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Filter-collected aerosol samples in the PM 1 and PM 10 fractions 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 PM 1 fraction with respect to PM 1–10 (with the exception of the first campaign, at S w = 1.01), and at water saturation ratio S w = 1.01 with respect to S w = 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 S w = 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 PM 10 fraction at S w = 1.01 (26 May, 8 a.m. and 1 p.m.; 27 May, 1 p.m.). [ABSTRACT FROM AUTHOR]

For the first time, falling ice crystals were collected on glass slides covered with a thin layer of 2% formvar in chloroform at the Dome Concordia site (Dome C), Antarctica. Samplings were performed in the framework of the 27th Italian Antarctica expedition of the Italian National Program for Research in Antarctica in the period 21 February–6 August 2012. Events of clear-sky precipitations and precipitations from clouds were considered and the replicas obtained were examined under Scanning Electron Microscope (SEM). Several shapes of ice crystals were identified, including “diamond dust” (plates, pyramids, hollow and solid columns), and crystal aggregates varying in complexity. Single events often contained both small (10 μm to 50 μm) and large (hundreds of microns) crystals, suggesting that crystals can form simultaneously near the ground (height of a few hundred metres) and at higher layers (height of thousands of metres). Images of sampled crystal replicas showed that single bullets are not produced separately, but by the disintegration of combinations of bullets. Rimed ice crystals were absent in the Dome C samples, i.e. the only mode of crystal growth was water vapour diffusion. On considering the aerosol in the sampled crystals, we reached the conclusion that inertial impaction, interception and Brownian motion were insufficient to explain the scavenged aerosol. We therefore presume that phoretic forces play a role in scavenging during the crystal growth process. [ABSTRACT FROM AUTHOR]

The paper provides a review of past and a few new measurements of Ice-forming Nuclei (IN) in Antarctica. The few available published data were mostly obtained adopting different devices and methods and for a limited period of time. Consequently, data are scattered and give an incomplete picture of the Antarctic situation. It should be pointed out, however, that ice nucleation is an intricate process, depending on many parameters (supersaturation relative to ice and water, aerosol physical–chemical properties, possible conditioning and preactivation of particles, different modes of nucleation). Therefore, the uncertainty does not concern the Antarctic continent alone, but all measurements performed world-wide. A comparison of the published data can be made between Saxena and Weintraub (1988) at Palmer Station, and Ardon-Dryer et al. (2011) at the South Pole, as both studies measured IN in the immersion mode, even if at different temperature. Saxena and Weintraub (1988) obtained in three filters IN concentrations of about 104 m−3 at T=−6°C, −11°C and −13°C, and 103 m−3 at T=−17°C, in an additional filter (February–December 1983). At the South Pole Ardon-Dryer et al. (2011) obtained a concentration of about 5×102 m−3 at T=−19°C, and the IN concentration increased until about 40×103 m−3 at the activation temperature of −26°C. Such values are higher than those measured by Bigg (1973) near Antarctica, using a thermal diffusion chamber (deposition or deposition–condensation modes). IN concentrations measured at Terra Nova Bay are lower than those reported above, and are comparable to values reported for the Scott Base, Byrd Station and cruises at latitude 60°–70° S. [ABSTRACT FROM AUTHOR]

Abstract: During austral summer 2006, in the framework of the XXII Italian Antarctic expedition of PNRA (Italian National Program for Research in Antarctica), aerosol particle number size distribution measurements were performed in the 10–500 range nm over the Nansen Ice Sheet glacier (NIS, 74°30′ S, 163°27′ E; 85m a.s.l), a permanently iced branch of the Ross Sea. Observed total particle number concentrations varied between 169 and 1385cm−3. A monomodal number size distribution, peaking at about 70nm with no variation during the day, was observed for continental air mass, high wind speed and low relative humidity. Trimodal number size distributions were also observed, in agreement with measurements performed at Aboa station, which is located on the opposite side of the Antarctic continent to the NIS. In this case new particle formation, with subsequent particle growth up to about 30nm, was observed even if not associated with maritime air masses. [Copyright &y& Elsevier]

Abstract: The work presents a characterisation of aerosol sampled during three campaigns conducted in the Venice Lagoon from 30 June to 21 July 2004, from 15 February to 10 March 2005 and from 8 May to 25 May 2006. The results yield information about the physical–chemical characteristics of fine aerosol, the possible sources and its fate. Sulphate (SO4 2−), nitrate (NO3 −) and ammonium (NH4 +) are identified as the main water soluble components. The sum of these ions in the spring campaign 2006 varies from 51% to nearly 100% of PM2.5 fraction aerosol. NH4 + is found to be significantly correlated to non-sea-salt sulphate (nss-SO4 2−) and NO3 −, thus indicating the prevalent presence of ammonium nitrate and sulphate. The overall lack of a clear diurnal and seasonal cycle of sulphate suggests a transport from the Po Valley, while the diurnal and seasonal profile of nitrate concentrations suggests a prevalence of local generation. Sulphates from sea water through oxidation of dimethylsulfide (DMS) are not negligible (about 10% of the anthropogenic sulphate). The marine aerosol contribution to PM1 and PM2.5 fractions, calculated using Na+ as a tracer of sea-salts, is low (range 1–6%). In some cases chlorine depletion is observed. The content of chlorine in the aerosol particle is mostly higher than expected for marine environments (considered on the basis of Cl−/Na+ ratio). Therefore, specific sources for the element have been suggested. Oxalic acid anion, which accounts for 55% of the organic compounds examined, presents mean values 180 ng m−3 and 161 ng m−3 in the winter and spring campaigns, comparable with reported values in low polluted urban areas. In the winter campaign there is a high correlation between formate and acetate (R 2 =0.93), suggesting that a common source makes a high contribution to the measured concentrations. [Copyright &y& Elsevier]

Laboratory generated aerosols and aerosol sampled in a field campaign were studied to assess the repeatability of heterogeneous ice activation processes and verify if the ice nuclei particles maintained their nucleating capability. Experiments were performed using a filter technique, i.e. aerosol particles were sampled on black-gridded cellulose nitrate membrane filters and assessed for ice nucleating particles in a dynamic filter-processing chamber at our ISAC-CNR laboratory. The laboratory investigations used Arizona Test Dust (ATD) generated by nebulizing a suspension of ATD. After nebulization, the wet aerosol in the carrier gas was dried and then sampled on a filter. Aerosol field sampling (size range PM1 and PM10) was carried out in the Arctic (Ny-Ålesund, Svalbard) from 12 to 23 April 2019. The activation processes were repeated twice for each filter at temperatures of −22 °C and −18 °C. For aerosol, ATD runs were performed only in conditions of supersaturation (saturation ratio S w = 1.02), while for the Arctic aerosol, conditions below saturation were also considered (S w = 0.96). The experiments showed that approximately 40% of the ATD particles maintain their initial nucleating efficiency, both at T = −22 °C and − 18 °C, and at S w = 1.02, i.e. in the condensation freezing mode. The percentage of repeatability was higher for ATD with respect to the Artic-sampled aerosol processed in the condensation mode. This may depend on greater particle morphology variation of the mixed organic-inorganic aerosol due to the hydration and dehydration cycle. • Experiments showed that about 40% of ATD particles maintains the nucleating efficacy. • The repeatability was higher for ATD with respect to aerosol sampled in Arctic. • Variation of ice nucleating capability of the aerosol has atmospheric implication. • The ice nucleation shows both stochastic and deterministic behavior. [ABSTRACT FROM AUTHOR]

In this work, the ice nucleating effectiveness of Arizona Test Dust (ATD), bare and coated with NaCl, not examined in previous published papers, was investigated. The ATD, NaCl coated ATD, and NaCl aerosols were generated by nebulizing water suspensions of bare ATD, ATD-NaCl and NaCl solution. Dried aerosols were sampled on black-gridded cellulose nitrate membrane filters for ice nuclei assessment through a Dynamic Filter Processing Chamber device (DFPC). After water evaporation ATD-NaCl droplets gave rise to ATD particles coated with NaCl. Processing of filters was performed at −22 °C and − 18 °C, under sub- and super-saturated conditions (water saturation ratio S w < 1 and S w > 1, respectively). Additional experiments were performed by modifying the storage conditions of the filters to simulate variation of atmospheric conditions, i.e. air temperature and relative humidity. For this purpose, the filters were housed in Petri dishes, with silica gel, at +10 °C and −10 °C. As silica gel can desorb water from the aerosol capillary at low temperatures, storage conditions with and without silica gel can simulate the aerosol pre-activation atmospheric process. The highest activated fraction (AF) and ice-active surface site density (n s) values were obtained for ATD at −22 °C and S w = 1.02. Results showed a decrease in the AF and n s between bare and ATD coated with NaCl both at S w = 1.02 and S w = 0.96 and for filters stored in the presence of silica gel. The results obtained show the great influence exerted by NaCl coated insoluble aerosol on the ice nucleation. • Results show a decrease in ice nucleation efficiency in case of NaCl coated ATD. • These results extend toward higher temperatures previous laboratory studies. • Preactivation process was simulated by storing the filters in low RH conditions. • Activated fraction increases from sub to super-saturated water conditions. [ABSTRACT FROM AUTHOR]

Abstract: The work presents the results of an experimental campaign performed at the Drop Tower Facility (Bremen) in microgravity conditions, concerning the scavenging process of an evaporating single droplet in stationary conditions. In the experimental conditions the thermo- and diffusiophoretic forces are the only ones that can determine the scavenging of the aerosol. The research is finalized to help solve the open question concerning the contribution of thermo- and diffusiophoretic forces in aerosol scavenging process due to cloud droplets. Although earlier theoretical and experimental papers have addressed this problem, the results are contradictory and inconclusive. As phoretic forces depend on aerosol diameter and water vapour pressure gradient, experiments were performed by changing the aerosol diameter (range 0.4μm–2μm) and the water vapour gradient. The experimental results show a prevalence of the diffusiophoretic over thermophoretic force, for the considered aerosol. The measured values of the particle velocities due to phoretic forces increase with increasing aerosol diameter and vapour pressure gradient. [Copyright &y& Elsevier]

Abstract: The work presents the results of an experimental campaign performed at a rural site (S. Pietro Capofiume, near Bologna July 2007) concerning measurements of ice nuclei in different size classes of aerosol: PM1, PM2.5, PM10 and total suspended particles (TSP). Simultaneous measurements of particle number concentrations were also performed. Aerosol in the PM1 fraction contributes about 50% of the measured ice nuclei number concentration, and in the PM10 fraction contributes about 70–90%. Consequently, the dominant fraction of aerosol that can be activated as ice nuclei involves particles with aerodynamic diameter less than 10 μm. A positive correlation is observed between higher supersaturation with respect to ice and water (S ice and S w, respectively) values, and ice nuclei number concentration. The variations of S w from 2 to 10% and S ice from 20 to 32% (T air =−15 °C) determine an approximately threefold increase (from 110 to 337 m−3) in the ice nuclei average number concentration. There is no correlation between ice nuclei measured in the different size ranges, either with the particle number concentration measured with the counter spectrometer (d >0.3 μm) or with the condensation nuclei counter. [Copyright &y& Elsevier]

Abstract: Brownian diffusion of aerosol particles was studied in microgravity conditions using a digital holographic velocimeter. Based on digital image processing, the observed volume, recorded on a charge-coupled device (CCD) camera, is reconstructed slice by slice in order to achieve a full focused volume. Three dimensional coordinates of the particles are retrieved by such procedures and particle trajectories are reconstructed by analysing the sequence of the particle position. We deduced that the displacement of particles in microgravity, due to Brownian motion, follows a Gaussian distribution, like at 1 g. Particle sizes obtained from SEM measurements were in good agreement with those calculated from the three dimensional trajectories provided by the holographic microscope. [Copyright &y& Elsevier]