Optimal estimation retrieval of aerosol microphysical properties from SAGE~II satellite observations in the volcanically unperturbed lower stratosphere D. Wurl1, R. G. Grainger2, A. J. McDonald1, and T. Deshler3 1Department of Physics and Astronomy, University of Canterbury, Christchurch, New Zealand 2Department of Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford, UK 3Department of Atmospheric Science, College of Engineering, University of Wyoming, Laramie, USA
Abstract. Stratospheric aerosol particles under non-volcanic conditions are typically
smaller than 0.1 μm. Due to fundamental limitations of the scattering
theory in the Rayleigh limit, these tiny particles are hard to measure by
satellite instruments. As a consequence, current estimates of global aerosol
properties retrieved from spectral aerosol extinction measurements tend to be strongly biased.
Aerosol surface area densities, for instance, are observed to be about 40%
smaller than those derived from correlative in situ measurements (Deshler et al., 2003).
An accurate knowledge of the global distribution of aerosol properties is,
however, essential to better understand and quantify the role they play in
atmospheric chemistry, dynamics, radiation and climate.
To address this need a new retrieval algorithm was developed, which
employs a nonlinear Optimal Estimation (OE) method to iteratively solve
for the monomodal size distribution parameters which are statistically
most consistent with both the satellite-measured multi-wavelength aerosol
extinction data and a priori information. By thus combining spectral
extinction measurements (at visible to near infrared wavelengths) with
prior knowledge of aerosol properties at background level, even the
smallest particles are taken into account which are practically invisible
to optical remote sensing instruments.
The performance of the OE retrieval algorithm was assessed based on
synthetic spectral extinction data generated from both monomodal and
small-mode-dominant bimodal sulphuric acid aerosol size distributions.
For monomodal background aerosol, the new algorithm was shown to fairly
accurately retrieve the particle sizes and associated integrated properties
(surface area and volume densities), even in the presence of large extinction
uncertainty. The associated retrieved uncertainties are a good estimate of the true errors.
In the case of bimodal background aerosol, where the retrieved (monomodal)
size distributions naturally differ from the correct bimodal values, the
associated surface area (A) and volume densities (V) are, nevertheless,
fairly accurately retrieved, except at values larger than
1.0 μm2 cm−3 (A) and 0.05 μm3 cm−3
(V), where they tend to underestimate the true bimodal values. Due to
the limited information content in the SAGE II spectral extinction
measurements this kind of forward model error cannot be avoided here.
Nevertheless, the retrieved uncertainties are a good estimate of the
true errors in the retrieved integrated properties, except where the
surface area density exceeds the 1.0 μm2 cm−3 threshold.
When applied to near-global SAGE II satellite extinction measured in
1999 the retrieved OE surface area and volume densities
are observed to be larger by, respectively, 20–50% and 10–40%
compared to those estimates obtained by the SAGE~II operational retrieval
algorithm. An examination of the OE algorithm biases with in situ data
indicates that the new OE aerosol property estimates tend to be more
realistic than previous estimates obtained from remotely sensed data
through other retrieval techniques.
Based on the results of this study we therefore suggest that the new Optimal Estimation retrieval algorithm
is able to contribute to an advancement in aerosol research by considerably
improving current estimates of aerosol properties in the lower stratosphere under low aerosol loading conditions.
Citation: Wurl, D., Grainger, R. G., McDonald, A. J., and Deshler, T.: Optimal estimation retrieval of aerosol microphysical properties from SAGE~II satellite observations in the volcanically unperturbed lower stratosphere, Atmos. Chem. Phys., 10, 4295-4317, doi:10.5194/acp-10-4295-2010, 2010.