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. Volcanic eruptions impact climate through the injection of sulfurdioxide ( SO2 ), which is oxidized to form sulfuric acidaerosol particles that can enhance the stratospheric aerosol opticaldepth (SAOD). Besides large-magnitude eruptions, moderate-magnitudeeruptions such as Kasatochi in 2008 and Sarychev Peak in 2009 canhave a significant impact on stratospheric aerosol and henceclimate. However, uncertainties remain in quantifying theatmospheric and climatic impacts of the 2009 Sarychev Peak eruptiondue to limitations in previous model representations of volcanicaerosol microphysics and particle size, whilst biases have beenidentified in satellite estimates of post-eruption SAOD. Inaddition, the 2009 Sarychev Peak eruption co-injected hydrogenchloride ( HCl ) alongside SO2 , whose potentialstratospheric chemistry impacts have not been investigated todate. We present a study of the stratospheric SO2 –particle– HCl processing and impacts followingSarychev Peak eruption, using the Community Earth System Model version1.0 (CESM1) Whole Atmosphere Community Climate Model (WACCM) – CommunityAerosol and Radiation Model for Atmospheres (CARMA) sectionalaerosol microphysics model (with no a priori assumption on particlesize). 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. Thepost-eruption evolution of the volcanic SO2 in space andtime are well reproduced by the model when compared toInfrared Atmospheric Sounding Interferometer (IASI) satellitedata. Co-injection of 27  Gg HCl causes a lengthening of the SO2 lifetime and a slight delay in the formation ofaerosols, and acts to enhance the destruction of stratospheric ozoneand mono-nitrogen oxides ( NOx ) compared to the simulationwith volcanic SO2 only. We therefore highlight the need toaccount for volcanic halogen chemistry when simulating the impact oferuptions such as Sarychev on stratospheric chemistry. Themodel-simulated evolution of effective radius ( reff )reflects new particle formation followed by particle growth thatenhances reff to reach up to 0.2  µm on zonalaverage. Comparisons of the model-simulated particle number andsize distributions to balloon-borne in situ stratosphericobservations over Kiruna, Sweden, in August and September 2009, andover Laramie, USA, in June and November 2009 show good agreement andquantitatively confirm the post-eruption particle enhancement. Weshow that the model-simulated SAOD is consistent with that derivedfrom the Optical Spectrograph and InfraRed Imager System (OSIRIS) whenboth the saturation bias of OSIRIS and the fact that extinctionprofiles may terminate well above the tropopause are taken intoaccount. Previous modelling studies (involving assumptions onparticle size) that reported agreement with (biased) post-eruptionestimates of SAOD derived from OSIRIS likely underestimated theclimate impact of the 2009 Sarychev Peak eruption.

atmospheric chemistry and physics

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