Although new particle formation (NPF) events have been studied extensively for some decades, the mechanisms that drive their occurrence and development are yet to be fully elucidated. Laboratory studies have done much to elucidate the molecular processes involved in nucleation, but this knowledge has yet to be conclusively linked to NPF events in the atmosphere. There is great difficulty in successful application of the results from laboratory studies to real atmospheric conditions due to the diversity of atmospheric conditions and observations found, as NPF events occur almost everywhere in the world without always following a clearly defined trend of frequency, seasonality, atmospheric conditions, or event development.
The present study seeks common features in nucleation events by applying a
binned linear regression over an extensive dataset from 16 sites of various
types (combined dataset of 85 years from rural and urban backgrounds as well
as roadside sites) in Europe. At most sites, a clear positive relation with the frequency of NPF events is
found between the solar radiation intensity (up to
The analysis of chemical composition data presents interesting results.
Concentrations of almost all chemical compounds studied (apart from O
New particle formation (NPF) events are an important source of particles in the atmosphere (Merikanto et al., 2009; Spracklen et al., 2010). These are known to have adverse effects on human health (Schwartz et al., 1996; Politis et al., 2008; Kim et al., 2015) and affect the optical and physical properties of the atmosphere (Makkonen et al., 2012; Seinfeld and Pandis, 2012). While NPF events occur almost everywhere in the world (Dall'Osto et al., 2018; Kulmala et al., 2017; O'Dowd et al., 2002; Wiedensohler et al., 2019; Chu et al., 2019; Kerminen et al., 2018), with some exceptions reported in forest (Lee et al., 2016; Pillai et al., 2013; Rizzo et al., 2010) and high-elevation sites (Bae et al., 2010; Hallar et al., 2016), great diversity is found in the atmospheric conditions within which they take place. The many studies conducted have included many different types of locations (urban, traffic, regional background) around the world, and differences were found in both the seasonality and intensity of NPF events. This variability may be related to the mix of conditions that are specific to each location, which obscures the general understanding of the conditions that are favourable for the occurrence of NPF events (Berland et al., 2017; Bousiotis et al., 2020). For example, solar radiation is considered one of the most important factors in the occurrence of NPF events (Kulmala and Kerminen, 2008; Kürten et al., 2016; Pikridas et al., 2015; Salma et al., 2011), as it drives the photochemical reactions leading to the formation of sulfuric acid (Petäjä et al., 2009; Cheung et al., 2013), which is frequently the main component of the formation and growth of the initial clusters (Iida et al., 2008; Stolzenburg et al., 2020; Weber et al., 1995). Nevertheless, in many cases NPF events do not occur in the seasons with the highest insolation (Park et al., 2015; Vratolis et al., 2019). Similarly, uncertainty exists over the effect of temperature (Yli-Juuti et al., 2020; Stolzenburg et al., 2018). Higher temperatures are considered favourable for the growth of newly formed particles as increased concentrations of both biogenic volatile organic compounds (BVOCs) and anthropogenic volatile organic compounds (AVOCs) (Yamada, 2013; Paasonen et al., 2013) as well as their oxidation products (Ehn et al., 2014) support growth of the particles. On the other hand, the negative effect of increased temperature upon the stability of molecular clusters should not be overlooked (Kürten et al, 2018; Zhang et al., 2012). The former factor appears frequently be dominant, as higher growth rates are found in most cases in the local summer (Nieminen et al., 2018), although the actual importance of VOCs in the occurrence of NPF events is still not fully elucidated, with oxidation mechanisms still under intense research (Tröstl et al., 2016; Wang et al., 2020). The effect of other meteorological variables is even more complex, with studies presenting mixed results on the effect of the wind speed and atmospheric pressure. Extreme values of those variables may be favourable for the occurrence of NPF events, as they are associated with increased mixing in the atmosphere but at the same time suppress nucleation due to increased dilution of precursors (Brines et al., 2015; Rimnácová et al., 2011; Shen et al., 2018; Siakavaras et al., 2016) or favour it due to a reduced condensation sink (CS).
The effect of atmospheric composition on NPF events is also a puzzle of
mixed results. While the negative effect of increased CS on the
occurrence of the events is widely accepted (Kalkavouras et al., 2017;
Kerminen et al., 2004; Wehner et al., 2007), cases are found when NPF events
occur on days with higher CS compared to average conditions (Größ et
al., 2018; Kulmala et al., 2005). Sulfur dioxide (SO
It is evident that while general knowledge of the role of the meteorological and atmospheric variables has been achieved, there is great uncertainty over the extent and variability of their effect (and for some of them even the direction of an effect) in the mechanisms of NPF in real atmospheric conditions, especially in the more complex urban environment (Harrison, 2017). The present study, using an extensive dataset from 16 sites in six European countries, attempts to elucidate the effect of several meteorological and atmospheric variables not only in general, but also depending on the geographical region or type of environment. While studies with multiple sites have been reported in the past (Dall'Osto et al., 2018; Kulmala et al., 2005; Rivas et al., 2020), to the authors' knowledge this is the first study that focuses directly on the effect of these variables upon the frequency of NPF events as well as the formation and growth rates of newly formed particles in real atmospheric conditions.
Location and data availability of the sites.
The present study uses a total of more than 85 years of hourly data from 16 sites from six countries in Europe with various land usage and climates. It was considered very important that at least a rural and an urban site would be available from each country to study the differences between the different land usage effects on NPF events throughout Europe. The sites were chosen to cover the greatest possible extent of the European continent, with sites from northern, central, and southern Europe, as well as from western and eastern Europe. The sites are located in the UK (London and Harwell), Denmark (greater Copenhagen area), Germany (greater Leipzig area), Finland (Helsinki and Hyytiälä), Spain (Barcelona and Montseny – a site in a mountainous area), and Greece (Athens and Finokalia). Unfortunately, not all sites had available data for all the variables studied, which to an extent may bias some of the results. An extended analysis of the typical and NPF event conditions, seasonal variations, and trends at these sites for the same period is found in other studies (Bousiotis et al., 2019, 2020). A list of the available data and a brief description for each site are found in Table 1 (for ease of reading the sites are named by the country of the site followed by the last two letters, which refer to the type of site: RU is for rural–regional background, UB is for urban background, and RO is for roadside site), while a map of the sites is found in Fig. 1. For all the sites, the data used in the present study are of either 1 h resolution or less. Data with coarser resolution were omitted for reliability.
Map of the sites in the present study.
Most of the data used in this analysis were also published in previous
studies. The data from the UK were published in Bousiotis et al. (2019,
2020), while some were also published in Beddows et al. (2015), Beddows and Harrison (2019). The data for the German sites and some of the data from the UK, Denmark,
and Finland were also published in von Bismarck et al. (2013, 2014, 2015).
Some of the measurements for the Spanish sites were used in Carnerero et
al. (2019) and Brines et al. (2015). The data for the Greek rural
background site were published in Kalivitis et al. (2019). Finally, the
data for the Greek urban background site were extracted from the European
database (EBAS –
NPF events were selected using the method proposed by Dal Maso et al. (2005).
An NPF event is identified by the appearance of a new mode or particles in
the nucleation mode (smaller than 20 nm in diameter), which prevails for
some hours and shows signs of growth. The events can then be classified into
classes I and II according to the level of certainty, while class I events
can be further classified to Ia and Ib. Events having both a clear formation
of a new mode of particles in the smallest size bins available (thus
excluding possible advected events) and a distinct and persistent
growth of the new mode of particles for at least 3 h were classified as
Ia, while Ib consists of rather clear events that fail by at least
one of the criteria set. Additionally, for the roadside sites, a formation
of particles in the nucleation mode accompanied by a significant increase in
the concentrations of pollutants was not considered an NPF event, as it
may be associated with mechanisms other than secondary formation. In the
present study, only events of class Ia were considered, with the
additional criterion of at least 1 nm h
The condensation sink (CS) is calculated according to the method proposed by
Kulmala et al. (2001) as
Growth rate (GR) is calculated as (Kulmala et al., 2012)
The formation rate
The NPF frequency was calculated as the number of NPF event days divided by
the number of days with available data in the given group (full dataset or
temporal and variable ranges, etc.). The results presented in this study were
normalised according to the data availability as
Due to the large datasets available and the large spread of the values, a
direct comparison between a given variable and any of the characteristics
associated with NPF events (NPF frequency, growth rate, and formation rate)
always provided results with low statistical significance. As a result, an
alternative method which can provide a reliable result without the
dispersion of the large datasets was used in the present study to
investigate the relationships between the variables considered to
be associated with NPF events. For this, a timeframe which is more
directly associated with the NPF events typically observed at the
mid-latitudes was chosen. For NPF frequency and GR the timeframe between
05:00 and 17:00 local time (LT) was chosen, which is considered the time when
the vast majority of NPF events take place and further develop with the
growth of the particles. For the formation rate a smaller timeframe was
chosen of 09:00 to 15:00 LT, which is
For the CS the timeframe 05:00 to 10:00 LT was chosen. This was done to avoid including the direct effect of the NPF events (the contribution of newly formed particles to CS) and to provide results for conditions which either promote or suppress the characteristics studied, which specifically for the CS are more important before the start of the events. The extreme values (very high or very low) which bias the results only carrying a very small piece (forming bins of very small size) of information were then removed, though 90 % of the available data was used for all the variables. The remaining data were separated into smaller bins, and a minimum of 10 bins was required for each variable (for example, if the difference between the minimum and the maximum RH is 70 %, then 14 bins each with a range of 5 % were formed). The variables of interest were then averaged for each bin and plotted, and a linear relation was considered for each one of them. While it is evident that not all relationships are linear, the specific type was chosen in the present analysis for all the variables studied. This was done because the aim was to elucidate the general positive or negative effect of the variables studied. Furthermore, the effect of many variables appears to vary between sites with large differences (either geographical or type of land use), and the choice of a single method to describe these relationships ensures the uniformity of the results, as it appears to better describe them in most cases.
The gradient of these linear relations (
In this study NPF events are generally observed as particles grow from a
smaller size (typically 3–16 nm depending on the size detection limit of
instruments used) to 30 nm or larger. They therefore reflect the result both
of nucleation, which creates new particles of 1–2 nm (not detected with the
instruments used in this study), and growth to larger sizes. In analysing
NPF events, we therefore consider three diagnostic features.
The first is the frequency of events occurring (i.e. days with an event divided by total
days with relevant data, depending on the variable and range studied). As
only class Ia events were considered, the frequency
of the events calculated should be lower than the expected one if all types
of events were included. This could result in values up to one-third of
those anticipated if all types of events were considered. For the extent of
this variation please refer to Bousiotis et al. (2019, 2020) in which there
is an extended analysis of the NPF events for each site, including the
special cases of NPF events that do not comply with the criteria set for
class Ia. The second is the rate of particle formation at a given size ( The third is the growth rate of particles from the lower measurement limit to 30 nm (or
50 nm for the UK sites), which was found to be greater during summer months
for most of the sites also studied in the aforementioned works.
From the analysis of the extended dataset a total of 1952 NPF events were
extracted and studied. The NPF frequency, growth, and formation rate for each
site is found in Table 2. The seasonal variation of NPF events is found in
Fig. S14.
Frequency (and number), growth, and formation rate of class Ia NPF events.
The gradients, coefficients of determination (
Normalised gradients (non-normalised for growth rate),
Continued.
Continued.
* Global solar irradiation measurements in kJ m
As mentioned earlier, solar radiation intensity is considered to be one of
the most important variables in NPF occurrence, as it contributes to the
production of H
The relationship of solar radiation with the growth rate was weaker in all
cases and did not present a clear trend. Only some rural background sites
(GERRU, FINRU, and GRERU) presented a strong correlation (
Relation of average downward incoming solar radiation
(
Plotting the normalised gradients for NPF event frequency
Normalised gradients
Relative humidity is considered to have a negative effect on the occurrence
of NPF events (Jeong et al., 2010; Hamed et al., 2011; Park et al., 2015;
Dada et al., 2017; Li et al., 2019). While water in the atmosphere is one of
the main compounds needed for the formation of the initial clusters either
on the binary or ternary nucleation theory (Henschel et al., 2016; Korhonen
et al., 1999; Mirabel and Katz, 1974), under atmospheric conditions it may
also play a negative role in suppressing the number concentrations of new
particles by increasing aerosol surface area (Li et al., 2019). Consistent
with this, a negative relationship of the RH with NPF frequency was found
for all the sites in this study, with very high
Relationship of average relative humidity and normalised gradients
The normalised gradients once again provide some additional information.
Regarding the NPF frequency, it is found that the
Temperature can have both a direct and indirect effect on the development of
NPF events, as it is directly associated with the abundance of both biogenic
and anthropogenic volatile carbon, which is an important group of compounds
whose oxidation products can participate in nucleation itself (Lehtipalo et
al., 2018; Rose et al., 2018) and in the growth of newly formed
particles. It may also have a negative effect on particle size
distributions or number concentrations through other processes such as
particle evaporation. Most of the sites in the present study presented a
strong relationship of NPF frequency with temperature, which in most cases
was positive, though in many cases (such as the Danish, Finnish, and Spanish
sites – Fig. S2b, d, and e) there seems to be a peak in the NPF frequency
at some temperature, after which a decline starts (though being at the
higher end, it does not greatly affect the results). Sites with smaller
Growth rate had a more uniform trend, with almost all sites having a
positive relationship with temperature (apart from GERRO, though with
Relationship of average temperature and normalised gradients
The normalised gradients for this variable did not present a clear trend
among the areas studied, other than presenting greater
Normalised gradients
Wind speed may have both a positive and a negative effect on the occurrence
of NPF events. On one hand, it may promote NPF events through the increased
mixing of condensable compounds in the atmosphere and by reducing
the CS. On the other hand, high wind speeds may suppress NPF events due to
increased dilution. It should be considered that the variability found is
also affected by the specific conditions found at each site. The wind speed
measurements in many cases, especially at urban sites, can be biased by the
local topography or specific conditions found at each site, thus
representing the local conditions for this variable rather than the regional
ones. Similarly, measurements of wind speed at well-sited meteorological
stations may be more representative of regional conditions than of those
affecting the sites of nucleation measurement. The sites in this study
presented mixed results for both the importance and the effect of the
wind speed variability. Three different behaviours were found in the
variation of NPF event frequency and wind speed, which appear to be
associated with local conditions as they are almost uniformly found among
the sites within close proximity. Some sites presented a steady increase in
NPF event frequency with wind speed (Danish sites – UKUB, FINRU, SPAUB, and
GRERU), while others were found to steadily decline with increasing wind
speeds (German sites – it should be noted that the German sites are the
only ones that are located at a great distance from the sea), and some
were found to reach a peak and then decline, which also leads to smaller
Similarly, the effect of different wind speeds upon the growth rate also
varied a lot, though it was found to be negative in all the cases in which
The normalised gradients did not have any notable relationship with either the NPF frequency or the formation rate, further confirming that the effect of the different wind speeds is not due to its variability only, but it is also influenced by the characteristics of the incoming air masses and specific local conditions found at each site.
At almost all the sites with available data (apart from the Spanish), the
NPF frequency presented a positive relationship with high significance at
all types of sites. The greater significance found at the rural sites (apart
from SPARU) indicates the increased importance of meteorological conditions
for the occurrence of NPF events at this type of site. The growth rate also
presented a similar picture, with positive relationships at all the
background sites in this study except the ones in Greece (
The normalised gradients did not present any clear trends, even for the NPF frequency for which the results presented significant relationships at almost all sites.
The gradients,
Normalised gradients (non-normalised for growth rate),
Continued.
Sulfur dioxide, as a precursor of H
The normalised gradients
Relationship of average SO
NO
The normalised gradients do not provide a significant result for the
relationship of this variable with either the frequency of the events or the
formation rate. The only noteworthy point is the more negative
Ozone is typically the result of atmospheric photochemistry and is itself a
source of the hydroxyl radical through photolysis or ozonolysis of alkenes during both
daytime and night-time (Fenske et al., 2000). It might therefore be
expected to act as an indicator of photochemical activity, which promotes the
oxidation of SO
Unlike the solar radiation intensity, however, the growth rate presents a
negative relationship at the sites where the relationship between these two
variables was significant (UKRU, UKUB, DENUB, and FINRU), which might either
be an indication of a polluted background that may have a negative effect on
the growth of newly formed particles (though the trends found for
NO
Relationship of average O
As the correlations found were strong, the normalised gradients for NPF
frequency, when plotted against the average concentrations of O
Organic carbon (OC) compounds in secondary aerosol typically enter
particles via condensational processes, with a role that becomes
increasingly important as the size of the particles becomes larger (Nieminen
et al., 2010; Zhang et al., 2012; Shrivastava et al., 2017). Particulate OC,
data for which are available in the present study, can be associated with
pollution, especially in the urban environment. Only a few of the sites in
the present study were found to have a relatively strong negative
relationship (
The normalised gradients for this variable did not present any noteworthy relationships with either the type of site or the concentrations of OC at a given site.
Many volatile organic compounds have been found to be associated with the
NPF process. Benzene, toluene, ethylbenzene,
Volatile organic compound data were available for three of the sites in this
study (Table S2). Two of the sites with VOC data were from the rural
background and the roadside site in the UK. Most of the compounds are
associated with combustion sources and were found to have a negative
relationship with NPF event occurrence at both sites, with high
Growth rate was found to have a positive relationship with VOCs in almost
all cases for both UK sites. A few exceptions were found (with only 1,3
butadiene having a relatively high
As Hyytiälä (FINRU) is a rural background site far from the direct
effect of combustion emissions, different VOCs were measured, which mainly
originate from biogenic sources rather than anthropogenic ones. The results
were mixed and less clear compared to those from the UK sites (mainly due to
the smaller dataset), and three groups were found depending on their
relationship with NPF frequency. The first group, including acetonitrile,
acetic acid, and methyl ethyl ketone (MEK), presented a slight positive
relationship. The second group presented a negative relationship, with the
VOCs in this group being monoterpenes, methacrolein, benzene, isoprene, and
toluene (only the last two have
Sulfate (SO
The normalised gradients cannot be used for any analysis of sulfate as the measurements available are from different particle size ranges.
Ammonia (NH
The CS is a measure of the rate at which molecules will condense onto pre-existing aerosols (Lehtinen et al., 2003). It is highly dependent on the number and size of the particles in the atmosphere, and as a result it is expected to be affected by both local emissions within the urban environment and the formation and growth of particles due to NPF events. As a result, for the specific metric a timeframe before the events are in full development was chosen (05:00 to 10:00 LT) to avoid including the effect of the NPF events and provide a picture of the atmospheric conditions that preceded the NPF events. With these data, the NPF frequency presented very strong relationships with the condensation sink. Two groups of sites were found: those which had a positive relationship and those with a negative relationship. In the first group are the sites in Germany and Greece, while all others had a negative relationship. This grouping follows the trend between the countries, the sites of which presented a greater or smaller CS on NPF event days according to the findings in Bousiotis et al. (2019, 2020) (having positive or negative gradients, respectively), though it is unknown what causes this behaviour (at the German sites and GREUB it may be associated with the very high formation rates on NPF event days). While the gradients from this analysis cannot be used for direct comparisons, a trend was found for which the gradients were more positive or negative at the rural sites compared to their respective roadside sites, which might indicate the greater importance of the variability of the CS at the rural sites for the occurrence of NPF events.
The growth rate was positively correlated with the CS for most of the sites,
with relatively strong relationships (
The normalised gradients
The Pearson correlation coefficients for the variables studied at each site
are found in Table S1. The relatively strong relationship between the solar
radiation intensity, temperature, and O
The findings of our study with respect to the background sites show many
similarities to the conclusions drawn in a previous multi-station study
in Europe by Dall'Osto et al. (2018) despite the two studies using several
different sampling stations in addition to some in common. Both studies point
towards the influence of variables such as solar radiation intensity and CS
upon the occurrence of NPF events. The previous study suggested that
different compounds participate in the growth of particles depending on
the area considered. Thus, for northern and southern sites the growth of the
particles is suggested to be driven mainly by organic compounds, while for
the sites in central Europe sulfate plays a more important role. These
findings are confirmed by the present study, as the growth rate was found to
correlate better with organic compounds for the rural sites in Finland and
Greece, while SO
The seasonality of NPF events at northern sites was hard to explain in the
previous study, and the possible effect of low temperature was considered.
In the present study, the Finnish background sites presented a double-peak
relationship of NPF frequency with temperature, with one of the peaks being
below 0
The present study attempts to explain the effect of several meteorological and atmospheric variables on the occurrence and development of NPF events by using a large-scale dataset. More than 85 site years of data from 16 sites from six countries in Europe were analysed for NPF events. A total of 1952 NPF events with consequent growth of newly formed particles were extracted, and with the use of binned linear regression, the relationship between three variables associated with NPF events (NPF event frequency, formation, and growth rate) and meteorological conditions as well as atmospheric composition was studied. Among the meteorological conditions, solar radiation intensity, temperature, and atmospheric pressure presented a positive relationship with the occurrence of NPF events at the majority of the sites (though exceptions were found as well, mostly in the southern sites), either promoting the formation or growth rate. RH presented a negative relationship with NPF event frequency, which in most cases was associated with it being a limiting factor on particle formation at higher average values. Wind speed, on the other hand, presented variable results, appearing to depend on the location of the sites rather than their type. This shows that while wind speed can be a factor in NPF event occurrence, the origin of the incoming air masses also plays a very important role. In most cases, meteorological conditions, such as temperature or RH, appeared to be more important factors in NPF event occurrence at rural sites compared to urban sites, suggesting that NPF events are driven more by them at this type of site compared to urban environments and the more complex chemical interactions found there. Additionally, while some meteorological variables appeared to play a crucial role in the occurrence of NPF events, this role appears to become less important at higher values when a positive relation is found (or lower when a negative relation is found).
The results for the levels of atmospheric pollutants presented a more
interesting picture, as most of these, which appear to be either directly or
indirectly associated with the NPF process, were found to have negative
relationships with NPF frequency. This is probably due to the fact that
increased concentrations of such compounds are associated with more polluted
conditions, which are a limiting factor in the occurrence of NPF events, as
was found with the negative relationship between the CS and NPF frequency in
most cases. Thus, SO
It should be noted that the variables considered are in many cases inter-related (e.g. temperature and RH), and this considerably complicates the interpretation in terms of causal factors. Large datasets are very useful in providing more uniform results by removing the possible bias of short period extremities, which may lead to wrong assumptions. This study, apart from providing insights into the effect of a number of variables on the occurrence and development of NPF events in atmospheric conditions across Europe, also shows the differences that climatic, land use, and atmospheric composition variations cause in those effects. Such variations are probably the cause of the differences found among previous studies. Following from this, the importance of a high-resolution measurement network, both spatially and temporally, is underlined, as it can help in elucidating the mechanisms of new particle formation in the real atmosphere.
Data supporting this publication are openly available from the UBIRA eData
repository at
The supplement related to this article is available online at:
The study was conceived and planned by RMH, who also contributed to the final paper, and DB, who also carried out the analysis and prepared the first draft of the paper. AM, JKN, CN, JVN, HP, NP, AA, GK, SV, and KE provided the data for the analysis. JB provided help with analysis of the data. FP provided advice on the analysis. MD'O, XQ, and TP contributed to the final paper.
The authors declare that they have no conflict of interest.
This work was supported by the National Centre for Atmospheric Science funded by the UK Natural Environment Research Council (R8/H12/83/011).
This research has been supported by the Natural Environment Research Council (grant no. R8/H12/83/011).
This paper was edited by Gordon McFiggans and reviewed by two anonymous referees.