Issues related to the retrieval of stratospheric-aerosol particle size information based on optical measurements

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, stratosphericaerosol 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.

A Method for Retrieving Stratospheric Aerosol Extinction and Particle Size from Ground-Based Rayleigh-Mie-Raman Lidar Observations

“…In particular, the second mode frequently seen in the in-situ OPC measurements by Deshler [20]-having a typical median radius of about 400 nm-has a significantly lower number density than the main mode with typical median radii of about 100 nm or less. For our work, a monomodal assumption is justified, even if further modes of very small particles are present because the measurement is dominated by the contribution of the main mode, as recently demonstrated by [14]. It also should be mentioned that, with every mode, additional parameters have to be taken into account.…”

“…Together with the extinction profiles retrieved in the previous section, all information is available to estimate the aerosol number density N A utilising the relationship between extinction coefficient, scattering cross section, and particle number density. Following Equations (7) and (14), the particle number density is calculated as:…”

“…Finally, we show the error of the aerosol number density (N A ) in Figure 13. The number density is very sensitive to uncertainties of r m as σ Mie (r m , λ) in Equation (16) depends exponentially on r m with about σ Mie (r m , λ) ∝ r κ m with κ of 4 to 5 (e.g., [14]).…”

“…The discrepancies between the aerosol size parameters retrieved from SAGE II solar occultation measurements to the other techniques may in part be a consequence of different sensitivities of these measurement techniques to the aerosol particle population in combination with errors in the assumed PSD. von Savigny and Hoffmann [14] recently investigated this aspect. They simulated color index measurements for lidar backscatter and solar occultation measurement geometries assuming a bi-modal log-normal aerosol PSD (following the results by Desher et al [28] and Deshler [20]) and retrieved the median radius of an assumed monomodal log-normal PSD with fixed width parameter.…”

“…The number of experimental studies on the size of stratospheric sulfate aerosols at high latitudes is quite limited and the published stratospheric aerosol size parameters in general cover quite a large range of values, even under background aerosol conditions (see, e.g., [14]). A major advantage of this new method is that the lidar ratio does not have to be assumed, but it is calculated from the measurements themselves.…”