Measurements of the ice nucleating ability of aerosol particles in air masses over Florida having sources from North Africa support the potential importance of dust aerosols for indirectly affecting cloud properties and climate. The concentrations of ice nuclei within dust layers at particle sizes below 1 pn exceeded 1/cu cm; the highest ever reported with our device at temperatures warmer than homogeneous freezing conditions. These measurements add to previous direct and indirect evidence of the ice nucleation efficiency of desert dust aerosols, but also confirm their contribution to ice nuclei populations at great distances from source regions.

/pdf/african-dust-aerosols-as-atmospheric-ice-nuclei-47omsvy2ji.pdf

African Dust Aerosols as Atmospheric Ice Nuclei

https://cyberleninka.org/article/n/1004566.pdf

The size distribution of desert dust aerosols and its impact on the Earth system

Toward the end of the Cirrus Regional Study of Tropical Anvils and Cirrus Layer–Florida Area Cirrus Experiment (CRYSTAL–FACE) field campaign held during July 2002, high concentrations of Saharan dust, which can serve as cloud condensation nuclei (CCN), giant CCN (GCCN), and ice-forming nuclei (IFN) were observed over the peninsula of Florida. To investigate the impacts of enhanced aerosol concentrations on the characteristics of convective storms and their subsequent anvil development, sensitivity tests are conducted using the Regional Atmospheric Modeling System (RAMS) model, in which the initialization profiles of CCN, GCCN, and IFN concentrations are varied. These variations are found to have significant effects on the storm dynamics and microphysical processes, as well as on the surface precipitation. Updrafts are consistently stronger as the aerosol concentrations are increased. The anvils cover a smaller area but are better organized and have larger condensate mixing ratio maxima in the cas...

/pdf/impacts-of-nucleating-aerosol-on-florida-storms-part-i-3rtqqstdnt.pdf

Impacts of Nucleating Aerosol on Florida Storms. Part I: Mesoscale Simulations

The design, laboratory calibrations, and flight tests of a new optical imaging instrument, the twodimensional stereo (2D-S) probe, are presented. Two orthogonal laser beams cross in the middle of the sample volume. Custom, high-speed, 128-photodiode linear arrays and electronics produce shadowgraph images with true 10-m pixel resolution at aircraft speeds up to 250 m s 1 . An overlap region is defined by the two laser beams, improving the sample volume boundaries and sizing of small (100 m) particles, compared to conventional optical array probes. The stereo views of particles in the overlap region can also improve determination of three-dimensional properties of some particles. Data collected by three research aircraft are examined and discussed. The 2D-S sees fine details of ice crystals and small water drops coexisting in mixed-phase cloud. Measurements in warm cumuli collected by the NCAR C-130 during the Rain in Cumulus over the Ocean (RICO) project provide a test bed to compare the 2D-S with 2D cloud (2D-C) and 260X probes. The 2D-S sees thousands of cloud drops 150 m when the 2D-C and 260X probes see few or none. The data suggest that particle images and size distributions ranging from 25 to 150 m and collected at airspeeds 100 m s 1 by the 2D-C and 260X probes are probably (erroneously) generated from out-of-focus particles. Development of the 2D-S is in its infancy, and much work needs to be done to quantify its performance and generate software to analyze data.

The 2D-S (Stereo) Probe: Design and Preliminary Tests of a New Airborne, High-Speed, High-Resolution Particle Imaging Probe

[1] Black carbon (BC) is the dominant aerosol absorber of solar radiation in the atmosphere and is an important component of anthropogenic climate forcing. BC's role is strongly dependent on its physical state, which can influence the way that BC particles may act as ice and cloud nuclei, as well as the way they interact with solar radiation. In situ measurements made with a single-particle soot photometer flown on a NASA high-altitude research aircraft show the mass and size of individual BC particles in the tropics, as well as their propensity to be found mixed with additional materials. Mie theory was used to connect observed light scattering off BC particles to the optical effects of coatings on the particles. The observations indicate that as BC from ground-based emission sources rises in altitude to the lower stratosphere, coatings on BC particles become both thicker and more prevalent, while BC mass mixing ratios decrease dramatically from their values near the ground. Coatings enhance light absorption by the ambient BC column by at least 30%. These results reveal the microphysical state of BC in the atmosphere while providing important constraints for models evaluating BC's role in climate change.

/pdf/coatings-and-their-enhancement-of-black-carbon-light-5c5zcrghhy.pdf

W Dawson

Papers

The cloud, aerosol and precipitation spectrometer: a new instrument for cloud investigations