ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-19-57-2019Aircraft-based measurements of High Arctic springtime aerosol show evidence for vertically varying sources, transport and compositionSpring Arctic aerosolWillisMegan D.megan.willis@mail.utoronto.cahttps://orcid.org/0000-0003-0386-0156BozemHeikohttps://orcid.org/0000-0003-2412-9864KunkelDanielhttps://orcid.org/0000-0002-9652-0099LeeAlex K. Y.SchulzHanneshttps://orcid.org/0000-0002-5151-6467BurkartJuliaAliabadiAmir A.HerberAndreas B.LeaitchW. RichardAbbattJonathan P. D.https://orcid.org/0000-0002-3372-334XDepartment of Chemistry, University of Toronto, Toronto, Ontario, CanadaInstitute for Atmospheric Physics, Johannes Gutenberg University of Mainz, Mainz, GermanyDepartment of Civil and Environmental Engineering, National University of Singapore, SingaporeAlfred Wegener Institute, Helmholtz Center for Polar and Marine Research, Bremerhaven, GermanyFaculty of Physics, Aerosol Physics and Environmental Physics, University of Vienna, Vienna, AustriaSchool of Engineering, University of Guelph, Guelph, Ontario, CanadaEnvironment and Climate Change Canada, Toronto, Ontario, Canadanow at: Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USAMegan D. Willis (megan.willis@mail.utoronto.ca)3January2019191577622June201824August201810December201811December2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://acp.copernicus.org/articles/19/57/2019/acp-19-57-2019.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/19/57/2019/acp-19-57-2019.pdf
The sources, chemical transformations and removal mechanisms of aerosol
transported to the Arctic are key factors that control Arctic
aerosol–climate interactions. Our understanding of sources and processes is
limited by a lack of vertically resolved observations in remote Arctic
regions. We present vertically resolved observations of trace gases and
aerosol composition in High Arctic springtime, made largely north of
80∘ N, during the NETCARE campaign. Trace gas gradients observed on
these flights defined the polar dome as north of 66–68∘ 30′ N
and below potential temperatures of 283.5–287.5 K. In the polar dome, we
observe evidence for vertically varying source regions and chemical
processing. These vertical changes in sources and chemistry lead to
systematic variation in aerosol composition as a function of potential
temperature. We show evidence for sources of aerosol with higher organic
aerosol (OA), ammonium and refractory black carbon (rBC) content in the upper
polar dome. Based on FLEXPART-ECMWF calculations, air masses sampled at all
levels inside the polar dome (i.e., potential temperature <280.5K, altitude <∼3.5km) subsided during transport
over transport times of at least 10 days. Air masses at the lowest potential
temperatures, in the lower polar dome, had spent long periods (>10 days)
in the Arctic, while air masses in the upper polar dome had entered the
Arctic more recently. Variations in aerosol composition were closely related
to transport history. In the lower polar dome, the measured sub-micron
aerosol mass was dominated by sulfate (mean 74 %), with lower contributions from rBC (1 %), ammonium (4 %) and OA
(20 %). At higher altitudes and higher potential temperatures, OA,
ammonium and rBC contributed 42 %, 8 % and 2 % of aerosol mass,
respectively. A qualitative indication for the presence of sea salt showed
that sodium chloride contributed to sub-micron aerosol in the lower polar
dome, but was not detectable in the upper polar dome. Our observations
highlight the differences in Arctic aerosol chemistry observed at
surface-based sites and the aerosol transported throughout the depth of the
Arctic troposphere in spring.
Introduction
Arctic regions are warming faster than the global average, with significant
impacts on local ecosystems and local people
e.g,. While Arctic warming is driven largely
by increasing concentrations of anthropogenic greenhouse gases and local
feedback mechanisms, short-lived climate forcing agents also impact Arctic
climate. In particular, short-lived species such as aerosol, tropospheric
ozone and methane are important climate forcers
e.g.,. The impact of pollution aerosol, transported
northward over long distances, on Arctic climate has been significant. For
example, a large fraction of greenhouse-gas-induced warming (∼60 %) has
been offset by anthropogenic aerosol over the past century, such that
reductions in sulfur emissions in Europe since 1980 can explain a large
amount of Arctic warming since that time (∼0.5K)
. These estimates are compelling, and at the
same time global models that form the basis of our predictive capability
often struggle to reproduce key characteristics of Arctic aerosol, such as
the seasonal cycle and vertical distribution
. Our incomplete
understanding of Arctic aerosol processes results in diverse and frequently
poor model skill in simulating Arctic aerosol both at the surface and through
the troposphere, and therefore also in accurately simulating aerosol–climate
interactions . This challenge arises in part due to a lack of
vertically resolved observations in Arctic regions.
Particle composition drives aerosol optical properties
e.g.,, ice nucleation efficiency
e.g.,, and heterogeneous chemistry that impacts
both gas and particle composition e.g.,. The
vertical distribution of aerosol and its chemical and physical properties can
impact Arctic regional climate in a number of ways. First, absorption of
incoming solar radiation by aerosol (e.g., black carbon) can lead to warming
in the lower troposphere when present near the surface. In contrast,
absorbing aerosol present at higher altitudes causes cooling at the surface
and impacts atmospheric stratification
. Further, the location
in the troposphere impacts deposition to high albedo surfaces, depending on
the mechanism of removal e.g.,. Absorbing aerosol
deposited to the surface has a strong impact on the albedo of ice and snow,
efficiently leading to warming .
Second, neutralization of acidic sulfate impacts aerosol water content and
aerosol phase, with implications for the magnitude of aerosol–radiation
interactions . Third, sulfuric acid coatings on
particles can decrease their ability to act as ice-nucleating particles
(INPs), leading to larger, more readily precipitating ice crystals
. This process can lead to enhanced atmospheric
dehydration, leading to diminished long-wave forcing
. Further, particles containing mineral
dust, organic species, sea salt or neutralized sulfate can act as ice nuclei
and increase ice crystal number, also leading to impacts on long-wave and
short-wave cloud forcing .
Finally, Arctic pollution aerosol can impact the micro-physical properties of
liquid-containing clouds, by increasing liquid water path and decreasing
droplet radius. Such micro-physical changes can result in enhanced long-wave
warming effects during winter and spring .
Observations at Arctic ground-based monitoring stations form the basis of our
current knowledge about Arctic aerosol seasonality, chemical composition and
sources. These long-term observations have demonstrated a pronounced seasonal
cycle in Arctic aerosol mass concentrations, particle size distribution and
composition, driven by seasonal variations in northward long-range transport
and aerosol wet removal
e.g.,.
Aerosol mass concentrations peak in winter to early spring when long-range-transported accumulation mode particles (200–400 nm mode
diameter), referred to broadly as “Arctic haze”, dominate the particle size
distribution e.g.,. A mixture of natural and
anthropogenic aerosol is transported to Arctic regions by near-isentropic
transport along surfaces of constant potential temperature that slope upwards
toward the Arctic . The sloping isentropic
surfaces form a closed “dome” over the polar region; this polar dome is
zonally asymmetric, extends further south in winter and contracts northward
in spring to summer .
Arctic haze observed near the surface is largely acidic sulfate, with fewer
contributions from organic aerosol (OA), dust, nitrate, ammonium and sea salt
. Aerosol acidity increases
during winter and reaches a peak in late spring
, before the return of wet removal brings the
Arctic toward near-pristine conditions with more neutralized aerosol
. Sea salt is
thought to be an important contributor to Arctic haze in winter to early
spring owing to stronger wind speeds over nearby oceans, potential
wind-driven sources in ice and snow-covered regions, and open leads
. The major source region of
near-surface Arctic haze in winter and early spring is northern Europe and
northern Asia/Siberia, but the magnitudes of sources in this region have been
decreasing in recent decades
.
Surface-based observations have provided substantial insight into Arctic
aerosol processes, but owing to the stability of the troposphere the surface
can be decoupled from the atmosphere above. Therefore, surface-based
observations may not represent the overall composition of aerosol transported
to the Arctic troposphere e.g.,. How
transported aerosol present throughout the troposphere is related to Arctic
haze observed near the surface remains an unresolved question
.
Vertically resolved observations of the Arctic atmosphere, in the last 20 years, have furthered our understanding of the properties, processes, and
impacts of Arctic aerosol. Some of the only seasonal airborne observations of
aerosol sulfate suggested that the aerosol seasonal cycle may differ aloft
compared to near the surface . Clean-out may
begin to take place near the surface in late April to May, before significant
changes occur aloft. Intensive observations were made during the
International Polar Year (IPY) in 2007–2008. During IPY, high concentrations
of aerosol and trace gases from biomass and fossil fuel burning were observed
in discrete layers that did not appear related to Arctic haze observed near
the surface e.g.,. Also during
IPY, aerosol ammonium content increased from near the surface toward the
middle to upper troposphere . The largest fraction of
sulfate was observed in the lower ∼2km, in general agreement
with long-term monitoring observations. In years with high burned area in the
Northern Hemisphere, such as 2008, biomass burning sources contribute a
significant fraction of black carbon and organic aerosol in the Arctic
troposphere . In years with
moderate burned area consistent with decadal mean conditions, anthropogenic
sources can still lead to enhanced absorbing aerosol in the Arctic
mid-troposphere . IPY observations in the Alaskan Arctic
demonstrated that background pollution aerosol (i.e., in air masses with
CO<170ppbv) and aerosol in the near-surface layer
(i.e., in air masses with depleted O3) contained a larger fraction of
sulfate compared to aerosol attributed to biomass or fossil fuel burning
. The properties of Arctic background air masses were
generally consistent with median observations at a nearby ground station,
Utqiaġvik (Barrow), Alaska . This background
aerosol often has diffuse source regions that are difficult to diagnose
precisely using 10-day backward trajectories
e.g,.
Our knowledge of the vertical distribution of Arctic aerosol source regions
has also been extended by recent airborne observations. Results from
modelling efforts generally agree that Arctic pollution aerosol is a result
of a combination of anthropogenic and natural sources from mid-latitudes in
the Northern Hemisphere; particularly a combination of European, north and
south Asian, and North American source regions
e.g.,. However, modelling efforts provide less
quantitative agreement on the magnitude of the contributions of each region
near the surface and as a function of altitude. Our emerging understanding is
of northern Eurasian sources dominating near the surface in winter, while
North America and southern and eastern Asia can be important in the middle to upper troposphere
e.g.,. In
spring, as the polar dome recedes northward, North American and Asian sources
become more important at all altitudes . Overall,
more southerly source regions become more important at higher altitudes
e.g.,, and the importance of Asian
sources above the Arctic surface is being increasingly recognized
e.g.,. The magnitude of Asian influence on
the lower troposphere inferred from models in spring varies significantly and
depends on emissions estimates and assumptions about aerosol removal
efficiency during transport e.g.,. Source
apportionment of recent vertically resolved Arctic black carbon observations
demonstrated that eastern and southern Asia make important contributions
throughout the troposphere in spring, with a more significant contribution at
higher altitudes . Northern Asia was a more important source
region near the surface . Changes in source strengths at
mid-latitudes and within the Arctic strongly impact the dominant source
regions for different aerosol species .
Previous vertically resolved observations of Arctic pollution aerosol
frequently focused on episodic events of high pollutant concentrations,
largely owing to their potential radiative impact
e.g.,. We know less about the
vertical distribution of Arctic aerosol properties within the High Arctic
polar dome and under conditions consistent with Arctic background conditions
(e.g., CO<170ppbv; ). Improved
understanding of different anthropogenic and natural contributions to Arctic
aerosol will provide a scientific basis for sustainable climate mitigation
and adaptation strategies. Within the framework of the NETCARE project,
airborne observations of Arctic haze aerosol were made across the North
American and European Arctic in April 2015. Observations of trace gas
gradients during this campaign were used by to define the
boundaries of the polar dome. The location of the maximum trace gas gradient
defined the polar dome as north of 66–68∘ 30′ N and below potential
temperatures of 283.5–287.5 K. Based on we use a
conservative definition of the polar dome area based on the interquartile
range of the location of maximum trace gas gradient: north of 69∘ 30′ N
and below 280.5 K. In this work, we quantify vertical changes in
sub-micron aerosol composition in the Canadian High Arctic within the
boundaries of the polar dome and in the absence of episodic transport events
of high pollutant concentrations. Using the Lagrangian particle dispersion
model FLEXPART, we explore the source regions that drive observed sub-micron
aerosol in the springtime polar dome. Finally, we examine the depth over
which aerosol consistent with surface monitoring observations extends
vertically in the polar dome, and assess the representativeness of
ground-based observations for aerosol transported to the polar dome in
spring.
MethodsHigh Arctic measurementsMeasurement platform and inlets
Measurements of aerosol, trace gases and meteorological parameters were made
in High Arctic spring aboard the Alfred Wegener Institute (AWI) Polar 6
aircraft, an unpressurized DC-3 aircraft converted to a Basler BT-67
, as part of the Network on Climate and Aerosols: Addressing
Key Uncertainties in Remote Canadian Environments project (NETCARE,
http://www.netcare-project.ca, last access: 20 December 2018), and in
partnership with the Polar Airborne Measurements and Arctic Regional Climate
Model Simulation Project (PAMARCMiP; ). Measurements on a
total of 10 flights took place from 4 to 22 April 2015, based at four
stations along the PAMARCMiP track: Longyearbyen, Svalbard
(78.2 ∘ N, 15.6 ∘ E); Alert, Nunavut, Canada
(82.5 ∘ N, 62.3 ∘ W); Eureka, Nunavut, Canada
(80.0 ∘ N, 85.9 ∘ W); and Inuvik, Northwest Territories,
Canada (68.4 ∘ N, 133.7 ∘ W). To focus our analysis on
aerosol within the polar dome, a subset of six flights in the High Arctic
during 7–13 April 2015 are considered in this analysis (Fig. ).
The vertical extent of these flights is shown in Fig. S1 in the Supplement.
During measurement flights aircraft speed was maintained at ∼75ms-1 (∼270kmh-1), with ascent and
descent rates of ∼150mmin-1.
Aerosol and trace gas inlets were identical to those used aboard Polar 6
during the NETCARE 2014 summer campaign and are described in
and . Briefly, aerosol was sampled approximately isokinetically
through a stainless steel shrouded diffuser inlet,
with near-unity transmission of particles 20 nm to ∼1µm in diameter at typical survey airspeeds and a total flow
rate of approximately 55 Lmin-1. Bypass lines off the main
inlet, at angles of 45∘, carried aerosol to various instruments.
Performance of the aerosol inlet used here was characterized by
. Aerosol was not actively dried prior to sampling; however,
the temperature in the inlet line within the aircraft cabin was at least
15 ∘C warmer than the ambient temperature so that the relative
humidity (RH) decreased significantly.
Map of the NETCARE 2015 campaign study area, showing sea ice
concentrations on 11 April 2015 . All stations along the
NETCARE/PAMARCMiP 2015 track are shown with yellow stars (Longyearbyen,
Svalbard; Alert, Nunavut; Eureka, Nunavut; Resolute Bay, Nunavut; and Inuvik,
Northwest Territories). Parallels are shown in dashed circles at 60, 70 and
80∘ N. Inset: flight tracks from six flights during 7–13 April
2015 based in Alert and Eureka, Nunavut, which are the focus of this work.
State parameters
State parameters and meteorological conditions were measured with an
AIMMS-20, manufactured by Aventech Research Inc. (Barrie, ON, Canada;
https://aventech.com/products/aimms20.html, last access: 20 December
2018). The AIMMS-20 consists of three modules: (1) an Air Data Probe, which
measures temperature and the three-dimensional aircraft-relative flow vector
(total air speed TAS, angle of attack, and side slip) with a three-dimensional accelerometer for
measurement of turbulence; (2) an Inertial Measurement Unit, which provides
the aircraft angular rate and acceleration; and (3) a Global Positioning
System for aircraft three-dimensional position and inertial velocity.
Vertical and horizontal wind speeds are measured with accuracies of 0.75 and
0.50 ms-1 respectively. Accuracy and precision of the
temperature measurement are 0.30 and 0.10 ∘C respectively. Potential
temperature was calculated using temperature and pressure measured by the
AIMMS-20.
Trace gases
Carbon monoxide. CO concentrations were measured at
1 Hz with an Aerolaser ultra-fast carbon monoxide monitor (model
AL 5002), based on VUV fluorimetry using excitation of CO at
150 nm. The instrument was modified such that in situ calibrations
could be conducted in flight. Measured concentrations were significantly
higher than the instrument detection limit. The measurement precision is ±1.5ppbv, with an instrument stability based on in-flight
calibrations of 1.7 %.
Water vapour and carbon dioxide. H2O and CO2
measurements were made at 1 Hz using non-dispersive infrared
absorption with a LI-7200 enclosed CO2/H2O analyzer from
LI-COR Biosciences. In situ calibrations were performed during flight at
regular intervals (15–30 min) using a NIST traceable CO2
standard with zero water vapour concentration. Measured concentrations were
significantly higher than the instrument detection limit. The measurement
precision for CO2 is ±0.05ppmv, with an instrument
stability based on in-flight calibrations of 0.5 %. The measurement precision
for H2O is ±18.5ppmv, with an instrument stability based
on in-flight calibrations of 2.5 %.
Ozone. O3 concentrations were measured, with a time
resolution of 10 s, using UV absorption at 254 nm with a
Thermo Scientific ozone analyzer (model 49i). The measurement uncertainty is
±0.2ppbv.
Particle concentrations
Aerosol number size distributions from 100 nm to 1 µm
were acquired with two instruments: (1) a Droplet Measurement Technology
(DMT) Ultra-High Sensitivity Aerosol Spectrometer (UHSAS) with a flow rate of
55 cm3min-1 from a bypass flow off the main aerosol inlet,
and (2) a GRIMM sky optical particle counter (Sky-OPC, model 1.129) with a
flow rate of 1200 cm3min-1 from a bypass flow off the main
aerosol inlet . In their overlapping size range, comparison of
UHSAS and OPC particle number concentrations suggested that the UHSAS
underestimated the concentration of larger particles (>500nm).
This comparison is presented in Fig. S2 and discussed further in Supplement
Sect. S1. We therefore present these observations as the number of particles
between 100 and 500 nm (N100–500) derived from UHSAS
observations and the number greater than 500 nm (N>500) from
the OPC. Recent work has highlighted the impact of rapid pressure changes,
during aircraft ascent and descent, on reported UHSAS particle concentrations
. However, comparison between particle measurements
during NETCARE 2015 suggests that these effects are not significant, likely
owing to the relatively slow vertical speed of the Polar 6 .
Owing to these instrumental discrepancies present at low particle number
concentrations, we emphasize that absolute particle number concentrations
should be treated with caution.
Particle composition
Refractory black carbon. Concentrations of particles containing
refractory black carbon (rBC) were measured with a DMT single-particle soot
photometer (SP2) . The SP2 uses a continuous
intra-cavity Nd:YAG laser (1064 nm) to classify particles as either
incandescent (rBC) or scattering (non-rBC), based on the individual
particle's interaction with the laser beam. The peak incandescence signal is
linearly related to the rBC mass. The SP2 was calibrated with Fullerene Soot
(Alfa Aesar) standard by selecting a narrow size distribution of particles
with a differential mobility analyzer upstream of the SP2 .
The SP2 efficiently detected particles with rBC mass of 0.6 to
328.8 fg, which corresponds to 85–704 nm mass equivalent
diameter (assuming a void free bulk material density of
1.8 gcm-3). rBC mass concentrations were not corrected for
particles outside the instrument size range, and the measurement uncertainty
is ±15 % . Measurements of rBC during NETCARE 2015 are
discussed in detail by .
Non-refractory aerosol composition. Non-refractory aerosol
composition was measured with an Aerodyne time-of-flight aerosol mass
spectrometer (ToF-MS) . Operation of the ToF-AMS aboard
Polar 6 and characterization of the pressure-controlled inlet system is
described in . The ToF-AMS deployed here was
equipped with an infrared laser vaporization module similar to that of the
DMT SP2 (SP laser) ; however, rBC concentrations during
the flights discussed here were generally below ToF-AMS detection limits
(∼0.1µgm-3 for rBC) so SP2 measurements of rBC are
used in this work. The instrument was operated up to an altitude of ∼3.5km, and the temperature of the ToF-AMS was passively maintained
using a modular foil-lined insulating cover. The ToF-AMS was operated in
“V-mode” with a mass range of m/z 3–290, alternating between ensemble
mass spectrum (MS) mode for 10 s (two cycles of 5 s MS open and 5 s
MS closed) with the SP laser on, MS mode with the SP laser off for
10 s, and efficient particle time-of-flight (epToF) mode with the SP laser on for 10 s (Supplement Table S1)
. Single-particle observations were made on two
flights; this ToF-AMS operation mode is described below. Only observations
made with the SP laser off are used to quantify non-refractory aerosol
composition. Filtered ambient air was sampled with the ToF-AMS at least 3 times per flight, for a duration of at least 5 min, to account for
contributions from air signals.
Species comprising non-refractory particulate matter are quantified by the
ToF-AMS, including sulfate (SO4), nitrate (NO3), ammonium (NH4),
and the sum of organic species (OA). The ToF-AMS is also capable of detecting
sea salt . The detection efficiency of sea-salt-containing particles is dependent on not only the ambient RH but also
the temperature of the tungsten vaporizer . A
quantitative estimate of sea salt mass is not possible with these
measurements and this species is not included in the calculation of aerosol
chemical mass fractions, such that the mass fractions presented represent
non-refractory aerosol species and rBC measured by the SP2. The vaporizer
temperature was calibrated with sodium nitrate particles and was operated at
a temperature of ∼650∘C. ToF-AMS signals for sea salt, in
particular NaCl+ (m/z 57.96), can be used to quantify sea salt
; however, here we use the NaCl+ signal only as
a qualitative indication for the presence of sea salt owing to uncertainties
in sea salt collection efficiency as a function of RH and the lack of RH
measurement in the sampling line. Ammonium nitrate calibrations
were carried out twice during the campaign as well as
before and after, owing to restricted access to calibration instruments
during the campaign. Air-beam corrections were referenced to the appropriate
calibration in order to account for differences in instrument sensitivity
between flights. The relative ionization efficiencies for sulfate and
ammonium (RIESO4 and RIENH4) were
0.9±0.1 and 3.4±0.3. The default relative ionization
efficiency for organic species (i.e., RIEOrg=1.4) was used,
which is appropriate for oxygenated organic aerosol
. Elemental composition was calculated using the
method presented in . Data were analyzed using the Igor
Pro-based analysis tool PIKA (v.1.16H) and SQUIRREL (v.1.57l)
. Detection limits and propagated uncertainties
(i.e., ±(detection limit + total uncertainty)) for sulfate, nitrate,
ammonium, and organics at a 10 s time resolution were
±(0.009 µgm-3+ 35 %), ±(0.001 µgm-3+ 33 %), ±(0.003 µgm-3+ 33 %), and
±(0.08 µgm-3+ 37 %), respectively. We note that ion
ratios commonly reported from ToF-AMS measurements of ammonium and sulfate
are not appropriate for estimating aerosol neutralization ,
so we do not report these here. A composition-dependent collection efficiency
(CDCE) was applied to correct ToF-AMS mass loadings for non-unity particle
detection due to particle bounce on the tungsten vaporizer
, which resulted in a median (quartile range) collection
efficiency correction of 18 % (12 %–28 %) applied uniformly to non-refractory aerosol
species.
ToF-AMS total non-refractory aerosol mass correlated well with estimated
aerosol mass from the UHSAS and OPC, but was generally higher by
approximately a factor of 2 (Fig. S4, assuming a mean density of
1.5 gcm-3). An important exception to this observation occurred
when the ToF-AMS measured significant NaCl+; at these times, the
ToF-AMS total aerosol mass was relatively constant while the estimated mass
increased, indicating that sea salt was an important contributor to aerosol
mass. These discrepancies are discussed further in Sect. S1 of the
Supplement. Owing to the discrepancies between measured and estimated
particle mass, we emphasize that absolute mass concentrations presented in
this work should be treated with caution; however, these discrepancies do not
prevent a useful interpretation of the ToF-AMS data based upon relative
changes in particle composition.
ToF-AMS single-particle measurements. The ToF-AMS was operated in
Event Trigger Single Particle (ETSP) mode on two flights (Table S1). ETSP is
run in the single-slit particle-time-of-flight (pToF) mode. A particle event
is defined as a single mass spectrum (MS) extraction or set of consecutive MS
extractions associated with a single particle being vaporized and producing
MS signals. The number of MS extractions obtained during a particle event is
determined by the pulser frequency, and thus the mass range, set during
acquisition; in this case 30.9 kHz, corresponding to a pulser period
of 32.4 µs (m/z 3–290). Under these conditions, at least a
single mass spectrum is collected per particle event. Saving mass spectra
associated with a particle event is triggered in real time based on the
signals present in up to three continuous ranges of mass-to-charge ratios,
called regions of interest (ROIs). Three ROIs were used in this work such
that a signal above a specified ion threshold in any ROI would trigger saving
a mass spectrum (Table S2). Ion thresholds were purposely set low to collect
a large number of false positives that are subsequently removed based on the
relationship between total aerosol ion signal (i.e., excluding air) and
particle size (Fig. S5), similar to the approach described in .
Two background regions in the particle size distribution (10–50 and
2000–4000 nm) were selected to determine the average background ion
signal excluding air peaks, and particle events considered “real” must be
between 80 and 1000 nm with ion signals above the mean background
plus 3 times its standard deviation (Fig. S5). A simplified fragmentation
table, described in , was applied to particle mass spectra
identified as “real” and fragmentation corrections were based on higher
mass resolution ensemble MS spectra collected concurrently. A total of 1677
“real” particle spectra were collected over two flights (8 and 13 April
2015). A k-means cluster analysis was applied to particle spectra to
explore different particle mixing states, following . A
two-cluster solution was selected to describe the 1677 total “real”
particle spectra. Owing to the small number of particle spectra and the lack
of specificity in organic aerosol peaks from highly oxygenated aerosol,
increasing the number of clusters did not yield physically meaningful
information. Mean mass spectra and mass spectral histograms for each particle class are shown in Fig. S6. ETSP
data were analyzed using the Igor Pro-based analysis tools Tofware
version 2.5.3.b (developed by TOFWERK and Aerodyne Research, Inc.),
clustering input preparation panel (CIPP) version ETv2.1b and cluster
analysis panel (CAP) version ETv2.1 (developed by Alex K. Y. Lee and
Megan D. Willis).
Air mass history from particle dispersion modelling
The Lagrangian particle dispersion model FLEXible PARTicle (FLEXPART)
driven by meteorological analysis data from the European
Centre for Medium-Range Weather Forecasts (ECMWF) was used to study the
history of air masses prior to sampling during NETCARE flights. The ECMWF
data had a horizontal grid spacing of 0.25∘ and 137 vertical levels.
Here, we use FLEXPART-ECMWF run in backward mode to study the origin of air
influencing aircraft-based aerosol and trace gas measurements. Individual
FLEXPART parcels were initialized along the flight track every 3 min
and then traced back in time for 10 days, providing time-resolved information
on source regions of trace species measured along the flight track.
FLEXPART-ECMWF output was provided every 3 h over the 10-day period,
with horizontal grid spacing of 0.25∘ and 10 vertical levels (50,
100, 200, 500, 1000, 2000, 4000, 6000, 8000 and 10 000 m). In
backward mode, the model provides an emission sensitivity function called the
potential emission sensitivity (PES). The PES in a particular grid cell, or
air volume, is the response function of a source–receptor relationship, and
is proportional to the particle residence time in that grid cell
e.g.,. PES values can be combined with emission
distributions to calculate receptor concentrations, assuming the species is
inert; however, we use the PES directly and show maps of PES with units of
seconds (i.e., proportional to air mass residence time). Absolute residence
times depend on the model output time step and the extent of spatial
averaging. Maps of PES represent integration of model output over a period of
time prior to sampling (i.e., 10 days), also referred to as the “time before
measurement”, and over a vertical range. We show maps of both the total
column PES (i.e., 0–20 km) and partial column PES (i.e.,
0–200 m), as emissions near the surface are of particular interest.
By integrating model output at each model release over specific pressure
levels and/or latitude ranges we used FLEXPART-ECMWF to calculate the
residence time of air in the middle-to-lower polar dome. The horizontal
extent of the polar dome was defined based on as north of
69∘ 30′ N. The vertical extent of the middle-to-lower polar dome was
defined based on trace gas profiles as below 265 K
(∼1550m). Calculation of this quantity is analogous to
calculating the PES (i.e., by integrating in time and space), with
constraints on altitude and location. This residence time is reported as a
relative residence time over the 10-day FLEXPART-ECMWF backward integration
time. Aircraft observations were sub-sampled to the model time resolution by
taking a 1 min average of measurements around the model release time,
when the aircraft altitude was within ±100m of the model release
altitude.
Mean potential temperature profiles of trace gases (CO,
CO2, O3 and H2O) and particle concentrations
(N100–500 and N>500) in the polar dome observed during
7–13 April 2015. Coloured lines indicate the mean profile for each flight,
the black line represents the mean profile over all flights, and gray shading
shows the range of observations in each potential temperature bin. Horizontal
dashed blue lines separate the lower, middle and upper polar dome defined as
245–252, 252–265 and 265–280 K.
Results and discussionTransport regimes in the polar dome
We focus on observations made on six flights in the High Arctic during
NETCARE 2015 over the period 7–13 April 2015. Figure
illustrates flight tracks during this period on a map of the sea ice
concentration from 11 April 2015. Observations of trace gas gradients during
this campaign defined the region inside the polar dome as north of
69∘ 30′ N and below 280.5 K (∼3.5km)
. Zonal mean potential temperature cross sections from ECMWF
for the period 7–13 April 2015 generally agree with this definition of the
polar dome, and this demonstrates that our observations were made in the coldest
air masses present in the Arctic region during this time (Fig. S7).
CO concentrations observed in the polar dome were consistent with
“Arctic background” air masses identified in previous airborne observations
and with monthly mean CO concentrations at Alert, Nunavut, Canada
(Fig. S8). This suggests that our observations during April 2015 in the
polar dome were not strongly impacted by episodic transport events of high
pollutant concentrations . We restrict our analysis to those
air masses residing in the polar dome, to determine the sources and processes
contributing to aerosol composition within this region during spring. When
discussing observations and model predictions, we use potential temperature
instead of height or pressure for two reasons. First, the location of the
polar dome and transport northward are dictated by potential temperature
rather than absolute height. Second, trace gases and aerosol observed in the
polar dome varied systematically with potential temperature, but showed less
systematic variability with pressure (Fig. S9). Altitude profiles of
absolute and potential temperature are shown in Fig. S10. In this section,
we discuss transport patterns inferred from trace gas observations and
FLEXPART-ECMWF air mass history, and in Sect. we
discuss observed aerosol composition in the context of these transport
patterns.
FLEXPART-ECMWF potential emission sensitivity (PES) and plume
centroid altitude averaged over three potential temperature ranges in the
polar dome. (a–c) Mean total column PES, (d–f) mean
partial column (<200 m) PES, (g–i) mean plume centroid
altitudes for 245–252 K (a, d, g), 252–265 K (b, e, h)
and 265–280 K (c, f, i). Fire locations during 28 March to
13 April 2015 from MODIS are purple points, gas flaring locations associated
with oil and gas extraction from the ECLIPSE emission inventory (V5) for 2015
are light blue points.
Parallels are shown in dashed circles at 45, 60 and 80∘ N.
Trends in trace gas concentrations with potential temperature illustrate
different transport regimes within the polar dome
(Fig. ). Based on the mean vertical profiles of trace
gases, we divided observed vertical profiles into three ranges of potential
temperature (Fig. : 245–252, 252–265 and 265–280 K) to guide interpretation of air mass
history, transport characteristics and aerosol composition in the polar dome.
We refer to these three ranges of potential temperature as the lower, middle
and upper polar dome, respectively (dashed horizontal lines in
Fig. ), and discuss the characteristics of each region in
turn. First, in the coldest and driest air masses (245–252 K), we
consistently observed temperature inversion conditions, with potential
temperature increasing by 37 Kkm-1 compared to
11 Kkm-1 above the lower polar dome (Fig. S10). Temperature
inversions are frequent in the High Arctic spring, with median inversion
strengths of ∼5–10 K occurring frequently in March, April
and May . Owing to the
static stability of the lower polar dome under these conditions, these air
masses may be isolated from the air aloft and may be sensitive to different
sources and transport history . Under these stable
conditions, CO and CO2 were relatively constant (mean
(quartile range), 144.5 (144.2–146.5) ppbv and 405.8
(405.4–406.2) ppmv, respectively) in the lower polar dome and
O3 was depleted to 11.4 (3.1–23.4) ppbv. Active halogen
production and resulting O3 depletion may occur largely at the
surface e.g.,. It follows
that the observed O3 profile could be interpreted as an indication of
mixing of O3-depleted air from the surface up to ∼252K
(∼400m). Particle number concentrations between 100 and 500 nm (N100–500) were relatively constant in the lower polar
dome (∼150cm-3), while larger accumulation mode particles
(N>500) were most abundant in the lower polar dome compared to higher
potential temperatures (∼4cm-3 compared to
<1cm-3). Second, in the middle polar dome (252–265 K), O3 increased toward ∼50ppbv and CO and CO2 remained relatively constant while water vapour
showed more variability. Finally, at the highest potential temperatures we
observed more variability in CO, CO2 and H2O, while
O3 concentrations were relatively constant at 49.6
(45.7–54.1) ppbv. N>500 was near zero in the upper polar
dome, while N100–500 showed more variability compared to colder
potential temperatures.
The importance of lower latitude source regions increases as potential
temperature increases in the polar dome. The distribution of FLEXPART-ECMWF
potential emission sensitivities (Fig. ) indicates that most
air masses in the lower and middle polar dome had resided there for at least
10 days, with significant sensitivity to the surface north of
80∘ N and some sensitivity to high-latitude Eurasia. The fraction of
the previous 10 days spent in the polar dome is highest in the middle and
lower polar dome, while above ∼265K this quantity decreases
significantly (Fig. , S8). This observation indicates a
clear separation in air mass history between the middle-to-lower polar dome and
the upper polar dome. Sensitivity to lower latitude regions increases as
potential temperature increases in the polar dome, particularly in high-latitude Eurasia and North America (Fig. ). Locations of
active fires during 28 March 2015–13 April 2015 and of oil and gas
extraction emissions (Fig. ) indicate that biomass burning
emissions likely had a stronger influence on the upper polar dome, while oil
and gas extraction emissions may be more important in the lower polar dome.
Total March–May 2015 fire counts in the Northern Hemisphere were comparable
to previous years (Fig. S12), but were significantly lower than 2008. This
suggests that biomass burning sources are often less important sources of
Arctic aerosol than has been suggested by previous observations from the year
2008 e.g.,.
Observed potential temperature (K)
versus FLEXPART-ECMWF-predicted fraction of the past 10 days in the polar
dome (i.e., below 280.5 K and north of 69∘ 30′ N). The
FLEXPART-ECMWF relative residence time is binned in the lower
(245–252 K), middle (252–265 K) and upper
(265–280 K) polar dome.
A prevalent feature of air mass histories in the lower and middle polar dome
is descent from aloft over at least 10 days prior to our measurements
(Fig. g, h). The FLEXPART-ECMWF-predicted plume centroid also
shows some evidence for descent in the upper polar dome
(Fig. i), though we note that descent from aloft in the plume
centroid does not preclude some sensitivity to the surface. Air mass descent
in the polar dome is likely caused by a combination of both radiative cooling
(on the order of 1 Kday-1; ) and orographic
effects over nearby elevated terrain on Ellesmere Island and Greenland. With
long aerosol lifetimes under cold and relatively dry conditions in the polar
dome, this suggests that aerosol in the upper polar dome can influence the
lower and middle polar dome on the timescale of 10 days and longer.
Transport times to the Arctic lower troposphere are likely longer than
10 days e.g.,, suggesting that a major
springtime transport mechanism may be lofting near source regions, followed by
northward transport and descent into the polar dome . In the
next section, we discuss observed aerosol composition in the context of these
transport patterns.
Aerosol composition in the polar dome
Vertical variability in aerosol composition was systematic across flights in
the polar dome during April 2015. Sub-micron aerosol present in the coldest
air masses of the lower polar dome contained the highest fraction of sulfate
(74 % by mass, Fig. ). This trend in the sulfate mass
fraction (mfSO4) was driven by both decreasing sulfate and
increasing organic aerosol concentrations as potential temperature increased
(Figs. , S5). This observation is broadly consistent with
previous vertically resolved measurements of aerosol sulfate in both the
Canadian Arctic and Alaskan Arctic during spring that have indicated
increasing sulfate concentrations toward lower altitudes
. Large contributions of sulfate to near-surface Arctic spring aerosol is also consistent with ground-based
observations at long-term monitoring stations including Zeppelin, Svalbard;
Alert, Nunavut; and Utqiaġvik (Barrow), Alaska
e.g.,. The mass fraction of
ammonium (mfNH4) increases with increasing potential temperature.
This trend is driven by both decreasing sulfate concentration and increasing
ammonium concentration as potential temperature increases
(Fig. ). This observation is broadly consistent with
previous vertically resolved measurements in the North American Arctic from
April 2008 that demonstrated increased ammonium relative to sulfate toward
higher altitudes . However, observed
significantly higher ammonium relative to sulfate compared to our
measurements. These differences may arise from the larger altitude range in
(up to ∼10km) and differences in source
regions or source strengths between 2008 and 2015.
Mean potential temperature profiles of relative aerosol composition,
including mass fractions of sulfate (mfSO4), organic aerosol
(mfOA), refractory black carbon (mfrBC), and ammonium
(mfNH4), in the polar dome observed during 7–13 April 2015.
Coloured lines indicate the mean profile for each flight, the black line
represents the mean profile over all six flights, and gray shading shows the
range of observations in each potential temperature bin.
Mean potential temperature profiles of absolute (STP) sub-micron
aerosol composition in the polar dome observed during 7–13 April 2015,
including sulfate, organics and ammonium from the ToF-AMS and refractory
black carbon (rBC) from the SP2. Nitrate concentrations were negligible, and
largely below detection limits. Coloured lines indicate the mean profile for
each flight, the black line represents the mean profile over all six flights,
and gray shading shows the range of observations in each potential
temperature bin. Single points at the lowest potential temperature represent
concentrations of sulfate, ammonium and rBC measured at Alert, NU, during
6–13 April 2015 from . Points represent the mean
concentration and error bars represent measurement uncertainty.
Organic aerosol and refractory black carbon were more abundant in the upper polar dome, while sulfate was
less abundant. On average, OA and rBC contributed 42 % and 2 % to
aerosol mass, respectively, in the upper polar dome. OA was highly oxygenated
throughout the polar dome, with oxygen-to-carbon (O / C) ratios above 0.5
in the majority of measurements (Fig. S13). High O / C ratios are
consistent with an abundance of highly functionalized organic acids observed
in Arctic haze aerosol at Alert, Nunavut, during spring
. Owing to
the lack of unique mass spectral fragments from this highly oxygenated OA,
our ToF-AMS spectra cannot distinguish differences in OA composition in the
polar dome. Overall, our observations suggest that surface-based measurements
may underestimate the contribution of OA, rBC and ammonium to aerosol
transported to the Arctic troposphere in spring.
Air masses spent the longest times in the middle to lower polar dome
(Fig. ), and aerosol composition varied systematically
with time spent in this portion of the polar dome. The mass fractions of OA
and rBC decrease with the FLEXPART-ECMWF-predicted fraction of the previous
10 days spent north of 69∘ 30′ N and below 265 K
(Fig. ). OA and rBC were well-correlated in the middle and
upper polar dome (Fig. S14), suggesting that these species have a similar
source region and/or have undergone similar processing. A dominance of
anthropogenic (fossil fuel) sources of black carbon to the High Arctic during
April 2015 may explain this relationship between rBC and OA. The importance
of anthropogenic emissions of black carbon from eastern and southern Asia to
measured Arctic black carbon in spring was recently demonstrated using a
chemical transport model constrained by our measurements of black carbon in
combination with surface sites and previous aircraft-based campaigns
. European and north Asian anthropogenic emissions contributed
significantly to Arctic black carbon in the lowest kilometre, with eastern and
southern Asian sources increasing in importance toward higher altitudes
. Southern Asian regions are not well-represented in 10-day
FLEXPART-ECMWF backward simulations, which likely do not capture transport
back to all source regions . OA and rBC are largely
uncorrelated in the lower polar dome, suggesting shifting source regions
and/or chemical processing of OA toward lower potential temperatures. This
observation is consistent with multi-year observations from Alert, Nunavut,
showing that black carbon and organic matter are correlated during winter,
but become uncorrelated during spring .
Sub-micron aerosol mass fractions versus FLEXPART-ECMWF-predicted
fraction of the previous 10 days prior to measurement spent in the middle-to-lower polar dome (north of 69∘ 30′ N, , and, based on
trace gas profiles, below 265 K, ∼1600m). Data points
corresponding to individual FLEXPART-ECMWF releases are shown as circles, and
summary statistics are shown as boxes (25th, 50th, 75th percentiles) and
whiskers (5th, 95th percentiles) for data binned by time spent in the middle
and lower polar dome.
In contrast to OA and rBC, the mass fraction of sulfate increases with
increasing time spent in the middle-to-lower polar dome
(Fig. ). In the upper polar dome the ammonium-to-sulfate molar
ratio is at times consistent with ammonium bisulfate, while more sulfuric
acid is likely present at lower potential temperatures. The enhanced fraction
of sulfate in the lower polar dome compared to higher potential temperatures
could arise from a combination of possible mechanisms. First, the stability
of the polar dome may cause systematic vertical variability in source regions
throughout the polar dome e.g.,. The observed
middle-to-lower polar dome aerosol composition could arise from
high-latitude, sulfur-rich emissions in the absence of significant ammonia
and organic aerosol sources. A complex mixture of natural and anthropogenic
sources has previously been shown to contribute to observed variations in
sulfate and ammonium with altitude in Arctic spring . In the
lowest 2 km, emissions from non-Arctic Russia and Kazakhstan
(included as part of eastern and southern Asia in ), along with
North American emissions, were the dominant
sources of sulfate in April 2008 . At higher altitudes, model
results suggested eastern Asian sources of sulfate became more important and,
along with European sources, were the main contributor of sulfate aerosol in
Arctic spring. Second, and possibly in addition to shifting source regions,
aerosol composition could be changing as a result of chemical processing over
the long aerosol lifetime. The fraction of sulfate could be increasing with
decreasing potential temperature as a result of oxidation of transported
sulfur dioxide and subsequent condensation of sulfuric acid onto existing
particles as air masses slowly descend (Fig. ). In addition,
oxidation of existing OA, resulting in fragmentation and loss of aerosol mass
to the gas phase, could contribute to a decrease in OA concentrations toward
lower altitudes e.g.,; however, this process may be less
important at low temperatures. Descent from aloft appears to be an important
transport mechanism influencing the lower polar dome in our flight area,
lending some support to this second set of processes. Finally, wet removal or
cloud processing of aerosol over long transport times likely impacts the
aerosol composition we observe, though we cannot distinguish this influence
with our measurements. In the next section, we examine the characteristics of
lower polar dome aerosol in detail and compare it to aerosol present in the
middle and upper polar dome.
Potential temperature profiles of the ToF-AMS NaCl+ signal
(top axis), as a qualitative indication of the presence of sea salt aerosol,
and N>500 (bottom axis). The solid lines represent the mean profile for
7–13 April 2015, and shading represents the range of measurements in each
potential temperature bin.
Characteristics of lower polar dome aerosol
Lower polar dome air masses had resided for the longest times within the
polar dome (Figs. , and S11), suggesting
that this aerosol likely had a lifetime of 10 days or longer. This aerosol
was comprised largely of sulfate, with smaller amounts of OA, rBC and
ammonium compared to aerosol present in the middle and upper polar dome
(Sect. ). In addition, ToF-AMS spectra provide
qualitative evidence for the presence of sea salt aerosol in the lower polar
dome, which decreases to negligible concentrations through the middle polar
dome (Fig. ). The ToF-AMS NaCl+ signal and N>500
have a similar profile, suggesting that sea salt may be associated with the
increase in larger accumulation mode particles observed in the lower polar
dome. This observation is consistent with previous airborne measurements in
the Alaskan Arctic during spring that showed the largest fraction of sea salt
particles were present in air masses identified as associated with the
“Arctic boundary layer” (i.e., identified by depleted O3
concentrations) . Sea salt contributes significantly to
aerosol observed at ground-based long-term monitoring stations
e.g.,, and peaks in
concentration during winter to early spring. Sources of sea salt at high
northern latitudes in spring include transport of sea salt from northern
oceans, production of sea salt aerosol from open leads in sea ice
e.g.,, and production of saline aerosol
through wind driven processes over ice and snow
. The strong decrease in NaCl+
signal and N>500 above the lower polar dome is suggestive of a near-surface source of sea salt in the High Arctic; open leads or wind-driven ice
and snow processes may contribute to lower polar dome aerosol. Recent
observations at Utqiaġvik (Barrow), Alaska, have demonstrated the
prevalence of sea salt aerosol in Arctic winter and significant mixing with
sulfate . Sulfate may be internally mixed with sea salt in
the lower polar dome; however, owing to low particle concentrations we are
unable to obtain an NaCl+ signal from size-resolved mass spectra.
(a) Normalized mean ToF-AMS size distributions of sulfate
subset by observed potential temperature: below 252 K (black), above
265 K (light blue). (b) ToF-AMS size distributions of
sulfate (red) and total organic aerosol (green) below 252 K. The
mass fraction of sulfate calculated from ToF-AMS size distributions is shown
on the right axis in black circles, and is calculated only between 200 and
600 nm owing to low OA signals at smaller and larger sizes. Shading
corresponds to ±1 standard deviation for sulfate and organic
aerosol size distributions, and the relatively large variation in
size-resolved composition indicates that the derived mass fraction of
sulfate as a function of size is uncertain.
Summary of k-means cluster analysis of 1677 single-particle (ETSP)
spectra obtained on two flights (8 and 13 April 2015). (a) Bar
chart of relative ion fraction scaled by the relative ionization efficiency
(RIE) of each species, for two particle classes obtained by k-means cluster
analysis. Particle class 1 is referred to as “organic-rich” and class 2 is
referred to as “sulfate-rich”. (b) Mean size distributions
(expressed as dSignal/dlogDva, Hz) of
the two particle classes (points) and Gaussian fits to the observations
(lines). (c) Mean relative abundance of class 1 (green,
organic-rich) and class 2 (red, sulfate-rich) binned by potential
temperature. Horizontal lines represent the divisions between the lower,
middle and upper polar dome.
Size-resolved observations of non-refractory aerosol composition provide
evidence for different particle mixing states across the size distribution.
On average, sulfate was present in larger particle sizes in the lower polar
dome compared to the middle and upper polar dome (Fig. ). In
contrast, OA size distributions were very similar in the lower and upper
polar dome (Fig. S15). In the lower polar dome, the fraction of sulfate
increases with particle size (Fig. ), implying the presence of
different particle mixing states, and different particle sources or chemical
processing in the polar dome. Single-particle observations from two flights
(Figs. , S16) are consistent with these bulk size-resolved observations. Accumulation mode particles were highly internally
mixed, consistent with very aged particles, but the presence of sulfate-rich
and organic-rich particles was discernible from cluster analysis of ToF-AMS
spectra. Sulfate-rich particles were dominant in the coldest air masses and
were larger in size compared to organic-rich particles
(Figs. and S16). Increasing sulfate fraction toward
larger particle sizes (Fig. ) suggests that sulfuric acid may
have condensed on existing particles, growing them to larger sizes. An
increase in aerosol sulfate toward lower altitudes and a simultaneous
decrease in gas phase SO2 has been observed previously in Arctic
spring; this could be consistent with oxidation of SO2 and
condensation on pre-existing particles in the lowest 1–2 km. estimated a mean oxidation rate of
SO2 to sulfuric acid in April of
2.4 %day-1–4.8 %day-1, which could explain
the enhanced concentrations of sulfate toward lower potential temperatures
in our observations. While the smaller OA size in the lower polar dome could
be consistent with loss of OA mass through fragmentation processes, similar
OA size distributions in the lower and upper polar dome appear to negate this
possibility (Fig. S15). Our observations do not distinguish unambiguously
between vertical variability in source composition, chemical processing
during descent in the polar dome, and wet removal or cloud processing during
transport. All processes likely contribute to the systematic vertical
variability in High Arctic aerosol composition that we observe.
Conclusions
In the Arctic spring polar dome, aerosol composition and trace gas
concentrations varied systematically with potential temperature. We defined
the lower (245–252 K), middle (252–265 K) and upper
(265–280 K) polar dome based on vertical profiles of trace gases.
The contribution of sulfate increased from the upper to lower polar dome
(mean mass fractions 48 % and 74 %, respectively), while organic aerosol,
refractory black carbon and ammonium were more abundant in the upper polar
dome (mean mass fractions 42 %, 2 % and 8 %, respectively). At the lowest
potential temperatures, in the lower polar dome, sulfate-rich particles were
present at larger accumulation mode sizes compared to the upper polar dome.
While observations at long-term monitoring stations provide the majority of
our knowledge about Arctic aerosol, decoupling of air masses near the surface
from the rest of the polar dome means that surface-based observations may not
represent the altitude-dependent composition of aerosol transported to the
Arctic troposphere. Our observations indicate that long-term, surface-based
measurements may underestimate the contribution of organic aerosol,
refractory black carbon and ammonium to aerosol transported to the High Arctic troposphere in spring, while overestimating the contribution of
sulfate. In addition, our observations of sea salt signals in the lower
polar dome suggest that the significant sea salt concentrations observed at
long-term monitoring stations in spring may not occur throughout the depth of
the polar dome.
Systematic differences in aerosol composition with potential temperature
likely arise through a combination of mechanisms. First, aerosol from
different source regions, with differing composition, arrives at a range of
potential temperatures in a stable atmosphere. Second, aerosol composition
can be altered by chemical processing of transported aerosol and sulfur
dioxide during descent into the polar dome over periods of 10 days or longer.
Third, wet removal and cloud processing near emission and along the transport
path may impact the composition of aerosol arriving in the polar dome, though
this influence is difficult to distinguish with our observations. Modelled
air mass history from FLEXPART-ECMWF demonstrates that this systematic
variation in aerosol composition is in part related to differing transport
regimes as a function of potential temperature. In the lower polar dome, air
masses had resided in the High Arctic region for at least 10 days prior to
measurement and had largely descended from higher altitudes. Some sensitivity
to the High Arctic surface could explain the observed sea salt in the lower
polar dome. Lower latitude source regions in Europe, Asia and North America
became more important toward higher potential temperatures in the upper polar
dome. Long transport times make source diagnosis difficult using 10-day
backward trajectories, and chemical processing during long Arctic residence
times contributes to challenges in identifying source regions of lower polar
dome aerosol. Using our observations, we cannot quantitatively distinguish
the relative importance of vertical variability in source composition,
chemical processing during descent in the polar dome, and removal or cloud
processing during transport. Our observations present a challenge to chemical
transport models for their representation of the processes impacting High Arctic aerosol in spring.
NETCARE data are available on the Government of Canada Open
Data Portal (https://open.canada.ca/data/en/dataset). Data from NETCARE
2015 are available at
https://open.canada.ca/data/dataset/efe0e41c-890d-404d-bb1b-421456022d51. Sea ice data shown in Fig. are available at
https://seaice.uni-bremen.de/sea-ice-concentration/.
Global MODIS active fire locations shown in Fig. are
available at
https://earthdata.nasa.gov/earth-observation-data/near-real-time/firms/active-fire-data. Gas-flaring locations from the ECLIPSE inventory V5
shown in Fig. are available at
http://www.iiasa.ac.at/web/home/research/researchPrograms/air/ECLIPSEv5.html.
The supplement related to this article is available online at: https://doi.org/10.5194/acp-19-57-2019-supplement.
MDW wrote the paper, with significant conceptual input
from DK, HB, WRL and JPDA, and critical feedback from all co-authors. MDW,
HB, JB, AKYL, HS and WRL operated instruments in the field and analyzed
resulting data. AAA analyzed flight data. WRL, JPDA and ABH designed the
field experiment. DK ran FLEXPART simulations, and MDW analyzed the resulting
data with input from DK and HB.
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.
Acknowledgements
Funding for this work was provided by the Natural Sciences and Engineering
Research Council of Canada (NSERC) through the NETCARE project
(https://www.netcare-project.ca/) of the Climate Change and Atmospheric Research
Program, the Alfred Wegener Institute (AWI) and Environment and Climate
Change Canada (ECCC). We gratefully acknowledge Kenn Borek Air Ltd, in
particular our pilots and crew Garry Murtsell, Neil Traverse and Doug Mackenzie, for their support of our measurements. Logistical and technical
support before and during the campaign was provided by a number of
contributors, in particular by Desiree Toom-Sauntry (ECCC), Ralf Staebler
(ECCC), Katherine Hayden (ECCC), Andrew Elford (ECCC), Anne Marie MacDonald
(ECCC), Maurice Watt (ECCC), Mohammed Wasey (ECCC), Jason Iwachow (ECCC),
Alina Chivulescu (ECCC), Ka Sung (ECCC), Dan Veber (ECCC), Julia Binder
(AWI), Lukas Kandora (AWI), Jens Herrmann (AWI) and Manuel Sellmann (AWI).
Extensive logistical and technical support was provided by Andrew Platt
(ECCC), Mike Harwood (ECCC) and Martin Gerhmann (AWI). We are grateful to CFS
Alert and Eureka Weather Station for supporting the measurements presented in
this work. We gratefully acknowledge funding by the Deutsche
Forschungsgemeinschaft (DFG, German Research Foundation) – project number
268020496 – TRR 172, within the Transregional Collaborative Research Center
“ArctiC Amplification: Climate Relevant Atmospheric and SurfaCe Processes,
and Feedback Mechanisms (AC)3”. Data were analyzed in Igor Pro 6.37
(https://www.wavemetrics.com/, last access: 20 December 2018) and Python 3.5.2
(https://www.python.org/downloads/release/python-352/, last access: 20 December 2018). We acknowledge Charles Brock,
Jennifer Murphy and Dylan Jones for their comments on an early version of
this paper.
Edited by: Barbara Ervens
Reviewed by: four anonymous referees
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