Abstract
Abstract. Stratospheric-sulfate aerosols play an important role in the physics and chemistry of the atmosphere. The radiative and chemical effects of stratospheric-sulfate aerosols depend critically on the aerosol particle size distribution and its variability. Despite extensive research spanning several decades, the scientific understanding of the particle size distribution of stratospheric aerosols is still incomplete. Particle size estimates (often represented by the median radius of an assumed monomodal log-normal distribution with a fixed width or by the effective radius) reported in different studies cover a wide range, even under background stratospheric conditions, and particle size estimates retrieved from satellite solar-occultation measurements in the optical spectral range show a tendency to be systematically larger than retrievals based on other optical methods. In this contribution we suggest a potential reason for these systematic differences. Differences between the actual aerosol particle size distribution and the size distribution function assumed for aerosol size retrievals may lead to systematic differences in retrieved aerosol size estimates. We demonstrate that these systematic differences may differ significantly for different measurement techniques, which is related to the different sensitivities of these measurement techniques to specific parts of the aerosol particle population. In particular, stratospheric-aerosol size retrievals based on solar-occultation observations may yield systematically larger particle size estimates (median or effective radii) compared to, e.g., lidar backscatter measurements. Aerosol concentration, on the other hand, may be systematically smaller in retrievals based on occultation measurements compared to lidar measurements. The results indicate that stratospheric-aerosol size retrievals based on occultation or lidar measurements have to be interpreted with caution, as long as the actual aerosol particle size distribution is not well known.
Highlights
According to Robock (2015) the variation of the stratospheric-aerosol particle size as a consequence of volcanic injections into the stratosphere is one of the key open science questions of current stratospheric-aerosol research
Particle size estimates reported in different studies cover a wide range, even under background stratospheric conditions, and particle size estimates retrieved from satellite solar-occultation measurements in the optical spectral range show a tendency to be systematically larger than retrievals based on other optical methods
For increasing values of the coarse-mode fraction, the median radius based on the occultation measurements increases significantly and reaches about 3.5 times the median radius of the main mode for a coarse-mode fraction of χ = 10−1
Summary
According to Robock (2015) the variation of the stratospheric-aerosol particle size as a consequence of volcanic injections into the stratosphere is one of the key open science questions of current stratospheric-aerosol research. Stratospheric-aerosol particle size retrievals – for volcanically quiescent periods – from satellite occultation measurements tend to yield relatively large median radii of up to several hundred nanometers (e.g., Bingen et al, 2003, 2004a, b; Wrana, 2019; Wrana et al, 2020), whereas retrievals based on several other measurement techniques typically lead to significantly smaller median radii (e.g., McLinden et al, 1999; Bourassa et al, 2008; Malinina et al, 2018; Ugolnikov et al, 2018; Zalach et al, 2019). One of the main issues of optical techniques to investigate the size of aerosol particles is the strong dependence of the aerosol scattering cross section on particle radius.
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