The Arctic is one of the most vulnerable regions affected by climate change.
Extensive measurement data are needed to understand the atmospheric
processes governing this vulnerability. Among these, data describing cloud
formation potential are of particular interest, since the indirect effect of
aerosols on the climate system is still poorly understood. In this paper we
present, for the first time, size-resolved cloud condensation nuclei (CCN)
data obtained in the Arctic. The measurements were conducted during two
periods in the summer of 2008: one in June and one in August, at the
Zeppelin research station (78
The Arctic represents a region of special interest for atmospheric research because it is (i) very sensitive to changes in radiative forcing owing to a direct feedback mechanism, (ii) expecting greater anthropogenic activity from increased shipping and natural resource explorations in the near future and (iii) still poorly understood in terms of climate controlling processes, largely due to the lack of observational data. One of the most significant uncertainties in climate prediction is the role of clouds, and in particular, the influence of anthropogenic activities on clouds. In general, clouds have the ability to both cool the surface by reflecting incoming solar radiation back to space, or warm the surface by re-emitting long-wave radiation back to the surface (Boucher et al., 2013). The formation of clouds is dependent on the presence of excess water vapour in the air and on the presence of aerosol particles having cloud condensation nuclei (CCN) properties. Such particles must have sufficient size and hygroscopicity to act as sites for cloud droplet formation. In this study, two short case studies are presented, based on observations conducted in June and August 2008 at the Zeppelin station, Svalbard. These data complement the existing CCN and aerosol measurements conducted in the Arctic, but for the first time the CCN properties here are determined online as a function of dry particle size. R. H. Moore et al. (2011) have provided a brief literature review of CCN measurements in the Arctic; however, to set our study in the context of other studies and to summarize the available information concerning Arctic CCN, we also present a short literature overview, including some of the most recent studies. For clarity, data are first grouped into land-based measurements, then measurements from ships and followed by aircraft measurements.
Shaw (1986) examined the CCN spectra of air masses characterized by Arctic
haze during January and February 1985 in central Alaska. The maximum
supersaturation (SS) was found to be around 0.33 %, and the dominant CCN
consisted of soluble particles at a concentration of a few hundred per
cm
Silvergren et al. (2014) presented chemical and physical properties of
aerosols collected at the Zeppelin research station, Svalbard from September 2007 to August 2008. Hygroscopic growth and cloud-forming potential were
examined on a monthly basis. From this, it was shown that during the summer
months, the SS has the greatest impact on the number of CCN. As the aerosol
sulphate and nitrate mass concentrations reached a maximum between March and
May, it was concluded that these months presented the most unfavourable
cloud-forming properties of the entire year. From September to February, sea
salt was present in the highest mass concentrations. Both the growth factor
and the values of the hygroscopicity parameter
Bigg and Leck (2001) reported the results from CCN measurements conducted
on an icebreaker at latitudes higher than 80
The results of CCN measurements conducted during 3 weeks in August and
September 2008 on board the icebreaker “Oden”, which was drifting passively
to the north of 87
Hoppel et al. (1973) present results of aircraft measurements from February 1972 above Alaska, approximately 160 km north of Fairbanks. A strong
temperature inversion was observed during the measurements, and an increase
in CCN concentration, approximately from 100 to 400 cm
CCN data from aircraft measurements conducted during April 1992 over the
Arctic Ocean were presented by Hegg et al. (1995). Measurements took place
around 350 km from the Alaskan coast between 0.03 and 4 km altitude; they
show CCN concentration to vary between 19.9 and 92.7 cm
Results of aircraft measurements made during 11 flights over Alaska in June 1995 were published by Hegg et al. (1996) and compared to measurements presented in Hegg et al. (1995). This further study concluded that the fraction of activated particles is, on average, approximately 0.10 at a SS of 1 %. They therefore suggested that the number of smaller particles is higher during June 1995 than in the spring of 1992.
Yum and Hudson (2001) presented vertical CCN profiles obtained at least 500 km north of the Alaskan coast during a flight campaign in May 1998. They
observed a clear increase in CCN concentration with an increase in altitude
when low stratus clouds were present. However, under non-cloudy conditions,
an increase in CCN concentration was only observed at heights with an air
pressure lower than 700 mbar. Average CCN concentrations measured at a SS of
0.8 % were 257
R. H. Moore et al. (2011) presented results from five research flights over the Alaskan Arctic during April 2008, beginning from Fairbanks and covering parts of the Beaufort Sea. The air masses sampled variously represented background conditions, biomass burning plumes, anthropogenic pollution and Arctic boundary layer conditions. Calculated activation curves with SS values ranging between 0.1 and 0.6 % showed that at least 70 % of the particles were activated for SS at around 0.2 % for all air masses. It was therefore concluded that this similarity in observed activation pattern, despite the differences in chemical composition, is a result of aerosol size, which largely determines CCN activity. However, the authors pointed out that for SS between 0.3 and 0.6 % it is likely that the particle chemical composition controls the maximum fraction of particles that can act as CCN.
Lathem et al. (2013) presented results of CCN measurements conducted during
research flights from 26 June to 14 July 2008. The flight campaigns set-off
from Cold Lake, Alberta, Canada and passed through the northeastern Canadian
Arctic before heading to the west coast of Greenland. During the flights,
the various air masses were characterized by biomass burning, boreal forest
background, Arctic background and anthropogenic industrial pollution. Median
CCN concentrations were highest for air masses influenced by fresh biomass
burning, at 7778 cm
During a flight campaign over the northern slopes of Alaska in April 2008, Hiranuma et al. (2013) collected ambient particles, dry residuals of mixed-phase cloud droplets and ice crystals. They analysed their size and chemical structure using an electron microscope in combination with various X-ray techniques. Note that the results should be interpreted with caution due to the limited number of samples. However, the limited data showed that the residuals of cloud droplets were enriched with respect to carbonate and black carbon, compared to the ambient particles. Significant mixing was also observed in the cloud droplet residuals. Additionally, during a period of high ice nucleation efficiency, residuals were enriched in sodium and magnesium salts compared to the ambient particles.
The studies described above reveal the significant variability in CCN concentration across the Arctic, likely resulting from differing locations of CCN production (upper troposphere vs. lower boundary layer), production mechanisms, in-cloud processing and the origins of air masses. Several studies indicate an increase in CCN with increasing altitude in the lower half of the troposphere. However, the controlling mechanism for this increase is still unclear. In this study, we compare bulk CCN properties with those found in previous studies, and we also explore the size dependence of CCN activation potential for the Arctic aerosols by combining a DMPS (Differential Mobility Particle Sizer) system with a CCN counter (CCNC). Although size-dependent CCN activation has been studied worldwide (Bhattu and Tripathi, 2014; Rose et al., 2010; Paramonov et al., 2013; Gunthe et al., 2009), according to our knowledge, this is the first study presenting size-resolved CCN activation in the Arctic.
Measurements were made at the Zeppelin research station (78
Particle number size distributions were measured using a closed-loop
Differential Mobility Particle Sizer (DMPS), consisting of a medium-sized
Hauke Differential Mobility Analyzer (DMA) in combination with a TSI
Condensation Particle Counter (CPC) 3010. Measurements were performed within
40 different size bins, with particle diameters ranging between 10 and 900 nm. Each particle size range was measured for 10 s, followed by a lag time
of 5 s before the next size range was measured. Simultaneously, total
particle number concentrations were precisely measured using a TSI CPC 3025
with a lower cut-off size of 3 nm, and by a TSI CPC 3010 with a lower
cut-off size of 10 nm. A commercially available DMT CCN counter connected to
a 1/4
In the standard configuration, these two instrument systems operate independently. In this study, however, we combined the two systems such that the DMA first selects a nearly mono-disperse aerosol, which is then supplied to the CCNC. For the CCN size-resolved concentration measurements, the CCNC was connected to the DMA and SS was fixed at 0.4 %. The number of size bins of the DMPS system was also reduced from 40 to 15, and the time each particle size was measured was extended from 10 to 35 s to improve counting statistics. The lower and upper bounds of the DMPS scans were also narrowed to 15 and 400 nm, respectively. The two different setups of the CCNC are shown in Fig. 1.
Two case studies are presented here, consisting of CCN size-resolved number
concentration measurements conducted during summer 2008. The measurement
period for the first case study lasted from around 09:40 UTC on 27 June to around
10:15 on 29 June during which about 290 size-resolved CCN scans were
conducted. The minimum and maximum temperatures for this period were 3.8 and
9.4
Scheme of the two different measurement modes for the cloud condensation nuclei counter (CCNC). When CCN size-resolved number concentration measurements took place, the CCNC was connected behind the Differential Mobility Analyzer and the supersaturation was set to 0.4 %. During normal operation, the CCNC was connected parallel to the DMA and SS alternated between 0.2 and 1.0 %.
Particle number size distributions observed from 27 to 30 June 2008 are
presented in Fig. 2a. The vertical purple lines in this figure indicate the
beginning and the end point of the measurement period for the size-resolved
CCN number concentration data. Based on the particle number size
distribution, at least three characteristic periods can be distinguished:
(i) from midnight to approximately midday of 27 June, when particles with
diameters of approximately 70 nm dominate the particle concentration; (ii) from midnight to approximately midday of 28 June, when particle number
concentrations are highest for particle diameters of approximately 20 nm and
(iii) from approximately midday on 29 June to the following midnight,
when the concentration of particles with diameters approximately between 20
and 70 nm increased to more than 1000 cm
The time series of particle number size distribution (Fig. 2a) is accompanied by two time series of total aerosol number concentrations for particles having a lower cut-off size of 3 and 10 nm, respectively (Fig. 2b). Although particles smaller than 10 nm are unlikely to be CCN, the combination of the two CPC instruments permit detection of particles that are a result of recent new particle formation. The combination of 5-day backward trajectory analyses, lidar measurements, particle number size distributions and total aerosol concentration time series gives a rounded picture of the conditions that prevailed during the experimental period.
The entire period from 27 to 30 June 2008 is characterized by a maximum of particle concentrations occurring at particle diameters below 100 nm. This is in line with the results of Tunved et al. (2013), who analysed long-term particle number size distributions at the Zeppelin station during the years 2000–2010. In their study, the authors concluded that the Arctic summer aerosol number size distribution (June–August) is characterized by a dominance of particles with diameters less than the accumulation diameter. It is proposed that these aerosols are most likely formed within the Arctic itself. This explanation of local production agrees with our calculated trajectories (Fig. 2), which show transport almost only within the Arctic. In addition, the lidar data from the period from 27 to 30 June 2008 does not indicate any cloud processing of the aerosols in the lower atmosphere boundary layer at the measurement site.
From midnight to approximately midday of 27 June, particles with diameters of approximately 70 nm dominate the particle concentration. The associated trajectory plot (Fig. 2) indicates that this pattern may result from a mixture of air masses, originating from the Norwegian Sea as well as from the Arctic Ocean.
Normalized relative backscatter (Level 1.0 data) based on lidar
measurements at Ny-Ålesund recorded during the period 27–29 June 2008
(modified from
During the early morning hours of 28 June 2008 a sharp increase in total particle number concentration is observed (Fig. 2b). The highest concentration of particle numbers is found for particles with dry diameters of less than 20 nm (Fig. 2a), which points, alongside the sharp increase in particle number concentration, towards new particle formation during previous hours.The process of particle formation is not yet fully understood (Komppula et al., 2003; Yli-Juuti et al., 2011; Ortega et al., 2012), but sulphuric acid and organic compounds have been found to be the key components (Riipinen et al., 2007; Kuang et al., 2008; Sipilä et al., 2010; Kulmala et al., 2013). Most nucleation events take place during the daylight hours, which indicates the importance of photochemistry in the nucleation process. However, at some locations particle formation events are also observed at night when there is no ambient light (Ortega et al., 2012). In Ny-Ålesund, the polar day lasts from around 18 April to 23 August; therefore, the measurements made herein during June 2008 lie within this daylight period. Tunved et al. (2013) presented averaged diurnal particle number size distributions for June, based on observations made during 2000–2010, and found that the concentrations of particles with diameters less than 20 nm predominantly begin to increase at around noon. Presented data indicate that an increase of particle concentration occured later in the day. In the Arctic environment, it has been suggested that dimethyl sulphide plays an important role as a condensing vapour for the nucleation process (Chang et al., 2011). Tunved et al. (2013) stated that another requirement of particle nucleation in the Arctic is a low condensation sink, which means a low concentration of particles in the accumulation mode. These authors showed that the particle mass is strongly related to accumulated precipitation along the transport path (cf. Fig. 15 in Tunved et al., 2013), and that conditions are favourable for new particle formation during the period of midnight sun. Integrated precipitation over the 5-day duration was calculated for each hourly trajectory. Over all there was little precipitation during the investigated periods with a median of less than 3.7 mm for the June case and less than 1.7 mm for the August case. The maximum integrated precipitation is an isolated event for a trajectory arriving 06:00 on 27 June. For this trajectory the integrated precipitation was 18.5 mm. From this we can conclude that recent precipitation within the last 5 days was not likely a dominant factor in shaping the aerosol properties during transport.
From midday on 29 June 2008 until approximately 22:30 on that day, the total
particle number concentrations of particles with diameters greater than 3 nm
increased approximately from 400 to 3860 cm
To place the period in which the size-resolved CCN measurements were
conducted in a long-term context, the median of the total particle number
concentration for particles with diameters greater than 10 nm during this
period is compared with the medians of the June data for the years
2001–2010 (Tunved et al., 2013). The long-term data have a time resolution
of 1 h, but around 9 % of these data are missing or are of poor quality
and are therefore not considered in the calculation. The data are available
within specific size distributions, and the total number was calculated by
integrating over the distinct size ranges. From 2001 to 2005 the lowest
measured size was 17.8 and the largest was 707.9 nm. From 2006 to 2007 a
size bin with a lower measurement range of 13.8 nm was added. For
2008–2010, the size distribution diameter range was again broadened, to range between
10 and 790 nm. The calculations resulted in a median particle number
concentration of 177 cm
Particle number size distributions from 21 to 25 August 2008 are presented in Fig. 4a. In this figure, the purple vertical lines indicate the start and end times for the CCN size-resolved concentration measurements. Difficulties with the DMPS measurements occurred approximately from 08:00 to 19:30 on 21 August and for short periods on 22 August; these time periods are omitted from the analysis. Time series of total particle number concentrations with dry diameters greater than 3 and 10 nm are showin in Fig. 4b. As with the measurements from June 2008, different periods with different characteristic particle number size distributions can be distinguished during the studied time period in August 2008 (Fig. 4a): (i) the final hours of 21 August, when particle number concentrations were highest for particles with diameters between 100 and 200 nm; (ii) the early morning hours of 23 August, when particle number concentrations were relatively low for all measured sizes (cf. Fig. 4b) and (iii) during the first half of 24 August, when total aerosol concentrations were relatively high for the period, but no particular size range clearly dominated. Calculated 5-day backward trajectories for each hour indicate that air masses arriving on the 21 August at the Zeppelin station mainly come from the southern part of the Norwegian Sea (Fig. 4). Air masses arriving from the 22 August until midday the 24 August at the Zeppelin station have a more northern origin, the Barents Sea air masses arriving between midday and midnight on 24 August at the Zeppelin station have again an origin over the Norwegian Sea.
As with the measurement period in June 2008, lidar data were consulted to investigate any local effects from clouds and precipitation (cf. Fig. 5). During the 21 August 2008, apparently clouds are present approximately between 0.7 and 9 km above the Zeppelin station. However, no precipitation reaching the station level could be detected. On 22 August low clouds (altitude < 2 km) were observed from approximately 09:00, and precipitation started at approximately 24:00, continuing until approximately 09:00. Only a few precipitation events are observed on 23 August 2008 for the most part, no clouds are observed at the altitudes above the Zeppelin station. On 24 August, clouds were only observed in Ny-Ålesund at altitudes higher than 0.8 km.
From around 20:00 to 23:00 on 21 August 2008, particles with dry diameters between 100 and 200 nm dominate the particle number size distribution. During the time period of 02:00 and 24:00 on 21 August, the Zeppelin research station was, according to the lidar measurements, very likely unaffected by clouds. The trajectories of the 21 August show that air masses originate from the mid-latitudes and lower their height when reaching the Zeppelin research station (Fig. 4). Therefore, it is likely that the peak in the particle number size distribution for particles with diameters between 100 and 200 nm is a result of particles being transported from the mid-latitudes to the Arctic and the processes taking place during transport rather than particles are being produced locally. The accumulation mode-dominated size distribution differs somewhat from the typical summer conditions. Tunved et al. (2013) demonstrated from their long-term average, during June–August, which locally produced particles with diameters in the nucleation and Aitken mode dominate the particle number size distribution. In the morning hours of 23 August 2008, air masses arriving at the Zeppelin station originated in the Barents Sea (Fig. 4) and resulted in relatively low total particle concentrations, compared to the concentrations observed between 20:00 and 23:00 on 21 August 2008 (Fig. 2b). Air masses in the morning of 24 August originated as well in the Barents Sea, but result in higher total particle concentrations than observed on 23 August.
Normalized relative backscatter (Level 1.0 data) based on lidar
measurements at Ny-Ålesund recorded during the period 21–24 August 2008
(modified from
To place our second case study data in a long-term context, we compare
median values of August 2008 with the 10-year climatology presented by
Tunved et al. (2013). Approximately 12 % of the hourly data were excluded
from calculations of the median integrated particle number concentration
from 2001 to 2010 August, owing to them being either missing or of poor
quality. The calculations produced a median particle number concentration of
127 cm
Overall, the June case is similar to the long-term climatology and appears to be more representative of the summer period, with air masses of Arctic origin. In contrast, the August case differs more from the long-term climatology and shows a more significant influence of lower latitudes and higher number densities of accumulation particles.
A CCN spectrum for a 5 h long period on the 27 June 2008 was obtained before
the size-resolved CCN measurements were begun. The data presented in this
section comprise medians calculated from one SS scanning cycle. The ratio,
as a function of SS, between CCN number concentration and the total particle
number concentrations for particles with diameters greater than 3 nm
(CN
CCN spectra obtained during 17 and 13 h observation periods on 21 and 24 August 2008, respectively, are shown on the right side of Fig. 6. The ratios
between CCN and CN as a function of SS are shown in Fig. 6c for the two
different days. For 21 August, a significant increase in the CCN to CN ratio
with an increase in SS was observed in all cases. For 24 August, the
increase in ratio was significant for all increases in SS, except for the
increase from 0.4 to 0.6 % SS. The absolute number of CCN for 21 and
24 August, as a function of SS, is shown in Fig. 6d. For both days, the
increase in CCN number from one SS to the next is significant. This is based
on applying the two-sample Kolmogorov–Smirnov test with a 5 %
significance level. As with the same data from 27 June 2008 (cf. Fig. 6b), a
power-law function of the same form was fitted to the data from 21 and 24 August 2008, as denoted by the red lines in Fig. 6d. The fittings resulted
in
Rogers and Yau (1996) demonstrated that the coefficients for maritime air
vary, with
Yum and Hudson (2001) estimated an average CCN concentration of 76 cm
Silvergren et al. (2014) presented CCN number concentrations as a function
of SS and as a function of the month from September 2007 to August 2008,
calculated based on aerosol collections on filters at the Zeppelin research
station. For June 2008, a CCN number concentration of around 100 cm
No clear separation can be made between the two CCN spectra from August 2008 and the one CCN spectrum from June 2008. In general, the CCN spectrum of June 2008 (Fig. 6b) lies between the two different spectra of August 2008 (Fig. 6d). Comparing backward trajectories arriving at the Zeppelin before midday the 27 June (Fig. 2) and before midday the 21 August and after midday the 24 August (Fig. 4), corresponding to the times when the CCN spectra were measured, show that the air masses' origin was for the most of the times southerly of Svalbard. However, even those air masses with similar origins can show differences in their aerosol characteristics (Park et al., 2014).
The size-resolved activation of particles having
Geometric means of size-resolved particle density measurements and
resulting CCN concentrations for the measurement period in
To establish the presented study contextually with other studies, the ratio
between CCN and CN as a function of dry particle diameter was calculated
(Fig. 8). Note that during June the CCN concentration exceeds the total
particle concentration for
After applying a spline interpolation to the measurement data, the dry
diameter at which 50 % of the total particle number concentration was
activated (
Activation ratio as a function of dry particle diameter (
Besides SS, the chemical composition and mixing state determines the ability
of particles to become activated to cloud droplets (Frosch et al., 2011;
M. J. K. Moore et al., 2011; Ervens et al., 2010; Sullivan et al., 2009).
Kreidenweis et al. (2005) summarize results of predicted and experimentally
determined critical diameters of ammonium sulphate and sodium chloride
particles. Predicted critical diameters for sodium chloride particles vary
between 44.6 and 39.4 nm (Kreidenweis et al., 2005 and references therein)
and the experimentally determined diameter for a SS of 0.4 % was reported
to be 40
The hygroscopicity parameter
First, the relationship between the activation diameter (
Second, the
Comparison of the “bulk
Measured diameters when CCN
Experimentally derived mass fractions (Silvergren et al., 2014),
densities
Silvergren et al. (2014) used three different approaches to calculate
Based on aerosol optical properties, Zieger et al. (2010) determined a mean
The main reason for the differences between the present study and both
Zieger et al. (2010) and Silvergren et al. (2014) is probably related to the
influence of large (> 400 nm) particles in determining
Anttila et al. (2012) reported a
For the first time, size-resolved CCN measurements in the Arctic have been
reported. Measurements were conducted at the Zeppelin research station,
Svalbard during two short periods in June and August 2008. A near-monodisperse aerosol having a
Trajectory analysis showed that during the measurement period in June 2008
air masses arriving at Zeppelin were dominated by Arctic air, while during
August 2008 air masses originated from the Norwegian Sea and from the
Barents Sea. A comparison of long-term June particle number size
distributions with those registered during the size-resolved CCN
measurements in June 2008 showed that the size distribution characterized by
a nucleation event and low particle concentrations for
We would like to thank Peter Tunved for providing the DMPS and CPC data and
for the help with trajectory calculation and plotting. We acknowledge that
the CCN measurements were supported by the KOPRI project NRF-2011-0021063.
The authors gratefully acknowledge the NOAA Air Resources Laboratory (ARL)
for the provision of the HYSPLIT transport and dispersion model and the READY
website (