Supersaturated Vapor Research Articles

Long-term observations of cloud condensation nuclei over the Amazon rain forest – Part 2: Variability and characteristics of biomass burning, long-range transport, and pristine rain forest aerosols

Abstract. Size-resolved measurements of atmospheric aerosol and cloud condensation nuclei (CCN) concentrations and hygroscopicity were conducted over a full seasonal cycle at the remote Amazon Tall Tower Observatory (ATTO, March 2014–February 2015). In a preceding companion paper, we presented annually and seasonally averaged data and parametrizations (Part 1; Pöhlker et al., 2016a). In the present study (Part 2), we analyze key features and implications of aerosol and CCN properties for the following characteristic atmospheric conditions: Empirically pristine rain forest (PR) conditions, where no influence of pollution was detectable, as observed during parts of the wet season from March to May. The PR episodes are characterized by a bimodal aerosol size distribution (strong Aitken mode with DAit ≈ 70 nm and NAit ≈ 160 cm−3, weak accumulation mode with Dacc ≈ 160 nm and Nacc≈ 90 cm−3), a chemical composition dominated by organic compounds, and relatively low particle hygroscopicity (κAit≈ 0.12, κacc ≈ 0.18). Long-range-transport (LRT) events, which frequently bring Saharan dust, African biomass smoke, and sea spray aerosols into the Amazon Basin, mostly during February to April. The LRT episodes are characterized by a dominant accumulation mode (DAit ≈ 80 nm, NAit ≈ 120 cm−3 vs. Dacc ≈ 180 nm, Nacc ≈ 310 cm−3), an increased abundance of dust and salt, and relatively high hygroscopicity (κAit≈ 0.18, κacc ≈ 0.35). The coarse mode is also significantly enhanced during these events. Biomass burning (BB) conditions characteristic for the Amazonian dry season from August to November. The BB episodes show a very strong accumulation mode (DAit ≈ 70 nm, NAit ≈ 140 cm−3 vs. Dacc ≈ 170 nm, Nacc ≈ 3400 cm−3), very high organic mass fractions (∼ 90 %), and correspondingly low hygroscopicity (κAit≈ 0.14, κacc ≈ 0.17). Mixed-pollution (MPOL) conditions with a superposition of African and Amazonian aerosol emissions during the dry season. During the MPOL episode presented here as a case study, we observed African aerosols with a broad monomodal distribution (D ≈ 130 nm, NCN,10 ≈ 1300 cm−3), with high sulfate mass fractions (∼ 20 %) from volcanic sources and correspondingly high hygroscopicity (κ< 100 nm ≈ 0.14, κ>100nm≈ 0.22), which were periodically mixed with fresh smoke from nearby fires (D ≈ 110 nm, NCN,10 ≈ 2800 cm−3) with an organic-dominated composition and sharply decreased hygroscopicity (κ<150nm≈ 0.10, κ>150nm≈ 0.20). Insights into the aerosol mixing state are provided by particle hygroscopicity (κ) distribution plots, which indicate largely internal mixing for the PR aerosols (narrow κ distribution) and more external mixing for the BB, LRT, and MPOL aerosols (broad κ distributions). The CCN spectra (CCN concentration plotted against water vapor supersaturation) obtained for the different case studies indicate distinctly different regimes of cloud formation and microphysics depending on aerosol properties and meteorological conditions. The measurement results suggest that CCN activation and droplet formation in convective clouds are mostly aerosol-limited under PR and LRT conditions and updraft-limited under BB and MPOL conditions. Normalized CCN efficiency spectra (CCN divided by aerosol number concentration plotted against water vapor supersaturation) and corresponding parameterizations (Gaussian error function fits) provide a basis for further analysis and model studies of aerosol–cloud interactions in the Amazon.

Differential heat and mass transfer rate influences on the activation efficiency of laminar flow condensation particle counters

Laminar flow condensation particle counters (CPCs) are uniquely sensitive detectors for aerosol particles in the nanometer size range (i.e. below 10 nm in size they can have single particle sensitivity). Their operation hinges upon the creation of supersaturation of a working fluid; particles exposed to supersaturated vapor grow by condensation to optically detectable sizes. The degree of supersaturation is fully controlled via differential rates of heat transfer and working fluid vapor mass transfer. Because of the Kelvin relationship governed vapor pressure of small particles, in all CPCs there is a critical size/cut-size (diameter), and particles smaller than this size do not grow and are not detected efficiently. While efforts have been made to control the CPC activation efficiency (i.e. the fraction of particles detected as a function of size), prior studies have not examined how differential heat and mass transfer in CPCs are governed by changes in gas composition. Here, we measure and model CPC activation efficiencies (with 1-butanol as the working fluid) in mixtures of gases of disparate thermophysical properties, namely helium and molecular nitrogen. Our experiments show that the activation efficiency of smaller particles (i.e. below 8 nm in the tested CPC) can be increased by adding a modest amount of helium to the aerosol (mole fractions near 0.20). This is expected based upon the increased Lewis number brought about by Helium addition, and supported by predictions of CPC activation efficiency based upon thermophysical property variable models of coupled heat, mass, and momentum transfer within the CPC condenser region. Interestingly, we find that when operating with a constant precision orifice diameter (choked flow), the activation efficiency for a given sub-10 nm particle diameter first increases with increasing Helium mole fraction and then decreases as the Helium mole fraction increases beyond 0.67. In comparison, experiments with constant mass transfer Peclet number (Pem = 77) show an increase in CPC activation efficiency up to a helium mole fraction of 0.67, but then the activation efficiency decreases more modestly beyond this helium mole fraction. We attribute these contrasting results to the increased flowrate through the instrument under constant orifice diameter conditions, which affects the performance of the CPC saturator. Finally, through modeling we show that the ability to enhance the activation efficiency of a CPC via a modest amount of helium addition is general, and can be applied with other heavy working fluids. The results presented in this study elucidate the importance of gas composition and Lewis number controlled differential heat and mass transfer rates on the performance of condensation based nanoparticle detectors.

Water nucleation in helium, methane, and argon: A molecular dynamics study.

Nucleation of highly supersaturated water vapor in helium, methane, and argon carrier gases at 350 K was investigated using molecular dynamics simulations. Nucleation rates obtained from the mean first passage time (MFPT) method are typically one order of magnitude lower than those from the Yasuoka and Matsumoto method, which can be attributed to the overestimation of the critical cluster size in the MFPT method. It was found that faster nucleation will occur in carrier gases that have better thermalization properties such that latent heat is removed more efficiently. These thermalization properties are shown to be strongly dependent on the molecular mass and Lennard-Jones (LJ) parameters. By varying the molecular mass, for unaltered LJ parameters, it was found that a heavier carrier gas removes less heat although it has a higher collision rate with water than a lighter carrier. Thus, it was shown that a clear distinction between water vapor-carrier gas collisions and water cluster-carrier gas collisions is indispensable for understanding the effect of collision rates on thermalization. It was also found that higher concentration of carrier gas leads to higher nucleation rate. The nucleation rates increased by a factor of 1.3 for a doubled concentration and by almost a factor of two for a tripled concentration.

Nucleation rates of droplets in supersaturated steam and water vapour–carrier gas mixtures between 200 and 450 K

We compare experimental nucleation rates for water vapour in various carrier gases, estimated nucleation rates for steam, and nucleation rates obtained from molecular simulations. The data for steam are deduced from empirical adjustments of the classical nucleation theory developed by various authors to reproduce pressure and optical data for condensing steam flows in converging-diverging nozzles and turbine stages. By combining the data for nucleation in carrier gases and the data for steam nucleation, an unprecedented temperature range of 250 K is available to study the temperature dependence of nucleation rate. Original results of molecular dynamic simulations for TIP4P/2005 force field in the NVE (system constrained by number of particles, volume and energy) conditions are provided. Correction of classical nucleation theory for non-isothermal nucleation conditions is applied to experimental and simulated data. The nucleation rate data for steam follow a similar temperature trend as the nucleation rate data for water vapour in carrier gases at lower temperatures. The ratio of observed nucleation rates to classical nucleation theory predictions decreases more steeply with temperature than the empirical correlation by Wölk et al. ( J Chem Phys 2002; 117: 4954–4960). On the contrary to experimental data, the ratios of nucleation rates computed from molecular simulations to classical nucleation theory predictions do not show a significant temperature trend.

Investigation of nucleation kinetics in H2SO4 vapor through modeling of gas phase kinetics coupled with particle dynamics.

We have developed a new model utilizing our existing kinetic gas phase models to simulate experimental particle size distributions emerging in dry supersaturated H2SO4 vapor homogeneously produced by rapid oxidation of SO2 through stabilized Criegee-Intermediates from 2-butene ozonolysis. We use a sectional method for simulating the particle dynamics. The particle treatment in the model is based on first principles and takes into account the transition from the kinetic to the diffusion-limited regime. It captures the temporal evolution of size distributions at the end of the ozonolysis experiment well, noting a slight underrepresentation of coagulation effects for larger particle sizes. The model correctly predicts the shape and the modes of the experimentally observed particle size distributions. The predicted modes show an extremely high sensitivity to the H2SO4 evaporation rates of the initially formed H2SO4 clusters (dimer to pentamer), which were arbitrarily restricted to decrease exponentially with increasing cluster size. In future, the analysis presented in this work can be extended to allow a direct validation of quantum chemically predicted stabilities of small H2SO4 clusters, which are believed to initiate a significant fraction of atmospheric new particle formation events. We discuss the prospects and possible limitations of the here presented approach.

Substantial convection and precipitation enhancements by ultrafine aerosol particles.

Aerosol-cloud interactions remain the largest uncertainty in climate projections. Ultrafine aerosol particles smaller than 50 nanometers (UAP<50) can be abundant in the troposphere but are conventionally considered too small to affect cloud formation. Observational evidence and numerical simulations of deep convective clouds (DCCs) over the Amazon show that DCCs forming in a low-aerosol environment can develop very large vapor supersaturation because fast droplet coalescence reduces integrated droplet surface area and subsequent condensation. UAP<50 from pollution plumes that are ingested into such clouds can be activated to form additional cloud droplets on which excess supersaturation condenses and forms additional cloud water and latent heating, thus intensifying convective strength. This mechanism suggests a strong anthropogenic invigoration of DCCs in previously pristine regions of the world.

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.

Statistical analysis of dimer formation in supersaturated metal vapor based on molecular dynamics simulation

We discuss the problem of dimer formation during the homogeneous nucleation of atomic metal vapor in an inert gas environment. We simulated nucleation with molecular dynamics and carried out the statistical analysis of double- and triple-atomic collisions as the two ways of long-lived diatomic complex formation. Close pair of atoms with lifetime greater than the mean time interval between atom–atom collisions is called a long-lived diatomic complex. We found that double- and triple-atomic collisions gave approximately the same probabilities of long-lived diatomic complex formation, but internal energy of the resulted state was essentially lower in the second case. Some diatomic complexes formed in three-particle collisions are stable enough to be a critical nucleus.

Surface effects in droplet nucleation

The classical nucleation theory (CNT) concept of a nucleus as a fragment of the bulk new phase fails for nanosized nuclei. An extension of CNT taking into account the properties of the transition region between coexisting bulk phases is proposed. For this purpose, the finite-thickness layer method which is an alternative to Gibbs’ one is used; the transition region is considered as a separate (surface) phase. The nucleus inhomogeneity (difference in the properties of surface and bulk phases) is shown to result in the dependence of surface tension on radius and temperature. An equation for the work of nucleus formation is derived which is basic for the multivariable theory of nucleation; it takes into account the dependences of surface tension on radius (the curvature effect) and temperature. The Kelvin formula for the equilibrium vapor pressure is extended to small radii. It is shown that the ratio of the isothermal nucleation rate to that of CNT can achieve several orders of magnitude due to the curvature effect. The analysis of different dependences of the Tolman length on radius, δ(R), obeying the condition of limiting (spinodal) vapor supersaturation suggests that the curvature effect in the nucleation region is determined by the value of δ(0) which is positive, rather than by the asymptotic value δ∞. The equilibrium temperature of a noncritical droplet is shown to differ from the vapor temperature due to the dependence of surface tension on temperature. Therefore, the calculated mean steady state overheat of droplets relatively the vapor temperature consists of the kinetic (due to the release of the condensation heat) and thermodynamic parts.

Tolman’s length and limiting supersaturation of vapor

The classical Kelvin formula for the equilibrium vapor pressure over a droplet of radius R is extended to small radii and vapor non-ideality, from where the limiting supersaturation condition is obtained by relating the point R=0 to the value of limiting (spinodal) supersaturation of vapor. The analysis of different dependences of the Tolman length on radius, δ(R), obeying this condition suggests that (i) the value of δ(0) is positive and the function δ(R) decreases with increasing radius; (ii) the curvature effect (the dependence of surface tension on radius) in the nucleation region is determined by the value of δ(0). At the same time, this effect is weakly sensitive to the form of the function δ(R) and insensitive to its asymptotic value δ∞ .

Precipitation regime influence on oxygen triple-isotope distributions in Antarctic precipitation and ice cores

The relative abundance of 17O in meteoric precipitation is usually reported in terms of the 17O-excess parameter. Variations of 17O-excess in Antarctic precipitation and ice cores have hitherto been attributed to normalised relative humidity changes at the moisture source region, or to the influence of a temperature-dependent supersaturation-controlled kinetic isotope effect during in-cloud ice formation below −20 °C. Neither mechanism, however, satisfactorily explains the large range of 17O-excess values reported from measurements. A different approach, based on the regression characteristics of 103ln(1+δ17O) versus 103ln(1+δ18O), is applied here to previously published isotopic data sets. The analysis indicates that clear-sky precipitation (‘diamond dust’), which occurs widely in inland Antarctica, is characterised by an unusual relative abundance of 17O, distinct from that associated with cloud-derived, synoptic snowfall. Furthermore, this distinction appears to be largely preserved in the ice core record. The respective mass contributions to snowfall accumulation – on both temporal and spatial scales – provides the basis of a simple, first-order explanation for the observed oxygen triple-isotope ratio variations in Antarctic precipitation, surface snow and ice cores. Using this approach, it is shown that precipitation during the last major deglaciation, both in western Antarctica at the West Antarctic Ice Sheet (WAIS) Divide and at Vostok on the eastern Antarctic plateau, consisted essentially of diamond dust only, despite a large temperature differential (and thus different water vapour supersaturation conditions) at the two locations. In contrast, synoptic snowfall events dominate the accumulation record throughout the Holocene at both sites.

Direct and indirect photoinduced gas–liquid phase transitions

The condensation of supersaturated vapors of various substances in air under UV irradiation in a cloud chamber has been studied. The irradiation and subsequent photodissociation greatly facilitate the condensation of vapors in comparison with unirradiated mixtures, wherein both a vapor that directly absorbs radiation (direct photoinduced phase transition) and, for example, water vapor in the presence of photodissociating impurities (indirect photoinduced transition) can be condensed.

Analysis of emission characteristics of gas turbine engines with some alternative fuels

ABSTRACTThe paper concerns the comparative analysis of combustion characteristics of different alternative fuels such as Fischer-Tropsch Synthetic Paraffinic Kerosene (FT-SPK), cryogenic methane, bioethanol, biomethanol, biobutanol, dimethyl ether, biodiesel and conventional aviation kerosene Jet-A as well as analysis of emissions of NOx, CO, CO2, H2O, HNOy (y = 2,3) and organics for gas turbine engine operating on these fuels. The analysis has shown that the usage of all considered alternative fuels results in the increase of H2O emission, compared to kerosene-fueled combustor, and, as consequence, in the growth of water vapor supersaturation that can increase the rate of the H2O vapor condensation and enhance the formation of contrails and cirrus clouds in the atmosphere. The usage of all considered alternative fuels except FT-SPK, cryogenic methane and dimethyl can increase the CO2 emission compared to using of kerosene. Emission of N-containing species can be reduced upon the usage of considered alternative fuels, except dimethyl ether, for which one can expect the increase in the emissions of HNO2 and HNO3 approximately by 10%. The emission of CO decreases for all fuels except biodiesel. The major decrease can be achieved upon the replacement of kerosene to bioethanol.

Aerosol removal and cloud collapse accelerated by supersaturation fluctuations in turbulence

AbstractPrior observations have documented the process of cloud cleansing, through which cloudy, polluted air from a continent is slowly transformed into cloudy, clean air typical of a maritime environment. During that process, cloud albedo changes gradually, followed by a sudden reduction in cloud fraction and albedo as drizzle forms and convection changes from closed to open cellular. Experiments in a cloud chamber that generates a turbulent environment show a similar cloud cleansing process followed by rapid cloud collapse. Observations of (1) cloud droplet size distribution, (2) interstitial aerosol size distribution, (3) cloud droplet residual size distribution, and (4) water vapor supersaturation are all consistent with the hypothesis that turbulent fluctuations of supersaturation accelerate the cloud cleansing process and eventual cloud collapse. Decay of the interstitial aerosol concentration occurs slowly at first then more rapidly. The accelerated cleansing occurs when the cloud phase relaxation time exceeds the turbulence correlation time.

UV laser synthesis of nanoparticles in the gas phase

This review deals with the UV laser photodissociation of metal carbonyls, ferrocene, carbon suboxide, and other precursors. The formation of supersaturated atomic vapors followed by the formation of carbon, metal, and metal–carbon nanoparticles is discussed. Application of UV laser synthesis to preparation of catalytic nanomaterials is considered.

First measurements of the number size distribution of 1–2 nm aerosol particles released from manufacturing processes in a cleanroom environment

This study was conducted to observe a potential formation and/or release of aerosol particles related to manufacturing processes inside a cleanroom. We introduce a novel technique to monitor airborne sub 2 nm particles in the cleanroom and present results from a measurement campaign during which the total particle number concentration (>1 nm and >7 nm) and the size resolved concentration in the 1 to 2 nm size range were measured. Measurements were carried out in locations where atomic layer deposition (ALD), sputtering, and lithography processes were conducted, with a wide variety of starting materials. During our campaign in the clean room, we observed several time periods when the particle number concentration was 105 cm−3 in the sub 2 nm size range and 104 cm−3 in the size class larger than 7 nm in one of the sampling locations. The highest concentrations were related to the maintenance processes of the manufacturing machines, which were conducted regularly in that specific location. Our measurements show that around 500 cm−3 sub 2 nm particles or clusters were in practice always present in this specific cleanroom, while the concentration of particles larger than 2 nm was less than 2 cm−3. During active processes, the concentrations of sub 2 nm particles could rise to over 105 cm−3 due to an active new particle formation. The new particle formation was most likely induced by a combination of the supersaturated vapors, released from the machines, and the very low existing condensation sink, leading to pretty high formation rates J1.4 nm = (9 ± 4) cm−3 s−1 and growth rates of particles (GR1.1–1.3 nm = (6 ± 3) nm/h and GR1.3–1.8 nm = (14 ± 3) nm/h).Copyright © 2017 American Association for Aerosol Research

Density functional description of size-dependent effects at nucleation on neutral and charged nanoparticles

A numerical study of size-dependent effects in the thermodynamics of a small droplet formed around a solid nanoparticle has been performed within the square-gradient density functional theory. The Lennard-Jones fluid with the Carnahan-Starling model for the hard-sphere contribution to intermolecular interaction in liquid and vapor phases and interfaces has been used for description of the condensate. The intermolecular forces between the solid core and condensate molecules have been taken into account with the help of the Lennard-Jones part of the total molecular potential of the core. The influence of the electric charge of the particle has been considered under assumption of the central Coulomb potential in the medium with dielectric permittivity depending on local condensate density. The condensate density profiles and equimolecular radii for equilibrium droplets at different values of the condensate chemical potential have been computed in the cases of an uncharged solid core with the molecular potential, a charged core without molecular potential, and a core with joint action of the Coulomb and molecular potentials. The appearance of stable equilibrium droplets even in the absence of the electric charge has been commented. As a next step, the capillary, disjoining pressure, and electrostatic contributions to the condensate chemical potential have been considered and compared with the predictions of classical thermodynamics in a wide range of values of the droplet and the particle equimolecular radii. With the help of the found dependence of the condensate chemical potential in droplet on the droplet size, the activation barrier for nucleation on uncharged and charged particles has been computed as a function of the vapor supersaturation. Finally, the work of droplet formation and the work of wetting the particle have been found as functions of the droplet size.

Organic Nanoaerosol in Coal Mines: Formation Mechanism and Explosibility

ABSTRACTThe mechanism of aerosol formation in coal mines during the operation of a longwall shearer was studied using a diffusion battery, optical counter, and by means of Transmission Electron Microscopy (TEM). The aerosol number concentration was measured to be (2–5) × 105 cm–3. The aerosol size spectrum contained three modes, at about 10, 100, and 1000 nm. The first mode relates to single (primary) particles formed by the homogeneous nucleation of supersaturated organic vapor. This vapor is formed by the evaporation of organic matter from coal due to the release of frictional heat at the interface between the cutting pick and coal. The second mode relates to the particles that are aggregates formed by coagulation of primary particles. The third mode relates to the particles formed by direct grinding of coal by the cutting picks. The laboratory studies of organic aerosol formation in a flow coal grinding machine showed that the size spectrum and morphology of aerosol from the grinding machine were close to those in the coal mine. This fact confirms the thesis that the release of frictional heat is the driving force for the formation of organic aerosol. The analysis of gas-phase products in the outflow of the grinding machine showed that along with aerosol formation, gas products such as CO, CO2, CH4, C2H6, H2O are released from coal due to the frictional heat. Methane and ethane concentrations in the flow reached 10 and 5 vol. %, respectively. To demonstrate the explosibility of organic aerosol, the combustion of organic aerosol in the air was studied. It was shown that the lower explosive limit for organic aerosol is less than 50 g m–3. A conclusion is made that the formation of organic aerosol is to be taken into account when estimating the safety limits in coal mines.

Self-assembly of MoO3 needles in gas current for cubic formation pathway.

Nucleation and subsequent rapid growth are enigmatic due to the unrevealed pathways. Despite the relatively simpler mechanism compared to nucleation and growth in solution, that in vapor has received little attention. The largest hindrance to unveiling this process may be observing the rapid and mesoscopic-scale phenomena. To overcome this hindrance, we combine an experimental approach with in situ spatial scanning Fourier-transform infrared spectroscopy, which reveals the nucleating and growing nanoparticles in vapor. The nanoparticles are then collected at different evolutionary stages and analyzed by ex situ transmission electron microscopy (TEM). Needle-shaped molybdenum oxide (MoO3) nanoparticles were formed within ∼0.1 s after homogeneous nucleation from a highly supersaturated vapor. Over one second, the needle particles gradually evolved into a cubic shape by fusion in a crystallographically favored orientation in a free-flying state in vapor. The similar sizes of the elongated axes of the needle and cubic structures suggest an additional growth stage, in which the needle particles become the growth units of the cubic particles. The morphology of a final crystal should reflect the formation environment of the particle because growing crystals are sensitive to the formation conditions such as temperature, concentration, and impurities. Although nucleation under very high supersaturation induces the anisotropic growth of the needle particles, this information of the initial nucleation environment is lost in the final cubic crystal. These findings enrich our understanding of pathways in the nucleation and growth of nanoparticles and provide new insights into the growth stages driven by oriented attachment.

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.