controlled by the
HO
Part of the ozone depletion can be related to the coupled
BrO
Our study provides key observations of the chemical perturbation in the lower stratosphere by the moderate Sarychev volcano eruption in June 2009. Three- and one-dimensional CTM simulations are performed to interpret balloon-borne observations of some key chemical species made in the summer high-latitude lower stratosphere. The modelled chemical response to the volcanic aerosols is treated by comparing simulations using background aerosol levels and simulations driven by volcanic aerosol amounts inferred from balloon-borne and space-borne observations.
Quantifying the impact of volcanic aerosols on stratospheric ozone chemistry is difficult as chemical and dynamical (radiative) effects occur simultaneously (Pitari and Rizi, 1993; Robock, 2000; Al-Saadi et al., 2001; Aquila et al., 2013). The model is a CTM driven by ECMWF offline meteorological data and does not describe radiative processes. In other words, volcanic aerosol radiative effects do not directly interact with the circulation computed by the model. Radiative processes from the injection of volcanic aerosols into the tropics have been shown to have an impact on mean meridional circulation and ozone transport (Brasseur and Granier, 1992; Pitari et Rizi, 1993). In our study, effects of the Sarychev aerosols on midlatitude stratospheric dynamics, if any, are at least of the first order intrinsically taken into account in the ECMWF analyses used for all simulations. REPROBUS does not take into account the aerosol impact on calculated photolysis rates, which is likely to result in some differences between models when this process is computed or ignored (Pitari and Rizi, 1993; Pitari et al., 2014). However, because the Sarychev eruption has only impacted the lower stratosphere at mid- and high latitudes the effect on the photolysis frequency of molecular oxygen and ozone due to absorption and backscattering of solar radiation by the volcanic aerosols is expected to be very small in these regions (Tie et al., 1994). Therefore, since all our simulations were driven with the same wind and temperature fields, our approach only estimates the chemical effects of the Sarychev aerosols.
The NO
Although direct comparisons in terms of solar illumination, latitude,
injection altitudes and temperature are not possible for distinct volcanic
eruptions such as Pinatubo and Sarychev, it is interesting to compare the
effect of both eruptions on the photochemistry of the lower stratosphere.
Overall, although different in magnitude, the eruptions of Pinatubo and
Sarychev show a similar observed and simulated depletion of NO
For the Pinatubo aerosols, ozone destruction was not observed throughout the
volcanic aerosol layer because N
However, limitations in our model simulations also contribute to some
model–measurement discrepancies. A first major difficulty is to drive the
model simulations with representative and consistent inputs in term of
volcanic aerosol loading. To address this issue, two different model runs
for aerosol forcing have been performed, one using OSIRIS satellite data
converted to aerosol SAD fields and the other one from in situ balloon-borne
observations. The OSIRIS satellite data represent zonally and daily averaged
values of SAD, which may vary from a 3-D construction based on the local
surface areas. The possible presence of aerosol streamers (geographical
variations in the aerosol content) resulting from the transport of the
volcanic aerosols over the Northern Hemisphere present from mid-July to
September 2009 is likely to affect the N
Secondly, adequate modelling of transport is also crucial for the
partitioning of NO
Thirdly, part of the discrepancies between model and observations might be
attributed to spatial resolution issues. It may be tricky to compare model
calculations with high-resolution in situ profiles and with remote-sensing
observations integrating over tens of kilometres (Berthet et al., 2007). For
instance, discrepancies between remote-sensing observations and model
calculations have been reported for stratospheric NO
In our study, no comprehensive sulfur chemistry is included in the model. We have also excluded dynamical and radiative effects on the ozone response, which have been shown to be of primary importance when dense volcanic clouds are present (e.g. Pitari and Rizi, 1993; Kinnison et al., 1994; Tie et al., 1994; Al-Saadi et al., 2001). In a forthcoming study it would be interesting to compare dynamical or radiative and chemical effects of moderate volcanic eruptions on stratospheric ozone using chemistry–climate models with full sulfur chemistry and aerosol–dynamics interactive calculations.
Finally, it might be interesting to investigate the effects of other volcanic plumes coming from moderate volcanic eruptions which are then transported to high-latitude regions when stratospheric temperatures are more favourable for chlorine activation and enhanced ozone loss (e.g. in winter). The activation of chlorine from volcanic sulfate aerosols and associated ozone depletion is arguably more significant in the cold temperature conditions of winter and spring, even above the formation threshold of polar stratospheric clouds (Hanson et al., 1994). The eruption of the Calbuco volcano in the Southern Hemisphere in April 2015 could be a good candidate for a study of this process (Solomon et al., 2016).
Balloon data can be accessed on the ESPRI database (ESPRI data Centre,
Aerosol size distributions are provided in the 0.4–5
A detailed description of the instrumental characteristics of SPIRALE and of
its operating mode can be found in Moreau et al. (2005). Six tunable laser
diodes emitting in spectral micro-windows (< 1 cm
Direct solar spectra from two UV–visible DOAS spectrometers are collected
onboard the azimuth-controlled LPMA/DOAS (Limb Profile Monitor of the
Atmosphere/Differential Optical Absorption Spectroscopy) balloon payload
which carries a sun tracker (Hawat et al., 1995). The solar reference
spectrum is usually the spectrum for which the air mass along the
line of sight and the residual trace gas absorption are minimal. The
residual absorption in the solar reference is determined using Langley's
extrapolation to zero air mass. Rayleigh and Mie scattering are accounted
for by including a third-order polynomial in the fitting procedure. The
relative wavelength alignment of the absorption cross sections and the solar
reference spectrum is fixed and only the measured spectrum is allowed to
shift and stretch. O
Bromine monoxide (BrO) is detected in the UV wavelength range from 346 to
360 nm as recommended by Aliwell et al. (2002). This wavelength range
contains the UV vibration absorption bands (4–0 at 354.7 nm and 5–0 at
348.8 nm) of the A(
The data presented in this study were obtained using a
Système d'Analyse par Observation Zénithale (SAOZ)-type UV–visible spectrometer
(Pommereau and Piquard, 1994) connected to a sun and moon tracker for the
detection of ozone and NO
For the flight presented in this study we have added an HR4000 UV
spectrometer from Ocean Optics to detect BrO absorption lines in the
346–360 nm range as done for the DOAS instrument. The spectrometer is
thermally insulated and regulated using Peltier devices to avoid spectral
shifts. It has its own connection to the sun tracker but collects the
sunlight simultaneously with a Jobin Yvon UV–visible spectrometer. We use
the same data reduction method as for DOAS as described in detail by Dorf et
al. (2006b) to retrieve SCDs and the vertical profile of BrO. In our case the
Wahner et al. (1988) BrO and Bremen ozone and NO
The REPROBUS 3-D CTM computes the evolution of 55 species by means of about 160 photolytic gas-phase and heterogeneous reactions, with a time step of 15 min in this study. A semi-Lagrangian code transports 40 species or chemical families, typically long-lived tracers but also more unstable compounds (Lefèvre et al., 1994, 1998).
Temperature, winds and surface pressure are specified from the 3-D ECMWF meteorological data from the surface up to 0.01 hPa (i.e. about 80 km in altitude) on 91 levels. This results in a vertical resolution of about 0.45 km in the lower stratosphere. REPROBUS is driven by 3-hourly ECMWF wind fields obtained by interleaving operational analysis and forecasts because in this way spurious calculation of transport is reduced in comparison with simulations based on 6-hourly analysis (Legras et al., 2005; Berthet et al., 2006).
Gas-phase kinetics parameters used in the present study are based on the
recommendation by the Jet Propulsion Laboratory (JPL) described in Sander
et al. (2011). In particular for nitrogen gas-phase chemistry, revised kinetic
data were recommended because, following a number of studies (e.g. Brown et
al., 1999; Gao et al., 1999; Jucks et al., 1999; Osterman et al., 1999;
Kondo et al., 2000; Prasad, 2003), a lower rate for the reaction of NO
The heterogeneous chemistry module includes reactions on liquid aerosols. An
analytical expression is used to calculate the equilibrium composition and
volume of the H
Initialized amounts of species are taken from a long-term simulation from
the UPMC 2-D model (Bekki and Pyle, 1994; Weisenstein and Bekki, 2006).
The initialization of stratospheric chlorine precursors is based on scenarios
defined by the World Meteorological Organization (WMO, 2014). Total
inorganic chlorine (Cl
Gaseous sulfur chemistry is not included in the REPROBUS CTM. The UPMC 2-D
model climatology (Bekki and Pyle, 1994) provides the initialization of
H
The authors declare that they have no conflict of interest.
The authors are grateful to the CNES (Centre National d'Etudes Spatiales) balloon launching team for successful operations and the Swedish Space Corporation at Esrange. The StraPolÉté project and the associated balloon campaign has been funded by the French “Agence Nationale de la Recherche” (ANR-BLAN08-1-31627), CNES, and the “Institut Polaire Paul-Emile Victor” (IPEV). The study is supported by the French Labex “Étude des géofluides et des VOLatils–Terre, Atmosphère et Interfaces – Ressources et Environnement” (VOLTAIRE) (ANR-10-LABX-100-01) managed by the University of Orleans. The ETHER database (CNES-INSU/CNRS) is a partner of the project. Further support for the DOAS balloon measurements came through the Deutsche Forschungsgemeinschaft, DFG (grants PF-384/5-1 and 384/5-1 and PF384/9-1/2) and the European projects EU projects Reconcile (FP7-ENV-2008-1-226365) and SHIVA (FP7-ENV-2007-1-226224). We thank Michel Van Roozendael and Caroline Fayt from BIRA/IASB in Belgium for making available the WINDOAS algorithm in a form very well-adapted to data reduction methods based on the differential optical absorption technique. We acknowledge the MIPAS/Envisat team from Karlsruhe Institute of Technology (KIT) for making IMK/IAA data available. Edited by: A. Engel Reviewed by: two anonymous referees