28 research outputs found
Review Of Recent Developments And Shortcomings In The Characterization Of Potential Atmospheric Ice Nuclei : Focus On The Tropics.
In this paper, we summarize the four main ice nucleating aerosol types: mineral dusts,
bioaerosols, soot, and glassy organics, with the aim of demonstrating the limitations in scientifi c
literature regarding their ice nucleation properties. Because the tropics are largely associated with
marine environments, such as the Atlantic, Pacifi c, and Indian Oceans, they are potential source of
ice nuclei, and therefore ice clouds can form in regions of high convective uplift. As a result, these
particles are able to infl uence both the planet’s radiative force and its hydrological cycle. Due to
our limited understanding of these effects, we would like to encourage the scientifi c community to
increase its efforts to study and characterize the tropical aerosol particles that may function as ice
nuclei. Through such efforts, we may reduce uncertainties in climate predictions, and improve our
understanding of global warming in the hopes of fi nding potential solutions to these issues
The immersion mode ice nucleation behavior of mineral dusts: A comparison of different pure and surface modified dusts
In this study we present results from immersion freezing experiments with size-segregated mineral dust particles. Besides two already existing data sets for Arizona Test Dust (ATD), and Fluka kaolinite, we show two new data sets for illite-NX, which consists mainly of illite, a clay mineral, and feldspar, a common crustal material. The experiments were carried out with the Leipzig Aerosol Cloud Interaction Simulator. After comparing the different dust samples, it became obvious that the freezing ability was positively correlated with the K-feldspar content. Furthermore, a comparison of the composition of the ATD, illite-NX, and feldspar samples suggests that within the K-feldspars, microcline is more ice nucleation active than orthoclase. A coating with sulfuric acid leads to a decrease in the ice nucleation ability of all mineral dusts, with the effect being more pronounced for the feldspar sample. Key Points The freezing ability of mineral dusts correlated with the K-feldspar contentAmong feldspars, microcline shows a better ice nucleation ability than orthoclaseAfter coating, all investigated dusts feature a similar ice nucleation abilit
Ice-nucleating ability of aerosol particles and possible sources at three coastal marine sites
Despite the importance of ice-nucleating particles (INPs) for climate and precipitation, our understanding of these particles is far from complete. Here, we investigated INPs at three coastal marine sites in Canada, two at mid-latitude (Amphitrite Point and Labrador Sea) and one in the Arctic (Lancaster Sound). For Amphitrite Point, 23 sets of samples were analyzed, and for Labrador Sea and Lancaster Sound, one set of samples was analyzed for each location. At all three sites, the ice-nucleating ability on a per number basis (expressed as the fraction of aerosol particles acting as an INP) was strongly dependent on the particle size. For example, at diameters of around 0.2µm, approximately 1 in 106 particles acted as an INP at −25°C, while at diameters of around 8µm, approximately 1 in 10 particles acted as an INP at −25°C. The ice-nucleating ability on a per surface-area basis (expressed as the surface active site density, ns) was also dependent on the particle size, with larger particles being more efficient at nucleating ice. The ns values of supermicron particles at Amphitrite Point and Labrador Sea were larger than previously measured ns values of sea spray aerosols, suggesting that sea spray aerosols were not a major contributor to the supermicron INP population at these two sites. Consistent with this observation, a global model of INP concentrations under-predicted the INP concentrations when assuming only marine organics as INPs. On the other hand, assuming only K-feldspar as INPs, the same model was able to reproduce the measurements at a freezing temperature of −25°C, but under-predicted INP concentrations at −15°C, suggesting that the model is missing a source of INPs active at a freezing temperature of −15°C
A marine biogenic source of atmospheric ice nucleating particles
The amount of ice present in clouds can affect cloud lifetime, precipitation and radiative properties1,2. The formation of ice in clouds is facilitated by the presence of airborne ice nucleating particles1,2. Sea spray is one of the major global sources of atmospheric particles, but it is unclear to what extent these particles are capable of nucleating ice3-11. Sea spray aerosol contains large amounts of organic material that is ejected into the atmosphere during bubble bursting at the organically enriched sea-air interface or sea surface microlayer12-19. Here we show that organic material in the sea surface microlayer nucleates ice under conditions relevant for mixed-phase cloud and high-altitude ice cloud formation. The ice nucleating material is likely biogenic and less than ~0.2 μm in size. We find that exudates separated from cells of the marine diatom T. Pseudonana nucleate ice and propose that organic material associated with phytoplankton cell exudates is a likely candidate for the observed ice nucleating ability of the microlayer samples. Global model simulations of marine organic aerosol in combination with our measurements suggest that marine organic material may be an important source of ice nucleating particles in remote marine environments such as the Southern Ocean, North Pacific and North Atlantic
Overview paper: New insights into aerosol and climate in the Arctic
Motivated by the need to predict how the Arctic atmosphere will
change in a warming world, this article summarizes recent advances made by
the research consortium NETCARE (Network on Climate and Aerosols: Addressing
Key Uncertainties in Remote Canadian Environments) that contribute to our
fundamental understanding of Arctic aerosol particles as they relate to
climate forcing. The overall goal of NETCARE research has been to use an
interdisciplinary approach encompassing extensive field observations and a
range of chemical transport, earth system, and biogeochemical models. Several
major findings and advances have emerged from NETCARE since its formation in
2013. (1)Â Unexpectedly high summertime dimethyl sulfide (DMS) levels were
identified in ocean water (up to 75 nM) and the overlying atmosphere (up to
1 ppbv) in the Canadian Arctic Archipelago (CAA). Furthermore, melt ponds,
which are widely prevalent, were identified as an important DMS source (with
DMS concentrations of up to 6 nM and a potential contribution to atmospheric
DMS of 20 % in the study area). (2)Â Evidence of widespread particle
nucleation and growth in the marine boundary layer was found in the CAA in
the summertime, with these events observed on 41 % of days in a 2016
cruise. As well, at Alert, Nunavut, particles that are newly formed and grown
under conditions of minimal anthropogenic influence during the months of July
and August are estimated to contribute 20 % to 80 % of the 30–50 nm
particle number density. DMS-oxidation-driven nucleation is facilitated by
the presence of atmospheric ammonia arising from seabird-colony emissions,
and potentially also from coastal regions, tundra, and biomass burning. Via
accumulation of secondary organic aerosol (SOA), a significant fraction of the new
particles grow to sizes that are active in cloud droplet formation. Although
the gaseous precursors to Arctic marine SOA remain poorly defined, the
measured levels of common continental SOA precursors (isoprene and
monoterpenes) were low, whereas elevated mixing ratios of oxygenated volatile
organic compounds (OVOCs) were inferred to arise via processes involving the
sea surface microlayer. (3)Â The variability in the vertical distribution of
black carbon (BC) under both springtime Arctic haze and more pristine
summertime aerosol conditions was observed. Measured particle size
distributions and mixing states were used to constrain, for the first time,
calculations of aerosol–climate interactions under Arctic conditions.
Aircraft- and ground-based measurements were used to better establish the BC
source regions that supply the Arctic via long-range transport mechanisms,
with evidence for a dominant springtime contribution from eastern and
southern Asia to the middle troposphere, and a major contribution from
northern Asia to the surface. (4)Â Measurements of ice nucleating particles
(INPs) in the Arctic indicate that a major source of these particles is
mineral dust, likely derived from local sources in the summer and long-range
transport in the spring. In addition, INPs are abundant in the sea surface
microlayer in the Arctic, and possibly play a role in ice nucleation in the
atmosphere when mineral dust concentrations are low. (5)Â Amongst multiple
aerosol components, BC was observed to have the smallest effective deposition
velocities to high Arctic snow (0.03 cm s−1).</p
The Roles of Mineral Dusts and Coastal Aerosol in Cold and Warm Cloud Formation
The indirect effect of atmospheric aerosol is one of the largest uncertainties in determining the Earth's radiative budget. This uncertainty has been attributed to our lack of understanding of processes leading to cloud formation. Consequently, this thesis investigates the abilities of two main types of aerosol to form warm and mixed-phase clouds.To study the mixed-phase cloud formation properties of 24 atmospherically-relevant minerals, their deposition ice nucleation properties were studied using a single experimental method. From a set of minerals present in mineral dusts it was found that feldspars were the most efficient ice nuclei. In addition, the warm cloud formation properties, or hygroscopicity (ê), of coastal ambient aerosol and its organic components were investigated in Ucluelet, BC. While the ê of 50 nm and 100 nm particles exhibited a wide size-independent variation (0.14 - 1.08), the ê of its organic fraction was estimated to be between 0.3 and 0.5.M.Sc
Feldspar minerals as efficient deposition ice nuclei
Mineral dusts are well known to be efficient ice nuclei, where the source of
this efficiency has typically been attributed to the presence of clay
minerals such as illite and kaolinite. However, the ice nucleating abilities
of the more minor mineralogical components have not been as extensively
examined. As a result, the deposition ice nucleation abilities of 24
atmospherically relevant mineral samples have been studied, using a
continuous flow diffusion chamber at −40.0 ± 0.3 °C and particles
size-selected at 200 nm. By focussing on using the same experimental
procedure for all experiments, a relative ranking of the ice nucleating
abilities of the samples was achieved. In addition, the ice nucleation
behaviour of the pure minerals is compared to that of complex mixtures, such
as Arizona Test Dust (ATD) and Mojave Desert Dust (MDD), and to lead iodide,
which has been previously proposed for cloud seeding. Lead iodide was the
most efficient ice nucleus (IN), requiring a critical relative humidity with
respect to ice (RH<sub>i</sub>) of 122.0 ± 2.0% to activate 0.1% of
the particles. MDD (RH<sub>i</sub>) 126.3 ± 3.4%) and ATD (RH<sub>i</sub> 129.5 ± 5.1%)
have lower but comparable activity. From a set of clay
minerals (kaolinite, illite, montmorillonite), non-clay minerals (e.g.
hematite, magnetite, calcite, cerussite, quartz), and feldspar minerals
(orthoclase, plagioclase) present in the atmospheric dusts, it was found
that the feldspar minerals (particularly orthoclase) and some clays
(particularly kaolinite) were the most efficient ice nuclei. Orthoclase and
plagioclase were found to have critical RH<sub>i</sub> values of 127.1 ± 6.3% and 136.2 ± 1.3%, respectively. The presence of feldspars
(specifically orthoclase) may play a significant role in the IN behaviour of
mineral dusts despite their lower percentage in composition relative to clay
minerals