Extinction Retrieval

Light scattering by aerosols is a function of the number, size, and composition of the particles. However, depending on the number and range of the measurement wavelengths there is rarely enough information to quantify more than a small number of these parameters. If only one wavelength is used then it is typical to retrieve the extinction, or amount of light being scattered by aerosol particles. While incomplete, this quantity is directly related to radiative impact of aerosols, and less impacted by assumptions on other microphysical parameters. [Bourassa et al., 2012]

For the extinction retrieval aerosols are assumed to have a composition of 25% H₂O and 75% H₂SO ₄. This is typical of sulfate aerosols in the stratosphere, but can be inaccurate after ash-heavy volcanic eruptions or extreme forest fires where ash and organic aerosols can dominate before becoming coated with sulfuric acid. The particle size distribution is assumed to have the form a lognormal distribution of the form:

\[n(r) = \frac{N}{r\ln{\sigma_g}\sqrt{2\pi}} \exp(-\frac{\ln{r} - \ln{r_g}}{2\ln^2{\sigma_g}})\]

where \(N\) is the number density, \(r_g\) is the median radius, and \(\sigma_g\) is the lognormal width. The extinction retrieval assumes the size distribution parameters, \(r_g=0.08\mu m\) and \(\sigma_g=1.6\), by default which is typical of background conditions. The number density is then varied, and converted to extinction using the optical cross section.