Airborne CCN Measurements

UoM administered thesis: Phd

  • Authors:
  • James Trembath

Abstract

This work tests the validity of using a commercial cloud condensation nuclei (CCN) counter (CCNc) on the Facility for Airborne Atmospheric Measurements (FAAM) research aircraft. The CCNc was suitable for aircraft work with sta- ble and repeatable supersaturation, temperature and pressure relationships. The sample architecture of the aircraft fitted CCNc was found to transmit particles with acceptable losses in the diameter range of interest as was a pressure control device designed for airborne work. Rosemount inlets, used to sample aerosol, were found to be sensitive to particle density resulting in disparate aerosol being sam- pled with different efficiencies. In dust dominated aerosol inlet efficiency peaks at 10.24 at an optical diameter of 2.91 micro metre, with a minimum inlet efficiency between 1.78 and 1.51 at 0.28micro metre. In less dense aerosol inlets sample representatively below 0.6 micro metre and comparably below 1.0 micro metre.The thorough testing of the CCNc, associated sampling architecture and mea- surement strategies, enabled vertical and horizontal CCN to be investigated along with other aerosol and cloud microphysical properties in the Southern Equato- rial Pacific (SEP). The primary source of particulates was the South American continent, with sulphate dominating composition. There were strong gradients in aerosol and gas phase chemistry concentration with distance from the coast and in the cloud microphysics measurements where highest droplet numbers and smallest diameters were close to the coast. These data represent an important validatory and parameterisation data set for models of all scales. CCN data were used to calculate the aerosol hygroscopicity parameter, the mean project value, kappa, was 0.21 ± 0.18 . There was no evident variation in hygroscopicity with distance from the Chilean coastline suggesting a single dominant source and a well mixed boundary layer up to 907km to the west. CCN measurements were also com- pared to predictions from multiple models of different composition and mixing state assumptions. The best CCN closure used an external mixture of inorganic and organic aerosol components, with a modelled to observed ratio of 1.37 ± 0.32. It was hypothesised that this large ratio and the relatively low bulk hy- groscopicity was influenced by an external mixture. Incorporating this external mixture is imperative if CCN are to be accurately modelled and any subsequent cloud processes accurately captured.

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Original languageEnglish
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Award date31 Dec 2013