Balloon Baseline Stratospheric Aerosol Profiles (B2SAP)—Perturbations in the Southern Hemisphere, 2019–2022.

Volcanic and pyrocumulonimbus (pyroCB) injections into the stratosphere perturb the aerosol layer and can have important radiative and chemical impacts on timescales spanning from months to several years. Repeated in situ balloon‐borne measurements of aerosol size and number concentration (>140 nm in diameter), ozone, water vapor, and atmospheric state variables made at midlatitudes in the southern hemisphere (SH) since 2019 enable us to better characterize such events. We use this record and coincident lidar extinction profiles to study several moderate to large stratospheric perturbations in the SH between 2019 and 2022 in detail, including the Australian New Year Super Outbreak (ANYSO) pyroCB in 2020. Median vertical profiles of aerosol number concentration, effective radius, and surface area in SH midlatitudes are also compared with those recorded in Northern Hemisphere midlatitudes under baseline conditions using an identical payload. These data depict the variability in stratospheric aerosol properties in the SH midlatitudes during this period and provide a benchmark for global sectional aerosol models. They reveal that sulfate particle size distributions under baseline conditions and in volcanic plumes are relatively well represented in the Community Earth System Model—Community Aerosol Radiation Model for Atmospheres (CESM‐CARMA), but more observations of biomass burning plumes are needed to improve model skill in simulating pyroCB. Comparisons between in situ and lidar observations also highlight a need for more observations of aerosol composition and refractive index in both fresh and aging biomass burning plumes. Plain Language Summary: Particles in the atmosphere scatter incoming sunlight and can influence surface temperatures. When large wildfire and volcanic plumes occasionally reach the upper atmosphere, they can alter the number and size of particles in this region of the atmosphere for months to years. Here, we describe results from repeated balloon‐based measurements, ground‐based measurements and model simulations of particle number, size, and other optical properties in the Southern Hemisphere between 2019 and 2022 to better understand the impact of such events. We also compare these measurements to similar ones made in the Northern Hemisphere to probe for interhemispheric differences. Key Points: Lower‐stratospheric perturbations over SH midlatitudes are linked to Australian New Year's Super Outbreak pyrocumulonimbus and volcanic eruptionsMedian stratospheric aerosol number concentration, surface area and effective radius are similar over SH and NH midlatitudes in 2019–2022Balloon particle size distribution measurements indicate work remains to correctly simulate aerosol microphysics in large wildfire plumes [ABSTRACT FROM AUTHOR]