Size-segregated particle samples were collected in the Arctic (Ny-Ålesund, Svalbard) in April 2011 both at ground level and in the free atmosphere exploiting a tethered balloon equipped also with an optical particle counter (OPC) and meteorological sensors. Individual particle properties were investigated by scanning electron microscopy coupled with energy dispersive microanalysis (SEM-EDS). Results of the SEM-EDS were integrated with particle size and optical measurements of the aerosols properties at ground level and along the vertical profiles. Detailed analysis of two case studies reveals significant differences in composition despite the similar structure (layering) and the comparable texture (grain size distribution) of particles in the air column. Differences in the mineral chemistry of samples point at both local (plutonic/metamorphic complexes in Svalbard) and remote (basic/ultrabasic magmatic complexes in Greenland and/or Iceland) geological source regions for dust. Differences in the particle size and shape are put into relationship with the mechanism of particle formation, that is, primary (well sorted, small) or secondary (idiomorphic, fine to coarse grained) origin for chloride and sulfate crystals and transport/settling for soil (silicate, carbonate and metal oxide) particles. The influence of size, shape, and mixing state of particles on ice nucleation and radiative properties is also discussed.
The Arctic is the world region mostly affected by climate change [
Arctic aerosol is believed to play a relevant role in climate-environment feedbacks by scattering and absorbing the solar radiation and by altering cloud properties [
An important consideration for both the radiation budget and cloud properties is the vertical distribution of aerosol particles in the troposphere. Studies on the planetary boundary layer (PBL) and the lower troposphere in the Arctic by lidar, radiometric and meteorological measurements revealed typical multilayered structures [
Based on above remarks, in spring 2011, an intensive field campaign was carried out at the Italian station Dirigibile Italia in Ny-Ålesund (79°N, Svalbard) in the framework of the “ARCTICA” PRIN (Progetto di Ricerca di Interesse Nazionale) project. The campaign was focused on study of chemical and physical properties of aerosols over there and long-range transport processes [
In this paper results of individual particle characterization by scanning electron microscopy coupled with energy dispersive microanalysis (SEM-EDS) are reported and discussed. These results are integrated with particle size and optical (scattering, absorption) measurements of the aerosols at ground level and with optical particle counter (OPC) measurements along the vertical profiles, in order to improve our knowledge of the structure and processes in the lower Arctic Boundary Layer.
The field campaign was carried out at the Gruvebadet site, Ny-Ålesund, between March 20 and May 5, 2011. Due to weather conditions, valid measurements were collected from April 5 to April 30. A total of 84 vertical profiles of variable height have been carried out up to 1 km using a tethered balloon equipped with a gondola developed for this field campaign. During each profile particle size distribution (0.5–20
The synoptic conditions at Ny-Ålesund in April 2011 were characterized by low pressure in the first half of the month followed by a high pressure system. April 18 marked the transition of weather conditions. Before April 18 the mean values of the geopotential height at 850 and 500 mbar in the Ny-Ålesund area were 1208 and 5119 m; they turned to 1346 and 5311 m after that date (ECMWF).
Accordingly at ground (Figure
Synopsis of April 2011. Data collected at the Amundsen-Nobile Climate Change Tower (CCT;
On April 18 the pressure level rapidly increased while the temperature reached a mean value of about −7.4°C. The wind speed at 10 m above ground was quite low (<4 m/s) from ESE direction. Sporadic cloud events were also registered between 3000 and 6000 m altitude. April 29 was a typical synoptic high pressure situation. During the day, ground pressure was rising and surface air temperature was constantly below or around the freezing point (Figure
Air mass backward trajectories (cluster means) on April 18 and April 29 are shown in Figure
Clustered 5 d back-trajectories (BTs) for Apr 18 (a) and Apr 29 (b), 2011, calculated every 1 h using the NOAA HYSPLIT transport model [
The Gruvebadet laboratory at the Dirigibile Italia Base in Ny-Ålesund is equipped with a series of instruments aimed at measuring physical and optical properties of aerosol at ground level. In particular size distribution measurements are carried out by an Aerodynamic Particle Sizer (TSI-APS 3321, 52 classes in the size range 0.5–20
The scattering coefficient (Bsca) at 530 nm is measured by a Radiance Research M903 nephelometer at 1 s time resolution and stored as minute average. The absorption coefficient (Babs) is measured using a Radiance Research Particle Soot Absorption Photometer (PSAP) at three wavelengths (467, 530, and 600 nm), with the same temporal resolution.
Aerosol properties at the ground level during Apr 2011 are shown in Figure
Integrated particle number concentration over the size range measured (cm−3) and optical properties (scattering, extinction, and absorption coefficient at different wavelength; cm−1) at ground level (Gruvebadet station) in April 2011. The grey areas in the figure mark April 18 and April 29.
Vertical profile measurements have been carried out by means of a helium-filled tethered balloon (VAISALA; volume ~7 m3; payload ~5 kg) equipped with (i) an optical particle counter (OPC, 1.107
Meteorological conditions (temperature:
Data |
|
RH (%) | WS (m/s) | WD | Profiles (UTC) | Ground (UTC) | Balloon (UTC) | Height (m asl) |
---|---|---|---|---|---|---|---|---|
18 Apr | −9 | 52 | 5 | NW | 17.25–17.35 | 15.20 (4 h) | 16.45 (3 h) | 250 m |
29 Apr | −1.2 | 99 | 1.6 | SE | 15.48–15.57 | 15.50 (4 h) | 16.45 (3 h) | 260 m |
For individual particle characterization, single portions (~10 × 10 mm) were cut from the central part of the sampling filters and mounted on to SEM aluminium stubs using double-sided carbon tape. Samples were then coated with a 100–150 Å carbon film to provide electrical conductivity and to prevent charge build-up during the exposure to the electron beam.
Conventional SEM-EDS microanalyses were performed at the Centro Universitario di Microscopia Elettronica (CUME), University of Perugia (Italy), using a Philips XL30 instrument equipped with an X-ray energy dispersive spectrometer (EDS-EDAX DX-4I, coupled with GENESIS software for data treatment). The instrument was operated at 20 kV acceleration voltage and variable magnification (2,000 to 20,000x) depending on particle size. EDS spectra (spot size 5, working distance 10 mm) were collected for 30 s live time and the X-ray count rates were corrected for matrix effects using the so-called “standardless” procedures provided by the GENESIS software. The analysis started in the middle of the sample, the scan probe was moved on a chosen direction, and the particles were analyzed manually one by one at the proper magnification. Using this procedure approximately 100 particles per filter were analysed. Indeed, this is a small number of particles in itself, but it was obtained on very large portions of the filters (~0.5 mm2) as the number density of particles on the filters was generally very low. On the other hand, individual particle analysis performed on a nonautomated instrument on disperse samples, like in this case, is very time consuming. We, thus, believe that the number of particles analysed per sample is an acceptable compromise to obtain representative results at reasonable analytical time.
The samples collected on 29 April 2011, which were particularly enriched in particles, underwent detailed individual particle characterization by computer controlled SEM analysis (CCSEM). In this procedure, software scans fields one at a time in automated mode, detecting the single particles and recording morphology (digitalized image) and chemical composition (spectrum) data on a large number of particles. Measurements were performed at the Department of Physics, University of Fribourg (Switzerland), using a FEI XL 30 Sirion FEG environmental scanning electron microscope equipped with an EDAX Pegasus EDS system. The system was operated at 25 kV acceleration voltage and fixed magnification (2000x). Analyses were performed on selected representative fields of the filter after evaluation of sample uniformity, adequate greyscale calibration, and proper definition of the working parameters. EDS spectra of more than 2000 particles were collected for 15 s live time and the chemical composition (elements with
All EDS spectra were manually reviewed and particles whose signal was too weak were discarded. Element concentrations lower than 0.1 weight % (SEM detection limit) were omitted. According to Kandler et al. [
Digitalized images underwent image analysis (IA). In the conventional procedure the manually selected particles were analysed using the software Image Tool 3.0 (
Examples of aerosol vertical profiles recorded on the 18 and 29 of April are reported in Figure
(a) Vertical profiles of particle number concentration of fine (solid lines) and coarse particles (dashed lines) on April 18 (blue lines) and April 29, 2011 (red lines); (b) temperature (solid lines) and relative humidity profiles (dashed lines) during the measurements. The black lines mark the height of sampling at the balloon level on both days. Data were obtained from measurements performed just before the sample collection for SEM microanalyses.
When looking to the particle size distributions (Figure
Aerosol particle size distribution at different height on April 18 (a, c) and April 29 (b, d), 2011. Data were obtained from measurements performed just before the sample collection for SEM microanalyses.
Five main particle types were observed in the samples, namely, silicates, carbonates, sulfates, chlorides, and metal oxides. Soot particles have been also evidenced in a few cases. Except for soot and Cu-oxides, which are both from vehicular/combustive sources, the rest of particles are from natural (soil) sources. Particles in most cases consist of internally or externally mixed solid phases. The pie plots in Figure
Relative abundance (% number) of the particle types in the total suspended particulate matter at ground and balloon level on April 18 and April 29, 2011. The height of balloon sampling was 250 m asl on April 18 and 260 m on April 29. Carbonaceous particles have been subtracted from the calculations. The number of particles analyzed is reported in parenthesis.
A great change in composition is observed at different levels in the lower troposphere on both days. At ground level, the chlorides (mostly Na chloride) are the main constituent of natural aerosol, followed by the silicates. Metal oxide particles and alkaline sulfates form significant parts of dust on April 18 and 29, respectively, while a few carbonate oxide particles are present in both samples. The chlorides are generally well below 1
SEM micrographs illustrating the characteristics of the particle types at ground level: (a) sodium chloride and alkaline sulfates; (b) silicate and sulfate deposits. In (b) the roundish area containing the grains marks the original shape of a water droplet. Qz: quartz; Gy: gypsum; silicates: undefined.
At 260 m asl the silicates, mostly in the form of quartz and/or clay sheet minerals, are the main constituent of airborne dust; at this height a remarkable part of gypsum along with significant amounts of metal oxides is also present on April 29 (Figure
SEM micrographs illustrating the characteristics of the particle types at 260 m asl: (a) silicates and metal oxides (Pl: plagioclase; Alk-feld: alkali feldspar; Qz: quartz; sheet: undefined sheet clay mineral; Fe-ox: iron oxide); (b) gypsum crystals.
In the samples collected on April 29, whose quantity was sufficiently high to obtain a significant number of particles for analysis, the mineral chemistry of the metal particles and the sheet silicates has been evaluated for source identification purposes.
Most metal particles in this sample are associated with Fe-Cr spinel oxides; a significant part of Fe-oxides and/or hydroxides is also present, along with a small number of Fe- and/or Ti-oxides (Figure
Chemical composition of the metal particles in the aerosol dust sampled at 260 m asl on April 29.
Sheet silicates sampled at ground level are slightly enriched in Al and Fe, while those sampled at 260 m asl are enriched in Mg (Figure
Chemical composition of the sheet minerals in the aerosol dust sampled at 260 m asl.
Ice crystals in the atmosphere have important impacts on radiative transfer, precipitation formation, and the microphysical and optical properties of clouds. Ice is formed in clouds either by homogeneous freezing of water and solution droplets at temperatures below about −35°C or by heterogeneous ice nucleation processes induced by insoluble aerosol particles [
The particle size distribution of the silicate minerals upon Ny-Ålesund shows two main modes in the submicron size range (Figure
Frequency plot of Feret diameter in the silicate particles collected at 260 m asl upon Ny-Ålesund on April 29. Data obtained by CCSEM individual particle image analysis.
Frequency plots of the shape factor, SF, of sulfate and chloride particles: (a) at 260 m asl; (b) at ground level.
The size, the morphology, and the composition of sea salt particles have been found to exert direct effects on the aerosol radiative impact [
In this study, some aspects of aerosol characterization upon Svalbard Islands have been discussed focusing on the properties of the constituent particle types at different height in the troposphere. Detailed analysis of selected samples representing typical synoptic conditions revealed significant differences in composition despite the similar structure (layering) and the comparable texture (size distribution) of the particles in the air column. These differences have been explained considering the combined action of transport (long, short range) and aging (settling by gravity, salt hygroscopic growth) on the particle properties.
The mineral chemistry of different particle types revealed interesting differences among the samples. In particular, in the case of the metal oxides, the specific geochemical features of different particle types led to distinguishing different geological sources for the particles. These features make metal oxides good markers of geographic provenance, provided a consistent geochemical databank of representative metal oxide particle populations from different geological sources/source regions is arranged. The question of provenance should be, thus, addressed following a more detailed analytical protocol as for the number of particles and, especially, the reference samples for comparison. Combined with bulk geochemical records and back-trajectory statistical evaluation, this approach could be exploited to evaluate the contribution of different source areas to the aerosol particles in the site.
Both the origin and the source regions of particles deeply mark their properties. Differences in the shape, size, and the state of mixing of the particles have been put into relationship with their origin and evolution within the air column over the site. In particular, the size of the silicates significantly affects their ability to act as cloud condensation and/or ice nucleation active sites, while the shape of sulfate minerals may influence the scattering and absorption properties of the aerosol at different level in the troposphere depending on the extent of aging processes (settling, layering). These latter are able, on turn, to influence the radiative budget in the troposphere through direct (light scattering and absorption) and indirect (cloud formation and evolution, cloud microphysical and optical properties) effects.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors thank Bernard Grobety and Christophe Neururer, University of Fribourg, for valuable support and precious help in the CCSEM analyses. This study was funded by the Italian Ministry of University and Research (PRIN-2009 project). The logistic assistance of the Polar Support Unit of the CNR Department of Earth and Environment (POLARNET) is gratefully acknowledged.