Stratospheric aerosol size reduction after volcanic eruptions

Abstract

Abstract. The stratospheric aerosol layer plays an important role in the radiative balance of earth primarily through scattering of solar radiation. The magnitude of this effect depends critically on the size distribution of the aerosols. The aerosol layer is in large part fed by volcanic eruptions strong enough to inject gaseous sulfur species into the stratosphere. The evolution of the stratospheric aerosol size after volcanic eruptions is currently one of the biggest uncertainties in stratospheric aerosol science. We retrieved aerosol particle size information from satellite solar occultation measurements from the Stratospheric Aerosol and Gas Experiment III mounted on the International Space Station (SAGE III/ISS) using a robust spectral method. We show that, surprisingly, some volcanic eruptions can lead to a decrease in average aerosol size, like the 2018 Ambae and the 2021 La Soufrière eruptions. In 2019 an intriguing contrast is observed, where the Raikoke eruption (48° N, 153° E) in 2019 led to the more expected stratospheric aerosol size increase, while the Ulawun eruptions (5° S, 151° E), which followed shortly after, again resulted in a reduction of the median radius and absolute mode width values in the lowermost stratosphere. In addition, the Raikoke and Ulawun eruptions were simulated with the aerosol climate model MAECHAM5-HAM. In these model runs, the evolution of the extinction coefficient as well as of the effective radius could be reproduced well for the first 3 months of volcanic activity. However, the long lifetime of the very small aerosol sizes of many months observed in the satellite retrieval data could not be reproduced.

Using OMPS-LP color ratio to extract stratospheric aerosol particle median radius and concentration with application to two volcanic eruptions

“…9, 10 show the best agreement is for 𝑠 =1.6. This is consistent with analysis of Wrana et al (2021Wrana et al ( , 2023 for SAGE III/ISS observations at altitudes above ~ 20 km where the particle distributions are unimodal. Both the OMPS-LP and balloon data particle radius are ~0.1 μm at all altitudes.…”

“…This assessment is also consistent with (2012). The exception appears to be the Hunga-Tonga eruption where SAGE measurements support a smaller distribution width of 1.2 (Wrana et al, 2023) which leads to larger particle sizes consistent with balloon observations (Baron et al, 2023). We also found that the uncertainty caused by the algorithm's assumed phase function is minimized when using typical aerosol CRs although the uncertainty can reach ±12-±32% for volcanic aerosol.…”

“…The HT aerosol plume has two distinct populations (Figure 11e), one lower extinction population with a CR of ~2.5 and a higher extinction population with a CR of ~1.5. Using the in situ measurements (Baron et al, 2023), we assume a log-normal particle size distribution width of 1.2 for this eruption event, which consistent with SAGE III/ISS measurements (Wrana et al, 2023). The two distinct populations translate into two size populations.…”

Observation of the Aerosol Plume From the 2022 Hunga Tonga—Hunga Ha'apai Eruption With SAGE III/ISS

“…The homogeneity and the persistence of the properties across the plume and in time are clearly visible. Using the same method, Wrana et al (2023) found σ values for other stratospheric volcanic plumes produced by the eruptions of Ambae in 2018, Raikoke and Ulawun in 2019, and La Soufrière in 2021, which are all larger, approaching 1.8-2.0 together with smaller sizes (Wrana et al, 2023), reinforcing the exceptional aspect of the HTHH eruption.…”

“…This density is not very different from that of the background, here around 2 cm −3 at the altitudes of the plume and 3 cm −3 for Wrana et al (2021) at 20 km in June 2017. For comparison, Wrana et al (2023) found density of 20-30 cm −3 in stratospheric volcanic plumes generated by other recent eruptions with similar sulfur injection but much smaller particles.…”