Ground level ice nucleating particles measurements at Capo Granitola, a Mediterranean coastal site

This study presents near-surface measurements of ice nucleating particle (INP) concentrations at a ground level Mediterranean site (Capo Granitola, CGR), located in southern Sicily. A coastal area like CGR can be influenced by marine and anthropogenic aerosols, and even by Saharan dust. To obtain the INP concentration in the PM1 and PM10 fractions, an experimental campaign was carried out in April 2016 at the WMO/GAW "I-AMICA" Observatory. Aerosol was sampled on nitrocellulose membranes twice a day. INP concentrations were detected by a dynamic filter processing chamber (DFPC), a replica of the Langer dynamic chamber, at -18 °C and - 22 °C activation temperatures, at different conditions of supersaturation with respect to ice and water. The sampling period was characterized by three main wind directions, along the coast (from North-West and from South-East), during daytime, and mainly from the land side during the night. INP concentration, in the PM10 size fraction, ranged 0.5-27, 3.3-65 and 13-115 m-3, at -18 °C below water saturation, at -18 °C at water supersaturation and at -22 °C at water supersaturation, respectively. Two weak dust transport events occurred during the experimental campaign, both characterized by an increase in coarse particle number and PM10 concentration, not associated with fine particles or other pollutants. Air mass back-trajectories, during the events, were observed to originate from the North African region. Results of the experimental campaign were compared with INP concentrations previously measured at San Pietro Capofiume, a rural site in the Po Valley, and at Mt. Cimone ("O. Vittori" Italian Climate Observatory, 2165 m a.s.l) in the northern Apennines. Both INP concentrations, in the PM1 and PM10 fractions, and activated fractions, at CGR, were prevalently lower than the concentrations obtained at the other two sites. This may be, likely, related to the lower nucleation efficiency of soluble marine aerosols and aged dust particles, partly coated and mixed with hygroscopic material, at CGR.