As part of the NETCARE project (Network on Climate and Aerosols: Addressing Key Uncertainties in Remote Canadian Environments, http://www.netcare-project.ca), measurements of aerosol physical and chemical properties, trace gases, and meteorological parameters were made aboard the Alfred Wegener Institute (AWI) Polar 6 aircraft, a DC-3 aircraft converted to a Basler BT-67 . Measurements aboard Polar 6 took place from 4 to 21 July 2014, based in Resolute Bay, Nunavut (74∘41′ N, 94∘52′ W). The survey speed was maintained at approximately ∼ 75 ms-1 for measurement flights, with ascent and descent rates of 150 mmin-1 for vertical profiles.

The main aerosol inlet was located on the starboard side of the fuselage ahead of the engines. Based upon a total flow drawn to instruments of 35 Lmin-1 and a measured flow at the exhaust of the sampling line of 20 Lmin-1, the total flow through the shrouded inlet diffuser was nearly isokinetic at 55 Lmin-1. Aerosol flowed into the cabin through a stainless steel manifold (outer diameter of 2.5 cm, inner diameter of 2.3 cm) and was directed to the various particle instruments through stainless steel lines that branched from the main inlet at angles less than 90∘. Aerosol was not dried prior to sampling; however, the temperature in the inlet line was approximately 10–15 ∘C warmer than the ambient temperature so that the relative humidity (RH) decreased significantly as the aerosol entered the sampling line. Exhaust from the main aerosol inlet flowed freely into the back of the cabin to keep the inlet from being over-pressured. Therefore, the total flow through the main aerosol inlet was dictated by the true airspeed (TAS). With the survey air speed noted above, transmission efficiency of aerosol through the main inlet was near unity for particles 20 nm to ∼ 1 µm in diameter.

Trace gases (CO, CO2, and H2O) were sampled through a second inlet consisting of a 0.40 cm (outer diameter) Teflon line, with a continuously measured sample of flow of ∼ 12 Lmin-1. The trace gas inlet used the forward motion of the aircraft to push ambient air into the line in combination with a rear-facing 0.95 cm Teflon exhaust line that lowered the pressure in the sampling line.

State parameters and meteorological conditions were measured with an AIMMS-20, manufactured by Aventech Research Inc. (Barrie, Ontario, Canada; http://aventech.com/products/aimms20.html). The AIMMS-20 consists of three modules: (1) an Air Data Probe, which measures temperature and the three-dimensional aircraft-relative flow vector (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.

Measurements of particle number concentrations and size were made aboard Polar 6 at a frequency of 1 Hz, unless otherwise indicated. Number concentrations of particles greater than 5 nm in diameter (N>5) were measured with a TSI 3787 water-based ultra-fine condensation particle counter (UCPC), sampling at a flow rate of 0.6 Lmin-1. Aerosol number size distributions from 20 nm to 1 µm were acquired with two instruments: a Brechtel Manufacturing Incorporated (BMI) scanning mobility system (SMS) coupled to a TSI 3010 condensation particle counter (CPC) measured from 20 to 100 nm (N20–100) with a 60 s time resolution, while a Droplet Measurement Technology (DMT) Ultra High Sensitivity Aerosol Spectrometer (UHSAS) measured number size distributions from 70 nm to 1 µm (N>70) with a time resolution of 1 Hz. The SMS sampled at a flow rate of 1 Lmin-1, with a dried (∼ 20 % RH) sheath flow of 6 Lmin-1. The UHSAS uses light-scattering signals from a 1054 nm laser for particle detection and sizing on a single-particle basis e.g.,, with a sample flow rate of 55 cm3min-1 from a bypass flow off the main aerosol inlet. Characterization and calibration of the UCPC, SMS, and UHSAS are described in detail in . Particle number concentrations from the SMS and UHSAS generally agreed within a factor of 2 over their overlapping size range (70 to 100 nm).

Particle number concentrations from all instruments are reported at ambient pressure and temperature. A characteristic size distribution is shown in Fig. . Values of N>80, N>100, and N>200 were derived from UHSAS measurements. Number concentrations from 5 to 20 nm (N5–20) were estimated by subtracting the sum of the SMS total number concentration (N20–100) and UHSAS N>100 from the total UCPC concentration (i.e. N>5). The number of particles greater than 50 nm (N>50) was determined by the sum of the SMS number from 50 to 100 nm (N50–100) and the UHSAS N>100. Here, we refer to N5–20 as the nucleation mode, N20–100 as the Aitken mode, and N>100 and larger as the accumulation mode.

CCN concentrations were measured using a DMT CCN counter (CCNC, model 100), sampling behind a DMT pressure-controlled inlet (PCI) at a reduced pressure of ∼6.5×104 Pa. The effective supersaturation (for a nominal water supersaturation of 1 %, at 6.5×104 Pa) was found to be 0.6 % and was held constant throughout the study to allow more measurement stability and the highest time resolution possible and to examine the hygroscopicity of small particles. Calibration and characterization of the CCNC is described in .

The effective aerosol hygroscopicity parameter (κ) was estimated according to , using the average aerosol composition from the aerosol mass spectrometer (Sect. 2.6.2) with ammonium sulfate and organic aerosol densities of 1770 and 1550 kgm-3 respectively e.g.,. Assuming a temperature of 298 K and the surface tension of pure water the dry diameter for activation was calculated at the supersaturation of our CCN measurements. The measured size distribution could then be integrated down to this dry diameter to produce predicted CCN concentrations for comparison with measured values.

Carbon monoxide (CO) concentrations were measured 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. CO concentrations are used here as a relative indicator of aerosol influenced by pollution sources, such as anthropogenic or biomass burning emissions.

Water vapour (H2O) measurements were based on infrared absorption using a LI-7200 enclosed CO2/H2O Analyzer from LI-COR Biosciences GmbH. In situ calibrations were performed during flight at regular intervals (15–30 min) using a NIST traceable CO2 standard with zero water vapour concentration. The measurement uncertainty is ±40 ppmv. H2O mixing ratios were used to calculate RH with pressure and temperature measured by the AIMMS-20.

The Lagrangian particle dispersion model FLEXPART-WRF (website: http://flexpart.eu/wiki/FpLimitedareaWrf) was used to study the history air masses prior to sampling during the flight. FLEXPART-WRF is based on FLEXPART but uses the limited area meteorological forecast from WRF , with the specific WRF forecast details for the NETCARE campaign provided in . Here, we use FLEXPART-WRF run in backward mode to study the origin of air influencing aircraft-based aerosol measurements. Further details of the FLEXPART-WRF simulations performed for NETCARE 2014 summer campaign are also found in .

In this case study we focus on the flight conducted on 12 July 2014 where Polar 6 travelled at ∼ 3 km altitude from Resolute Bay, past the marginal ice zone and out over open water to the eastern end of Lancaster Sound (Fig. a) as far as was permitted by our aircraft range, at which point it descended and returned west. The relevant portion of this flight, over open water in Lancaster Sound, is highlighted in Fig. a (79.7 to 86.5∘ W, 20:00–21:20 UTC). During this time the aircraft flew to the west below 100 m a.g.l. and covered a distance of approximately 175 km at a survey speed of 75 ms-1 under clear sky conditions. Profiles were carried out at three different locations to characterize the vertical structure of the troposphere (Fig. a, triangles): one profile from 60 to 3000 m near Resolute Bay (∼ 95∘ W, denoted as “west” in Fig. b and c); a second, shallower profile to ∼ 900 m near the marginal ice zone (∼ 88∘ W, “central”); and a third profile down from 3000 to 60 m in eastern Lancaster Sound (∼ 80∘ W,“east”).

Meteorological observations and measurements of trace gases and black carbon indicate a stable and clean atmosphere. Temperature profiles in all three locations indicated a shallow surface-based temperature inversion of 2–4 ∘C, reaching up to ∼ 800 m over the ice near Resolute Bay and to only ∼ 100 m in the eastern profile (Fig. b). Applying the method of bulk Richardson number with Polar 6 meteorological observations, and radiosondes conducted concurrently at Resolute Bay and aboard the CCGS Amundsen, estimated boundary layer heights of 275 ± 164 m during the NETCARE summer campaign. Here, we will refer to the portion of the boundary layer with a positive vertical gradient in the temperature profile as the “lower boundary layer.” Both within and above the lower boundary layer winds were predominantly from the west, with measured wind speeds near the surface averaging 6.5 ± 1.8 ms-1. Similarly, surface winds from WRF indicate predominately west-north-west winds at this time, with wind speeds of 4–8 ms-1 (Fig. S4 in the Supplement). CO profiles in all three locations demonstrated very clean background conditions with concentrations ranging from 73 to 78 ppbv and little variation with altitude (Fig. c). RH was generally high near the surface, with an average of 80 % in the lower boundary layer (Fig. S5 in the Supplement). Refractory black carbon concentrations also indicate a very clean atmosphere, well below the threshold for a clean marine boundary layer discussed by . Average (± standard deviation) rBC mass loadings during the period of interest and over the entire flight were 1.6 ± 0.9 and 2.5 ± 1.5 ngm-3 respectively, with slightly higher concentrations found aloft.

Air mass history from FLEXPART-WRF indicates a strong local Arctic influence on the sampled air mass. FLEXPART-WRF air mass origin is shown as the column-integrated air mass residence time prior to sampling, also referred to as the column-integrated potential emission sensitivity (PES), for the release time and location of this case study (Fig. a). The column-integrated PES supports that the locally influenced air mass originated from generally clean conditions with no pollution sources. The air mass encountered by the aircraft at 82.2∘ W and ∼ 85 m a.g.l. resided over a snow and ice-covered island (Devon Island; ) for approximately 1 week before descending into Lancaster Sound within 1 day of sampling (Fig. ). The model also indicates that the sampled air mass had a residence time within the lowest 300 m of 4 to 5 h prior to sampling, providing at least 4 h of transport and chemistry within the boundary layer (Fig. b and c). Overall, FLEXPART-WRF air mass history suggests that the sampled air mass had little exposure to fresh sea emissions until 4 to 5 h prior to sampling, when it moved from above the snow- and ice-covered land and was exposed to the ocean surface within the lower boundary layer.

Our observations of particle number concentration, over the size range from 5 nm to 1 µm, indicated the simultaneous presence of nucleation mode and Aitken mode particles near the ocean surface. At low altitude near 85∘ W we observed an enhancement in N5–20 above background levels, indicating the presence of nucleation mode particles (Figs. , a). Upon entering the lower boundary layer further downwind (Fig. d) and to the east (82.5–81∘ W), we observed a sharp increase in N5–20 concurrently with an increase in N20–100 (Fig. a). We do not directly observe the formation of the smallest particles; however, we hypothesize that they were formed through nucleation in a very clean atmosphere.

Particle number size distributions from 20 to 1000 nm illustrate that particles below 20 nm (Fig. a, c) grow to form a mode centred at 30–40 nm (Fig. d–f). Beyond 86∘ W we observe N20–100 at background levels of ∼ 100 cm-3. These observations suggest that the aerosol size distribution develops as the air mass moves downwind to the east. The advection timescale from 85.8 to 81.1∘ W is 6.7 h, given an average wind speed of 6.5 ms-1, and the sampling time of the aircraft over this distance is 35 min. Given the substantial changes in aerosol size and number concentration observed over this relatively short time period, our observations suggest that a source of condensible material contributing to aerosol growth is present to the west of the sampling region and it is unlikely that a wider source region contributed. An estimate of the growth rate in this case is associated with a large uncertainty since it is complicated by a number of factors, including the 1 min time resolution of the SMS that corresponds to a sampling distance of ∼ 4 km, and uncertainties in the advection time. Therefore, it is difficult to quantitatively follow the evolution of the size distribution. Compounded by our lack of knowledge of the spatial uniformity of the condensible material, we do not present an estimate of the growth rate here.

The boundary layer was characterized by a low pre-existing aerosol surface area (i.e. a small condensation sink). A small number of particles above 200 nm in diameter (∼ 10–15 cm-3) were present within the lower boundary layer and show a time variation distinct from that of N>50 and N>100 (Fig. b). These larger particles are present during both sampling periods within the lower boundary layer (i.e. near 85 and 82.5∘ W), where winds speeds were relatively constant (6.5 ± 1.8 ms-1), and could be from ejection of primary sea-spray aerosol (see Sect. 3.3.2). The small N>200 provides a low pre-existing aerosol surface area (average ± standard deviation: 3.8 ± 2.0 µm2cm-3), which assists particle nucleation.

N>50 and N>100 show a time variation distinct from that of the nucleation mode (N5–20) and larger accumulation mode particles (N>200) (Fig. b). In our eastern-most observations within the lower boundary layer (82.5–81∘ W), N>50, which is accounted for largely by N50–150, is enhanced above background levels of ∼ 200 to ∼ 400 cm-3 (Fig. b). At the same time, CCN concentrations are elevated to > 200 cm-3, above background levels of ∼ 100 cm-3 (Fig. b). N>50, N>100, and CCN concentrations remain somewhat elevated up to ∼ 900 m (Fig. b, near 80.5∘ W ), suggesting that some mixing above the lower boundary layer occurred during some time prior to our observations, possibly due to katabatic winds off Devon Island suggested from the FLEXPART-WRF analyses (Fig. c). Profiles of aerosol number and composition are presented in Fig. S5 in the Supplement.

The variation in the size distribution from west to east in Lancaster Sound suggests that particles between ∼ 30 and greater than ∼ 50 nm are forming through secondary processes. In our western-most observations in the lower boundary layer (85.6–84.4∘ W) the size distribution is dominated by particles below 20 nm (Fig. a, c). Further to the east in the lower boundary layer (82.5–81∘ W), N5–20 is high and subsequently decreases moving east (Fig. a), while N>50 and N>100 do not increase until 81.9∘ W (Fig. b). The aircraft covered a distance of 19 km between entering the lower boundary layer (82.5∘ W) and observing this increase in N>50 and N>100. With a wind speed of 6.5 ms-1 near the surface, the advection time over this distance is approximately 50 min. If the aerosol size distribution was dominated by primary sea-spray aerosol, given constant wind speed, there would be no reason for such a delay in our observations of N>50 and N>100. Indeed, given the decreasing abundance of N>300, the evidence suggests that the sea-spray source, which is associated with larger particles (e.g., , see Sect. 3.3.2), is becoming less important as N>50 is increasing. These observations are suggestive of a secondary process growing particles from less than 20 nm into larger sizes, above 50 nm.

CCN concentrations are elevated above background levels during the growth event and are well-correlated with the number of particles greater than 80 nm (N>80, Fig. ). If the particles contributing to CCN concentrations at this time were only composed of ammonium sulfate, under our experimental conditions (i.e. 0.6 % supersaturation), we would expect the CCN-activation diameter to be ∼ 40 nm . A CCN-activation diameter of approximately 80 nm therefore indicates that a species less hygroscopic than ammonium sulfate is contributing to the CCN we observe. This is consistent with the elevated OA mass loading we measure when CCN concentrations are high (Fig. , colour scale), while sulfate was relatively low compared to other time periods (Fig. , marker size).

Since the aerosol was not actively dried and the supersaturation was held constant in the CCNC, in order to allow for rapid measurements, a calculation of the effective aerosol hygroscopicity parameter (κ) in this case carries a large uncertainty . In particular, measured particle diameters may be slightly larger than the corresponding dry diameter. The temperature in the inlet line was 10–15 ∘C warmer than the ambient temperature so that the RH decreased significantly as the aerosol entered the sampling line (i.e during the case study period, the ambient RH was 80 % at 8–10 ∘C, and the RH decreased within the inlet to approximately < 30 %). Using the measured aerosol composition, we estimate that measured particle diameters are up to 10 % larger than the corresponding dry diameter.

Nonetheless, this calculation is still illustrative of the organic aerosol properties in this environment. If the κ value of the organic aerosol (κOrg) is 0.1, and κ for the whole aerosol is calculated based on the HR-ToF-AMS organic and sulfate loadings (average organic volume fraction = 0.85 ± 0.03) and the known κ for ammonium sulfate, then the resulting dry diameter for activation is ∼ 60 nm. From our measurements, the activation diameter seems to be larger than 60 nm so that κOrg of 0.1 could be regarded as an upper limit. When we overestimate aerosol size by 10 %, due to incomplete drying in our sampling line, then our estimated CCN-activation diameter and the calculated dry diameter for activation become more similar. Overall, this illustrates that the organic aerosol was relatively non-hygroscopic with κOrg∼0.1. This estimate is within the range of κOrg recently measured in a coastal, marine-influenced environment by .