Small Field Campaign: Aerosol-Ice Formation Closure Pilot Study

The formation of ice crystals in the atmosphere by aerosol particles inducing nucleation of ice from water or the vapor phase represents one of the least understood atmospheric processes. Our insufficient understanding of the physical and chemical processes underlying ice nucleation hampers our predictive capability of cloud formation processes and climate. This knowledge is crucial since atmospheric ice formation impacts the distribution of water vapor which is the strongest greenhouse gas, the hydrological cycle including precipitation, and the radiative properties of clouds thereby modulating Earth’s energy budget. To improve our fundamental understanding of the processes leading to ice formation, in a multi-institutional field measurement and modeling effort, we conduct a first-of-its-kind aerosol-ice formation closure study. The basic idea is to use detailed information on authentic aerosol particles to predict the measured number of ice nucleating particles (INPs) in sampled air mass. Predictability of INP types and number concentrations will allow cloud and climate models to forecast ice crystal number concentrations.A key difference from previous studies is to base the study on the total ambient aerosol population rather than on a limited portion of it.This pilot study will be conducted at the Atmospheric Radiation Measurement (ARM) user facility at Southern Great Plains (SGP) to test a field observational approach for an aerosol-ice formation closure study. In this pilot study we focus on one pathway how ice forms in the atmosphere, termed immersion freezing, where the INP is engulfed by liquid water at temperatures below the ice melting point. Immersion freezing is considered a major formation pathway of ice in the atmosphere, in particular, for mixed-phase clouds where ice particles and water droplets co-exist. Mixed-phase clouds can have a significant impact on the radiative budget of our planet and the hydrological cycle. The closure concept is straightforward to test any physical model: measure all model inputs as well as predicted outputs, and then evaluate whether the model can predict the measured outputs when accounting for input and output measurement uncertainties.Here, we aim to simultaneously characterize ambient immersion-mode INPs and leading aspects of the aerosol population relevant to ice crystal formation via immersion freezing. This includes the size distribution and composition of the ambient aerosol population and INPs. The aerosol data will serve as input for prediction of INP number concentrations using various state-of-the-art ice nucleation parameterizations whose agreement with the INP measurements can then be evaluated within propagated uncertainties. These parameterizations will account in varying degrees for the size and composition of the particles for prediction of INP number concentrations. The objective is to provide an end-to-end test of the aerosol-ice formation closure measurement approach and its suitability to better constrain INP prediction by current climate models. We expect that the results will also provide new insights into the following fundamental questions regarding prediction of INP numbers in the atmosphere: What are the crucial aerosol property measurements to accurately guide ice nucleation representations in models? What level of parameter details need to be known to achieve aerosol-ice formation closure within current measurement uncertainties? What are the leading causes for climate model bias in INP predictions? What ancillary aerosol property observations are most useful to accompany long-term INP measurements for the combined purpose of constraining INP parameterizations and climate model skill?