Air ion concentrations influence new particle formation and
consequently the global aerosol as potential cloud condensation nuclei. We
aimed to evaluate air ion concentrations and characteristics of new particle
formation events (NPF) in the megacity of Paris, France, within the MEGAPOLI
(Megacities: Emissions, urban, regional and Global Atmospheric
Pollution and climate effects, and Integrated tools for assessment and
mitigation) project. We measured air ion number size distributions (0.8–42 nm) with an
air ion spectrometer and fine particle number concentrations (> 6 nm) with a twin differential mobility particle sizer in an urban site of
Paris between 26 June 2009 and 4 October 2010. Air ions were size classified
as small (0.8–2 nm), intermediate (2–7 nm), and large (7–20 nm). The
median concentrations of small and large ions were 670 and 680 cm
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 cm
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 (
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.
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
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).
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 cm
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
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
(
We also measured the total number concentration of fine aerosol particles by
using a condensation particle counter (CPC, TSI 3772, dp50: 6 nm, accuracy
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 cm
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 (
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
The median of the daily means, and the median of the hourly means of
particle number concentration in the LHVP were 12 900 cm
The mean number concentrations of small ions at the LHVP site were 330
and 390 cm
In Nanjing, China, the total concentration of small ions, aerosol particles
and CS were 840 cm
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
The median concentrations of positive and negative large ions were 410
and 270 cm
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
Figure A1 shows correlations between particle number and ions. Particle
number correlated the highest with large ions of both polarities (
Diurnal cycle of particle number concentrations (
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
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
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
Seasonal variations of particle number
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
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
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 (
The number concentrations of positive large ions were also fairly stable
throughout the seasons (between 400 and 450 cm
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 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
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 (
Examples of NPF event days observed in the LHVP site. The first
row of figures represent positive ions measured using AIS (dp: 0.8–42 nm)
with a time resolution of 3 min. The second row represents mean number
concentrations of particle total number (
We selected four NPF event days of various intensities and duration to observe the behaviour of ions and aerosol particles during the bursts (Fig. 6). In all the 4 days, a “banana” shaped NPF event was observed. This type of NPF event is likely of regional nature as it requires uniform air masses to last for at least a few hours (Manninen et al., 2010). Thus, the gaps in the “bananas” (Fig. 6b–c) could be caused by some degree of heterogeneity in the regional air masses. According to Hussein et al. (2009), regional NPF events may spread for over 200 km and the newly formed particles may be traced for as long as 30 h before they merge into background levels. Pikridas et al. (2015) analyzed NPF events in LHVP and in two suburban sites near Paris, GOLF and SIRTA (20 km NE and 20 km SE from Paris, respectively). The authors measured particle number size distributions in all the three sites during the summer of 2009 and the winter of 2010. The results showed that nearly all the NPF events observed in SIRTA in the summer were also observed in LHVP, and roughly half of these events (6 event days) were also observed at GOLF, thus covering at least 40 km in extension. The results by Pikridas et al. (2015) indicate that at least half of the NPF events observed in LHVP in the summer were regional in nature.
The diurnal behaviour of ions varied considerably among the 4 days. On
example days, NPF started between 08:00 and 12:00 (UTC) (Fig. 6). A “pool”
of small ions was observed in all the 4 days suggesting the constant
presence of these ions, in agreement with previous studies (Manninen et al.,
2009). No significant changes in small ion number concentrations were
observed during the bursts (Fig. 6e–h). The number concentrations of
intermediate ions (both polarities) however increased 4–15 times
(depending on the day) during the bursts in comparison to the number
concentrations immediately before the bursts, reaching mean values as high
as 420 cm
Growth rates of ions (mean of positive and negative) calculated
from 21 NPF event days (9 workdays and 12 weekends). The total growth rates
(GR
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
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
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 PM
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
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.
Correlation between particle number concentrations and ions (small: 0.8–2 nm; intermediate: 2–7 nm; large: 7–20 nm).
Median size distribution of ion on workdays: early morning (02:00–04:00), rush hours (07:00–09:00) and noon (12:00–14:00).
Diurnal cycle of condensation sink (CS) based on data from 1 to 31 July 2009 and 15 January to 15 February 2010 (1 h resolution) and particle number concentrations. The markers represents median of hourly means.
Monthly variations of ions and particles in Paris. The edges of the boxes represent 25th and 75th percentiles, the central line is the median, the whiskers represent the highest concentrations (not considered outliers). The data span the period 1 July 2009–30 September 2010.
Correlation between intermediate ions and small ions.
Correlation between the ratio intermediate ions/small ions and particle number and small ions.
This project was partially developed in the frame of the European Union's Seventh Framework Programme FP/2007-2011 within the project MEGAPOLI (grant agreement no. 212520). We also gratefully acknowledge the support by the Academy of Finland Centre of Excellence Program (grant no. 1118615 and no. 272041), and the support by the French data centre for atmospheric chemistry, created and co-directed by CNES (the French Space Agency) and INSU-CNRS (National Institute of Sciences of the Universe) of the MEGAPOLI database. We would also like to acknowledge Katrianne Lehtipalo for her contribution on the MEGAPOLI measurements and data processing.Edited by: M. Beekmann