Advances in Understanding the Role of Aerosols in Cirrus Formation from the Fifth International Ice Nucleation (FIN) Workshop (Invited Presentation)

The relationship of ambient aerosol particles to the formation of ice-containing clouds is one of the largest uncertainties in understanding the Earth's climate. This is particularly true of completely glaciated, i.e., cirrus, clouds. The uncertainty is due to several poorly understood processes and measurements including, but not limited to, the microphysics of how particles nucleate ice, the number of effective heterogeneous ice forming particles (i.e., ice nuclei) as a function of atmospheric properties such as temperature and relative humidity, the atmospheric distribution of ice nuclei, the role of anthropogenic activities in producing or changing the behavior of ice forming particles, and the interplay between the effective heterogeneous ice nuclei and homogeneous ice formation. The ways in which ice forming particles can impact climate is also multi-faceted. More ice nuclei can suppress homogenous freezing and lead to clouds with fewer, but often larger, ice crystals and different optical properties. More effective ice nuclei can also lead to ice at higher temperature and/or lower saturation, resulting in clouds at lower altitude or latitude which also changes the Earth's radiative balance. Ice nucleation also initiates the majority of precipitation, even in the mid- and low-latitudes, since cloud-top temperatures are often below freezing. We have recently completed a three-part international workshop to improve our understanding of atmospheric ice formation. The combined activities, termed the Fifth International Ice Nucleation (FIN) Workshop, were motivated by the limited number of measurements and a lack of understanding of how to compare measurements made by different groups The first activity, termed FIN1, addressed the characterization of ice nucleating particle size, number and chemical composition by co-locating groups performing mass spectrometry in the laboratory and field. The second activity, FIN2, addressed the determination of ice nucleating particle number density by co-locating international groups making these measurements. Groups modeling ice nucleation joined FIN2 to provide insight on measurements critically needed to model atmospheric ice nucleation and to model the performance of the ice chambers intercompared in this activity. Both FIN1 and FIN2 took place at the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) chamber at the Karlsruhe Institute of Technology, the world's foremost facility for producing ice clouds in a controlled setting. In both FIN1 and FIN2 a particular emphasis was the use of ‘blind' intercomparison of instrument performance using a highly characterized, but unknown the instrument operator, aerosol sample. The third activity, FIN3, took place at the Desert Research Institute's Storm Peak Laboratory (SPL). A high elevation site not subject to local emissions, SPL allowed for a comparison of ice chambers and subsequent analysis of the ice residuals under the challenging conditions of low particle loading, temperature and pressure found in a remote atmosphere. Among the salient results is an understanding of how mass spectra from different instruments can be compared (with a particular focus placed on the role of coatings in ‘deactivating' this effective ice nucleus) and the complementary nature of different ice chamber to access the diverse regimes of temperature and relative humidity space (e.g. heterogeneous versus homogeneous freezing). As a result of the FIN Workshop we believe the performance of instruments in the field can now be quantified and compared.