While atmospheric new particle formation (NPF) has been observed in various
environments and was found to contribute significantly to the total aerosol
particle concentration, the production of new particles over open seas is
poorly documented in the literature. Nucleation events were detected and
analysed over the Mediterranean Sea using two condensation particle counters
and a scanning mobility particle sizer on board the ATR-42 research aircraft
during flights conducted between 11 September and 4 November 2012 in the
framework of the HYMEX (HYdrological cycle in Mediterranean EXperiment)
project. The main purpose of the present work was to characterize the spatial
extent of the NPF process, both horizontally and vertically. Our findings
show that nucleation is occurring over large areas above the Mediterranean
Sea in all air mass types. Maximum concentrations of particles in the size
range 5–10 nm (
New particle formation (NPF) is a widespread phenomenon in the atmosphere which results from a complex sequence of multiple processes including two major steps (Kulmala and Kerminen, 2008): (1) the formation of clusters from the gaseous phase and (2) the growth of these clusters up to sizes at which they may influence the climate through cloud related radiative processes (Kerminen et al., 2012; Makkonen et al., 2012). Observations of the phenomenon in various environments are reported in the literature (Kulmala et al., 2004), including boundary layer (BL) polluted locations (e.g. Brock et al., 2003; Wiedensohler et al., 2009), clean or rural sites (e.g. Suni et al., 2008), high-altitude stations (e.g. Venzac et al., 2008; Boulon et al., 2010; Rose et al., 2015a), polar areas (e.g. Asmi et al., 2010), and coastal sites (e.g. O'Dowd et al., 1998, 2002). NPF events' characteristics, such as spatial extent, both vertical and horizontal (Crumeyrolle et al., 2010; Boulon et al., 2011), particle formation and growth rates (Manninen et al., 2010; Yli-Juuti et al., 2011), are known to be affected by atmospheric parameters, including the number of gaseous precursors, the concentration of pre-existing aerosol particles, and meteorological variables (temperature, relative humidity, solar radiation). However, current knowledge of the theory that lies beyond NPF remains poor, especially at high altitudes, i.e. above 1000 m, and a more profound understanding of the mechanisms and precursors involved in nucleation and particle growth is currently required, especially to improve the accuracy of climate modelling studies.
Marine aerosol particles, as one of the main contributors to the natural aerosol emissions, have been the focus of the attention in the scientific community for several decades (e.g. Heintzenberg et al., 2000, and references therein). In these pristine environments, changes in the aerosol burden might have significant impacts on cloud properties, as shown in recent studies (Tao et al., 2012; Koren et al., 2014; Rosenfeld et al., 2014). Marine aerosol has a primary component, known as “sea spray aerosol”, which results from the interaction between wind and water surface, and a secondary component, which is in the scope of the present work. Current knowledge regarding NPF in the marine boundary layer mainly concerns coastal regions (O'Dowd et al., 1998, 2002; O'Dowd and de Leeuw, 2007), where observations of NPF are more abundant than over the open ocean. Many studies have concentrated on the comprehension of the mechanisms and the identification of the precursors involved in the NPF process from coastal macroalgae fields (O'Dowd and de Leeuw, 2007, and references therein), including both model studies (Pirjola et al., 2000) and chamber experiments (Sellegri et al., 2005). A schematic view of the marine aerosol production in coastal areas based on observations from the Mace Head station (Ireland) is given by Vaattovaara et al. (2006). In contrast, the scarcity of open-ocean nucleation studies reported in the literature might indicate that the NPF process does not occur with a high probability over the open ocean.
While ground-based stations allow indirect analysis of the horizontal extent of NPF (Kristensson et al., 2014; Rose et al., 2015b), airborne aerosol measurements can add relevant information on both the horizontal and vertical extent of NPF. Such measurements were conducted over the boreal environment, in the vicinity of the Hyytiälä SMEAR-II station, using aircraft (O'Dowd et al., 2009; Schobesberger et al., 2013) as well as motorized hang glider/microlight aircraft (Junkermann, 2001, 2005) or balloons (Boy et al., 2004; Laakso et al., 2007). These studies, based on a limited number of observations, delivered contrasting results indicating that nucleation could occur not only throughout the boundary layer but also in the free troposphere in some cases (Laakso et al., 2007), with no clear trend or preference. Airborne studies conducted in different environments are also reported in the literature, showing evidence for the occurrence of NPF in the free troposphere above Europe during the EUCAARI-LONGREX project (Mirme et al., 2010) and above the Arctic region, up to 7 km, by Khosrawi et al. (2010) during the project ASTAR 2004. In contrast, using a restricted number of aircraft vertical soundings, Crumeyrolle et al. (2010) suggest that over the North Sea, NPF could be limited to the top of the boundary layer.
Giving more insights into the vertical development of the NPF process in the marine troposphere is one of the main objectives of the present study based on aircraft measurements conducted above the Mediterranean sea between 11 September and 4 November 2012 in the framework of the HYMEX (HYdrological cycle in Mediterranean EXperiment) project. We report particle size distributions and concentrations measured with a scanning mobility particle sizer (SMPS) and condensation particle counters (CPCs) deployed on board the French ATR-42 aircraft. The occurrence of NPF is investigated, with a special focus on the horizontal and vertical extents of the process, coupled with an analysis of several atmospheric parameters expected to explain these extents.
Overview of ATR-42 flights performed during the HYMEX campaign between 11 September and 4 November 2012 and discussed in the present study. The range of latitudes (longitudes) corresponds to the minimum and maximum latitudes (longitudes) reached during the flight. Dominant air mass origins are reported in the last column according to the following abbreviations: WE for western Europe, SMS for southern Mediterranean Sea, EMS for eastern Mediterranean Sea and NE for northern Europe.
As part of the HYMEX project, the ATR-42 research aircraft, operated by SAFIRE (Service des Avions Français Instrumentés pour la Recherche en Environnement), was based at Montpellier airport, in the south of France. A total of 28 flights were performed between 11 September and 4 November 2012 (Table 1).
The instrumental setup deployed on board and used for the analysis of NPF consisted of two CPCs and an SMPS. The CPCs, developed at the Max Planck Institute for Polymer Research, Mainz, Germany, are specifically dedicated to aircraft measurements (Weigel et al., 2009). Their nominal 50 % lower cut-off diameter can be varied by changing the temperature difference between saturator and condenser. During HYMEX, one of the CPCs was measuring with a cut-off diameter of 5 nm and the second with a cut-off diameter of 10 nm. The time resolution of the CPCs was set to 1 s. The SMPS system is described in Crumeyrolle et al. (2010) and consists of a CPC (TSI, 3010), a differential mobility analyser (DMA) and a krypton aerosol neutralizer. The SMPS provided particle size distributions in the diameter range 20–485 nm, with a time resolution of 130 s.
The ATR-42 was also equipped with additional instruments dedicated to the
analysis of aerosol and cloud properties: a particle soot Absorption photometer
allowing the monitoring of black carbon concentration (PSAP; Bond et al.,
1999), an optical particle counter (OPC, GRIMM), a compact aerosol time-of-flight mass spectrometer (AMS; Drewnick et al., 2005) and a fast
forward-scattering spectrometer probe (Fast-FSSP). The aerosol instrumentation was
connected to the community aerosol inlet (CAI), which has a 50 % sampling
efficiency for particles larger than 5
The main objective of this study was to investigate the occurrence of NPF in
the marine troposphere. For that purpose, all measurements conducted above
land were removed from the database. An arbitrary threshold distance of 1 km
to the coast was set to consider measurement as “marine”. The distance,
Method for the determination of the “marine”/“land” flag. The
red point corresponds to the location of the aircraft. The blue points
represent the 1000
In order to track the occurrence of nucleation above the Mediterranean Sea
and to evaluate the strength of the events, particle concentration in the
size range 5–10 nm (
The influence of air mass history on the occurrence of nucleation was studied
using three days air mass back trajectories computed with the HYSPLIT
transport and dispersion model (Draxler and Rolph, 2003). Three days before
sampling were chosen based on previous work by Tunved et al. (2005), who
estimated the turnover time of aerosol particles to be around 1.6–1.7 days
for nuclei size ranges and 2.4 days for 200 nm particles. Back trajectories
were calculated every 5 min along the flight path (i.e. 30 km for a typical
aircraft speed of 100 m s
Illustration of the air mass back trajectories calculation along the flight path (black points) for flight 39 (23 September 2012). The colour coding of the trajectories corresponds to the sectors as given by the text colours.
When considering all measurements recorded in clear-sky conditions above the
sea, 26 % of
The location where the significant
Location of
According to the geographical distribution of the nucleation points in
Fig. 3, it is likely that nucleation could occur at variable distances to the
coast. Small particles observed close to the littoral may be expected to be
produced during events initiated above land or involving gaseous precursors
originating from the continental boundary layer (BL). In contrast,
significant
As reported in Table 1, most of the studied flights were performed under western Europe and southern Mediterranean Sea atmospheric flows (15 and 7 out of 28, respectively), while northern and local types were only observed during three flights. The three remaining flights were performed under variable conditions, however often dominated by western Europe and southern Mediterranean Sea flows.
Nucleation probability and
Significant concentrations of small particles in the size range 5–10 nm
were detected in all types of air masses (Table 2), but the highest
nucleation probability was found in northern air masses, where 60 % of
Median
In order to further investigate this aspect, a more detailed analysis of the
number of nucleated particles as a function of the fetch, i.e. the time spent
by air masses above the sea before sampling, was conducted and is reported in
Fig. 4, separately for western Europe and southern Mediterranean Sea flows.
Reported
Profile of the
In western Europe and southern Mediterranean Sea air masses, fetches up to 60 h
were calculated. In these air masses, larger concentrations are typically
recorded for smaller fetches, with maximum
The purpose of the next section is now to investigate the vertical extent of nucleation.
A total of 39 vertical soundings were performed during the studied flights,
giving the opportunity to examine profiles of the
For most of the soundings, the BLH was lower than 1000 m, being on average
662
The vertical limits of the nucleation process are clearly visible for
profile nos. 6 and 27. For profile no. 6, nucleation is observed between 1680
and 3170 m, with concentrations significantly increased between 2400 and
2900 m. During sounding no. 27, nucleation is detected on a slightly more
restricted altitude range, 1350–1780 m, with higher concentrations between
1660 and 1710 m. Other profiles, such as nos. 18, 23 and 34, also display
In order to further investigate the vertical extension of nucleation, all
measurements were considered, i.e.
Statistics on the detection of significant particle concentration in the size range 5–10 nm as a function of altitude (left panel; the number of data points is indicated on the plot for each altitude range). Corresponding median concentrations are reported on the right panel; left and right limits of the error bars stand for the 1st and 3rd quartile, respectively.
However, it seems that, when nucleation events are detected, the number of
nucleated particles does not significantly vary with altitude, especially
above 500 m, where median concentrations are in the range of
307–376 cm
The fact that nucleation could be favoured in the FT compared to the BL contradicts the results by Crumeyrolle et al. (2010), who found that NPF events were limited to the top of the BL in the North Sea. However, it is worth noting that the number of vertical profiles included in the Crumeyrolle et al. study was limited (13 profiles), and most of them were performed close to the coast.
The purpose of this section is to further investigate atmospheric parameters and/or processes which are associated with the higher probability of observation of small particles at higher altitudes.
Meteorological parameters, such as temperature and relative humidity (RH), as
well as global radiation, were previously reported to influence the
nucleation process. Global radiation, which is expected to be more intense at
higher altitudes and thus favour photochemical processes, including the
oxidation of gaseous precursors involved in the nucleation process, could
provide a first explanation for the observed
Statistics concerning temperature and RH recorded during the studied flights
are presented as a function of altitude range in Fig. 7. It is very clear
that temperature is decreasing with altitude, especially above 3000 m, where
most of the temperatures are found to be below zero. The same trend is
observed for RH, but with higher variability.
Median temperature and relative humidity (RH) as a function of altitude range; left and right limits of the error bars stand for the 1st and 3rd quartile, respectively.
Figure 8 shows, for the different altitude ranges previously introduced, the
median condensation sink (CS) calculated from SMPS size distributions
recorded at constant altitudes, i.e. apart from vertical soundings. Up to
2000 m, the median CS does not significantly vary with altitude, being in
the range 3.1–3.9
Median CS as a function of altitude range; left and right limits of the error bars stand for the 1st and 3rd quartile, respectively. The number of data points included in the statistics is indicated on the plot for each altitude range.
A more complete analysis focussed on altitudes above 2000 m was then
conducted to highlight the role of the CS in the nucleation process at higher
altitudes. Figure 9 shows the correlation between
Particle concentration in the size range 5–10 nm as a function of condensation sink (CS) for altitude ranges above 2000 m.
In the present study, we may hypothesize that some gaseous compounds are transported, together with the pre-existing particles, from lower altitudes, and that they may be further oxidized to more condensable species involved in the nucleation process. As previously mentioned, such processes might be favoured by convection associated with clouds and their outflow. In that case, the lifted air parcels where small particles are detected are expected to have different chemical signature from free-troposphere air, as well as different water vapour content. Also, the fact that clusters might be formed at lower altitudes and then be transported together with larger particles above 2000 m cannot be excluded. In addition, it has been previously reported by several studies that the mixing of two air parcels showing contrasting levels of RH, temperature, condensation sink and precursors could favour the occurrence of nucleation (Nilsson and Kulmala, 1998; Khosrawi and Konopka, 2003; Dall'Osto et al., 2013).
We further investigated the contribution of cloud processes and BL intrusions regarding the formation of new particles using tracers such as the black carbon (BC) concentration and the specific humidity, in addition to cloud cover. Unfortunately, there was no measurement available regarding the composition of the gas phase. Our analysis was focussed on the vertical soundings performed by the ATR-42 that allow for a direct comparison of the vertical distribution of the parameters of interest (Figs. 5 and B1).
Among the 17 profiles previously associated with NPF in the FT (profile nos. 2,
6, 13, 17, 18, 19, 26, 27, 29, 30, 31, 32, 33, 34, 35, 36 and 38), although
cloudy conditions were filtered from our analysis, we retrieved that clouds
were observed in the same altitude range as small particles in four cases
(profile nos. 29, 30, 34 and 38). For three of them, collocated increases in the
specific humidity (profile nos. 29 and 30) and/or BC concentration (profile
nos. 30 and 34) were also found. These observations suggest that for those
four particular cases, the formation of small particles was most probably induced
in recently lifted air from convective clouds. For the remaining soundings,
clouds were detected at lower altitudes: for sounding nos. 2, 6, 13, 18, 19,
31, 33, 35 and 36, the vertical cloud profile was sparse, while it was denser
for profile nos. 17, 26, 27 and 32. Missing data did not allow for a complete
analysis of sounding nos. 18 and 36, which will thus not be further
discussed. During sounding no. 31, high
We have shown so far that, above the Mediterranean Sea, NPF was observed over large areas and could be favoured at higher altitudes, since particles in the size range 5–10 nm are mostly seen above 1000 m. However, the previous analysis did not always unambiguously answer the question regarding the conditions associated with the initial cluster (1–2 nm particles) formation, especially in terms of the degree of BL influence/intrusions. Nonetheless, these particles, regardless of whether transported to or formed in the FT, in more or less polluted conditions, are expected to grow to larger diameters and might reach climate-relevant sizes in the FT. The purpose of the next section is to investigate this growth process above 2000 m by analysing the shape of the SMPS size distributions.
SMPS size distributions (20.4–484.5 nm) recorded at constant altitude were fitted with four Gaussian modes, with the initial guess on the modal diameters being 25, 50, 110 and 220 nm for the nucleation, Aitken, first and second accumulation modes, respectively. These four modes were chosen because they most frequently came out of the fitting procedure when run without initial guesses. These diameters were previously found in the literature to describe particle size distributions in the marine atmosphere (Heintzenberg et al., 2000). The geometric mean diameters of these modes found by the fitting procedure vary from the initial guesses with time and altitude. In the following sections, the parameters of the modes (geometric mean diameter and concentration) will be used to describe the evolution of the particle size distribution.
Figure 10 shows the average fitted distributions as a function of altitude and daytime. The analysis is focussed on the highest altitude ranges, where nucleation was more frequently observed (the altitude range 1000–2000 m is not considered because of too few data points). The parameters of the Gaussians used for these fits are given as additional information in Table A1. The “closed” shape of the reconstructed size distributions shown in Fig. 10 is the result of the originally “open-type” size distributions that were fitted with a nucleation mode that extended below the SMPS lower size cut (20 nm). These reconstructed size distributions do represent the real size distributions measured below 20 nm.
Averaged fitted SMPS size distributions as a function of daytime and altitude.
During the time period 11:00–17:00 UTC, the size distributions are
dominated by the nucleation mode. In fact, this mode includes 42 % of the
particles measured by the SMPS between 2000 and 3000 m, and 48 % above
3000 m. However, both the concentration and the diameter of the nucleation
mode appear to be larger between 2000 and 3000 m compared to higher
altitudes (Table A1). Since
At night (17:00–05:00 UTC), the contributions of nucleation and Aitken modes to the total particle concentration are very similar between 2000 and 3000 m, being around 34 % each, whereas above 3000 m the nucleation mode is dominant (46 % against 36 % for the Aitken mode). These observations suggest that between 2000 and 3000 m, nucleated particles are growing during the course of the day, leaving the nucleation mode, which thus includes a decreasing fraction of the total particle concentration, to reach the Aitken mode. In contrast, it is likely that particle growth is not as fast above 3000 m, since the nucleation mode displays particle concentrations which remain on average higher compared to the Aitken mode, even in the evening. Again, this observation suggests that particle growth could get slower with increasing altitudes.
In the morning (05:00–11:00 UTC), the average size distribution above
3000 m displays a nucleation mode whose diameter is similar to the diameter
observed at night (
As previously mentioned, nanoparticles in the size range 5–10 nm were detected during night-time (17:00–05:00 UTC). The purpose of this last section is to investigate their origin, and more particularly to examine the possibility for the night-time particles to originate from nucleation events triggered earlier during daytime.
The time interval 09:00–12:00 UTC, during which the formation of cluster
particles is most frequently observed (Rose et al., 2015a, b), was considered
as a reference nucleation period, and only the significant 130 s
averaged
Particle GRs were estimated from the shift of the nucleation-mode diameter
observed on the average SMPS size distributions between night-time
(17:00–05:00 UTC) and morning hours (05:00–11:00 UTC) for the altitude
range 2000–3000 m (Table A1, Fig. 10). An average value of
0.31 nm h
Estimations of
We investigated the occurrence of nucleation events above the Mediterranean Sea using data obtained during research flights performed in the framework of the HYMEX project between September and November 2012.
Based on our observations, nucleation takes place over large areas above the
Mediterranean Sea in all air mass types. Nucleation probability slightly
varies with air mass origin, but the signature of the different air mass
types is, however, complex to distinguish in terms of
The analysis of the vertical extension of nucleation showed that the process
could be promoted above 1000 m, and especially between 2000 and 3000 m.
Simultaneous analysis of the boundary layer height indicated that these
altitudes often corresponded to free-tropospheric conditions. Vertical
gradient of the condensation sink, together with temperature and humidity,
might explain the increasing nucleation probability with altitude. The mixing
of two air parcels with contrasting properties (temperature, RH, condensation
sink, precursors) could also support the occurrence of nucleation at
preferential altitudes. In addition, for a given altitude range, larger
The investigation of the global shape of the particle size distributions derived from SMPS measurements finally allowed us to study the particle growth above 2000 m, and more particularly the relative contribution of the different particle modes to the total concentration as a function of time and altitude. After they formed, particles appear to grow to larger sizes above 2000 m, reaching the Aitken-mode range, but with growth rates which seem to decrease with altitude. This slow growth, coupled with low coagulation sinks, may favour longer subsistence for nanoparticles in the size range 5–10 nm, and could explain the detection of these small particles during night-time, several hours after their formation.
Our findings demonstrate that higher altitudes could promote the occurrence of NPF, not only over continental areas, as previously suggested by Boulon et al. (2011), but also over open seas, with indications of marine precursors. This result supports the model study by Makkonen et al. (2012), which predicts that nucleation could have a significant contribution to the cloud condensation nuclei (CCN) concentration over the Mediterranean Sea, and indicates that this contribution could be even more decisive at higher altitudes, where clouds form.
Parameters of the Gaussians used to fit the SMPS size distributions as a function of daytime for the altitude ranges above 2000 m.
Profiles of
HyMeX SOP1 was supported by CNRS, Météo-France, CNES, IRSTEA and INRA
through the large interdisciplinary international programme MISTRALS
(Mediterranean Integrated STudies at Regional And Local Scales), dedicated to
the understanding of the Mediterranean Basin environmental processes
(