Ice nuclei variability, relation to ambient aerosol properties, and impacts on mixed-phase clouds

It is expected that ice nuclei (IN) play a critical role in determining the conditions required for the onset of ice crystal formation and initial concentrations of ice crystals in supercooled clouds. Although some progress has been made in identifying potentially dominant sources of ice nuclei, the spatial and temporal distributions of ice nuclei concentrations are not well understood. Such knowledge is critical for understanding and quantifying present-day ice nuclei impacts on cloud properties, precipitation formation, and cloud lifetimes, and in making informed predictions about the effects of changing atmospheric aerosol particle concentrations and compositions on cold cloud processes.

This paper seeks to use past field measurements of IN and other aerosol concentrations toward a more comprehensive understanding of IN in the troposphere. We first document retrospective analyses of measurements of ice nuclei made over more than a decade with a continuous flow ice-thermal diffusion chamber (CFDC). These data were collected at different locales and during different seasons, and confirm the strong roles of temperature and relative humidity in controlling ice nuclei concentrations. However, these analyses further suggest regional and seasonal variability in ice nuclei concentrations, likely tied to meteorological control on transport of ice nuclei from sources and such things as removal of IN through precipitation scavenging. In all studies, mineral components were commonly detected in the particles serving as IN. Focused studies during different seasons at one locale and chemical analyses of collected ice nuclei confirm the major role of mineral dust particles. Correlation of IN measurements to available aerosol size distribution data indicates a relationship between concentrations of particles having diameters larger than about 0.3 microns and ice nuclei concentrations, also largely consistent with a major IN source from mineral and soil dust particles.

In a few recent cases of CFDC sampling, a counterflow virtual impactor was used for selectively sampling the evaporated residual nuclei of cloud particles during aircraft measurements of ice clouds. A clear correlation between the presence or absence of ice nuclei and presence and number concentrations of ice crystals is demonstrated in some cases, while in other cases the role of secondary ice formation processes is apparent. These observations also confirm a correlation between number concentrations of larger aerosol particles and IN within nucleated ice crystals, corroborating the relationships evident from processing ambient aerosol particles.

The summarized data thus far suggest several hypotheses with regard to dominant IN sources and their variability that can and should be tested with additional field observations. The results of this research will ultimately be applied to the improvement of current numerical modeling parameterizations of ice nucleation, by suggesting appropriate relationships of [IN] to specific aerosol types, size distributions and mass concentrations.