Impact of a moderate volcanic eruption on chemistry in the lower stratosphere: balloon-borne observations and model calculations

Abstract

Abstract. 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 2 , HNO 3 and BrO from balloon-borne infrared and UV-visible spectrometers launched in Sweden in August-September 2009. It is shown that differences between observations and threedimensional (3-D) chemistry-transport model (CTM) outputs are not due to transport calculation issues but rather reflect the chemical impact of the volcanic plume below 19 km altitude. Good measurement-model agreement is obtained when the CTM is driven by volcanic aerosol loadings derived from in situ or space-borne data. As a result of enhanced N 2 O 5 hydrolysis in the Sarychev volcanic aerosol conditions, the model calculates reductions of ∼ 45 % and increases of ∼ 11 % in NO 2 and HNO 3 amounts respectively over the August-September 2009 period. The decrease in NO x abundances is limited due to the expected saturation effect for high aerosol loadings. The links between the various chemical catalytic cycles involving chlorine, bromine, nitrogen and HO x compounds in the lower stratosphere are discussed. The increased BrO amounts (∼ 22 %) compare rather well with the balloon-borne observations when volcanic aerosol levels are accounted for in the CTM and appear to be mainly controlled by the coupling with nitrogen chemistry rather than by enhanced BrONO 2 hydrolysis. We show Published by Copernicus Publications on behalf of the European Geosciences Union. 2230G. Berthet et al.: Impact of a moderate volcanic eruption on chemistry in the lower stratosphere that the chlorine partitioning is significantly controlled by enhanced BrONO 2 hydrolysis. However, simulated effects of the Sarychev eruption on chlorine activation are very limited in the high-temperature conditions in the stratosphere in the period considered, inhibiting the effect of ClONO 2 hydrolysis. As a consequence, the simulated chemical ozone loss due to the Sarychev aerosols is low with a reduction of −22 ppbv (−1.5 %) of the ozone budget around 16 km. This is at least 10 times lower than the maximum ozone depletion from chemical processes (up to −20 %) reported in the Northern Hemisphere lower stratosphere over the first year following the Pinatubo erupti...

Model simulations of the chemical and aerosol microphysical evolution of the Sarychev Peak 2009 eruption cloud compared to in situ and satellite observations

“…The impacts were more modest than found for large-magnitude eruptions such as the 1991 Mt. Pinatubo eruption, but simulations suggest ozone depletion up to 4 % in the lower stratosphere at high latitudes, with local NO 2 depletion up to 40 % (Berthet et al, 2017), consistent with balloon-based and satellite observations (Adams et al, 2017).…”

“…Pinatubo eruption were efficiently washed out and therefore did not reach the stratosphere (Mankin et al, 1992;Tabazadeh and Turco, 1993), though the washout for the Sarychev case was not necessarily as efficient (von Glasow et al, 2009). Observational evidence of stratospheric NO 2 depletion following moderate-magnitude volcanic eruptions is provided by Adams et al (2017) based on satellite remote sensing, and Berthet et al (2017) by balloon-borne observations following the Sarychev Peak eruption. Our study builds on these recent works in two aspects.…”

“…In summer, greater depletions of up to −60 % for NO 2 are found at higher latitudes. This is primarily due to the higher aerosol loadings in these regions, and to favourable solar illumination conditions for which the catalytic ozone loss cycles (through OH radical production) are enhanced (Berthet et al, 2017). A more detailed description of the involved chemical processes is provided in Berthet et al (2017).…”

“…This is primarily due to the higher aerosol loadings in these regions, and to favourable solar illumination conditions for which the catalytic ozone loss cycles (through OH radical production) are enhanced (Berthet et al, 2017). A more detailed description of the involved chemical processes is provided in Berthet et al (2017). Our results are broadly consistent with the observations of Adams et al (2017), who reported that stratospheric NO 2 abundances were reduced by up to ≈ 45-55 % over 40-80 • N as consequence of the Sarychev eruption.…”

“…Previous modelling studies of the Sarychev 2009 eruption focused on the injection of SO 2 , formation of volcanic aerosol, and its radiative and atmospheric chemistry impacts (Haywood et al, 2010;Kravitz et al, 2011;Berthet et al, 2017). However, the models did not explicitly simulate the aerosol microphysical evolution of the volcanic cloud; rather, they used bulk aerosol schemes and/or assumed size distributions.…”