A REtrieval Method for optical and physical Aerosol Properties in the stratosphere (REMAPv1)

Abstract. The stratospheric aerosol is an important climate forcing agent as it scatters some of the incoming solar radiation back to space, thus cooling the Earth’s surface and the troposphere. At the same time it absorbs some of the upwelling terrestrial radiation, which heats the stratosphere. It also plays an important role in stratospheric ozone chemistry by hosting heterogeneous reactions. Major volcanic eruptions can cause strong perturbations of stratospheric aerosol, changing its radiative and chemical effects by more than an order of magnitude. Many global climate models require prescribed stratospheric aerosol as input to properly simulate both climate effects in the presence and absence of volcanic eruptions. This paper describes REMAP, a retrieval method and code for aerosol properties that has been used in several model intercomparison projects (under the name SAGE-3/4λ). The code fits a single-mode log-normal size distribution for a pure aqueous sulfuric acid aerosol to aerosol extinction coefficients from observational or model data sets. From the retrieved size distribution parameters the code calculates the effective radius, surface area density, as well as extinction coefficients, single-scattering albedos and asymmetry factors of the aerosol within the wavelength bands specified by individual climate models. We validate the REMAP using balloon observations after the Pinatubo and Hunga-Tonga Hunga Ha’apai volcanic eruptions, as well as four decades of LIDAR measurements. Within the constraints of a single-mode log-normal distribution REMAP generates realistic effective radii and surface area densities after volcanic eruptions and generally matches the LIDAR backscatter time series within measurement uncertainty. Deviations in aerosol backscatter up to a factor of 2 arise when (non-volcanic) tropospheric intrusions (e.g. from wildfires) are present and their composition deviates significantly from the background type. We describe the products that have been used in CCMI, CMIP6 and other model intercomparison projects, and provide practical instructions for use of the code.