Retrieval of aerosol size distributions from satellite extinction spectra using constrained linear inversion

Abstract

Abstract. We examine the retrievability of stratospheric aerosol size distributions from stratospheric aerosol and gas experiment (SAGE) II extinction measurements. Analytical size distributions, which have been fitted to in situ measurements, are used as a model in conjunction with a Mie scattering code to produce synthetic extinction spectra at the SAGE II observational wavelengths. These synthetic data are analyzed using a constrained linear inversion procedure to deduce the variety of particle size distributions which can satisfy them. Fidelity of the retrieval depends on the range of radii over which the inversion is carried out and the range of wavelengths over which measurements are available, but is less sensitive to the number of wavelength channels. As expected, solutions are not unique and depend upon the weighting function used to constrain the solution and the degree of "smoothness" imposed. A given set of extinction measurements may be attributed to either a unimodal or bimodal size distribution, and the selection of a single solution is always subjective to some degree. For bimodal distributions the larger particle mode can be recovered reasonably well; however, the smaller particle mode of a bimodal distribution or the single mode of the background aerosol, which lie below the experiment's sensitivity range, cannot be retrieved accurately. The retrieval of any physically reasonable solution becomes increasingly difficult in the presence of experimental error. In many simulations in which a 10% random error was added to the extinction measurements, only solutions which were strongly influenced by the weighting function could be obtained. Integrated aerosol properties, such as total surface area and volume, can be retrieved to within 25 and 15% for particles in the size range between 0.1 and 1.0/•m from the SAGE II data using this technique. However, these error estimates must be increased to account for particles lying outside this range. It is these latter parameters which are most directly applicable to global chemistry and climate change studies.

Optimal estimation retrieval of aerosol microphysical properties from SAGE~II satellite observations in the volcanically unperturbed lower stratosphere

“…4.2) shows that both cover basically the same range and are therefore not distinguishable prior to the retrieval analysis. This agrees with results by Steele and Turco (1997) who found that it is possible for bimodal size distributions to account for extinctions generated from monomodal distributions and vice versa. The OE retrieval algorithm was then applied to the synthetic extinction data to retrieve monomodal size distribution parameters.…”

“…The "+" indicates that the value is an estimate of partial errors only and that the total error is expected to be higher due to other disregarded uncertainty components. The uncertainties in A as reported by Steele et al (1999) and Steele and Turco (1997), for instance, account for propagated random errors only. The total errors are expected to be higher by about 50% due to disregarded systematic (method bias) errors and contributions from particles smaller than 0.1 µm.…”

“…The total errors are expected to be higher by about 50% due to disregarded systematic (method bias) errors and contributions from particles smaller than 0.1 µm. The methods and the conditions under which these uncertainties were achieved are described in: (1) Wurl (2008), (2) Steele et al (1999), (3) Thomason and Poole (1993), (4) Steele and Turco (1997), (5) Anderson et al (2000), (6) Bingen et al (2004b), (7) Wang et al (1989), and (8) Bauman et al (2003a) et al ascribe their low number densities and overestimated particle sizes to the inability of SAGE II optical measurements to discriminate very thin particles in the Rayleigh limit of scattering. This suggests that the larger OE number densities and smaller OE median radii tend to be more realistic than those presented by Bingen et al (2004b).…”

SAGE II measurements of stratospheric aerosol properties at non-volcanic levels

“…This will be demonstrated later in this section. Many techniques have been applied to this calculation for SAGE II data, each of which filled this "blind spot" in a somewhat different manner (Wang et al, 1989;Thomason et al, 1997;Steele et al, 1997;Yue et al, 1999;Bingen et al, 2004). The operational SAD retrieval puts relatively little material at radii smaller than 50 nm and thus is likely to produce SAD values that are biased low to some degree (Thomason et al, 1997).…”

Toward a combined SAGE II-HALOE aerosol climatology: an evaluation of HALOE version 19 stratospheric aerosol extinction coefficient observations

“…HALOE makes use of N 2 O and CH 4 (from near the tropopause and below) from this dataset which are based on data from the Stratospheric and Mesospheric Sounder (SAMS) mission in 1979 and balloon-based profiles (Jackman et al, 1989;Jones and Pyle, 1984). Many climatologies of SAD have been derived from SAGE II and HALOE independently (Thomason et al, 1997;Steele and Turco, 1997;Wang et al, 1989;Yue, 1999;Hervig et al, 1998). The goal of this paper is to evaluate the HALOE aerosol extinction coefficient data as the first necessary step in a process that will eventually allow the production of a combined HALOE/SAGE II aerosol climatology of derived aerosol products including surface area density (SAD) and a mechanism to produce complete aerosol extinction/absorption spectra suitable for use in climate modeling.…”