The Fifth International Workshop on Ice Nucleation Phase 3 (FIN-03): Field Intercomparison of Ice Nucleation Measurements

Abstract. The third phase of the Fifth International Ice Nucleation Workshop (FIN-03) was conducted at Storm Peak Laboratory in Steamboat Springs, Colorado in September 2015 to facilitate the intercomparison of instruments measuring ice nucleating particles (INPs) in the field. Instruments included a subset of two online and four offline measurement systems for INPs, a subset of those utilized in the laboratory study that comprised the second phase of FIN (FIN-02). Composition of total aerosols were characterized by the Particle Ablation by Laser Mass Spectrometry (PALMS) and Wideband Integrated Bioaerosol Sensor (WIBS) instruments, and aerosol size distributions were measured by a Laser Aerosol Spectrometer (LAS). The dominant total particle compositions present during FIN-03 were composed of sulfates, organic compounds, and nitrates, as well as particles derived from biomass burning. Mineral dust containing particle types were ubiquitous throughout and represented 67 % of supermicron particles. Total WIBS fluorescing particle concentrations for particles with diameter > 0.5 µm were 0.04±0.02 cm^-3 (0.1 cm^-3 highest, 0.02 cm^-3 lowest), typical for the warm season in this region and representing ~9 % of all particles in this size range as a campaign average.

The primary focus of FIN-03 was the measurement of INP concentration via immersion freezing at temperatures > –33 °C. Additionally, some measurements were made in the deposition nucleation regime at these same temperatures, representing one of the first efforts to include both mechanisms within a field campaign. INP concentrations via immersion freezing reported by all ice nucleation instruments generally agreed to within one order of magnitude for measurement and sampling times coordinated to within three hours. Sometimes, much better agreement was obtained. Outliers of up to two orders of magnitude occurred between –25 °C and –18 °C; better agreement was seen at higher and lower temperatures. INP activity in the immersion freezing mode was generally found to be an order of magnitude or more efficient than in the deposition regime at 95–99 % water relative humidity, although this limited data set should be augmented in future efforts.

To contextualize the study results an assessment was made of the composition of INPs during the late Summer to early Fall period of this study, inferred through comparison to existing ice nucleation parameterizations and through measurement of the influence of thermal and organic carbon digestion treatments on immersion freezing ice nucleation activity. Consistent with other studies in continental regions, biological INPs dominated at temperatures > –20 °C and sometimes colder, while arable dust-like or other organic-influenced INPs were inferred to dominate at most times below –20 °C.