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

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 array based on the design of the Colorado State University Ice Spectrometer (CSU-IS) but 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) for aerosol standards as well as ambient samples, 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). ... mehrOur 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 higher INP concentrations detected with both Quartz methods compared to the Polycarbonate method, which could be related to the particle extraction efficiency of this method. Differences between the three methods varied depending on the sample, with general good agreement for polluted and background conditions and different levels of agreement for dust particles. 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 potential alternative to the Quartz 96-punch method for analyzing INP concentrations using quartz filters.