2.1 Measurements on board Cessna

As a platform for aerosol instruments, we used a light one-engine Cessna FR172F aircraft. The measurement instruments were installed on an aluminium rack in the middle part of the plane inside the cabin (Fig. 1). A steel inlet line (with a 32 mm inner diameter) was mounted onto the top of the rack and lifted in and out of the window on the left side of the plane. The sample was collected from a 50 cm distance from the fuselage of the plane. The main flow in the steel tube was kept constant at 47 L min⁻¹ during the measurement flight and was produced by suction in the venturi and forward motion of the airplane. Each instrument took the actual inlet flow from the central line of the main flow, minimizing the diffusional losses of the smallest particles. The measurements were performed with an airspeed of 125 km h⁻¹. More details about the instrumentation and layout can be found in Schobesberger et al. (2013) and Väänänen et al. (2016). The data were collected within a 30 km distance from SMEAR II and the area is covered mainly by coniferous forest.

Figure 1The instrumentation rack was installed inside the cabin (a) and the sample air for the instrumentation was taken from a steel tube at a 50 cm distance from the fuselage of the plane (b). The main instruments (ultrafine CPC, PSM and SMPS) are shown in the figure.

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2.2 Instrumentation

The main instrumentation for this study consisted of several different particle counters. An ultrafine condensation particle counter (uCPC, model TSI-3776) is an instrument that detects the total concentration of particles larger than about 3 nm in diameter. Particles larger than the threshold diameter are grown into large droplets by condensing butanol vapour onto their surface, after which they are detected optically with a laser-diode photodetector. The ultrafine CPC has an internal vacuum pump that draws the aerosol sample with a flow rate of 1.5 L min⁻¹ into the instrument.

Airmodus Ltd has developed a mixing-type particle size magnifier (PSM). The instrument is able to directly detect sub-3 nm atmospheric particles using diethylene glycol (DEG) as a condensing fluid (Vanhanen et al., 2011). Compared with the typically used working fluids in CPCs, water and butanol, the advantages of using DEG as a condensing fluid are its lower saturation vapour pressure and higher surface tension, which enables it to detect particles down to 1 nm. The PSM requires a separate water or butanol counter (CPC) for optically detecting the grown particles. The PSM in this study was a model A10, operating with a butanol CPC (model TSI-3010). During the flight measurements presented here, the instrument was used in the fixed-saturator flow-rate mode, measuring the total particle concentration with a 1.5 nm cut-off size.

The instrumentation also included a custom-built scanning mobility particle sizer (SMPS), which measures the particle number size distribution in the diameter size range of 10–400 nm with a 2 min time resolution. Before the classification of an aerosol population, the particles are transported to a radioactive source in which they reach a constant bipolar charge equilibrium. The SMPS contains a differential mobility analyser (DMA, Hauke type), while particle number concentrations are measured with a butanol CPC (model TSI-3010).

The concentrations of water vapour (H₂O) and carbon dioxide (CO₂) were measured with a Li-Cor (LI-840) gas analyser located in the instrumentation rack. Basic meteorological variables, including the ambient temperature (with PT-100 temperature sensor), relative humidity (RH) (with Rotronic HygroClip-S sensor) and static pressure (with Vaisala PTB100B), were measured. Pressure was measured inside the plane while the temperature and RH sensor was located in the right wing of the plane. The location of the plane was recorded by a GPS receiver.

2.3 SMEAR II

The research Station for Measuring Ecosystem–Atmospheric Relations (SMEAR) II in Hyytiälä, southern Finland, was established in 1995 (see Hari and Kulmala, 2005). The station is equipped with several aerosol and gas instruments together with flux, irradiation and meteorological measurements. The long-term measurements provide reliable and comprehensive knowledge about ambient conditions at a relatively clean coniferous forest site. The station includes ground-based measurements, tower measurements at a 35 m height above the ground level right above the canopy and measurements conducted from a mast at different altitudes up to 128 m.

In this study, we mainly used particle data from the ground level as reference data to which we compare our flight measurement data. The number concentrations in the size range of 1.5–3 nm were calculated from the difference between the measured total particle concentration at the 1.5 nm cut-off size (from the PSM) and total concentration at the 3 nm cut-off size (from DMPS). The distance between the PSM and DMPS is vertically a few metres and horizontally a few tens of metres, which causes some uncertainties in 1.5–3 nm particle number concentrations, especially during poorly mixed BL times in the morning when the two instruments do not always measure the same air mass.

The sensible heat flux (SHF) was measured at a 23 m height, and we used these data to get qualitative information on the strength of vertical mixing in the measured air masses.

2.4 Data analysis

The particle number concentration in the size range of 1.5–3 nm was calculated as the difference of the total particle concentrations measured with the PSM and uCPC on board the Cessna. The cut-off sizes of these instruments were 1.5 and 3 nm. The cut-off size of the SMPS was 10 nm. The number concentration in the size range of 3–10 nm was calculated as the difference in the total particle number concentrations measured with uCPC and SMPS.

Total particle number concentrations measured on board the Cessna were first converted into standard temperature and pressure conditions (STP; 273.15 K, 100 kPa) and were then corrected with the maximum detection efficiency of the instrument based on laboratory calibrations. The maximum detection efficiency of the PSM used in airborne measurement was 0.75 and that of uCPC was 0.99. The maximum detection efficiencies of the PSMs used at the station were 0.8. Finally, the particle number concentrations were corrected with respect to diffusional losses in the inlet part (Fig. 1) and inside the sampling lines on the plane, assuming a constant correction factor for each size bin. The ground and tower data were assumed to have negligible inlet line losses because of core sampling (Kangasluoma et al., 2016). The correction factor for the inlet part was 0.716 for 1.5–3 nm particles and 0.720 for 3–10 nm particles based on simulation results using COMSOL multiphysics software. Penetration efficiencies through the sampling lines were 0.70 and 0.88 in the size ranges of 1.5–3 and 3–10 nm, respectively.

All the results presented here are reported vertically as metres above the ground level, and all the data were collected within a distance of 30 km from SMEAR II in Hyytiälä. A typical measurement flight includes a linear ascent from 100–200 m (a.g.l.) up to the FT region, 2500–3500 m, and a descent back to near the canopy top level.

In this study, we analysed altogether 53 measurement profiles during 18 days. The flights were conducted during three measurement campaigns: May–June 2015, August 2015 and April–May 2017, either in the morning (07:00–12:00 UTC+2) or in the afternoon (12:00–15:00). The days were classified as event, non-event or undefined days based on the NPF event classification method by Dal Maso et al. (2005).

Well-mixed boundary layers are capped by a stable layer. The boundary layer height (BLH) was visually estimated from the in situ measurements on board the Cessna aircraft for each vertical measurement profile. The BLH was estimated from the minimum vertical gradient in H₂O and RH and the maximum vertical gradient in the potential temperature. The estimated BLH was evaluated visually with Doppler lidar profiles when possible (due to a very low lidar signal-to-noise ratio in the clean-air environment) and was found to agree very well. When the sun is rising, the mixing of air mass starts from near the ground, and aerosol particles originating from the surface get mixed upwards within the rising mixed layer. Inside the mixing layer, higher concentrations of H₂O are sometimes seen when the turbulence mixes up the moisture from the surface. CO₂ tends to be higher in the morning boundary layer due to respiration and decreases in the residual layer. The vertical profile of the potential temperature is almost constant in the surface mixed layer and rapidly increases with increasing altitude under stable conditions.

2.5 Uncertainties

As described above, all the results were converted into standard temperature and pressure (100 kPa, 273.15 K) conditions and corrected for the instrumental maximum detection efficiency and line losses according to the laboratory characterizations of the flight set-up. However, there are several factors causing uncertainties in the measured concentrations. The flight speed, main flow rate, air pressure, relative humidity and temperature change rapidly during a flight, which can cause variations in the inlet flows and the performance of the instruments. It is poorly known how the uCPC and PSM behave under quickly varying operational conditions. The reduced pressure at high altitudes may change the maximum detection efficiency and cut-off size of laminar-flow CPCs (e.g. Zhang and Liu, 1991; Herrman and Wiedensohler, 2001). The pressure effect on the PSM cut-off size has been observed to be small (<0.1 nm until 60 kPa) compared to the uncertainty caused by a changing relative humidity and particle composition (Kangasluoma et al., 2016). We measured up to altitudes with the pressure going down to ∼70 kPa, which gives an uncertainty of ±5 % for the aerosol flow rate of the CPC 3776 and thus directly for the concentrations, as shown by Takegawa et al. (2017), Fig. 3. In addition, we compared the concentrations measured by PSM, uCPC and SMPS in the FT where the occurrence of small particles (below 10 nm, which has been under discussion in this paper) is very low. These concentrations matched well with each other, so we may assume that the pressure effect on the measured concentrations for any of the instruments was not significant in our study.

Because of the uncertainties in the instrument cut-off sizes, the true size range for the 1.5–3 nm concentration may vary with altitude and between different flights. This would also slightly affect the particle sampling losses, which were here assumed to be constant for the whole size range, although in reality there is a size dependency. Because of these uncertainties in the determined concentrations, we should focus on the relative behaviour of median values rather than absolute concentrations.

Figure 2All-day median particle concentrations in two size ranges, 3–10 nm (pink) and 1.5–3 nm (blue), and 25th and 75th percentiles (dashed lines) of the 1.5–3 nm particle concentration as a function of altitude for afternoon profiles over 17 event days (a), 34 undefined days (b) and 2 non-event days (c). The concentrations were calculated from the differences between three instruments (PSM, uCPC and SMPS) at different cut-off sizes: 1.5, 3 and 10 nm, respectively. The data were collected from near (<30 km) SMEAR II during spring and August flight measurement campaigns in 2015 and a spring campaign in 2017. Median boundary layer heights are marked by green lines.

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3.1 General features and vertical profiles

The median values of particle concentrations, sensible heat flux, measurement height and estimated BLH were calculated for the 27 cases when comparable data were available at SMEAR II (Table 1). The values on board the aircraft (inside the BL) indicate the observations on board the Cessna, which means that the minimum limit for altitude was around 100 m from ground level. The values on the ground level were measured inside the forest canopy.

Table 1Numerical statistics for boundary layer height (BLH) and sensible heat flux (SHF), indicating the mixing of air mass, and concentrations of 1.5–3 nm particles together with median measurement heights during measurement flights in 2015 and 2017. The morning flights were conducted between 07:00 and 12:00 and afternoon flights at 12:00–15:00. The low number of flights during non-event days is caused by cloudiness, which makes the operation of the aircraft impossible.

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On average, we found that the concentrations of 1.5–3 nm particles were higher on board the aircraft (inside the BL) (1400 cm⁻³) than at the ground station level (1100 cm⁻³) (hereafter referred to as “ground”). The observation of having somewhat lower concentrations of small particles at the ground level is probably due to higher sinks of particles and their precursors inside the canopy compared with above-canopy air (Tammet et al., 2006; Zha et al., 2018). The values were the highest on NPF event days (1500 cm⁻³ on board the aircraft (inside the BL) and 1300 cm⁻³ on the ground) and undefined days (1450 cm⁻³ on board the aircraft (inside the BL) and 1130 cm⁻³ on the ground) and clearly the lowest on non-event days both on board the aircraft (inside the BL) (890 cm⁻³) and on the ground level (740 cm⁻³). It should be noted that two non-event profiles were measured during the same afternoon in the spring of 2015.

The median BLH of all the profiles was 1400 m, being lower in the morning (1100 m) and higher during the afternoon flights (2000 m). Indicative of stronger vertical mixing, the median value of the sensible heat flux was the highest on the NPF event days, especially during the afternoon (286 W m⁻²).

Figure 2 shows the median vertical profiles of the total particle number concentration in the size ranges of 1.5–3 and 3–10 nm separately for the NPF event days, undefined days and one non-event day. The profiles typically contain data from 100 up to 2700 m a.g.l. It is noticeable that the non-event profile consists of only two vertical profiles and that both of them were measured on the same afternoon. We found that airborne 1.5–3 nm particle concentrations were similar between the event and undefined days, whereas substantially lower concentrations were observed on non-event days. We also observed that during the event days there were clearly more 3–10 nm particles inside the BL (on board the aircraft) than during undefined days (Fig. 2a and b). The reason for this could be that during the undefined days the formation of sub-3 nm particles took place, yet the conditions were not suitable for particle growth to larger sizes (see Buenrostro Mazon et al., 2009; Kulmala et al., 2013). Our findings are consistent with earlier observations of high sub-3 nm particle concentrations in Hyytiälä on both event and undefined days compared with non-event days (Lehtipalo et al., 2009; Dada et al., 2017).

During NPF event days, the median and 25th and 75th percentiles show that the concentration of sub-3 nm particles was relatively the highest right above the canopy top. This indicates that the sources of particles and their precursor vapours are near the ground level. During the undefined days, the origin of sub-3 nm particles was not necessarily at the ground level, as their concentration decreased right above the ground level (from 100 to 200 m). In addition, reviewing the median values in Table 1, the concentration of 1.5–3 nm particles was observed to be higher inside the BL (on board the aircraft) during morning on undefined days (2800 cm⁻³) than during afternoon (1150 cm⁻³), opposite to event days (1070 and 3020 cm⁻³, respectively), which supports this hypothesis (Table 1). The concentrations of both sub-3 nm and 3–10 nm particles were very low during the non-event days and we did not observe any clear layers for these particles. However, it should be noted that our study included only two such profiles, since the flight measurements were not possible to conduct during non-event days due to meteorological conditions, especially cloudiness.

The measurement flights were conducted either in the morning (07:00–12:00, UTC+2) or in the afternoon (12:00–15:00). We studied the median vertical particle concentrations separately for those two times in order to estimate the effect of mixing strength on the vertical profile of particles on NPF event and undefined days. As expected based on observed SHF fluxes, we found that the concentrations of 1.5–3 nm particles inside the BL (on board the aircraft) were, on average, the most homogenous vertically during the afternoons of the NPF event days (Fig. 3).

Figure 3Median concentrations in two size ranges (1.5–3 and 3–10 nm) and 25th and 75th percentiles of 1.5–3 nm particle concentration over measurement profiles during event and undefined days separately for morning (a, c) (07:00–12:00) and afternoon (b, d) (12:00–15:00). The median vertical profiles were defined over 9 event mornings, 8 event afternoons, 18 undefined mornings and 16 undefined afternoons. Median boundary layer heights are marked by green lines.

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On NPF event days, we can see an interesting layer of 3–10 nm particles in the morning above the BL at 2400 m. From this layer, the particles can mix down into the evolving BL. A similar behaviour is seen on undefined days, when an increase in concentration of 1.5–3 nm particles is observed in the layer right below 2500 m in the morning and the particles are grown in size and mix downward until afternoon.

3.2 Diurnal variation of particle concentration at different altitudes in the lower atmosphere

We studied the median diurnal variation of total particle concentration (all particles >1.5 nm) and, separately, particle concentration in the size range 1.5–3 nm at different altitudes from 100 to 2700 m a.g.l. around the SMEAR II area. The study included 17 vertical measurement profiles during event days and 34 during undefined days. From Fig. 4a it can be seen that the total particle number concentration over all measurement profiles was the highest near the ground in the morning. The aerosol population mixed with cleaner air within the evolving BL after the morning, which led to a decreasing particle number concentration, whereas the concentration increased again towards the afternoon, presumably as a result of NPF. The highest particle number concentrations were observed at 11:30–14:30 inside the BL (on board the aircraft), which coincides with the peak time of NPF in Hyytiälä (Dada et al., 2018).

Figure 4Panel (a) shows median total particle number concentration at different altitudes calculated over 51 measurement flight profiles (for 17 event days and 34 undefined days) during spring and August 2015 and spring 2017 campaigns at a 30 km maximum distance from SMEAR II. The total particle number concentration was measured with PSM with a cut-off size of 1.5 nm. Colour scale indicates total number concentration. Panel (b) shows median particle number concentration in the size range of 1.5–3 nm at different altitudes. The value is defined as the difference of total number concentrations with different cut-off sizes: PSM (1.5 nm) and uCPC (3 nm). Panel (c) shows the number of data points in each cell of panels (a)–(b). Estimated boundary layer heights are marked as crosses in panels (a)–(b) over flight profiles. Each cell includes the median value of all measurement points inside the 100 m bin and 30 min.

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Figure 5New particle formation event at SMEAR II in Hyytiälä on 13 August 2015. Panel (a) shows the number size distribution measured by a differential mobility particle sizer at ground level inside the forest canopy. Start and end times of two measurement flights are marked by vertical lines in figure. Panel (b) shows the particle flux measured at 23 m a.g.l. at the station. Negative particle flux indicates particle flux downwards.

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Figure 6Vertical profiles during the first measurement flight at 07:30–09:00 on 13 August 2015 (marked in Fig. 5). Panels (a) and (b) show data from the ascent and panels (c) and (d) from the descent. Panels (a) and (c) show the number concentration of 1.5–3 nm (black solid line) and 3–10 nm (dashed line) particles and the carbon dioxide concentration (red). Panels (b) and (d) show water vapour concentration (black), relative humidity (red) and potential temperature (dashes line) profiles. The green line is the estimated boundary layer height.

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The sub-3 nm particle number concentrations (Fig. 4b) were the highest in the morning near the ground level, with a second maximum around noon. Later in the afternoon, the sub-3 nm particle concentration was clearly lower, probably because these particles apparently grew efficiently to larger sizes and contributed significantly to the total particle concentration (Yli-Juuti et al., 2011). Both total particles and sub-3 nm particles had the highest concentrations near the ground level throughout the day, even though especially the total particle population seems to have been spread within the whole mixed layer.

Figure 4c shows the data availability for this analysis. It is noticeable that the number of data in each 100 m 30 min cell varies considerably. In addition, one intense NPF event day with strong particle formation in the early morning dominated the distribution due to the low number of flights at around 07:00–08:00. Most of the data were collected either during the morning (08:30–11:30) or afternoon (13:30–15:00). As we know, the BLH, mixing of air and meteorological conditions can also differ significantly even within 1 day, especially between the NPF event and undefined days.

Figure 7Measurement profiles like in the previous figure, but during the second measurement flight on 13 August 2015 at 11:00–12:45

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3.3 Case study – NPF in evolving BL

13 August 2015 was an intense NPF event day in Hyytiälä (Fig. 5a). During that day we conducted two measurement flights around SMEAR II and observed the particle concentration in size range of 1.5–3 nm to follow the development of BL and turbulent mixing (Figs. 6a, c, 7a, c). During the first measurement flight at 07:30–09:00, we observed a clear layer of 3–10 nm particles near the FT region above 2300 m. These particles were mixed down before the afternoon flight, as this population was no longer observed during that flight. The negative (downwards) particle flux at SMEAR II after 12:00 supports this hypothesis (Fig. 5b).

The estimated BLH was ∼500–700 m during the first flight in the morning and had risen up to 1500–1700 m by the afternoon flight. Below the FT, the vertical variation of the 1.5–3 nm particle concentration was larger compared to the stable conditions in the FT. The median concentration of 1.5–3 nm particles inside the BL (on board the aircraft) decreased from the morning flight (7300 cm⁻³ during the ascent and 6300 cm⁻³ during the descent; Fig. 6a and c) to the afternoon flight (∼2500 cm⁻³; Fig. 7a and c), whereas 3–10 nm particles seemed to behave in an opposite manner (350, 200, 850 and 1450 cm⁻³). The sub-3 nm particle concentrations were clearly higher inside the BL (on board the aircraft) than in the FT, and the concentration increased towards the ground. This is consistent with organic vapours emitted from the ground vegetation participating in NPF and growth (Kulmala et al., 2013; Ehn et al., 2014).