Determination of marine aerosol composition, mass distribution, and optical properties
Semiempirical models of electrolyte solution water activity, density, and refractive index were combined with a Mie scattering program to determine aerosol scattering characteristics from inorganic ion analyses of cascade impactor samples.^ Five samples of marine aerosol chosen to represent diverse conditions showed that mass scattering efficiency for non-sea salt sulfate varies by more than a factor of two, from 5.2 to 12 m$\sp2$/g at 80% humidity, depending on the details of the chemical size distribution. Estimates used by recent direct forcing models appear to use a reasonable average value, but may experience errors as large as 30% over much of the ocean. Dry particle scattering and the effect of humidity on that scattering both vary significantly, but the variations tend to cancel, so measuring scattering from dry aerosol and applying a fixed f(RH) may overstate the variability in scattering due to NSS.^ To test our sampling procedures, we participated in a shipboard intercomparison between our Micro-Orifice Uniform Deposit Impactor (MOUDI), a Berner low-pressure impactor, and a Sierra high volume slotted impactor. To resolve inconsistencies in the results, I performed a second intercomparison between two MOUDIs and the Berner impactor.^ The Sierra collected sea salt particles far more efficiently than the others, which had severe inlet losses for 7 $\mu$m and larger particles. The MOUDI and Berner showed no significant differences in accumulation mode mass median diameters ($\sim$0.30 $\mu$m) while the Sierra indicated diameters almost twice as large ($\sim$0.58 $\mu$m).^ Impactor data must be inverted to determine size distributions. As the inversion problem is ill-posed, it is not generally possible to demonstrate that any given solution is actually the correct size distribution. Several methods to perform inversions were tested for accuracy, reliability, sensitivity to analytical errors, and physical plausibility of the results.^ Histogram inversions and lognormal fitting were not generally accurate representations of the ambient size distribution. An iterative nonlinear algorithm and a constrained regularization technique could produce inversions that fit the input data well, but were sometimes subject to implausible-appearing artifacts such as spurious peaks and valleys. ^
Physical Oceanography|Physics, Atmospheric Science|Environmental Sciences|Geochemistry
Steven G Howell,
"Determination of marine aerosol composition, mass distribution, and optical properties"
Dissertations and Master's Theses (Campus Access).