The airborne campaign “Boundary layer and Aerosol and Cloud Study in the Arctic, based on aircraft and T-Bird Measurements II” (BACSAM II) was conducted from 5 April to 2 May 2024. We carried out eleven research flights (RFs) with the German research aircraft Polar 6 P6,, operated by the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI; see Supplement Fig. S1 for a map of all flight paths). With the operational base in Longyearbyen (Svalbard), target areas included the Fram Strait, the Svalbard Archipelago, and the vicinity of Villum Research Station in north-eastern Greenland. Flights were conducted between 7 and 29 April 2024. This time period was characterized by persistent warm air advection driven by low pressure systems south-west of Svalbard, resulting in above-average surface temperatures in Svalbard. Two exceptions with typical cold conditions and high pressure at the beginning of the campaign from 7 to 11 April and from 15 to 17 April occurred. Note that our sampling is potentially biased toward viable flight conditions, i.e., high visibility at Longyearbyen Airport. Furthermore, we mostly preferred cloud-free conditions in the target regions or elevated cloud layers (cloud base ⪆200m), and no precipitation.

Aboard the P6 aircraft, we measured aerosol particle number size distributions (PNSDs) with a mobility particle size spectrometer MPSS,. The custom-built MPSS measures the PNSDs in the particle diameter size range between 10 and 850 nm based on the electric mobility of aerosol particles with a time resolution of about 300 s. In this paper, we only use data up to 600 nm since on some measurement days arcing inside the differential mobility analyzer (DMA) led to artifacts in the PNSDs appearing at larger diameters. The system features a Vienna-type DMA (built by the Leibniz Institute for Tropospheric Research) and a condensation particle counter (CPC, TSI model 3760A, TSI Incorporated, USA). For further details on the MPSS system see . The flow in the sampling tubing leading to the MPSS was manually adjusted depending on the true airspeed of the aircraft (typically ≈60 m s⁻¹ during legs in the target area and ≈90 m s⁻¹ during transit flights) to achieve near-isokinetic sampling. As the onboard-temperature was significantly higher than the ambient temperature, the relative humidity of the sampled air in the aerosol inlet was below 40 %. Consequently, all reported particle diameters refer to dry conditions . All particle number concentration data have been corrected to standard conditions (1013 hPa and 273.15 K). Particle losses inside the sampling line from the aerosol inlet to the MPSS due to diffusion, sedimentation, turbulent inertial deposition, and inertial deposition in bends and contractions have been corrected for, based on calculations utilizing the particle loss calculator Sect. S2.

Since the minimum diameter of aerosol particles that can be detected with our measurement instrumentation is 10 nm, it is not feasible to clearly identify the exact region where NPF occurred, i.e., where the process of the initial nucleation of particles with sizes of about 2 to 3 nm from condensable gases took place . Nevertheless, particle sizes of 10 nm still imply that these particles were newly formed by secondary particle formation i.e., by NPF,. We hence refer to these particles as newly formed particles and, in this paper, instead on the process of NPF, focus on the occurrence of such secondarily formed particles.

To detect newly formed particles, we fitted mono-modal, bi-modal, or tri-modal log-normal functions to the MPSS-measured PNSDs, with the different modes representing nucleation, Aitken, and accumulation modes, respectively. Following , we refer to aerosol particles in the measured dry diameter range from below 10 to 20 nm as nucleation mode particles, from 20 to 90 nm as Aitken mode particles, and from 90 to 600 nm as accumulation mode particles. We consider an MPSS-measured PNSD as indicative for newly formed particles if the fitted distribution features a nucleation mode with a geometric mean diameter below 10 nm, i.e., a mode with a maximum below the lower size limit of the MPSS. The detection of newly formed particles was verified by visual inspection of the fitted PNSDs. Only distributions measured at a constant altitude were considered for quantitative analysis, i.e., PNSDs measured during the aircraft's ascents and descents were not taken into account in quantitative terms. Figure  shows examples of three fitted PNSDs, with the data displayed in Fig. a and c indicating the presence of newly formed particles, and the data in Fig. b the absence of newly formed particles.

A nose boom was installed at the tip of the aircraft to measure air pressure, temperature, and the three-dimensional wind vector. The nose boom features an Aventech five-hole probe to measure static, dynamic, and differential pressures to derive the wind vector from these data, and an open-wire Pt100 in an unheated Rosemount housing to record air temperature data. These data were measured with a time resolution of 100 Hz. Humidity data are available at 20 Hz from a Vaisala HMT333 in a Rosemount housing installed aboard the aircraft. Surface skin temperature data at 20 Hz are derived from measurements by a KT-19 infrared pyrometer (Heitronics KT-19.85 II). Position and altitude were measured by combining Global Positioning System (GPS) and Inertial Navigation System (INS) data, which were recorded at 100 Hz. See for further details on the instrumentation and data processing.

Backward trajectories were calculated from ERA5 wind fields utilizing the Lagrangian analysis tool LAGRANTO . The trajectories were initiated every minute along the flight track of the P6 at the corresponding flight altitude. The trajectories were calculated for 120 h backward in one minute time steps and several meteorological parameters from ERA5 were traced along the trajectory. Based on these parameters, we carry out an analysis of air mass history in Sect. . Among others, we determine the last contact of these air masses with clouds based on the specific cloud liquid water content (CLWC) and the specific cloud ice water content (CIWC). Here, we consider an air mass to have been in contact with a cloud if the total water content (CWC=CLWC+CIWC) exceeds the threshold of CWC>0.01 g kg⁻¹ see Sect. S3.

Throughout the entire BACSAM II campaign, we frequently observed newly formed particles over large areas of the Arctic region studied (Fig. ). Newly formed particles were detected during all eleven research flights, with the first observation of such particles on 7 April 2024, and the last on 29 April 2024. We encountered newly formed particles at various locations (approximately between 76 to 85° N and 15° W to 26° E) and altitudes ranging from about 60 to 3900 m a.s.l. (Fig. ). The time periods during which we identified newly formed particles amount to an overall duration of about 10 h and 30 min, which corresponds to approximately 29 % of the total measurement time (at constant altitude) of all research flights and a distance flown of about 2770 km.

Analyzing all measurements featuring newly formed particles in the course of our campaign, we could identify four distinct environmental conditions (based on the in situ data, see Sect. S3; Fig. ). These cases include newly formed particles in the free troposphere (without clouds being present, Sect. ), newly formed particles at low altitudes in the ABL over sea ice (Sect. ), newly formed particles in the vicinity of clouds (Sect. ), and newly formed particles inside the ABL over ocean with sea ice floes, i.e., the sea surface consisted of both open water and sea ice during the latter observations (Sect. S5). No indications of the presence of newly formed particles were found within or near the ABL over open, completely ice-free ocean (Sect. S5).

Figure a depicts the median size distributions of the PNSDs indicating the presence of newly formed particles, for each of the four environments. The highest particle number concentrations in the nucleation mode are found during observations of newly formed particles in the free troposphere. Inside the ABL over sea ice and in cloud vicinity, concentrations in the nucleation mode are smaller by a factor of roughly two. The smallest nucleation mode concentrations are observed inside the ABL over ocean with sea ice floes, with the concentrations being reduced by about one order of magnitude compared to the free tropospheric case.

In the accumulation mode, roughly similar concentrations are observed during newly formed particle-occurrence inside the ABL over ocean with sea ice floes and in cloud vicinity. Inside the ABL over sea ice, accumulation mode concentrations are slightly higher. The lowest accumulation mode concentrations are found in the free troposphere, with the mode being decreased by factors of roughly two to four compared to the other cases.

The reader is referred to Sect. S4 and Fig. S3 regarding the related temperature and relative humidity conditions prevailing during observations of newly formed particles. We will discuss the observations for each of the four categories, which combined represent 92 % of all observations of newly formed particles, in more detail in Sects. , , , and S5.

As mentioned earlier, newly formed particles were detected in a wide range of altitudes between approximately 60 and 3900 m (Fig. b). Here, 60 and 3900 m correspond to the minimum and maximum flight altitudes at which measurements were carried out (Fig. b). Consequently, we cannot rule out that newly formed particles could be present at altitudes even higher than 3900 m in the target regions during our campaign.

In the free troposphere, newly formed particles were detected at altitudes between about 900 and 3900 m (Fig. b). These observations amount to measurement times of approximately 4 h and 40 min, corresponding to distances flown of ≈1460 km. Inside the ABL over sea ice, at altitudes between roughly 60 and 200 m, newly formed particles were identified during about 1 h and 40 min of measurements (≈350 km), and inside the ABL over ocean with sea ice floes, at altitudes between 60 and 300 m, during approximately 40 min (≈130 km). During about 2 h and 40 min (≈630 km) of measurements in the vicinity of clouds, newly formed particles were observed. Here, these particles were found at altitudes between 300 and 2700 m (Fig. b).

Table summarizes the durations of newly formed particle-occurrence for the four environments. Furthermore, based on ERA5 reanalysis data, we present total measurement times spent in the free troposphere, in the ABL over sea ice and over ocean with sea ice floes, and in cloud vicinity throughout the campaign (see Sect. S3 for details). Additionally, relative frequencies of occurrence of newly formed particles for each environment are provided in Table . As discussed in Sect. S3, the ERA5-based values are associated with large uncertainties. Consequently, the total measurement times throughout the campaign as well as the frequencies of occurrence should be interpreted solely semi-quantitatively and only provide a very rough estimate.

It should be noted that, for all case studies described below, as well as for the assignment of measurement periods featuring newly formed particles to the four categories free troposphere, cloud vicinity, ABL over sea ice, and ABL over ocean with sea ice floes, we only considered flight sections for which the prevailing conditions could be clearly identified based on the in situ data. If newly formed particles were detected but either, e.g., the ABL height could not be determined for the target region or the absence/presence of clouds close to the aircraft could not be verified, these observations are assigned to the unclassified cases (Sect. S3). Such observations represent 8 % of all PNSDs featuring newly formed particles (≈50 min, ≈200 km of measurements).

Naturally, during the campaign, sampling times and consequently numbers of PNSDs measured, as well as distances flown at certain altitudes, were not evenly distributed over the investigated altitudes, geographical locations, and synoptic conditions (Fig. b). For example, while measurements in the lowest 100 m were carried out for a total duration of almost 400 min (over a distance of about 1480 km), measurements at 3500–3600 and 3800–3900 m were merely performed during one research flight, and at each altitude range for merely 5 to 10 min (27 to 54 km, to feature one full MPSS scan; Fig. b). The values of duration and relative frequency of newly formed particle-occurrence reported in this section are thus not intended to constitute representative values for the lower Arctic troposphere. They rather provide a rough overview and represent (semi-quantitative) statistics of the observations made during the BACSAM II campaign.

Nevertheless, despite these limitations with respect to the statistical analysis, our measurements clearly demonstrate that newly formed particles were frequently found throughout the lower Arctic troposphere, and suggest that NPF in the Arctic occurred across large geographical areas and over a wide range of altitudes during our campaign in April 2024.

To further elucidate the atmospheric scenarios under which newly formed particles were observed, and to gain insights into potential precursor gases and their origins, in the following sections, the results of case studies focusing on newly formed particles in the free troposphere, inside the ABL over sea ice, and in the vicinity of clouds will be presented. First, specific examples for these scenarios will be given. Second, to put the given examples into a larger perspective, we will discuss respective backward trajectory analyses for all air masses featuring newly formed particles for each of these three environments. Regarding the presence of newly formed particles inside the ABL over ocean with sea ice floes, as well as the absence of such particles inside the ABL over completely ice-free ocean, the backward trajectory analysis required is somewhat different compared to the other three cases (for example, due to the prevailing meteorological conditions, e.g., air masses passing over the island of Spitsbergen, see Sect. S5 for details). To keep the main text both consistent and concise, these latter observations (inside the ABL over ocean with sea ice floes and over open ocean) are discussed comprehensively in the Supplement in Sect. S5.

Here, as an illustrative example, we present results concerning newly formed particles in the free troposphere which were encountered on 13 April 2024 during RF04 while transiting from Longyearbyen (Svalbard) to Villum Research Station (northern Greenland) at an altitude of 2890 m (Fig. a).

In the two days ahead of the flight, a low pressure system located over the east coast of Greenland advected warm and moist air over the sea ice edge in the Fram Strait (Fig. b). Within this warm-air intrusion, North Atlantic ABL air was lifted up to the free troposphere. The uplifting over the sea ice edge was accompanied with the formation of precipitating clouds, which dissipated before sampling of the air mass. Consequently, the sampled free tropospheric air was cloud-free. The cumulus clouds visible in the satellite image in Fig. a capped the marine ABL, which was located more than 2 km beneath the flight track. Therefore, we expect no influence of these low-level clouds on our measurements in the free troposphere. In addition to the southerly air masses from the Atlantic, air masses originating at higher latitudes close to Svalbard were probed during RF04 (Fig. c).

During the time period from about 06:23 to 07:40 UTC, flying at high altitudes (≈ 2890 m), and covering a distance of about 409 km, we encountered elevated number concentrations in the lower MPSS size bins (Fig. ). The measured PNSDs featured a sharp decline in the size range from 10 to about 60 nm, i.e., we observed the “upper” end of a pronounced nucleation mode (see Fig. S8 for the related median PNSD). Detected particle number concentrations in the size range from 10 to 20 nm reached on average values of about N10-20≈778 cm⁻³, which constituted about 68 % of the total particle number concentrations measured in the range from 10 to 600 nm (Fig. c). In other words, we detected newly formed particles in the Arctic free troposphere across a large geographical area.

For the time period 07:40 to 08:19 UTC, encountered PNSDs featured no distinct nucleation mode, and number concentrations in the size range from 10 to 20 nm were substantially lower (N10-20≈27 cm⁻³, 17 % of the total MPSS-measured number concentrations, Fig. c), i.e., no newly formed particles were present. Notably, the newly formed particles observed during the first part of the transit flight were associated with air masses originating from lower latitudes which were transported into the Arctic within a warm-air intrusion (Fig. b), while in the northerly air masses no newly formed particles were detected (Fig. c).

Inside the ABL, newly formed particles were detected, e.g., on 15 April 2024 during RF05 while flying east of Svalbard at low altitudes (approximately 70 and 155 m) over sea ice, where cracks as well as open and partly refrozen leads were present (Fig. S9). The associated flight track is shown in Fig. a, with Fig. b, c depicting the backward trajectories for the air masses encountered along the flight track.

The area investigated was subject to the anti-cyclonic flow around a high pressure system north of Svalbard, causing cold air advection from the central Arctic to the target region. This meteorological situation resulted in air masses that exclusively originated in the high Arctic and traveled toward the P6 flight track from higher latitudes over sea ice-covered regions (Fig. b, c). All the way between waypoint 1 (WP1) and WP3, over a distance of about 115 km, clear sky conditions prevailed (Fig. a).

The presence of newly formed particles is indicated by the occurrence of a distinct nucleation mode, i.e., elevated aerosol particle number concentrations in the lower size bins of the PNSDs, which we observed during three consecutive legs at altitudes of ≈70 and ≈155 m between WP1 and WP3 (Fig. ). Mean number concentrations in the nucleation mode size range reached N10-20≈206 cm⁻³ (time period 10:03 to 10:39 UTC), N10-20≈185 cm⁻³ (11:08 to 11:22 UTC), and N10-20≈125 cm⁻³ (12:17 to 12:40 UTC), accounting for about 40 %, 38 %, and 31 % of the total MPSS-measured aerosol particle number concentrations. In contrast, during the leg at ≈850 m altitude (11:30 to 12:00 UTC) between WP1 and WP3, as well as during the transit to and from the target region at ≈1360 m altitude, the PNSDs featured no nucleation mode and, accordingly, low number concentrations in the sub-20 nm size range (Fig. , N10-20≈3 cm⁻³, 1 % of the total MPSS-measured particle number concentration at 850 m).

From the vertical profiles of potential temperature obtained during the ascents and descents at WP1, WP2, and WP3 (Fig. a), we determine an ABL height of about 190 m in the region of interest (see Fig. S10). Thus, the three legs at 70 and 155 m, during which newly formed particles were detected, were carried out inside the ABL, while the leg at higher altitude (≈850 m), as well as the transits to and from the region of interest, took place in the free troposphere.

Integral aerosol particle number concentrations in the total measured size range (10 to 600 nm) and in the size range from 10 to 20 nm decreased from WP1 to WP3, i.e., from south to north (Figs. a, ). The spatial variability of the number concentration of nucleation mode particles points toward newly formed particles as the result of regional formation rather than long-range transport. Nucleation mode particles have short atmospheric residence times due to their high diffusion, coagulation with larger particles, and condensational growth out of the nucleation mode size e.g.,. These effects tend to smooth out concentration gradients of such small particles, thereby making a contribution from long-range transport to the observed events less likely .

Interestingly, the particle number concentrations in the nucleation mode size range from 10 to 20 nm recorded during the legs inside the ABL between WP1 and WP3 (i.e., in the same area) decreased not only spatially, but also over time (Fig. c). In addition to the integral values shown above, Fig. S11 depicts median PNSDs for each of the three legs close to the surface individually, which likewise shows that number concentrations of particles in the nucleation mode reduce over time. This temporal variability points toward a regional source as well, since it suggests that we observed an NPF event that becomes weaker throughout the day. However, we are not able to unambiguously resolve if the precursors involved in NPF originated directly from the sea ice-covered measurement region, with various cracks and leads being present (Fig. S9), or if the precursors were transported to the target area from the higher Arctic. But, in our view, it appears feasible to conclude that the spatial and temporal variability of the newly formed particles encountered here (i.e., the decrease of sub-20 nm particles from WP1 to WP3 and throughout the day) and the similar origin of air masses featuring newly formed particles (Fig. b) and air masses without newly formed particles (Fig. c) rather indicate a regional source.

Newly formed particles in the vicinity of clouds were, e.g., encountered on 24 April 2024 during RF08, while flying north of Svalbard over sea ice (open leads and cracks were present, see Fig. S12) with an initially slightly broken and then closed stratocumulus cloud deck (Fig. a). Measurements were carried out below cloud base and at cloud top.

On 24 April, a high pressure system was centered directly over Svalbard, resulting in westerly flow in the target region. The air mass probed at altitudes between approximately 600 and 1200 m originated either in the North Atlantic close to Greenland, near Iceland, or northern Scandinavia (Fig. b), and was advected from the North Atlantic marine ABL and lifted above the sea ice, where this layer was decoupled from the surface (Sect. S8.2, Fig. S13). At the top of that elevated mixed layer, stratocumulus clouds were present (Fig. a), with a cloud base at about 800 m altitude, and cloud top at about 1150 m. The air mass that was probed at about 2100 m in the free troposphere originated in northern Canada and passed over Greenland on its way to the P6 position (Fig. c).

Here, we specifically discuss the data collected between WP1 and WP2 over a distance of 75 km in the time period between 10:28 and 11:35 UTC. This time period includes a descent from approximately 2100 m down to below cloud level (10:29 to 10:35 UTC), a leg at cloud base (10:35 to 10:52 UTC), an ascent through cloud with subsequent drying away of the ice collected inside cloud, and finally a leg at cloud top (11:07 to 11:35 UTC). With respect to the aerosol particle measurements, the time period from 10:52 to 11:07 UTC, i.e., the ascent through cloud and the subsequent drying away of ice, is excluded from further analysis, as erroneous measurements due to icing of the aerosol inlet and/or droplet shattering cannot be ruled out. The MPSS data from this period are grayed out in Fig. b, c.

The PNSDs at cloud top and at cloud base featured a pronounced nucleation mode and correspondingly enhanced number concentrations in the nucleation mode size range from 10 to 20 nm, regardless of the cloud cover being closed or broken (Fig. ). In contrast, in the PNSD measured during the descent toward cloud level around 10:30 UTC, which we assume to be representative for the free troposphere, concentrations in the sub-20 nm size range were low. This PNSD should be considered semi-quantitative only, as it represents an average over an altitude range of more than 1000 m. The presence/absence of particles in the size range from 10 to 20 nm is also clearly visible in Fig. c, with particles in the size range below 20 nm contributing approximately 67 %, 60 %, and 9 % to the total aerosol particle number concentrations at cloud top, cloud base, and in the free troposphere, respectively. A more detailed comparison between PNSDs measured at cloud top and at cloud base reveals an interesting effect (Fig. ). The overall shapes of the distributions are qualitatively similar, with the cloud top and cloud base accumulation mode concentrations following each other rather closely, while the number of Aitken and nucleation mode particles is reduced by roughly a factor of two to three at cloud base, resulting in a more pronounced Hoppel-minimum for the PNSDs measured at cloud base. Even a small shift of the accumulation mode at cloud base to larger sizes can be seen, which could be due to particle growth inside cloud. For the comparison of the PNSDs measured at cloud top and cloud base, we only considered measurements carried out in the exact same geographical area, i.e., measurements between WP1 and WP2 from approximately 10:38 to 10:52 UTC (cloud base) and 11:17 to 11:31 UTC (cloud top). However, as differences are small and spatial and/or temporal variability cannot fully be ruled out, we cannot draw solid quantitative conclusions. To indicate the absence of nucleation mode particles in the free troposphere, the PNSD measured during the descent from 2100 m toward cloud level is also shown in Fig. .