New particle formation (NPF) events have different patterns of development
depending on the conditions of the area in which they occur. In this study,
particle size distributions in the range of 16.6–604 nm (7 years of
data) were analysed and NPF events occurring at three sites of differing
characteristics – rural Harwell (HAR), urban background North Kensington
(NK), urban roadside Marylebone Road (MR), London, UK – were extracted and
studied. The different atmospheric conditions in each study area not only
have an effect on the frequency of the events, but also affect their
development. The frequency of NPF events is similar at the rural and urban
background locations (about 7 % of days), with a high proportion of events
occurring at both sites on the same day (45 %). The frequency of NPF
events at the urban roadside site is slightly less (6 % of days), and
higher particle growth rates (average 5.5 nm h-1 at MR compared to 3.4
and 4.2 nm h-1 at HAR and NK respectively) must result from
rapid gas-to-particle conversion of traffic-generated pollutants. A general
pattern is found in which the condensation sink increases with the degree of
pollution of the site, but this is counteracted by increased particle growth
rates at the more polluted location. A key finding of this study is that the
role of the urban environment leads to an increment of 20 % in
N16–20 nm in the urban background compared to that of the rural area in
NPF events occurring at both sites. The relationship of the origin of
incoming air masses is also considered and an association of regional events
with cleaner air masses is found. Due to lower availability of condensable
species, NPF events that are associated with cleaner atmospheric conditions
have lower growth rates of the newly formed particles. The decisive effect
of the condensation sink in the development of NPF events and the
survivability of the newly formed particles is underlined, and influences
the overall contribution of NPF events to the number of ultrafine particles
in an area. The other key factor identified by this study is the important
role that pollution, both from traffic and other sources in the urban
environment (such as heating or cooking), plays in new particle formation
events.
Introduction
Ultrafine particles (particles with diameter smaller than 100 nm) typically
make the greatest contribution in the total particle count, especially in
urban environments (Németh et al., 2018), but a very small contribution
to total volume and mass (Harrison et al., 2000). Research studies have
indicated that ultrafine particles can cause pulmonary inflammation, may
contribute to cardiovascular disease (Oberdörster, 2000) and have
an increased possibility to penetrate the brain and central nervous system
(Politis et al., 2008) compared to fine and coarser particles. Since some
studies report that toxicity per unit mass increases as particle size
decreases (Penttinen et al., 2001; MacNee et al., 2003; Davidson et al.,
2005), it is considered possible that particle number concentrations may be a
better predictor of health effects than mass concentrations (Harrison et al.,
2000; Atkinson et al., 2010; Kelly et al., 2012; Samoli et al., 2016).
Additionally, new particle formation (NPF) events have an impact on climate (Makkonen et al., 2012)
either by increasing the number of cloud condensation nuclei (Spracklen et
al., 2008; Merikanto et al., 2009; Dameto de España et al., 2017;
Kalkavouras et al., 2017) or directly affecting the optical properties of
the atmosphere (Seinfeld and Pandis, 2012).
The sources of ultrafine particles in urban areas can either be primary
particles from emission sources such as traffic (Shi et al., 1999; Harrison et
al., 2000), airports (Masiol et al., 2017) and other combustion-related
processes (Keuken et al., 2015; Kecorius et al., 2016), or by NPF from gaseous precursors. New particle formation as described
by Kulmala et al. (2014) is the process of production of low-volatility
vapours, clustering of these vapours, nucleation, activation of the clusters
with a second group of vapours and condensational growth to larger sizes.
This process can occur both locally or on a larger scale; in the latter case
the events are characterized as regional. Regional events have been found to
take place on a scale of hundreds of kilometres (Németh and Salma, 2014;
Shen et al., 2018), without being affected by air mass advection (Salma et
al., 2016). NPF is one of the main contributors of particles in the
atmosphere (Spracklen et al., 2010; Kulmala et al., 2016; Rahman et al.,
2017) and this relative contribution increases when moving from a kerbside to a
rural area (Ma and Birmili, 2015). While NPF events in rural and remote areas
have been widely studied for many years (O'Dowd et al., 2002; Dal Maso et
al., 2005; Ehn et al., 2010; Dall'Osto et al., 2017; Kalkavouras et al.,
2017), in urban areas intensive studies have started mainly in recent years
(Jeong et al., 2010; Minguillón et al., 2015; Peng et al., 2017;
Németh et al., 2018). Early studies in Birmingham, UK, highlighted the
connection of NPF events with solar radiation (Shi et al., 2001) and a low
condensation sink (Alam et al., 2003), a measure of pre-existing aerosol
loading (Dal Maso et al., 2002). The importance of a low condensation sink
was further underlined by later studies as being one of the most influential
variables in the occurrence of NPF in all types of environment (Wehner et
al., 2007; Park et al., 2015; Pikridas et al., 2015). An important
contributor to many NPF pathways is SO2 (Woo et al., 2001; Berndt et
al., 2006; Laaksonen et al., 2008), which in the presence of solar radiation
forms H2SO4, often the main component of the initial clusters
(Kuang et al., 2008; Kulmala et al., 2013; Bianchi et al., 2016; Kirkby et
al., 2016). Dall'Osto et al. (2013) pointed out that the role of SO2 is
less significant in urban areas compared to rural and background areas.
SO2 concentration variability in urban areas was found to have a small
impact on the frequency of NPF events (Alam et al., 2003; Jeong et al.,
2010), though it can have an effect on the number of particles formed
(Charron et al., 2007). Furthermore, Dall'Osto et al. (2018) in their
research at 24 sites in Europe pointed out the different role SO2 seems
to play depending on its concentration, and that of other species. Jayaratne
et al. (2017), however, found that in the heavily polluted environment of
Beijing, China, NPF events were more probable in sulfur-rich conditions
rather than sulfur-poor conditions. Apart from its role in the initial formation of the
clusters, H2SO4 seems to participate in the early stages of growth
of the newly formed clusters (Kulmala et al., 2005; Iida et al., 2008; Xiao
et al., 2015). In later stages of growth, low or extremely low volatility
organic compounds (O'Dowd et al., 2002; Laaksonen et al., 2008; Metzger et
al., 2010; Kulmala et al., 2013; Tröstl et al., 2016; Dall'Osto et al.,
2018) appear to be more important, while the role of ammonium nitrate in
particle growth is also considered (Zhang et al., 2017). While in rural areas
the organic compounds are mainly of biogenic origin (Riccobono et al., 2014;
Kirkby et al., 2016), in urban areas they mainly originate from combustion
processes (Robinson et al., 2007; Gentner et al., 2012). Many comparative
studies have reported higher growth rates in urban areas compared to
background sites (Wehner et al., 2007; Jeong et al., 2010; Salma, et al.,
2016; Wang et al., 2017), as well as greater particle formation rates (Salma,
et al., 2016; Nieminen et al., 2018) and a higher frequency of NPF events
(Peng et al., 2017), which was attributed to the higher concentration of
condensable species. Salma et al. (2014), however, reported fewer NPF events in
the city centre of Budapest compared to the urban background, due to the
higher condensation sink. Due to the complexity of the conditions and
mechanisms within an urban area (Harrison, 2017), NPF events are harder to
study and factors that drive them are harder to attribute. Increased concentrations of particles in
the size range 1.3–3 nm were measured at a kerbside site when downwind
from the road, following the trends in traffic-related nucleation mode
particles, associating them with traffic emissions and thus not resulting
from homogeneous nucleation mechanisms (Rönkkö et al., 2017; Hietikko
et al., 2018); studies in Barcelona, Spain (Dall'Osto et al., 2012;
Brines et al., 2014), and Leicester, UK (Hama et al., 2017), attributed a
larger portion of nucleation mode particles to vehicular emissions compared
to photochemically induced nucleation. As the condensation sink is higher
within an urban environment, NPF events are less favoured. Their occurrence
is attributed to either ineffective scavenging or the higher growth rate of
the newly formed particles (Kulmala et al., 2017). The latter may occur when sufficient
concentrations of precursors are present in the atmosphere (Fiedler et al.,
2005), as particle formation was found to take place on both event and
non-event days with variable intensity. Particle formation though is not always followed by
survival or growth of the newly formed particles and in such cases does not qualify as a NPF
event (Riipinen et al., 2007).
In this study, NPF events in three areas of different land use in the
southern UK are analysed. Studies for NPF events have been conducted in the
past for Harwell, Oxfordshire (Charron et al., 2007, 2008), and the effect of
NPF upon particle size distributions was also considered for North Kensington,
London (Beddows et al., 2015). A combined study including all three sites has
also been conducted, but in the aspect of ultrafine particle variation (Von
Bismarck-Osten et al., 2013). The present study is the first to use a
combined long-term database for all three sites, focusing on the trends and
conditions of NPF events at these sites, as well as the first which
identifies NPF events at the high-traffic Marylebone Road site, as up to
this point ultrafine particles were attributed only to traffic (Charron and
Harrison, 2003; Dall'Osto et al., 2011). As in this study a rural and an
urban background area are studied alongside a kerbside site in the city of
London in close proximity, the conditions and development of NPF events in a
mid-latitude European region are discussed in relation to the influence of
different local environments.
Map of the measuring stations.
Data and methodsSite description and data availability
This study analysed NPF events in three areas in the southern United Kingdom
(Fig. 1). Harwell in Oxfordshire is located about 80 km west of the greater
London area. The site is in the grounds of the Harwell Science Centre in
Oxfordshire (51∘34′15′′ N, 1∘19′31′′ W) and is
representative of a rural background area; a detailed description of the site
was given by Charron et al. (2013). North Kensington is a suburban area in
the western side of London, UK, 4.5 km west of Marylebone Road. The site is
located in the grounds of Sion Manning School (51∘31′15′′ N,
0∘12′48′′ W) and is representative of the urban background of
London. A detailed description of the site was given by Bigi and Harrison
(2010). Marylebone Road is located in the centre of London, UK. The site is
located on the kerbside of Marylebone Road (51∘31′21′′ N;
0∘9′16′′ W), a very busy arterial route within a street canyon. A
more detailed description of the area can be found in Charron and Harrison
(2003).
At all three sites, 7 years (2009–2015) of particle number size
distributions in the range of 16.6–604 nm have been measured and recorded
as 15 min averages, using a scanning mobility particle sizer (SMPS),
comprised by an electrostatic classifier (EC, TSI model 3080) and a
condensation particle counter (CPC, TSI Model 3775), operated on behalf of
the UK Department for Environment, Food and Rural Affairs (DEFRA). At
all sites the inlet air is dried, and the operation is in accordance with the
EUSAAR/ACTRIS protocol (Wiedensohler et al., 2012). These 15 min
measurements were averaged to an hourly resolution. In Harwell there were
46 930 h of available SMPS data (76.5 % coverage), in North Kensington
51 059 (83.3 % coverage) and at Marylebone Road 45 562 (74.3 % coverage).
Detailed data availability is found in Table S1 in the Supplement. A free-standing CPC (TSI
model 3022A) was also operated alongside the EC for most of the years of the survey and
was used to give an estimate of particles in the 7–16.6 nm range by
difference from the SMPS.
Additionally, air pollutants and other gas and particle chemical composition
data (NOx, SO2, SO42-, Cl, Na, Mg, gaseous ammonia and
volatile organic compounds – VOCs) were extracted from the DEFRA website
(https://uk-air.defra.gov.uk/, last access: 14 May 2018); daily measurements of particulate
organic carbon (OC) were also extracted from the DEFRA website which are
determined using the method described in the Annual report of the National
Physical Laboratory (Beccaceci et al., 2015). Meteorological data for Harwell
and Heathrow airport (used for North Kensington and Marylebone Road) were
available from the Met Office, while solar radiation data from Benson station
(for Harwell) and Heathrow airport (for North Kensington and Marylebone Road)
were extracted from the Centre for Environmental Data Analysis (CEDA) site
(http://www.ceda.ac.uk, last access: 16 May 2017). Back trajectory data calculated using the
HYSPLIT model (Draxler and Hess, 1998) were extracted by the NOAA Air
Resources Laboratory (https://ready.arl.noaa.gov/READYtransp.php, last access: 8 December 2017) and
were processed using the Openair package for R (Carslaw and Ropkins, 2012).
MethodsNPF events selection
The identification of the NPF event days was made by visual inspection of
SMPS data, supplemented with the use of CPC data to confirm the formation of
a new mode of particles, using the criteria set by Dal Maso et al. (2005).
NPF events are considered when a distinctly new mode of particles, which
appears in the size distribution at nucleation mode size, prevails for some
hours and shows signs of growth. Using these criteria, NPF events are
classified into two classes, I and II depending on the level of certainty.
Class I events are further classified to Ia and Ib, with class Ia containing
very clear and strong particle formation events, while Ib contains less clear
events. In this study only the events of class Ia are considered to be the
most suitable for analysing case studies of NPF events (Fig. S1 in the Supplement). At this
point it should be mentioned that due to the particle size range available,
NPF events in which newly formed particles failed to grow beyond 16.6 nm (if
any) could not be identified. Bursts of new particles in the size range
<16.6 nm that were identified using the CPC data but did not appear
in the SMPS dataset were ignored as their development was unknown. This type
of development was rare and mainly found at the rural background site,
occurring on a few days per year mainly in summer. Its main feature was the
short duration of the bursts compared to event days. In the urban sites, this
type of development was almost non-existent. High time resolution data for
gaseous pollutants and aerosol constituents were used to identify pollution
events affecting particle concentrations and these were removed from the data
analysis. This analysis took account of the fact that nanoparticle emissions
from Heathrow Airport affect size distributions at London sites (Harrison et
al., 2018), and such primary emission influences were not included as NPF
events.
Calculation of the condensation sink and growth rate
For the calculation of the condensation sink the method proposed in Kulmala
et al. (2001) was used in which the condensation sink is calculated as
CS=4πD∑βMrN,
where r is the radius of the particles and N is the number concentration of
the particles. D is the diffusion coefficient calculated (for T=293 K
and P=1013.25 mbar) according to Kuuluvainen et al. (2010):
Dvap=0.00143⋅T1.75Mair-1+Mvap-1PDx,air13+Dx,vap132,
where P is air pressure, M is the molar mass and Dx is the diffusion
volume for air and H2SO4. βM is the Fuchs correction
factor calculated as follows (Fuchs et al., 1971):
βM=1+Kn1+43a+0.377Kn+43aKn2,
where Kn is the relation of the particle diameter and the mean free
path of the gas λm, called the Knudsen number.
The growth rate of the particles on nucleation event days was also calculated
as proposed by Kulmala et al. (2012), using the formula
GR=DP2-DP1t2-t1
for the size range 16.6–50 nm. The number of points taken depended on the
development of the event and were considered from the start of the event
until (a) growth stopped, (b) GMD reached 50 nm or (c) the day ended (this
cut-off was chosen as the development of an event in its later stages is
heavily biased by the local conditions, especially at the urban sites).
Calculation of the urban increment (UI)
The urban increment (UI) is defined as the ratio of the number concentration of
particles below 20 nm for event days to the average (for the period April–October,
when the majority of the events take place) for North Kensington to
that at Harwell. This provides a measure of the new particles formed in
each area in comparison to the average conditions, and is calculated by
UI=NKNucMax-NKAvgHWNucMax-HWAvg,
where NKNucMax is the maximum concentration of particles below 20 nm
found in the diurnal cycle on event days (found at 13:00 GMT) and NKAvg is the average
mean concentration at the same time (same for Harwell in
the denominator).
Calculation of nucleation strength factor (NSF) and the <inline-formula><mml:math id="M51" display="inline"><mml:mi>P</mml:mi></mml:math></inline-formula> parameter
The nucleation strength factor (NSF) was proposed by Salma et al. (2014) as
a measure of the effect nucleation events have in the composition of
ultrafine particles in an area. Two factors were proposed. First is the
NSFNUC. This is calculated as
NSFNUC=Nsmallestsizeavailable-100N100-largestsizeavailablenucleationdaysNsmallestsizeavailable-100N100-largestsizeavailablenon-nucleationdays
and provides of a measure of the concentration increment on nucleation days
exclusively caused by NPF. The second factor is
NSFGEN calculated as
NSFGEN=Nsmallestsizeavailable-100N100-largestsizeavailablealldaysNsmallestsizeavailable-100N100-largestsizeavailablenon-nucleationdays
and gives a measure of the overall contribution of NPF over a longer time span (Salma et al., 2017).
The dimensionless survival parameter P, as proposed in Kulmala et al. (2017),
was calculated as
P=CS′GR′,
where CS′= CS / (10-4 s-1) and GR′= GR / (1 nm h-1). CS
and GR values used were calculated with the methods mentioned at Sect. 2.2.2. An
increased P parameter is an indication that a smaller percentage of newly
formed particles will survive to greater sizes. Hence this is the inverse of
particle survivability, and values of P<50 are typically required
for NPF in clean or moderately polluted environments, although higher values
of P are observed in highly polluted atmospheres (Kulmala et al., 2017).
Number of NPF events per season for all 7 years of the present
study (winter – DJF; spring – MAM; summer – JJA; autumn – SON) at Harwell
(rural), North Kensington (urban background) and Marylebone Road (urban
roadside).
Number of NPF events per site (in parenthesis the number of days
with available data).
1 Refers to events occurring simultaneously at Harwell and
North Kensington. 2 Refers to events which occur simultaneously at all three
sites.
Results and discussionNPF events in the background areasConditions and trends of NPF events
The number of NPF event days for each site per year, those that took place
simultaneously at both urban and rural background sites, and those
events that took place at all three sites simultaneously appear in Table 1.
Given that overall data recovery was in the range of 74 %–83 %, results from
individual years are unreliable, but the 7-year runs should average out
most of the effects of incomplete data recovery. The number of events is
similar for Harwell and North Kensington, with a frequency of about 7 % of all
days with data. There is a clear seasonal variation favouring summer and
spring (Fig. 2) for both areas of the study. A similar pattern of variation
was found for North Kensington by Beddows et al. (2015). In general, higher
solar radiation, lower relative humidity, low cloud cover and higher pressure
conditions, lower concentrations of pollutants, and lower condensation
sinks are found when NPF events took place in all areas (Fig. S2), as was
also reported by Charron et al. (2007) for Harwell. While SO2 is one of
the main factors for NPF events to occur, concentrations are lower when
events take place. This is indicative that SO2 concentrations in these
areas are sufficient for events to take place, and higher concentrations are
likely to be associated with higher pollution and a higher condensation sink.
The proxy for [H2SO4] was calculated for the background sites using
the method outlined in Petäjä et al. (2009) and was found to be
higher on event days for both background sites (results not included). This
indicates the possible positive effect of increased concentrations of
H2SO4 in the occurrence of NPF events as well as, since SO2
concentrations were found lower, the increased role of either the solar
radiation (via the formation of OH radical) or the reduced condensation sink
to its formation. For the case of gaseous ammonia (results not included) for
Harwell where data were available, as there was no distinct variation found
between event and non-event days, but as the concentration of ammonia in the
UK is in the range of few parts per billion (Sutton et al., 1995), it is sufficient
according to ternary nucleation theory (Korhonen et al., 1999) for NPF events
not to be limited by ammonia. The average growth rate for Harwell was found
to be 3.4 nm h-1, within the range given by Charron et al. (2007), and
higher at North Kensington at 4.2 nm h-1, a trend found for all seasons
(Fig. 3). The increased growth rate in the urban area can be related to the
greater presence of organic matter and other condensable species. In both
areas NPF events had higher growth rates in summer than in spring, as was
also found in previous studies (Kulmala et al., 2004; Nieminen et al., 2018).
This may be associated with the higher concentration of organic compounds
emitted by trees during summer (Riipinen et al., 2007), or faster oxidation
rates due to higher concentrations of hydroxyl radical and ozone (Harrison et
al., 2006).
Growth rate per season at the three sites.
About 45 % of the events took place simultaneously in both background
areas. These events are characterized as regional, as NPF took place on a
larger scale, regardless of the local conditions of the given area. In this
case, meteorological conditions were even clearer, indicative of the greater
dependence of regional events on synoptic conditions rather than local. While
most chemical constituents were also lower in concentration during regional
events, different patterns were found for organic compounds and sulfate for
each background area. In Harwell sulfate was higher during regional events,
while in North Kensington organic compounds were higher during regional events.
This may be indicative of the variable role that specific chemical species
have in condensational nanoparticle growth (Yue et al., 2010). In all cases
though, the concentrations of these species were lower compared to the
average conditions. Despite these differences, the growth rate of particles
was found to be higher for local events in North Kensington (4.4 nm h-1)
compared to regional events (3.9 nm h-1), though within the margin of
uncertainty. In Harwell, no difference was found in the growth rate between
regional and local events.
Urban increment and particle development
The urban environment, depending on the conditions, may have a positive or
negative effect in the number of the particles formed and their consequent
survival and growth. Both Harwell and North Kensington are in background areas,
rural and urban respectively. As a result, while the concentrations of
pollutants are higher in North Kensington than Harwell, their effect is smaller
compared to that of Marylebone Road. A comparison of the particles smaller
than 20 nm gives insight into the formation and survival of the newly
formed particles in the early stages. Calculating the urban increment
(Eq. 5) using the two background sites showed around 20 % more
particles of size 16–20 nm in North Kensington than Harwell for event days,
an increment that is even stronger when solely local events are considered
(Fig. 4). As the sizes of the particles in the calculation are relatively
large and due to the higher condensation sink found in North Kensington, this
increment is expected to be larger for smaller size particles. A possible
explanation for this result may be the greater concentration of organic
compounds which is observed in North Kensington, as discussed earlier, which
leads to more rapid formation of secondary condensable species that enhances
the nucleation process in the more polluted area.
Diurnal variation of N16–20 nm at each site: annual mean and NPF
event days.
Considering the local events, most of the pollutant concentration data
available appear to be higher, which is reflected in the condensation sink as
well. The role of the polluted background appears to be decisive in the
further growth of the newly formed particles, especially for Harwell. At
both sites, this causes the number of particles of greater size to be smaller
for the later hours in the days of local events (Fig. S3). Another
possible reason for this difference in the larger size ranges could be the
higher concentration of organic content on the days of regional events at North Kensington (as discussed earlier). On the other hand, for Harwell all
hydrocarbons with available data are lower throughout the day (apart from
ethane) during regional events. Unlike North Kensington, at Harwell particles
smaller than 20 nm as well as the growth rate of the newly formed particles
are almost the same for regional and local events.
The calculation of the increment on Marylebone Road provided negative
results; particles smaller than 20 nm were less abundant on event days
compared to the average, throughout the day. This is due to the fact that
Marylebone Road is heavily affected by traffic pollution and, on average,
conditions do not promote NPF events due to the high condensation sink,
unless clear conditions prevail, which are also associated with a low
particle load.
NPF events at Marylebone Road
For many years, NPF events were thought not to take place in heavily
polluted urban areas, as the effect of the increased condensation sink was
considered crucial in suppressing the formation and growth of new particles.
Recent long-term analyses have shown this is not the case and nowadays an
increasing number of studies confirm the occurrence of NPF events in urban
areas. In this study, for the same period of 7 years as for the two
background areas, NPF events were found to occur for 6.1 % of days at
Marylebone Road, lower than in the background areas. Though due to the
particle size range available there cannot be a definitive answer to whether
the formation of the particles takes place in the specific locality of the
sampling site, due to the observed increase in particle concentrations in
the range 7–16 nm (provided by the CPC data) and the increased growth
rates found in urban areas in general, it can be assumed that the formation
takes place either in the area of the measuring site or in its close
vicinity, while the growth of the particles persists in the area for several
hours, despite the high condensation sink. Seasonal variation is similar to
that at the background sites, but day-of-the-week variation is stronger at
Marylebone Road, further favouring weekends (Fig. S4), as on these days
traffic intensity is lower.
In general, similar conditions found to affect NPF events at the background
sites are also found at Marylebone Road, despite a much larger condensation
sink (Fig. S2). As a result, less particles of size smaller than 20 nm
were found on NPF event days than the average for the site, as the sum of
background particles plus those formed on these days were less than that on
an average day. The growth rate of the newly formed particles (5.5 nm h-1) is higher than that of the background sites, which is in agreement
with the findings in the study of the background areas on the possible role
of the condensable species, the concentrations of which are even greater at
the urban kerbside. About 15 % of NPF event days at Marylebone Road
presented particle shrinkage after the initial growth; the study of these
cases though is outside of the context of the present work. At Marylebone
Road, the number of NPF days which were common with the background sites was
fewer, as local conditions (high condensation sink) are detrimental to the
occurrence of NPF events and thus the days of regional events including
Marylebone Road were separately studied for this site. There were 37 regional event days that
were common for all three sites (31 % of events at
Marylebone Road) (Table 1). As with the other two areas, the growth rate is
higher during local events, but the conditions are mixed, with lower
concentrations of sulfate and organic compounds but higher SO2, NOx
and elemental carbon. The relationship with higher wind speed (mainly
western; Fig. S6), solar radiation (which results in greater
H2SO4 formation) and lower relative humidity indicates the
stronger relation of the regional events with synoptic conditions than the
local events in the heavily polluted environment of Marylebone Road.
Connection of NPF events with incoming air massesAir mass back trajectory clustering and connection with NPF
events
The origin of the air masses plays a very important role in the occurrence
of NPF events. Air masses of different origins have different
characteristics. Back trajectories provide excellent insight into the source
of the air masses. Air mass back trajectories were calculated both for all
days and for NPF event days for each site separately. This analysis gives a
view of the frequency of NPF events within different air mass types. The
initial air mass back trajectory clustering ended up with an optimal
solution of nine clusters of different air masses. As many of these clusters
had similar characteristics and origin, solutions with fewer clusters were
attempted. As the number of clusters was decreasing clusters became a
mixture of different origins, thus making the distinction of different
sources harder. As a result, the method chosen was to merge clusters of
similar origin and characteristics, which kept the detail of the large
number of clusters and made the separation of the different origins more
distinct.
Map and frequency of incoming air mass origin – average and for NPF
events per site.
The resulting four merged clusters (Fig. 5), using the characterisation
proposed by McIntosh et al. (1969), are as follows:
An arctic cluster, which originates mainly from the northerly sector. It
occurs about 10 % of the time and consists of cold air masses, which
either passed over northern parts of the UK or through the Irish Sea.
A tropical cluster, which originates from the central Atlantic. It occurs
25 % of the time and contains warmer air masses. A small percentage of
this cluster contains masses that have passed over countries south of the
UK. Even though these days were more polluted, the clustering method was
unable to clearly distinguish these days as it does not take into account
particle numbers or composition, even when the nine-cluster solution was
applied.
A polar cluster, which originates from the north Atlantic. It is the most
common type of air mass arriving in the areas of study and occurs about
40 % of the time, bringing fast-moving, “clean” air masses with increased
marine components (Cl, Na, Mg) from the west. This cluster also contains
air masses that have passed through Ireland, though an effect on particle
size and chemical composition is not distinct.
A continental cluster, which originates from the east. It occurs about
25 % of the time and consists mainly of slow-moving air masses,
originating from the London area (for the background areas) and/or
continental Europe. It has higher concentrations of most pollutants as well
as the highest condensation sink.
The occurrence of each air mass class for average and event days for Harwell
and London (both sites) can also be found in Fig. 5, while their main
characteristics for each site can be found in Table S2. Though in this case
the air mass grouping for each site was done in a different analysis, the
resulting groups are almost identical in their characteristics and
frequency, as the sites are close to each other.
The polar cluster is the one prevailing on both average and event days. This
consists of clean fast-moving air masses originating mainly from middle and high
latitudes of the Atlantic, and this cluster presents favourable conditions
for NPF events. The association of NPF events with air masses from the
mid-Atlantic at North Kensington was also found by Beddows et al. (2015). Cool
Arctic air masses on average are not clean as they may have passed over the
northern UK. The event days associated with this air mass type have the
lowest concentrations of the pollutants within available data for all areas.
The increased percentage of events with this air mass at all sites indicates
that lower temperatures, in a clear atmosphere with sufficient solar
radiation, are favourable for NPF events as found in previous studies (Napari
et al., 2002; Jeong et al., 2010; Kirkby et al., 2011). A similar trend of
increased probability with polar and arctic maritime air masses was also
found for Hyytiälä, Finland, by Nilsson et al. (2001). Tropical air
masses have a lower probability for NPF events, which is associated with the
fact that a number of these days are associated with air masses which have
passed from continental areas south of the UK (France, Spain etc.).
Specifically for Marylebone Road the NPF probability is a lot lower (11 %
versus 17 % for North Kensington and 20 % for Harwell). This is due to the
fact that these air masses are more related to southerly winds which on
Marylebone Road are associated with a street canyon vortex which causes
higher pollutant concentrations at this site. Finally, the Continental
cluster presents the lowest probability for NPF events. The air masses in
this group originate from continental Europe and for the background areas in
most cases have passed over the London region as well. This results in both a
higher condensation sink and concentration of pollutants, which limits the
number of days with favourable conditions for NPF events. The growth rate for all
sites though appears to be higher for air masses originating from more
polluted areas (Fig. 6), which appear to enhance the growth process due to
containing a higher concentration of condensable species (after oxidation).
Variability of the origin of the air masses on NPF events
As both background sites are relatively close to each other (about 80 km)
and had a similar number of event days, a combined clustering of back
trajectories for the event days (only) in these two areas was attempted.
This would provide an insight into the origin of air masses for local and
regional events, as well as the conditions for these air masses. The data
for local North Kensington events and both local and regional events in Harwell
were clustered together and the results along with the characteristics of
the air mass clusters are found in Fig. S5.
Growth rate per incoming air mass origin at each of the sites.
Cluster C3, which is placed between C2 and C4 among those originating from
the Atlantic Ocean, has the highest percentage for both area-specific and
regional events. Specifically, for regional events the percentage is over
35 %, much higher compared to all others, showing a clear “preference” of
regional events for cleaner and faster-moving air masses from mid-latitudes
of the Atlantic Ocean. This “preference” explains the lower production and
growth rate of the new particles found for regional events, compared to local
ones, as for air masses from this area lower organic carbon and SO2
concentrations were found at both sites in this study. Cluster C5,
originating straight from the north but representing air masses that have
crossed the Irish Sea and have not extensively gone over land, presents a
similar case. These cold and clean air masses are associated with a low
growth rate and consequently low survivability of the newly formed particles.
Local events for both sites apart from those in Cluster C3 are highly
associated with Clusters C1 and C2. C1, which contains slow and polluted air
masses, presents the highest growth rate and as a result high particle
survivability, as given by the P parameter (see later). On the other hand,
C2,
which consists of warm and moist air masses from lower latitudes, is the least
common for regional events and presents a high growth rate and survival
probability of the particles. Apart from the weak relation found with
particulate organic carbon concentrations and growth rate (Fig. S5), there
appears to be an inverse relation between the temperature and survivability
of the particles. Warmer air masses seem to be related to higher particle
survival probability, which may be attributable to greater growth rates as
temperature increases (Yli-Juuti et al., 2011).
The NSF (Eqs. 6 and 7) is used to describe the effect nucleation events
have on the number of particles at a site. The values of NSF for each site
and for spring and summer are shown in Table 2. The decrease in the
contribution of NPF events to particle number, moving from the rural area to
the kerbside was also found in previous studies (Salma et al., 2014, 2017).
This is explained by the increased contribution to the particle number
concentrations of other sources, mainly combustion in the urban environment,
compared to rural areas. Apart from this trend, in the background areas the
increase in N16–100 was greater in spring than summer. This effect seems
stronger in the urban background area compared to the rural, as in that area
the variability of N16–100 is greater for event days compared to that of
the rural area. On the other hand, the contribution of NPF events in the
longer term, as is illustrated by the NSFGEN, appears to favour summer
for all areas, showing the increased formation and survivability of particles
in this season.
For Marylebone Road the result for the increase in the N16–100 is
greater in summer than in spring, in contrast to what was found for the
background sites. This is due to the fact that in summer the traffic
intensity is decreased, giving the contribution from NPF events a stronger
effect compared to the other sources. The very small increase found in NPF
events on Marylebone Road, with a factor of just 1.26, a lot lower than that
found in the urban area of Seoul, South Korea (Park et al., 2015), is
indicative of the reduced effect of NPF events in an area which is heavily
affected by traffic, as also pointed out by Von Bismarck-Osten et al. (2013)
in their study on particle composition on Marylebone Road.
Survival parameter P(a) per season, (b) for regional and local
events (for Marylebone Road regional is for all three sites), and (c) by incoming
air mass origin.
The survival parameter <inline-formula><mml:math id="M119" display="inline"><mml:mi>P</mml:mi></mml:math></inline-formula>
The average values of the P parameter for each of the areas of this study are
10.5 for Harwell, 15.8 for North Kensington and 28.9 for Marylebone Road. The
values found place Marylebone Road at the upper end of heavily polluted areas
in Europe and North Kensington at the same level as many other urban areas in
Europe, while Harwell somehow had higher values compared to other rural
background areas in Europe, as calculated by Kulmala et al. (2017). The
seasonal, air mass origin and local-versus-regional variations can be found
in Fig. 7 (winter is excluded due to very low number of events). While the
increasing trend of the P parameter as we move from rural background to
kerbside was expected, it can be seen that there is a clear seasonal pattern
in all three areas, with summer having the lowest P parameter (greatest
survivability) compared to the other two seasons. This is associated with the
higher growth rate found in summer for all areas of this study, as the
differences in the condensation sink on event days are negligible between
seasons. The case is similar for regional and local events. The result per
air mass origin is related to the different conditions and parameters of each
incoming air mass in each area. For example, the higher P parameter for
tropical air masses at Marylebone Road is associated with the higher
condensation sink found for this kind of air mass, due to the street canyon
effect which is specific for Marylebone Road for southerly wind directions
with which these air masses are mainly related, while the higher values for
the rather clean Arctic air masses for the other two areas are associated
with the lower growth rates found for this kind of air mass in these areas.
The more polluted continental air masses seem to have a different effect for
rural and urban areas. Their higher condensation sinks and concentrations of
pollutants have a negative effect on P values for the rural site and a
positive effect at the urban sites. The exact opposite is found for the
cleaner air masses of the Polar cluster, which appear to result in reduced
P values of the newly formed particles at the urban sites. This is related to
the lower condensation sink associated with this air mass type.
Conclusions
A total of 7 years of particle size distributions in the range 16.6–604 nm and
other meteorological and chemical composition data from three distinct areas
(regional background, urban background, kerbside) in the southern UK were
analysed and the conditions associated with NPF events were studied. NPF
events were found to occur on about 7 % of days at background sites and
less at the kerbside site. The conditions on event days for all three areas
were similar, with clear atmospheric conditions and a lower condensation
sink. While the condensation sink appears to be the most important factor
limiting NPF events at the kerbside site, SO2 was found to have smaller
concentrations on event days for all areas, which indicates that either on
average it is in sufficient concentrations for NPF events to occur, or that
other variables that participate in the production mechanism of
H2SO4 are more important. The growth rate of the newly formed
particles increases from the rural site to the kerbside and is greater in
summer compared to other seasons for all three sites. Almost half of the NPF
events at the rural and urban background sites were found to happen
simultaneously. In these cases, the atmospheric conditions were cleaner,
which resulted in slower growth rates. While most of the chemical species
available were at lower concentrations in regional events, a difference in
the behaviour with respect to sulfate and organic compounds was found
between the two background site types.
The prevailing origin of air masses in the southern UK is from middle and
high latitudes of the Atlantic Ocean. These fast-moving air masses present
an increased probability for NPF to occur. The case is similar for the
cooler and cleaner arctic air masses, while air masses from the tropics and
continental Europe, having greater pollutant content, have decreased NPF
probability, but a higher growth rate of particles when NPF events occurred.
Regional events appear to be more associated with cleaner air masses,
presenting a smaller growth rate and condensation sink compared to local
events. The difference in growth rate is probably related to the greater
content of condensable species; a positive relation of particle survival
probability with temperature was also found.
Comparing the background areas in this study, particles of 16–20 nm were
found to be about 20 % greater in concentration (above long-term average)
on NPF event days at the urban background site compared with the rural site.
This is associated with a higher abundance of condensable species in the
urban environment, which enhances the nucleation and growth process. This
effect though is limited as particle size increases and NPF events have a
greater effect on the overall N<100nm in the rural areas, compared
to urban areas, as calculated by the NSF. The effect becomes even smaller at the
kerbside as the number of background particles emitted by traffic is a lot
greater.
The occurrence of NPF events at the highly polluted Marylebone Road site is
at first sight surprising given the elevated condensation sink. This must be
counteracted by an abundance of condensable material, which is surprising
given the generally modest rate of atmospheric oxidation processes in
comparison to residence times in a street canyon (Harrison, 2017). However,
Giorio et al. (2015), using aerosol time-of-flight mass spectrometry,
reported rapid chemical processes within the Marylebone Road street canyon
leading to production of secondary particulate matter from road traffic
emissions. They postulated that this resulted from very local gas-to-particle conversion from vehicle-emitted pollutants. Condensation of such
reaction products upon pre-existing particles could explain the enhanced
particle growth rates observed at Marylebone Road (Fig. 3).
Finally, particle survival probability was found to decrease moving from
rural to urban areas. While formation and initial growth of new particles is
increased in urban areas, their survivability reduces as their size
increases. The probability of particles to survive to greater sizes was
found to be increased in summer for all areas, which is also explained by
the higher growth rate. The probability is also different depending upon the
origin of the air masses and is related to conditions specific for each
area.
In the present work, the effects of atmospheric conditions upon the NPF
process are studied. NPF is a complex process, highly affected by
meteorological conditions (local and synoptic), the chemical composition and the pre-existing conditions in an area. For this reason, the study
of NPF events in one area cannot provide safe assumptions for other areas,
as the mixture of conditions found in different places is unique and alters
the occurrence and development of NPF events. Thus, more studies on the
conditions and the trends in NPF events should be conducted to better
understand the effect of the numerous variables that affect those processes.
Data availability
Data supporting this publication are openly available from
the UBIRA eData repository at
https://doi.org/10.25500/edata.bham.00000307 (Bousiotis and Harrison, 2019).
The supplement related to this article is available online at: https://doi.org/10.5194/acp-19-5679-2019-supplement.
Author contributions
This study was conceived by MDO and RMH, who also contributed to the final
paper. The data analysis was carried out by DB with guidance from DCSB,
and DB also prepared the first draft of the manuscript. FDP provided advice
on the analysis.
Competing interests
The authors declare that they have no conflict of
interest.
Acknowledgements
The authors acknowledge financial support (to DCSB) from the Natural
Environment Research Council's funding of the National Centre for Atmospheric
Science (NCAS) (grant number R8/H12/83/011).
Review statement
This paper was edited by Tuukka Petäjä and reviewed by
two anonymous referees.
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