Evaluation of different sampling methods to determine the ice-nucleating particle concentration in the atmosphere using the GRAnada Ice Nuclei Spectrometer (GRAINS)

Abstract. This work deals with the analysis of different filter sampling methods to obtain INP concentration spectra using the GRAnada Ice Nuclei Spectrometer (GRAINS), a droplet freezing assay based on the design of the Colorado State University Ice Spectrometer (CSU-IS) with droplet volumes of 100 μL. GRAINS was first validated with NX Illite, showing spectra consistent with literature, and also compared with FrESH (Freezing Experiment Setup Helsinki), INSEKT (Ice Nucleation Spectrometer of the Karlsruhe Institute of Technology), and PINE (Portable Ice Nucleation Experiment), with results generally within confidence intervals or a factor of 5. To assess the filter sampling methods, we simultaneously sampled ambient aerosol on polycarbonate filters (commonly used for INP analysis) and microfiber quartz filters (used for chemical analysis) over three months, with 27 filters of each type. Three analysis approaches were tested: washing the polycarbonate filters (Polycarbonate method), randomly punching the quartz filters (Quartz 96-punch method), and washing a larger punch of the quartz filter (Quartz punch washed method). Our results showed a good performance of the three methods, obtaining similar results for the INP concentrations, with approximately 89 % of the data within a factor of 5. Differences between methods become more evident at lower temperatures, with lower INP concentrations detected with the Polycarbonate method compared to the other two, which could be related to the particle extraction efficiency of this method. Differences between the three methods varied depending on the sample, so these differences could originate from the nature of the particles being analyzed. Still, there is a clear correlation between the three methods, with Spearman's coefficients of around 0.9 (p < 0.05). The Quartz punch washed method allows to perform sample dilutions similar to the Polycarbonate method, making it a potentially better alternative to the Quartz 96-punch method for analyzing INP concentrations using quartz filters.