NEW PARTICLE FORMATION INSIDE THE EVOLVING BOUNDARY LAYER
K. LEINO1, J. LAMPILAHTI1, S. BUENROSTRO MAZON1, A. MANNINEN1, R. VÄÄNÄNEN1, H.E.
MANNINEN1,2, T. PETÄJÄ1 and M. KULMALA1
1
Department of Physics, P.O. Box 64, FI-00014 University of Helsinki, Finland.
2
CERN, CH-1211Geneva 23, Switzerland.
Keywords: ATMOSPHERIC BOUNDARY LAYER, CLUSTERS, NEW PARTICLE FORMATION.
INTRODUCTION
New particle formation (NPF) events frequently occur inside the continental boundary layer over the
boreal forest region of Southern Finland (Kulmala et al., 2001, Kulmala et al., 2004, Kulmala et al.,
2013). The phenomenon has been investigated near the ground level for over 20 years in the SMEAR II
(Station for Measuring Ecosystem-Atmosphere Relations, Hari and Kulmala, 2005) field station in
Hyytiälä, Finland.
Airborne measurements of aerosol particles have been conducted near the SMEAR II station (61°51’N,
24°17’E, 181 m above sea level) since the year 2003 using small aircraft (Laaksonen et al., 2008, O’Dowd
et al., 2009, Schobesberger et al., 2013, Väänänen et al., 2016) and hot-air balloons (Laakso et al., 2007,
Petäjä et al., 2013). As a short overview, Laakso et al. (2007) observed the new particle formation to
occur in the mixed boundary layer but also in the free troposphere with no connection to the boundary
layer nucleation. They used a hot-air balloon as a platform for particle and cluster measurements. O’Dowd
et al. (2009) observed the nucleation throughout the boundary layer over SMEAR II and the nucleation
mode number concentration peaked above the forest canopy.
Schobesberger et al. (2013) observed the new particle formation inside the planetary boundary layer. The
highest concentrations of nucleation mode particles were found to be in the upper parts of the planetary
boundary layer, which indicates that the nucleation does not necessarily occur only close to the surface.
Väänänen et al. (2016) studied the vertical and horizontal extent of new particle formation in the lower
troposphere near to SMEAR II station. They observed that the air masses within 30 km from SMEAR II
only differed slightly from the ground-based observations at the station, though the variability of the
differences was larger for the nucleation mode particles than for the larger particles. Furthermore,
Väänänen et al. (2016) detected the nucleation to take place both inside the boundary layer as well as
some times separately in the free troposphere.
In addition to near to surface nucleation inside the planetary boundary layer and free troposphere
nucleation, another suggestion is that the nucleation takes place near the clouds (Wehner et al., 2015).
Episodes of new-particle formation might take place also in elevated air layers that are not influenced by
the surface, provided that the necessary gaseous precursors were transported there earlier.
Measurement techniques that enable detection of the smallest aerosol particles have developed during the
last years. Total particle concentration can be measured with Condensation Particle Counters (CPC). A
history of the CPCs is given in McMurry (2000). Aerosol particle size distributions can be measured by
Pulse Height Condensation Particle Counter (PH-CPC) in the size range of 1.3 – 5 nm (Sipilä et al., 2008,
Lehtipalo et al., 2009). The smallest ions and charged particles we can measure with Balanced Scanning
Mobility Analyzer (BSMA) (in the size range of 0.8 – 7.5 nm) (Tammet, 2006) and Neutral Cluster and
Air Ion Spectrometer (NAIS, Airel Ltd, Estonia, Manninen et al., 2009) (in the size range of 0.8 – 42 nm).
In addition to naturally charged particles, the NAIS measures also total particle distribution in the size
range of 2 – 42 nm charging the neutral particles with a unipolar charger. One of the newest particle
counters is the Particle Size Magnifier (PSM) which detects directly sub-3 nm atmospheric particles
(Vanhanen et al., 2011).
In this work, we study the vertical distribution of the smallest clusters in the size range below 3 nm with a
PSM onboard the Cessna aircraft. We have successfully measured small aerosol particles as small as 1.5
nm in diameter, as well as followed their growth and mixing in the evolving boundary layer using as small
aircraft as the measurement platform.
METHODS
In this study, we have measured the total number concentrations of aerosol particles with a Particle Size
Magnifier (PSM a10) (Vanhanen et al., 2011) including CPC TSI 3010 as a counter. The instrument had a
1.5 nm cut-off size determined in laboratory verifications. Total particle concentrations were measured
with an ultrafine Condensation Particle Counter (uCPC, TSI 3776) that had a 3 nm cut-off size. The total
particle number-size distributions were measured with a Scanning Mobility Particle Sizer (SMPS) in the
size range of 10–400 nm. The instrument setup contained devices measuring meteorological parameters
(pressure, relative humidity, temperature and wind), and concentrations of H2O and CO2 gases. This setup
was used in two measurement campaigns: the first campaign was between May and June and the second
was in August, in 2015.
The instruments were installed inside the cabin of a Cessna 172 airplane. The flights started from
Tampere-Pirkkala airport. The flights were carried out above the SMEAR II station in Hyytiälä, Southern
Finland with an altitude range of 300–3000 m above sea level. For detailed description of the
instrumentation and sampling, see also Schobesberger et al. (2013) and Väänänen et al. (2016).
RESULTS
During two measurement flights on Aug 13, 2015 we observed that the vertical profile of the cluster
concentration (1.5 – 3 nm) was influenced by the boundary layer evolution and convection (Figure 1). The
figure includes four measurement profiles from the ground level up to 2700 m above ground. The first two
profiles were flown between 7:30 –9:00 a.m. and the last two profiles between 10:45 a.m. – 12:45 p.m.
The flights were performed around the SMEAR II station in Hyytiälä where the new particle formation
event started around 10 a.m.
The small clusters were inside the mixed layer already before the event and they also extended into the
residual layer during the first ascent. The mixed layer height was around 400 meters during the first profile
and rose up to 1200 meters during the second profiles. During the second ascent and descent the NPF
event was on-going and the cluster concentration increased throughout the boundary layer. In all the
measurement profile figures we see that the total concentration measured by PSM was higher than the
total concentration measured by uCPC inside the planetary boundary layer and about equal in higher
altitudes. This indicates that the smallest clusters were not present above the mixed layer.
CONCLUSIONS
Based on the initial tests, we illustrated that the PSM can be operated as a part of aerosol instruments
onboard an aircraft. Our results suggest that the clusters, which participate in new particle formation,
originate from near the forest canopy and their number concentration decreases as the altitude increases.
The source near the surface is likely linked to the emissions from the vegetation. The decreasing
concentration as a function of altitude is probably caused by vertical transport combined with limited
cluster lifetime. A more detailed analysis is required to draw further conclusions on the vertical and spatial
variability of the clusters.
Figure 1. Vertical profiles of total particle concentrations measured with PSM (green) with 1.5 nm cut-off
size and ultrafine-CPC with 3 nm cut-off size (blue) on 13.8.2015 above the boreal forest site in Hyytiälä.
The first ascent and descent were flown at 7:30 – 9:00 a.m. and the last ascent and descent at 11:00 a.m. –
12:45 p.m.
ACKNOWLEDGEMENTS
This work was supported by the Academy of Finland Centre of Excellence in Atmospheric Science (grant
no. 272041) and the project has received funding from the European Union’s Horizon 2020 research and
innovation programme under grant agreement No 654109.
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