Properties of Sarychev sulphate aerosols over the Arctic

Abstract

[1] Aerosols from the Sarychev Peak volcano entered the Arctic region less than a week after the strongest SO 2 eruption on June 15 and 16, 2009 and had, by the first week in July, spread out over the entire Arctic region. These predominantly stratospheric aerosols were determined to be sub-micron in size and inferred to be composed of sulphates produced from the condensation of SO 2 gases emitted during the eruption. Average (500 nm) Sarychev-induced stratospheric optical depths (SOD) over the Polar Environmental Atmospheric Research Laboratory (PEARL) at Eureka (Nunavut, Canada) were found to be between 0.03 and 0.05 during the months of July and August, 2009. This estimate, derived from sunphotometry and integrated lidar backscatter profiles was consistent with averages derived from lidar estimates over Ny-Ålesund (Spitsbergen). The Sarychev SOD e-folding time at Eureka, deduced from lidar profiles, was found to be approximately 4 months relative to a regression start date of July 27. These profiles initially revealed the presence of multiple Sarychev plumes between the tropopause and about 17 km altitude. After about two months, the complex vertical plume structures had collapsed into fewer, more homogeneous plumes located near the tropopause. It was found that the noisy character of daytime backscatter returns induced an artifactual minimum in the temporal, pan-Arctic, CALIOP SOD response to Sarychev sulphates. A depolarization ratio discrimination criterion was used to separate the CALIOP stratospheric layer class into a low depolarization subclass which was more representative of Sarychev sulphates. Post-SAT (post Sarychev Arrival Time) retrievals of the fine mode effective radius (r eff,f ) and the logarithmic standard deviation for two Eureka sites and Thule (Greenland) were all close to 0.25 mm and 1.6 respectively. The stratospheric analogue to the columnar r eff,f average was estimated to be r eff,f (+) = 0.29 mm for Eureka data. Stratospheric, Raman lidar retrievals at Ny-Ålesund, yielded a post-SAT average of r eff,f (+) = 0.27 mm. These results are $50% larger than the background stratospheric-aerosol value. They are also about a factor of two larger than modeling values used in recent publications or about a factor of five larger in terms of (per particle) backscatter cross section.

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

“…At mid-latitudes, a temporary decrease in r eff can also be seen immediately following the eruption: this is due to new particle formation (nucleation) of particles of a few nm-size. The latitudinal trend in r eff simulated by our 25 model is broadly consistent with the trend reported from ground-based remote sensing at Eureka (Nunavut, Canada) that found r eff = 0.29 µm (O'Neill et al, 2012). Modelled absolute values of r eff are also globally consistent with balloon-borne observations in August 2009 (Jégou et al, 2013).…”

“…The volcanic aerosol evolution is discussed further in Section 3.5 below. As pointed out by Fromm et al (2014), it is impossible to revert this process of data degradation; the best we can achieve to perform consistent model-to-observation comparisons is to degrade the extinctions derived from the model in order to 25 derive SAOD "as OSIRIS would detect it". It must nonetheless be emphasised that such a comparison is not a complete evaluation of the model performance: any agreement found cannot fully validate aspects of the model output that are removed in the degradation process.…”

“…3.5 Post-eruption effective radius simulated using a sectional aerosol scheme 20 Discrepancies in the magnitude and e-folding times between model and OSIRIS SAOD's have been mentioned elsewhere, notably in Haywood et al (2010); Kravitz et al (2011);O'Neill et al (2012). This led to a consequent questioning of the models' reliability to simulate sulfuric acid particle formation accurately in terms of timing, thought to be caused by the 21 Atmos.…”

“…9, Table 3). Recent studies (Haywood et al, 2010;Kravitz et al, 2011;O'Neill et al, 2012) quantifying volcanic impacts have tended to (only) incriminate their models' particle formation schemes because the comparisons with OSIRIS's satellite retrievals were poor specifically regarding the timing of the SAOD 20 maximum. As a matter of fact, caveats on OSIRIS's measurement, as outlined in Fromm et al (2014), are the key point to any model-observation comparison.…”

“…We neglected the minor, low-altitude (inferior to 5 km) explosions reported on 11 and 16 June, and injected SO 2 continuously for a 24-hr period on 15 June spread evenly between 11 km and 15 km altitude a.s.l. The timing of the SO 2 emissions is based on SO 2 satellite retrievals from IASI (Clarisse et al, 2012;Carn et al, 2016;Carboni et al, 2016), 25 MODIS (MODerate-resolution Imaging Spectroradiometer) (Rybin et al, 2011;Realmuto and Berk, 2016), and OMI (Ozone Monitoring Instrument) (Theys et al, 2015) which all show that the majority of high altitude SO 2 was released on the 15 th (and possibly in the early morning of the 16 th ). Haywood et al (2010) used a total injection mass of 1.2 Tg SO 2 , which was the SO 2 total mass value retrieved on 16 June with IASI.…”