Model simulations of the chemical and aerosol microphysical evolution of the Sarychev Peak 2009 eruption cloud compared to in situ and satellite observations
Authors: Lurton Thibaut , Jégou Fabrice , Berthet Gwenaël , Renard Jean-Baptiste , Clarisse Lieven , Schmidt Anja , Brogniez Colette , Roberts Tjarda J. 
Published: March 2018

Abstract. Volcanic eruptions impact climate through the injection of sulfur
dioxide (SO2), which is oxidized to form sulfuric acid
aerosol particles that can enhance the stratospheric aerosol optical
depth (SAOD). Besides large-magnitude eruptions, moderate-magnitude
eruptions such as Kasatochi in 2008 and Sarychev Peak in 2009 can
have a significant impact on stratospheric aerosol and hence
climate. However, uncertainties remain in quantifying the
atmospheric and climatic impacts of the 2009 Sarychev Peak eruption
due to limitations in previous model representations of volcanic
aerosol microphysics and particle size, whilst biases have been
identified in satellite estimates of post-eruption SAOD. In
addition, the 2009 Sarychev Peak eruption co-injected hydrogen
chloride (HCl) alongside SO2, whose potential
stratospheric chemistry impacts have not been investigated to
date. We present a study of the stratospheric
SO2–particle–HCl processing and impacts following
Sarychev Peak eruption, using the Community Earth System Model version
1.0 (CESM1) Whole Atmosphere Community Climate Model (WACCM) – Community
Aerosol and Radiation Model for Atmospheres (CARMA) sectional
aerosol microphysics model (with no a priori assumption on particle
size). The Sarychev Peak 2009 eruption injected 0.9 Tg of
SO2 into the upper troposphere and lower stratosphere
(UTLS), enhancing the aerosol load in the Northern Hemisphere. The
post-eruption evolution of the volcanic SO2 in space and
time are well reproduced by the model when compared to
Infrared Atmospheric Sounding Interferometer (IASI) satellite
data. Co-injection of 27 Gg HCl causes a lengthening of the
SO2 lifetime and a slight delay in the formation of
aerosols, and acts to enhance the destruction of stratospheric ozone
and mono-nitrogen oxides (NOx) compared to the simulation
with volcanic SO2 only. We therefore highlight the need to
account for volcanic halogen chemistry when simulating the impact of
eruptions such as Sarychev on stratospheric chemistry. The
model-simulated evolution of effective radius (reff)
reflects new particle formation followed by particle growth that
enhances reff to reach up to 0.2 µm on zonal
average. Comparisons of the model-simulated particle number and
size distributions to balloon-borne in situ stratospheric
observations over Kiruna, Sweden, in August and September 2009, and
over Laramie, USA, in June and November 2009 show good agreement and
quantitatively confirm the post-eruption particle enhancement. We
show that the model-simulated SAOD is consistent with that derived
from the Optical Spectrograph and InfraRed Imager System (OSIRIS) when
both the saturation bias of OSIRIS and the fact that extinction
profiles may terminate well above the tropopause are taken into
account. Previous modelling studies (involving assumptions on
particle size) that reported agreement with (biased) post-eruption
estimates of SAOD derived from OSIRIS likely underestimated the
climate impact of the 2009 Sarychev Peak eruption.