the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Stratospheric residence time and the lifetime of volcanic stratospheric aerosols
Abstract. The amount of time that volcanic aerosols spend in the stratosphere is one of the primary factors influencing the climate impact of volcanic eruptions. Stratospheric aerosol persistence has been described in different ways, with many works quoting an approximately 12 month “residence time” for aerosol from large tropical eruptions. Here, we aim to develop a framework for describing the evolution of global stratospheric aerosol after major volcanic eruptions and quantifying its persistence, based on global satellite-based aerosol observations, tracer transport simulations and simple conceptual modeling. We show that the stratospheric residence time of air, which is estimated through passive tracer pulse experiments and is one factor influencing the lifetime of stratospheric aerosols, is strongly dependent on the injection latitude and height, with an especially strong sensitivity to injection height in the first four kilometers above the tropical tropopause. Simulated stratospheric tracer evolution is best described by a simple model which includes a lag between the injection and initiation of removal from the stratosphere. Based on analysis of global stratospheric aerosol observations, we show that the stratospheric lifetime of stratospheric aerosol from the 1991 Pinatubo eruption is approximately 22 months. We estimate the potential impact of observational uncertainties on this lifetime finding it unlikely the lifetime of Pinatubo aerosol is less than 18 months.
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Status: open (until 19 Nov 2024)
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RC1: 'Comment on egusphere-2024-2400', Daniele Visioni, 31 Oct 2024
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This manuscript provides a very interesting perspective on the issue of the persistence of stratospheric aerosols through residence time, by developing a new framework to better understand (and potentially separate) the combined effect of stratospheric circulation and aerosol microphysics when quantifying the lifetime of volcanic (or potentially of SAI) aerosols. The manuscript is of extremely high value and quality, and I fully endorse publication. I have a few very minor comments and notes that the authors might wish to consider, but the manuscript is essentially publishable in its present form.
I want to add that I particularly liked the clear explanations in Section 2, which make this a very valuable "entry-point" manuscript for researchers interested in this topic. The analyses in Section 3 are also very robust and thorough.
On Figure 4, I would also note the consistency in the results with the experiments shown in Visioni et al. (2019): in that case, injections of SO₂ at 24 km at different latitudes and seasons were also tested in CESM-WACCM (for SAI simulations purposes, so more a three-months injection than a pulse) but there is a very robust similarity between the results (here, measured in terms of lifetime, there, in terms of burden) showing higher residence times for June than for December.
In Section 4.2 I would suggest that when the authors say "the HIRS estimates might be too small through this period or the GloSSAC estimates too large" they acknowledge that, for instance, the Quaglia et al. (2023) manuscript elsewhere cited point to the fact that the data [...] "are affected by a systematic error of 10 % due to the sensitivity of the retrieved method and uncertainties in the background.".
Finally, I will note in the conclusions that the framework here developed would be of great use not just to " aid in the interpretation of simulated geoengineering scenarios", but also in the development of more robust climatic emulators for SAI. In particular, there are now some efforts such as Farley at al (2024) to develop climatic emulators to simulate a broader range of interruptions of terminations of stratospheric aerosol injections, and adding a more robust representation of residency time could greatly improve our understanding (i.e. see their figure 6).
References
Farley et al 2024 Environ. Res.: Climate 3 035012 DOI 10.1088/2752-5295/ad519c
Visioni, D., MacMartin, D. G., Kravitz, B., Tilmes, S., Mills, M. J., Richter, J. H., & Boudreau, M. P. (2019). Seasonal injection strategies for stratospheric aerosol geoengineering. Geophysical Research Letters, 46, 7790–7799. https://doi.org/10.1029/2019GL083680
Citation: https://doi.org/10.5194/egusphere-2024-2400-RC1
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