AgI is one of the best-investigated ice-nucleating substances. It has relevance for the atmosphere since it is used for glaciogenic cloud seeding. Theoretical and experimental studies over the last ...60 years provide a complex picture of silver iodide as an ice-nucleating agent with conflicting and inconsistent results. This review compares experimental ice nucleation studies in order to analyze the factors that influence the ice nucleation ability of AgI. The following picture emerges from this analysis: the ice nucleation ability of AgI seems to be enhanced when the AgI particle is on the surface of a droplet, which is indeed the position that a particle takes when it can freely move in a droplet. The ice nucleation by particles with surfaces exposed to air depends on water adsorption. AgI surfaces seem to be most efficient at nucleating ice when they are exposed to relative humidity at or even above water saturation. For AgI particles that are completely immersed in water, the freezing temperature increases with increasing AgI surface area. Higher threshold freezing temperatures seem to correlate with improved lattice matches as can be seen for AgI–AgCl solid solutions and 3AgI·NH4I·6H2O, which have slightly better lattice matches with ice than AgI and also higher threshold freezing temperatures. However, the effect of a good lattice match is annihilated when the surfaces have charges. Also, the ice nucleation ability seems to decrease during dissolution of AgI particles. This introduces an additional history and time dependence for ice nucleation in cloud chambers with short residence times.
Measurements of the concentration and variability of ice
nucleating particles in the subtropical maritime boundary layer are reported.
Filter samples collected in Cabo Verde over the period 2009–2013 ...are
analyzed with a drop freezing experiment with sensitivity to
detect the few rare ice nuclei active at low supercooling. The data set is
augmented with continuous flow diffusion chamber measurements at temperatures
below −24 ∘C from a 2-month field campaign in Cabo Verde in 2016.
The data set is used to address the following questions: what are typical
concentrations of ice nucleating particles active at a certain temperature?
What affects their concentration and where are their sources? Concentration
of ice nucleating particles is found to increase exponentially by 7 orders of
magnitude from −5 to −38 ∘C. Sample-to-sample variation in the
steepness of the increase indicates that particles of different origin, with
different ice nucleation properties (size, composition), contribute to the
ice nuclei concentration at different temperatures. The concentration of ice
nuclei active at a specific temperature varies over a range of up to 4 orders
of magnitude. The frequency with which a certain ice nuclei concentration is
measured within this range is found to follow a lognormal
distribution, which can be explained by random dilution during transport. To
investigate the geographic origin of ice nuclei, source attribution of air
masses from dispersion modeling is used to classify the data into seven
typical conditions. While no source could be attributed to the ice nuclei
active at temperatures higher than −12 ∘C, concentrations at lower
temperatures tend to be elevated in air masses originating from the Sahara.
Desert dust is one of the most abundant ice nucleating particle types in the atmosphere. Traditionally, clay minerals were assumed to determine the ice nucleation ability of desert dust and ...constituted the focus of ice nucleation studies over several decades. Recently some feldspar species were identified to be ice active at much higher temperatures than clay minerals, redirecting studies to investigate the contribution of feldspar to ice nucleation on desert dust. However, so far no study has shown the atmospheric relevance of this mineral phase.For this study four dust samples were collected after airborne transport in the troposphere from the Sahara to different locations (Crete, the Peloponnese, Canary Islands, and the Sinai Peninsula). Additionally, 11 dust samples were collected from the surface from nine of the biggest deserts worldwide. The samples were used to study the ice nucleation behavior specific to different desert dusts. Furthermore, we investigated how representative surface-collected dust is for the atmosphere by comparing to the ice nucleation activity of the airborne samples. We used the IMCA-ZINC setup to form droplets on single aerosol particles which were subsequently exposed to temperatures between 233 and 250 K. Dust particles were collected in parallel on filters for offline cold-stage ice nucleation experiments at 253–263 K. To help the interpretation of the ice nucleation experiments the mineralogical composition of the dusts was investigated. We find that a higher ice nucleation activity in a given sample at 253 K can be attributed to the K-feldspar content present in this sample, whereas at temperatures between 238 and 245 K it is attributed to the sum of feldspar and quartz content present. A high clay content, in contrast, is associated with lower ice nucleation activity. This confirms the importance of feldspar above 250 K and the role of quartz and feldspars determining the ice nucleation activities at lower temperatures as found by earlier studies for monomineral dusts. The airborne samples show on average a lower ice nucleation activity than the surface-collected ones. Furthermore, we find that under certain conditions milling can lead to a decrease in the ice nucleation ability of polymineral samples due to the different hardness and cleavage of individual mineral phases causing an increase of minerals with low ice nucleation ability in the atmospherically relevant size fraction. Comparison of our data set to an existing desert dust parameterization confirms its applicability for climate models. Our results suggest that for an improved prediction of the ice nucleation ability of desert dust in the atmosphere, the modeling of emission and atmospheric transport of the feldspar and quartz mineral phases would be key, while other minerals are only of minor importance.
Aerosol particles acting as cloud condensation nuclei (CCN) or ice-nucleating particles (INPs) play a major role in the formation and glaciation of clouds. Thereby they exert a strong impact on the ...radiation budget of the Earth. Data on abundance and properties of both types of particles are sparse, especially for remote areas of the world, such as the Southern Ocean (SO). In this work, we present unique results from ship-borne aerosol-particle-related in situ measurements and filter sampling in the SO region, carried out during the Antarctic Circumnavigation Expedition (ACE) in the austral summer of 2016–2017. An overview of CCN and INP concentrations over the Southern Ocean is provided and, using additional quantities, insights regarding possible CCN and INP sources and origins are presented.
CCN number concentrations spanned 2 orders of magnitude, e.g. for a supersaturation of 0.3 % values ranged roughly from 3 to 590 cm−3.
CCN showed variable contributions of organic and inorganic material (inter-quartile range of hygroscopicity parameter κ from 0.2 to 0.9).
No distinct size dependence of κ was apparent, indicating homogeneous composition across sizes (critical dry diameter on average between 30 and 110 nm).
The contribution of sea spray aerosol (SSA) to the CCN number concentration was on average small.
Ambient INP number concentrations were measured in the temperature range from −5 to −27 ∘C using an immersion freezing method. Concentrations spanned up to 3 orders of magnitude, e.g. at −16 ∘C from 0.2 to 100 m−3.
Elevated values (above 10 m−3 at −16 ∘C) were measured when the research vessel was in the vicinity of land (excluding Antarctica), with lower and more constant concentrations when at sea. This, along with results of backward-trajectory analyses, hints towards terrestrial and/or coastal INP sources being dominant close to ice-free (non-Antarctic) land.
In pristine marine areas INPs may originate from both oceanic sources and/or long-range transport.
Sampled aerosol particles (PM10) were analysed for sodium and methanesulfonic acid (MSA). Resulting mass concentrations were used as tracers for primary marine and secondary aerosol particles, respectively.
Sodium, with an average mass concentration around 2.8 µg m−3, was found to dominate the sampled, identified particle mass.
MSA was highly variable over the SO, with mass concentrations up to 0.5 µg m−3 near the sea ice edge.
A correlation analysis yielded strong correlations between sodium mass concentration and particle number concentration in the coarse mode, unsurprisingly indicating a significant contribution of SSA to that mode.
CCN number concentration was highly correlated with the number concentration of Aitken and accumulation mode particles. This, together with a lack of correlation between sodium mass and Aitken and accumulation mode number concentrations, underlines the important contribution of non-SSA, probably secondarily formed particles, to the CCN population. INP number concentrations did not significantly correlate with any other measured aerosol physico-chemical parameter.
Heterogeneous ice formation by immersion freezing in mixed-phase clouds can be parameterized in general circulation models (GCMs) by classical nucleation theory (CNT). CNT parameterization schemes ...describe immersion freezing as a stochastic process, including the properties of insoluble aerosol particles in the droplets. There are different ways to parameterize the properties of aerosol particles (i.e., contact angle schemes), which are compiled and tested in this paper. The goal of this study is to find a parameterization scheme for GCMs to describe immersion freezing with the ability to shift and adjust the slope of the freezing curve compared to homogeneous freezing to match experimental data. We showed in a previous publication that the resulting freezing curves from CNT are very sensitive to unconstrained kinetic and thermodynamic parameters in the case of homogeneous freezing. Here we investigate how sensitive the outcome of a parameter estimation for contact angle schemes from experimental data is to unconstrained kinetic and thermodynamic parameters. We demonstrate that the parameters describing the contact angle schemes can mask the uncertainty in thermodynamic and kinetic parameters. Different CNT formulations are fitted to an extensive immersion freezing dataset consisting of size-selected measurements as a function of temperature and time for different mineral dust types, namely kaolinite, illite, montmorillonite, microcline (K-feldspar), and Arizona test dust. We investigated how accurate different CNT formulations (with estimated fit parameters for different contact angle schemes) reproduce the measured freezing data, especially the time and particle size dependence of the freezing process. The results are compared to a simplified deterministic freezing scheme. In this context, we evaluated which CNT-based parameterization scheme able to represent particle properties is the best choice to describe immersion freezing in a GCM.
The role of black carbon (BC) in ice crystal formation via immersion freezing relevant for mixed‐phase cloud formation is uncertain. Previous studies report either negligible or significant ...contributions of BC particles to cloud glaciation via immersion freezing. Despite conflicting evidence, immersion freezing by BC particles is included in several cloud models. Here we show that fossil fuel soot and commercially available hydrocarbon BC is inactive as immersion freezing nuclei for atmospherically relevant particle sizes and surface areas. Instead, temperatures <235 K are necessary for freezing droplets with immersed soot particles, implying homogeneous freezing, rather than immersion freezing by soot. A comparison of the results to previous studies using larger soot aggregates and dust reveals the ineffectiveness of soot as immersion ice nucleating particles. We conclude that soot particles with properties like those investigated here can be neglected for simulating ice nucleation in mixed‐phase clouds.
Plain Language Summary
Small subvisual particles in the atmosphere are responsible for cloud formation. Some of these particles, called ice nucleating particles (INPs), promote the freezing of individual cloud droplets. The chemical and physical properties that render particles INP are highly variable and poorly constrained. Here we show carbonaceous nanoparticles also called soot or black carbon, produced from various industrial processes and fossil fuel combustion do not contribute to ice crystal formation in clouds that form at temperatures between 0°C and −38°C unlike, for example, mineral dust. The findings suggest that climate and cloud models can neglect any contributions of soot to the formation of the ice phase in the atmosphere for temperatures greater than −38°C.
Key Points
Laboratory‐generated black carbon and soot particles from hydrocarbon and fossil fuel do not nucleate ice at mixed‐phase cloud conditions
Homogeneous freezing temperatures are required for droplets containing soot to freeze
Studies quantifying the aerosol contribution to ice nucleation do not need to include this INP type for temperatures warmer than 235 K
Sea spray aerosols (SSA) greatly affect the climate system by scattering solar radiation and acting as seeds for cloud droplet formation. The ecosystems in the Arctic Ocean are rapidly changing due ...to global warming, and the effects these changes have on the generation of SSA, and thereby clouds and fog formation in this region, are unknown. During the ship-based Arctic Century Expedition, we examined the dependency of forced SSA production on the biogeochemical characteristics of seawater using an on-board temperature-controlled aerosol generation chamber with a plunging jet system. Our results indicate that mainly seawater salinity and organic content influence the production and size distribution of SSA. However, we observed a 2-fold higher SSA production from waters with similar salinity collected north of 81°N compared to samples collected south of this latitude. This variability was not explained by phytoplankton and bacterial abundances or Chlorophyll-a concentration but by the presence of glucose in seawater. The synergic action of sea salt (essential component) and glucose or glucose-rich saccharides (enhancer) accounts for >80% of SSA predictability throughout the cruise. Our results suggest that besides wind speed and salinity, SSA production in Arctic waters is also affected by specific organics released by the microbiota.
Ambient concentrations of ice-forming particles measured during ship expeditions are collected and summarised with the aim of determining the spatial distribution and variability in ice nuclei in ...oceanic regions.
The presented data from literature and previously unpublished data from over 23 months of ship-based measurements stretch from the Arctic to the Southern Ocean and include a circumnavigation of Antarctica. In comparison to continental observations, ship-based measurements of ambient ice nuclei show 1 to 2 orders of magnitude lower mean concentrations. To quantify the geographical variability in oceanic areas, the concentration range of potential ice nuclei in different climate zones is analysed by meridionally dividing the expedition tracks into tropical, temperate and polar climate zones. We find that concentrations of ice nuclei in these meridional zones follow temperature spectra with similar slopes but vary in absolute concentration. Typically, the frequency with which specific concentrations of ice nuclei are observed at a certain temperature follows a log-normal distribution. A consequence of the log-normal distribution is that the mean concentration is higher than the most frequently measured concentration. Finally, the potential contribution of ship exhaust to the measured ice nuclei concentration on board research vessels is analysed as function of temperature. We find a sharp onset of the influence at approximately −36 ∘C but none at warmer temperatures that could bias ship-based measurements.
The relation between the mineralogical characteristics of
size-selected feldspar particles from 50 to 800 nm and their ability to act
as ice-nucleating particles (INPs) in the immersion mode is ...presented. Five
polymorph members of K-feldspar (two microclines, orthoclase, adularia and
sanidine) and four plagioclase samples (three labradorites and a pericline
sample) are tested. Microcline was found to be the most active INP in the
immersion mode consistent with previous findings. Samples were selected for
their differences in typical feldspar properties such as crystal structure,
bulk and trace elemental composition, and ordering of the crystal lattice.
The properties mentioned are related to the temperature of feldspar
crystallization from the magma during formation. Properties characteristic of low-temperature feldspar formation coincide with an increased ability to
nucleate ice. Amongst the samples investigated, ice nucleation is most
efficient on the crystallographically ordered, triclinic K-feldspar species
microcline, while the intermediate and disordered monoclinic K-feldspar
polymorphs orthoclase and sanidine nucleate ice at lower temperatures. The
ice nucleation ability of disordered triclinic Na∕Ca-feldspar is comparable
to disordered K-feldspar. The conditions of feldspar rock formation also
leave a chemical fingerprint with varying abundance of trace elements in the
samples. X-ray fluorescence spectroscopy analysis was conducted to determine
metal oxide and trace elemental composition of the feldspar samples. The
analysis revealed a correlation of trace metal abundance with median
freezing temperatures (T50) of the K-feldspar samples allowing us to sort
them for their ice nucleation efficiency according to the abundance of
specific trace elements. A pronounced size dependence of ice nucleation
activity for the feldspar samples is observed, with the activity of smaller-sized particles scaling with surface area or being even higher compared to
larger particles. The size dependence varies for different feldspar samples.
In particular, microcline exhibited immersion freezing even for 50 nm
particles which is unique for heterogeneous ice nucleation of mineral dusts.
This suggests that small microcline particles that are susceptible to
long-range transport can affect cloud properties via immersion freezing far
away from the source. The measurements generally imply that temperatures at
which feldspars can affect cloud glaciation depend on the transported
particle size in addition to the abundance of these particles.
Abstract
African desert dust is emitted and long-range transported with multiple effects on climate, air quality, cryosphere, and ecosystems. On 21–23 February 2021, dust from a sand and dust storm ...in northern Africa was transported to Finland, north of 60°N. The episode was predicted 5 days in advance by the global operational SILAM forecast, and its key features were confirmed and detailed by a retrospective analysis. The scavenging of dust by snowfall and freezing rain in Finland resulted in a rare case of substantial mineral dust contamination of snow surfaces over a large area in the southern part of the country. A citizen science campaign was set up to collect contaminated snow samples prepared according to the scientists’ instructions. The campaign gained wide national interest in television, radio, newspapers and social media, and dust samples were received from 525 locations in Finland, up to 64.3°N. The samples were utilised in investigating the ability of an atmospheric dispersion model to simulate the dust episode. The analysis confirmed that dust came from a wide Sahara and Sahel area from 5000 km away. Our results reveal the features of this rare event and demonstrate how deposition samples can be used to evaluate the skills and limitations of current atmospheric models in simulating transport of African dust towards northern Europe.
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