This work presents an analysis of the physical properties of sub-micrometer aerosol particles measured at the high Arctic site Villum Research Station, Station Nord (VRS), northeast Greenland, between July 2010 and February 2013. The study focuses on particle number concentrations, particle number size distributions and the occurrence of new particle formation (NPF) events and their seasonality in the high Arctic, where observations and characterization of such aerosol particle properties and corresponding events are rare and understanding of related processes is lacking.
A clear accumulation mode was observed during the darker months from October
until mid-May, which became considerably more pronounced during the prominent
Arctic haze months from March to mid-May. In contrast, nucleation- and
Aitken-mode particles were predominantly observed during the summer months.
Analysis of wind direction and wind speed indicated possible contributions of
marine sources from the easterly side of the station to the observed
summertime particle number concentrations, while southwesterly to westerly
winds dominated during the darker months. NPF events lasting from hours to
days were mostly observed from June until August, with fewer events observed
during the months with less sunlight, i.e., March, April, September and October.
The results tend to indicate that ozone (O
Climate
change driven by anthropogenic greenhouse gas emissions is a global
challenge. In the Arctic, the warming climate has already led to an earlier
onset of spring-ice melt, later freeze-up and decreasing sea-ice extent
(Zwally et al., 2002; Markus et al., 2009; Stroeve et al., 2012). The
reduction of the Earth's albedo due to ice loss subsequently impacts the
radiative balance of the Earth through a positive feedback, leading to
further warming. As a result, the Arctic has been considered a manifestation
of global warming with the rate of temperature increase in the region being
twice as high as the rest of the world (IPCC, 2013; ACIA, 2005), up to
8–9
Aerosol particles influence the radiative balance in the Arctic in many ways, through their ability to absorb and scatter incoming solar radiation or by acting as cloud condensation nuclei to form cloud and fog droplets. The presence of low-level liquid clouds above bright ice- and snow-covered surfaces in the Arctic could lead to increasing near-surface temperature as opposed to a cooling effect observed in most other global regions (Shupe and Intrieri, 2004; Bennartz et al., 2013), though the effect is probably small (AMAP, 2011). At the same time, deposition of black carbon on Arctic snow- and ice-covered surfaces accelerates surface heating and ice melting in early spring (Hansen and Nazarenko, 2004; Flanner et al., 2007, 2009). It is thus crucial to investigate the dynamics of atmospheric aerosol particles observed in the Arctic (involving the formation, concentration, physico-chemical properties, temporal variability and transport) to understand their direct and indirect effects on the radiation budget.
It is well known that during each winter extending into spring, Arctic aerosol particles containing mineral dust, black carbon, heavy metals, elements and sulfur and nitrogen compounds are detected in elevated concentrations. This has been attributed to the annually recurring Arctic haze phenomenon, which is related to distant latitude anthropogenic pollution (Li and Barrie, 1993; Quinn et al., 2002; Ström et al., 2003; Heidam et al., 2004, 1999; Nguyen et al., 2013). The focus was thus on long-range transported aerosols, which are expected to be aged due to the long transport distance from midlatitude source regions.
A number of studies have reported in situ formation of new aerosol particles
in the Arctic, which mostly involved new particle formation in the Arctic
boundary layer. The first observations of the occurrence of an ultrafine
particle mode (
VRS, Station Nord is a unique coastal station located close to sea level, representing the conditions of the high Arctic throughout the year. To date, there is only one observation and characterization of NPF events at Alert, Nunavut (Leaitch et al., 2013). Understanding of particle size distribution and seasonality, as well as related mechanisms and processes of NPF events, are thus lacking from the high Arctic region.
This study aims to characterize the formation, concentration, physical
properties and seasonality of atmospheric aerosols based on particle number
size distributions at VRS. The occurrence of NPF events was investigated in
detail. The events were classified and analyzed together with ozone (O
Aerosol particles and trace gases were measured at the measurement site
“Flyger's Hut”, VRS, Station Nord, in northeast Greenland
(81
Measurement of particle number size distributions at Station Nord was
initiated in July 2010 using a TROPOS-type Mobility Particle Size
Spectrometer as described in Wiedensohler et al. (2012). Briefly, the
instrument consists of a medium Vienna-type Differential Mobility Analyzer
(DMA) followed by a butanol-based Condensation Particle Counter (CPC 3772 by
TSI Inc., Shoreview, USA). The DMA design is described in Winklmayr et
al. (1991). The system is operated at 1 L min
The high Arctic site Villum Research Station, Station Nord
(81
The instrument was specifically designed to allow long-term operation with minimum maintenance as follows. The DMA sheath air flow rate was continuously measured using a calibrated mass flow sensor. The DMA aerosol flow rate was monitored by a pressure drop measurement over a calibrated capillary. A computer-based control program adjusted the sheath air flow rate after each measurement of the particle number size distribution. Systematic deviations in the sample flow rate, which was controlled by a critical orifice in the CPC, were monitored and corrected for in the successive size distribution evaluation. Additionally, absolute pressure was measured at the inlet of the system to detect any substantial technical problems such as clogging of the inlet. Temperature and relative humidity (RH) were monitored at several positions inside the instrument. The RH inside the DMA is the most critical parameter, since excessive moisture would allow particles to grow much beyond their nominal dry diameter. At VRS, Station Nord, RH is usually not a critical issue, as the climate is cold and arid with low humidity most of the year. The temperature in the laboratory is mostly considerably higher than outdoor temperature, implying that substantial drying of the aerosol is not needed most of the time during sample intake into the laboratory.
Sampling was provided from a conductive flow tube. An air blower was used to
suck a main air flow (much higher than the sample flow) into the main
sampling inlet, and the air sampling was probed from this main air flow using
a 1/4 inch tube directed into the main air flow. The main sampling inlet
was not heated; however, no icing issue was observed for the inlet. The main
sampling inlet did not have any size cutoff. Sampling was performed at
standard conditions of about 20
The raw particle electrical mobility distributions collected by the Mobility Particle Size Spectrometer were processed by a linear inversion algorithm presented in Pfeifer et al. (2014). A specific DMA transfer function was used for inverting the data, while CPC efficiency and diffusion losses were corrected for during the inversion.
As a first part of quality control, any data associated with DMA excess air
RH above 50 % and sheath air temperature above 30
Subsequently, daily particle number size distributions were plotted to
inspect any sudden increase in the particle number concentration above the
background. If such sudden increase in particle number concentration (without
any detectable particle growth) coincided with sudden elevation of
NO
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NO
Wind speed and wind direction data were obtained from a sonic anemometer
(METEK, USA-1, heated) for the period from April 2011 to April 2013. The
sonic is placed on a horizontal boom at the top of a 9 m mast. The mast is
situated about 36 m east–southeast from the measurement hut at ca.
62 m a.s.l. This means that the fetch-limited wind direction is
300
SMPS, O
NPF events were identified and classified following a scheme adapted from Dal Maso et al. (2005). A brief description is given here.
A plot was compiled for each day with available particle number size
distribution data, plotting the particle diameter on the
Time series of particle number size distributions as
This section presents the observed overall seasonality of particle number size distributions measured at VRS, Station Nord, during the time period from July 2010 to February 2013, with an analysis of NPF event cases together with the atmospheric oxidation capacity at the station. Analysis of local wind speed, wind direction and air mass back trajectories was used to support the interpretation of the seasonality of particle number size distributions and the dynamics of NPF events.
Three modes were fitted to the average monthly data of 2012 using
log-normal fitting. The parameters shown for each mode include the modal
number concentration (
A clear seasonality of particle number size distributions was observed during 2012 (Figs. 3–4). A persistent accumulation mode appeared at the end of September, which became more prominent at the end of February lasting until mid-May. The Arctic summer (June–August) was coupled with a higher abundance of nucleation-mode and Aitken-mode aerosol particles and a very low abundance of accumulation-mode particles (Table 1). The small particles were also observed to a lesser extent in September and only during one episode in mid-October. This observation of strong seasonality was supported by observations from the available scattered data in the other years 2010, 2011 and 2013. The elevated concentrations of accumulation-mode particles observed in this study generally followed the varying pattern of aged total suspended particles during the Arctic haze period previously reported at VRS, Station Nord (Heidam et al., 2004; Nguyen et al., 2013), and other Arctic stations (Quinn et al., 2002; Ström et al., 2003). It should also be noted that the sun rises at the end of February at Station Nord, so the period thereafter is affected by photochemical processes. Observations of smaller particles during this period were in accordance with previous studies in the Arctic (Ström et al., 2003; Tunved et al., 2013; Wiedensohler et al., 1996; Covert et al., 1996; Quinn et al., 2002; Heintzenberg et al., 2015; Leaitch et al., 2013). During this period, the Arctic is considerably cleaner with respect to long-range transport of atmospheric pollutants and is characterized by constant daylight.
Monthly median particle number size distribution at Station Nord during 2012. The corresponding log-normal fitting parameters are shown in Table 1. The shade area shows the 75th (upper) and 25th (lower) percentile of the actual data.
Median and average particle number concentration (
Figure 4 and Table 1 describe detailed statistics of the particle number size
distributions measured at the site, showing the prominent accumulation mode
during February–May and the prominent nucleation/Aitken mode during
June–August. Table 2 provides detailed median and average particle number
concentration (
Newly formed particles are usually high in number and therefore significantly
influence the total number concentration
The total particle mass concentrations
Wind roses showing monthly wind direction and wind speed at Station Nord during 2012. The concentric rings show the percentage of wind arriving from a particular direction.
Similar distribution of the major modes was also observed at the Zeppelin
mountain site by Tunved et al. (2013). However, the nucleation mode and Aitken
mode observed during the summer months seemed considerably more pronounced at
VRS, Station Nord, compared to Zeppelin. This indicates higher number
concentrations of smaller particles at Station Nord, which were visible until
October (Figs. 3–4). In regard to the total particle mass concentration,
Tunved et al. (2013) reported summer
Analysis of wind direction and wind speed was performed to investigate the impacts of wind pattern on the particle number size distributions at the station. Figure 5 demonstrates monthly wind roses during 2012, where two distinct patterns could be identified during the darker (September–April) and the summer (June–August) periods. The early haze months (January and February) and the prominent haze months (March and April) showed prevailing wind arriving from the southwesterly to westerly direction. During May, some northerly wind was observed, while the frequency of southwesterly wind seemed to decrease. During the summer period (June–August), when smaller and freshly formed particles were observed, easterly wind became more prominent, especially during July and August. September marked a prompt change in the wind direction back to a southwesterly direction. The wind speed became higher during November–December, which is probably due to increasing katabatic winds from the ice sheet. During the other years, 2011 and 2013 (data not shown), considerably similar patterns were observed for the corresponding months.
Earlier studies on source apportionment of total suspended particles observed during the Arctic haze period at VRS mostly identified Siberian industries and long-range transport from midlatitudes as major factors (Nguyen et al., 2013; Heidam et al., 2004). However, the wind pattern shown here may indicate an immediate impact of the adjacent southwesterly to westerly regions contributing to the properties of particles prior to arrival at the station.
Based on the summer wind pattern, the smaller particles observed during June–August were probably linked to sources from the easterly side of the station, with some marine contribution. During summer, the marine contribution from the easterly direction is possibly driven by the retreat of sea-ice cover, which exposes areas of open waters (“open leads”) and meltwater on top of sea ice to wind stress, especially along the coastline of Greenland due to the presence of first-year ice in these regions. This would result in enhanced primary emissions of sea spray particles (Korhonen et al., 2008). Surface active organic species in the ocean surface layer, which are more abundant due to increased biological activity during summer, could also be released into the atmosphere by bubble bursting (Middlebrook et al., 1998; Tervahattu et al., 2002) and become mixed with other sea spray particles. It was suggested by Sellegri et al. (2006) that this could also alter the number size distributions of particles. Another study by Karl et al. (2013) proposed that new nanoparticles in the high Arctic could be marine granular nanogels injected into the atmosphere from evaporating cloud droplets. Recent analysis of particle number size distributions and back trajectories during summer cruises in the Arctic by Heintzenberg et al. (2015) also showed a strong coupling of newly formed particles and the traveling of air masses over open water. At the same time, it must be noted that wind measurements using the sonic anemometer were confined to local observations at ground level, which according to radio sound measurements by Batchvarova et al. (2013), do not capture activities such as transport of air masses at higher altitudes, or regional transport of air masses. The extent of wind impacts on the particle size distributions at the station is thus not well constrained.
Previous studies reported a dependence of particle number concentrations on
wind speed in the Arctic (Leck et al., 2002) and North Atlantic (Odowd and
Smith, 1993). However, in this study, the accumulation-mode particles
(110–900 nm) only showed positive correlation with wind speed during 8
out of 12 months of 2012, with a moderate Pearson correlation coefficient
range of 0.05–0.38. The reason could be partly attributed to the larger size
ranges (500 nm up to 16
Demonstration of the impacts of O
NPF events were observed at the station during the sunlit months, especially during the summer months of June–August, though events were also identified during the months with relatively low sunlight, i.e., March and October. The onset of NPF events was observed during various hours of the day (Fig. S1 in the Supplement). Examples of three events are shown in Fig. 6. As apparent from the figure, the events showed clear but slow growth over considerably long periods up to a few days.
Figure 6 also shows an overlay of O
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In this work, O
During 15 June, the O
In the late hours of 19 June, the O
During the Event A case study, the NO and NO
The three events seemed to visually display an anti-correlation between the
concentration level of O
It is generally agreed that particle nucleation involves sulfuric acid
(H
The source of VOCs at VRS, Station Nord, is unclear. There might be some biogenic emissions of VOCs at the station during summer, expected due to retreated snow and ice cover, exposed bare ground and thus possibly increased biogenic activity. However, since this area is arid, this is expected to be extremely limited. Meanwhile, the presence of VOC oxidation products such as organic acids and organosulfates at the station has been reported by Hansen et al. (2014), though at very low concentrations. The low mass or surface loading of organic materials (Nguyen et al., 2014) and total suspended particles (Nguyen et al., 2013) and thus low condensation sink observed at the station during summer would inhibit removal of small particles by condensation and also coagulation to a lesser extent, thus allowing particle growth and prolonged NPF events. At the same time, no considerable difference in particle mass or surface was observed at the onset of events compared to the average particle mass or surface of the corresponding months during 2012.
As mentioned above, sparks of particle formation, which did not grow further,
were considered as local pollution events, which were related to NO
The summer period of June–August was associated with a lower level of
background NO
Demonstration of air mass back trajectories calculated hourly using HYSPLIT for arrival at 50 m and 500 m at the station for the case study events.
As mentioned above, the Mobility Particle Size Spectrometer system employed at VRS, Station Nord, is limited to particles larger than 10 nm in size, capturing only aged nucleation particles. It is thus uncertain whether the formation of the freshly nucleated particles actually occurred at the site, or whether they were transported from elsewhere or produced aloft.
Air mass back trajectories were analyzed in order to investigate possible source regions for the observed events. The trajectories were calculated using HYSPLIT (Draxier and Hess, 1998). The model runs were based on meteorological data obtained from the Global Data Assimilation System (GDAS), which is maintained by the US National Centers for Environmental Prediction (NCEP).
In order to facilitate the interpretation of the events shown in Fig. 6, hourly air mass back trajectories were calculated going 72 h backwards for air masses arriving at the station at 50 and 500 m above sea level. The trajectories are presented in Fig. 7, with the names of the events kept consistent with those in Fig. 6. Calculations of air mass back trajectories were performed for 3-day periods, in order to minimize the uncertainties associated with calculating longer trajectories.
As can be seen in Fig. 7, westerly air masses were arriving at the station during the hours before the onset of Event A. At 21:00 on 15 June, air masses started to originate from the southwesterly direction instead, which also marks the observation of the first NPF event. In fact, during both NPF events identified on 15 and 17 June, during Event A, air masses seemed to be fast-moving, originating from longer distances in the southwesterly direction. During the late hours of 17 June to early 19 June, the station started to receive more air masses arriving from a northerly direction (for example 19 June, 06:00 local time), which may associate with the faded nucleation-mode particles observed during this exact time period. The “interrupted period” observed on 19–20 June also seemed to overlap with the time period where air masses were locally confined (for example 19 June, 15:00 local time), and nucleation-mode particles started to be observed again as the air masses started to arrive from a westerly direction instead (20 June, 16:00 local time). It should be noted that this interrupted period was off by about 2 h compared to changes in HYSPLIT air mass trajectories, which might be attributed to uncertainties in HYSPLIT output, especially for calculating air mass movement over small distances in an area with few meteorological measurement data.
The trajectories for Event B (Fig. 7) show that from 05:00 to 18:00 on 2 August, air masses seemed to arrive constantly along the coastline from the northerly direction (which is shown by the example at 06:00, Event B, Fig. 7), compared to the non-event period on that same day, where air masses were arriving from inland instead (2 August, 03:00 and 18:00 local time). The air masses thus might involve the Arctic sea-ice region (Fig. S2 in the Supplement) and related sources such as open leads or meltwater on top of sea ice due to wind stress as discussed above.
At the same time, the onset of an observed event cannot always be traced
using HYSPLIT air mass back trajectories. For example, Event C was observed
at the site around 00:00 on 9 August (Fig. 7, Event C) despite no
clear changes in HYSPLIT air mass back trajectories. This was a rather weak
event which seemed to stem from particle sizes below 10 nm, which were not
able to be captured by the Mobility Particle Size Spectrometer. This also
highlights the uncertainty with using HYSPLIT to trace the onset of the NPF
event, as the onset time might only be for particles above 10 nm in
diameter, whereas the air masses transporting particles below 10 nm in size
might have arrived at the site prior to this so-called onset time. On the
other hand, the interruption of Event C was easier to trace, as
it seemed to coincide with the time where the air masses were confined to the
inland westerly region prior to arriving at the station (10 August, 04:00 local time).
Air mass back trajectories were also calculated 3 days backwards, at 1 h after the starting time of each identified event using
HYSPLIT, whereas for the other days, trajectories arriving at 00:00.
local time were used. The region around Station Nord was split into 1
The probability of observing an event at Station Nord (bottom tip of the black triangle) as a function of air mass origin. This figure uses all available data (62 events) from the study period July 2010–February 2013.
As apparent from the figure, the probability of observing an event at the
station is low when the air masses arrive from the southwesterly direction
over Greenland. Other directions of air mass origin, however, showed relatively
similar probability of registering an event. A slightly higher probability
range was observed for southeasterly air masses that passed over the region,
where open waters and melt ponds on ice are more likely to occur. As
particles typically grow very slowly at Villum Research Station, the time gap
from particle nucleation occurring around 1.5 nm in diameter until the point
when they are observed at the site (
Monthly variation of total number of days with good data (left vertical axis) and frequency percentages (%) of event days, non-event days and undefined days (right vertical axis) during the study period (July 2010–February 2013).
The wind pattern was also investigated on specific event days in 2011 and
2012 (figure not shown). However, these patterns were found to be very similar to the general
wind patterns of the corresponding month or period. Therefore, it is unlikely
that any change in local wind direction during the specific event days could
have an impact on the occurrence of new particle formation events observed at
the site. This indicates the possibility of other factors, which may have
changed during the event days, affecting new particle formation such as
precursors. In fact, Quinn et al. (2002) indicated that the abundant dimethyl
sulfide (DMS) could affect particle production during summer, as evidenced by
a strong correlation between particle number concentrations and
methanesulfonate (MSA
Percentage of total new particle formation events (marked in blue) vs. non-events and undefined days during the period July 2010 to February 2013. The total events were further divided into class I and class II events. A column of total days (by month) over the studied years was also provided.
In general, the event days accounted for 17–38 % of the classified days during June–September, with the highest percentages of event days observed in August (38 %) and July (33 %) (Fig. 9, Table 3). The period from June to early September was also the period during which longer events up to several days were observed and most class I events were identified (Table 3).
The observed frequencies of event days during these months at VRS, Station
Nord, were relatively high compared to reported values from subarctic
stations during the same months, such as Värriö (20–25 %) Kyro
et al., 2014), Pallas (10–20 %) (Asmi et al., 2011) or Abisko
(
In this work, the seasonality of particle number size distributions, total particle number, volume and mass concentrations was examined. A strong seasonal pattern was found, showing the abundance of smaller particles during the sunlit period of the year, especially during summer, and a persistent accumulation mode during the darker months caused by long-range transport of particles to the Arctic. Analysis of wind data showed a dominance of easterly winds during the summer months and southwesterly winds during the darker months of the year.
The observed NPF events at the station lasted from hours to days with various
onset times. O
The data used in this work were obtained and managed by the Department of Environmental Science at Aarhus University, Denmark, and can be accessed via correspondence to Quynh T. Nguyen (quynh@eng.au.dk).
This work was financially supported by the Danish Environmental Protection Agency with means from the MIKA/DANCEA funds for Environmental Support to the Arctic Region, which is part of the Danish contribution to “Arctic Monitoring and Assessment Program” (AMAP) and to the Danish research project “Short-lived Climate Forcers” (SLCF). The findings and conclusions presented here do not necessarily reflect the views of the Agency. This work was also supported by the Nordic Centre of Excellence Cryosphere–Atmosphere Interactions in a Changing Arctic Climate (CRAICC). The Villum Foundation is acknowledged for funding the construction of Villum Research Station, Station Nord. The authors are also grateful to the staff at Station Nord for their excellent support.
The Royal Danish Air Force is gratefully acknowledged for providing free transport to Station Nord. The authors are also grateful to the staff at Station Nord for their excellent and unwavering support. Edited by: M. Boy Reviewed by: two anonymous referees