Intercomparison of Ice Nucleation Measurements at a Mountaintop Field Observatory (Invited Presentation)

The ability to accurately measure ice nucleating particles (INP), and to parameterize INP concentrations arising from the immersion freezing and deposition freezing mechanisms are major hurdles in understanding and quantifying the role of ice nucleation in cold cloud microphysics and climate. Fortunately, a surge in development of new instruments and methods which will increase the availability and reliability of INP field measurements is underway.

In September of 2015, the field phase of the Fifth International Ice Nucleation Workshop (FIN-03) was conducted at the Storm Peak Laboratory in Steamboat Springs, CO. While several successful INP comparisons have been conducted in the last 30 years, this is the first large scale INP comparison conducted under field conditions. Online and offline INP measurements were conducted by research groups from five participating Universities. INP were sampled under a variety of meteorological conditions including during precipitation events, during a post storm aerosol recovery period and in dry and stable conditions. In addition to changes in meterological conditions, changing aerosol composition were encountered throughout the intercomparison. The agreement between INP concentrations reported by instruments operated in immersion freezing mode was satisfactory, with a spread of ~1 order of magnitude in INP concentration observed during the ~2 week sampling period. This agreement is similar to the FIN-02 INP intercomparison conducted on more controlled aerosol sources within an aerosol chamber. During FIN-03, a subset of the instruments was also operated in deposition mode, and good agreement between online and offline deposition measurements were observed. Concentrations of immersion INP were 1 to 2 orders of magnitude larger than those of deposition INP. According to aerosol composition data determined by the Particle Analysis By Laser Mass Spectrometry (PALMS), a wide variety of ambient aerosol types were present and available to act as INP during the intercomparison, including dust, biomass burning particles, and biological aerosols.