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Aerosol‐cloud interactions are an important source of uncertainty in current climate models. To understand and quantify the influence of ice‐nucleating particles in cloud glaciation, it is crucial to have a reliable estimation of the hemispheric and seasonal contrast in cloud top phase, which is believed to result from the higher dust aerosol loading in boreal spring. For this reason, we locate and quantify these contrasts by combining three different A‐Train cloud‐phase products for the period 2007–2010. These products rely on a spaceborne lidar, a lidar‐radar synergy, and a radiometer‐polarimeter synergy. We show that the cloud‐phase from the product combination is more reliable and that the estimation of the hemispheric and seasonal contrast has a lower error compared to the individual products. To quantify the contrast in cloud‐phase, we use the hemispheric difference in ice cloud frequency normalized by the liquid cloud frequency in the southern hemisphere between −42 °C and 0 °C. In the midlatitudes, from −15 to −30 °C, the hemispheric contrasts increase with decreasing temperature. At −30 °C, the hemispheric contrast varies from 29% to 39% for the individual cloud‐phase products and from 52% to 73% for the product combination. Similarly, in the northern hemisphere, we assess the seasonal contrast between spring and fall normalized by the liquid cloud frequency during fall. At −30 °C, the seasonal contrast ranges from 21% to 39% for the individual cloud‐phase products and from 54% to 75% for the product combination., Plain Language Summary: The influence of atmospheric particles on clouds is one of the main unknowns in climate predictions. Particularly, the cloud glaciation process and its dependence on desert dust and soot particles are not well‐understood. To better understand the differences in cloud glaciation between hemispheres, we counted liquid and ice cloud tops, as observed from four different satellites, during 4 years. Combining these observations, we could confirm a higher frequency of ice cloud tops during spring in the northern hemisphere. We found that the contrast between hemispheres is higher than previously thought. These results will help to improve our understanding of cloud glaciation processes, which can be valuable for future climate predictions and for understanding the impact of aerosols on radiation and precipitation., Key Points: A satellite product ensemble was used to locate and quantify the hemispheric and seasonal contrast in cloud top thermodynamic phase. At −30 °C, half of the liquid cloud tops observed in the southern hemisphere would glaciate in the northern hemisphere. The new product ensemble is more reliable than the individual products and suggests a previous underestimation of the cloud‐phase contrasts.

Heterogeneous ice nucleation is one element inside the overall complexity of the Earth's atmosphere, however, it has a profound impact on our representation of cloud properties: this process affects the optical thickness and lifetime of mixed-phase clouds and cirrus clouds, and it is responsible for a significant proportion of precipitations formed globally. Heterogeneous ice nucleation is related to the presence of specific aerosol particles, named ice nuclei particles (INP), with the unique ability of lowering the energy barrier required for the formation of ice crystals, especially where cloud’s temperatures are >-38 °C. In the last decades, significant advancements have been made to the fundamental understanding of ice nucleation, however the lack of knowledge on the cloud ice phase still contributes to major uncertainties in climate model prediction of radiative forcing. This is partly due to limited observational data quantifying INP distributions and properties all over the world, especially in remote locations. In the first part of this thesis, field observations of ice nucleating particles have been performed at the Italian Climate Observatory “O. Vittori” on Mountain Cimone (2165 m above sea level), in the spring 2014 and autumn 2015, within the Bacchus and Air Sea Lab projects. For the first time we report the results of offline INP measurements, performed at a high altitude site within the Mediterranean basin. In the period 19-29 May 2014, a parallel campaign took place at the low-altitude station San Pietro Capofiume, a rural site in the Po Valley. The two campaigns were concerned, for a few days, by a Saharan Dust transport Event, which was recorded simultaneously at the high and the low-level station. We investigated the ambient number concentration of INP under condensation freezing activation mechanism (at -18 °C and above water saturation). In the second part of this thesis, we present the observations that were performed during the Arctic campaign Parcs-Maca, in the period of transition among the polar night and the polar day. We could characterise for the first time the ice nucleating and physical/chemical properties of the Arctic Primary Marine Aerosol, in a laboratory-controlled generation approach, that was combined to a mesocosm experiment. The aim of the mesocosm experiment was to adopt a multidisciplinary approach to study the effect of marine pollution on marine emissions. We found a moderate but significant decrease of the ice nuclei concentration in the polluted seawater (with respect to the control seawater) recorded in the freezing range between -8.5 and -19 °C and activated through immersion-freezing. Within the seaspray our measurements have indicated a relation among INP active at warm temperature (above -15 °C through immersion-freezing) and a calcium enrichment detected in PM1 filters (and followed by an apparent Chloride depletion). On the basis of our observations, and the results reported from other studies, a few suggestions on the nature of these marine ice nuclei have been suggested. In summary, the measurements made for this thesis provide new information on the concentrations of ice nuclei in ambient aerosol particles in remote regions (a high-altitude observatory in the central Mediterranean region) and in relation to a specific source (the Arctic sea spray).; La nucléation hétérogène de la glace est un processus, entre autres, qui concerne les phénomènes complexes de l'Atmosphère terrestre, cependant son impact est déterminant pour les propriétés et le temps de vie des nuages : ce processus affecte l'épaisseur optique et la durée de vie des nuages en phase mixte et froide et il est responsable d'une proportion significative des précipitations à l'échelle mondiale. La nucléation hétérogène de la glace est liée à la présence de particules d'aérosol spécifiques, nommées noyaux glaçogènes (acronyme INP depuis l’anglais), avec la propriété de réduire l’énergie de seuil exigée pour la formation des cristaux de glace, et dont le rôle est influent pour des températures de nuage >-38 °C. Dans les dernières décennies, des avancements significatifs ont été faits dans la description des noyaux glaçogènes, cependant leur compréhension reste incomplète et ceci influence les incertitudes sur les estimations de forçage radiatif dans les modèles climatiques. Il manque un grand nombre d’observations sur les distributions et propriété des 'INP' à l’échelle globale, spécialement en région éloignée. Dans la première partie de cette thèse, sont présentées des mesures de noyaux glaçogènes faites à l'Observatoire Climatique Italien "O. Vittori" sur la Montagne Cimone (2165 m au-dessus du niveau de la mer), pendant le printemps 2014 et l’automne 2015, sous les projets Bacchus et Air Sea Lab. Pour la première fois des mesures de INP hors ligne, reliés à un site de haute altitude du bassin méditerranéen sont présentées. Pendant la période du 19 au 29 mai 2014, une autre campagne a eu lieu en parallèle à la station San Pietro Capofiume, un site rural de basse altitude dans la Vallée du Po. Pendant quelques jours, les deux campagnes ont été concernées par un phénomène de Transport de Poussière Saharienne, qui a été enregistré simultanément dans la station de bas et de haute altitude. Nous avons examiné la concentration atmosphérique de INP activés à travers une congélation par condensation de vapeur (à -18°C et au-dessus de la pression de vapeur saturante). Dans la deuxième partie de cette thèse, sont présentées les observations qui ont été menées pendant la campagne arctique Parcs-Maca, dans la période de transition entre la nuit polaire et le jour polaire. Pour la première fois on reporte la caractérisation des propriétés glaçogènes et physiques/chimiques de l’aérosol marin primaire Arctique, dans une approche de génération contrôlée en laboratoire, qui a été combinée à une expérience de mesocosm. Le but de l'expérience de mesocosm était d'adopter une approche pluridisciplinaire afin d’étudier l'effet de la pollution marine sur les émissions marines. Nous avons trouvé une diminution modérée mais significative de la concentration de noyaux glaçogènes dans l'eau de mer polluée (par rapport à l'eau de mer du contrôle) pour des noyaux glaçogènes actives en congélation par immersion entre -8.5 et -19 °C. En ce qui concerne le spray marin, nos mesures indiquent une relation parmi les INP actifs à des températures chaudes (en congélation par immersion et au-dessus de -15 °C) et un enrichissement du Calcium détecté dans les filtres PM1 (suivi par un apparent épuisement du Chlorure). Sur la base de nos observations et des résultats publiés en littérature, quelque indication a été suggéré sur la nature de ces noyaux glaçogènes marins. En conclusion, les mesures effectuées dans cette thèse fournissent des nouvelles informations sur les concentrations de noyaux glaçogènes pour des particules d’aérosols en régions éloignées (un observatoire à haute altitude dans la région méditerranéenne centrale) et par rapport à une source spécifique (le spray marin Arctique).

Heterogeneous ice nucleation is one element inside the overall complexity of the Earth's atmosphere, however, it has a profound impact on our representation of cloud properties: this process affects the optical thickness and lifetime of mixed-phase clouds and cirrus clouds, and it is responsible for a significant proportion of precipitations formed globally. Heterogeneous ice nucleation is related to the presence of specific aerosol particles, named ice nuclei particles (INP), with the unique ability of lowering the energy barrier required for the formation of ice crystals, especially where cloud’s temperatures are >-38 °C. In the last decades, significant advancements have been made to the fundamental understanding of ice nucleation, however the lack of knowledge on the cloud ice phase still contributes to major uncertainties in climate model prediction of radiative forcing. This is partly due to limited observational data quantifying INP distributions and properties all over the world, especially in remote locations. In the first part of this thesis, field observations of ice nucleating particles have been performed at the Italian Climate Observatory “O. Vittori” on Mountain Cimone (2165 m above sea level), in the spring 2014 and autumn 2015, within the Bacchus and Air Sea Lab projects. For the first time we report the results of offline INP measurements, performed at a high altitude site within the Mediterranean basin. In the period 19-29 May 2014, a parallel campaign took place at the low-altitude station San Pietro Capofiume, a rural site in the Po Valley. The two campaigns were concerned, for a few days, by a Saharan Dust transport Event, which was recorded simultaneously at the high and the low-level station. We investigated the ambient number concentration of INP under condensation freezing activation mechanism (at -18 °C and above water saturation). In the second part of this thesis, we present the observations that were performed during the Arctic campaign Parcs-Maca, in the period of transition among the polar night and the polar day. We could characterise for the first time the ice nucleating and physical/chemical properties of the Arctic Primary Marine Aerosol, in a laboratory-controlled generation approach, that was combined to a mesocosm experiment. The aim of the mesocosm experiment was to adopt a multidisciplinary approach to study the effect of marine pollution on marine emissions. We found a moderate but significant decrease of the ice nuclei concentration in the polluted seawater (with respect to the control seawater) recorded in the freezing range between -8.5 and -19 °C and activated through immersion-freezing. Within the seaspray our measurements have indicated a relation among INP active at warm temperature (above -15 °C through immersion-freezing) and a calcium enrichment detected in PM1 filters (and followed by an apparent Chloride depletion). On the basis of our observations, and the results reported from other studies, a few suggestions on the nature of these marine ice nuclei have been suggested. In summary, the measurements made for this thesis provide new information on the concentrations of ice nuclei in ambient aerosol particles in remote regions (a high-altitude observatory in the central Mediterranean region) and in relation to a specific source (the Arctic sea spray).; La nucléation hétérogène de la glace est un processus, entre autres, qui concerne les phénomènes complexes de l'Atmosphère terrestre, cependant son impact est déterminant pour les propriétés et le temps de vie des nuages : ce processus affecte l'épaisseur optique et la durée de vie des nuages en phase mixte et froide et il est responsable d'une proportion significative des précipitations à l'échelle mondiale. La nucléation hétérogène de la glace est liée à la présence de particules d'aérosol spécifiques, nommées noyaux glaçogènes (acronyme INP depuis l’anglais), avec la propriété de réduire l’énergie de seuil exigée pour la formation des cristaux de glace, et dont le rôle est influent pour des températures de nuage >-38 °C. Dans les dernières décennies, des avancements significatifs ont été faits dans la description des noyaux glaçogènes, cependant leur compréhension reste incomplète et ceci influence les incertitudes sur les estimations de forçage radiatif dans les modèles climatiques. Il manque un grand nombre d’observations sur les distributions et propriété des 'INP' à l’échelle globale, spécialement en région éloignée. Dans la première partie de cette thèse, sont présentées des mesures de noyaux glaçogènes faites à l'Observatoire Climatique Italien "O. Vittori" sur la Montagne Cimone (2165 m au-dessus du niveau de la mer), pendant le printemps 2014 et l’automne 2015, sous les projets Bacchus et Air Sea Lab. Pour la première fois des mesures de INP hors ligne, reliés à un site de haute altitude du bassin méditerranéen sont présentées. Pendant la période du 19 au 29 mai 2014, une autre campagne a eu lieu en parallèle à la station San Pietro Capofiume, un site rural de basse altitude dans la Vallée du Po. Pendant quelques jours, les deux campagnes ont été concernées par un phénomène de Transport de Poussière Saharienne, qui a été enregistré simultanément dans la station de bas et de haute altitude. Nous avons examiné la concentration atmosphérique de INP activés à travers une congélation par condensation de vapeur (à -18°C et au-dessus de la pression de vapeur saturante). Dans la deuxième partie de cette thèse, sont présentées les observations qui ont été menées pendant la campagne arctique Parcs-Maca, dans la période de transition entre la nuit polaire et le jour polaire. Pour la première fois on reporte la caractérisation des propriétés glaçogènes et physiques/chimiques de l’aérosol marin primaire Arctique, dans une approche de génération contrôlée en laboratoire, qui a été combinée à une expérience de mesocosm. Le but de l'expérience de mesocosm était d'adopter une approche pluridisciplinaire afin d’étudier l'effet de la pollution marine sur les émissions marines. Nous avons trouvé une diminution modérée mais significative de la concentration de noyaux glaçogènes dans l'eau de mer polluée (par rapport à l'eau de mer du contrôle) pour des noyaux glaçogènes actives en congélation par immersion entre -8.5 et -19 °C. En ce qui concerne le spray marin, nos mesures indiquent une relation parmi les INP actifs à des températures chaudes (en congélation par immersion et au-dessus de -15 °C) et un enrichissement du Calcium détecté dans les filtres PM1 (suivi par un apparent épuisement du Chlorure). Sur la base de nos observations et des résultats publiés en littérature, quelque indication a été suggéré sur la nature de ces noyaux glaçogènes marins. En conclusion, les mesures effectuées dans cette thèse fournissent des nouvelles informations sur les concentrations de noyaux glaçogènes pour des particules d’aérosols en régions éloignées (un observatoire à haute altitude dans la région méditerranéenne centrale) et par rapport à une source spécifique (le spray marin Arctique).

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 (RHamb) on the INP concentration was analyzed. The number of INPs activated varied from 1L-1 at RHamb of 25% to 30L-1 at RHamb of 90%. In general, a linear trend between the INP concentration and the RHamb 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 RHamb 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. Fil: Lopez, Maria Laura. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina Fil: Avila, Eldo Edgardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina

[1] Cirrus clouds can form by homogeneous and heterogeneous ice nucleation mechanisms at temperatures below 235 K. Here we evaluate the effectiveness of heterogeneous freezing versus homogeneous freezing using a newly developed parameterization of heterogeneous freezing that is restricted to immersion freezing as the most likely pathway for heterogeneous ice formation in cirrus conditions [Karcher and Lohmann, 2003]. In addition to a reference simulation considering homogeneous nucleation with temperature-dependent freezing thresholds, we discuss two idealized model experiments. We conduct a scenario that hypothetically assumes that the aerosol particles available for homogeneous freezing could act as freezing nuclei commencing freezing at 130% with respect to ice and contrast that by a scenario that only considers black carbon and mineral dust as immersion nuclei with the same freezing relative humidity of 130%. These idealized simulations serve to delimit possible climate responses. If the number of freezing nuclei is limited by the number of black carbon and dust aerosols, then heterogeneous freezing results in fewer ice crystals than formed by homogeneous freezing. These fewer ice crystals grow more readily to precipitation size and with that increase the global mean precipitation, decrease the ice water path, and trap less outgoing longwave radiation at the top of the atmosphere.

Factors controlling the microphysical link between distributions of relative humidity above ice saturation in the upper troposphere and lowermost stratosphere and cirrus clouds are examined with the help of microphysical trajectory simulations. Our findings are related to results from aircraft measurements and global model studies. We suggest that the relative humidities at which ice crystals form in the atmosphere can be inferred from in situ measurements of water vapor and temperature close to, but outside of, cirrus clouds. The comparison with concomitant measurements performed inside cirrus clouds provides a clue to freezing mechanisms active in cirrus. The analysis of field data taken at northern and southern midlatitudes in fall 2000 reveals distinct differences in cirrus cloud freezing thresholds. Homogeneous freezing is found to be the most likely mechanism by which cirrus form at southern hemisphere midlatitudes. The results provide evidence for the existence of heterogeneous freezing in cirrus in parts of the polluted northern hemisphere, but do not suggest that cirrus clouds in this region form exclusively on heterogeneous ice nuclei, thereby emphasizing the crucial importance of homogeneous freezing. The key features of distributions of upper tropospheric relative humidity simulated by a global climate model are shown to be in general agreement with both, microphysical simulations and field observations, delineating a feasible method to include and validate ice supersaturation in other large-scale atmospheric models, in particular chemistry-transport and weather forecast models.

[1] A physically based parameterization of cirrus cloud formation by heterogeneous freezing is developed along with a novel method to compute associated nucleation rates. The analysis is restricted to immersion freezing, possibly the dominant pathway for heterogeneous cirrus formation under cold ( 1.3–1.4 triggers cirrus formation, cloud properties are not very susceptible to changes of IN properties, as in the case of homogeneous freezing. In contrast, much stronger indirect aerosol effects on cirrus clouds are possible if at least two types of IN with distinct freezing thresholds compete during the freezing process, most likely leading to a suppression of ice crystal concentrations.

This study presents the sensitivity of global ice crystal number concentration, Nc, to the parameterization of heterogeneous ice nuclei (IN). Simulations are carried out with the NASA Global Modeling Initiative chemical and transport model coupled to an analytical ice microphysics parameterization. Heterogeneous freezing is described using nucleation spectra derived from theoretical considerations and empirical data for dust, black carbon, ammonium sulfate, and glassy aerosol as IN precursors. When competition between homogeneous and heterogeneous freezing is considered, global mean Nc vary by up to a factor of twenty depending on the heterogeneous freezing spectrum used. IN effects on Nc strongly depend on dust and black carbon concentrations and are strongest under conditions of weak updraft and high temperature. Regardless of the heterogeneous spectrum used, dust is an important contributor of IN over large regions of the Northern Hemisphere. Black carbon however exhibits appreciable effects on Nc when the freezing fraction is greater than 1%. Compared to in situ observations, Nc is overpredicted at temperatures below 205 K, even if a fraction of liquid aerosol is allowed to act as glassy IN. Assuming that cirrus formation is forced by weak updraft addressed this overprediction but promoted heterogeneous freezing effects to the point where homogeneous freezing is inhibited for IN concentrations as low as 1 L-1. Chemistry and dynamics must be considered to explain cirrus characteristics at low temperature. Only cloud formation scenarios where competition between homogeneous and heterogeneous freezing is the dominant feature would result in maximum supersaturation levels consistent with observations. Copyright 2010 by the American Geophysical Union.