99 research outputs found
Kaolinite particles as ice nuclei: learning from the use of different kaolinite samples and different coatings
Kaolinite particles from two different sources (Fluka and Clay
Minerals Society (CMS)) were examined with respect to their ability
to act as ice nuclei (IN). This was done in the water-subsaturated regime
where often deposition ice nucleation is assumed to occur, and for
water-supersaturated conditions, i.e., in the immersion freezing
mode. Measurements were done using a flow tube (the Leipzig Aerosol Cloud Interaction Simulator, LACIS) and
a continuous-flow diffusion chamber (CFDC). Pure and coated
particles were used, with coating thicknesses of a few nanometers or
less, where the coating consisted of levoglucosan, succinic
acid or sulfuric acid. In general, it was found that the coatings
strongly reduced deposition ice nucleation. Remaining ice formation
in the water-subsaturated regime could be attributed to immersion
freezing, with particles immersed in concentrated solutions formed
by the coatings.
In the immersion freezing mode, ice nucleation rate coefficients
jhet from both instruments agreed well with each other, particularly when
the residence times in the instruments were accounted for. Fluka
kaolinite particles coated with either levoglucosan or succinic acid
showed the same IN activity as pure Fluka kaolinite particles;
i.e., it can be assumed that these two types of coating did not alter
the ice-active surface chemically, and that the coatings were
diluted enough in the droplets that were formed prior to the ice
nucleation, so that freezing point depression was
negligible. However, Fluka kaolinite particles, which were either coated
with pure sulfuric acid or were first coated with the
acid and then exposed to additional water vapor, both showed
a reduced ability to nucleate ice compared to the pure
particles. For the CMS kaolinite particles, the ability to nucleate
ice in the immersion freezing mode was similar for all examined
particles, i.e., for the pure ones and the ones with the different
types of coating. Moreover, jhet derived for the CMS
kaolinite particles was comparable to jhet derived for
Fluka kaolinite particles coated with sulfuric acid. This is suggestive
for the Fluka kaolinite possessing a type of ice-nucleating surface
feature which is not present on the CMS kaolinite, and which can be
destroyed by reaction with sulfuric acid. This might be potassium
feldspar
Electronic structure and magnetic properties of RMnX (R= Mg, Ca, Sr, Ba, Y; X= Si, Ge) studied by KKR method
Electronic structure calculations, using the charge and spin self-consistent
Korringa- Kohn-Rostoker (KKR) method, have been performed for several Mn
compounds ( = Mg, Ca, Sr, Ba, Y; = Si, Ge) of the CeFeSi-type structure.
The origin of their magnetic properties has been investigated emphasizing the
role of the Mn sublattice. The significant influence of the Mn-Mn and Mn-
interatomic distances on the Mn magnetic moment value is delineated from our
computations, supporting many neutron diffraction data. We show that the marked
change of with the Mn-Mn and Mn- distances resulted from a
redistribution between spin-up and spin-down -Mn DOS rather than from
different fillings of the Mn 3-shell. Bearing in mind that the neutron
diffraction data reported for the Mn compounds are rather scattered, the
KKR computations of are in fair agreement with the experimental
values. Comparing density of states near obtained in different magnetic
orderings, one can notice that the entitled Mn systems seem to 'adapt'
their magnetic structures to minimize the DOS in the vicinity of the Fermi
level. Noteworthy, the SrMnGe antiferromagnet exhibits a pseudo-gap behaviour
at , suggesting anomalous electron transport properties. In addition,
the F-AF transition occurring in the disordered LaYMnSi alloy for
the range is well supported by the DOS features of
LaYMnSi. In contrast to the investigated Mn compounds,
YFeSi was found to be non-magnetic, which is in excellent agreement with the
experimental data.Comment: 10 pages + 14 figures, to appear in Eur. Phys. Jour.
The Ny-Ă lesund Aerosol Cloud Experiment (NASCENT): Overview and First Results
The Arctic is warming at more than twice the rate of the global average. This warming is influenced by clouds, which modulate the solar and terrestrial radiative fluxes and, thus, determine the surface energy budget. However, the interactions among clouds, aerosols, and radiative fluxes in the Arctic are still poorly understood. To address these uncertainties, the Ny-Ă
lesund Aerosol Cloud Experiment (NASCENT) study was conducted from September 2019 to August 2020 in Ny-Ă
lesund, Svalbard. The campaignâs primary goal was to elucidate the life cycle of aerosols in the Arctic and to determine how they modulate cloud properties throughout the year. In situ and remote sensing observations were taken on the ground at sea level, at a mountaintop station, and with a tethered balloon system. An overview of the meteorological and the main aerosol seasonality encountered during the NASCENT year is introduced, followed by a presentation of first scientific highlights. In particular, we present new findings on aerosol physicochemical and molecular properties. Further, the role of cloud droplet activation and ice crystal nucleation in the formation and persistence of mixed-phase clouds, and the occurrence of secondary ice processes, are discussed and compared to the representation of cloud processes within the regional Weather Research and Forecasting Model. The paper concludes with research questions that are to be addressed in upcoming NASCENT publications
Integrating laboratory and field data to quantify the immersion freezing ice nucleation activity of mineral dust particles
Data from both laboratory studies and atmospheric measurements are used to
develop an empirical parameterization for the immersion freezing activity of
natural mineral dust particles. Measurements made with the Colorado State
University (CSU) continuous flow diffusion chamber (CFDC) when processing
mineral dust aerosols at a nominal 105% relative humidity with respect
to water (RHw) are taken as a measure of the immersion freezing
nucleation activity of particles. Ice active frozen fractions vs. temperature
for dusts representative of Saharan and Asian desert sources were consistent
with similar measurements in atmospheric dust plumes for a limited set of
comparisons available. The parameterization developed follows the form of one
suggested previously for atmospheric particles of non-specific composition in
quantifying ice nucleating particle concentrations as functions of
temperature and the total number concentration of particles larger than
0.5 ÎŒm diameter. Such an approach does not explicitly account for
surface area and time dependencies for ice nucleation, but sufficiently
encapsulates the activation properties for potential use in regional and
global modeling simulations, and possible application in developing remote
sensing retrievals for ice nucleating particles. A calibration factor is
introduced to account for the apparent underestimate (by approximately 3, on
average) of the immersion freezing fraction of mineral dust particles for CSU
CFDC data processed at an RHw of 105% vs. maximum fractions
active at higher RHw. Instrumental factors that affect activation
behavior vs. RHw in CFDC instruments remain to be fully explored
in future studies. Nevertheless, the use of this calibration factor is
supported by comparison to ice activation data obtained for the same aerosols
from Aerosol Interactions and Dynamics of the Atmosphere (AIDA) expansion
chamber cloud parcel experiments. Further comparison of the new
parameterization, including calibration correction, to predictions of the
immersion freezing surface active site density parameterization for mineral
dust particles, developed separately from AIDA experimental data alone, shows
excellent agreement for data collected in a descent through a Saharan aerosol
layer. These studies support the utility of laboratory measurements to obtain
atmospherically relevant data on the ice nucleation properties of dust and
other particle types, and suggest the suitability of considering all mineral
dust as a single type of ice nucleating particle as a useful first-order
approximation in numerical modeling investigations
A comprehensive characterization of ice nucleation by three different types of cellulose particles immersed in water
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 (1ââC) data over a wide T range (â36ââCâ<T<â4ââC). 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 (Îlog(ns,geo)/ÎT) 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 ââ10ââC. 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)
The relevance of nanoscale biological fragments for ice nucleation in clouds
Most studies of the role of biological entities as atmospheric ice-nucleating particles have focused on relatively rare supermicron particles such as bacterial cells, fungal spores and pollen grains. However, it is not clear that there are sufficient numbers of these particles in the atmosphere to strongly influence clouds. Here we show that the ice-nucleating activity of a fungus from the ubiquitous genus Fusarium is related to the presence of nanometre-scale particles which are far more numerous, and therefore potentially far more important for cloud glaciation than whole intact spores or hyphae. In addition, we quantify the ice-nucleating activity of nano-ice nucleating particles (nano-INPs) washed off pollen and also show that nano-INPs are present in a soil sample. Based on these results, we suggest that there is a reservoir of biological nano-INPs present in the environment which may, for example, become aerosolised in association with fertile soil dust particles
Climate warming and decreasing total column ozone over the Tibetan Plateau during winter and spring
The long-term trends of the total column ozone (TCO) over the Tibetan Plateau (TP) and factors responsible for the trends are analysed in this study using various observations and a chemistryâclimate model (CCM). The results indicate that the total column ozone low (TOL) over the TP during winter and spring is deepening over the recent decade, which is opposite to the recovery signal in annual mean TCO over the TP after mid-1990s. The TOL intensity is increasing at a rate of 1.4 DU/decade and the TOL area is extending with 50,000 km2/decade during winter for the period 1979â2009. The enhanced transport of ozone-poor air into the stratosphere and elevated tropopause due to the rapid and significant warming over the TP during winter reduce ozone concentrations in the upper troposphere and lower stratosphere and hence lead to the deepening of the TOL. Based on the analysis of the multiple regression model, the thermal dynamical processes associated with the TP warming accounts for more than 50% of TCO decline during winter for the period 1979â2009. The solar variations during 1995â2009 further enlarge ozone decreases over the TP in the past decade. According to the CCM simulations, the increases in NOx emissions in East Asia and global tropospheric N2O mixing ratio for the period 1979â2009 contribute to no more than 20% reductions in TCO during this period
The study of atmospheric ice-nucleating particles via microfluidically generated droplets
Ice-nucleating particles (INPs) play a significant role in the climate and hydrological cycle by triggering ice formation in supercooled clouds, thereby causing precipitation and affecting cloud lifetimes and their radiative properties. However, despite their importance, INP often comprise only 1 in 10Âłâ10ⶠambient particles, making it difficult to ascertain and predict their type, source, and concentration. The typical techniques for quantifying INP concentrations tend to be highly labour-intensive, suffer from poor time resolution, or are limited in sensitivity to low concentrations. Here, we present the application of microfluidic devices to the study of atmospheric INPs via the simple and rapid production of monodisperse droplets and their subsequent freezing on a cold stage. This device offers the potential for the testing of INP concentrations in aqueous samples with high sensitivity and high counting statistics. Various INPs were tested for validation of the platform, including mineral dust and biological species, with results compared to literature values. We also describe a methodology for sampling atmospheric aerosol in a manner that minimises sampling biases and which is compatible with the microfluidic device. We present results for INP concentrations in air sampled during two field campaigns: (1) from a rural location in the UK and (2) during the UKâs annual Bonfire Night festival. These initial results will provide a route for deployment of the microfluidic platform for the study and quantification of INPs in upcoming field campaigns around the globe, while providing a benchmark for future lab-on-a-chip-based INP studies
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