A two-component parameterization of marine ice-nucleating particles based on seawater biology and sea spray aerosol measurements in the Mediterranean Sea

Ice-nucleating particles (INPs) have a large impact on the climate-relevant properties of clouds over the oceans. Studies have shown that sea spray aerosols (SSAs), produced upon bursting of bubbles at the ocean surface, can be an important source of marine INPs, particularly during periods of enhanced biological productivity. Recent mesocosm experiments using natural seawater spiked with nutrients have revealed that marine INPs are derived from two separate classes of organic matter in SSAs. Despite this finding, existing parameterizations for marine INP abundance are based solely on single variables such as SSA organic carbon (OC) or SSA surface area, which may mask specific trends in the separate classes of INP. The goal of this paper is to improve the understanding of the connection between ocean biology and marine INP abundance by reporting results from a field study and proposing a new parameterization of marine INPs that accounts for the two associated classes of organic matter. The PEACETIME cruise took place from 10 May to 10 June 2017 in the Mediterranean Sea. Throughout the cruise, INP concentrations in the surface microlayer (INPSML) and in SSAs (INPSSA) produced using a plunging aquarium apparatus were continuously monitored while surface seawater (SSW) and SML biological properties were measured in parallel. The organic content of artificially generated SSAs was also evaluated. INPSML concentrations were found to be lower than those reported in the literature, presumably due to the oligotrophic nature of the Mediterranean Sea. A dust wet deposition event that occurred during the cruise increased the INP concentrations measured in the SML by an order of magnitude, in line with increases in iron in the SML and bacterial abundances. Increases in INPSSA were not observed until after a delay of 3 days compared to increases in the SML and are likely a result of a strong influence of bulk SSW INPs for the temperatures investigated (T=−18 ∘C for SSAs, T=−15 ∘C for SSW). Results confirmed that INPSSA are divided into two classes depending on their associated organic matter. Here we find that warm (T≥−22 ∘C) INPSSA concentrations are correlated with water-soluble organic matter (WSOC) in the SSAs, but also with SSW parameters (particulate organic carbon, POCSSW and INPSSW,−16C) while cold INPSSA (T<−22 ∘C) are correlated with SSA water-insoluble organic carbon (WIOC) and SML dissolved organic carbon (DOC) concentrations. A relationship was also found between cold INPSSA and SSW nano- and microphytoplankton cell abundances, indicating that these species might be a source of water-insoluble organic matter with surfactant properties and specific IN activities. Guided by these results, we formulated and tested multiple parameterizations for the abundance of INPs in marine SSAs, including a single-component model based on POCSSW and a two-component model based on SSA WIOC and OC. We also altered a previous model based on OCSSA content to account for oligotrophy of the Mediterranean Sea. We then compared this formulation with the previous models. This new parameterization should improve attempts to incorporate marine INP emissions into numerical models

Aerosol Scavenging during the Early Growth Stage of Ice Crystal Formation

n/

Discrepancy between Ice Particles and Ice Nuclei in Mixed Clouds: Critical Aspects

Measurements of ice crystal concentrations in mixed clouds tend to exceed ice nucleus concentrations measured in nearby clear air. This discrepancy is a source of uncertainty in climate change projections as the radiative properties of mixed phase clouds are largely determined by their liquid and ice water content. The ice enhancement process can sometimes depend on secondary ice production, which can occur through ice crystal fracture during sublimation, cloud drop shattering during freezing or following collision with ice particles. However, the discrepancy is observed even in mixed clouds where only primary ice nucleation processes occur. Several hypotheses have been suggested for the observed discrepancies. One factor could be the existence in clouds of pockets of high vapor supersaturation formed by droplet freezing or removal of small droplets by collision with larger droplets, associated with the fact that ice crystal concentration increases with water supersaturation. However, ice crystal concentrations are usually measured at near water saturation. Additional factors could be drop freezing during evaporation and activation of droplet evaporation residues. Here we suggest that a major factor could be underestimation of the contact freezing mode as it is not measured in experimental campaigns and seldom considered in nucleation models. Laboratory experiments give only incomplete answers to the important questions concerning the contact freezing mode, e.g. what fraction of the aerosol particles that come into contact with the droplet surface results in a freezing event and what is the influence of particle type and size, air temperature and relative humidity. As supercooled droplets grow or evaporate in mixed clouds, phoretic forces should play an important role in the collision efficiency between aerosol and droplets, and consequently in contact freezing. A further question is the possibility that aerosol, usually not active in deposition or condensation/ immersion freezing, can trigger ice nucleation by colliding with supercooled droplets

Influence of supersaturation on the concentration of ice nucleating particles

There is a consensus on the increase in ice nucleating particles (INP) concentration from subsaturated to supersaturated water conditions typically associated with clouds (1 ÷ 2%). However, it is important to evaluate the INP concentration trend when water supersaturation further increases, as supercooled clouds contain pockets of high water vapor supersaturation. Three laboratory dry-generated aerosols, two biological (microcrystalline and fibrous cellulose) and one mineral (Arizona test dust), and a field aerosol, sampled on filters, were investigated. Atmospheric aerosol (PM1 and PM10 fractions) was sampled at Capo Granitola (CG, coastal site in Sicily) and the National Research Council (CNR) research area in Bologna (urban background site). The dynamic filter processing chamber (DFPC) was used to explore the ice nucleation of the sampled aerosol in the deposition and condensation freezing modes. Experiments were performed from water subsaturated conditions (water saturation ratio Sw = 0.94) to Sw = 1.1, at T = −22 °C. At CG we considered separately events with a prevalent contribution of marine aerosol, and those showing a contribution of both marine and continental aerosols. An increase in INP concentration, the aerosol activated fraction (AF) and ice nucleation active surface site density (ns) from water subsaturated conditions to Sw = 1.02 was measured in both laboratory and field campaigns. This increase is due to the transition from deposition nucleation to condensation freezing. The highest increases in AF and ns from Sw = 1.02 to Sw = 1.1 were obtained for urban and mixed aerosol and the lowest for marine aerosol. Samplings performed in Bologna showed a high increase in the average INP concentration from PM1 to PM10. Our results show the importance of performing measurements of ice nucleation efficiency for continental aerosol even at supersaturation values higher than those typically associated with clouds, and also considering the contribution of coarse aerosol particles

Observation of macroscopic aerosol motion due to thermal creep on chamber walls at low Knudsen number in microgravity

In a rarefied gas the temperature field and the gas motion are closely related, and the temperature field can cause, in a confined flow geometry, a steady flow without the help of external forces. This is due to the creep of the fluid along the walls induced by a wall temperature gradient, as a consequence of the molecular transfer of momentum to the wall. Experiments performed in microgravity conditions in two small cells, with mono- and bi-atomic carrier gases (Ar, N2), and a thermal gradient between upper and bottom horizontal plates, allowed the measurement of the width of the thermal creep and the induced velocity of the gas near the vertical cell wall, due to thermal gradient. In addition, experiments demonstrated the existence of the thermal creep flow in the cells, even with Knudsen number as low as 10-5. The work evidences experimentally, and for the first time, the thermal creep flow in small cells, due to wall temperature gradient, with Knudsen number as low as 10-5. In view of these results, the no-slip boundary conditions of the Navier-Stokes law in the "continuum" regime can be inadequate in non-isothermal flow geometry and in microgravity conditions. © 2014 Elsevier Inc.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Phoretic forces on aerosol particles surrounding an evaporating droplet in microgravity conditions

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 0.4. μm-2. μm) 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. © 2013 Elsevier B.V.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Thermophoretic measurements in presence of thermal stress convection in aerosols in microgravity conditions of drop tower

info:eu-repo/semantics/publishe

Measurements of thermophoretic velocities of aerosol particles in microgravity conditions in different carrier gases

Measurements of the thermophoretic velocities of aerosol particles (paraffin) in different carrier gases (helium, nitrogen, argon, xenon) were performed in microgravity conditions (the drop tower facility, in Bremen). The experiments permitted the study of thermophoresis in conditions which minimize the impact of gravity. Monodisperse aerosol particles were observed through a digital holographic velocimeter, a device allowing the determination of 3-D coordinates of particles in the viewing volume. Particle trajectories, and consequently particle velocities, were reconstructed by analysing the sequence of particle positions. We successfully observed thermophoretic velocities in low-gravity conditions. The experiments show that the thermophoretic velocity decreases from helium (He) to nitrogen (N2), argon (Ar), and xenon (Xe). Talbot et al. [1980. Thermophoresis of particles in a heated boundary layer. Journal of Fluid Mechanics 101, 737-758] predict thermophoretic velocities that nearly equal the observed values in Xenon, but are larger than observed values in N2 and Ar and smaller than the observed values in He. Yamamoto and Ishihara [1988. Thermophoresis of a spherical particle in a rarefied gas of a transition regime. Physics of Fluids 31, 3618-3624] predict thermophoretic velocities that are smaller than observed values and also predict negative values in N2, Ar and Xe. Beresnev's theory [1995. Thermophoresis of a spherical particle in a rarefied gas: Numerical analysis based on the model kinetic equations. Physics of Fluids 7, 1743-1756] fits the experimental data well when the coefficient of tangential momentum accommodation is set to one and the coefficient of energy accommodation is set to a value between 0.4 and 0.9, depending upon the gas. © 2007 Elsevier Ltd. All rights reserved.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Measurements of phoretic velocities in microgravity conditions of parabolic flights

info:eu-repo/semantics/publishe

Measurements of phoretic velocities of aerosol particles in microgravity conditions

none6Measurements of thermo- and diffusio-phoretic velocities of aerosol particles (camauba wax, paraffin and sodium chloride) were performed in microgravity conditions (Drop Tower facility, in Bremen, and Parabolic Flights, in Bordeaux). In the case of thermophoresis, a temperature gradient was obtained by heating the upper plate of the cell, while the lower one was maintained at environmental temperature. For diffusiophoresis, the water vapour gradient was obtained with sintered plates imbued with a water solution Of MgCl2 and distilled water, at the top and at the bottom of the cell, respectively. Aerosol particles were observed through a digital holographic velocimeter, a device allowing the determination of 3-D coordinates of particles from the observed volume. Particle trajectories and consequently particle velocities were reconstructed through the analysis of the sequence of particle positions. The experimental values of reduced thermophoretic velocities are between the theoretical values of Yamamoto and Ishihara [Yamamoto, K., Ishihara, Y., 1988. Thermophoresis of a spherical particle in a rarefied gas of a transition regime. Phys. Fluids. 3 1, 3618-3624] and Talbot et al. [Talbot, L., Cheng, R.K., Schefer, R.W., Willis, D.R., 1980. Thermophoresis of particles in a heated boundary layer. J. Fluid Mech. 101, 737-758], and do not show a clear dependence on the thermal conductivity of the aerosol. The existence of negative thennophoresis is not confirmed in our experiments. Concerning diffusiophoretic experiments, the results obtained show a small increase of reduced diffusiophoretic velocity with the Knudsen number.noneF. PRODI; G.SANTACHIARA; S.TRAVAINI; A.VEDERNIKOV; F.DUBOIS; J.C.LEGROSProdi, Franco; G., Santachiara; S., Travaini; A., Vedernikov; F., Dubois; J. C., Legro