Baseline Balloon Stratospheric Aerosol Profiles (B2SAP)—Systematic Measurements of Aerosol Number Density and Size

Stratospheric aerosol plays an important role in Earth's radiative balance. Systematic measurements of stratospheric aerosol number concentration and size are needed to characterize their baseline values and variability, improve understanding of aerosol microphysics, and assess the representation of stratospheric aerosol processes in global models. In situ measurements of particle size distribution provide insight into the responses of stratospheric aerosol processes to perturbations from volcanoes and wildfires, anthropogenic emissions from rockets and aircraft, or intentional injections related to climate intervention. The Baseline Balloon Stratospheric Aerosol Profiles (B2SAP) project uses compact, lightweight payloads carried by meteorological balloons to measure aerosol number density and size distributions, water vapor, ozone, and meteorological data from the surface to the middle stratosphere. The long‐term goal for the B2SAP project is to produce climatologies of aerosol number and size distributions into the middle stratosphere at latitudinally distributed measurement sites. Since March 2019, B2SAP payloads have been launched from Boulder, CO, USA once to twice per month and four to six times per year from Lauder, NZ. These measurements provide a new record of in situ stratospheric aerosol observations and are being used to help validate satellite‐based estimates of stratospheric aerosol size distributions. Here we present 50 vertical profiles from Boulder, CO, from March 2019 to March 2022, to build a NH mid‐latitude climatology of stratospheric aerosol and its variability and investigate the impact of natural perturbations on stratospheric aerosol microphysics. Plain Language Summary Particles in the stratosphere play important roles in chemistry and climate. Lightweight instruments carried by weather balloons can provide systematic measurements of the particle number and size, atmospheric water vapor and ozone (two gases that also influence atmospheric chemistry and climate) to inform these areas of research. Without weather in the stratosphere, the sources and sinks of particles are few, and concentrations of particles change slowly over time, in the absence of powerful volcanic eruptions or large wildfires, which can inject or loft material into the stratosphere. Particularly in these cases, balloon‐borne observations can be critical to validating satellite retrievals and improving global climate models. As part of the Baseline Balloon Strato