Abstract
In climate and weather models, the quantitative description of aerosol and cloud processes relies on simplified assumptions. This contributes major uncertainties to the prediction of global ...and regional climate change. Therefore, models need good parameterizations for heterogeneous ice nucleation by atmospheric aerosols. Here the authors present a new parameterization of immersion freezing on desert dust particles derived from a large number of experiments carried out at the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) cloud chamber facility. The parameterization is valid in the temperature range between −12° and −36°C at or above water saturation and can be used in atmospheric models that include information about the dust surface area. The new parameterization was applied to calculate distribution maps of ice nuclei during a Saharan dust event based on model results from the regional-scale model Consortium for Small-Scale Modelling–Aerosols and Reactive Trace Gases (COSMO-ART). The results were then compared to measurements at the Taunus Observatory on Mount Kleiner Feldberg, Germany, and to three other parameterizations applied to the dust outbreak. The aerosol number concentration and surface area from the COSMO-ART model simulation were taken as input to different parameterizations. Although the surface area from the model agreed well with aerosol measurements during the dust event at Kleiner Feldberg, the ice nuclei (IN) number concentration calculated from the new surface-area-based parameterization was about a factor of 13 less than IN measurements during the same event. Systematic differences of more than a factor of 10 in the IN number concentration were also found among the different parameterizations. Uncertainties in the modeled and measured parameters probably both contribute to this discrepancy and should be addressed in future studies.
Abstract
Based on results of 11 yr of heterogeneous ice nucleation experiments at the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) chamber in Karlsruhe, Germany, a new empirical ...parameterization framework for heterogeneous ice nucleation was developed. The framework currently includes desert dust and soot aerosol and quantifies the ice nucleation efficiency in terms of the ice nucleation active surface site (INAS) approach.
The immersion freezing INAS densities nS of all desert dust experiments follow an exponential fit as a function of temperature, well in agreement with an earlier analysis of AIDA experiments. The deposition nucleation nS isolines for desert dust follow u-shaped curves in the ice saturation ratio–temperature (Si–T) diagram at temperatures below about 240 K. The negative slope of these isolines toward lower temperatures may be explained by classical nucleation theory (CNT), whereas the behavior toward higher temperatures may be caused by a pore condensation and freezing mechanism. The deposition nucleation measured for soot at temperatures below about 240 K also follows u-shaped isolines with a shift toward higher Si for soot with higher organic carbon content. For immersion freezing of soot aerosol, only upper limits for nS were determined and used to rescale an existing parameterization line.
The new parameterization framework is compared to a CNT-based parameterization and an empirical framework as used in models. The comparison shows large differences in shape and magnitude of the nS isolines especially for deposition nucleation. For the application in models, implementation of this new framework is simple compared to that of other expressions.
A porous composite of 3-dimensional (3D) reduced graphene oxide (rGO) and silica dioxide nanoparticles (PrGO-SN) was synthesized via a single-step hydrothermal process, which can initiate facile ice ...nucleation and growth starting from temperature as high as −8 °C and 5–8% RH supersaturation and sustain rapid ice crystal growth. The excellent ice nucleation activity of the PrGO-SN composite demonstrates a novel means of ice nucleation relative to known materials, attributed to not only the lattice match between the ice and crystalline structure of the PrGO-SN composite but also higher specific surface area, larger water vapor adsorption capacity, better porosity, and more hydrophilic surface of the composite than rGO. Moreover, environmental scanning electron microscope (E-SEM) in situ observation confirmed detailed growth patterns of ice crystals on the composite, which were affected by the regions with different surface roughness. These findings enabled further understanding of the factors that affected the heterogeneous ice nucleation process and shed light on the design and fabrication of more efficient functional porous ice nucleation materials for many practical applications such as cloud seeding.
Ice-nucleating particles (INPs) trigger the formation of cloud ice crystals in the atmosphere. Therefore, they strongly influence cloud microphysical and optical properties and precipitation and the ...life cycle of clouds. Improving weather forecasting and climate projection requires an appropriate formulation of atmospheric INP concentrations. This remains challenging as the global INP distribution and variability depend on a variety of aerosol types and sources, and neither their short-term variability nor their long-term seasonal cycles are well covered by continuous measurements. Here, we provide the first year-long set of observations with a pronounced INP seasonal cycle in a boreal forest environment. Besides the observed seasonal cycle in INP concentrations with a minimum in wintertime and maxima in early and late summer, we also provide indications for a seasonal variation in the prevalent INP type. We show that the seasonal dependency of INP concentrations and prevalent INP types is most likely driven by the abundance of biogenic aerosol. As current parameterizations do not reproduce this variability, we suggest a new mechanistic description for boreal forest environments which considers the seasonal variation in INP concentrations. For this, we use the ambient air temperature measured close to the ground at 4.2 m height as a proxy for the season, which appears to affect the source strength of biogenic emissions and, thus, the INP abundance over the boreal forest. Furthermore, we provide new INP parameterizations based on the Ice Nucleation Active Surface Site (INAS) approach, which specifically describes the ice nucleation activity of boreal aerosols particles prevalent in different seasons. Our results characterize the boreal forest as an important but variable INP source and provide new perspectives to describe these new findings in atmospheric models.
About half of present-day cloud condensation nuclei originate from atmospheric nucleation, frequently appearing as a burst of new particles near midday. Atmospheric observations show that the growth ...rate of new particles often accelerates when the diameter of the particles is between one and ten nanometres. In this critical size range, new particles are most likely to be lost by coagulation with pre-existing particles, thereby failing to form new cloud condensation nuclei that are typically 50 to 100 nanometres across. Sulfuric acid vapour is often involved in nucleation but is too scarce to explain most subsequent growth, leaving organic vapours as the most plausible alternative, at least in the planetary boundary layer. Although recent studies predict that low-volatility organic vapours contribute during initial growth, direct evidence has been lacking. The accelerating growth may result from increased photolytic production of condensable organic species in the afternoon, and the presence of a possible Kelvin (curvature) effect, which inhibits organic vapour condensation on the smallest particles (the nano-Köhler theory), has so far remained ambiguous. Here we present experiments performed in a large chamber under atmospheric conditions that investigate the role of organic vapours in the initial growth of nucleated organic particles in the absence of inorganic acids and bases such as sulfuric acid or ammonia and amines, respectively. Using data from the same set of experiments, it has been shown that organic vapours alone can drive nucleation. We focus on the growth of nucleated particles and find that the organic vapours that drive initial growth have extremely low volatilities (saturation concentration less than 10(-4.5) micrograms per cubic metre). As the particles increase in size and the Kelvin barrier falls, subsequent growth is primarily due to more abundant organic vapours of slightly higher volatility (saturation concentrations of 10(-4.5) to 10(-0.5) micrograms per cubic metre). We present a particle growth model that quantitatively reproduces our measurements. Furthermore, we implement a parameterization of the first steps of growth in a global aerosol model and find that concentrations of atmospheric cloud concentration nuclei can change substantially in response, that is, by up to 50 per cent in comparison with previously assumed growth rate parameterizations.
Iodic acid (HIO
) is known to form aerosol particles in coastal marine regions, but predicted nucleation and growth rates are lacking. Using the CERN CLOUD (Cosmics Leaving Outdoor Droplets) chamber, ...we find that the nucleation rates of HIO
particles are rapid, even exceeding sulfuric acid-ammonia rates under similar conditions. We also find that ion-induced nucleation involves IO
and the sequential addition of HIO
and that it proceeds at the kinetic limit below +10°C. In contrast, neutral nucleation involves the repeated sequential addition of iodous acid (HIO
) followed by HIO
, showing that HIO
plays a key stabilizing role. Freshly formed particles are composed almost entirely of HIO
, which drives rapid particle growth at the kinetic limit. Our measurements indicate that iodine oxoacid particle formation can compete with sulfuric acid in pristine regions of the atmosphere.
As part of the A-LIFE (Absorbing aerosol layers in a changing climate: aging, LIFEtime and dynamics) campaign, ground-based measurements were carried out in Paphos, Cyprus, to characterize the ...abundance, properties, and sources of aerosol particles in general and cloud condensation nuclei (CCN) and ice-nucleating particles (INP) in particular. New particle formation (NPF) events with subsequent growth of the particles into the CCN size range were observed. Aitken mode particles featured κ values of 0.21 to 0.29, indicating the presence of organic materials. Accumulation mode particles featured a higher hygroscopicity parameter, with a median κ value of 0.57, suggesting the presence of sulfate and maybe sea salt particles mixed with organic carbon. A clear downward trend of κ with increasing supersaturation and decreasing dcrit was found. Super-micron particles originated mainly from sea-spray aerosol (SSA) and partly from mineral dust. INP concentrations (NINP) were measured in the temperature range from −6.5 to −26.5 ∘C, using two freezing array-type instruments. NINP at a particular temperature span around 1 order of magnitude below −20 ∘C and about 2 orders of magnitude at warmer temperatures (T>-18 ∘C). Few samples showed elevated concentrations at temperatures >-15 ∘C, which suggests a significant contribution of biological particles to the INP population, which possibly could originate from Cyprus. Both measured temperature spectra and NINP probability density functions (PDFs) indicate that the observed INP (ice active in the temperature range between −15 and −20 ∘C) mainly originate from long-range transport. There was no correlation between NINP and particle number concentration in the size range >500 nm (N>500 nm). Parameterizations based on N>500 nm were found to overestimate NINP by about 1 to 2 orders of magnitude. There was also no correlation between NINP and particle surface area concentration. The ice active surface site density (ns) for the polluted aerosol encountered in the eastern Mediterranean in this study is about 1 to 3 orders of magnitude lower than the ns found for dust aerosol particles in previous studies. This suggests that observed NINP PDFs such as those derived here could be a better choice for modeling NINP if the aerosol particle composition is unknown or uncertain.
Dust particles from high latitudes have a potentially large local, regional, and global significance to climate and the environment as short-lived climate forcers, air pollutants, and nutrient ...sources. Identifying the locations of local dust sources and their emission, transport, and deposition processes is important for understanding the multiple impacts of high-latitude dust (HLD) on the Earth’s systems. Here, we identify, describe, and quantify the source intensity (SI) values, which show the potential of soil surfaces for dust emission scaled to values 0 to 1 concerning globally best productive sources, using the Global Sand and Dust Storms Source Base Map (G-SDS-SBM). This includes 64 HLD sources in our collection for the northern (Alaska, Canada, Denmark, Greenland, Iceland, Svalbard, Sweden, and Russia) and southern (Antarctica and Patagonia) high latitudes. Activity from most of these HLD sources shows seasonal character. It is estimated that high-latitude land areas with higher (SI ≥ 0.5), very high (SI ≥ 0.7), and the highest potential (SI ≥ 0.9) for dust emission cover > 1 670 000 km2 , > 560 000 km2 , and > 240 000 km2 , respectively. In the Arctic HLD region (≥ 60◦ N), land area with SI ≥ 0.5 is 5.5 % (1 035 059 km2), area with SI ≥ 0.7 is 2.3 % (440 804 km2), and area with SI ≥ 0.9 is 1.1 % (208 701 km2). Minimum SI values in the northern HLD region are about 3 orders of magnitude smaller, indicating that the dust sources of this region greatly depend on weather conditions. Our spatial dust source distribution analysis modeling results showed evidence supporting a northern HLD belt, defined as the area north of 50◦ N, with a “transitional HLD-source area” extending at latitudes 50–58◦ N in Eurasia and 50–55◦ N in Canada and a “cold HLD-source area” including areas north of 60◦ N in Eurasia and north of 58◦ N in Canada, with currently “no dust source” area between the HLD and low-latitude dust (LLD) dust belt, except for British Columbia. Using the global atmospheric transport model SILAM, we estimated that 1.0 % of the global dust emission originated from the high-latitude regions. About 57 % of the dust deposition in snow- and ice-covered Arctic regions was from HLD sources. In the southern HLD region, soil surface conditions are favorable for dust emission during the whole year. Climate change can cause a decrease in the duration of snow cover, retreat of glaciers, and an increase in drought, heatwave intensity, and frequency, leading to the increasing frequency of topsoil conditions favorable for dust emission, which increases the probability of dust storms. Our study provides a step forward to improve the representation of HLD in models and to monitor, quantify, and assess the environmental and climate significance of HLD.
The mineralogy and mixing state of dust particles originating from the
African continent influences climate and marine ecosystems in the North
Atlantic due to its effect on radiation, cloud ...properties and biogeochemical
cycling. However, these processes are difficult to constrain because of large
temporal and spatial variability, and the lack of in situ measurements of
dust properties at all stages of the dust cycle. This lack of measurements is
in part due to the remoteness of potential source areas (PSAs) and transport
pathways but also because of the lack of an efficient method to report the
mineralogy and mixing state of single particles with a time resolution
comparable to atmospheric processes, which may last a few hours or less.
Measurements are equally challenging in laboratory simulations where dust
particles need to be isolated and characterised in low numbers whilst
conditions are dynamically controlled and monitored in real time. This is
particularly important in controlled expansion cloud chambers (CECCs) where
ice-nucleating properties of suspended dust samples are studied in cold and
mixed phase cloud conditions. In this work, the mineralogy and mixing state of the fine fraction (<2.5 µm)
in laboratory-suspended dust from PSAs in north Africa were made using
novel techniques with online single-particle mass spectrometry (SPMS) and
traditional offline scanning electron microscopy (SEM). A regional
difference in mineralogy was detected, with material sourced from Morocco
containing a high number fraction of illite-like particles in contrast to
Sahelian material which contains potassium- and sodium-depleted clay minerals
like kaolinite. Single-particle mixing state had a much greater local
variation than mineralogy, particularly with respect to organic–biological
content. Applying the same methods to ambient measurement of transported dust
in the marine boundary layer at Cabo Verde in the remote North Atlantic
enabled the number fractions of illite/smectite clay mineral (ISCM),
non-ISCM and calcium-containing particles to be reported at a 1 h time
resolution over a 20-day period. Internal mixing of silicate particles with
nitrate, chlorine and organic–biological material was also measured and
compared to that in the suspended soils. The results show SPMS and SEM techniques are complementary and demonstrate
that SPMS can provide a meaningful high-resolution measurement of
single-particle mineralogy and mixing state in laboratory and ambient
conditions. In most cases, the differences in the mineralogical composition
between particles within a soil sample were small. Thus, particles were not
composed of discrete mineral phases. In ambient measurements, the ISCM and
nitrate content was found to change significantly between distinct dust
events, indicating a shift in source and transport pathways which may not be
captured in offline composition analysis or remote sensing techniques.
We present the laboratory results of immersion freezing efficiencies of
cellulose particles at supercooled temperature (T) conditions. Three types of
chemically homogeneous cellulose samples are used ...as surrogates that
represent supermicron and submicron ice-nucleating plant structural
polymers. These samples include microcrystalline cellulose (MCC), fibrous
cellulose (FC) and nanocrystalline cellulose (NCC). Our immersion freezing
dataset includes data from various ice nucleation measurement techniques
available at 17 different institutions, including nine dry dispersion
and 11 aqueous suspension techniques. With a total of 20 methods, we
performed systematic accuracy and precision analysis of measurements from
all 20 measurement techniques by evaluating T-binned (1 ∘C)
data over a wide T range (−36 ∘C <T<-4 ∘C). Specifically, we intercompared the geometric surface
area-based ice nucleation active surface site (INAS) density data derived from
our measurements as a function of T, ns,geo(T). Additionally, we also
compared the ns,geo(T) values and the freezing spectral slope parameter
(Δlog(ns,geo)/ΔT) from our measurements to previous
literature results. Results show all three cellulose materials are
reasonably ice active. The freezing efficiencies of NCC samples agree
reasonably well, whereas the diversity for the other two samples spans
≈ 10 ∘C. Despite given uncertainties within each
instrument technique, the overall trend of the ns,geo(T) spectrum traced
by the T-binned average of measurements suggests that predominantly
supermicron-sized cellulose particles (MCC and FC) generally act as more
efficient ice-nucleating particles (INPs) than NCC with about 1 order of
magnitude higher ns,geo(T).
PDF
