The vertical distribution of black carbon (BC) particles
in the Arctic atmosphere is one of the key parameters controlling their
radiative forcing and thus role in Arctic climate change. This work
investigates the presence and properties of these light-absorbing aerosols
over the High Canadian Arctic (
Generally, the rBC mass concentration decreased from spring to summer by a factor of 10. Such depletion was associated with a decrease in the mean rBC particle diameter, from approximately 200 to 130 nm at low altitude. Due to the very low number fraction, rBC particles did not substantially contribute to the total aerosol population in summer.
The analysis of profiles with potential temperature as vertical coordinate
revealed characteristic variability patterns within specific levels of the
cold and stably stratified, dome-like, atmosphere over the polar region. The
associated history of transport trajectories into each of these levels showed
that the variability was induced by changing rates and efficiencies of rBC import.
Generally, the source areas affecting the polar dome extended southward with increasing
potential temperature (i.e. altitude) level in the dome. While the lower dome was mostly
only influenced by low-level transport from sources within the cold central and marginal
Arctic, for the mid-dome and upper dome during spring it was found that a cold
air outbreak
over eastern Europe caused intensified northward transport of air from a corridor over
western Russia to central Asia. This sector was affected by emissions from gas flaring,
industrial activity and wildfires. The development of transport caused rBC concentrations in the second lowest
level to gradually increase from 32 to 49 ng m
Our work provides vertical, spatial and seasonal information of rBC characteristics in the polar dome over the High Canadian Arctic, offering a more extensive dataset for evaluation of chemical transport models and for radiative forcing assessments than those obtained before by other Arctic aircraft campaigns.
Climate change in the Arctic is more rapid than on global scale and a
significant loss of the summertime sea-ice extent has been observed over
recent decades
Model studies of the Arctic climate system by
Consequently, in order to provide accurate radiative forcing estimation in
the Arctic region, it is necessary to understand what controls the vertical
distribution of BC particles in the Arctic atmosphere. Import of polluted air
from lower latitudes is controlled by the cold air mass that lies over the
Arctic like a dome with sloping isentropes, the isolines of potential
temperature
Despite their important implications, measurements of the vertical
distribution of BC and its variability in the Arctic atmosphere are very
sparse
This paper will discuss a set of measurements from the spring and summer
aircraft campaigns in the NETCARE project (Network on Climate and Aerosols:
Addressing Key Uncertainties in Remote Canadian Environments,
Aerosol observations were carried out with the Alfred Wegener Institute's
(AWI) research aircraft Polar 6, a DC-3 fuselage converted to a turboprop
Basler BT-67
Maps of all flight tracks evaluated in this study. The summer
measurement flights were operated out of Resolute Bay on the northern shore
of Lancaster Sound, southern Canadian Arctic Archipelago
The vertical atmospheric profile measurements were performed during the
aircraft campaigns of the NETCARE project in summer 2014 and spring 2015
An overview of all measurement flights of the NETCARE summer
campaign 2014 and the spring campaign 2015 that are evaluated in this study.
The stations are the Arctic airfields the plane started from and returned to
(see Fig.
A Single-Particle Soot Photometer (SP2; 8-channel) by Droplet Measurement
Technologies Inc. (DMT, Longmont, CO, USA) was used to detect BC particles.
The operation principle and evaluations of the method are given by
The incandescence light detector, a photomultiplier tube with a 350–800 nm
band-pass filter, used two gain stages. It was calibrated with a fullerene
soot standard from Alfa Aesar (stock no. 40971, lot no. FS12S011) by selecting a
narrow size distribution of particles with a differential mobility analyser
upstream of the SP2
The measured
The particle number size distributions and number concentration of the total
aerosol (TA) were measured with a DMT Ultra-High Sensitivity Aerosol
Spectrometer (UHSAS). As described in
The air inlet for aerosol sampling was a shrouded inlet diffuser (diameter
0.35 cm at intake point) on a stainless steel tube (outer diameter of
2.5 cm, inner diameter of 2.3 cm) mounted to the top of the cockpit and
ahead of the engines to exclude contamination. In-flight air was pushed
through the line with a regulated flow rate of approximately
55 L min
Carbon monoxide (CO) was measured with an Aerolaser ultra fast CO monitor
model AL 5002 based on vacuum ultraviolet fluorimetry, using the excitation of CO at a
wavelength of 150 nm. The instrument was modified for applying in situ
calibrations during in-flight operations. Calibrations were performed on a
15–30 min time interval during the measurement flights, using a NIST
traceable calibration gas. The total uncertainty relative to the working
standard of 4.7 ppbv (summer) or 2.3 ppbv (spring) can be regarded as an
upper limit. Further details of calibrations and corrections are presented by
Atmospheric BC and CO are often co-emitted from the same combustion sources
The meteorological state parameters pressure, humidity and temperature were
recorded at 1 Hz resolution with the basic meteorological sensor suite and
data acquisition of Polar 6. The ambient air temperature was measured with a
PT100-type sensor mounted to the aircraft fuselage in a Goodrich/Rosemount
102 EK 1BB housing with de-icing facility. Corrections for the de-icing heat
and adiabatic temperature increase due to pressurisation of the airflow
inside the sensor housing (RAM raise and recovery factor) were applied to the
temperature readings
A Forward Scattering Spectrometer Probe (FSSP), model 100, by Particle
Measuring Systems (PMS Inc., Boulder, CO) was used for the measurement of
cloud particles. Data from the probe, which was mounted in a canister on a
wing pylon, were analysed in more detail in
The ERA-Interim re-analysis data
Weather charts for the NETCARE spring
campaign in April 2015 showing the 750 hPa geopotential height
With the focus on the polar dome and the vertical distribution of rBC
therein, subsets of the flights in spring 2015 and summer 2014 were selected
for this analysis. The subset selections are based on the variability of the
polar dome's position and southern border. The structure and extent of the
polar dome in both seasons has been evaluated by
Weather charts for the NETCARE summer campaign in July 2014 showing
the 750 hPa geopotential height
The meteorological situation in April 2015 was dominated by a pool of very
cold air centred over the Canadian Arctic Archipelago that surrounded the
stations Alert and Eureka on Ellesmere Island. The cyclonic flow surrounding
the cold air stabilised this system by blocking perturbations of
low-pressure systems (Fig.
The NETCARE summer campaign 2014 operated in an area of the high Canadian
Arctic that was situated within the summer polar dome. The first half of the
campaign (4–12 July) was characterised by a northern influence
(Fig.
In this section, the vertical distribution of rBC is examined with a focus on
changes from spring to summer. For each ascent or descent of the flights
listed in Table
Mean regional vertical profiles of
The absolute and relative presence of rBC was generally reduced during
summer. Ground-based observations at High Arctic sites like Alert show a
pronounced seasonal cycle in rBC concentrations
During spring, mean profiles from the Alert and Eureka regions showed a
similar
The spring profiles of MMD show a nearly steady decrease with altitude from
206 to 162 nm for Alert and 202 to 140 nm for Eureka
(Fig.
In order to fully understand the vertical variability of the aerosol
distribution, it is important to consider the vertical structure of the polar
dome with its core of cold, dense air at ground level and successive dome-shaped layers of warmer air above.
The measurement periods in both seasons each covered an evolution cycle of a
low-pressure system, causing a disturbance of the polar dome's structure (see
Sect.
Flight profiles averaged over intervals of potential temperature
from the spring polar dome of
The spring mean flight profiles from Alert and Eureka averaged over intervals of potential
temperature are shown in Fig.
The profiles in Fig.
Although rBC represented a minor component of the total aerosol in the
respective size range by number, with an averaged
Concentrations and mixing ratios of rBC increased from the surface to level
II, which was in the potential temperature range of about 255–265 K. The
mean
The highest variability in rBC abundance and its properties was present
between 265 and 277 K (level III). At the beginning of the observation
period (7 April), low mean
The potential temperature range 277–285 K (level IV) was in the transition
zone to the air mass above the dome
Different transport pathways between the described levels as well as the
temporal variability of transport to each layer are investigated in
Sect.
Flight profiles averaged over intervals of potential temperature
from the summer polar dome of
The variability of aerosol properties was also investigated for summer as
a function of the potential temperature within the polar dome over the area of
Resolute Bay (Fig.
Close to the surface, within air at potential temperatures between 273 and
284 K (level I), the 75th percentile
A weakly stable to neutral atmospheric level was present above the stable
near-surface level and up to a strong temperature gradient aloft (level II).
The highest investigated level (III) of the atmosphere was characterised by
potential temperatures above 294 K, and most probably represented a strong
temperature gradient separating the polar dome from free tropospheric
conditions. In fact,
Air parcel back-trajectories were analysed for Sect.
Heat maps of normalised back-trajectory overpass frequencies in
each
Kinematic back-trajectories were calculated in order to discern different
contributions of potential source regions to the changing characteristics of
aerosol properties observed within the potential temperature levels
identified above in Sect.
Heat maps of normalised back-trajectory overpass frequencies in each
The aerosol over Alert and Eureka in the period 7–13 April was influenced by
air transport from eastern Europe, central Asia and Siberia as well as North
America (Fig.
Confined by the cyclonic winds around the polar dome and due to the lower
wind speed within it (Sect.
Compared to the surface, the mass concentration of rBC significantly
increased in the two levels between 255–265 K (II) and 265–277 K (III).
In fact, the highest
The transport patterns to level IV between 277 and 285 K showed a high
degree of complexity and some patterns visible in Fig.
The air sampled at potential temperatures
Interpreting the back-trajectories and vertical profiles together makes
it apparent that the regions contributing most to the enhanced presence of
combustion-generated particles were Russia and central Asia, while the
contribution of North America was mostly negligible or significantly smaller
than from the Eurasian side at all levels. This conclusion agrees with the
findings presented by
The MMD and
The vertical profiles in Fig.
The aerosol features observed in summer and described in Sect.
Most of the back-trajectories initiated in level I (capped by an inversion at 284 K) stayed for at least 10 days in
the close vicinity of Lancaster Sound (map in Fig.
More Arctic-wide air exchange was possible in level II (potential temperature
between 284 and 294 K; Fig.
Level III (potential temperatures
It is established that the minimum BC mass concentration as observed in
ground-based measurements appears in summer in both the European
As already discussed in Sect.
Averaged rBC mass size distributions normalised with the total mass concentration under the curve for levels I–V from the spring polar dome observations around Alert (A) and Eureka (E), as well as for level II from the summer polar dome observations around Resolute Bay.
All size distributions were log-normally distributed with the main mode
located between 160 and 220 nm. The MSDs from the upper polar dome in summer
(level II) and spring (levels III-V) showed a surprising resemblance, with
their mean mode peaking below 200 nm of
Two aircraft campaigns within the NETCARE project allowed observation of the
vertical distribution of black carbon aerosol over the High Canadian Arctic
during spring and summer. A seasonal difference was first and foremost
noticed in the concentration and properties of rBC from the vertical profile
flights. rBC mass concentration at low altitude decreased by 1 order of
magnitude from spring (mean of
The vertical variability of rBC in the polar dome was investigated as a function of potential temperature, which highlighted that the distribution of aerosols is constrained by the temperature structure of the cold and stable air mass over the Arctic. Prominent patterns of variability in rBC mass, particle size and particle presence relative to total aerosol number and CO mixing ratio were identified within five potential temperature levels in spring and three levels in summer. Back-trajectories initialised within each level generally showed that with increasing potential temperature in the polar dome, air pollution from warmer, more southern areas can affect the High Canadian Arctic. Low-pressure systems caused strong southward disruptions of the polar front and thus extended the area affecting the cold polar air mass for more emission sources.
The lowest levels in spring (with potential temperatures
In summer, by contrast, the motion of air masses was mostly confined to the
region north of the Arctic Circle, largely preventing entrainment of
pollution from lower latitudes during the measurement period. The averaged
rBC mass concentration increased from the surface to the upper atmospheric
levels by a factor of 2–3, but remained below 12 ng m
Regarding rBC properties, a remarkable and monotonic decrease in rBC particle size with altitude was observed in spring, while no evident trend was observed in summer. The change in rBC particle size in spring might be associated either with changes in source type, facilitated by the southward extent of the polar dome, or with atmospheric processing such as wet removal, which is enabled during uplift of an air mass and consequent cloud formation. A similar discussion applies to the ratio of rBC over CO, which showed, at least in spring, distinct deviations from an otherwise general decreasing trend.
The vertical profiles presented here captured the variability of rBC concentrations and properties imposed by cyclonic disturbances to the polar dome over the course of 1 week in spring and 2 weeks in summer, which to our knowledge has not been achieved during an aircraft campaign in the High Arctic before. The back-trajectory analysis suggests that Eurasian emissions represent the most probable source of combustion-generated aerosol in the polar dome. The contribution of individual sources and the effects of removal ultimately resulting in the observed high variability in the vertical presence and properties of rBC could not be given a concluding explanation by the analysis presented here. Despite this, the discussion of this dataset along the meteorological context represents an extensive insight into the vertical distribution of rBC and its properties in the Arctic, providing new input for the validation of chemical transport models and radiative forcing assessments.
The NETCARE project (Network on Climate and Aerosols:
Addressing Key Uncertainties in Remote Canadian Environments,
The supplement related to this article is available online at:
HS wrote the paper, with significant conceptual input from MZ, WRL and ABH and critical feedback from all co-authors. HS, HB, MDW, JB and WRL operated instruments in the field and analysed resulting data. HB, DK and PMH ran LAGRANTO simulations and HS analysed the resulting data with input from HB. WRL, JPDA and ABH designed the field experiment.
The authors declare that they have no conflict of interest.
This article is part of the special issue “NETCARE (Network on Aerosols and Climate: Addressing Key Uncertainties in Remote Canadian Environments) (ACP/AMT/BG inter-journal SI)”. It is not associated with a conference.
This research was funded jointly by NETCARE through the Climate Change and
Atmospheric Research (CCAR) program at the Natural Sciences and Engineering
Research Council of Canada (NSERC), by the Alfred Wegener Institute (AWI), and by Environment and Climate Change Canada (ECCC). We gratefully
acknowledge the funding by the Deutsche Forschungsgemeinschaft (DFG, German
Research Foundation) – project no. 268020496 – TRR 172, within the
Transregional Collaborative Research Center “ArctiC Amplification: Climate
Relevant Atmospheric and SurfaCe Processes, and Feedback Mechanisms
(AC)