Introduction
In the last decade, with the threat of climate change, a growing number of
researchers have focused on understanding the association between aerosol
particles and the climate. Aerosol particles are either directly emitted
into the atmosphere (primary particles) or formed in the atmosphere
(secondary particles). Freshly formed secondary aerosol particles may grow
within a day or two up to sizes where they can act as cloud condensation
nuclei (CCN) and affect the radiation budget of the Earth (Makkonen et al., 2012; Kerminen et al., 2012; Wiedensohler et al., 2009). Merikanto et al. (2009)
estimated that 45 % of the global tropospheric CCN at 0.2 % super
saturation are originated from secondary particle formation. In addition to
the climatic effects, the formation and growth of secondary aerosol
particles contributes to the deterioration of the air quality as aerosol
particles are associated to adverse health effects (Oberdörster et al.,
2005). Despite its importance, the mechanisms underlying secondary new
particle formation are not yet fully understood (see Kulmala et al., 2014).
In the atmosphere, new particle formation (NPF) occurs in different steps
including formation of low volatile vapours, clustering of vapour molecules
and subsequent growth (see Kulmala et al., 2014).The presence of air ions
can facilitate the formation and growth of new particles by aiding the
stabilization of the molecular clusters during the initial stages of
nucleation (so called ion-induced nucleation) (e.g. Yu and Turco, 2000). The
magnitude of the contribution of ions to atmospheric NPF however is still
under investigation. On one hand, several studies reported a rather low
contribution of ion-induced nucleation to the total NPF events, 10–30 %
(Hirsikko et al., 2011, and references therein), with even lower values
observed in urban areas, 0.2–1.3 % (Gagné et al., 2012; Iida et
al., 2006; Herrmann et al., 2014). On the other hand, some models and
chamber studies suggest that ion-mediated nucleation (which considers
ion–ion recombination) may be a significant path for NPF (Yu and Turco,
2011; Yu, 2010; Svensmark et al., 2007; Nagato and Nakauchi, 2014). Chamber
studies in the CLOUD project have shown that in low temperatures and at low
precursor species concentrations, ion-induced nucleation can have a
significant contribution to total nucleation rates (Kirkby et al., 2011;
Riccobono et al., 2014). Based on earlier urban studies by Gagné et al. (2012), Iida et al. (2006) and Herrmann et al. (2014), we assume that ions
and charged particles detected in Paris are the naturally charged fraction
of total aerosol particles.
In this study, the air ions were mobility-classified as small or cluster
ions (3.2–0.5 cm2 V-1 s-1), intermediate
(0.5–0.034 cm2 V-1 s-1),
and large ions (0.034–0.0042 cm2 V-1 s-1), which correspond to mobility diameters of 0.8–2,
2–7 and 7–20 nm, respectively. Small ions are always present in the
air and are responsible for the atmospheric electrical conductivity (e.g.
Harrison and Carslaw, 2003; Hirsikko et al., 2011). They are mainly formed
from ionizing radiation of decaying radon, gamma radiation and galactic
cosmic radiation. Thunderstorms, water splashing, and rain and snow storms
also contribute to the formation of air ions in the atmosphere (Virkkula et
al., 2007; D'Alessandro, 2009; Tammet et al., 2009; Kolarž et al.,
2012). Increased small ion concentrations have been observed in the vicinity
of power lines (Jayaratne et al., 2008). Small ion emissions in fuel burning
processes in engines or via nucleation from exhaust gas have been studied
theoretically and experimentally (e.g. Yu and Turco, 1997; Haverkamp et al.,
2004; Gopalakrishnan et al., 2005; Lähde et al., 2009; Jayatane et al.,
2010; Ling et al., 2013). The most important sinks for ions are ion–ion
recombination to form neutral particles, and attachment to pre-existing
aerosol particles (Hoppel, 1985; Hoppel and Frick, 1986).
Urban areas are important sources for global aerosol and CCN load because
they emit both primary particles and precursors for secondary particle
formation. Nevertheless, the number of studies focusing on the behaviour of
air ions and particularly its association to NPF in urban areas around the
world is still somewhat limited (e.g. Tiitta et al., 2007; Hirsikko et al.,
2007b; Retalis et al., 2009; Tammet et al., 2014; Gagné et al., 2012;
Herrmann et al., 2014; Backman et al., 2012; Crilley et al., 2014; Jayaratne
et al., 2010, 2014; Ling et al., 2013; Ling et al., 2010; Siingh et al.,
2013; Lee et al., 2012; Iida et al., 2006, 2008; Pikridas et
al., 2015), and actually only some of them measured ion size distributions.
The main aim of this study was to determine the frequency and seasonal
variations of NPF events in a megacity based on ion number size distribution
measurements. Our research was developed within the framework of the
project “Megacities: Emissions, urban, regional and Global Atmospheric
Pollution and climate effects, and Integrated tools for assessment and
mitigation (MEGAPOLI)”, which aimed to improve the understanding of the
impacts of megacities on the climate. In this context, Paris, one of the
largest cities in Europe, was chosen as case study. Although some
publications on aerosol particles in Paris already exist (e.g. Crippa et
al., 2013; Freutel et al., 2013; Freney et al., 2014; Sciare et al., 2010; Pikridas et al., 2015), only Pikridas et al. (2015) considered air ion
number size distributions (> 0.8 nm), which allows the evaluation
of ion number concentrations in early stages of NPF. Pikridas et al. (2015)
provided valuable information on the spatial variation of NPF events and
particle number concentrations as well as on factors affecting NPF in Paris
and surrounding areas. However, their study was based on rather short
campaigns (about 2 months of data) and air ion number size distributions
were used only to classify NPF events and to determine their duration and
frequency. Our study complements Pikridas et al. (2015) by providing
detailed information on the behaviour of air ion concentrations of both
polarities in three different size ranges, and particle number
concentrations in Paris for over 1 year.
Materials and methods
We measured air ion size distributions (0.8–42 nm) and aerosol particle
number (6–740 nm) at an urban background site in Paris from 26 June 2009
to 4 October 2010, using an air ion spectrometer (AIS), and a combination
of a twin differential mobility particle sizer (TDMPS) and condensation
particle counter (CPC). In addition to seasonal variations and frequency of
NPF events, we also analyzed seasonal variations and diurnal cycles of air
ions and aerosol particles on workdays and weekends, and on NPF event and
NPF non-event days. Furthermore, we estimated the average condensation
sinks, and the growth rates of ions on workdays and weekends, and provided a
statistical summary of air ions and aerosol particle number concentrations
in Paris.
Description of the site
Paris is a megacity with 12.2 million inhabitants in its urban area (2.2 million
in the centre alone) (INSEE, 2010). Our measurements of air ion size
distributions and particle total number concentrations were located at the
Laboratoire d' Hygiène de la Ville de Paris building (LHVP) on 13th
Arrondissement (latitude 48.83∘; longitude 2.36∘) in
Paris (Fig. 1), from July 2009 to October 2010. Particle number size
distributions were measured from a container on the ground of the LHVP
building, whereas air ion size distributions were measured on top of the
building (about 15 m high). LHVP is located about 400 m away from busy
intersections and is considered an urban background site (Sciare et al.,
2010; Favez et al., 2007). The site was surrounded by a small street, a park
and restaurants (Freutel et al., 2013). According to Crippa et al. (2013),
important anthropogenic sources of particles in the site are traffic,
cooking (from restaurants around noon and evenings), and biomass burning in
general, whereas an important natural source is secondary particle
formation.
Location of the LHVP site in Paris (on the rooftop of Laboratoire
d'Hygiène de la Ville de Paris, Paris 13 arrondissement, 11 Rue George
Eastman, 75013 Paris).
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Description of the instruments
Air ion number size distributions
We used an Air Ion Spectrometer (AIS, Airel Ltd.) (Mirme et al., 2007) to
measure the size distributions of naturally charged particles and ions of
both polarities simultaneously during 26 June 2009–4 October 2010 in
Paris, France. The AIS comprises of two identical differential mobility
analysers (DMA), one for each polarity. Particle size is determined based on
the electrical mobility of the particle in the electric field, and particle
number concentration is calculated based on the intensity of the currents
measured by the electrometers at an outer cylinder of the DMA. The AIS
measures electrical mobilities varying from 3.2 to 0.0013 cm2 V-1 s-1, which is equivalent to mobility diameters of 0.8–42 nm
(Mäkelä et al., 2006).
The main sampling line of the AIS was 0.6 m long (inner diameter: 35 mm)
with a total inlet flow rate of 60 L min-1 which was equally divided
between both DMAs. A metallic grid was used in front of the inlet to prevent
for instance large dust particles to enter the system. The sheath air flow
of the DMAs was cleaned using corona chargers and electrical filters, and
reused in a closed loop at 60 L min-1 (Gagné et al., 2011). The
instrumental setup and calibration are described in more details by Mirme et al. (2007) and Asmi et al. (2009), respectively. The accuracy of the
particle number concentration of the AIS was estimated to be 10 %, which
was mainly due to flow rate uncertainties (Mirme et al., 2007). During the
campaign the accumulating air pollution inside the instrument causes
decreasing flow rates between the maintenance periods. This may further
increase the uncertainty of measured particle size and number especially at
the larger end of the measured spectra.
Number size distributions and total concentrations of fine aerosol
particles
We used a twin differential mobility particle sizer (TDMPS) to measure the
particle number size distribution (diameter 3–740 nm) during July 2009.
The instrument comprised of a neutralizer, two Hauke DMAs (lengths: 110
and 280 mm; both with inner and outer diameters of 50 and 67 mm,
respectively) and two condensation particle counters (CPC), models TSI 3025A
(d50: 3 nm, accuracy ±10 % at 105 cm-3) and TSI 3010
(d50: 10 nm, accuracy ±10 % at 104 cm-3). The first
DMA classified particles from 3 to 72 nm, while the second DMA classified
particles from 25 to 740 nm. The sampling and sheath flow rates were 2
and 20 L min-1, respectively, for the first DMA, and 0.5 and 5 L min-1 for the second DMA. The sampled air was dried
using an automated diffusion dryer (Tuch et al., 2009). According to
Wiedensohler et al. (2012) the drier is estimated to cause about 28 % and
8 % of losses for particles of 3 and 10 nm, respectively, for the given
flow rate through the drier. The TDMPS data were averaged per hour.
We also measured the total number concentration of fine aerosol particles by
using a condensation particle counter (CPC, TSI 3772, dp50: 6 nm, accuracy
±10 % at 104 cm-3 and ±20 % at 5×104 cm-3) during 11 August 2009–4 October 2010. In order to
reduce the cut-off diameter from typical 10 nm to 6 nm, the condenser of the
CPC was operated at 10 ∘C instead of the common operational
temperature of 22 ∘C. The sampled air was dried using a Nafion
dryer.
Data treatment and definitions
Air ion data containing negative concentrations (positive ions: 0.64 % of
all data; negative ions: 1.18 %), concentrations measured during unstable
flow rates (optimum range: 1000 cm3 s-1±8 %) and very
noisy data were considered invalid. A three-point median filter was applied
to the ion concentrations to reduce noise as suggested by Kulmala et al. (2012). The air ions were mobility-classified as small or cluster ions (3.2–0.5 cm2 V-1 s-1), intermediate
(0.5–0.034 cm2 V-1 s-1),
and large ions (0.034–0.0042 cm2 V-1 s-1), which correspond to mobility diameters of
0.8–2,
2–7 and 7–20 nm, respectively. In these size ranges charged
particles are assumed to be single charged. In this study, we apply the
mobility diameter, i.e. Millikan diameter, when converting the measured
mobility to particle diameter (see Mäkelä et al., 1996).
The particle total number concentrations for the entire campaign were
obtained by combining the total concentrations measured by the TDMPS
(calculated from 6 to 740 nm, 1 h means, period: 1–31 July 2009) with
the concentrations measured by the CPC (> 6 nm, 1 h means, period:
11 August 2009–4 October 2010). Total concentrations below 100 cm-3
were considered invalid as these values are unrealistic for urban areas.
To analyse the behaviour of the ion population during NPF we plotted air ion
size distributions as a function of time, from 27 June 2009 to 3 October 2010. Based on the plots, we classified the days into NPF events, NPF
non-events, or undefined days according to the procedure described by
Hirsikko et al. (2007a). NPF event days referred to days where new particle
formation and growth was clearly observed for several hours; NPF non-event
days comprised days of no particle formation, and undefined days referred to
days in which the occurrence of NPF was unclear.
The growth rates (GR) of ions were calculated based on the
maximum-concentration method described in Kulmala et al. (2012): (1) we
manually selected the time of peak concentrations during NPF for each
particle size range, (2) applied a Gaussian fit to the manually selected
peak to determine the time of maximum concentration of that particle size
range, and (3) calculated the GR by linear regression (least-squares fit) to
the data points of particle size vs. time of maximum concentration.
Condensation sink (CS) was calculated based on the equations described by
Dal Maso et al. (2005) using dry particle number size distributions. The
approach estimates the loss rate of the condensable vapours during the
change from the gas-to-particle phase (Kulmala et al., 2001). A high CS
indicates the presence of large number of aerosol particles acting as both
condensing nuclei for vapours and coagulation surfaces for particles.
Months were classified into seasons as follows: winter – December, January,
and February; spring – March, April, and May; summer – June, July, and August;
autumn – September, October, and November. The air ion data were originally
averaged every 3 min; however, as the particle number data from the TDMPS
was provided as hourly means, to facilitate comparison the air ion data and
the particle number concentration data from the CPC were also presented as
hourly means. The only exceptions were Fig. 6 (a, b, c, d) and Appendix Fig. A2,
where the air ion data were shown in the original format (3 min means).
Moreover, all the data in this study were presented at UTC (Paris local time:
UTC+1 h in the winter, and UTC+2 h in the summer), and when calculating
concentrations on workdays and weekends, national holidays were classified
as weekends.
Results and discussion
Concentrations of ions and particles at the LHVP site
The median of the daily means, and the median of the hourly means of
particle number concentration in the LHVP were 12 900 cm-3 (data not
shown) and 12 500 cm-3 (Table 1), respectively. Aalto et al. (2005) and
Puustinen et al. (2007) observed daily medians of particle number
concentrations ranging from 9000 to 38 500 cm-3 (both studies
combined) in urban background sites of European cities, including Augsburg,
Stockholm, Helsinki, Amsterdam, Birmingham, Athens, Barcelona, and Rome. The
mean particle number concentrations in urban and suburban areas of São
Paulo, Nanjing, and Beijing were 23 500, 23 000, and 23 900–32 800 cm-3 (combined studies), respectively (Backman et al.,
2012; Herrmann et al., 2014; Wu et al., 2008; Wang et al., 2013). Thus,
particle number concentrations in Paris (LHVP) were in range with the daily
medians of other European cities and were lower roughly by a factor of 2
(mean: 13 700 cm-3) compared to busy cities of other continents.
Pikridas et al. (2015) evaluated mean particle number concentrations during
the summer and winter in the LHVP site and reported similar concentration
for particles of 10–500 nm (mean of both seasons: 13 500 cm-3). In
general, particle number concentrations tend to vary considerably among
cities due to differences in meteorology, spatial and temporal distribution
of local sources, emission-cleaning technologies, and air quality
regulations.
The mean number concentrations of small ions at the LHVP site were 330
and 390 cm-3 for positive and negative polarities, and are
close to the lower range reported in the review by Hirsikko et al. (2011)
for sites around the world, 200–2500 cm-3. Shortly after their
formation the small ions are removed from the air by ion-recombination and
by coagulation with larger particles. Thus in polluted environments, where
the load of aerosol particles is high (leading to high condensation sink),
the concentrations of small ions are often lower than in cleaner
environments (Hirsikko et al., 2011, and references therein; Tiitta et al.,
2007; Hirsikko et al., 2007b).
In Nanjing, China, the total concentration of small ions, aerosol particles
and CS were 840 cm-3 (sum of polarities), 23 000 cm-3 and
5.4×10-2 s-1, respectively (Herrmann et al., 2014).
Considering only the period of July 2009 and 15 January/15 February 2010, when CS calculations were possible, the mean small ion concentrations,
particle number, and CS in LHVP were 800, 14 460 cm-3 and 1.43×10-2 s-1, respectively. The small ion
concentrations in Nanjing were similar to that of the LHVP despite the
considerably higher particle total number and CS in Nanjing. The large
particle surface area acting as coagulation and condensation sinks in
Nanjing should result in lower concentrations of small ions in comparison to
LHVP. Since this was not observed, the results suggest that Nanjing may have
a higher production rate of small ions than Paris. Other studies around the
world reported mean and median concentrations of small ions to be in the
same range with those observed in LHVP, varying from 183 to 860 cm-3
for positive and 151–720 cm-3 for negative ions near traffic and in
urban background of cities such as Athens (Greece), Kuopio (Finland),
Helsinki (Finland) and Brisbane (Australia) (Retalis et al., 2009; Tiitta et
al., 2007; Hirsikko et al., 2007b; Ling et al., 2013).
The concentrations of intermediate ions were in general very low.
Intermediate ions were mostly present on NPF event days in comparison to NPF
non-event days (Sect. 3.5). The mean concentrations of intermediate ions
during the whole campaign were 20–30 cm-3 per polarity, and were
similar to the annual mean observed by Tammet et al. (2014) in the city of
Tartu, Estonia, 35–40 cm-3 (per polarity), but roughly half of that
observed by Tiitta et al. (2007) (40–70 cm-3 per polarity) near a
road in Kuopio, Finland. One explanation for the higher concentrations in
Kuopio could be the proximity of the road, as some studies (Jayaratne et
al., 2010; Ling et al., 2013; Ling et al., 2010; Lee et al., 2012) reported
ion concentrations near traffic to be higher than in sites away from
traffic.
The median concentrations of positive and negative large ions were 410
and 270 cm3, respectively. In Helsinki, Hirsikko et al. (2007b) reported weekday median concentrations of large ions (10–40 nm)
of 510 and 540 cm-3 for positive and negative polarities, respectively.
When the hourly data were segregated into workdays and weekends, the weekday
concentrations of large ions (10–40 nm) were 1220 and 850 cm-3, for positive and negative polarity, thus considerably higher than
the observations in Helsinki. As the concentrations of large ions are
influenced by traffic-related aerosols (Hirsikko et al., 2007b; Tiitta et
al., 2007), the difference in large ion concentrations could be due to a
larger number of sources of aerosol particles, i.e. vehicle emissions, in
LHVP surroundings in comparison to Helsinki.
In July 2009, 41 % of the total particles in the size range of 3–23 nm
were comprised of naturally charged particles (sum of positive and negative
polarities). The month-to-month median concentrations of ions from 0.8 to 42 nm
varied between 1000 and 2000 cm-3 per polarity (data not
shown).
Figure A1 shows correlations between particle number and ions. Particle
number correlated the highest with large ions of both polarities (r=0.58–0.61, p<0.01)
as expected, since it is very likely that these
aerosol particles were in charge balance most of the time (aerosol particles
quickly attach to ions forming large ions). Weak or no correlation was found
between particle number and small or intermediate ions (r<|0.18|,p<0.01). Accumulation mode aerosol particles that
have large surface area (thus creating high condensation sinks) and small
ions are often negatively associated as the first act as sink for the
latter. Aerosol particles in urban areas are mainly originated from human
activities such as traffic, while small ions are originated from natural
ionizing radiation, hence the weak correlation.
Diurnal cycles and number size distributions of ions
Diurnal cycle of particle number concentrations (> 6 nm) (g), and small (0.8–2 nm), intermediate (2–7 nm), and large ions (7–20 nm) (a–f) for workdays and weekends. The markers show the hourly
median concentrations and the whiskers show 25th and 75th
percentiles. The dashed lines represent mean concentrations, and the
rectangles (06:00–09:00) indicate the morning peak of particle number.
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Figure 2 shows the diurnal variations of ions and particle number
concentrations. On workdays, particle number concentrations peaked in the
morning (07:00–08:00) and in the evening (19:00–20:00) (Fig. 2g)
reflecting traffic rush hours. This pattern was consistent with the findings
of Pikridas et al. (2015) in Paris during summer and winter. The evening
peak was fairly constant regardless of the day, whereas the morning peak on
workdays was about 50–60 % higher than on the weekends, when traffic
intensity is generally lower. The constant presence of an evening peak
suggests constant nocturnal activities in the area, e.g. traffic and/or
cooking emissions from restaurants as suggested by Freutel et al. (2013). A
decrease in boundary layer mixing height also plays a role in accumulating
air pollutants in the evening due to poor dilution, as suggested by Pikridas
et al. (2015). Cimini et al. (2013) shows that the mixing height of the
boundary layer in 15 August 2011 in SIRTA, a site 20 km away from LHVP,
increased at 08:00 and decreased at 18:00 (UTC), roughly the time when the
evening peak begins.
Large ions had maximum median concentrations of 400–600 cm-3 per
polarity and a diurnal cycle very similar to that of particle number (Fig. 2e–f),
undergoing an abrupt increase from the night to the morning rush
hours and from weekends to workdays. As traffic produces aerosol particles,
the concentrations of large ions are likely resulting from the coagulation
between neutral aerosol particles and small or intermediate ions. Because
busy intersections were located about 400 m away, it is possible that
particulate traffic emissions from the intersections enhanced concentrations
of large ions in LHVP. Note that as the instruments measuring particle
number and ions overlap from 6 to 20 nm, some of the intermediate and all of
the large ions were also detected by the CPC. If we compare the diurnal
cycle of particle number concentrations to that of large ions (sum of
polarities), the latter comprised about 5.5 % of the total particle number
concentrations (in 6–20 nm size range) in the morning of workdays.
Small ion number concentrations of both polarities peaked early in the
morning (Fig. 2a–b) and decreased during the day in agreement with some
studies reviewed by Hirsikko et al. (2011). The higher concentrations on
early mornings may be attributed to both the accumulation of ionizing
radiation from radon decay, as the boundary layer mixing height is usually
lower before sunrise (Hirsikko et al., 2011), and the lower condensation
sinks early in the mornings (Fig. A3), which decrease the removal rate of
small ions.
On workdays, the peak median number concentrations of small ion were between
380 and 430 cm-3 per polarity. On weekends, the number
concentrations of small ions were slightly higher (400–490 cm-3) and
the elevated concentrations of positive small ions lasted a few hours longer
than on workdays, indicating that the production rate of small ions (i.e.
from radon radiation) was similar throughout the week but the removal rates
were lower on weekends (lower level of coagulation sink). The simultaneous
decrease in small ion concentrations and increase in large ion
concentrations and particle number (Fig. 2e–g) suggests that part of the
small ion population was lost by attachment to aerosol particles as observed
in previous studies (Hirsikko et al., 2007b; Jayaratne et al., 2014). The
concentrations of positive small ions in the mornings of workdays (07:00)
were about 26 % lower in comparison to the mornings of weekends (07:00)
indicating that this fraction may have been lost by coagulation to
pre-existing aerosol particles.
The median number concentrations of intermediate ions (Fig. 2c–d) were
low and were considerably different from the mean indicating a large
variability. On workdays, the median concentrations of positive intermediate
ions showed two peaks (04:00–05:00 and 12:00–13:00), while in the
weekends only one shallow peak was observed. The decrease in concentrations
of intermediate ions in the mornings of workdays between 06:00 and 08:00
coincided with the peak in particle number and CS (Figs. 2 and A3),
indicating that coagulation sinks from traffic emissions scavenged the
intermediate ions. On weekends, with the decrease in the number of aerosol
particles, the number concentrations of intermediate ions remained elevated
for several hours. Thus, NPF along with the decrease of particle number
concentrations (condensation sinks) in the afternoon enhanced concentrations
of intermediate ions around 12:00–13:00. As intermediate ions are
directly associated to NPF, the results indicate that NPF was more likely to
occur on weekends than on workdays in LHVP. Negative intermediate ions
showed a similar diurnal cycle as the positive intermediate ions, only with
lower concentrations. Despite the effects of traffic on the ion number
concentrations, traffic intensity did not seem to influence the median ion
size distribution (Fig. A2) in agreement with Tiitta et al. (2007).
Studies near busy roads (10–100 m away) in Finland reported that traffic
emissions caused a decrease in small ion concentrations and an increase in
both intermediate and large ions (Hirsikko et al., 2007b; Tiitta et al.,
2007) which agrees with our results for small and large ions but disagree
for intermediate ions. In Helsinki, the weekday diurnal peak concentrations
of small, intermediate, and large ions were roughly 750–900, 80–90 and 950–1000 cm-3, respectively per polarity. The
number concentrations were thus higher in Helsinki despite similar CS range
between the sites (Helsinki: 1–50×10-3 s-1, LHVP: 1.7–51×10-3 s-1). The discrepancy is likely caused by
higher radon decay and gamma radiation emissions rates from soils in
Helsinki. The World Health Organization estimates higher levels of indoor
radon emissions in Finland (120 Bq m-3) than in several other European
countries, including France (89 Bq m-3) (WHO, 2009). In addition to
differences in radon emission rates, the size-classification of intermediate
and large ions in Hirsikko et al. (2007b) (3–10 and 10–40 nm,
respectively) was different than our classification, which could explain the
larger concentrations. Also the study in Helsinki was developed in the
summer while ours represents an average of all seasons.
Seasonal variations of ions and particles
Seasonal variations of particle number (d) and positive/negative
ions (a–c). The bars represent median concentrations, the whiskers
represent 25th and 75th percentiles, and n (±) represents
the number of hours included in each season (winter/spring/summer/autumn).
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The number concentrations of small ions of both polarities (Fig. 3a) were the
highest in the summer and autumn (maxima between July and September,
depending on the polarity, Fig. A4) and lowest in the spring.
Concentrations in January and February were also relatively high. Lopez et
al. (2012) measured concentrations of 222Rn for 8 years in
Gif-sur-Yvette, 20 km away from the LHVP site, and reported the highest
radon concentrations in autumn and the lowest in the summer/spring. As radon
and gamma radiations are major sources of small ions in continental areas
(Hirsikko et al., 2011), the seasonality of small ions is partially
associated to the seasonality of radon exhalation, which depends for
instance on boundary layer mixing height, presence of fog, snow coverage and
soil humidity (Lopez et al., 2012). Despite the differences in altitude,
Rose et al. (2013) also observed the lowest concentrations of small ions in
spring in Puy de Dôme, a mountain in central France (1465 m a.s.l.). In Athens, the highest concentrations of small ions were observed in
the summer (Retalis et al., 2009). As radon emissions depend on several
factors, concentrations of small ions are expected to vary between sites. In
addition to radon concentrations, in spring the higher frequency of NPF may
also have increased the scavenging of small ions.
Statistical summary of particle number concentration (6–740 nm),
small (0.8–2 nm), intermediate (2–7 nm), and large ion (7–20 nm) number
concentrations in Paris for the entire campaign. Total ions represent ions in
the size range of 0.8–42 nm in size. Concentrations were presented as
particles cm-3 and were based on 1 h means.
Mean
SD
5 %
25 %
50 %
75 %
95 %
No. of hours
(1 h scale)
Small ions (+)
330
150
130
230
310
400
600
7810
Small ions (–)
390
180
160
270
360
470
740
7820
Intermediate ions (+)
30
40
0
0
10
30
100
10 310
Intermediate ions (–)
20
70
0
0
10
10
60
10 310
Large ions (+)
460
240
160
290
410
590
910
10 310
Large ion (–)
310
180
80
180
270
410
650
10 310
Total ions (+)
1640
660
780
1180
1530
1980
2880
10 310
Total ions (–)
1270
540
590
900
1180
1530
2290
10 310
Particle number concentrationa
13 690
6430
5590
9200
12 460
16 840
26 000
9310
CS (3–740 nm)b (×10-3 s-1)
14.3
8.4
4.7
7.9
12.7
18.1
31.3
1520
a Particle number: combined TDMPS (6–740 nm; 29 June 2009–31 July 2009)
and CPC (dp50: 6 nm; 11 August 2010–4 October 2010) measurements.
b CS was calculated based on the TDMPS size distribution from 29 June 2009 to 31 July 2009 and from 15 January 2010 to 19 February 2010.
The median number concentrations of positive intermediate ions (Fig. 3b)
varied with season showing the highest median number concentrations in
spring, whereas the median number concentrations of the negative
intermediate ions were lower (< 10 cm-3) and more stable
throughout the year. For positive intermediate ions, the highest monthly
median concentrations were observed in February, March and May (peak), while
for the negative polarity, the highest were observed in February, March
(peak) and November (Fig. A4). The concentrations of positive
intermediate ions were highly variable in July (Fig. A4), with 75th
percentile reaching nearly 200 cm-3. Because intermediate ions are
mostly observed during NPF events (Tammet et al., 2014) and these events
have been observed to occur more often in the spring/summer around Europe
(Manninen et al., 2010), high number concentrations of intermediate ions
during these seasons were expected. In general, the results suggest that
positive intermediate ions were more affected by seasonality than the
negative intermediate ions.
The number concentrations of positive large ions were also fairly stable
throughout the seasons (between 400 and 450 cm-3), whereas the number
concentrations of negative large ions were less stable (between 230 and 310 cm-3) showing lowest in the summer and highest in
the winter and autumn (Fig. 3c), resembling the seasonal variations of
particle number (Fig. 3d). Aalto et al. (2005) showed that in several
European cities particle number concentrations were highest in the winter
and lowest in the summer, in agreement with our study. Pikridas et al. (2015), also reported this pattern for Paris and surrounding areas. The
lower mixing height of the boundary layer and the need for heating are
possible drivers for the increase in particle number concentrations in the
winter.
Frequency of NPF events
To analyse new particle formation events we classified days into NPF event,
NPF non-event, and undefined as described in Hirsikko et al. (2007a). The
monthly frequency of NPF events in LHVP is shown in Fig. 4 as percentage of
NPF events per number of days. On average, NPF events occurred between
February and October, being most frequent in the spring and summer (highest
in May and July) and least frequent in the winter. Undefined and NPF
non-event days on the other hand occurred throughout the year. Manninen et
al. (2010) analyzed NPF based on ion concentrations in 12 European sites
and reported that several sites showed the highest frequency of NPF event days
in spring/summer and minimum in the winter, in agreement with our study.
Studies from urban areas such as Helsinki, Budapest, Beijing, and Pittsburgh
also reported high incidence of NPF in spring (Salma et al., 2011; Hussein
et al., 2008; Wu et al., 2007; Stanier et al., 2004). Pikridas et al. (2015)
also observed considerably higher frequency of NPF events in the summer than
in the winter in Paris and in two surrounding suburban sites.
Monthly frequency (%) of NPF events, NPF non-events and
undefined days. Data collected continuously from July 2009 to September
2010.
![]()
The higher incidence of solar radiation favours photochemical reactions in
the atmosphere in spring and summer which may consequently increase, the
frequency of NPF, as observed by Pikridas et al. (2015). In addition to
meteorological conditions, the air in LHVP and in several other sites in
Europe is cleaner in the summer than in the winter (Aalto et al., 2005;
Pikridas et al., 2015). Thus, NPF was likely favoured by fewer aerosol
particles acting as condensation sinks (Salma et al., 2011; Wu et al., 2007;
Stanier et al., 2004; Pikridas et al., 2015) in the summer.
In our study, air ions were monitored for a total of 442 days, out of which
57 days were NPF events (about 13 %), 94 were undefined days, and 291 were
NPF non-event days. In non-urban environments, NPF was observed to occur
somewhere between 21 and 57 % of the days depending on the site (Manninen
et al., 2010). In urban areas, however, NPF is expected to be less frequent
due to the higher number of condensation sinks competing for condensing
vapours (Hussein et al., 2008). In cities such as Nanjing (China), São
Paulo (Brazil), Helsinki (Finland), Shanghai (China), Pune (India), Kanpur
(India), Birmingham (UK), and Budapest (Hungary), the frequency of NPF events
was between 5 and 27 % (Herrmann et al., 2014; Backman et al., 2012;
Hussein et al., 2008; Du et al., 2012; Leng et al., 2014; Xiao et al., 2015;
Kanawade et al., 2014; Zhang et al., 2004; Salma et al., 2011) which is
within range of the observations in Paris (13 %). However, NPF frequencies
as high as 40–55 % were observed in Beijing (China), Pittsburgh (USA),
Brisbane (Australia), and Nanjing (Wu et al., 2007, 2008; Stanier et al.,
2004; Crilley et al., 2014; Yu et al., 2015), although not all the studies
comprised an entire year of measurements.
Diurnal cycle of ions during new particle formation
Diurnal cycle of aerosol particles and ions (small: 0.8–2 nm;
intermediate: 2–7 nm; large: 7–20 nm) on strong NPF event days and NPF
non-event days. The markers show the hourly median number concentrations and
the whiskers show 25th and 75th percentiles (1 h data points).
![]()
Figure 5 shows the differences in diurnal cycles of ions and particles on
NPF events and NPF non-event days. In this section, only strong NPF events
were considered (21 NPF event days). On NPF event days, a clear peak was
observed between 09:00 and 11:00 (UTC) for intermediate ions and at 12:00–14:00 for large ions and particle number, whereas on NPF non-event days
these “noon” peaks were completely absent. As NPF is often observed at
noon, an increase in concentrations around this time was expected. The
time-lag in peak concentrations between intermediate and large ions was
likely caused by growth of intermediate ions. During NPF, the highest
increase in concentrations occurred for intermediate ions, with median
maxima of 50–80 cm-3 (10:00–11:00), about 8.5–10 times higher
than on the same hour of NPF non-event days, depending on the polarity.
Because the median concentrations of intermediate ions were very low on NPF
non-event days (< 12 cm-3), the results suggest that
intermediate ion concentrations may be used as indicator for NPF events in
Paris. The median particle number and large ion concentrations increased 1.2
times (12:00) and 1.5–1.8 times (12:00–13:00), respectively, on NPF
event days in comparison to the same hour on NPF non-event days. Thus,
despite its relatively low frequency (13 %), NPF was an important source
for intermediate ions, large ions and particles in Paris around noon. In
cities such as Pittsburgh, USA, where the frequency of NPF events is high
(50 %) (Stanier et al., 2004), particle number concentrations roughly
doubled during NPF bursts (45 000 cm-3) in comparison to the morning
rush hours of workdays (23 000 cm-3). The authors estimate that the
particle number daily averages on NPF event days was about 40 % higher
than on NPF non-event days. In LHVP, the mean of the daily mean
concentrations on event days (11 744 cm-3) was in fact lower than on
NPF non-event days (14 259 cm-3) probably due to the lower
concentrations of aerosol particles in the mornings of NPF event days.
As mentioned, in the morning of event days the concentrations of large ions
and especially aerosol particles (Fig. 5e–g) were lower than on NPF
non-event days, which may have favoured NPF. This result is consistent with
the idea that NPF can be favoured on weekends due to the lower condensation
sink. The cleaner atmospheric conditions illustrated in Fig. 5 could have
been caused for instance by enhanced turbulent vertical mixing on NPF days
(Nilsson et al., 2001). According to Wehner et al. (2010) and Nilsson et al. (2001) a higher vertical mixing could favour NPF not only by increasing the
dilution of condensation sinks in the atmosphere, but also by mixing
condensable vapours with cooler air from higher altitudes, thus increasing
supersaturation, or even by transporting clusters formed at higher altitudes
downwards.
New particle formation did not affect the small ion concentrations as much
as it did the other particle sizes. On event days, the concentrations of
positive small ions decreased roughly around noon in comparison to NPF
non-event days, indicating scavenging of these ions by the newly formed
particles. This decrease around noon was also observed for negative small
ions; however, the number concentrations of these ions were in general
slightly lower on NPF event days in comparison to NPF non-event days.
Winkler et al. (2008) indicates that ion-induced nucleation is formed
preferably onto negative ions, thus, the decrease in negative small ion
concentrations could indicate that part of these ions were used during
ion-induced nucleation. Yet, we only observed a weak positive correlation (r=0.10 to 0.25, p< 0.01) between intermediate and small ions
(Fig. A5). In general, no clear correlation between NPF (intermediate
ion/small ions) and small ions or particle number was observed (Fig. A6).
Growth rates of ions
Growth rates of ions (mean of positive and negative) calculated
from 21 NPF event days (9 workdays and 12 weekends). The total growth rates
(GRtot) include both workdays and weekends. The unit for GR is nm h-1. The CS calculations were based on TDMPS data from Jul 2009 and
January/February 2010 (hourly means).
Mean
5 %
25 %
50 %
75 %
95 %
No. days
GRtot (1.9–3 nm)
4.0
1.3
2.5
3.4
5.7
7.4
21
GRtot (3–7 nm)
7.6
1.9
3.9
5.9
9.1
24.1
21
GRtot (7–20 nm)
8.5
4.0
6.3
6.9
10.8
17.6
21
Workdays
GR (1.9–3 nm)
4.1
2.1
2.8
3.4
5.7
6.9
12
GR (3–7 nm)
7.1
3.1
4.4
6.8
9.3
12.2
12
GR (7–20 nm)
8.8
6.4
6.8
8.0
9.1
16.5
12
CS(3–740 nm) (×10-3 s-1)
14.9
5.2
8.5
13.1
18.5
33.2
51
Weekends
GR (1.9–3 nm)
3.9
1.0
2.1
3.3
5.5
7.9
9
GR (3–7 nm)
8.0
1.7
3.2
5.0
8.4
28.4
9
GR (7–20 nm)
8.3
3.6
4.5
6.5
11.6
18.6
9
CS(3–740 nm) (×10-3 s-1)
12.7
3.7
7.1
11.1
16.7
26.7
19
Particles growth rate (GR) is proportional to the concentrations of
condensing vapours in the air. We calculated GR for ions in diameters of 1.9–3, 3–7, and 7–20 nm. A total of 21 strong NPF events were used
in the calculations, 9 of which were workdays and 12 were weekends. Thus,
the results once again suggest that NPF (in this case strong NPF events) may
be favoured on weekends due to the lower load of condensation sinks. In general, the GR of ions (Table 2) increased with ion size (median: 1.9–3 nm:
3.4 nm h-1; 3–7 nm: 5.9 nm h-1; 7–20 nm: 6.9 nm h-1) in agreement with previous studies, including urban areas
(Yli-Juuti et al., 2011; Manninen et al., 2010; Kulmala et al., 2012, 2004b; Backman et al., 2012; Herrmann et al., 2014). The
results support the theory that condensing vapours aiding the growth of ions
from 3 to 20 nm may differ in composition from vapours aiding the growth of
smaller ions, as suggested by previous studies (Manninen et al., 2010). In
addition to different chemical composition, Kulmala et al. (2004b) suggests
that the increase in GR with particle size could also relate to the diurnal
availability of condensing vapours and Nano-Köhler effect (Kulmala et
al., 2004a). If the diurnal peak in vapour concentration occurred after NPF,
there would be fewer vapours available to grow the smaller particles in
comparison to the larger ones (growing later). The Kelvin effect (Kulmala et
al., 2004a, b; Yli-Juuti et al., 2011) and the
Nano-Köhler effect (Kulmala et al., 2004a) may also influence the GR as
they favour evaporation of small particles and growth of larger ones.
Moreover, the median GR was higher on workdays than on weekends for ions
from 3 to 7 and 7 to 20 nm. This pattern was not as evident for ions from
1.9 to 3 nm nor for mean GR values.
The GR of ions from 3 to 20 nm were higher on workdays likely due to the
higher availability of traffic-emitted condensable vapours. In cities such
as São Paulo, Nanjing, and Helsinki, the reported mean GR for ions were
2.1–5.3, 6.3–9.7, and 8.0–11.4 nm h-1
for size ranges of 1–3, 3–7, and 7–20 nm (7–30 nm in
Nanjing), respectively, and were in range with the GR observed in the LHVP
site (Table 2). Manninen et al. (2010) reported median GR of ions in
European sites (mostly rural and coastal sites) to be 2.8 nm h-1 for
particles of 1.5–3 nm; 4.3 nm h-1 for particles of 7–20 nm, and
5.4 nm h-1 for particles of 7–20 nm, which are mostly lower than the
values observed in the urban areas. Hussein et al. (2008) compared NPF
characteristics between Helsinki and Hyytiälä, a rural area in
Finland. The authors observed higher GR in Helsinki and concluded that the
higher availability of condensing vapours and the large number of aerosol
particles in Helsinki probably enhanced growth by condensation and
coagulation in comparison to Hyytiälä. Note that, as Hussein et al. (2008) and Yli-Juuti et al. (2011) pointed out, the GR calculation method is
somewhat subjective and thus also influences GR values. Moreover, GR can
also vary depending on the instruments used (Yli-Juuti et al., 2011).
The median CS concentrations were only slightly higher on workdays in
comparison to weekends (Table 2) indicating that part of the particle
surface area may also originate from long range transport. Sciare et al. (2010) analyzed the composition of PM2.5 in Paris and reported that
the city receives polluted air masses (PM2.5) from north-western and
central Europe. Note that CS calculations were based on roughly 2 months
of data, and thus are not representative of the entire campaign.
Conclusions
We analyzed frequency and seasonal variations of NPF events, diurnal and
seasonal cycles of ions and aerosol particles, as well as the behaviour of
ions and their growth rates during NPF events in an urban background site of
Paris, France. Condensation sinks were also calculated. Our measurement
period extended over 16 months: June 2009–October 2010. We were
especially focusing on atmospheric ions: small (0.8–2 nm), intermediate
(2–7 nm), and large ions (7–20 nm).
On workdays, particle number concentrations peaked in the mornings and
evenings, reflecting the traffic rush hours. During the morning peak, the
concentrations of small and intermediate ions decreased, whereas the
concentrations of large ions increased. This indicates that aerosol
particles from traffic acted as scavengers for small and intermediate ions.
Both ions and aerosol particle concentrations varied with season, and these
variations differed with ion polarities. Number concentrations of small ions
were lowest in the spring, when number concentrations of positive
intermediate ions were highest. The results thus indicate that when
comparing ion concentrations from different studies, one should consider the
season in which the study was conducted and also the polarity regarded.
NPF was occurred on 13 % of the days (34 weekdays and 23 weekends).
Seasonally, NPF occurred mainly in late spring and summer, and were
completely absent from November to January. Undefined days, however,
occurred throughout the year. Higher frequency of photochemical reactions
along with lower number concentrations of aerosol particles may have
enhanced the frequency of NPF in the summer. The growth rates of ions during
NPF events increased with ion size and had median values varying between 3 and 7 nm h-1 in Paris. Moreover, the median GR's of ions were higher on
workdays than on weekends for ions from 3 to 20 nm, but this pattern was
unclear for ions from 1.9 to 3 nm and for mean GR values. A higher GR during
workdays suggests higher availability of condensing vapours in comparison to
weekends.
The diurnal cycle of ions and particles during NPF events and NPF non-event
days suggest that NPF was an important contributor for both ions and aerosol
particles in Paris. On average, the NPF bursts caused an extra peak between
09:00 and 14:00 in the diurnal cycles of intermediate ions, large ions and
particle number. The intermediateions were by far the most affected by NPF,
with median concentrations increasing 8.5 to 10 times during the bursts in
comparison to the same hour on NPF non-event days. Because the median number
concentrations of intermediate ions were so low on NPF non-event days
(< 12 cm-3) in comparison to NPF event days (50–80 cm-3), the results suggest that intermediate ion number concentrations
could be used as an indicator for NPF in Paris. The intermediate ions
produced during the bursts grew to larger sizes on average within a few
hours, increasing the median number concentrations of large ions and aerosol
particles by a factor of 1.5–1.8 (depending on the polarity) and 1.2,
respectively, in comparison to NPF non-event days. The results indicate that
NPF in Paris was favoured on weekends, when the load of aerosol particles
was lower. This idea was reinforced by the statistics of strong NPF events.
Out of the 21 strong NPF events, 9 were observed on workdays and 12 were on
weekends.
In general, as aerosol particles are associated to adverse health effects,
the results suggest that NPF events influenced the air quality in Paris
around noon (increasing the total particle number concentration, not so much
the total particle mass as these are nucleation mode particle), especially
during the spring and summer, when the frequency of NPF was highest.