Aerosol particle size distribution in the stratosphere retrieved from SCIAMACHY limb measurements

Information about aerosols in the Earth’s atmosphere is of a great importance in the scientific community. While tropospheric aerosol influences the radiative balance of the troposphere and affects human health, stratospheric aerosol plays an important role in atmospheric chemistry and climate change. In particular, information about the amount and distribution of stratospheric aerosols is required to initialize climate models, as well as validate aerosol microphysics models and investigate geoengineering. In addition, good knowledge of stratospheric aerosol loading is needed to increase the retrieval accuracy for the key trace gases (e.g. ozone or water vapor), when interpreting remote sensing measurements of the scattered solar light. There are several parameters which are commonly used to describe stratospheric aerosols, such as the aerosol extinction coefficient and Ångström coefficient. However, the use of particle size distribution parameters coupled with the aerosol number density is an unambiguous and more optimal approach. In this manuscript we present a new retrieval algorithm to obtain the particle size distribution of the stratospheric aerosol from space borne observations of the scattered solar light in the limb viewing geometry. While the mode radius and width of the aerosol particle size distribution are retrieved, the aerosol particle number density remains unchanged. The latter is justified by a lower sensitivity of the limb-scattering measurements to changes in this parameter. To our knowledge this the first data set providing two parameters of the particle size distribution of the stratospheric aerosol from space borne measurements of the scattered solar light. Generally, the mode radius and absolute distribution width can be retrieved with the uncertainty of less than 20 %. The algorithm was successfully applied to the tropical region (20° N–20° S) for 10 years (2002–2012) of SCIAMACHY observations in limb viewing geometry, establishing a unique data set. Analysis of this new climatology for the particle size distribution parameters showed clear increases of the mode radius after the tropical volcanic eruptions, whereas no distinct behaviour oft the absolute distribution width could be identified. A tape recorder, which describes the time lag as the perturbation propagates to higher altitudes, was identified for both parameters after the volcanic eruptions. A Quasi Biannual Oscillation (QBO) pattern at upper altitudes (28–32 km) is prominent in the anomalies of the analyzed parameters. A comparison of the aerosol effective radii derived from SCIAMACHY and SAGE II data was performed. The average difference is found to be around 30 % at the lower altitudes decreasing with increasing height to almost zero around 30 km. The data sample available for the comparison is, however, relatively small.