Ground level ice nuclei particle measurements including Saharan dust events at a Po Valley rural site (San Pietro Capofiume, Italy)

Filter-collected aerosol samples in the PM1and PM10fractions and particle number concentration were measured during experimental campaigns in a rural area near Bologna (Italy) in the periods 10–21 February 2014 and 19–30 May 2014. Ice nuclei particle (INP) concentrations measured off-line showed prevalently higher average values in the morning with respect to the afternoon, in the PM1fraction with respect to PM1–10(with the exception of the first campaign, at Sw= 1.01), and at water saturation ratio Sw= 1.01 with respect to Sw= 0.96. The aerosol in the coarse size range (1–10 μm) contributed significantly to the total INP concentration. In the first campaign, the average INP concentration in the coarse fraction was 80% of the total in the morning and 74% in the afternoon, at Sw= 1.01. In the second campaign, the contribution of the coarse size fraction to the INP number concentration was lower. On the whole, the results showed that the freezing activity of aerosol diameters larger than 1 μm needs to be measured to obtain the entire INP population. Sahara dust events (SDEs) took place during both campaigns, in the periods 17–20 February and 21–23 May 2014. Results show that the averaged particle number concentration was higher during SDE than during no-Saharan dust events. A low correlation between INP and total aerosol number concentration was generally measured, except for SDEs observed in February, in which the correlation coefficient between aerosol concentration in the coarse fraction and INP in the same range, at water supersaturation, was about 0.8. Precipitation events influenced the aerosol concentration. In the February campaign, lower values of INP and particle concentrations were measured in case of heavy rain events. During the May campaign, an average number concentration of the aerosol in the range 0.5–10 μm was slightly higher than on days when no precipitation was measured, the rainfall intensity being low. Only in a few cases did we note a sharp drop in INP in the PM10fraction at Sw= 1.01 (26 May, 8 a.m. and 1 p.m.; 27 May, 1 p.m.)