Results
Long-term Variability of Stratospheric Aerosols at Mauna Loa, Hawaii
As part of the Network for the Detection of Atmospheric Composition Change (NDACC), ground-based measurements obtained from the Jet Propulsion Laboratory (JPL) stratospheric ozone lidar at Mauna Loa, Hawaii (MLSOL) over the past two decades were used to investigate the impact of volcanic eruptions and pyro-cumulonimbus smoke plumes on the stratospheric aerosol load above Hawaii. Between 1 January 1999 and 1 November 2019, MLSOL conducted 2732 measurement sessions with an average duration of 2 h. The retrieved aerosol extinction profiles and corresponding stratospheric aerosol optical depth (AOD) are presented in Figure 1a, together with the AOD derived from a co-located aerosol lidar (NOAA lidar) and satellite measurements (OMPS/GloSSAC) in Figure 1b. All datasets reveal a relatively quiescent period between 1999 and 2005, followed by an active period of multiple eruptions (e.g., Nabro) until early 2012. Another quiescent period, with slightly higher aerosol background, lasted until mid-2017, when a combination of extensive wildfires 15 and multiple volcanic eruptions caused a significant increase in stratospheric aerosol loading. This loading maximized at the very end of the time period considered (fall 2019) as a result of the Raikoke eruption, the plume of which ascended to 26 km altitude in less than three months. The short-term variability in the stratospheric aerosols load over Mauna Loa is dominated by volcanic eruptions that occurred in the tropics and northern hemisphere with a VEI>4 (indicated by red arrows). As can be seen, some of those eruptions had a strong impact in the stratospheric extinction profiles and caused an abrupt increase in the stratospheric AOD, followed by a decay over a period of 3 to 5 months.
Figure 1. a) Stratospheric aerosol extinction coefficient over Mauna Loa measured by MLSOL. (b) Stratospheric AOD derived from MLSOL, the co-located NOAA aerosol lidar and the satellite-based datasets (OMPS and GloSSAC).
The Raikoke Eruption Plume
Among the several eruptions that impacted the stratospheric AOD records over Mauna Loa, the aerosol plume observed on September 2019 over Mauna Loa at 26 km represented an anomaly due to its strong backscatter coefficient and compactness (Figure 2b). The back-trajectories (Figure 2a) pointed the origin of this plume back to the Raikoke eruption, and showed an unprecedent ascent rate that took the plume to 26 km in less than 3 months.
Figure 2. (a) Aerosol backscatter coefficient profiles corresponding to the Raikoke (red) and Nabro (blue) eruption aerosol plumes over Mauna Loa, as well as typical background conditions (dashed black). (b) Estimated locations of the Raikoke plume observed at Mauna Loa on September 24, derived from CALIOP L1 attenuated backscatter profiles between 17 July and 24 September 2019.
A 25-year High in Global Stratospheric Aerosol Loading
The presence of aerosol in the stratosphere is mainly driven by the injection from below of tropospheric aerosols and sulfur-containing compounds which act as precursors for the formation of sulfuric acid droplets in the stratosphere. The period 2017-2020 showed several volcanic and large biomass burning events that loaded the stratosphere with aerosol levels unprecedented since the Mount Pinatubo era (early 1990s). Although the occurrence and frequency of large volcanic eruptions remain random, certain favorable conditions such as extended drought or the increase in surface temperature may lead to an increase in the occurrence and strength of pyro-cumulonimbus events and its associated stratospheric aerosol injection. For example, estimates of the 2019/2020 Australian wildfires total mass injected in the stratosphere point to the equivalent of a mid-size volcanic eruption. Whether of fire-type or volcanic-type, an increase in the strength and/or frequency of aerosol injection into the stratosphere may have an impact on climate and ozone that is yet to be fully quantified and understood. The continuation of key stratospheric aerosol measurements such as those described here is of crucial importance.
References
Chouza, F., T. Leblanc, J. Barnes, M. Brewer, P. Wang, and D. Koon (2020), Long-term (1999–2019) variability of stratospheric aerosol over Mauna Loa, Hawaii, as seen by two co-located lidars and satellite measurements, Atmos. Chem. Phys., 20(11), 6821-6839. https://doi.org/10.5194/acp-20-6821-2020
Leblanc, T., F. Chouza, G. Taha, S. Khaykin, J. Barnes, J.-P. Vernier, and L. Rieger, 2019: A 25-year high in global stratospheric aerosol loading [Sidebar 2.2. in “State of the Climate in 2019"]. Bull. Amer. Meteor., 101 (8), S88–S89, https://doi.org/10.1175/BAMS-D-20-0104.1