Influence of the ambient humidity on the concentration of natural deposition-mode ice-nucleating particles

Abstract. This study reports measurements of deposition-mode ice-nucleating particle (INP) concentrations at ground level during the period July–December 2014 in Córdoba, Argentina. Ambient air was sampled into a cloud chamber where the INP concentration was measured at a temperature of −25 °C and a 15 % supersaturation over ice. Measurements were performed on days with different thermodynamic conditions, including rainy days. The effect of the relative humidity at ground level (RH_amb) on the INP concentration was analyzed. The number of INPs activated varied from 1 L⁻¹ at RH_amb of 25 % to 30 L⁻¹ at RH_amb of 90 %. In general, a linear trend between the INP concentration and the RH_amb was found, suggesting that this variability must be related to the effectiveness of the aerosols acting as INPs. From the backward trajectories analysis, it was found that the link between INP concentration and RH_amb is independent of the origin of the air masses. The role of biological INPs and nucleation occurring in pores and cavities was discussed as a possible mechanism to explain the increase of the INP concentration during high ambient relative humidity events. This work provides valuable measurements of deposition-mode INP concentrations from the Southern Hemisphere where INP data are sparse so far.