Number
concentrations of ice-nucleating particles (NINP) in the Arctic
were derived from ground-based filter samples. Examined samples had been
collected in Alert (Nunavut, northern Canadian ...archipelago on Ellesmere
Island), Utqiaġvik, formerly known as Barrow (Alaska), Ny-Ålesund
(Svalbard), and at the Villum Research Station (VRS; northern Greenland). For
the former two stations, examined filters span a full yearly cycle. For VRS,
10 weekly samples, mostly from different months of one year, were included.
Samples from Ny-Ålesund were collected during the months from March until
September of one year. At all four stations, highest concentrations were
found in the summer months from roughly June to September. For those stations
with sufficient data coverage, an annual cycle can be seen. The spectra of
NINP observed at the highest temperatures, i.e., those obtained
for summer months, showed the presence of INPs that nucleate ice up to
−5 ∘C. Although the nature of these highly ice-active INPs could
not be determined in this study, it often has been described in the
literature that ice activity observed at such high temperatures originates
from the presence of ice-active material of biogenic origin. Spectra observed
at the lowest temperatures, i.e., those derived for winter months, were on
the lower end of the respective values from the literature on Arctic INPs or
INPs from midlatitude continental sites, to which a comparison is presented
herein. An analysis concerning the origin of INPs that were ice active at
high temperatures was carried out using back trajectories and satellite
information. Both terrestrial locations in the Arctic and the adjacent sea
were found to be possible source areas for highly active INPs.
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The presented data from literature and previously unpublished data from over 23 months of ship-based measurements stretch from the Arctic to the Southern Ocean and include a circumnavigation of Antarctica.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.
Ice crystal formation in atmospheric clouds has a strong effect on precipitation, cloud lifetime, cloud radiative properties, and thus the global energy budget. Primary ice formation above 235 K is ...initiated by nucleation on seed aerosol particles called ice-nucleating particles (INPs). Instruments that measure the ice-nucleating potential of aerosol particles in the atmosphere need to be able to accurately quantify ambient INP concentrations. In the last decade several instruments have been developed to investigate the ice-nucleating properties of aerosol particles and to measure ambient INP concentrations. Therefore, there is a need for intercomparisons to ensure instrument differences are not interpreted as scientific findings.In this study, we intercompare the results from parallel measurements using four online ice nucleation chambers. Seven different aerosol types are tested including untreated and acid-treated mineral dusts (microcline, which is a K-feldspar, and kaolinite), as well as birch pollen washing waters. Experiments exploring heterogeneous ice nucleation above and below water saturation are performed to cover the whole range of atmospherically relevant thermodynamic conditions that can be investigated with the intercompared chambers. The Leipzig Aerosol Cloud Interaction Simulator (LACIS) and the Portable Immersion Mode Cooling chAmber coupled to the Portable Ice Nucleation Chamber (PIMCA-PINC) performed measurements in the immersion freezing mode. Additionally, two continuous-flow diffusion chambers (CFDCs) PINC and the Spectrometer for Ice Nuclei (SPIN) are used to perform measurements below and just above water saturation, nominally presenting deposition nucleation and condensation freezing.The results of LACIS and PIMCA-PINC agree well over the whole range of measured frozen fractions (FFs) and temperature. In general PINC and SPIN compare well and the observed differences are explained by the ice crystal growth and different residence times in the chamber. To study the mechanisms responsible for the ice nucleation in the four instruments, the FF (from LACIS and PIMCA-PINC) and the activated fraction, AF (from PINC and SPIN), are compared. Measured FFs are on the order of a factor of 3 higher than AFs, but are not consistent for all aerosol types and temperatures investigated. It is shown that measurements from CFDCs cannot be assumed to produce the same results as those instruments exclusively measuring immersion freezing. Instead, the need to apply a scaling factor to CFDCs operating above water saturation has to be considered to allow comparison with immersion freezing devices. Our results provide further awareness of factors such as the importance of dispersion methods and the quality of particle size selection for intercomparing online INP counters.
Within the framework of the RACEPAC (Radiation–Aerosol–Cloud Experiment in
the Arctic Circle) project, the Arctic aerosol, arriving at a ground-based
station in Tuktoyaktuk (Mackenzie River delta ...area, Canada), was
characterized during a period of 3 weeks in May 2014. Basic meteorological
parameters and particle number size distributions (PNSDs) were observed and
two distinct types of air masses were found. One type were typical Arctic
haze air masses, termed accumulation-type air masses, characterized by a
monomodal PNSD with a pronounced accumulation mode at sizes above
100 nm. These air masses were observed during a period when back
trajectories indicate an air mass origin in the north-east of Canada. The
other air mass type is characterized by a bimodal PNSD with a clear minimum
around 90 nm and with an Aitken mode consisting of freshly formed
aerosol particles. Back trajectories indicate that these air masses, termed
Aitken-type air masses, originated from the North Pacific. In addition, the
application of the PSCF receptor model shows that air masses with their
origin in active fire areas in central Canada and Siberia, in areas of
industrial anthropogenic pollution (Norilsk and Prudhoe Bay Oil Field) and
the north-west Pacific have
enhanced total particle number concentrations (NCN). Generally,
NCN ranged from 20 to 500 cm−3, while cloud
condensation nuclei (CCN) number concentrations were found to cover a range
from less than 10 up to 250 cm−3 for a supersaturation (SS)
between 0.1 and 0.7 %. The hygroscopicity parameter κ of the
CCN was determined to be 0.23 on average and variations in κ were
largely attributed to measurement uncertainties. Furthermore, simultaneous
PNSD measurements at the ground station and on the Polar 6 research
aircraft were performed. We found a good agreement of ground-based PNSDs
with those measured between 200 and 1200 m. During two of the four
overflights, particle number concentrations at 3000 m were found to
be up to 20 times higher than those measured below 2000 m;
for one of these two flights, PNSDs measured above 2000 m showed a
different shape than those measured at lower altitudes. This is indicative of
long-range transport from lower latitudes into the Arctic that can advect
aerosol from different regions in different heights.
For three austral summer seasons (2013–2016, each from December to February) aerosol particles arriving at the Belgian Antarctic research station Princess Elisabeth (PE) in Dronning Maud Land in East ...Antarctica were characterized. This included number concentrations of total aerosol particles (NCN) and cloud condensation nuclei (NCCN), the particle number size distribution (PNSD), the aerosol particle hygroscopicity, and the influence of the air mass origin on NCN and NCCN. In general NCN was found to range from 40 to 6700 cm−3, with a median of 333 cm−3, while NCCN was found to cover a range between less than 10 and 1300 cm−3 for supersaturations (SSs) between 0.1 % and 0.7 %. It is shown that the aerosol is dominated by the Aitken mode, being characterized by a significant amount of small, and therefore likely secondarily formed, aerosol particles, with 94 % and 36 % of the aerosol particles smaller than 90 and ≈35 nm, respectively. Measurements of the basic meteorological parameters as well as the history of the air masses arriving at the measurement station indicate that the station is influenced by both marine air masses originating from the Southern Ocean and coastal areas around Antarctica (marine events – MEs) and continental air masses (continental events – CEs). CEs, which were defined as instances when the air masses spent at least 90 % of the time over the Antarctic continent during the last 10 days prior to arrival at the measurements station, occurred during 61 % of the time during which measurements were done. CEs came along with rather constant NCN and NCCN values, which we denote as Antarctic continental background concentrations. MEs, however, cause large fluctuations in NCN and NCCN, with low concentrations likely caused by scavenging due to precipitation and high concentrations likely originating from new particle formation (NPF) based on marine precursors. The application of HYSPLIT back trajectories in form of the potential source contribution function (PSCF) analysis indicate that the region of the Southern Ocean is a potential source of Aitken mode particles. On the basis of PNSDs, together with NCCN measured at an SS of 0.1 %, median values for the critical diameter for cloud droplet activation and the aerosol particle hygroscopicity parameter κ were determined to be 110 nm and 1, respectively. For particles larger than ≈110 nm the Southern Ocean together with parts of the Antarctic ice shelf regions were found to be potential source regions. While the former may contribute sea spray particles directly, the contribution of the latter may be due to the emission of sea salt aerosol particles, released from snow particles from surface snow layers, e.g., during periods of high wind speed, leading to drifting or blowing snow. The region of the Antarctic inland plateau, however, was not found to feature a significant source region for aerosol particles in general or for cloud condensation nuclei measured at the PE station in the austral summer.
There are strong indications that particles containing secondary organic aerosol (SOA) exhibit amorphous solid or semi-solid phase states in the atmosphere. This may facilitate heterogeneous ice ...nucleation and thus influence cloud properties. However, experimental ice nucleation studies of biogenic SOA are scarce. Here, we investigated the ice nucleation ability of viscous SOA particles. The SOA particles were produced from the ozone initiated oxidation of α-pinene in an aerosol chamber at temperatures in the range from −38 to −10 °C at 5–15 % relative humidity with respect to water to ensure their formation in a highly viscous phase state, i.e. semi-solid or glassy. The ice nucleation ability of SOA particles with different sizes was investigated with a new continuous flow diffusion chamber. For the first time, we observed heterogeneous ice nucleation of viscous α-pinene SOA for ice saturation ratios between 1.3 and 1.4 significantly below the homogeneous freezing limit. The maximum frozen fractions found at temperatures between −39.0 and −37.2 °C ranged from 6 to 20 % and did not depend on the particle surface area. Global modelling of monoterpene SOA particles suggests that viscous biogenic SOA particles are indeed present in regions where cirrus cloud formation takes place. Hence, they could make up an important contribution to the global ice nucleating particle budget.
Ice surface properties can modify the scattering properties of atmospheric
ice crystals and therefore affect the radiative properties of mixed-phase and
cirrus clouds. The Ice Roughness Investigation ...System (IRIS) is a new
laboratory setup designed to investigate the conditions under which roughness
develops on single ice crystals, based on their size, morphology and growth
conditions (relative humidity and temperature). Ice roughness is quantified
through the analysis of speckle in 2-D light-scattering patterns.
Characterization of the setup shows that a supersaturation of 20 % with
respect to ice and a temperature at the sample position as low as
−40 ∘C could be achieved within IRIS. Investigations of the influence
of humidity show that higher supersaturations with respect to ice lead to
enhanced roughness and irregularities of ice crystal surfaces. Moreover,
relative humidity oscillations lead to gradual “ratcheting-up” of
roughness and irregularities, as the crystals undergo repeated
growth–sublimation cycles. This memory effect also appears to result in
reduced growth rates in later cycles. Thus, growth history, as well as
supersaturation and temperature, influences ice crystal growth and
properties, and future atmospheric models may benefit from its inclusion in
the cloud evolution process and allow more accurate representation of not
just roughness but crystal size too, and possibly also electrification
properties.
The marine aerosol arriving at Barbados (Ragged Point) was characterized during two 3-week long measurement periods in November 2010 and April 2011, in the context of the measurement campaign CARRIBA ...(Cloud, Aerosol, Radiation and tuRbulence in the trade wInd regime over BArbados). Through a comparison between ground-based and airborne measurements it was shown that the former are representative of the marine boundary layer at least up to cloud base. In general, total particle number concentrations (Ntotal) ranged from as low as 100 up to 800 cm−3, while number concentrations for cloud condensation nuclei (NCCN) at a supersaturation of 0.26 % ranged from some 10 to 600 cm−3. Ntotal and NCCN depended on the air mass origin. Three distinct types of air masses were found. One type showed elevated values for both Ntotal and NCCN and could be attributed to long-range transport from Africa, by which biomass burning particles from the Sahel region and/or mineral dust particles from the Sahara were advected. The second and third type both had values for NCCN below 200 cm−3 and a clear minimum in the particle number size distribution (NSD) around 70 to 80 nm (Hoppel minimum). While for one of these two types the accumulation mode was dominating (albeit less so than for air masses advected from Africa), the Aitken mode dominated the other and contributed more than 50 % of all particles. These Aitken mode particles likely were formed by new particle formation no more than 3 days prior to the measurements. Hygroscopicity of particles in the CCN size range was determined from CCN measurements to be κ = 0.66 on average, which suggests that these particles contain mainly sulfate and do not show a strong influence from organic material, which might generally be the case for the months during which measurements were made. The average κ could be used to derive NCCN from measured number size distributions, showing that this is a valid approach to obtain NCCN. Although the total particulate mass sampled on filters was found to be dominated by Na+ and Cl−, this was found to be contributed by a small number of large particles ( > 500 nm, mostly even in the super-micron size range). Based on a three-modal fit, a sea spray mode observed in the NSDs was found to contribute 90 % to the total particulate mass but only 4 to 10 % to Ntotal and up to 15 % to NCCN. This is in accordance with finding no correlation between Ntotal and wind speed.
The second phase of the Fifth International Ice Nucleation Workshop (FIN-02)
involved the gathering of a large number of researchers at the Karlsruhe
Institute of Technology's Aerosol Interactions ...and Dynamics of the Atmosphere
(AIDA) facility to promote characterization and understanding of ice
nucleation measurements made by a variety of methods used worldwide.
Compared to the previous workshop in 2007, participation was doubled,
reflecting a vibrant research area. Experimental methods involved sampling of
aerosol particles by direct processing ice nucleation measuring systems from
the same volume of air in separate experiments using different ice nucleating
particle (INP) types, and collections of aerosol particle samples onto
filters or into liquid for sharing amongst measurement techniques that
post-process these samples. In this manner, any errors introduced by
differences in generation methods when samples are shared across laboratories
were mitigated. Furthermore, as much as possible, aerosol particle size
distribution was controlled so that the size limitations of different methods
were minimized. The results presented here use data from the workshop to
assess the comparability of immersion freezing measurement methods activating
INPs in bulk suspensions, methods that activate INPs in condensation and/or
immersion freezing modes as single particles on a substrate, continuous flow
diffusion chambers (CFDCs) directly sampling and processing particles well
above water saturation to maximize immersion and subsequent freezing of
aerosol particles, and expansion cloud chamber simulations in which liquid
cloud droplets were first activated on aerosol particles prior to freezing.
The AIDA expansion chamber measurements are expected to be the closest
representation to INP activation in atmospheric cloud parcels in these
comparisons, due to exposing particles freely to adiabatic cooling. The different particle types used as INPs included the minerals illite NX and
potassium feldspar (K-feldspar), two natural soil dusts representative of arable sandy loam
(Argentina) and highly erodible sandy dryland (Tunisia) soils, respectively,
and a bacterial INP (Snomax®). Considered
together, the agreement among post-processed immersion freezing measurements
of the numbers and fractions of particles active at different temperatures
following bulk collection of particles into liquid was excellent, with
possible temperature uncertainties inferred to be a key factor in determining
INP uncertainties. Collection onto filters for rinsing versus directly into
liquid in impingers made little difference. For methods that activated
collected single particles on a substrate at a controlled humidity at or
above water saturation, agreement with immersion freezing methods was good in
most cases, but was biased low in a few others for reasons that have not been
resolved, but could relate to water vapor competition effects. Amongst
CFDC-style instruments, various factors requiring (variable) higher
supersaturations to achieve equivalent immersion freezing activation dominate
the uncertainty between these measurements, and for comparison with bulk
immersion freezing methods. When operated above water saturation to include
assessment of immersion freezing, CFDC measurements often measured at or
above the upper bound of immersion freezing device measurements, but often
underestimated INP concentration in comparison to an immersion freezing
method that first activates all particles into liquid droplets prior to
cooling (the PIMCA-PINC device, or Portable Immersion Mode Cooling chAmber–Portable Ice Nucleation Chamber), and typically slightly underestimated INP
number concentrations in comparison to cloud parcel expansions in the AIDA
chamber; this can be largely mitigated when it is possible to raise the
relative humidity to sufficiently high values in the CFDCs, although this is
not always possible operationally. Correspondence of measurements of INPs among direct sampling and
post-processing systems varied depending on the INP type. Agreement was best
for Snomax® particles in the temperature regime
colder than −10 ∘C, where their ice nucleation activity is nearly
maximized and changes very little with temperature. At temperatures warmer than
−10 ∘C, Snomax® INP measurements (all
via freezing of suspensions) demonstrated discrepancies consistent with
previous reports of the instability of its protein aggregates that appear to
make it less suitable as a calibration INP at these temperatures. For
Argentinian soil dust particles, there was excellent agreement across all
measurement methods; measures ranged within 1 order of magnitude for INP
number concentrations, active fractions and calculated active site densities
over a 25 to 30 ∘C range and 5 to 8 orders of corresponding
magnitude change in number concentrations. This was also the case for all
temperatures warmer than −25 ∘C in Tunisian dust experiments. In
contrast, discrepancies in measurements of INP concentrations or active site
densities that exceeded 2 orders of magnitude across a broad range of temperature
measurements found at temperatures warmer than −25 ∘C in a previous study were
replicated for illite NX. Discrepancies also exceeded 2 orders of magnitude at
temperatures of −20 to −25 ∘C for potassium feldspar (K-feldspar), but these coincided
with the range of temperatures at which INP concentrations increase rapidly at
approximately an order of magnitude per 2 ∘C cooling for
K-feldspar. These few discrepancies did not outweigh the overall positive outcomes of the
workshop activity, nor the future utility of this data set or future similar
efforts for resolving remaining measurement issues. Measurements of the same
materials were repeatable over the time of the workshop and demonstrated
strong consistency with prior studies, as reflected by agreement of data
broadly with parameterizations of different specific or general (e.g., soil
dust) aerosol types. The divergent measurements of the INP activity of illite
NX by direct versus post-processing methods were not repeated for other
particle types, and the Snomax® data
demonstrated that, at least for a biological INP type, there is no expected
measurement bias between bulk collection and direct immediately processed
freezing methods to as warm as −10 ∘C. Since particle size ranges
were limited for this workshop, it can be expected that for atmospheric
populations of INPs, measurement discrepancies will appear due to the
different capabilities of methods for sampling the full aerosol size
distribution, or due to limitations on achieving sufficient water
supersaturations to fully capture immersion freezing in direct processing
instruments. Overall, this workshop presents an improved picture of present
capabilities for measuring INPs than in past workshops, and provides
direction toward addressing remaining measurement issues.
The second phase of the Fifth International Ice Nucleation Workshop (FIN-02)involved the gathering of a large number of researchers at the Karlsruhe Institute of Technology's Aerosol Interactions and ...Dynamics of the Atmosphere(AIDA) facility to promote characterization and understanding of ice nucleation measurements made by a variety of methods used worldwide.Compared to the previous workshop in 2007, participation was doubled,reflecting a vibrant research area. Experimental methods involved sampling of aerosol particles by direct processing ice nucleation measuring systems from the same volume of air in separate experiments using different ice nucleating particle (INP) types, and collections of aerosol particle samples onto filters or into liquid for sharing amongst measurement techniques tha tpost-process these samples. In this manner, any errors introduced by differences in generation methods when samples are shared across laboratories were mitigated. Furthermore, as much as possible, aerosol particle size distribution was controlled so that the size limitations of different methods were minimized. The results presented here use data from the workshop to assess the comparability of immersion freezing measurement methods activating INPs in bulk suspensions, methods that activate INPs in condensation and/or immersion freezing modes as single particles on a substrate, continuous flow diffusion chambers (CFDCs) directly sampling and processing particles well above water saturation to maximize immersion and subsequent freezing of aerosol particles, and expansion cloud chamber simulations in which liquid cloud droplets were first activated on aerosol particles prior to freezing.The AIDA expansion chamber measurements are expected to be the closest representation to INP activation in atmospheric cloud parcels in these comparisons, due to exposing particles freely to adiabatic cooling. The different particle types used as INPs included the minerals illite NX and potassium feldspar (K-feldspar), two natural soil dusts representative of arable sandy loam(Argentina) and highly erodible sandy dry land (Tunisia) soils, respectively,and a bacterial INP (Snomax®). Considered together, the agreement among post-processed immersion freezing measurements of the numbers and fractions of particles active at different temperatures following bulk collection of particles into liquid was excellent, with possible temperature uncertainties inferred to be a key factor in determining INP uncertainties. Collection onto filters for rinsing versus directly into liquid in impingers made little difference. For methods that activated collected single particles on a substrate at a controlled humidity at or above water saturation, agreement with immersion freezing methods was good inmost cases, but was biased low in a few others for reasons that have not been resolved, but could relate to water vapor competition effects. Amongst CFDC-style instruments, various factors requiring (variable) higher supersaturations to achieve equivalent immersion freezing activation dominate the uncertainty between these measurements, and for comparison with bulk immersion freezing methods. When operated above water saturation to include assessment of immersion freezing, CFDC measurements often measured at or above the upper bound of immersion freezing device measurements, but often underestimated INP concentration in comparison to an immersion freezing method that first activates all particles into liquid droplets prior to cooling (the PIMCA-PINC device, or Portable Immersion Mode Cooling chAmber–Portable Ice Nucleation Chamber), and typically slightly underestimated INP number concentrations in comparison to cloud parcel expansions in the AIDA chamber; this can be largely mitigated when it is possible to raise the relative humidity to sufficiently high values in the CFDCs, although this isnot always possible operationally. Correspondence of measurements of INPs among direct sampling and post-processing systems varied depending on the INP type. Agreement was best for Snomax® particles in the temperature regime colder than -10 ° C, where their ice nucleation activity is nearly maximized and changes very little with temperature. At temperatures warmer than -10 ° C, Snomax® INP measurements (all via freezing of suspensions) demonstrated discrepancies consistent with previous reports of the instability of its protein aggregates that appear to make it less suitable as a calibration INP at these temperatures. For Argentinian soil dust particles, there was excellent agreement across all measurement methods; measures ranged within 1 order of magnitude for INP number concentrations, active fractions and calculated active site densities over a 25 to 30 ° C range and 5 to 8 orders of corresponding magnitude change in number concentrations. This was also the case for all temperatures warmer than -25 ° C in Tunisian dust experiments. In contrast, discrepancies in measurements of INP concentrations or active site densities that exceeded 2 orders of magnitude across a broad range of temperature measurements found at temperatures warmer than -25 ° C in a previous study were replicated for illite NX. Discrepancies also exceeded 2 orders of magnitude at temperatures of -20 to -25 ° C for potassium feldspar (K-feldspar), but these coincided with the range of temperatures at which INP concentrations increase rapidly at approximately an order of magnitude per 2 ° C cooling forK-feldspar. These few discrepancies did not outweigh the overall positive outcomes of the workshop activity, nor the future utility of this data set or future similar efforts for resolving remaining measurement issues. Measurements of the same materials were repeatable over the time of the workshop and demonstrated strong consistency with prior studies, as reflected by agreement of data broadly with para meterizations of different specific or general (e.g., soil dust) aerosol types. The divergent measurements of the INP activity of illiteNX by direct versus post-processing methods were not repeated for other particle types, and the Snomax® data demonstrated that, at least for a biological INP type, there is no expected measurement bias between bulk collection and direct immediately processed freezing methods to as warm as -10 ° C. Since particle size ranges were limited for this workshop, it can be expected that for atmospheric populations of INPs, measurement discrepancies will appear due to the different capabilities of methods for sampling the full aerosol size distribution, or due to limitations on achieving sufficient water supersaturations to fully capture immersion freezing in direct processing instruments. Overall, this workshop presents an improved picture of present capabilities for measuring INPs than in past workshops, and provides direction toward addressing remaining measurement issues.
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