An enhanced aerosol layer, known as the Asian Tropopause Aerosol Layer (ATAL), has been observed within the seasonal Asian monsoon anticyclone (AMA) since the late 1990s. Given the apparently abrupt appearance of this layer based on observations, it has been speculated that it originates from increasing human made emissions in Asia. However, the ATAL confinement is a result of a dynamical feature and does not purely consist of human made components. We herein investigate the possible existence of an ATAL earlier than the late 1990s. We exploit earliest possible, high quality space-based aerosol observations from Stratospheric Aerosol and Gas Experiment, or SAGE (1979-1981), SAGE III/ISS (2017, ongoing) and revisit SAGE II (1984-2005) data analysis. We find that seasonal averaged solar occultation aerosol measurements (past and present) can neither be used to exclude the existence of the ATAL, nor to infer a significant trend. However, first CAM5-MAM7 simulations indicate the presence of an ATAL signal for the tested years 1979 and 1980, with a human made component. We hypothesize that the human made component of the ATAL likely occurred since at least the 1970s, while the natural ATAL component (e.g. from dust) has always existed. Extended simulation based ATAL evolution studies are therefore the most reliable source for early ATAL investigations.

During an extended volcanic unrest starting in 2017, two main moderate stratospheric eruptions occurred at the Ambae volcano (15°S and 167°E), Vanuatu, in April and July 2018. Observations from a geostationary orbit show that the April and July eruptions injected a volcanic plume into the lower stratosphere. While aerosol enhancements from the April eruption have only had an impact on the Southern Hemisphere, the plume from the July eruption was distributed within the lower branch of the Brewer-Dobson circulation to both hemispheres. Satellite, ground-based and in situ observations show that the background aerosol is enhanced throughout the year after the July eruption on a global scale. A volcanic-induced perturbation of the global stratospheric aerosol optical depth up to 0.012 is found, in the ultraviolet/visible spectral range. This perturbation is comparable to that of recent moderate stratospheric eruptions like from Kasatochi, Sarychev and Nabro. Top of the atmosphere radiative forcing values are estimated between -0.45 and -0.6 W/m2 for this event, showing that the Ambae eruption had the strongest climatic impact of the year 2018. Thus, the Ambae eruption in 2018 has to be taken into account when studying the decadal lower stratospheric aerosol budget and in climate studies.

Following the Hunga Tonga eruption (20.6°S, 175.4°W, mid-January 2022), we present a balloon-borne characterization of the stratospheric aerosol plume one week after its injection (on 23 and 26/01/2022, La Réunion island at 21.1°S, 55.3°E). Satellite observations show that flight #1 took place during the overpass of a denser plume of sulfate aerosols (SA) compared to a more diluted plume during flight #2. Observations show that the sampled plumes (at around 22, 25 and 19 km altitude, respectively) consist exclusively of very small particles (with radius < 1 µm). Particles with radii between 0.5 and 1.0 µm show optically transparent features pointing to predominant SA. Particles with radii below 0.5 µm are partly absorbing, which could point to small sulfate coated ash particles, a feature not identified with space-borne observations. This shows that in situ observations are necessary to fully characterize the microphysical properties of the plumes tracked by space-borne instruments.